Physical geography in diagrams [4th edn] 9789353433758, 9353433754

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R. B. BUNNETT SEEMA MEHRA PARIHAR

Fourth GCSE Edition

Physical Geography in Diagrams by R. B. Bunnett was first published in 1965. The fourth edition of this book came in 1988. This internationally renowned title has now been adapted after 30 years, as per the requirements of the Indian students and their curriculums. This annotated version retains all the distinctive features of the original edition. Consequently, it should be read as an updated edition, where it will not only help students but faculties also who are teaching Geography, for various levels of students, will find this book extremely useful in their day to day lesson plans. Even students who have not studied Geography earlier can find this book extremely engaging for their further reading or for competitive readiness. The core objective of this title is to explain geographical principles and concepts through illustrations and engage students in the learning process of the physical aspects of geography through several line diagrams, 3D/2D artwork, field-based photographs, and locations of features marked on satellite imageries as it is. This book examines the systems and their processes, the landforms associated with these, and the relationships between living organisms and the inorganic environment within specific natural ecosystems. Every attempt is made to focus attention on all the main components of the physical environment as well as on the associated inter-relationships. Whenever possible, a wide range of landforms from within the framework of the British and Indian environment are examined.

HIGHLIGHTS w Includes more than 1,150 diagrams and 3D/2D artwork along with google maps, NASA satellite images w Separate section on contemporary environmental challenges and human activities w Important data, statistics, reports are presented in tabular format, along with numerous flow charts for effective preparation

`650.00 in.pearson.com

Fourth GCSE Edition

BUNNETT PARIHAR

MRP Inclusive of all Taxes

PHYSICAL GEOGRAPHY IN DIAGRAMS

Fourth GCSE Edition

w Updated content along with new pedagogical elements, chapter-end questions supported by many new diagrams, maps, visual aids

Cover image: emperorcosara. Shutterstock

Whilst the main concern of this book is to study the physical environment, where appropriate, the effects of human activities on the environment on global perspective have also been included. The present book addresses key concerns from the students’ point of view and in each chapter, there are few sections which pro-actively connect students to their role as a stake holder in creation and sustenance of different geographies around them.

PHYSICAL GEOGRAPHY IN DIAGRAMS

PHYSICAL GEOGRAPHY IN DIAGRAMS

ISBN 978-93-534-3375-8

This edition is manufactured in India and is authorized for sale only in India, Bangladesh, Bhutan, Pakistan, Nepal, Sri Lanka and the Maldives.

Size: 203x254 mm Spine: 20 mm

ISBN: 9789353433758

9 789353 433758

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All In-House Artworks by: DiacriTech for Pearson India Education Services Pvt. Ltd This title carries certain photographs which are the part of Dr. (Mrs.) Seema Mehra Parihar’s private collection, and are not to be circulated outside of this title. All copyrights to these are vested in Dr. (Mrs.) Seema Mehra Parihar, and these have been used with her permission.

Copyright © 2019 Pearson India Education Services Pvt. Ltd Published by Pearson India Education Services Pvt. Ltd, CIN: U72200TN2005PTC057128. No part of this eBook may be used or reproduced in any manner whatsoever without the publisher’s prior written consent. This eBook may or may not include all assets that were part of the print version. The publisher reserves the right to remove any material in this eBook at any time. ISBN 978-93-534-3375-8

eISBN: Head Office: 15th Floor, Tower-B, World Trade Tower, Plot No. 1, Block-C, Sector-16, Noida 201 301,Uttar Pradesh, India. Registered Office: 4th Floor, Software Block, Elnet Software City, TS-140, Block 2 & 9, Rajiv Gandhi Salai, Taramani, Chennai 600 113, Tamil Nadu, India. Fax: 080-30461003, Phone: 080-30461060 www.in.pearson.com, Email: [email protected]

Brief Contents 1 The Solar System: Positions and Time 2 Plate Tectonics: The Earth’s Structure and Landforms 3 Weathering of Slopes 4 Water on the Surface 5 Underground Water and Limestone Features 6 Glacial Processes 7 Desert Processes 8 Coastal Processes 9 The Oceans 10 Atmosphere: Temperature 11 Atmosphere: Pressure and Wind 12 Atmosphere: Water 13 The Weather Station and Weather Maps 14 Climate, Weather, and Natural Environment

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Contents Preface xi Preface to the Indian Edition  xiii About Seema Mehra Parihar  xiv Acknowledgement xv

1 The Solar System: Positions and Time  1.1 Introduction  1.2 The Solar System  1.2 Shape of the Earth  1.4 Phases of the Moon  1.6 The Sun as an Input into the Earth’s System  1.6 Position and Time  1.8

The position of a place on the earth’s surface  1.8 Rotation and Time  1.10

2 Plate Tectonics: The Earth’s Structure and Landforms  2.1 Introduction  2.2 Structure of the Earth  2.2 Isostasy   2.4 Collision of Plates  2.11 Plate Boundary  2.12 Rocks  2.17 Classification of rocks  2.17 A Global Pattern Through Plate Tectonics  2.20 Rock system in Great Britain  2.22 Rock system in India  2.23 Vulcanicity and Landforms  2.28 Distribution of Volcanoes and Volcanic Activity  2.29

Volcanic features formed in the crust  2.33 Volcanic features formed on the surface  2.34 Vent eruptions and the types of volcanoes  2.34 Craters and calderas  2.35 Composite cones or stratovolcanoes  2.37 Fissure eçruptions and the landforms they produce  2.38 Other forms of volcanic activity  2.40 Is a Volcanic Landscape Hostile?  2.40

Major Landforms  2.42 Earthquakes  2.43 Faults  2.49 Joints  2.53 Folds  2.53 Earth Movements Behind Landforms  2.55 Major Landforms  2.55 Mountains  2.56 Rift valley  2.63 Plateaus and related landforms  2.64 Plains and related landforms  2.66

3 Weathering of Slopes  3.1 Introduction  3.2 Denudation and Weathering  3.2 Types of Weathering  3.3 Geomorphic Cycles of Slope Development  3.11 Rejuvenated and Polycyclic Landforms  3.13

Mass Wasting and Slope Processes  3.14 Types of Slope Movement  3.15 Concave Slope  3.18 Other Types of Slope Movement  3.21 Vegetation Protects the Slopes  3.21

viii  Contents

4 Water on the Surface  4.1 Introduction  4.2 Global Water and the Atmosphere  4.3 River Basin Drainage—an Open System  4.5 Dynamics of Water Supply  4.7 Supply and demand  4.7 Human Impact on Drainage Basins  4.8 Water storage  4.8 Irrigation  4.12 Flooding  4.18 River system  4.20 Stream system  4.21 River Transport  4.24 River erosion  4.24 River deposition  4.25 Development of a River Valley  4.26 Long profile  4.26 Adjustment to erosion and deposition  4.27 Grade  4.27 Influence of gradient  4.27 River valley characteristics—processes and landforms  4.28 Drainage patterns  4.38 Water conservation  4.49

5 Underground Water and Limestone Features  5.1 Introduction  5.2 Sources of Groundwater  5.2 Wells and Artesian Basins  5.4 Karst Cycle of Erosion  5.9 Limestone Landforms  5.11

6 Glacial Processes  6.1 Introduction  6.2 Regional Distribution of Glaciers  6.3 Accumulation of ice and the snow line  6.3 Classification of glaciers  6.5

Glacial System  6.6 Glacial movement  6.7

Surface features and moraines  6.7 Glacial processes  6.8

Landforms Produced by Glacial Erosion  6.9 Landforms Produced by Glacial Deposition  6.14 Boulder clay deposits  6.15 Ice-Dammed Lakes and Overflows  6.17 Examples of Glaciated Landscapes  6.19 Economic Value of Glaciated Landscapes  6.25 Glacial landforms of specific value  6.25

Melting Permafrost  6.26

7 Desert Processes  7.1 Introduction  7.2 Desert Locations  7.3 Action of Winds in a Desert  7.8

Features produced by wind erosion  7.13 Features produced by wind deposition  7.15 Features produced by water in desert regions  7.18

Are Deserts Expanding?  7.20

8 Coastal Processes  8.1 Introduction  8.2 Coasts  8.2 Terms related to Coastal Geography  8.3 Formation of waves  8.3 The nature of waves  8.4 Types of waves  8.5 Wave Erosion  8.9 Landforms produced by wave erosion  8.9 Materials Transported by Waves  8.16

Contents  ix

Landforms Produced by Wave Deposition  8.17 Beach  8.17 Spit  8.18 Coastal Dunes  8.25 Changing Sea Levels  8.26

9 The Oceans  9.1 Introduction  9.2 Oceanic Zones  9.3 Ocean Current  9.4

Ocean currents and winds  9.5

Coral Reefs  9.7

Major reef types  9.8 Nature of Tides  9.10 Tidal influences  9.11 Energy from the Oceans  9.13 Natural Hazards of Oceans  9.13 Beneficial Influences of the Oceans  9.16

10 Atmosphere: Temperature  10.1 Introduction  10.2 Structure of Atmosphere  10.2 Troposphere  10.3 Atmospheric System  10.3 Heating of Atmosphere  10.4

Heating of the Earth  10.7 Latitude  10.7 Altitude  10.8 Nature of the surface  10.8 Distance from the sea  10.8 Winds  10.9 Cloud cover and humidity  10.9 Aspect  10.10 Length of day  10.11 Ocean currents  10.11 Temperature Changes within the Atmosphere  10.14 Measurement of temperature  10.15 Maximum thermometer  10.15

Minimum thermometer  10.16 Six’s thermometer  10.17

How Temperature is Shown on a Map?  10.18 World Distribution of Temperature  10.20

11  Atmosphere: Pressure and Wind  11.1 Introduction  11.2 Origin of pressure  11.2

Influence of altitude on pressure  11.2 Influence of temperature on pressure  11.3 Influence of rotation on pressure  11.3 Actual pressure systems  11.5

Measurement of Air Pressure  11.7 Mercury barometer  11.7 Aneroid barometer  11.7 Barograph  11.7 How Pressure is Shown on a Map?  11.9 Winds  11.9 The origin of winds  11.9 Measurement of wind direction and velocity  11.10 How winds and wind velocity are shown on a map?  11.11

Planetary Winds  11.13

Monsoon winds of the Asian region  11.17

Local Winds  11.18

Land and sea breezes  11.19 Descending winds  11.19 Valley breeze and mountain breeze  11.20 Convection winds  11.21 Depression winds  11.21

Air Masses and Fronts  11.23 Depression  11.26

Development of a depression  11.26 Weather associated with a depression  11.27 Weather and depressions  11.28

Tropical Cyclone  11.31

Development of a tropical cyclone  11.33 Weather associated with a tropical cyclone  11.34 Tornado  11.35

x  Contents

Anti-cyclone  11.38

Weather maps and anti-cyclones  11.38

12 Atmosphere: Water  12.1 Introduction  12.2 Humidity  12.2

Measurement of humidity  12.2

Condensation  12.5

Types of condensation  12.5

Clouds  12.7

High clouds  12.8 Middle clouds  12.8 Low clouds  12.9 Clouds of great vertical extent  12.10 Formation of clouds  12.10

Precipitation  12.12

Types of precipitation  12.13 How air is cooled  12.15

Types of Rain  12.16 Convection rain  12.16

Depression or cyclonic or frontal rain  12.17 Relief or orographic rain  12.17 Thunderstorms  12.17

Measurement of Rainfall  12.19 How Rain is Shown on a Map  12.19 World Patterns for Rainfall Distribution  12.22 Global pattern for May to October  12.23 Global pattern for November – April  2.23 Seasonal Distribution of Rainfall  12.25 Seasonal Rainfall and Type of Rainfall  12.26 Annual Global Rainfall  12.27

13 The Weather Station and Weather Maps  13.1 Introduction  13.2 Weather Station  13.2 Stevenson Screen  13.2

Visibility  13.4 Recording Weather  13.5 Gathering Information  13.7 NASA and Weather Information  13.9 National Remote Sensing Centre (NRSC) and Atmospheric Observations  13.11

14 Climate, Weather, and Natural Environment  14.1 Introduction  14.2 Weather and Climate  14.2 Factors Affecting Climate and Climatic Types  14.2 Fairly uniform climates  14.3 Seasonal climates  14.7 Continentality  14.9 Ecosystem  14.11

Linkages and interactions in an  ecosystem  14.12 Adaptation in plants  14.15 Influence of temperature and water on plants  14.15

Main Types of Vegetation  14.16 Tropical rainforest  14.19 Soil  14.20 The formation of soil  14.20 Water movement in the soil  14.21 Soil profile  14.21 Destruction of tropical rainforests  14.24 Soil productivity  14.25 Soil Erosion  14.25 By water  14.26 By wind  14.28 Soil Conservation  14.28 Types of soil conservation  14.28 Photo credit  PC.1 Index  I.1

Preface The physical environment has enormous variety and is of great complexity. It is forever changing and to­ understand the nature and causes of the changes, it is necessary to study the individual components involved. The ­processes operating in the many systems of the environment produce changes. An examination of the systems and of the interactions among their component processes results in a clearer understanding of both the diversity and the unity that characterize the physical environment. This book examines the systems and their processes, the landforms associated with these, and the ­relationships between living organisms and the inorganic environment within specific natural ecosystems. Every attempt is made to focus attention on all the main components of the physical environment as well as on the associated inter-relationships. Whenever possible, a wide range of landforms from within the framework of the British environment are examined. Whilst the main concern of this book is a study of the physical environment, where appropriate, mention is made of the effects of human activities on the environment. Although these activities have been going on for a very long time, it is only in the last five decades that the changes have reached a global dimension. The most widespread of these are air and water pollution and it is important that proper attention be given to these, especially in respect of the adverse direct and indirect affects they have on vast numbers of plant and animal species which threaten the delicate balance of many natural ecosystems. The text is extensively illustrated with diagrams and photographs, which are numbered on a chapter basis for easy reference. Varied exercises and a set of key facts are given at the end of all chapters.  R B Bunnett 1987

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Preface to the Indian Edition ‘Light precedes every transition. Whether at the end of a tunnel, through a crack in the door or the flash of an idea, it is always there, heralding a new beginning’ — Teresa Tsalaky Physical Geography in Diagrams by R. B. Bunnett was first published in 1965. The fourth edition of this book came in 1988 (ISBN: 9780582225077). When Pearson India, editorial team asked me to adapt the 4th edition for Indian students, I felt ­overwhelmed. I am being entrusted to bring this wonderful book into life again! I feel privileged for being given a chance to work on this book. It took me more than a year to make certain changes. While I was working on them, I realized how much effort had gone into the original version of the book to make it a most established title in this subject!. This annotated version retains all the distinctive features of the original edition. Consequently, it should be read as an updated edition to the original work and in no way should be interpreted as a completely new book in itself. I am associated with teaching profile for nearly 30 years, and can claim with confidence that this adaptation work will not only help students but faculties who are teaching Geography, for various levels of students, too will find this book extremely useful in their day to day lesson plans. Even students who have not studied Geography earlier can find this book extremely engaging for their further reading or for competitive readiness. The core objective of this title is to explain geographical principles and concepts through illustrations and engage students in the learning process of the physical aspects of geography through several line diagrams, 3D/2D artwork, field-based (i.e., real-life) photographs, and locations of features marked on satellite imageries as it is. With all said that, after using this book for classroom resources, it convinced me that some parts of the content need to be reworked and data should be updated as per the latest developments in today’s World. In last 30 years, there were no substantial changes in the content of this book, so it was a necessity to take this book forward for our next generation of students. We have tried our best to update the content along with new pedagogical elements, chapter-end questions, and also included as many new diagrams, maps, visual aids wherever possible. The goal has been to attempt to incorporate new technologies and methods to make the book relevant and useful for the current generation of students. I believe that present book addresses key concerns from the student’s point of view and in each chapter there are few sections which pro-actively connect students to their role as a stake holder in creation and sustenance of different geographies around them. The book which was earlier in black and white format is now available in its four-colored version. I have tried to include more recent photographs clicked during my field visits with students, family, and friends and have attempted to make visible the location of the places and physical features photographed through Google maps. As no single photograph was available from the original text, we had to buy many of these photographs which were not readily available from different libraries and museums of the world. Now the book is almost double the size with more than 500 pages and covers diverse areas all with enriched explanation visualized through more than 1,150 diagrams and 3D/2D artwork. The journey of coming out with the present version was ­beautiful . . . there was so much to know, read, and understand. It has not only added to my knowledge, but also has enabled me to grow as a human being. Again, I wish to thank the Pearson team for trusting me with this work. I also wish luck to all those students, researchers, teachers, and practitioners who are going to use this book in future. Seema Mehra Parihar

About Seema Mehra Parihar Dr. Seema Mehra Parihar is an Associate Professor at Kirori Mal College, University of Delhi. She has more than 30 years of experience in academics in the specific area of Geo-Informatics, Natural Resource Management, Physical geography and Gender analytics. Dr Seema earned her Ph.D from the Department of Geography, Delhi School of Economics, University of Delhi. The title of her PhD research was ‘Natural Resource Management in the Bhagirathi Basin’ Her Post-Doctoral Fellowship at the Department of Geo-informatics, Cartography and Geo-visualisation, ITC, Enschede, The Netherlands involved designing and d ­ eveloping a web based course in Web G ­ eo-informatics. Her specific interest lies in field-based research and geospatial ­ mapping using remote sensing and GIS. She has recently coordinated 40 module e-learning course and recorded 20 ­lessons for postgraduate (epg) pathshala for Ministry of Human Resource Development, Government of India. She has been a Principal Investigator of 14 research Projects sanctioned by national & ­international funding agencies. She has authored more than 30 articles in Geospatial Journals and is credited for Gender Atlas of India (Series 1 & 2) sanctioned by Ministry of Women and Child Development, Government of India. She has been the Convenor of the Gender Forum in the Bandung Conference, Indonesia and is currently working on a project entitled ‘Mapping Geospatial Dimension of Hydro-politics in Jammu and Kashmir’ and guiding research students. Dr. Seema has coordinated number of training workshops, seminars, conferences and refresher programs of UGC for University teachers in the field of Geospatail technologies; Geo-analytics; field work and Gender. Dr Parihar has also been a Trainer of Trainers and a resource person for National workshops on Capacity Building of Women Leaders in Higher Education. A trained behavioral assessor and an avid trekker, Dr. Seema has also been actively associated for more than thirty years in spreading the movement of national integration. Dr. Seema has guided Parivartan—a gender forum on issues surrounding gender and intersectionality in the Indian context and has been a driving force behind the events that the forum organises. Dr. Parihar was also a founder Chairperson of Central Placement Cell, University of Delhi(DU); Deputy Dean Students Welfare, University of Delhi; Joint Director, Developing Countries Research Centre (DCRC), DU and Fellow Institute of Life Long Learning (ILLL), University of Delhi. Dr Seema has recently been awarded by an ‘Annual Award 2018’ during IIRS Academia meet 2019 by Indian Institute of Remote Sensing, Indian Space Research Organization, Government of India. She has also been awarded by Bhoo Samman an award for contributions in geosciences during a conference on ‘Rural IndiaMillenium Development Goals’ by Bhoovikas Foundation.

Acknowledgement ‘We often take for granted the very things that most deserve our gratitude.’ — Cynthia Ozick This a wonderful moment, when I am getting an opportunity to acknowledge R.B Bunnett trust, United Kingdom, and Pearson Team, United Kingdom, for giving me a chance to unveil physical geography through the lens of R.B Bunnet, (first published in 1965). In this title, I could rework on each chapter, each diagram, each photograph and present those in a form that exist today. However, the presence of particular person at special place has only enabled me in adding almost double the pages and more than double images and diagrams covering diverse areas, and each one has been a key to the completion of this book and deserves a separate acknowledgement. When Pearson India approached me initially to work on the 4th edition of Physical Geography, I felt overwhelmed. I would like to thank, first and foremost, the Pearson India team for entrusting me the responsibility to bring this wonderful book into life again! I am more than grateful to H.R. Nagaraja, who not only encouraged for an contemporary adaptation, but albeit retaining its ­identity, understood the need for an overhaul with latest examples, cases-studies and pictures as there was no ­existing repository of diagrams and pictures present in earlier editions and more than fifty years had gone between the original text and current requirement of students. It is only because of that, a structure of each chapter was relooked into and visualized in four colours. I would equally like to thank Nandini Basu, for her continuous support, meticulous suggestions, giving new ideas and inputs to the book (analyzing chapter content, developing pedagogy, creating art works, modifying chapter end questions, etc.), ­flexibility and understanding the necessity of including satellite based images with features marked and adding new pictures from different sources, field experiences including mine. I also would like to thank Priyankia Dey, R&P Project Manager for taking permission clearances wherever required and putting it all together in a systematic way. My thanks also extends to Vipin Kumar from the production team for the creativity and patience in handling each page, all team members of the publishing team are most responsible for the coherent, well designed book that evolved from my initial drafts. I must express my gratitude to our new Principal Dr. Vibha Singh Chauhan for enabling academic ­environment for pursuing additional academic works. I am also grateful to my Ph.D student Peerzada Raouf Ahmad for his helpful comments and my student research assistants of different projects including Rohit Kumar, JitendraTiwari and Jitender Rathore for their constructive feedback on each diagram, each image and early versions of chapters. ­ ndergraduate I like to thank Rohit Kumar for reviewing the chapter end questions. I am further grateful to my u students at Kirori Mal College who have undertaken many field works with me to places in India, Nepal and Bhutan—thereby adding value to chapters through pictures, graphics and deep insights within different geographies. Special thanks to my husband Premendra, son Dushyant and daughter Jayashree who have always been there and without their help and support it would have been impossible to dedicate time to complete this book. Every effort has been made by publishing team to trace and contact copyright holders for their permissions to reprint material in this book. The publishers would be grateful to hear from any copyright holder who is not acknowleged and will undertake to rectify any errors or omissions in future editions of this book. To all of these people, heartfelt thanks. Seema Mehra Parihar

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1

The Solar System: Positions and Time

Learning Outcomes After completing this chapter, you should be able to: ● ● ● ●

Visualize solar system, inner and outer planets and their positioning. Locate the position of a place on the earth’s surface. Understand the importance of Sun as the main input in the earth’s energy system. Compute different time zones in different locations of the world.

Keywords Solar System, Earth’s radiation, Latitude, Longitude, Geoid, Greenwich Meridian Time and International Date Line.

1

1.2  Chapter 1

Introduction In our solar system, there is the sun and eight planets. The planets are categorized in two different groups—the terrestrial planets (innermost planets) and gas giants (the outer planets). We are going to study about these terrestrial planets and outer planets of the solar system and understand the relevance of positions and time on the earth’s surface in this chapter. The terrestrial planets include Mercury, Venus, Earth and Mars. These planets are composed of silicate rocks. The other four planets, i.e., gas giants or outer planets are Jupiter, Saturn, Uranus and Neptune. These four gas giants are huge in size and are composed mostly of helium and frozen hydrogen (no solid surface).

The Solar System

According to NASA, ‘two of the outer planets beyond the orbit of Mars—Jupiter and Saturn— are called gas giants; the more distant Uranus and Neptune are called ice giants.’ This is because where the first two are dominated by gas, the last two have more ice. All forms contain mostly hydrogen and helium.

Contemporary observations are changing our understanding of planetary system. The International Astronomical Organization (IAU) in 2006 resolved that ‘planets and other bodies in our solar system be defined into three distinct categories: ‘planet,’ ‘dwarf planet,’ and ‘small solar system bodies.’ A planet is a celestial body that is in the orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round, hydrostatic equilibrium shape, and has cleared the neighbourhood around its orbit. However, the ‘dwarf planet,’ unlike the planet, has neither cleared the neighbourhood around its orbit nor is a satellite. ‘As per Resolution 5A of IAU’ all other objects, except satellites, orbiting the sun shall be referred collectively as small solar system bodies.’ The eight planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (Figure  1.1). The IAU (2006) has further resolved ‘Pluto’ as a dwarf planet by the above definition, thereby recognized as the prototype of a new category of trans-Neptunian objects. The sun has a central position in our solar system and all planets revolve in elliptical orbits around it. The first time Nicolaus Copernicus (an astronomer, mathematician and scientist from Poland), in 1514, proposed the heliocentric theory of the solar system in his work Commentariolus. The ordering of planets in increasing

FIGURE 1.1  The Sun and the Eight Primary Planets of our Solar System.

The Solar System: Positions and Time   1.3

order of distance from the sun is: Mercury—57.9 million km; Venus—108.2 ­million km; Earth—149.6 million km; Mars—227.9 km; Jupiter—778.3 million km; Saturn—1427.0 million km; Uranus—2871.0 million km; and Neptune—4497.1 ­million km. The size of the planets is as follows in decreasing order: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury. In our solar system, the earth is a unique planet which supports life and is thus termed as the living planet. It is the third planet nearest to the sun, Mercury is the nearest planet to the sun and Jupiter is the largest planet of our solar system. Almost all the energy of the solar system is derived from the sun. The surface of the sun is covered with burning gases at a temperature of about 6000°C. Mercury, the smallest planet, is nearest to the sun. Some of the planets, e.g., Earth, Jupiter and Saturn, have small celestial bodies called satellites in orbit around them. The moon is the satellite of the earth. Each planet takes a different amount of time to complete one orbit around the sun. This is because their distances from the sun vary. Mercury completes its orbit in 88 days, which means that 1 year on Mercury lasts for 88 days. The earth completes its orbit in 365¼ days—the length of 1 year on earth. The moon takes about 27 days to revolve about the earth. Table 1.1 provides a brief overview of the eight primary planets in our solar system, in order from the inner solar system to outward.

Table 1.1 PLANET

Overview of the eight primary planets in our solar system DISCOVERY

NAMED FOR

DIAMETER

ORBIT

DAY

Mercury

Known to the ancients and visible to the naked eye

Messenger of the Roman Gods

3031 miles (4878 km)

88 Earth days

58.6 Earth days

Venus

Known to the ancients and visible to the naked eye

Roman Goddess of Love and beauty

7521 miles (12,104 km)

225 Earth days

241 Earth days

Earth

Known to the ancients and visible to the naked eye

7926 miles (12,760 km)

365.24 days

23 hours 56 minutes

Mars

Known to the ancients and visible to the naked eye

Roman God of war

4,217 miles (6,787 km)

687 Earth days

Jupiter

Known to the ancients and visible to the naked eye

Ruler of the Roman Gods

86,881 miles (739,822 km)

11.9 Earth years

9.8 Earth hours

Saturn

Known to the ancients and visible to the naked eye

Roman God of agriculture

74,900 miles (120,500 km)

29.5 Earth years

About 10.5 Earth hours

Uranus

1781 by William Herschel

Personification of heaven on Earth

31,763 miles (51,120 km)

84 Earth years

18 Earth hours

Neptune

1846

Roman God of water

30,775 miles (49,527.5 km)

165 Earth years

19 Earth hours

Just more than one Earth day 24 hours 37 minutes

Note: Pluto was considered as the ninth planet till 2006, when the International Astronomical Union (IAU) decided to call Pluto a dwarf Planet, reducing the list of real planets in our solar system to eight.

1.4  Chapter 1

Shape of the Earth In reality shape of the earth is not a perfect sphere, but an oblate spheroid—a sphere with a bulge around the equator (Figure 1.2). The earth is bulged outward at its equator because of the centripetal force occurring due to rapid rotation of earth on its axis. Similarly, the earth is flattened at the poles, and the equatorial diameter is large than the polar diameter by about 43 km. This actually makes a pretty big difference. Important dimensions of the earth are given in Table 1.2. Therefore, the shape of the earth is referred as earth-like, i.e., ‘geoid.’ In addition, there are intervening highlands and oceans on the Surface of the Earth earth’s surface. The geoid is the equipotential surface that defines sea level, and is expressed relative to the Land reference ellipsoid (Figure 1.3). Temporal variations in the geoid are caused by lateral variations in the internal densities of the earth, and by the distribution of masses Sea (primarily hydrological) upon the surface of the earth. Geoid Mass excess (either sub-surface excess density or positive topography) deflects the geoid upward. Ellipsoid The gravity map (Figure  1.4) is what is known as a geoid; based on data, it was created by a European satellite called the Gravity field and steady-state Ocean Circulation Explorer (GOCE). Studying the geoid FIGURE 1.2  Shape of the Earth (Figure 1.5) can help us understand tectonic processes

Table 1.2 PARAMETER

VALUE

Equatorial circumference

40,075 km

Equatorial diameter

12,742 km

Polar circumference

40,024 km

Polar diameter

12,713 km

Total surface area

51,09,00,000 km2

GPS H

Important Dimensions of the Earth

Topography h

N

soid Ellip Oceans

H

Orthometric Height

Geo

id

=

h

-

N

Ellipsoidal Height Geoid from GPS Height

FIGURE 1.3  Illustration of Earth’s Geoid Shape and Reference of Ellipsoid

FIGURE 1.4  A Model of Earth’s Gravity Field Made with Data from European Space Agency’s GOCE Satellite.

The Solar System: Positions and Time   1.5

Observed Geoid (EGM96)

10

2 40 0

60

20

0

–100 –20 10

–4 0

–4 0

–2

20

–10

–120 –100 –80

–60

10

–40

–20

–10

–10

0 20

40

–20

10 0

0

10

60

40

–80

–10

–6 0

0

–40

–10

20

–20

–40

–20

0

10

20

40

60

80

100

120

Geoid Height (m) Observed Geoid (EGM96, degree 4–25)

–10 –20

0

–20 –10

20 0

20 10

20

0

0 –1

0

0

20

0

10

–1

0

0

20

–150 –100 –80

–60

–40

–20

20 –20

–1

–10

0

0 –1

10

–1

10

–10

0

10

Geoid Height (m) FIGURE 1.5  Earth’s Figure: Gravity and Geoid

20

10

40

0

60

80

100

150

1.6  Chapter 1

and different natural phenomena like earthquakes. Scientists have established that large earthquakes move enough mass to change the gravity field. The change guides the mechanism of the quake and how much slip and uplift occurred, especially in offshore areas where it is difficult to observe (earth’s crust) directly.

Phases of the Moon The illuminated part of the moon appears to vary in size as it revolves around the earth. In Figure 1.6, the two circles represent moon positions. The outer circle clearly shows that exactly half of the moon is illuminated all the time. The inner circle shows what the moon looks like to us on earth during its different positions, e.g., at full moon it is a circle. Look at the moon on different nights in any 1 month, and find out whether the part of the moon that is not illuminated can be seen. Because eclipses of the sun or moon happen relatively infrequently, they were always a cause of wonder, even fear to early peoples. Their explanations are fairly simple as shown in Figure 1.7. There were two solar eclipses during 1987, neither of which was visible from Great Britain.

Full Moon

Moon Earth Sun Eclipse of the Sun (Moon Comes Between Earth and Sun)

Last Quarter

Earth

First Quarter

Partial Eclipse Sun

Total Eclipse Moon Corona

Eclipse of the Sun as Seen from the Earth Earth

New Moon

Moon

Sun

Sun’s Rays FIGURE 1.6  The Way the Moon Appears from Earth During its Revolution Around the Earth.

Eclipse of the Moon (Earth Comes Between Moon and Sun)

FIGURE 1.7  Eclipses of the Moon and Sun.

The Sun as an Input into the Earth’s System Of all the solar radiation reaching the earth’s atmosphere, 33 per cent is reflected back into the space by the upper atmosphere while the remaining 67 per cent proceeds into the atmosphere. Figure 1.8 shows the image of Sun producing energy. Out of this, 14 per cent is absorbed by the atmosphere and 53 per cent reaches the surface.

The Solar System: Positions and Time   1.7

Out of the 53 per cent that reaches the surface, some is reflected back into the atmosphere and some is absorbed by the surface soil and water, which raises the surface temperature. The amount reflected into the atmosphere depends on the nature of the surface, e.g., snowfields reflect up to 80 per cent of the radiation; water surfaces reflect from 5 to 40 per cent according to whether the sun’s rays are vertical or oblique. A soil surface covered with vegetation usually reflects about 10–30 per  cent. All of this is shown in Figure 1.9 (a), but it must be noted that the energy values given in this diagram are averages for the earth as a whole. As we shall see later, they vary according to the season, the latitude, the amount of water vapour in the air, and the amount of cloud cover. Figure  1.9(b) shows that the energy reflected and re-radiated back into space equals the energy received. ­ This incoming and outgoing energy is sometimes called the global energy balance. We shall see later that this energy balance fuels the earth’s other major cycles—the ocean ­currents and atmospheric circulation, as well as the hydro- FIGURE 1.8  Sun Produces an Enormous Amount logical cycle and the food cycle of which we all form a part. of Energy. Nearly White Areas are the Hottest, The solar energy input to the earth’s ­surface is vast in White Deep-Red Regions are the Coolest. This amount but is dissipated in various ways. Figure 1.10 illus- Image was Taken in Extreme Ultraviolet Light by the trates this process. You will see that some of the energy is Earth-Orbiting Solar and Heliospheric Observatory converted into heat, some powers the hydrological cycle, (SOHO) Satellite. atmospheric circulation and the waves and currents of the oceans, and some powers the food cycle through the process of photosynthesis, which in turn links with the fossil fuels. It is important to remember that the bulk of the energy used by humankind throughout the world is solar energy that has been locked up, for vast periods of time, in the fossil fuels (oil, including natural gas and coal). The rest of the energy used by humankind comes from nuclear fission, flowing water, the wind, and to a lesser degree, geothermal activity. (a) 100% Solar Radiation 33% Reflected into Space from Top of Atmosphere 14% Absorbed by Atmosphere Reflected 2%

20% Absorbed 53% Reaches Earth’s Surface

Of the 53% Reaching Earth’s Surface 95% Absorbed from Vertical Rays 60% from Oblique Rays 5% Reflected with Vertical Rays 40% Reflected with 80% Oblique Rays Reflected

70% – 90% Absorbed

10% – 30% Reflected

51% Absorbed by Surface Snow

Water

Vegetation

FIGURE 1.9  (a) The Amount of Solar Energy Reflected from and Absorbed by the Earth’s Surface Depends on The Nature of the Surface;(continued)

1.8  Chapter 1

(b)

Absorbed by the Atmosphere Solar Radiation

14

100 iation t Rad Direc 34

17

ed Diffus n tio ia d a R

Reflected by the Atmosphere

Absorbed by Earth

6

Reflected and Deflected

Reflected by the Clouds

61

Radiated Transferred from Earth to Atmosphere

34

27

Deflecte d by the E art

h

Directly to

Space

Radiated from Atmosph ere

35

2 17

Absorbed and re-Radiated 48

65

Tropopause (continued) FIGURE 1.9  (b) Earth’s Solar Energy Budget—Notice What Happens to 100 Units of Solar Energy When it Reaches the Earth’s Atmosphere. Solar Radiation

Shortwavelength Radiation

Longwavelength Radiation

Direct Radiation

The position of a place on the earth’s surface

Direct Conversion to Heat Evaporation and Precipitation

Storage Water Ice

Winds, Waves, Currents Photosynthesis

Position and Time

Decay Storage Plants Animals

Fossil Fuels

FIGURE 1.10  The Dissipation of the Solar Energy Input.

Take a large ball (to represent the earth) and mark two points on it in the centre so that they are exactly opposite to each other. Draw a line right a round the ball so that it is midway between the points all the way. The line divides the ball into two equal parts and because the ball is a sphere, each part can be called a hemisphere. In the case of earth, we call this line the equator, and you can see that it is a circle. One of the points is called North Pole and the other the South Pole (Figure 1.11). We can now draw more circles parallel to the north and south of the equator. These can be called parallels or lines of ­latitude. Latitude refers to the angular distance from north or south of the equator.

The Solar System: Positions and Time   1.9

This idea is applied to the earth. The equator is given a North Pole value of 0°. The North Pole has a latitude of 90°N and the Northern South Pole has a latitude of 90°S. Every other place on the North Hemisphere earth’s surface has a latitude of so many degrees north or south of the equator (Figure 1.12). Notice that the equator is the longest parallel. Figure 1.13 shows what the parallels look like on a globe from (a) the side and (b) a pole. O or Equatorial Plane ˚ We can draw another set of circles on the ball, all of which Equat pass through the two poles. That part of each circle between the South Southern poles can be called a meridian or line of longitude. This idea is Hemisphere South Pole applied to the earth also. The meridian, which passes through Greenwich, near London, is given a value of 0°; the opposite meridian, therefore, will have a value of 180° (Figure  1.14). FIGURE 1.11  The Poles, the Equator and the Longitude refers to the angular distance east or west of the Two Hemispheres. Greenwich Meridian. All places except those on Meridian 180° will have longitudes so many degrees east or west of Greenwich (Figure  1.15). Figures  1.14 and  1.16 show what the meridians look like from the side and from a pole, respectively.

North Pole 90˚ N

North Pole

90˚ N 80˚ N 60˚ N 40˚ N 20˚ N Equator Equator 20˚ N

45˚ N 90˚ Equator

0˚

45˚ 45˚ 45˚S

South Pole

(a)

FIGURE 1.12  A Line of Latitude Gives the Angular Distance of a Place North or South from the Equator.

0˚

45˚

45˚

E

enwich Meredian 0 Gre ˚

Equator

North Pole

180˚

(a)

(b)

(b)

90˚ S

FIGURE 1.13  Parallels of Latitude on a Globe (A) from the Side and (B) from the North Pole.

North Pole

t

North Pole

60˚ N 80˚ N

South Pole

W es

20˚ N 40˚ N 60˚ N 80˚ N

tor

ua East Eq

South Pole

FIGURE 1.14  (a) Lines of Longitude; (b) The Greenwich Meridian.

45˚

45˚

South Pole

45˚

FIGURE 1.15  A Line of Longitude Gives the Angular Distance of a Place East or West of Greenwich.

1.10  Chapter 1

13

180˚

13

90˚ E

S

5 ˚W

90˚ W

45 ˚E

45 ˚W

0˚

N

5 ˚E

FIGURE 1.16  Lines of Longitude from Above the South Pole. North Pole Arctic Circle 66 1/2˚ N Tropic of Cancer 23 1/2˚ N Equator 0˚ Tropic of Capricorn 23 1/2˚ S Antarctic Circle 66 1/2˚ S South Pole

90˚ N

North Pole 90˚

66 1/2˚

N

1 / 2˚

66

23 1/2˚

0˚

90˚ O

Arc NAS = 20,000 km Arc NA = 10,000 km

A

S FIGURE 1.17  The Length of 90° of Latitude Along a Line of Longitude.

How long is 1° of latitude? Figure 1.17 is a diagram of a hemisphere. Angle NOA is 90° and this is the longitude of the North Pole or angular distance from the equator (0°). This angle is subtended by arc NA whose length is one-half of a meridian. On the earth, arc NA has a length of 10,000 km approximately. If an arc of 10,000 km subtends 90° then an arc of 10,000/90 km subtends 1°, i.e., 1° of latitude represents 111 km approximately. How long is 1° of longitude? Every parallel has an angle of 360° at its centre, and every half-parallel an angle of 180°. If the length of the parallel or the half-parallel is known, then the length of the arc subtended by 1° can be calculated. For the equator, this is 111 km, but for other parallels it is less than this because parallels decrease in size away from the equator. Important parallels of latitude are shown on Figure 1.18.

1 23 /2˚ N

Rotation and Time 90˚ S

1 S 23 /2˚ S 1 / 2˚ 6 6

The sun reaches its highest position in the sky for the Greenwich Meridian when it lies under the sun. When this happens, it is said to be 1200 noon local time along the meridian. Local time is sometimes called sun time. South Pole Figure  1.19 shows that all places on the Greenwich FIGURE 1.18  Important Parallels of Latitude. Meridian have noon at the same time. It follows that all places on the same meridian have the same local time. Local time at Greenwich is called Greenwich Mean Time or abbreviated to GMT. The highest position of the sun for any place can be observed from a study of the lengths of the shadows cast by a vertical stick. The shortest shadow is cast by the sun when it is in its highest position in the sky. Study Figure 1.20, which is a sun path diagram for London.

The Solar System: Positions and Time   1.11

th’s Rotatio Ear n 1800 90˚ E

Noon

00 21 5 ˚E 13

00 15 5 ˚E 4 2400 1200 (Noon) N 180˚ 0˚ 09 0 00 0 03 5 ˚W 3 1

Sun’s Rays

S

0600 90˚ W

˚W 45

23 1 /2˚ N

E

FIGURE 1.19  When the Earth’s Rotation Brings Each Meridian Under the Sun, it is Noon Along that Meridian.

W

N FIGURE 1.20  Sun Path Diagram for London for 21St June.

Behind and ahead of GMT All meridians to the east of Greenwich Meridian have sunrise before that meridian. Local times along these meridians are therefore ahead of GMT. Meridians to the west of Greenwich Meridian have sunrise after this meridian and therefore their local times are behind GMT. Figure 1.21 explains this. How to calculate longitude from local time and GMT The local time at X is 1600 hours and GMT is 1400 hours. The difference in time between X and Greenwich is therefore 2 hours. This represents a difference of 30° of longitude between the two places (15° of longitude represents 1 hour). Since the local time at X is ahead of that at Greenwich, then X is east of Greenwich. The longitude of X is 30°E. How to calculate the local time from the longitude and GMT

Night

Day N

Tr

av W to Cl e lli n o c g f r o m E ack ks a r e P ut B

N

15 ˚ E ours H 1100 Noon 15˚ 0˚ 15˚ 1300 H 15 W ours ˚

Figure  1.22 shows what happens when two travellers set off at the same time, say, 1600 hours, on a Monday from a place A (longitude 0°). FIGURE 1.21  Time Ahead and Time Behind GMT.

Ahead Behind

The significance of the International Date Line

from W lling ve are Adv To a an E Tr ocks ce d Cl R rth’s otation Ea

The longitude of Harare (Zimbabwe) is 31°E. This means that there is a difference of 2 hours 4 minutes between the time at Harare and Greenwich (1° represents 4 minutes). If GMT is 0800 hours then the local time at Harare is 1004 hours because Harare is ahead of GMT. Note  : In each of these two examples, three facts are given. If any two of these are known, the third can always be calculated.

1.12  Chapter 1

1000 Hours Monday

90ºW 1600 hours Monday

A

0º

0400 hours Monday 180º B

N

0400 hours Tuesday

90ºE 2200 Hours Monday

160ºE

180º

160ºW

Standard time and time zones

This Side Is ‘Ahead’ in Time

60ºN

40ºN International

20ºN

0º te

Da

Line

This Side Is ‘Behind’ in Time

One traveller goes westwards and the other eastwards to a place  B (­longitude  180°). The traveller going west calculates the local times at 90°W and 180° to be 1000 hours Monday and 0400 hours Monday, respectively. The traveller going east calculates the local time at 90°E and 180° to be 2200 hours Monday and 0400 hours Tuesday, respectively. In theory, along Meridian 180° it is both 0400 hours Monday and 0400 hours Tuesday. The traveller going west crosses this meridian and finds that it is 0400 hours on Tuesday, i.e., one day has been lost. The traveller going east crosses this meridian and finds that it is 0400 hours on Monday, i.e., one day has been gained. The line at which a day is lost or gained is called the International Date Line. This line follows Meridian 180° except where it crosses land surfaces. To avoid confusion to the peoples of these regions the line bends around them so passing over a sea surface.

20ºS

40ºS

60ºS FIGURE 1.22  The International Date Line.

Each meridian has its own local time. Thus when it is 1200 noon local time in London, whose longitude is 0°, it is 1253 hours and 40 seconds local time in Berlin whose longitude is 13°25’E. (Note 1°E equals 4 minutes local time ahead. Why?) Great confusion would arise if all places used local time. Work out what the local time of a town approximately 650 km to the east of you would be in relation to your own local time. To avoid problems such as these, the world is divided into 24 belts, each 15° of longitude wide (Figure  1.23), and the local time of the central meridian for each belt is applied to that belt, which is called a time zone. The local time of the central meridian is called standard time. A country of limited longitudinal width has only one standard time, which is based on its central meridian, e.g., the standard time for Great Britain is set by meridian 0°. A country of great longitudinal width has several time zones, e.g., the USSR, which has a longitudinal width of about 165°, is divided into 11 time zones. Note: The boundaries of time zones are often adjusted to conform to political boundaries.

FIGURE 1.23  The World’s Time Zones, Shown at GMT 12 Noon.

The Solar System: Positions and Time   1.13

1.14  Chapter 1

Key Facts ●●

●● ●● ●● ●● ●● ●● ●● ●●

The IAU is the international astronomical organization that brings together distinguished astronomers from all nations of the world with a mission to promote and safeguard the science of astronomy in all its aspects through international cooperation. The solar system consists of the sun, the eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of dwarf planets was introduced in IAU Resolution in 2006. The first member of the ‘dwarf planet’ category are Ceres, Pluto, and 2003 UB313. The sun is the main input into the earth’s energy system. Position on the earth’s surface is defined by latitude and longitude. 1° of longitude represents 4 minutes of time. A country of great longitudinal width has several time zones as, e.g., USSR is divided into 11 time zones. The whole of India operates to a single time zone (IST). It is 5:30 hours (5 hours 30 minutes) ahead of Greenwich Meridian Time (GMT + 5.5)

The Solar System: Positions and Time   1.15

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. 1. An important announcement was broadcasted from London at 3.40 p.m. GMT. This was picked up by the navigator of a ship anchored off the coast of West Africa in longitude 10°W. What was the local time for the ship? (a) 4.20 p.m. (b) 3.20 p.m. (d) 3.00 p.m. (c) 4.00 p.m. (e) 2.40 p.m. 2. Which of the following does not belong to the solar system? (a) Asteroids (b) Comets (c) Nebula (d) Planets 3. The scientist who discovered that the earth revolves round the sun was (a) Newton (b) Copernicus (c) Einstein (d) Dalton 4. Which one of the following statement is correct about the innermost Planets? (a) The surfaces of these planets are almost solids. (b) They are composed of silicates rocks. (c) They are called terrestrial planets. (d) All of the Above. 5. Arrange the following in decreasing order of size and select correct answer from the code given below: (a) Saturn (b) Jupiter (d) Uranus (c) Earth Code: (1) b, a, d, c. (2) b, d, a, c. (3) b, c, a, d. (4) b, c, d, a. 6. Which of the two given planets are between Mars and Uranus in order of distance from sun? (a) Earth and Jupiter (b) Saturn and Neptune (d) Saturn and Earth (c) Jupiter and Saturn 7. Which of the following is not a great circle on the globe (a) Prime Meridian Line (b) 60°North Latitude (c) 60°East Longitude (d) Equator 8. Among the following cities which one lies in the farthest North? (a) Beijing (b) New Delhi (c) New York (d) Rome 9. When it is mid-day in the Greenwich, local time of a place is 5 O’clock evening. At which of the following longitudes (meridians) will that place be located (a) 75° West (b) 150° East (c) 75° East (d) 150° West 10. The basis of deciding standard time of any place is (a) Longitude (b) International Date Line (c) Prime Meridian (d) Latitude

1.16  Chapter 1

11. Zero degree longitude and 0° latitude lies in the (a) Atlantic ocean (b) Arctic ocean (c) Pacific ocean (d) Indian ocean 12. Time of which of the following places is equal to the time of GMT. (i) Accra (ii) Dublin (iii) Madrid (iv) Lisbon Code: (a) (ii), (iii), (iv). (b) (i), (ii), (iv). (c) (i), (iii), (iv). (d) (i), (ii), (iii). 13. International Date line passes through: (a) Pacific ocean (b) Asia (d) Africa (c) Atlantic ocean 14. When it is midnight at IST meridian, a place observes 6 a.m. the meridian on which the said place is located is (a) 7°31´E (b) 7°30´E (d) 127°30´W (c) 172°32´E 15. Which one of the following cities does not have the same clock time as that of the other three cities at any given instance? (a) London (UK) (b) Lisbon (Portugal) (c) Addis Ababa (Ethiopia) (d) Accra (Ghana)

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Suppose an international boxing match is to be held in Birmingham (latitude 52°30’N; longitude 1°50’W), and is scheduled to start at 9.00 p.m. local time on 26th June. The contestants will come from France and Australia and the referee will be from Manila, in the Philippines (latitude 14°36’N; longitude 120°59’E). Calculate the time and date that listeners in the following cities will have to tune in their radios for the start of the contest: (a) Paris, France (latitude 48°52’N; longitude 2°20’E); (b) Sydney, Australia (latitude 33°55’S; longitude 151°10’E); (c) Manila, the Philippines. 2. Imagine the whole of South America became federated into one state with Buenos Aires as its capital. What arguments can you produce for and against the whole state being in one time zone with a common federal time, that of the capital? 3. Briefly explain the meaning of global energy balance and account for what happens to the input of solar energy. 4. Give a detailed account of earth’s radiation budget with diagram. 5. Discuss the shape of the earth with reference to a geoid

The Solar System: Positions and Time   1.17

Answer key Exercise 1 1. (d) 6.  (c) 11.   (a)

2. (d) 7. (b) 12. (b)

3. (b) 8. (d) 13. (a)

4.  (d) 9.  (c) 14.  (c)

5. (1) 10. (d) 15. (c)

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2

Plate Tectonics: The Earth’s Structure and Landforms

Learning Outcomes After completing this chapter, you will be able to: ● ● ●

●

Describe the structure of the earth and concept of isostasy Understand the relevance of plate boundaries and plate tectonics Describe continental drift theory and appreciate changing position of continental masses Comprehend different internal process such as volcanism, earthquake, folding and faulting

Keywords Lithosphere, Tectonic Plates, Isostasy, Crust, Plate Boundary, Plate Movement, Rocks, Vulcanicity, Volcano, Crater, Earthquake, Faults, Folds, Mountain, Plains, Rift Valley and Plateau

1

2.2  Chapter 2

1990

Crust Mantle Liquid Outer Core

Introduction 1996

Fastest Route for Seismic Waves Solid Inner Core

Earth’s structure and tectonic activity is the base for understanding the living planet that shapes its surface. We start our discussion in this chapter with concept of structure of earth, buoyancy principle, i.e., isostasy, plate motion with reference of theory of plate tectonics, major rocks and other internal process, i.e., volcanism, earthquake, ­folding and faulting. Earth’s systems operates within its interior and are structured in three parts crust, mantle and core that play a key role in forming the rigid slabs of lithospheric plates, seismic flow of energy and rock material of its surface. Moreover, the topography of earth’s surface is shaped by the movement of mass and energy and its interaction between atmosphere and hydrosphere with the lithosphere.

Structure of the Earth Earthquakes, volcanic eruptions, deep mines and boreholes all ­provide clues to the nature of the earth’s interior. These clues indicate that the earth is made of three parts— the core (or barysphere), the mantle(or mesosphere) and the crust (or lithosphere). The crust is the solid surface on which we live. The ­ hydrosphere refers to the water Continental Crust masses (oceans and seas) on the Oceanic Crust surface. The atmosphere is composed of a mixture of gases, which Continental Crust - “Granitic” form an envelope around the Crust Oceanic Crust - “Basaltic” Crust earth. All of these are shown in Mohorovivic Figure 2.1. The thickness of the Discontinuity (Moho) earth’s crust relative to the whole Mantle Mantle Peridotite (Olivine & Augite) Gutenberg earth is shown in Figure 2.2. Discontinuity The earth’s crust consists of basalt rocks and it ‘floats’ on the Core mantle. It is in constant motion Core Iron & Nickel moving perhaps only 1 or 2 cm, horizontally or vertically, a year. But over millions of years this has added up to considerable distances, suffiFigure 2.2  Thickness of the Earth’s Crust in Relation to the Whole Earth cient to cause the oceans and conti(Not Drawn to Scale).(continued) nents to change their shapes, their sizes and their positions. The crust forms a continuous covering to the mantle but its thickness varies from 5 to 7 km under the oceans from 20 to 75 km under the continents. The thinner crust below the oceans is denser than the thicker crust, which forms the continents. Also the thinner crust is weaker and it is here that the new crust forms and the old crust is destroyed. From time to time, semi-fluid and molten rocks beneath the crust force their way through the crust and onto its surface. These rocks beneath the crust are called magma but when they reach the surface of the crust they are called lava. FIGURE 2.1  A Sectional View of the Three Zones of the Earth—the Barysphere (Core), the Mesosphere (Mantle) and the Lithosphere (Crust).

Plate Tectonics: The Earth’s Structure and Landforms   2.3

Mohorovičić Discontinuity— Between Lower Crust & Upper Mantle

Conrad Discontinuity— Between Upper & Lower Crust Crust Upper Mantle

Repetti Discontinuity— Between Upper & Lower Mantle

Mantle

Outer Core Gutenberg Discontinuity— Between Lower Mantle & Outer Core

Inner Core

(continued) Figure 2.2  Thickness of the Earth’s Crust in Relation to the Whole Earth (Not Drawn to Scale).

Lehmann Discontinuity— Between Outer & Inner Core

Juan de fuca plate

Eurasian plate

North American plate

Cocos plate

Caribbean plate

African plate

Filipino plate Pacific plate Indian plate

Pacific plate Nazca plate

Arabian plate

South American plate Australian plate

Easter plate Juan Fernandez plate

Scotia plate

Antarctic plate

FIGURE 2.3  The Boundaries of the Major and Minor Plates. There are Either Ocean Ridges or Ocean Trenches Along the Boundaries of the Plates.

The outpourings of lava, which are known as volcanic eruptions(refer figure 2.24) are mainly confined to the zones of weakness in the crust. These zones are also f­ requently shaken, often violently, by rapid movements in the crust. Such m ­ ovements are called earthquakes. The zones of weakness divide the crust into s­ everal large pieces called plates (Figure 2.3).

2.4  Chapter 2

Compared with mantle, the crust consists of solid stable rocks, which are lighter than the viscous magma of the mantle. In this respect, the crustal plates can be regarded as 'floating' on the magma. There is evidence that movements in the magma are dragging along the overlying crustal plates. The continental plates carrying South America and Africa are being dragged apart causing the crust beneath the central part of the Atlantic to be stretched. In this zone, the magma thrusts into the ocean floor forming a chain of volcanoes and the outpourings of lava spread out to form new crust. These areas are sometimes called submarine ridges or spreading zones. We shall see later how some of these volcanoes reach the surface in Iceland. Similar movements are taking place beneath the Indian and Pacific Oceans. Since the crust forms a complete covering to the mantle, plates which move apart result in other plates moving toward each other. Where plates collide, one plate will be dragged down as the other plate rides up over it. As the edge of the plate which subsides is forced down into the underlying magma, a trench forms in which there is considerable volcanic activity (Figure 2.4). The plate concept is known as plate tectonics. It is what is happening along the plate boundaries that is important. Spreading Zone or Fold Mountains Submarine Ridge Continent Ocean Trench

Plate O

Continent Ocean

Plate O

Spreading Zone

Zone of Ocean Crust Subduction

Continental Crust

Plate C

Mid-Oceanic Ridge

Magma Plate C Rises Mantle

FIGURE 2.4  A Submarine Ridge, Often with Volcanoes, Forms as Magma Bursts to the Surface when Two Oceanic Plates (O) Move Apart. The Zone Where this Happens is Called the Spreading Zone, and it is here that the New Crust is Formed. When an Oceanic Plate and a Continental Plate C Collide, the Oceanic Plate is Thrust Down into the Magma Where it Melts in a Subduction Zone. A Trench Forms here and Active Volcanoes Develop.

Isostasy The word Isostasy is derived from the Greek word ‘iso-stasios,’ which means “equal standing” (in equipoise) that attaining hydrostatic ­equilibrium—the position of lithospheric plates alters vertically as per its thickness and density. American geologist Clarence Dutton introduced “Isostasy” term in 1889 to describe this state of balance or state of equilibrium that exists between mountain ranges, plateaus, and large upstanding areas of the earth's surface. The concept of isostasy came in the mind of geologists but the concept grew out of attraction of giant mountainous masses (Figure 2.5).

Plate Tectonics: The Earth’s Structure and Landforms   2.5

Field Experiences Two field experiences with one expedition led by Pierre Bouguer and second by Sir George Everest noted some discrepancy in the latitudinal value of both the locations. Pierre Bouguer during his expedition of the Andes in 1735–1745 noted that “the towering volcanic peak of Chimborazo was not attracting the plumb line, as it should have done and thus maintained that the gravitational attraction of the Andes is much smaller than that to be expected from the mass represented by these mountains”. Similar discrepancies were noted by Sir George Everest (1859), the then Surveyor General of India during the geodetic survey of the Indo-Gangetic plain noted the latitudinal position of Kalianpur and Kaliana (370-miles apart) near the Himalayas using both triangulation and astronomical methods. The difference between the two results amounted to 5.236 seconds with the result obtained through triangulation noted as 5° 23’ 42.294 and the result obtained through astronomical method as 5° 23’37.058.

Kaliyana

This concept was also proposed by Sir George Airy and Archdeacon Pratt. According to Airy, the inner part of the mountains cannot be hollow; rather the excess weight of Kalianpur the mountains is compensated (balanced) by lighter materials below. According to him, the crust of relatively lighter material is floating in the substratum of denser material. Less Dense In other words, “sial” is floating over “sima.” Thus, the Rock Rock Beneath Plain Must Be Himalayas are floating in denser glassy magma. According Denser than Mountain to Airy, “the great mass of the Himalayas was not only a surface phenomenon—the lighter rocks of which they are composed do not merely rest on the level surface of denser Towards Centre Observed Expected material beneath, but as a boat in water, sink into the denser of the Earth Deflection Deflection material. Airy postulated, “if the land column above the substratum is larger, its greater part would be submerged in FIGURE 2.5  Sir George Everest Experiment— the substratum and if the land column is lower, its smaller Deflection of Plumb Bob Due to Gravity. part would be submerged in the substratum.” According to Airy, the density of different columns of the land (e.g., mountains, plateaus, plains, etc.) remains the same. In other words, density does not change with depth, that is, “uniform density with varying thickness.” It is a gravitational adjustment of earth’s crust that is based on the principle of “buoyancy” (Archimedes). In the case of isostatic equilibrium, one unit of lithospheric plates will stand higher than another due to its lower density (Pratt model or crustal density) and if it is of the same density but thicker (Airy model or crustal thickness) or it is a combination of both greater thickness and lower density (figure). In other words, it considers rigid slabs of the earth, i.e., crust to be buoyantly maintained in an underlying fluid medium, i.e., mantle and free to move vertically. The movement of these slabs will continue until their weight is exactly balanced by their buoyancy and this state is called isostatic equilibrium. Due to the greater thickness and lower density of continental crusts, they stand higher in comparison with oceanic crust. In most cases, the great differences in elevation are related to variation in crustal thickness within the continents and the area of high elevations commonly being caused by deep roots of floating crustal plates in mantle. Pratt postulated, “Elevation is inversely proportionate to density” it means, the higher the altitude of the mountain, the lower its density. Airy postulated, “the roots of the mountain is extended down into the mantle.” The same pattern may be demonstrated by taking wooden pieces of varying lengths (Figure 2.6) and if we put them into the basin of water, these would sink in the water according to their lengths.

2.6  Chapter 2

Airy thus maintained that the crustal parts (landmasses) were floating, like a boat, in the magma of the substratum. Applying therefore, the law of floatation as stated by Airy, we have to assume that for the 8848-m height of the Himalayas there must be a root, High High Density Density Low Density nine times more in length than the height of the Himalayas, in the substratum. Thus, for 8848 m part Sea level Sea Level of the Himalayas above, there must be downward projection of lighter material beneath the mountain reaching a depth of 79,632 m (roughly 80,000 m). Whereas, according to Archdeacon Pratt, there is a level of compensation above which there is variation in the density of different columns of land but there is no change in density below this level. Density does not change within one column but it Level of Compensation changes from one column to other columns above the level of compensation. In other words, Pratt’s concept of inverse relationship between the height of different columns and their respective densities FIGURE 2.6  Airy Model of Isostasy—Wooden Pieces of may be expressed as ‘bigger the column lesser the Uniform Density. density and smaller the column, greater the density’. Thus, Pratt’s concept of isostasy was related to the ‘law of compensation’ and not to ‘the law of floatation.’ According to Pratt, different relief features are standing only because of the fact that their respective mass is equal along the line of compensation because of their varying densities (Figure 2.7). While making a comparative analysis of the views of Airy and Pratt on isostasy, Bowie has observed that “the fundamental difference between Airy’s and Pratt’s views is that the former postulated a uniform density with varying thickness and the latter a uniform depth with varying density (Steers, 1937).” Figure 2.8 explains the fundamental difference between the concepts of Airy and Pratt on isostasy. Bowie made a comparative study of the views of Airy and Pratt on isostasy and concluded that there was a great deal of similarity in their views. In fact, “both the Uniform Density

Rose wood

Oak wood

Balsa wood Pine wood

Oak wood

Rose wood

Density 3.0

3.0

Varying Density

3.0

3.0

4.0

5.0

Line of compensation Sub Stratum Water Level

Pratt

Level of compensation Airy

FIGURE 2.7  Pratt’s Model of Isostasy.

FIGURE 2.8  Comparison between the Views of Airy and Pratt on Isostasy

Plate Tectonics: The Earth’s Structure and Landforms   2.7

views appeared to him similar but not the same.” According to Hayford and Bowie, there is an inverse relationship between the height of columns of the crust and their respective densities (as assumed by Pratt) above the line of compensation. The plane of compensation (level of compensation) is supposedly located at the depth of about 100 km. The columns having the rocks of lesser density stand higher than the columns having the rocks of higher density. This statement may be understood with the help of Figure 2.9. Joly (1925) disapproved the view of Hayford and Bowie about the existence of level of compensation at the depth of about 100 km on the ground since at this depth, the temperature would be so high that it would cause complete liquefaction with no possibility of ‘level of compensation.’ He further refuted the concept of Hayford and Bowie by saying, “density varies above the level of compensation but remains uniform below the level of compensation” on the ground that such a condition would not be possible in practice because such a condition would be easily disturbed by the geological events and thus the level of compensation would be disturbed. According to Joly, there exists a layer of 10-mile (16 km) thickness below a shell of uniform density. The density varies in this zone of 10-mile thickness. Thus, Joly assumed the level of compensation as not a linear phenomenon but a zonal phenomenon. In other words, he did not believe in a “line (level) of compensation” rather he believed in a “zone of compensation” (of 10-mile thickness).

Antimony

Offshore Plain

Coastal Plain

Plateau

Interior Plain

Iron

Level of Compensation

Zinc

Lead Level of Compensation

FIGURE 2.9  The Concept of Isostasy as given by Hayford and Bowie

Therefore, we can understand that isostasy is a gravitational adjustment of the earth’s crust that is based on the principle of “buoyancy” (Archimedes). In the case of isostatic equilibrium, one unit of lithospheric plates will stand higher than another due to its lower density (Pratt model or crustal density), due to same density but thicker (Airy model or crustal thickness), or due to a combination of both greater thickness and lower density. In other words, it is considered that rigid slabs of the earth, i.e., crust to be buoyantly maintained in an underlying fluid medium, i.e., mantle and free to move vertically. Continents, which drift from one latitude to another experience ­climatic changes. These are reflected in the rocks that develop under the different climates. There is evidence of glacial deposits in the Congo Basin, fossilized trees in Antarctica and coral limestones in Greenland, all of which show that the climates of these regions were once quite different from what they are today. Continental drift appears to be the only satisfactory explanation of these climatic changes. Other Scientific evidences though explained later included the Jigsaw-fit shapes match, finding similar sequence of rocks at numerous locations, presence of identical fossils of plants and animals, Ice matches—Glaciers and Tillite, among few (Figure 2.10).

Mercury

2.8  Chapter 2

A Closer Look  ▼

It is possible to match rock types in West Africa with rock types in Brazil, so these two regions may have been part of one continental region when the rocks formed.

(a)

Crust in motion Alfred Lothar Wegener, a 32-year-old lecturer delivered a lecture on “The Formation of the major Features of the Earth’s Crust (Countries and Oceans)” at Frankfurt in an eminent geological association. In his lecture in 1912, he suggested that continents had once been combined in the form of original single sialic landmass and had afterwards broken apart and drifted into their present position. Geologist considered his theory as an impossible hypothesis. Tuzo Wilson was first to introduce the moving plate idea in the year 1965. But his purpose was to explain the transform faults, which later became an important feature to delineate the plate boundary. W. Jason Morgan, while working on the idea of plate movement picked up the idea of “Seafloor spreading” coined by Robert Dietz and Harry Hess (1962) and applied the Leonhard Euler’s theorem to calculate the movements of plate on the earth with reference to axis of rotation of the plate. In this way, the movement of crustal plates has caused continental masses to change position. This is called continental drift.

(b) After the break-up of Pangea

Early Tertiary Period

North America

Europ

e

Laurasia

Asia

Africa South America

Gondwanaland

India lia a stra ctic u A tar An

(c)

(d)

Last Ice Age

Eu

rop

North America

e Asia

Africa South America

India

a

li ra

st Au

Antartica

Today

Plate Tectonics: The Earth’s Structure and Landforms   2.9

(f)

(e) The Shape Match: Jig-Saw-Fit

Identical Fossils of Plant and Animals

Cynognathus Africa (A Triassic Land Reptile) South America ca a

Africa South America

Mesosaurus (Freshwater Reptile) Lemurs (India, Madagascar and Africa)

Lystrosaurus (A Triassic Land Reptile) India Australia

Antarctica

Glossopteris (Gondwana Plant Fossil)

(h) The identical fossils of Cynognathus and Mesosaurus (g)

The Identical Fossil of Lemurs

Africa

Lemurs (India, Madagascar and Africa)

Cynognathus Africa (A Triassic land reptile) South America

I India Mesosaurus (Freshwater reptile) (j) (i)

The Ice Matches: Glaciers and Tillite

The Identical Fossil Glossopteris Africa South America

India IIn n

Glossopteris (Gondwana Plant Fossil)

Antarctica Australia

Africa

South America

India Antarctica

Australia

FIGURE 2.10  How the Present Continents may have been Formed by Continental Drift (a) The Possible Shapes and Positions of the Continents after the Break-Up of Pangea; (b) The Continents begin to Drift Apart; (c) The Last Ice Age; (d) The Continents Today—the Spreading Zones Between Plates are shown using Arrows; (e) The Shapes Match: Jigsaw-Fit; (f) The Identical Fossils of Plants and Animals. Snider-Pellegrini Recorded that Fossil Plants from the Carboniferous Period (354–290 Million Years Ago) in Africa and the Americas are Largely Identical. Later Paleontologists were Satisfied to Accept that Present-Day Continents were Connected by Land Bridges and Later theses Land Bridges Sank Beneath the Sea. The Land Bridges in the Past Would Have Been Helpful for the Animals to Cross Since it was not Possible to Cover the Distance Via Atlantic Ocean. Another Possibility for Migration of Specifies Across the Atlantic was Ruled Out by the Balancing Act of Isostasy; (g) For Instance, Lemurs Found in India, Madagascar and Africa Validates that India was Connected with Africa Via Madagascar in the Past; (h) The Remains of Cynognathus a Triassic Land Reptile and Mesosaurus a Type of Freshwater Reptile are Found in South America and Africa; (i) The Identical Fossils of Plants and Animals in Europe and Northeastern North America were not very Apparent, but Wegener Noticed that Earthworms are Found in Northeastern Areas of North America. The Earthworms cannot Swim Across Atlantic Oceans. It Suggests that Atlantic Ocean did not Exist in the Past (j) Ice Matches—Glaciers and Tilite.

2.10  Chapter 2

However, the strongest evidence for continental drift comes from paleomagnetic dating. Rocks become magnetized in the direction of magnetic north at the time they solidify, and so by examining the m ­ agnetism of very old rocks, it is possible to place the position on the earth’s surface where they were formed. British geophysicists Frederick Vine and Drummond Matthews in the year 1963 found that same patterns of magnetized rocks exist on both the sides of mid-ocean ridges belonging to the same period. They  together discovered “normal” and “reverse” polarity or the ocean floor, i.e., either side of ocean ridges. It indicated that both sides of the ridge were created during the same period. Paleomagnetic studies indicate that the present continents developed from a single continent, and that the breakup of this continent started in the Mesozoic era, about 200 million years ago (Figure 2.11). Wegener was the first person to suggest that the positions of the continents have changed, and it was he who gave the name Pangea to the original continent, which later split up into the continents of today. The northern part of Pangea is called Laurasia, and the southern part is called Gondwanaland. Africa developed from the latter. Figure 2.10 suggests how continental drift may have taken place. Gondwanaland probably began to break up in the Cretaceous era, and it was at that time that the present day southern continents came into being. (a)

200 Million Years Ago During Mesozoic Era PANGAEA

and

PANTHALASSA

(b)

Africa

a

di

In

South America

Australia

Antarctica

Plate Tectonics: The Earth’s Structure and Landforms   2.11

(c)

Continent Some 200 Million Years From Now

A tectonic plate is sometimes called lithospheric plate.

FIGURE 2.11  (a) 200 Million Years Ago During Mesozoic Era (b) Present Position and (c) Future Continents Some 200 Million Years from now (from Meadows, 2007). The Continents in the Present Time are Attaining their Maximum Distances. Their Continued Motion in the Future Must bring them Closer together again. In Future, 200 Million Years from now South America, Antarctica, and Australia Likely to Join Together in the Southern Hemisphere. Similarly, Africa, Europe, and Asia are Likely to Unite in Future (Meadows, 2007).

Collision of Plates When an oceanic plate and a continental plate move towards each other, the continental plate rides up over the edge of the heavier oceanic plate (see Figure 2.12), and the rocks along the edge of the oceanic plate are absorbed into the mantle. Regions where this happens are called zones of subduction. Active zones of subduction occur off the coasts of Japan, California, and the west coast of South America. In these Trench is formed Here Zone of Subregions, the edge of the oceanic plate is duction bent down into the mantle as the continental plate “rides” up over it. These are the Sediment zones of destruction. Trenches are formed, Land ion e.g., the Java Trench, which in time fills up os r E with sediments derived through erosion of the continent. But as the plates continue to approach, the sediments are crushed and Sea folded, and they may give rise to a range of mountains or a chain of islands. Volcanic activity, often of great intensity, is a characteristic feature of these regions. Mantle When two continental plates app­roach Continental each other, neither sinks because they Oceanic Plate are of equal density. Instead, they col- Plate Plate Movement lide, their edges are fractured, and the Produces Compression sediment of their continental margins is folded to produce vast mountain ranges. FIGURE 2.12  A Trench Forms in a Zone of Subduction; this The Himalayas were formed in this way, as Eventually Becomes Filled with Sediment, which Later Gives Rise to Fold Mountains. shown in Figure 2.13.

2.12  Chapter 2

(a)

Plate Boundary

Erosion

Ocean

Mantle Continental Plate

(b)

Sediment

Continental Plate

Sediment Here is Metamorphosed by Temperature and Pressure

Intensely Folded Sediment

Mantle Continental Plate

Granite Forms Mountain Roots

Continental Plate

FIGURE 2.13  When Continental Plates Collide, the Edges Fold to Give Mountains.

Mountains and volcanoes are often found where plates converge.

A tectonic plate is a gigantic, irregularly shaped, rigid slab of rock, which moves slowly over the asthenosphere. The edges of the plate boundary can be delineated or identified by three features which are described below. 1.  Ocean Ridges: They are situated along constructive plate margins and they represent the linear features that exist between two tectonic plates that are moving away from each other. Trenches: They are situated along con2.  vergent or destructive m ­ argin; here the oceanic lithosphere is destroyed and recycled back into the interior of the earth as one plate dives under another. Transform faults: In case of transform 3.  faults, there is neither construction nor destruction of the plate. The relative motion is generally parallel to the fault line.

Types of Plate Boundaries

Tectonic plates are constantly moving with respect to each other. They may move apart, or collide together, and slide and grind against each other. For each of these events, geomorphologists recognize a different type of boundary. One is divergent or extensional boundary or constructive margin when two plates slide past each other. For example, Mid-Atlantic Ridge separates the North and South American plates from the Eurasian and African Plates. This pulling apart causes “seafloor spreading” as new material is added to the oceanic plates. Second is the Convergent plate boundary where plates slide towards each other and is also known as destructive plate boundary. In general, there are three types of convergent boundaries: (1) ­oceanic–continental convergence; (2) oceanic–oceanic convergence, and (3) between two continental plates. Transform boundary or strike-slip boundary occurs where plates slide past each other g ­ enerally along a horizontal transform fault (Figure 2.14).

Distribution of major and minor plates

The Pacific plate is the largest plate and is almost oceanic in character.

The lithosphere is divided into six large and many smaller plates. In major categories except for the Pacific plate, the remaining major plates are named after the continents embedded in them. Many plates are comprised both continental and oceanic crust. The list of major and minor plates is as follows: ●●

Major plates ➤➤ Pacific plate: It is an entirely oceanic lithosphere. It covers the Pacific Ocean Basin. The relative motion of this plate is northwesterly, which

Plate Tectonics: The Earth’s Structure and Landforms   2.13

●●

has resulted in the formation of subduction zones. The southern and eastern boundary of this plate is characterized by spreading boundary. In the northeast, this plate makes active and transform fault in California region of the USA(Strahler and Strahler, 1992) ➤➤ American plate covers most of the North and South American continents as well as eastern part of Russia including Kamchatka peninsula. The Western edge is characterized by converging boundary and eastern boundary is situated along the western sides of Mid-Atlantic ridges. ➤➤ Eurasia plate: It is mostly continental in nature but its eastern and northern region is characterized by oceanic lithosphere. ➤➤ African Plate: It covers entire African continent surrounded by oceanic lithosphere. ➤➤ Antarctic plate covers the entire lithospheric Antarctica continent surrounded by oceanic lithosphere. This plate is surrounded by spreading boundary. ➤➤ India—Australia—New Zealand plate: it is an elongated rectangular plate, which is mostly covered by oceanic lithosphere. This continental lithosphere contains Australia, peninsular India and New Zealand. Minor Plates ➤➤ Cocos plate: Between Central America and Pacific plate ➤➤ Nazca plate: Between South America and Pacific plate ➤➤ Arabian plate: Mostly the Saudi Arabian landmass. It has two transform boundaries. ➤➤ Philippine plate : Between the Asiatic Eurasia plate and Pacific plate. It has subduction boundaries on both eastern and western sides of the plate. ➤➤ Caroline plate: Between the Philippines and Indian plate (North of New Guinea) ➤➤ Fiji plate: Northeast of Australia.

Plate Plate Asthenosphere Convergent Transform Plate Boundary Plate Boundary

Divergent Plate Boundary

Convergent Plate Boundary

Continental Rift Zone (Young Plate Boundary)

Island Arc

Trench StratoVolcano

Shield Volcano

Oceanic Spreadind Trench Ridge Lithosphere Asthenosphere Hot Spot

FIGURE 2.14  Types of Plate Boundary

Oceanic Crust Subducting Plate

Continental Crust

2.14  Chapter 2

San Andreas Fault North American Plate Caribbean Plate Cocos

Eurasian Plate Arab

Nazca Plate

S. American Plate

African Plate

Pacific Plate IndoAustralian Plate Antorctic Plate

FIGURE 2.15  Location of Major and Minor Plates; Lithospheric Plate

Divergent Boundaries

Scotia Plate

Convergent Boundaries Transform Boundaries

North American plate

Eurasian plate

Eurasian plate

Juan de Fuca plate Caribbean plate

Philippine plate

Australian plate

Cocos plate

Pacific plate

Nazca plate

South American plate

Arabian plate

Indian plate

African plate Australian plate

Scotia plate Antarctic plate

FIGURE 2.16  Map Showing Tectonic Plate Boundaries and Geological Activity Around Plate Boundaries.

When plates move apart on the ocean floor When plates move apart on the ocean floor (Figure 2.17), the up-welling magma spreads out forming mountain ridges such as the Mid-Atlantic ridge (Figure 2.18), which is 14,300-km long.

Plate Tectonics: The Earth’s Structure and Landforms   2.15

(a)

Trench

Continent Sea

Continent

(b)

Ridge Develops from the Magma

Continent

Mantle

Mantle Continental Plate

Magma Wells up into Trench Plate Movement (Tension)

Continent

Sea

Continental Plate

Spreading Centre Forms Continental From Upwelling Magma Plate Plate Movement (Tension)

Continental Plate

FIGURE 2.17  When Plates move Apart on the Ocean Floor, a Trench Forms, which Gradually Fills with Magma from the Mantle to form a Spreading Zone. Northern Boundary of map of India

Red Sea is located in a rift valley

FIGURE 2.18  Mid-Atlantic Ridge.

2.16  Chapter 2

(a)

Faults Continent Breaks up

Sea

Mantle Continental Plate

Continental Plate Plate Movement (Tension) Rift Valley

(b)

Sea

FIGURE 2.19  Faults Frequently Develop when a Continental Plate Fractures; a Rift Valley May Develop Between the Faults.

There are similar ridges in the Indian and Pacific Oceans. As the magma erupts, there is violent volcanic activity and frequent earthquakes. The moving apart of plates is sometimes called spreading. This results in the formation of new crust.

When a continental plate fractures When this happens (Figure 2.17), the tensional force of the moving parts produces cracks, called faults, and the continent is slowly broken up. The zone of faults often develops into a valley, called a rift valley (Figure 2.20), which may be occupied by a sea. We can say therefore that when continents move towards each other, oceans shrink, but at the same time, new oceans are formed in the places previously FIGURE 2.20  Picture Depicting Great Rift Valley from occupied by the continents. And, the moving apart Google Map. of plates on the ocean floor result in the upwelling of magma which forms igneous rocks when it cools and solidifies. All the ocean floors are made of igneous rocks, which are covered with sediment around the edges. The sediment comes from the erosion of the continental masses, which we shall discuss in Chapter 3.

Plate Tectonics: The Earth’s Structure and Landforms   2.17

Rocks Rocks are made of various minerals in differing combinations.

Classification of rocks Some rocks are hard and compact, e.g., sandstone and granite; and some are made up of loose particles, e.g., gravels and muds. Rocks can be put into three groups according to the way in which they were formed: (1) igneous, (2) sedimentary, (3) metamorphic (Table 2.1).

Table 2.1 ROCKS Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

Classification and description of rocks. CLASSIFICATION

Silicic or Felsic rocks

CHARACTERISTICS High amount of silica, potassium, and sodium and Low in calcium, iron, and magnesium

EXAMPLES

Granite, rhyolite

Intermediate rocks

Moderate amounts of sodium, potassium, calcium, iron, silica, and magnesium

Andesite/dacite (extrusive) and diorite/ granodiorite (intrusive).

Mafic rocks

Low in silica, potassium, and sodium content and rich amount of iron, magnesium, and calcium

Basalt, gabbro

Ultramafic rocks

Rich in olivine

Dunite

Clastic rocks

Eroded rocks which settle out of water or air and broken due to weathering, which can take place under the effect of wind, rain, etc.

Sandstone, mudrocks

Chemical Sedimentary rocks

Chemicals or elements, which get mixed or dissolved with water and when evaporation takes place, are left behind as deposits.

Halite, rock gypsum

Organic sedimentary Rock

Collection of plant matter and accumulation of sedimentary debris must be caused by organic process

Coal

Foliated

Wavy bands or planes which occur due to platy or elongated arrangement of minerals

Slate and Schist

Non-foliated

Exhibit coloured bands which occur due to minute impurities present in the rock

Marble and Quartzite

2.18  Chapter 2

e & Cemen Pressur tatio n

Years of erosion

Pressure

Sediments

Erosion

Heat

Sedimentary rock layers

Metamorphic rocks

lgneous rocks

Heat & Pressure

He

g

lin

at

oo

C

Magma Chamber

FIGURE 2.21  Inter-Conversion of Three Types of Rocks

The approximate volume proportions of these three rock types throughout the earth's crust are igneous 65%, metamorphic 27% and sedimentary 6%.

Igneous rocks These form when molten rock cools and solidifies. Crystals form on cooling and the rocks are called crystalline rocks. Some igneous rocks, e.g., basalt, have cooled quickly and they contain small crystals. Others, e.g., granite have cooled slowly and they contain large crystals. All igneous rocks originate inside the earth where they are under great pressure. If a crack develops in the earth’s crust, the pressure forces some of the rocks into the crack. Rock that moves into the crust and towards the surface is called magma. If it reaches the surface, it is called lava. Sometimes the magma solidifies in the crust where it cools slowly. Granite is formed in this way. If the magma reaches the surface, it cools and solidifies quickly. There are three main types of igneous rocks: as mentioned below 1. Plutonic: These have solidified deep in the crust and they are seen on the surface only after being exposed by prolonged erosion. 2. Volcanic: These have been poured on to the earth’s surface where they are called lavas. 3. Hypabyssal: These have solidified at depths between those of 1 and 2.

Sedimentary rocks In the beginning, the earth’s crust consisted only of igneous rocks, which formed from cooling magma. Over the hundreds of millions of years that followed, rain and running water broke down the igneous rocks into small particles, which

Plate Tectonics: The Earth’s Structure and Landforms   2.19

subsequently were carried away and deposited in Land the sea. These rock particles formed sediments and Sedimentary this became the stuff from which most sedimentary Layers Sea rocks are made. The sediment was laid down in layers or strata, one on top of the other, and in time, Edges of these layers turned into rock as they became comBadding pacted (hardened by compression). Rock layers of Planes this type are called stratified rocks (Figure  2.22). The plane between the two layers is called the bed- FIGURE 2.22  Stratified Rocks Laid Down in Water ding plane. There are many types of sedimentary rocks according to the types of rock debris and the variations in compaction of sediment. Some sedimentary rocks appear to be more resistant to erosion than others. You can sometimes see different types of sedimentary rocks on the sides of deep cuttings or better still, river gorges where these are not covered with vegetation. Most sedimentary rocks are non-crystalline and many contain fossils. Some sedimentary rocks are formed in water, e.g., inorganic rocks such as sandstone and mudstone; organic rocks such as coral, peat, and coal (the latter two formed in swamps). Some are formed on land, e.g., boulder clay, moraines, and loess (all inorganic). Those formed in water developed from inorganic sediment. Some sedimentary rocks are formed chemically and not from sediment. Limestones of either chemical or organic origin are sedimentary rocks.

Metamorphic rocks These are formed from rocks whose structure and appearance have been changed by heat or by pressure or by both. Any rock can be changed into a metamorphic rock. Examples of metamorphic rocks are slate, marble, and graphite. Figure 2.23 summarizes the relationships of the three rock types. Volcano Weathering and Erosion Transport

Sea Sediment

Brought to Surface by Uplift

ft pli U y eb ac f r Su Buried, Heated t to h g and Changed u o Br

Sedimentary Rocks Buried, Heated and Changed

Metamorphic Rocks

Plutonic Igneous Rocks

Volcanic Igneous Rocks

(Cools and Solidifies Below the Crust) Magma New Rocks From Below

FIGURE 2.23  Relationship Between the Three Rock Types. Igneous and Sedimentary Rocks can both be Turned into Metamorphic Rocks. Metamorphic, Igneous, and Sedimentary Rocks Through Uplift and Erosion can be Turned into Sediment and then into Sedimentary Rocks.

2.20  Chapter 2

Extrusive Igneous Rock

(Magma comes out as lava and cools on the surface)

Intrusive Igneous Rock (Magma cools off beneath the surface)

Earth’s surface Magma

FIGURE 2.24  Volcanic eruptions

Large parts of most of the continents consist of a basement or platform of very old igneous rocks which form the bedrock of the continents. Some parts of these basements are exposed; other parts are covered with sedimentary rocks. These basements are sometimes called shields, e.g., the Baltic Shield.

A Global Pattern Through Plate Tectonics The key to an understanding of the present landscape pattern of the world is the past movement of tectonic plates. The location and arrangement of the different types of rocks, and the distribution of the world’s mountains, plains, and oceans together with the locational pattern of continental landmasses, can be explained by these past plate movements.

Great Britain in the Global Pattern During the early part of the Palaeozoic era a huge land mass lay to the northwest of England and a wide sea-filled depression separated this from another land mass (now England and Wales) as shown in Figure 2.25. The two landmasses slowly approached each other and in doing so, the sediment in the depression was compressed and intensely folded into a huge mountain range. This took place in the Caledonian Orogeny, which is thought to have lasted about 200 million years. At this time, most of Scotland, northern Ireland, Wales, and northern England probably had a landscape very similar to that of the Himalayas today. Erosion by rain, water, and ice has cut into and removed the tops of the mountains so that what we see today are only the remnants, the roots of the mountains. Even so, these give Great Britain some of its finest scenery – Snowdonia (in Wales), the Lake District (in England), and the Grampians (in Scotland). The vast quantities of sediment derived from the Caledonian Mountains were carried by huge river systems and deposited in another sea-filled depression that formed across central and

Plate Tectonics: The Earth’s Structure and Landforms   2.21

southern England and much of England Europe. This took place in the Volcano Scotland Devonian and Carboniferous Ocean Volcano periods. Sedimentary rocks, Ancient especially sandstones and lime- Land stones, formed in this depression Mass during these periods. Swamp forests flourished in large areas of the shallow sea of the depression, and it was these forests that gave rise to the coal seams, which today provide Great Britain with FIGURE 2.25  An Ancient Land Mass Lay to the Northwest of most of its energy. Towards the end of the England Across a Wide Ocean in the Palaeozoic Era. The Two Carboniferous period, another Landmasses Gradually Moved Together as Shown. collision took place, which raised the mountain masses of southern Ireland, south Wales and Devon, and Cornwall. The Pennines were pushed up into a huge arch. This collision also gave rise to the Brittany mountains in France, the Harz Mountains in Germany, and several mountain ranges such as the Altaid Mountains in Central Asia. This mountain building period is called the Hercynian Orogeny. It lasted for about 100 million years. Subsequent erosion once again produced vast amounts of sediments, which was deposited in nearby seas. The last mountain building period, known as the Alpine Orogeny, resulted from a collision of the African plate and the Australian and Eurasian plates and caused the formation of the Alps and Himalayas. As in the earlier mountain building periods, faulting, and folding were extensive. The Alpine Orogeny caused ripples in the earth’s crust, which reached as far as Great Britain where they caused the pushing up of the southeastern part of England to form a low dome called the Weald of Kent. They also created a corresponding downfold called the Thames Basin. The ripples caused the old rock masses of northern Ireland, northern England and Scotland to fault, i.e., crack, and out of some of these there were great flows of lava. It was at this time that the lava plateaus of Antrim in northern Ireland, and similar plateaus in western Scotland were formed (see Figure 2.26). The ripples also caused the old mountain remnants of northern Great Britain to be pushed up to their present heights. Figure 2.27 shows the positions and effects of the Caledonian and Hercynian Orogenies as they affected Great Britain. Finally, over the last 100 million years, the North and South American plates have been moving away from the Eurasian and African Plates. This movement created the Atlantic Ocean. But this does not mean that all plate movements have stopped. They are as active as they have always been. Africa and Europe are still colliding as are India and Asia, and mountains are still being formed. Northern England is rising by about 5 mm a year, while southeastern England is sinking by about 1 mm a year.

2.22  Chapter 2

Tertiary Mesozoic and Permian Upper Palaeozoic Lower Palaeozoic and Pre-Cambrian Metamorphic Intrusive (Did not Reach the Surface) Extrusive (Reached the Surface as Lava Flows) Old Rocks of the Highland Zone Dykes

0

200 km

FIGURE 2.26  Distribution and Types of Rocks of Great Britain.

Geologists rarely use a timescale of years when comparing the ages of rocks. Instead, they use five eras, each of which is divided into a number of periods. The five eras are the Precambrian (this contains the oldest rocks known), the Palaeozoic, the Mesozoic, the Cenozoic, and the Quaternary (this contains the youngest rocks). Plate movement throughout the earth’s history has culminated in three major periods of mountain building as shown on pages 2.24–2.25.

Rock system in Great Britain Great Britain has a great variety of rocks of all ages r­ anging from Precambrian rocks in northwest Scotland to Tertiary and more recent rocks in southeast England. The major rock types are shown in Figure 2.26. Each rock type tends to give a specific type of scenery, which in turn influences the uses to which the land is put. This explains why Great Britain, though only small, has a large variety of landscapes. All three rock types mentioned earlier in this ­chapter occur in Great Britain.

Plate Tectonics: The Earth’s Structure and Landforms   2.23

Igneous rocks These are seen in the volcanic rocks of the Antrim Plateau and in the islands of Mull and Staffa off the west coast of Scotland. The rocks here form vertical columns in the cliffs. Plutonic rocks of granite occur at Shap in Cumbria and in Dartmoor and Bodmin Moor in southwest England. All of these areas form bleak, heath-covered uplands, which are of limited value for farming, other than rough grazing for sheep.

Caledonian Orogeny Gave Rise to Mountains in this Area

Sedimentary rocks These occur in many areas, especially in the areas of red desert sandstone in Devon, in parts of central England, and in the Vale of Eden. The sandstones have weathered to give fertile soils. Clays occur in parts of southeast England and in many river valleys. They are easily eroded and because of this, they form vales in the scarplands (where alternate layers of clay and more resistant limestone occur). Elsewhere, clays form lowlands. They weather to a stiff heavy soil, which remains damp for most of the year. Such areas support good pasture. More recently formed sedimentary rocks include river-deposited alluvium, clay, and sandstone, and ice-deposited boulder clay (mainly in eastern England).

Hercynian Orogeny Gave Rise to Mountains in this Area

Alpine Orogeny Gave Rise to Mountains in this Area

FIGURE 2.27  The Effects of the Main Mountain Building Movements on Great Britain.

Metamorphic rocks The slates of Borrowdale and Skiddaw in Cumbria and those of Snowdonia in Wales are examples of metamorphic rocks. Because metamorphic rocks are resistant to erosion, they usually stand up as areas of highland. These rocks are associated with volcanic activity and most of them in Great Britain are in the areas of old igneous rocks, i.e., in Scotland, parts of northern England, Wales, the southwest peninsula of England, and in northern Ireland.

Rock system in India The influence of rock type and structure on landscape development will be examined in more detail in later chapters of this book. Geologically, the Indian subcontinent is called craton. The term Craton is used here to denote a stable portion of a continent, commonly of Precambrian age, and not deformed for a longer period of time. Indian craton is one of the constituent units of the Super Continent-Pangaea and is existing, now, as a separate

2.24  Chapter 2

plate in the earth’s crust. The Indian plate got separated from Madagascar, about 90 million years ago, colliding with the Eurasian Plate. This tectonic movement has closed the Tethys Sea. India is a country with oldest geological bodies and features. It has a very unique geological and structural condition of almost all ages of the geological timescale. All kinds of rock masse mineral deposits, mineral fuels including coal and oil resources occur in India. The oldest known rocks in India are found to be 4500 million years age. The Archaean and Proterozoic eras belong to the Precambrian Period and all other units belong to the Phanerozoic period. Different eras can be categorized as mentioned in the following diagram: Plate Movements

Quaternary

Eurasian Plate American African Plate Plate Australian Plate Antarctic Plate

Present Time

Shallow Water

Period Recent Pleistocene

Pliocene Cenozoic

Pacific Plate

Era

Miocene Oligocene

65 Million Years Ago

Mesozoic (Tertiary)

Eocene Cretaceous

Jurassic Triassic Permian

135 Million Years Ago Laurasia

nd

Devonian Silurian Ordovician Cambrian

wa

na 180 Million Years Ago

Pre-cambrian

Go

Palaeozoic

Carboniferous

Proterozoic Archaeozoic Azoic

Tethys Ocean

200 million years ago

FIGURE 2.28  The evolution of plates through Archaean era; Proterozoic era; Palaeozoic era; Mesozoic era and Cenozoic era.

Plate Tectonics: The Earth’s Structure and Landforms   2.25

Archaean era (which includes the systems up to 2,500 million years); Proterozoic era (which includes the systems between 2500 and 570 million years); Palaeozoic era (which includes the systems between 570 and 245 million years); Mesozoic era and (which includes the systems between 245 and 66 million years) and Cenozoic era (which includes the systems between 66 and 0.01 million years).



Table 2.2

Important physical and organic events in relation to earth movements

IMPORTANT EVENTS – PHYSICAL AND ORGANIC

EARTH MOVEMENTS

TIME IN MILLIONS OF YEARS

Glaciers melted; many mammals disappeared; warmer climates Glaciation; invertebrates; large mammals; humankind

1

Mountain building; uplift of Rocky Mountains Formation of Alps and Himalayas with extensive volcanic activity in Rocky Mountains; basalt plateaus formed in Antrim (Northern Ireland), Skye, Faroes, and Iceland

10 Alpine

25

Tropical/mild climates; all modern mammals

40

Widespread lowlands; mild climates; flowering plants; insects; extinction of giant reptiles

60

Widespread lowlands; Europe under seas; mild climates; dinosaurs

135

Continents mountainous; widespread deserts; eruptions in North America

180

Appalachian Mountains (North America) formed; volcanic activity in Midland Valley (Scotland); sills and dykes formed in northern England Lowlands under seas; tropical coal swamps; large reptiles and amphibians; granites formed in Devon, Cornwall, and Midland Valley of Scotland. Granite intrusions in Scottish Highlands, e.g., Ben Nevis, in Lake District and in Wales

220 Armorican 270 350 Caledonian 400

Flat continents; mild climates; slate deposits

440

Mild climates; low continents; shallow seas; some mountains

500

Seas in geosynclines; mild climates; algae and trilobites

600

Seas in geosynclines; mild to cold; few fossils; Lake Superior iron deposits formed

1,000

Extensive mountain building; earliest known life

3,000

Formation of earth’s crust; no rocks have been Found

4,500–6,000

2.26  Chapter 2



Table 2.3

Geological era

QUATERNARY (1.6 – 0.01)

ECONOMIC MINERALS

1. Alluvium / 2. Alluvial Upland (Old)

Sedimentary

Placer Gold, Placer Diamond Placer Tin, Placer Heavy Minerals

3. Siwalik / 4. Marine Tertiaries

Sedimentary

Petroleum, Natural Gas, Bauxite, Kaolin

6. Intrusives/

9. Upper Gondwana/

PALAEOZOIC (570 - 245) PROTEROZOIC (2500 - 570)

Igneous

Sedimentary

12. Vindhyan

Sedimentary

14. Granites

Coal / Lignite

Sedimentary

11. Lower Gondwana

13. Malani Volcanics/

ARCHAEAN (Pre - 2500)

Igneous

7. Granites 8. Rajmahal Traps 10. Marine Trias

PRECAMBRAIAN

ROCK TYPES

TERTIARY (66 - 1.6)

INDIAN SUB-DIVISIONS (GROUPS)

5. Deccan Trap/ MESOZOIC (245 - 66)

PHENEROZIC

CENOZOIC (66 - 0.01)

MAJOR ERAS

Coal, Gypsum, Rock Salt

Igneous

15. Aravalli / 16. Delhi

Igneous

17. Cuddapah

Sedimentary

18. Granites (Old )

Igneous

19. Dharwar

Metamorphic

20. Khondalites

Metamorphic

21. Chamockites

Metamorphic

22. Un-Classified Crystallines

Metamorphic

Building Stones, Kaolin, Iron Limestone Uranium

Gold, Silver, Iron, Nickel Chromite, Copper, Lead-Zinc, Tin, Tungsten, Asbentos, Diamond, Graphite, Kyanite, Sillimanite

The Geological systems of India are always analysed with reference to their geographical locations. The following physiographic divisions of India are used for referencing the geological formations: a) The Himalayan Ranges           b) The Indo-Gangetic Plains c) The Extra Peninsular India          d) The Peninsular India and e) The Coastal sedimentary sequences.

Plate Tectonics: The Earth’s Structure and Landforms   2.27

GEOLOGICAL IMAGE OF INDIA

Sedimentary

Recent and Pleistocene

Sedimentary

Tertiary Deccan Trap

Igneous Metamorphic

Gondwana and Vindhyan

Metamorphic

Pre-Cambrian

FIGURE 2.29  Geological Image of India



Table 2.4

Classification of rock system in India Rock System in India

Archaean Rock System (PreChambrian Rocks) Very Old Plutonic Rocks

Archaean Gneisses & Schists

Purana Rock System (Pre-Chambrian Rocks) [Oldest metamorphosed Rocks]

Dharwar System

Cuddapah System

Vindhyan System

Dravidian Rock System [Palaeozoic]

Carboniferous rocks [European and North American Coal]

Aryan Rock System [Geologically Recent] Gondwana System [Indian Coal] Deccan Trap

Jurassic System

Tertiary System [Formation of Himalayas]

2.28  Chapter 2

Vulcanicity and Landforms We frequently hear about earthquakes that have violently shaken some part or other of a distant country, often causing considerable damage. Fortunately, we rarely feel the tremors in Great Britain; although in 1984 some parts of the country did. Also, from time to time we hear of volcanic eruptions that are taking place in which huge quantities of molten lava or equally huge quantities of cinders and ash, together with enormous clouds of steam and gases are thrown out of the earth’s surface. Hundreds of such eruptions have taken place in different parts of the world in historic times but we only remember the names of the really spectacular ones. The most recent big eruption occurred in Washington state in the United States when, in 1980, Mount St. Helens suddenly erupted, although a few hours before this happened, an earthquake shook the surrounding area. Mount St. Helens eruption. It is estimated that the explosion that took place inside the volcano was equal to 10 million tonnes of TNT, and that this blasted away 6 km3 of rock from the top of the volcano. The rock that was blasted away disintegrated into small pieces, which formed a gigantic mass of ash and dust, some of which reached a height of over 18 km as the explosion and hot gases towered into the atmosphere. The enormous heat melted millions of tonnes of ice and snow and the water from this mixed with the ash forming Mt st Helens mudflows, which swept across the countryside blocking rivers, burying homes, and smashing down forests. Very little lava was ejected Ash from the volcano. As you can see from Figure 2.30 an enormous Cloud cloud of ash was caught by the westerly winds and blown right across the United States and out over the Atlantic. So large was the cloud, which stayed in the atmosphere for many many months, that it was thought to have had a profound effect on the weather in parts of the northern hemisphere. A photograph (Figure 2.31) taken at the FIGURE 2.30  The Extent of the cloud of Ash time of the explosion clearly shows a large part of the volcano being from Mount St. Helens. blasted away.

(a)

(b)

FIGURE 2.31  (a) St. Helens. (b) The Eruption of Mount St. Helens in 1980 Caused a Large Part of the Cone to be Blown Away.

Plate Tectonics: The Earth’s Structure and Landforms   2.29

Distribution of Volcanoes and Volcanic Activity Most of the world’s volcanoes and volcanic activity can be sighted along the plate boundaries. The distribution can be classified into one of the following tectonic settings:

Divergence zones: volcanoes of the Mid-Atlantic ridge and over the continents In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. For example, the Mid-Atlantic Ridge separates the North and South American Plate from the Eurasian and African Plate. The figure demonstrates that, this pulling apart is causing “seafloor spreading” as new volcanic material is added to the oceanic plates. The spreading sites are the common sites of basaltic lava eruption. On the whole, seafloor spreading is basically volcanic, but it is a very slow and regular process, without the explosive outbursts of the volcanoes on land. Magma rises through the cracks and leaks out onto the ocean floor like a long, thin, undersea volcano. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. This process along the divergent boundary has created longest topoVolcanoes of graphic feature in the form of mid-oceanic ridges Atlantic Ridge Belt under the Oceans of the world Most of this activity is out of sight under the oceans, which is less hazardous to people (Figure 2.32). The map shows that over the continents, the divergence zones with fissure type of volcanic eruptions are represented by the East African Rift Valley Zone (Figures 2.33). This belt extends from Ethiopia to Tanzania. The Kilimanjaro in Tanzania is a well- FIGURE 2.32  Volcanoes of the Mid-Atlantic Ridge in known ­example of this belt (Figure 2.33). Divergent Zones.

Kilimanjaro in Tanzania FIGURE 2.33  East African Rift Valley Zone.

2.30  Chapter 2

Intra-plate oceanic volcanism (Hawaiian chain and other oceanic volcanic seamounts) Intra-plate oceanic volcanism can be represented by a single oceanic volcano, or lines of volcanoes such as the Hawaiian–Emperor seamount chains. They are also popular as hotspots and are located within the tectonic plates instead of plate margins. The map demonstrate that the Hawaiian volcanoes are located well within the Pacific plate rather than near a plate boundary (Figure 2.34).

Mid-Continental belt and Volcanoes in the Mediterranean region This belt is extended from the Mediterranean Alps to the Himalayan Region. Most often visited active volcanoes are found in this belt. Vesuvius and Stromboli are wellknown example of this belt. Mount Etna in Sicily (Figure 2.35) is Europe’s largest volcano. Its frequent eruptions often attract visitors. Movements within the earth usually produce several phenomena, which often form a chain reaction. This was shown in the Mount St Helens eruption (Figures 2.31 and 2.35). Slow movement along ­tectonic plates eventually resulted in a sudden movement (the earthquake) which was followed by the explosion and eruption. This in turn gave rise to mudflows and flooding by some rivers, forest fires and the production of a huge cloud of ash, which affected subsequent weather patterns.

FIGURE 2.34  Location of Hawaiian Volcanoes within the Pacific Plate

FIGURE 2.35  Etna in Sicily, Europe’s Largest Volcano. .

Plate Tectonics: The Earth’s Structure and Landforms   2.31

Helgafell eruption Quite a different volcanic eruption occurred on the island of Heimaey in Iceland in 1973 (Figure 2.36). The Helgafell volcano on this island had been dormant (had not erupted) for a very long time, but in 1973 great rumbles were heard which were followed by the opening of a long fissure (deep crack) in the surface of the island. Large quantities of molten lava and black ash poured out of Helgafell the fissure and spread out over much of the island. The fissure extended beneath the sea and the molten lava caused the water to boil. This eruption was a lava eruption. The lava flowed freely. This type of lava is called basalt. It does not build steep-sided conical 0 150 km mountains commonly associated with volcanoes. The Helgafell eruption, like others in Iceland before it, resulted from the slow FIGURE 2.36  Helgafell Erupted in 1973. The Lines show the Zone movement apart of the North American and Where the American and Eurasian Plates were Moving Apart. Eurasian plates. The Helgafell volcano is located in a zone of weakness known as the Mid-Atlantic Ridge while Mount St. Helens is located in another zone of weakness that rings the Pacific Ocean. This zone is called the Pacific Ring of Fire (Figure 2.37).

Subduction zones in the circum-Pacific belt The zones where one plate goes down under the other due to density difference are the sites of most of the world’s active and explosive volcanoes. The oceanic plate having higher density is subducted under the continental crust. The subducted slab melts under the increasing pressure and temperature to produce magma, which comes out through andesitic chain of volcanoes. The volcanoes are mainly situated on the continental side of the trenches.

FIGURE 2.37  Pacific Ring of Fire.

2.32  Chapter 2

The figure portrays that the so-called “Pacific Ring of Fire,” which is the collection of volcanoes bordering the Pacific Ocean. This zone is in fact a ring of subduction zones. It includes some of the deadliest volcanoes known, such as Pinatubo and Mt. St. Helens. The figure demonstrates that it starts from the Andean region of South America and extends northwards through Central America, Mexico, Western United States, and Canada to Alaska. From Alaska, it extends through Aleutian Islands towards the islands off the eastern coast of Asia and passes through Batholith Sill Kamchatka, the Kurile Islands, Japan, FIGURE 2.38  The More Important Types of Volcanic Intrusions and the Philippines, and further south to Extrusions. the New Guinea, Solomon Islands, New Zealand, and Antarctica. The volcanic belt of the Indian Ocean, which passes through Andamans, Sumatra, Java, Bali, Height Sunda, and Burma meets the Pacific belt of near the Malacca Island. Eruption We will now look at other aspects of and Viscosity Vulcanicity. This refers to all the various of ways by which molten rock and gases are Plinian Magma forced into the earth’s crust and on to its Hawaiian Vulcanian surface. Vulcanicity therefore includes Strombolian volcanic eruptions (the formation of volExplosiveness canoes and lava plateaus and geysers), and the formation of volcanic features such as FIGURE 2.39  Types of Volcanoes on the Basis of Nature of Eruption. batholiths, sills, dykes, etc. in the crust. Rocks below the crust have a very high temperature, but the great pressure exerted on them by the crust, keeps the rocks in a semi-solid state. Friction along rock surfaces at the boundaries of tectonic plates raises the temperature, and this, plus a reduction in pressure caused by faulting and folding associated with the movement of tectonic plates, causes these rocks to become molten and semi-fluid. Such rocks are called magma. As the magma rises, it forces its way into the cracks of the crust. The magma may stay in the crust where it forms batholiths, sills, and dykes (Figure 2.38) or it may reach the surface either quietly, or with great violence. If the magma contains a lot of gases, especially steam, then as the magma approaches the surface, the pressure on the gases is reduced. This causes the gases to expand rapidly, which gives rise to violent explosions. When magma reaches the surface, it loses its gases and is called lava. The most commonly used classification by volcanologist is that originally proposed by Lacroix in 1908. There are four principle types of eruptions: (1) Hawaiian, (2) Strombolian, (3) Vulcanian, and (4) Plinian. The ­figure demonstrates that degree of explosiveness, height of eruption, and viscosity of magma increases from Hawaiian to Plinian type of volcano (Figure 2.39). Lava Flow

Laccolith

Volcano

Dyke

Plate Tectonics: The Earth’s Structure and Landforms   2.33

Volcanic features formed in the crust Batholith This is a very large mass of magma, which accumulates in the crust. Sometimes it forms the root or core of a mountain. Batholiths are made of granite and they form surface features only after they have been exposed by denudation, as shown in Figure 2.40. Batholiths are (a) exposed at the surface in southwest England where they form Dartmoor, Bodmin Moor, Land’s End, and the Scilly Isles. These are the cores of an ancient mountain chain the tops of which have long since Sill been removed by erosion.

Low Escarpment

Bedding Planes

Sill When a sheet of magma lies along a bedding plane, it forms a structure called a sill. Some sills form ridgelike escarpments when exposed by erosion. The Great Whin Sill in northern England was quickly recognized by the Romans as being useful for defence and on it they built Hadrian’s Wall, large stretches of which remain to this day. Figure 2.41(a) shows a sill. The point at which a river crosses a sill is sometimes marked by a waterfall. High Force in Durham county is formed by the River Tees where it crosses the Great Whin Sill. See Figures 2.41(b). Figure 2.42 shows that sill is the tabular or sheetlike intrusive body formed when magma is injected Batholith

(b)

Over-lying Metamorphic and Sedimentary Rocks Removed by Erosion

FIGURE 2.40  A Batholith is Sometimes Exposed by Weathering and Erosion.

Sill

Magma Chamber FIGURE 2.42  Illustration of a Sill

FIGURE 2.41  (a) An Escarpment Formed by A Sill, the Dimensions of which are Similar to those of a Dyke. (b) High Force, the Highest Waterfall in England.

2.34  Chapter 2

A Few cm to Many m Depression

A Few m to Many km

along sedimentary bedding surfaces. They are usually formed from low viscosity magma. The Great Whin Sill situated in Great Britain is a well-known example. Dyke

When a mass of magma cuts across bedding planes, it forms a wall-like feature called a dyke. Sometimes the rocks on either side of a dyke are more resistant to erosion. When this happens, the dyke forms a depression (Figures 2.43 and 2.44). Dykes sometimes occur in swarms, as in Arran and parts of western Scotland and northern Ireland. See Figure 2.22. Dyke is Easily Dyke Resists The figure demonstrates that dyke is an igneous Eroded to Form a Erosion to Form a intrusion that cuts across the bedding of country Depression Low Ridge rock through near-vertical fissures. Hundreds of parFIGURE 2.43  Two Dykes—One Forming A Ridge and the allel dykes can be traced in northwestern Scotland, Other a Depression. especially in the Islands of Mull and Arran. The denudation processes can expose the comparatively harder Dyke to from “walls” or Cliff across the beaches. Sometimes a zone of dykes may surround a circular or dome-shaped intrusion in more or less arcuate from; these are known as ring-dykes. Dykes

Volcanic features formed on the surface Magma sometimes reaches the surface through a vent (hole), or a fissure (crack) in the surface rocks. When magma emerges at the surface, it is called lava. If lava emerges via a vent, it usually builds up a volcano, which is a cone-shaped mound. If it emerges from a fissure, it may build up a lava plain, or a lava plateau. Volcanic eruptions also take place on some ocean floors.

Magma Chamber FIGURE 2.44  Dyke

Crater Pipe Lava Layer

Conelet Dyke Feeding Conelet Layered Ash and Cinders and Broken Lava

Earth’s Crust Vent FIGURE 2.45  A Generalized Diagram of a Volcano Showing its Characteristic Features.

Vent eruptions and the types of volcanoes The mound of a volcano is called the cone and this may consist of lava, a mixture of lava and rocks torn from the crust by the molten magma, or it may consist of ash and cinders (small fragments of lava). The shape and size of the cone largely depend on the nature of the cone’s material, and the type of eruption. The channel through which the lava rises is called the pipe, and the exit of the pipe, which is usually a shallow depression, is called the crater. These features are all shown in Figure 2.45. Kilimanjaro has a well-formed, almost circular crater.

Plate Tectonics: The Earth’s Structure and Landforms   2.35

Craters and calderas The crater is a bowl or funnel-shaped depression or cavity usually of volcanic origin. It is usually more or less circular in the plan at the summit of the volcanic mountain. The diameter of the crater is commonly less than 1.6 km. Craters may result from either explosive activity or from subsidence. The figure exhibits an aerial view of a crater at the summit of Mt. St. Helens (USA). It should be noted that craters may also form due to impact of meteorites and mining process but in this case, it will not be associated with volcanic activity (Figure 2.46). The huge carter-like depression is called Caldera. The diameter of a caldera is usually several times that of a crater. The figure demonstrates the formation of caldera due repeated volcanic eruption. The  caldera may also form due to coalescence of several small craters (Figure 2.47). The collapsing of summit of the volcano due to development of underground cavity may also form caldera. Figure 2.44 shows formation of caldera and caldera lake on the on the northern Kuriles Islands of Russia. It shows that the roof of former volcano has collapsed and created a caldera in which a younger volcano starts to form from the old vent within the caldera lake (Figure 2.48). The Buldir caldera between the islands of the Aleutian island chain is the largest known caldera in the world Ash and cinder cone lava is blown to great heights when it is violently ejected, and it breaks into small fragments, which fall back to earth and build up a cone (Figure 2.49). Cinder cones are the most abundant of all volcanoes. Small cones consisting mostly of pyroclastic debris each having a single vent are called cinder cones. When pyroclastic fragments fall and accumulate close to the vent, they may pile up to form a very symmetric cinder cone. The figure shows a classic example of Cinder cones on the Lanzarote Island of Canary Islands (Figure 2.50). Good examples of ash and cinder cones are Volcano De Fuego (Guatemala) and Paricutin (Mexico).

Crater

Leschi Glacier The Breach Sugar Bowl

Toutle Glacler

The Boot

Central lava dome

Dogs Head

Lava Dome Crater Glacier Mt Saint Helens

FIGURE 2.46  Areal View of a Crater at the Summit of Mt. St. Helens (Usa). Image credit: ©Google earth 2015).

Caldera with Lake e Tim

After the violent volcanic eruption

Before the volcanic eruption

FIGURE 2.47  Formation of Caldera Due to Repeated Volcanic Explosions.

Lava cones The slope of a volcanic cone depends on whether the lava forming it was fluid or viscous when it was molten. A fluid lava builds a gently sloping cone, e.g., Helgafell volcano in Iceland. The most famous of all lava cones of this type is Mauna Loa (Figure 2.51) in Hawaii. This reaches a height of 9,100 m on a base of diameter 400 km on the seabed.

FIGURE 2.48  On The Northern Kuriles Islands (Russia) The Roof of a Former Volcano Collapsed and Created A Caldera in which a Younger Volcano Starts to form from the Old Vent within the Caldera Lake (Image Credit: ©Google Earth 2015).

2.36  Chapter 2

Crater Cone Made of Layers of Ash

3048 m

Earth’s Surface

9144 m

FIGURE 2.49  The Active Boqueron Volcano on the Island of San Banedicto in the Pacific Ocean.

4115 m

112 km Crater

Layers of Lava Sea Level

About 400 km FIGURE 2.51  The Fluid Lava Cone of Mauna Loa in Hawaii. Crater Pipe Viscous Lave

FIGURE 2.52  A Cone Made of Viscous Lava. Spine of Viscous Lava

Volcano

FIGURE 2.53  A Diagrammatic View of Mont Pelée, in Martinique, Just Before the Spine Disintegrated in 1903. In the Great Eruption that Took Place in 1902, 30,000 People were Killed and the Capital of St. Pierre was Completely Destroyed.

FIGURE 2.50  Cinder Cones on Lanzarote Island, Canary Islands, Spain (Image Credit: ©Google Earth 2015).

Mont Pelée eruption Viscous lava produces a steeply sloping cone such as that shown in Figure 2.52. Sometimes the lava is so viscous that when it is forced out of the volcano it forms a spine or plug dome that may completely block the vent. Mont Pelée in Martinique in the West Indies was an excellent example of a spine or plug dome. A few weeks after Mont Pelée started to erupt in 1902, violent explosions took place and molten lava forced its way into the base of the water-filled crater. The crater lake boiled, its water spilled over the rim of the crater, and swept down one side of the volcano turning into a mudflow as it went. Later clouds of ash and gases burst from the volcano followed by an immense outpouring of frothy lava, which advanced down the slopes at over 150 km per hour. Within minutes St Pierre, the port and capital of Martinique was overwhelmed causing the deaths of over 30,000 people. In the autumn of 1902, the plug of lava in the volcano’s pipe was forced out to form a plug dome over 300 m high. By the middle of the following year, the spine had broken into pieces and disappeared. Figure 2.53 indicates what the spine looked like. There are several plug domes in the Hoggar Mountains (Figure 2.54) in Algeria. Composite cone This type of cone is formed of alternate layers of lava and ash. The volcano begins each eruption with great violence forming a layer of ash. As the eruption proceeds, the violence ceases and lava pours out forming a layer on top of the ash. Lava often escapes from the sides of the cone where it builds up small conelets. Figure 2.55 illustrates the structure of this type of volcano. Mount Kilimanjaro, in Tanzania with a height of 5,895 m, and Vesuvius, Etna, and Stromboli, all in Italy, are examples of composite cones.

Plate Tectonics: The Earth’s Structure and Landforms   2.37

A composite cone sometimes has its top blown off by violent volcanic explosions and the top disintegrates into a mass of rocks and ash, leaving the crater greatly enlarged. The huge crater-like depression is now called a caldera. A caldera may also form through subsidence. After a major eruption, the supply of magma is depleted causing a huge chasm to form beneath the volcano. The weight of the cone sometimes causes faults to develop and in time, the whole cone collapses into the chasm beneath. Some calderas are formed in part by subsidence. Figure 2.56 shows how a caldera may form.

FIGURE 2.54  A Volcanic Plug in the Hoggar Mountains of Algeria. Notice the Well-Developed Talus Around the Base of the Plug.

Composite cones or stratovolcanoes Sometimes pyroclastic material either flows through Crater break in the crater wall or comes out from edges of the Ash Layer cone which forms composite volcanoes or alternatively, Conelet Lava Layer stratovolcanoes. Figure 2.57 shows that they are known as composite volcanoes because they are built up of lay- Dyke Lava Flow ers of more than one kind of material. The mix of lava Pipe and pyroclastics allows them to grow larger than either cinder cones or volcanic domes. The composite volcaEarth’s Surface noes have a crater at the summit, which generally contains a central vent or a clustered group of vents. Some FIGURE 2.55  The Structure Of A Composite Volcanic of the most beautiful volcanic mountains in the world are composite volcanoes, including Mount Cotopaxi in Cone. Ecuador and Mount St. Helens in the United States.

Gases and Lava Bombs Ejected

Top of Cone Breaks up

(a) (b)

Caldera

(c)

Top of Cone Has Sunk into the Magma

New Cones Caldera

(d)

FIGURE 2.56  This Shows how a Caldera May Develop by Violent Volcanic Explosion: (a) Before Eruptions Take Place; (b) Violent Eruption; (c) Eruptions have Ceased; and (d) Caldera with New Cones.

2.38  Chapter 2

Lateral Cone

Crater Sill

Buried Cone

Dykes Magma Chamber FIGURE 2.57  Composite Cones or Stratovolcanoes.

A caldera may become the site of a lake, e.g., Crater Lake in the United States. Radiocarbon dating of the carbonized wood from trees killed by the eruption that produced the crater now occupied by Crater Lake, puts the eruption that blew off the top of the volcano at about 7,000 years ago. The outer slopes of the caldera walls bear evidence that the volcano supported glaciers; from this evidence, it has been calculated that the cone must have been about 3,500 m high. This means that more than 65 km3 of volcanic cone must have been destroyed in the eruption.

Krakatoa eruption For two centuries, Krakatoa remained dormant and then in 1883, it began to erupt. The explosions became increasingly violent. Dense clouds of volcanic gases and ash reached a height of 26 km. Heavy rains turned into liquid mud. The eruption reached a climax in August 1883 when a series of stupendous explosions occurred, which could be heard in parts of Australia 4800-km away. A glowing cloud of white, hot ash rose 80 km into the air, and much of this was carried around the world several times in the upper layers of the atmosphere. The explosion caused two-thirds of Krakatoa island to disappear. It simply sank into a deep submarine depression formed by the eruption. All that remained was a submarine caldera.

Fissure eçruptions and the landforms they produce Eruption of lava from fissures usually takes place very quietly. The lava spreads out over the surrounding countryside, and successive outpourings cause layer upon layer of lava to form. These may completely cover up the features of the surrounding region. At times, the layers of lava are hundreds of metres thick and they form

A Closer Look  ▼ Are volcanoes always active? Volcanoes usually pass through three stages in their life cycle. Eruptions are frequent in the early stage. This is when the volcano is active. Later, eruptions become very infrequent, when the volcano is said to be ­dormant (sleeping). When the volcano has not erupted in recent times but is fresh looking, it is regarded as dormant. A dormant volcano exhibits no indication for future eruption but they may erupt suddenly and violently causing enormous damage to life and property. Mount Vesuvius is a classic example of this category which erupted in 79 CE and destroyed the Roman cities of Pompeii and Herculaneum. It again erupted in 1631 CE the frequency of eruption again increased in 19th century. A volcano is considered extinct when it has no recent eruptive history. Impact of erosion can be seen on this type of volcano. They are unlikely to erupt again. For example, Arthur’s Seat in Scotland. This is followed by a long period of inactivity. Volcanoes, which have not erupted in historic times are said to be extinct. Like all landforms, a volcano is attacked by weathering and erosion, and by the time it is extinct, most, if not all of the volcano may have been removed.

Plate Tectonics: The Earth’s Structure and Landforms   2.39

(b)

(a)

Original Rock Surface

Fissures Layer of Lava

FIGURE 2.58  Two Stages in the Formation of a Lava Plateau: (a) Original Relief; (b) Original Relief Buried Beneath Lava Flows, which form on Plateau.

high, fairly level features called lava plateaus (Figure 2.58). Subsequent erosion by rivers sometimes results in the original surface becoming exposed on the floors of deep valleys (Figure 2.59). The largest lava plateau in Great Britain is in Antrim (Northern Ireland). It has an area of about 1,50,000 km2. The Columbia and Snake Plateau in the United States is over 5,00,000 km2 whilst the Deccan lava plateau is almost 6,50,000 km2. When rivers cross a lava plateau, they often carve out deep valleys. Sometimes, depending on the nature of the lava, the action of weathering breaks down the lava into fertile soil. This has happened in the northwestern part of the Deccan Plateau, India.

Lava Plateau

Original Surface FIGURE 2.59  When Vertical Erosion by a River Crossing a Lava Plateau is Very Active, the River May Reach down to and Expose the Underlying Original Surface.

A Closer Look  ▼ Why do some volcanoes erupt violently? Most magma contains gases, which is under great pressure. In some cases, there is a sudden decrease in pressure in the rising magma and this causes the gases in it to expand very rapidly.This sudden expansion can cause violent explosions. Water vapour is often one of the gases, and it may have originated in the magma or from water in the crust with which the magma came into contact. Many of the gases burn with a fierce heat and some of them, such as sulphur gases, form a dense cloud, which rolls down the side of the volcano killing everything in its path. When eruptions are violent, the lava explodes into small pieces, which are blown to great heights. The sizes of the pieces vary from grains to small chunks of rock. The latter are called volcanic bombs. If the explosions are particularly violent, the fine dust can reach such great heights that it gets carried along by the air currents of the upper atmosphere. When Krakatoa exploded in 1883, some of its dust passed around the world causing vivid sunsets in many countries.

2.40  Chapter 2

India

On the surface of the Earth many e­ xtensive fairly leveled lava plateaus have been built by fissure eruption. They have ­completely covered up the surrounding region. (Figure 2.60). shows that the Deccan lava plateau covers almost 6,50,000 km2 of India’s g ­ eographical area

Other forms of volcanic activity

FIGURE 2.60  Deccan Lava Plateau.

Emissions of gases and steam periodically take place from dormant volcanoes. Similar emissions of gases and steam take place in some volcanic regions where active lava eruptions have long since ceased. Superheated water may flow quietly, as in hot springs, or it may be thrown out with great force and accompanied by steam, as in geysers. Thus, a geyser differs from a hot spring that its water is ejected explosively. Geysers often form natural fountains. Hot springs and geysers are common in Iceland, the North Island of New Zealand, and the Yellowstone National Park of the United States. Figure 2.62 shows two ways in which a geyser may form.

Is a Volcanic Landscape Hostile? Most people would agree that volcanic eruptions cause nothing but destruction. They can destroy settlements and farmland, burn down forests and cause flooding, and they can kill people. In this respect, they would appear to be utterly destructive. And yet, people who live in some regions of volcanic activity do not quit the regions for long. This happens in regions where lavas weather to give fertile soils. There are many such regions in the humid tropics, in Indonesia and the West Indies where high daily temperatures and abundant rainfall work together to turn some lavas into rich soils. Yellowstone National Park, U.S.A

Hot Spring Geyser

Geyser

Pressure Chamber FIGURE 2.61  Location of geyser at Yellowstone National park in the USA

Plate Tectonics: The Earth’s Structure and Landforms   2.41

(a) Dormant

Ejecting Steam

Restricted Opening Causes Steam to Escape Under Pressure Pressure Builds up Until a Jet Of Steam is Ejected

Rapidly Circulating Steam

Boiling water

Jointed Rocks Supply the Water

Very Hot Rocks

(b) Domant

Ejecting Water and Steam

When the Jet of Steam has been Ejected the Geyser Returns to the Dormani Phase The Sump Fills Up with Boiling Water Entering from the Jointed Rocks

Jointed Rocks Supply the Water

Very Hot Rocks

Very Hot Rocks

Boiling Water

Boiling Water

Pressure Builds Up

Boiling Water and Steam are Ejected Until the Sump is Cleaned Very Hot Rocks

Pressure is Strong Enough to Push the Water Out of the Sump

FIGURE 2.62  Two Ways in which a Geyser May Form: (a) Based on Underground Cavern; (b) Based on Underground Cavern and Sump.

Volcanic activity sometimes results in the formation of precious stones and minerals, which occur in some igneous and metamorphic rocks, e.g., diamonds of Kimberley in South Africa, copper deposits of Butte in the United States, and nickel deposits of Sudbury in Canada. We have seen that volcanic activity sometimes results in the formation of hot springs and geysers. In Iceland and New Zealand, this superheated water is used for heating buildings. In Iceland, a power station has been built, which taps the steam to drive turbines for generating electricity. This type of energy development is called geothermal energy.

2.42  Chapter 2

Major Landforms  We have seen that plate movement gives rise to the formation of trenches and ocean ridges on the sea floor, and to the development of lofty mountain ranges. From this, it appears that two types offorces are taking place, which together are called internal earth movements. They are listed as follows. 1.  Radial (vertical or up and down movements). 2.  Tangential (sideways movements). See Figure 2.63. Radial forces pull either towards or away from the centre of the earth. They cause large areas of the crust to be uplifted or lowered, which results in faulting. The landforms, which are produced as a result, include plateaus, block mountains (horsts), basins, and some types of escarpment. Tangential forces operate at right angles to the radial forces, i.e., parallel to the earth’s surface. These forces are (a) tensional, i.e., pulling away from one another and (b) compressional, i.e., pushing together. These forces produce rift valleys (which may be the product of tensional or compressional forces), block mountains, and fold mountains (the product of compressional forces). Earth movements, which usually take place very slowly, are responsible for the formation of most of the earth’s mountains, its plateaus and plains. They also cause sedimentary rocks to be displaced, i.e., to be pushed out of the horizontal plane so that the rocks are tilted or inclined. The inclination of the rocks is called the dip. The direction parallel to the bedding plane and at right angles to the dip is called the strike. Figure 2.64 has had the upper sedimentary layer of rock removed so as to show the bedding plane, dip, and strike. It takes a long time for forces in the earth’s crust to build up to breaking point but when this is reached the rocks snap, there is a violent shaking in the crust, and sudden movements of masses of rock take place. This is called an earthquake.

B2

Di

p

A Radial Forces B Tangential Forces 1 Tensional 2 Compressional

Strike

wn thro lock B

Up

e rik Dip

St

Dip Angle

B1 FIGURE 2.63  Sectional View of the Earth to show the Directions in Which Radial and Tangential Forces Operate at Right Angles to Each Other.

60º

Fault Scarp

A

Downthrown Block Hanging Wall

FIGURE 2.64  Tilted Rocks Showing the Relationship of the Dip to the Strike.

Plate Tectonics: The Earth’s Structure and Landforms   2.43

Earthquakes Earthquakes are sudden movements or vibrations in the earth’s crust. They cause the ground to shake violently, making the walls of buildings crack or bulge, or even tumble down. Whole settlements have been destroyed, sometimes with great loss of life, and at times deep faults have appeared on the surface with the rocks on either side being displaced horizontally or vertically. Earthquakes are caused by 1. one tectonic plate sliding over or past another plate along the line of a fault 2. volcanic eruptions – the movement of molten rock below, or on to the earth’s surface, which in turn is caused by the movement of plates.

Occurrence of earthquakes The majority of earthquakes occur in narrow belts, which mark the boundaries of tectonic plates. The main types of regions where they occur are 1. Mid-ocean ridges 2. Ocean trenches (deeps) and volcanic islands 3. Regions of crustal compression. Major earthquakes are caused by the movement of tectonic plates, e.g., the North American and Pacific plates result in horizontal movements along the San Andreas Fault, in California, and similar faults, which extend northward into Alaska and the Aleutians. Such movements involve no more than a few centimetres, but the sliding of two plates past each other, in a horizontal or ­vertical ­direction, produces violent waves, which cause the earth tremors. In some cases, there may be a displacement, horizontally or vertically, of several metres. Figure 2.65 shows the locations of the world’s main earthquake zones and volcanic belts. Helgafell (Iceland)

CircumPacific Ring

Be ge Agadir

Atl

Mauna Loa

Vesuvius

Rid ant ic

Mt St Helens

lt

Columbia-Snake River Plateau

Mont Pelee Popocatepetl Cotopaxi

Arctic Circle Siberian Plateau Mediterranean and South West Asian Belt CircumPacific Ring

Etna El Asnam Ethiopian Plateau

Deccan Plateau

Equator

Kihmanjaro Krakatoa

Volcanic Belt Earthquake Lava Plateaus

Parana Plateau

Kimberley Plateau Drakensberg Plateau

Volcanoes

FIGURE 2.65  The Major Earthquake and Volcanic Belts of the World. The Dotted Parts of the Belts Represent Areas where Earthquakes of Great Intensity Occur. The More Important Volcanic Plateaus and Volcanoes are Shown.ç

2.44  Chapter 2

JUAN DE FUCA PLATE

Vancouver

Canada

Seattle Trench

United States

San Francisce

N

SA

NORTH AMERICAN PLATE

AN

DR E

AS

Los Angeles

FA U

LT

PACIFIC PLATE Mexico

FIGURE 2.66  Trace of the San Andreas Fault, Southern California. The Map shows the Extent of the San Andreas Fault System and the Eastern California Shear Zone (a) and (b) a Diagram, which Shows that at the Northern End of the San Andreas Fault Lies a Subduction Zone Under the Pacific Northwest Trace of the San Andreas Fault, Southern California. The Map shows the Extent of the San Andreas Fault System and the Eastern California Shear Zone (a) and (b) a Diagram, which Shows that at the Northern End of the San Andreas Fault Lies a Subduction Zone Under the Pacific Northwest

There is a great deal of seismic activity when the lithospheric plates slide past each other laterally, across vertical fractures commonly referred to as transform faults. This zone is famous for occurrence of shallow focus earthquakes. The transform fault mostly occurs along short offsets related to slight bends in the mid-oceanic ridge system. But in some cases such as the San Andreas Fault in California (Figure 2.66), the transform fault extends through continental lithosphere. On 18 April 1906, an earthquake of magnitude 7.9 struck California. Other examples of transform fault in continental lithosphere

Plate Tectonics: The Earth’s Structure and Landforms   2.45

Oceanic ridge and transformers

Trenches

Shallow-focus (0–50km)

Intermediate-focus (50–300km)

Deep-focus (300–600km) FIGURE 2.67  Earth’s Seismicity and the Plate Boundaries.

are the Alpine Fault in New Zealand, Queen Charlotte fault in North America, and Anatolian fault in Turkey, which are witness to several shallow focus earthquakes. An earthquake of magnitude 7.8 struck Canada along the Queen Charlotte fault in 2012. The only parts of Africa, which have earthquakes, are located in the Great Rift Valley region of East Africa, and in parts of northwest Africa. Most of the earthquakes occurring in Africa are relatively mild. However, serious earthquakes occurred in El Asnam in 1954 and in Agadir in 1960.

Nature of earthquakes The point at which an earthquake originates is called the focus and sometimes it is several kilometres below the surface. The point on the earth’s surface immediately above the focus is called the epicentre. The above Map (Figure 2.67) shows the concentration of shallow-focus, intermediate-focus, and deep-focus earthquakes along the circum-Pacific belt, also referred to as the Ring of Fire. The Pacific plate subducts into the North American plate forming the Aleutian trench – where earthquakes are a common feature. As the Nazca plate subducts beneath the South American plate, the Peru–Chile trench is created. In the Indian Ocean, the Tsunami of 2004 was triggered by an earthquake in Sumatra, Indonesia, of magnitude of 9.1, along the megathrust where the Indo-Australian Plate subducts under the Burma Plate that is considered part of Eurasian plate. Large subduction zone earthquakes have continued to occur along this belt – the Nias Island earthquake of magnitude 8.6 in 2005, 8.5 and 7.9 magnitude earthquakes in 2007, and 7.8 ­magnitude earthquake on ­portion of megathrust west of Mentawai Islands in 2010. In case of oceanic–oceanic convergence, one oceanic plate subducts under the other leading to the formation of oceanic trenches. Earthquakes of both shallow and deep focus occur during this convergence. Mariana trench is the result of

convergence of fast-­moving Pacific plate against the slow-moving Philippine plate, which is also zone of earthquakes. The continental–continental convergence has

2.46  Chapter 2

no subduction due to buoyancy of the continental crust, but shallow-­focus earthquakes are quite common. The formation of huge mountain ranges such as Alps and Himalayas is attributed to this type of convergence. Earthquakes in Himalayas are common as the Indian plate and the Eurasian plates continue to converge at relative rate of 40–50 mm/year. In Nepal, on 5 April 2015, a shallow-focus earthquake of magnitude of 7.8 occurred that devastated the small Himalayan nation (Figure 2.69). The epicentre was located 36 km east of Khudi in Nepal and was followed by aftershocks of magnitude 6.1 and 6.6, on April 25th itself and 6.7 on 26 April 2017. On 3 January 2016, a 6.7 magnitude earthquake occurred 30 km east of Imphal in India. The point at which the energy is first released is the focus of the earthquake or the hypocentre. Epicentre on the other hand, is the place on the surface of the earth, which is directly above the focus (Figure 2.70). For instance, the 26 December 2004 earthquake that triggered the devastating tsunami in the Indian Ocean had an epicentre 160 km off the west coast of northern Sumatra and a focal depth of 30 km. Depending upon the focal depth, three categories of earthquakes can be recognized: shallow focus, intermediate focus, and deep focus. Shallow- and ­intermediate-focus earthquakes have focal depths of less than 70 km and between 70 and 300 km from the surface, respectively. While those with the foci located at a depth of more than 300 km are categorized as deep-focus earthquakes. Shallow focus earthquakes are the most destructive as the energy has little time to dissipate before reaching the surface.

CHINA

Epicentre Mount Everest Pokhara

Kathmandu Bharatpur Bhaktapur

50km

NEPAL

20 miles INDIA

Quake intensity Severe Very strong Strong Moderate Google

FIGURE 2.68  Nepal earthquake intensity.

Aftershock

Plate Tectonics: The Earth’s Structure and Landforms   2.47

This is where the shock waves first hit the surface. It is the shock waves, which give rise to an earthquake. There are two types of shock waves: 1. Body waves. These travel through the crust and are of two types: a. Primary waves, which cause the crustal rock to move back and forth in the direction of wave movement. b. Secondary waves, which cause the crustal rock to move from side to side, i.e., at right angles to the direction of wave movement. 2. Surface waves. These travel through the surface rocks and are of two types: a. Love (L) waves, which cause the surface rocks to move from side to side, at right angles to the direction of wave movement b. Rayleigh (R) waves, which cause the surface rocks to have a vertical circular movement very similar to that of water in a sea wave (refer Chapter 8).

Oceanic ridge and transformers Trenches Shallow-focus (0–50 km) Intermediate-focus (50–300 km) Deep-focus (300–600 km) FIGURE 2.69  The Epicentre of Nepal Earthquake, May 2015. 32

4.5

30s 31

Latitude (º)

30 29

20s

4

10s

3.5

0s

3 2.5

28

2

27

1.5

26

1 0.5

25 80

81

82

83

86 84 85 Longitude (º)

87

88

89

90

0

FIGURE 2.70  Focus and Epicentre of Earthquake.

2.48  Chapter 2

As the shock waves travel out from the focus they give rise to vibrations that may be as high as 200 per minute. Figure 2.71 shows the paths taken by the body and surface waves. The vibrations caused by the surface waves produce most of the damage that occurs in an earthquake. The violent shaking of the surface rocks often causes great damage to buildings and sometimes, considerable loss of life. The intensity of an earthquake is measured by an instrument called a seismograph. This instrument records the vibrations produced by an earthquake. The magnitude of an earthquake refers to the total amount of energy released, and the scale, which gives the magnitude is called the Richter Scale. This scale ranges from 0 to over 8. A magnitude of 2.0 is 10 times greater than that of 1.0, and one of 5.0 is 10,000 times greater than one of magnitude 1.0. The intensity of an earthquake refers to the effect produced by the earthquake. Of course, this varies from place to place, so while the intensity of a specific earthquake varies, its magnitude does not vary. It is important not to confuse magnitude with intensity. Most of the damage caused by an earthquake results from the effects of the surface waves. Figures 2.72 and 2.73 show Çthe type of damage that a severe earthquake can cause. Epicentre Earth’s Surface

aves ace W Surf Focus

Body Waves Fo cus

(a) FIGURE 2.71  (a) The Paths Taken by Earthquake Waves Through the Rocks below the Surface, and at the Surface; (b) the Waves Move Away from the Focus in a Cyclic Manner; the Movements are in all Planes.

(b)

Effects of earthquakes Given below is the lost of possible effects and consequences that take place in event of an earthquake.

FIGURE 2.72  Devastation of Earthquake 2015 at Sindhupal Chok, Nepal as Seen in March 2018.

1. T  hey can displace parts of the earth’s crust vertically or laterally. 2. They can raise or lower parts of the sea floor. The Agadir earthquake in Morocco in 1960 raised the sea floor off the coast. In some areas, the depth of the sea decreased from 400 to 15 m after the earthquake. 3.  They can raise or lower coastal rocks. In the Alaskan earthquake of 1899, some coastal rocks were raised by 16 m. 4. They can cause landslides and open deep cracks in the surface rocks. The El Asnam earthquake in Algeria, in 1954, destroyed an area of radius 40 km and opened surface cracks up to 3 m deep.

Plate Tectonics: The Earth’s Structure and Landforms   2.49

5. S ubmarine earthquakes may produce waves in the oceans that travel from 150 to 1000 km/h. The waves may be only 1  m high but when they enter shallow water, i.e., near the coast, they gain in height, sometimes up to 2.5 m. These waves are called tsunamis.

Faults Faulting can be caused by either radial or tangential forces (tensional and compressional). The rocks of the earth’s crust are subjected to tension and compression when radial or tangential forces operate. If one part of the crust is compressed then clearly another part must be stretched, FIGURE 2.73  In I960, a Violent Earthquake Struck Morocco and i.e., put under tension. Rocks under tenMuch of the Town of Agadir was Destroyed. sion usually fault, but under compression, they may fault or fold. Figure 2.74 gives the names of the parts of a fault. Tension causes a normal fault, compression causes a reverse fault and lateral movement (parallel to the fault) causes a tear fault. Escarpments, called faith scarps, develop if faulting is accompanied by upward or downward movements of adjoining parts of the crust as shown in Figures 2.74 and 2.75. Photographs of a normal fault and a tear fault are shown in Figures 2.76 and 2.78, respectively. The various terms used with reference to faults are given in Figure 2.79.

(a)

(ii) Normal Fault

Fa

ul

(i)

Li n Fa e o ul f t

Tension Compression

tP

Tension

Tension

(iii)

la

ne

Tension

Reverse Fault

Compression

Compression

Compression

FIGURE 2.74  (a) The Formation of Normal and Reverse Faults. (b) The Formation of Normal and Reverse Faults.(continued)

2.50  Chapter 2

(b) Stages in the Development of A Normal Fault (i) Fault Plane Develops.

Stages in the Development of a Reverse Fault (i) Fault Plane Develops. Compression

Tension

Fa

Fa

ul

Stages in the Development of a Tear Fault (i) The Fault Plane is Often Almost Vertical.

ul

t

Pl an

t

Fault Plane

Pl

an

e

(ii) Rocks on Each Side of the Fault Plane are Usuallu Displaced as Shown by the Arrows. Note Surface Area is Increased.

e

(ii) Rocks are Displaced as Shown by the Arrows. Those on One Side of the Fault Plane Ride up Over Those on the Other Side. Note That the Surface Area is Reduced.

Escarpment

(iii) Faulting Sometimes Produces an Escarpment. This is Sometimes Removed by Erosion

(ii) The Rocks Are Displaced Horizontally as Shown by the Arrows. There is no Vertical Displacement. Tear Faults Usually Occur During Earthquakes. The Earthquakes Which Wrecked San Francisco Produced Tear Faults. Note Earthquakes Sometimes Produce Vertical Movements.

(iii) An Escarpment May Mark the Fault. Erosion May Later Remove This.

(continued) FIGURE 2.74  (a) The Formation of Normal and Reverse Faults. (b) The Formation of Normal and Reverse Faults.

Heave Upthrow Throw Downthrow

Hade FIGURE 2.75  The Parts of a Fault.

FIGURE 2.76  A Normal Fault in Sedimentary Rocks in East Yorkshire.

Plate Tectonics: The Earth’s Structure and Landforms   2.51

FIGURE 2.77  San Andreas Fault.

FIGURE 2.78  Faults in Xinjiang (Google Earth, July 30, 2013)

Tear Fault Reverse Fault Horst

Normal Fault

Thrust Fault

Rift Valley

FIGURE 2.79  Some Faults and Associated Features.

2.52  Chapter 2

Major Faults in Great Britain A long time ago, powerful forces in the earth’s crust slowly tore Scotland into two parts along a tear fault (Figure 2.80). The rocks along the fault were crushed and long periods of subsequent erosion turned them into a depression which became occupied by three lochs. Figure 2.80 shows two areas of granite, which, before the fault developed, formed one granitic mass. Their present positions indicate the amount of lateral movement that has taken place since the fault developed. Similar faults developed in northern England where the Pennines were pushed up and tilted to the east. This line of faults is well shown in the northwest part of the Pennines by the steep slopes which overlook the Vale of Eden. The Vale separates the Pennines from the Lake District. This is shown in Figure 2.81.

Granite Mass North-West Highlands

ult

Moray Firth

Gr

ea

tG

len

Fa

Skye

Direction of Rock Movement

Mull

FIGURE 2.80  The Great Glen Fault of Scotland. The Two Granite Masses were Once One Mass.

0

100 km

Vale of Eden

Lake District

Northern Pennines

ine

L ult

Fa

FIGURE 2.81  The Development of Normal Faults Produced the Vale of Eden. the Northern Pennines were Tilted to Form a Block Mountain, E. G. Xinjiang Fault.

Limestone Sandstone

Limestone Sandstone

Plate Tectonics: The Earth’s Structure and Landforms   2.53

Joints Cracks often develop in rocks when they are subjected to strain produced by compression or tension. The strain may be caused by earth movements, by contraction when molten rocks solidify, or by the shrinking of sedimentary rocks on drying. The cracks formed are called joints. In sedimentary rocks, joints are often at right angles to the bedding plane (Figures 2.82 and 2.83). Sometimes, more than one set of joints develops. When this happens, the rock becomes broken into blocks, e.g., limestone and sandstone. Joints can also develop in igneous rocks. Polygonal joints often form in layers of basalt on cooling. These result from the process of contraction. See Figure 2.84.

Folds When tangential (lateral) forces operate as forces of compression, folding usually takes place. Figure 2.85 shows the process of folding. Figure 2.85(a) shows the forces of compression beginning to push towards each other across an area of horizontal

You should remember the difference between a joint and a fault. A joint is a crack in a rock which coincides with a line of weakness. There has been no movement of the rock along the joint. A fault is a break in a rock along which there has been movement of the rock.

FIGURE 2.82  Horizontal Bedding Planes and Vertical Jointing in Devonian Metamorphosed Rocks Near St. Agnes Head in Cornwall.

FIGURE 2.83  Horizontal Bedding at Tato Pani, Nepal.

FIGURE 2.84  The Giant’s Causeway in Northern Ireland. The Polygonal Jointing is Very Conspicuous.

2.54  Chapter 2

sedimentary rocks. In time, the sedimentary rocks will bend or fold as shown in Figure  2.85(b). The layers of rock, which bend up, form an upfold or anticline. Those that bend down form a downfold or syncline. The sides of the fold are called the limbs. The line of the highest points along the anticline is called the crest of the anticline. See Figure 2.85. If compression continues, then a simple fold is first changed into an asymmetrical fold where one limb is steeper than the other (Figure 2.86(b)), then into an ovcrfold (Figure 2.86(c)), and finally into an overthrust fold (Figure 2.86(d)). Folds can often be seen in the rocky cliffs along the coasts of Great Britain. See Figure 2.87.

(a) Horizontally Bedded Sedimentary Rocks

Crest

(b)

Downfold or Syncline

Simple Fold

Force of Compression

Limp

Upfold or Anticline

Force of Compression

FIGURE 2.85  Formation of a Simple Fold.

Lim

p

(b)

(d) Limp

(c)

Lim

Lim

p

p

Lim

p

Lim

(a)

Thrust

p

Line of Fractur e

FIGURE 2.86  The Formation Of (a) A Simple Fold; (b) An Asymmetrical Fold; (c) An Overfold; and (d) A Thrust Fault. This Happens when the Pressure is so Great that a Fracture Occurs in the Fold Causing the Top Part of The Fold to Override the Bottom Part Along a Fault Plane.

Plate Tectonics: The Earth’s Structure and Landforms   2.55

FIGURE 2.87  Folded Strata of the Purbeck Beds Near to Lulworth Cove in Dorset. Notice how Wave Erosion has Attacked Along the Bedding Planes.

Earth Movements Behind Landforms The face of the earth is forever changing. Earth movements within and below the crust produce landforms of great area and often of great height, while the agents of denudation, i.e., rain, frost, river, ice, wind, and wave, constantly work on the landforms modifying their surfaces and sometimes completely changing their appearances. In doing so, the agents of denudation transport vast quantities of sediment (rock debris which they have derived from the land) most of which is eventually deposited in the seas. For example, the Mississippi River carries about 1 tonne of sediment, which is deposited in its mouth forming a delta, in every 1,200 tonnes of water. The removal of rock debris from the land results in its elevation being lowered but as this happens re-adjustment takes place in the earth’s crust, which results in a slow upward movement of the land. This means that there is interaction between the processes of denudation, sedimentation, and uplift. This is shown by Figure 2.88. As denudation wears away the continents, their weight decreases but this does not necessarily mean that they are being lowered. It is thought that a re-adjustment takes place in the crust, which causes the continents to rise.

Major Landforms We have seen that earth movements cause rocks to fault and fold, and that they are the cause of earthquakes and vulcanicity. In general terms, lateral (tangential) movements produce features such as fold mountains while up and down (radial) movements produce features such as block mountains and rift valleys, plateaus, and basins. Major landforms can be put into three main groups. These are mountains, ­plateaus, and plains.

2.56  Chapter 2

Surface Processes Processes Below Surface

Denudation

Deposition Sedimentation

Uplift

Processes Below Crust

Burial and Physical and Chemical Change

FIGURE 2.88  Interaction Between the Processes of Denudation, Sedimentation, and Uplift.

Mountains A mountain is a huge piece landform that rises above the neighbouring land. Generally steeper than a hill, mountains are formed as a result of tectonic forces or volcanism.

Fold mountains In earlier sections, we saw that there have been three large mountain-building periods – the Caledonian, the Hercynian, and the Alpine, which occurred in that order in time. Subsequent plate movements and denudation have together made vast changes to the original mountain masses so that some of the Caledonian mountains are now no more than worn-down stumps of their former selves. The formation of fold mountains Fold mountains consist of masses of folded sedimentary rocks, which may have a thickness of 12,000 m or more. This suggests that in the beginning, vast areas of sediment must have been laid down in fairly horizontal layers, probably in an ancient sea. Movement of the continental plates on either side of such a sea towards each other crumpled and folded the sedimentary deposits. The dragging down of one continental plate, as discussed, earlier in this chapter resulted in the development of a trench or geosyncline in the zone of subduction (Figure 2.89(a)). The accumulation of further sediment from rivers entering the sea in the slowly subsiding geosyncline would account for the vast thicknesses of sedimentary rocks involved in mountain building. Figure 2.89 illustrates how some fold mountains may have developed. It is thought that the Alps developed in this way. When an oceanic plate converges on a continental plate, both a trench and a mountain system develop. This has happened on the western side of South America as shown in Figure 2.90. The Himalayan mountains are thought to have developed as a result of the collision of two continental plates, one bearing the Indian sub-continent and the other Asia, as shown in detail in the sections discussed above.

Plate Tectonics: The Earth’s Structure and Landforms   2.57

(a)

Continent

Continent ion ros

Eros

ion

Fold Mountains Begin to Form

(b)

E

Sea

Mantle (c)

Mantle

Sedimentary Rocks Continental Plate Fold Mountains

Sedimentary Rocks Are Intensely Folded

Mantle

Continental Plate

Sedimentary Rocks are Intensely Folded Volcano

(d)

Batholith

FIGURE 2.89  (a) Initial Stage in the Development of a Fold Mountain. Colliding Plates Produce a Trench or Geosyncline; (b) As the Continental Blocks Approach each other, the Sedimentary Rocks are Folded and Pushed Up; (c) As Compression Continues, Intense Folding Takes Place. Sometimes One Continental Block Rises Up and Over the Other (Not Shown in this Diagram), the Folded Rocks Continue to Rise; (d) the Folded, Uplifted Sedimentary Rocks now form Lofty Mountains, and Volcanic Activity, Often Violent, Takes Place.

Folding in rocks ranges from simple to complex. In simple folding, the anticlines form the mountains and the synclines the valleys. This is shown by Figure 2.91(a). Figure 2.91 also shows how an anticline can be turned into a valley by erosion leaving the neighbouring synclines standing up as mountains. This can be explained by the “weakness” of the rock structure of the anticlines, which were stretched by the process of folding. The rock structure of the synclines is more resistant to erosion because it was compressed during the folding process. Snowdon in Wales is a good example of a synclinal mountain (Figure 2.92). The Pennines and the Weald were once simple fold mountains. (See Figures 2.93 and 2.94.)

2.58  Chapter 2

Andes Trench

Mantle FIGURE 2.90  Convergence of Oceanic and Continental Plates off the Western Coast of South America. The Oceanic Plate Melts as it is Drawn Down into the Mantle Below the Trench. Volcanic Eruptions and Earthquakes are Common in this Region.

FIGURE 2.91  Stages In The Formation Of Synclinal Mountains: (a) Anticlines And Synclines, Simple Folding Such As Those In The Jura Mountains In France; (b) Anticline Opened By Erosion (Neighbouring Anticlines Will Be Opened Up Later By The Same Process); (c) Anticlinal Valley Develops Rapidly As River Excavates Weak Strata; (d) Synclines Stand Up As Mountains.

Ma

Continental Plate

ntl

e

Oceanic plate Sediment

(b) Anticline Syncline

(a)

Flank of Neighbouring Anticline River

Syncline

(c)

Escarpments and Valleys (d) Well Developed

Snowdon Former Folds

FIGURE 2.92  Snowdon is a Synclinal Peak.

Oceanic plate Melt Here

Escarpment

Plate Tectonics: The Earth’s Structure and Landforms   2.59

The fate of fold mountains

Anticline

The forces of denudation (combined action of weathering and e­ rosion – see Chapter 3) attack fold mountains as soon as they begin Yorkshire Coalfield to appear. The building of fold mountains like the building of all other major landforms takes vast periods of time to complete. W E To begin with, earth movements are more powerful than the forces of denudation and the mountains reach great heights, often of several thousand metres, but as earth moveSandstone Sandstone Limestone ments weaken, denudation becomes domiLancashire nant. In time, the mountains are reduced Coalfield to an almost level surface not far above sea FIGURE 2.93  The South Pennines were Once a Simple Fold level. This surface is called a peneplain. Mountain.

Block mountains

Anticline

Faults usually occur in series and their formation is sometimes accompanied by an upward or a downward movement of Limestone blocks of the crust. The upraised blocks (Chalk) are fault-bordered. They are called block mountains. Other blocks are depressed. This process is called block faulting. The uplifted blocks may be tilted when they form tilt blocks, or they may be horizontal when they are called horsts. These two Clay Sandstone types are shown in Figure 2.95. The northSandstone Clay ern Pennines is a tilt block (Figure 2.81) while the Black Forest in Western Germany FIGURE 2.94  The Weald was Once a Simple Fold Mountain. is a horst. One of the best examples of block faulting can be seen in the Great Basin between the Sierra Nevada and Wasatch mountains in the USA (Figure 2.96). There are other types of mountains, e.g., mountains of accumulation (volcanoes) which were discussed earlier and residual mountains.

Residual mountains Prolonged denudation lowers mountains and other landforms by removing the weaker rocks. The more resistant rocks remain as residual mountains. Such mountains occur in many parts of the world. Some residual mountains are formed by rivers actively cutting downward into old plateau surfaces. The Scottish Highlands are residual mountains that have been formed in this way. Another landform that looks like a mountain is known as an inselberg. Inselbergs. Inselbergs are of different shapes, sizes and structure but all have the characteristic feature of steep sides, which are usually concave at the base. Some inselbergs are dome-shaped (Figure 2.97) while others consist of a pile of massive boulders (Figure  2.98). Figure  2.99 shows an inselberg that  is made of massive granite boulders.

2.60  Chapter 2

(a)

Fault Scarp

Sea

Gulf

of

Aden

SE

NW Fault

(b)

Horst

Rift Valley

Horst

Alluvium

Fault

Fault

FIGURE 2.95  (a) Tilt Block in Somalia (East Africa). The Faulted Side Forms a Fault Scarp. Other Tilt Blocks are the Northern Pennines and the Brazilian Plateau (b) a Horst. the Black Forest is a Horst.

Basin of Inland Drainage

Escarpments

Block Mountains

FIGURE 2.96  Block Faulting in the Great Basin of the United States.

Plate Tectonics: The Earth’s Structure and Landforms   2.61

Inselberg

Surrounding Surface

The Rocks of Some Inselbergs are More Resistant Than the Surrounding Rocks FIGURE 2.97  A Dome-Shaped Inselberg.

FIGURE 2.98  Another Type of Inselberg that Often Looks Like a Mound of Gigantic Boulders – the Jos Plateau, in Nigeria.

FIGURE 2.99  Bismarck Rock in Tanzania is a Partially Drowned Inselberg. Hydrolysis Operates below the Surface (refer Chapter 3).

2.62  Chapter 2

Many African inselbergs appear to have been formed under arid or semi-arid conditions by the wearing away of an old, fairly high-level surface (plateau or plain), Scarp Slope by river action. River erosion produces a lower surface, surrounding the higher one, the two being separated by New Surface a steep slope, called a scarp as shown in Figure  2.100. Active erosion and weathering take place along the scarp which is pushed back further, widening the lower, FIGURE 2.100  The Appearance of a Scarp Slope new surface or pediment (Figure  2.101). The low grain the Early Stage of Development of an Inselberg. dient of the pediment prevents downward erosion but it permits lateral (sideways) erosion (Figure 2.101(b)). The area of the pediment increases while that of the old surface decreases as the scarps are pushed back. As the process continues, the old surface lying between two pediments which are extending their area, stands up as an inselberg (Figure 2.101(c)). Original Surface

(a) Original Surface

River Erodes Vertically

(b) Scarp

Pediment

(c) Inselberg Pediment

Pediment

FIGURE 2.101  Stages in the Formation of an Inselberg by River Action; Notice that the Width of the Pediment Increases as the Inselberg Develops.

Plate Tectonics: The Earth’s Structure and Landforms   2.63

Pediments may have formed under humid conditions, possibly by extensive and deep chemical weathering (see Chapter 3), combined with lateral erosion by rivers. A feature, large enough to be called a major landform, is often associated with block mountains. It is called a rift valley.

Rift valley The most impressive rift valley system in the world extends through eastern Africa northwards through the Red Sea. It is known as the Great Rift Valley of Africa and it extends for just over 7,200 km, of which 5,600 km is in Africa (Figures 2.102 and 2.103). From the Red Sea it extends via the Gulf of Aqaba into Jordan. The width of the valley varies from 30 to 100 km and its sides are both steep and high. It is in this valley, at Chesowanja near to Nairobi, that archaeologists are currently unearthing the remnants of what they think may be the site of humankind’s original civilization. The origin of rift valleys. Two theories have attempted to explain the origin of rift valleys. One relies on the forces of tension the other on the forces of compression. Both theories depend on up-warped swells, along the sides of which faults develop. The tension theory (Figure 2.104(a)) suggests that the forces of tension produce faults

5

28 27 26

African plate (Nubian)

Red Sea

29

Arabian plate 25

Afar Triple junction

Gulf of Aden

24

Indian Aust Plate

6

12

East African African plate Gulf (Somali) Indian Ridge

17

Antarctic Plate

FIGURE 2.102  East African Rift Valley.

2.64  Chapter 2

and that the crust between two parallel faults subsides and produces a rift valley. The compression theory (Figure 2.104(b)) suggests that the forces of compression also produce faults and that when these are parallel, the crust on the outside of the faults “rides” up, partially over the crust between the faults, thus producing a rift valley. The Rhine flows through a rift valley between the Vosges and Black Forest in its upper course. In Great Britain, the remnants of a rift valley can be seen in the Central Lowlands of Scotland. This is bounded on the north by the steep face of the Northern Highlands and on the south by the Southern Uplands. FIGURE 2.103  The Great Rift Valley of East Africa. The Deepest Part of the Rift Valley is in the Western Arm. The Floor of Lake Tanganyika is Just Over 600 M Below Sea Level. in Contrast, a Block of Ancient Rock has Been Pushed Up to a Height of 4,870 M Above Sea Level. This Block Forms the Ruwenzori Mountains.

Plateaus and related landforms

Vertical earth movements can cause the crust to warp, and sometimes large areas of it are uplifted whilst others are depressed. The uplifted areas form plateaus, sometimes called tectonic plateaus, and the depressed areas basins as shown in Figure 2.105. There are two types of tectonic plateaus. Some slope down to surrounding lower land, e.g., the Deccan Plateau of India. Other plateaus slope up to surrounding mountains and these are called intermontane plateaus. The Tibetan and Bolivian plateaus are examples. The former lies between the KunlunShan and the Himalayas. The latter lies between folds in the Andes.

(b)

(a)

Fault

Fault Removed by Erosion

FIGURE 2.104  Formation of a Rift Valley by (a) Tension and (b) Compression.

Plate Tectonics: The Earth’s Structure and Landforms   2.65

Lava plateau Lava plateaus are of a special type and they were discussed on page 2.38. Although they result from earth movements, their development is indirect in that there is no upward or downward movement of the crust.

Dissected plateau Vertical erosion is the dominant action of some rivers that cross a plateau. These rivers carve deep valleys, which break up the surface into many steep-sided pieces. A plateau of this type is called a dissected plateau. Figure 2.106 shows how a dissected plateau may develop. If the surface rocks are resistant to erosion, mesas may occur. When a plateau whose surface is capped by horizontal, resistant rocks such as sandstone or limestone is dissected, blocks of the plateau stand up with a distinctive shape. The tops of the blocks are level and they have steeply sloping sides. They are called mesas. Further erosion results in pieces of the mesas becoming separated from the rest. These pieces are known as buttes. Figure 2.107 is a photograph of a mesa and buttes in the Hombori Mountains in Mali, Africa. Mesas are usually well formed in arid climates, and their slopes remain steep because of the almost complete absence of weathering. Mesas sometimes form in humid regions but this only happens when the plateau has a resistant rock capping.

Basin

Plateau

Sediment FIGURE 2.105  A Tectonic Plateau and Basin. (b)

(a)

Butte

Mesa Resistant Rock

FIGURE 2.106  Stages in the Formation of a Dissected Plateau: (a) Vertical Dissection of Plateau Surface Begins; (b) Prolonged Vertical Erosion Leads to Formation of Mesas and Buttes, Separated by Flat-Bottomed Wide Valleys.

2.66  Chapter 2

FIGURE 2.107  Some of the Huge Buttes and Mesas, Which are Characteristic of the Hombori Mountains, in Mali (Africa).

Plains and related landforms A plain is an area of level or gently undulating land usually near to sea level or a few hundred metres above it. Plains are one of the most important landforms because they are the home of the bulk of the world’s population. Most of the fertile soils are located on plains and they are both extensively and intensively cultivated. Plains are basically the product of denudation and because of this, they will be examined in relation to the water cycle.

Depositional plain These plains owe their origin to the deposition by rivers, waves, glaciers, and wind. They are composed of sediment. Some rivers are capable of depositing vast amounts of sediment in their lower courses, which gradually build up gently sloping surfaces called flood plains (refer chapter 4). Sediment deposited in a river’s mouth builds a similar surface, called a delta. There are large deltas in the mouths of the Nile, Mississippi, and Ganges. River plains and deltas consist of sediment that has been sorted by river action. The finest particles are deposited last, usually in the delta. Ice sheets and glaciers often carry tremendous quantities of sediment, pebbles, and boulders all of which are deposited when the ice melts. This material, which is called boulder clay, is unsorted. The East Anglian Plain is a boulder clay plain. Also, rivers flowing from beneath ice masses carry gravel and sand, which they deposit along the edges of the ice. Weathering and wind erosion in desert regions produce fine particles of rock, called loess, which is carried by strong out-blowing winds and deposited in surrounding regions. Loess deposits sometimes cover low-lying regions to form loess plains, e.g., parts of the Pampas in Argentina, and parts of northern China. Sediment is also deposited by waves on some parts of the continental shelf, and if the land becomes uplifted, the sediment-covered shelf forms a gently sloping coastal plain. Plains of this type form large parts of Holland and Belgium, and they also occur along the Florida coast, in the United States.

Plate Tectonics: The Earth’s Structure and Landforms   2.67

Erosional plain Over millions of years, the agents of denudation smooth out and lower the surfaces of highlands, and if they operate uninterrupted for a sufficiently long time, the highlands will be reduced to an undulating low-lying landform called a peneplain. This process is shown in Figure 2.108. The Great Lakes Plain of the Hudson Bay area, in North America, is a peneplain. Peneplains are usually very large, i.e., of continental scale. River plains, though of smaller size, are produced by rivers through the deepening and widening of their valleys. The slow but steady movement of meandering rivers across their valley floors, pushes back the sides of the valleys thus widening the floors. Part of the Amazon Basin is a river-eroded plain. Wind erosion in desert and semi-desert regions often gives rise to very fine rock particles, called desert dust, being blown out of the regions. The removal of desert sediment in this way is known as deflation, and it results in plain-like landforms developing. The stony areas of the Sahara are called reg, and these are examples of wind-eroded plains. All of these features are more fully discussed in Chapter 7.

Emergent coastal plain A fall in the level of the sea, or an uplift of the land sometimes results in the exposure of a part of the continental shelf. Further information on coastal plains is given in Chapter 8.

(a)

(b)

(c)

FIGURE 2.108  Stages In the Development of Peneplain: (a) Original Landscape; (b) Landscape After Long Denudation; (c) Landscape After Long, Uninterrupted Denudation.

2.68  Chapter 2

Key facts ●● ●● ●● ●● ●● ●● ●● ●● ●●

●● ●●

●● ●● ●● ●●

●● ●● ●● ●●

●●

●● ●● ●●

The main geological zones of the earth are core (barysphere), mantle (mesosphere) and crust (lithosphere). The crust consists of plates, which are slowly moving—some are converging; others are diverging. There are continental and oceanic plates. A trench (deep) forms when a continental plate and an oceanic plate move towards each other. Fold mountains and volcanic activity may also occur. A fold mountain range forms when two continental plates approach each other. When two plates diverge on the ocean floor, lava flows take place which produce oceanic ridges. When a continental plate fractures, a rift valley may form. The movement of plates produces vulcanicity and earthquakes. The crust is composed of rocks, and rocks are composed of minerals. There are three basic rock types according to origin: igneous, sedimentary and metamorphic. Any rock can be changed into a metamorphic rock through prolonged heat and pressure. Rocks can also be classified according to age into eras, which are sub-divided into periods. The continents are slowly moving (continental drift)– some are converging; others are diverging. The movement results from the movement of ­tectonic plates. The movement of magma (molten rocks below the surface), into the crust or on to its surface is called vulcanicity. The products of vulcanicity are (a) internal: sills, dykes, batholiths and (b) external: volcanoes, lava plateaus, geysers, and hot springs. Some vent eruptions are accompanied by violent explosions. Vent eruptions produce volcanoes, and these are of three types: (a) ash and cinder; (b) lava; and (c) composite. Very viscous lava sometimes produces a plug, which projects out of the volcano where it solidifies. This type of volcano is called a plug dome. Sometimes the top of a volcano is blown off by a violent explosive eruption, and the crater is turned into a huge depression called a caldera. Calderas sometimes become sites of lakes. Fissure eruptions produce lava plateaus. Some volcanoes are active (i.e., they are liable to erupt at any time), some are dormant (i.e., they have not been known to erupt for a long time), and some are extinct (i.e., they have not erupted in historic times). There are two types of earth movement: radial and tangential. Radial forces cause vertical movements in the earth’s crust. Tangential forces cause horizontal movements. Earthquakes result from a movement of tectonic plates. Earthquakes tend to be concentrated in belts marking the boundaries of plates. Body waves (through the crust) and surface waves (through the surface rocks) radiate outwards from the focus (centre of the earthquake).

Plate Tectonics: The Earth’s Structure and Landforms   2.69

●● ●● ●●

●●

●●

●●

●●

●●

●●

●● ●● ●● ●●

The waves set up vibrations in the crust. Those caused by surface waves produce most of the damage. Earthquakes often cause parts of the earth’s crust to be displaced, vertically or horizontally. Faults are cracks in the earth’s crust along which the rocks have moved, either upwards, downwards, or sidewards. This movement produces three types of fault: normal fault, reverse fault, and tear fault. Joints are cracks developed in the rocks as a result of either earth movements or contraction due to the cooling of molten rock. They usually occur in sets and they divide the rock into blocks. There is no movement of rocks along joints. Folding is caused by either two landmasses moving towards each other or by only one landmass moving towards the other. Any sediment that has collected in the area between the two landmasses (i.e., in the geosyncline) will be folded to form mountains. The types of folds formed differ according to the amount of pressure exerted. They include simple folds, asymmetrical folds, over folds, and over thrust folds. Fold mountains are known as old fold mountains or young fold mountains according to when they were folded. Old fold mountains were folded between 440 and 350 million years ago. They have been worn down by weathering and erosion. Therefore, they do not form very high mountains. Examples are the Appalachians (North America) and the Eastern Highlands (Australia). Young fold mountains were only folded about 25 million years ago. They form the highest mountains, e.g., Himalayas (Indian sub-continent), Rockies (North America), and Andes (South America). Where there is more than one fault, (i.e., block faulting) movement of the earth’s crust along the faults may result in the formation of horsts (e.g., Korea), tilt blocks (e.g., Deccan Plateau), and rift valleys (e.g., East African Rift Valleys). Major physical features are called landforms, and they are the result of internal forces (earthquakes and volcanic eruptions). These features are modified by external forces (denudation and deposition). Vertical movements of large areas of the earth’s crust result in the formation of basins, e.g., Tariam Pendi (Tarim Basin) in central Asia and plateaus, e.g., Xizang Gaoyuan (Tibetan Plateau). Steep sides characterize all inselbergs. Plateaus are of several types, e.g., lava plateaus, tectonic plateaus. Some plateaus are dissected. Mesas and buttes can develop from a plateau made up of horizontal layers of resistant rock when this is dissected. There are several types of plain, e.g., depositional, erosional, emergent.

2.70  Chapter 2

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct answer. 1. Which one of the following statements is incorrect? (a) Igneous rocks are formed when molten rocks from the mantle cool and solidify. (b) Rocks that are changed by great pressure and heat are called metamorphic rocks. (c) Sedimentary rocks are formed from sediment that is deposited in layers either by water or by wind. (d) Basalt and granite are examples of igneous rocks. (e) Sedimentary rocks do not contain fossils. 2. Basement rocks refer to rocks that (a) are never seen at the surface. (b) can sometimes be seen at the base of an escarpment overlooking a rift valley. (c) form a continental platform on which other rocks have formed. (d) have originated in the earth’s mantle. (e) form the core of fold mountains. 3. The process of rock stratification is directly related to (a) the breakdown of sedimentary rocks by erosion. (b) the folding of rocks. (c) the formation of sedimentary rocks. (d) the forces of tension in the earth’s crust. (e) the conversion of igneous rocks into metamorphic rocks. 4. Which of the following, by origin and composition, is not a form of igneous rock? (a) sill (b) gypsum (c) dyke (d) lava plain (e) laccolith 5. Which one of the following best describes the world distribution of active and recently active volcanoes? (a) They are found in association with young fold mountain chains. (b) They occur in river flood plains. (c) They are associated with old eroded mountain chains. (d) They are located on the western sides of continents. (e) They tend to form chains around ocean basins. For each question, one or more of the responses given is/are correct. Decide which of the responses is/ are correct and then choose: (a) if 1 only is correct. (b) if 1 and 2 only are correct. (c) if 1, 2 and 3 are all correct. (d) if 2 and 3 only are correct. (e) if 3 only is correct. 6. Fold mountains develop as a result of 1. forces of compression. 2. submergence of sediment in a geosyncline. 3. movement of two continental plates away from each other.

Plate Tectonics: The Earth’s Structure and Landforms   2.71

7. The formation of a caldera is caused by 1. collapse of a volcanic cone. 2. violent volcanic eruption. 3. outpouring of lava and subsidence. 8. A zone of seduction is associated with 1. the movements of a continental and an oceanic plate. 2. a trench on the ocean floor. 3. a rift valley. 9. The movement of plates away from each other on the ocean floor may give rise to 1. earthquakes. 2. the formation of a rift valley. 3. up-welling of magma. 10. The shape of a volcanic cone is directly related to 1. the nature of the volcanic eruption. 2. erosional forces. 3. the nature of the lava. 11. The theory of plate tectonics can be used to explain 1. the locational pattern of continental landmasses. 2. the present day appearance of the Scottish Highlands. 3. the location and distribution of the different types of rocks. 12. Which of the following combinations are applicable to block mountains? (i). sometimes bordered by fault scarps (ii). level or horizontal (iii). caused by differential erosion (iv). always associated with faulting (a) i, ii and iv only (b) i, iii and iv only (c) i and ii only (d) ii, iii, and iv only (e) i, ii, and iii only 13. Some mountains owe their origin mainly to denudation. Such mountains are called (a) horsts. (b) fold mountains. (d) volcanoes. (c) mesas. (e) residual mountains. 14. Some plains are caused by erosion, some by deposition, and some result from a fall in sea level. Which of the following plains is of erosional origin? (a) coastal plain (b) deltaic plain (d) peneplain (c) loess plain (e) boulder clay 15. Large masses of volcanic rocks often form the “roots” of fold mountains. These volcanic rock masses are called (a) sills. (b) laccoliths. (c) volcanic cones. (d) batholiths. (e) dykes. Directions for questions from 16 to 24: For each question, one or more of the responses given is/are correct. Decide which of the responses is/are correct and then choose: (a) if 1 only is correct. (b) if 1 and 2 only are correct. (c) if 1, 2, and 3 are all correct. (d) if 2 and 3 only are correct. (e) if 3 only is correct. 16. Tensional and compressional forces both operate in the earth’s crust and they can result in the formation of 1. fold mountains. 2. rift valleys. 3. block mountains.

2.72  Chapter 2

17. The phenomenon of earthquakes is associated with 1. body waves. 2. focus. 3. faulting. 18. A fault and a joint can develop in all types of rocks but their origins are different. The main characteristics of both are 1. the displacement of rocks. 2. they form along lines of weakness. 3. they are at right angles to the bedding plane. 19. A block mountain is associated with 1. horizontal rock strata. 2. differential erosion. 3. faulting. 20. A range of fold mountains is usually characterized by 1. having a core of igneous rocks. 2. having a location near to a stable area of old igneous rocks. 3. having great altitude. 21. The formation of a fold mountain is dependent upon 1. a geosyncline. 2. continuous deposition of sediment. 3. a fall in the level of the sea. 22. The relationship between earth movements and faulting is best seen in landforms such as 1. horsts. 2. fold mountains. 3. rift valleys. 23. Crustal warping is caused by vertical earth movements and this can result in the formation of 1. lava plateaus. 2. intermontane plateaus. 3. continental basins. 24. Which of the following are relevant when explaining the factors involved in the formation of young fold mountains? 1. sediment is deposited in a geosyncline; 2. continental plates move apart; 3. subsidence of the floor of the geosyncline. 25. Great Ice-Age is related to (a) Holocene (b) Eocene (c) Oligocene (d) Pleistocene 26. Continents have drifted apart because of (a) tectonics activates (b) folding and faulting of rocks (d) All of the Above (c) volcanic eruptions 27. Which of the following phenomena might have influenced the evolution of organisms? 1. Continental drift 2. Glacial cycles Select the correct answer using codes given below. (a) 1 only       (b)   2 only (c) Neither 1 nor 2      (d)  Both 1 and 2 28. Consider the following statement about earthquake 1. Intensity of earthquake is measured on Mercalli scale. 2. Every integer on Richter scale shows a 100 times increase in energy released. 3. Magnitude of earthquake depends directly upon amplitude of the earthquakes wave. 4. An earthquake’s magnitude is measurement of energy released. Which of the statement are true? (a) 1, 2, and 3 (b) 1, 3, and 4 (c) 2 and 4 (d) 1 and 2

Plate Tectonics: The Earth’s Structure and Landforms   2.73

29. The intensity of earthquakes is measured (a) on the Richter scale (b) (c) on the Kelvin scale (d) 30. Arakan Yoma is the extension of the Himalaya located in (a) Kashmir (b) (c) Myanmar (d)

in Pascal in Decibel Afghanistan Balochistan

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Explain the main differences in appearance and origin between the members of each of the following pairs of features: (a) batholith and lava flow; (b) sill and dyke; (c) crater and caldera; (d) hot spring and geyser. Illustrate your answer with diagrams. 2. Carefully explain the differences between the appearance and the origin of (i) a lava volcano, and (ii) a composite volcano. Draw a large diagram for each type of volcano and on this mark and name the main parts. 3. Locate, by shading on a map of the world, two important volcanic and earthquake regions. Briefly describe one major volcanic eruption and one major earthquake, which have occurred in the last 20 years. Your description should indicate the causes and effects of these natural catastrophes. (a) Carefully explain the differences between (i) igneous and sedimentary rocks; (ii) sedimentary and metamorphic rocks. (b) Carefully explain how a sedimentary rock originates and name two common examples of this rock. 4. Illustrating your answer with diagrams, describe the characteristic mode of formation of any two of the following landforms: ash and cinder cone; plug dome; caldera; geyser. B

A

FIGURE 2.109  Features Caused by Earth Movements

Study Figure 2.109 which shows some of the features caused by earth movements in the crust. Use this figure to answer question 5–7.

2.74  Chapter 2

5. (a)  Name the landforms at A and B making your selection from the following: horst, dyke, rift valley, block mountain, fault scarp. (b) Explain how landform A was formed. 6. Choosing a specific example, describe the effects of an earthquake on the landscape and on the inhabitants of that landscape. 7. (a)  Briefly describe the difference between a fault and a joint in terms of their origin. (b) Name a common sedimentary rock in which joints occur. (c) Name one igneous rock landform in which joints occur. 8. With the aid of well-labelled diagrams, explain the differences in origin and appearance of the following: (a) fault and fold; (b) rift valley and block mountain; (c) basin and plateau. 9. For each of the following features: young fold mountain, rift valley, and block mountain: (a) draw a clear diagram to show its main features; (b) explain its possible origin; (c) name and locate a region where a good example may be found. 10. Write a short account of the formation of a rift valley. Your answer must be illustrated with relevant diagrams and you must name a specific example of the landform. 11. Carefully study Figure 2.110. (a) Briefly describe and account for the pattern of earthquake belts. (b) Earthquakes usually occur in narrow belts. Explain why this happens. (c) Describe the various effects of earthquakes upon human activities. (d) Explain why earthquakes often precede volcanic eruptions. (e) Name one region in southwest Asia where earthquakes have occurred frequently in the last 15 years.

FIGURE 2.110  Map Showing Earthquake Belts

Major earthquake belts

12. Earthquakes and volcanic eruptions are closely associated and sometimes they occur together. (a) Name the instrument by which an earthquake is recorded. (b) Name the scale, which gives the magnitude of an earthquake. (c) Briefly explain what is meant by the intensity of an earthquake. (d) What type of earthquake waves cause most of the damage? (e) Name two changes in the surface rocks that can result from an earthquake.

Plate Tectonics: The Earth’s Structure and Landforms   2.75

13. Volcanic eruptions are usually destructive although in the long term they may be beneficial. (a) Name two aspects of the destructive nature of a volcanic eruption and describe the results of these (i) to the natural landscape, (ii) to the cultural landscape. (b) Explain how a volcanic eruption can affect the weather pattern. (c) Name one human activity that can benefit from a volcanic eruption and name one region where this has happened. (d) Explain what is meant by the statement “A volcanic eruption can help date history.” (e) Name one region in Great Britain which has evidence of past volcanic eruptions, and name the nature of the evidence. 14. Inselbergs are of different shapes, sizes, and structure and they occur in several parts of Africa. (a) Describe two ways by which an inselberg may form. (b) Describe the characteristic features of an inselberg. (c) Draw a diagram of an inselberg to show its characteristic features.

Answer key Exercise 1 1.  (c) 6.   (2) 11.    (2) 16.   (c) 21.   (b) 26.  (a)

2.  (d) 7.   (3) 12.   (a) 17.  (e) 22.   (c) 27.   (d)

3.   (c) 8.  (3) 13.   (e) 18.   (b) 23.  (d) 28.   (b)

4.   (b) 9.   (d) 14.  (d) 19.   (e) 24.   (e) 29.  (a)

5.  (e) 10.   (1) 15.   (d) 20.  (c) 25.   (d) 30.   (c)

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3

Weathering of Slopes

Learning Outcomes After completing this chapter, you will be able to: ● ● ● ●

Understand the nature of agents of erosion and exogenetic forces Describe the concept of denudation and related process Comprehend geomorphic cycles of slope development Explain mass wasting and various slope processes

Keywords Denudation, Weathering, Slope Development, Slope Movements

1

3.2  Chapter 3

Introduction In Chapter 2, we examined the forces at work within the earth and the effects that these have on the nature of the earth’s surface. It was also mentioned that other forces such as rain, rivers, wind and ice are at work on the earth’s surface and that these have a profound effect on the form that surface features take. In this chapter and in the following chapters, we shall examine the nature of these forces, their interaction with the surface rocks, and the products of this interaction. The natural processes affecting breakdown are collectively called weathering, and it is to these processes that we now turn our attention to.

Denudation and Weathering

Denudation term is derived from a Latin word, which means, ‘to lay bare’. This term is used to those external agents which break down the rocks at the earth’s surface.

‘Denudation is the name for the processes of erosion, leaching, stripping and reduc­ ing the mainland due to removal of material from higher to lower areas like valleys, river valleys, lakes and seas with a permanent filling of low lands.’ At the time of their formation, all surface rocks were subjected to either high tem­ peratures or high pressures or both in the absence of atmospheric water and air. Once at the surface, these rocks come under the influence of a moist atmosphere, atmo­ spheric pressure, and a temperature ranging from more than 20°C below freezing point to a few degrees over 50°C. The rocks become adjusted to the surface environ­ ment through the process of weathering, which results in the decomposition and disin­ tegration of the rocks. A layer of broken rock particles called regolith, extending from the surface to the unaltered parent rock beneath, is the end product of weathering. Denudation is a long-term blend of multiple processes like weathering, erosion and mass wasting that cause wearing away of the earth’s surface. This leads to a reduction in elevation of landscapes and landforms. They include both endogenetic denudation processes, e.g., earthquakes, volcanoes, and plate tectonics uplift and exogenetic denu­ dation processes, e.g., erosion, weathering and mass wasting. Therefore, it is important to understand that weathering is only one process of denudation, whereas denudation is a blend of various processes. Weathering is a general term applied to the combined action of all processes that cause rocks to disintegrate physically and decompose chem­ ically and it strongly promotes and assists the denudation processes. Weathering also includes the physical disintegration and chemical decomposition of bedrock. Denudation involves four processes: 1. 2. 3. 4.

Weathering, Mass wasting, Erosion and Transportation.

Weathering is the first process in the evolution of landscape and it provides the raw materials or the input for the agents of erosion and transportation, which are run­ ning water, ice sheets and glaciers, the sea and the wind. It is important to understand that there is a general relationship between the sedi­ ment yield of major denudation regions and the history of weathering as recorded in the major element composition of suspended sediment. Natural and anthropogenic

Weathering of Slopes   3.3

processes cause variability in the denudation processes. Estimates of pre-human ­sediment yields are nearly a factor of two lower than currently measured yields. Weathering also sets the stage for the development of slopes. The agents of ero­ sion and transportation will be examined in Chapter 4.

Types of Weathering Although weathering occurs all over the earth’s surface, it is not always easy to see, especially in new towns. If there are very old stone or brick buildings near to where you live, look at them closely next time you pass by. You will probably see that the surface of the stone or brick is pitted, with bits flaking off. This breakdown of the sur­ face may have been caused by frost action, by rain and wind, by alternate heating and cooling between summer and winter, by the exhaust fumes of cars, by smoke from industrial plants, or by a combination of these. This type of breakdown is the result of weathering. The tombstones in old churchyards are sometimes so weathered that it is not possible to read the names of the deceased.

Table 3.1

Suspended sediment yield of some major rivers of the world denudation areas of the world.

RIVERS AND DENUDATION AREA

DRAINAGE AREA (106/km2)

SEDIMENT DISCHARGE (106 T/YR)

SEDIMENT YIELD (T/km2/YR)

River Amazon

6.15

1,200

195

Congo

3.72

43

12

Mississippi

3.27

210

64

Nile

3.03

0

0

Parana

2.83

79

30

Mackenzie

1.81

42

23

Ganges/Brahmaputra

1.48

1060

716

Huanghe

0.75

1050

1400

Indus

0.97

59

61

Alpine Europe

2.60

36

1.4

Ocean Islands

0.55

297

540

2.5

7175

2870

0.26

442

1,700

Selected Denudation Areas Western Europe

(Excluding New Zealand) New Zealand (south Island)

3.4  Chapter 3

There are three natural processes, which make up weathering: 1. Physical, 2. Chemical and 3. Biotic. In general, physical processes are dominant on bare rock surfaces; chemical processes are dominant where the rock is buried beneath soil, and biotic processes are visible where the roots of plants and many animals force apart joints and cracks in rocks.

Physical Weathering By temperature changes When an arid region has high day and low night temperatures, rock surfaces alter­ nately heat and cool. This causes the outer parts of the rocks to expand during the day and to contract during the night setting up powerful internal stresses in the top few centimetres of the rocks. The heating and cooling process does not penetrate far below the surface, but once cracks appear, the stresses operate to greater depths. The stresses produce fractures, which cause the outer layer of rock to pull away from the layer beneath. In well-jointed rocks, flat or curved plates of rock (sometimes called shells) break away from the main rock body. When curved plates of rock peel off in this way, it is called exfoliation (Figure 3.1). The separated plates fall to the ground and are themselves broken into smaller pieces by the forces of alternate expansion and contraction. Exfoliation results due to the extreme change in temperature as depicted in Figure 3.2. By unloading A large mass of rock formed far below the surface either as a result of mountain building or igneous intrusion, is compressed by the great pressure of overlying rock. As denudation removes this rock, pressure is reduced and the compressed rock slowly expands causing cracks to develop. This process is called unloading. As denu­ dation continues, the rock expands further resulting in large sheets of rock to split away from the main rock body. This process is called sheeting. Eventually, a domelike landform called an exfoliation dome (Figure 3.3) is produced.

FIGURE 3.1  Half Dome Displaying Exfoliation, Yosemite National Park, California.

FIGURE 3.2  Exfoliation Results Due to Extreme Temperature Variation.

Weathering of Slopes   3.5

A Few to Several Hundred m

FIGURE 3.4  Mountain Slopes in the Mountainous Country Known as the Lake District, in Northwest England. Screes, Consisting of Angular Rocks, Litter the Lower Slopes. The Screes Result from Frost Action on the Higher Slopes.

FIGURE 3.3  Mounds of Rock Fragments Formed by Physical Weathering Cover the Lower Slopes of these Exfoliation Domes.

FIGURE 3.5  A Dome-Shaped Volcanic Plug in the Hoggar Mountains of Algeria.

Mechanical weathering takes place at the same time as unloading and sheeting, and it plays an important part in breaking down the sheets of exfoliation domes into mounds of rock fragments. The mounds accumulate on the lower slopes of the domes. These mounds are called talus, or sometimes scree, but the latter term is better used for angular rock fragments produced by frost action (Figure 3.4). The mound of rock fragments at the foot of a dome-shaped volcanic plug in the Hoggar Mountains is shown in Figure 3.5. This is a mound of talus. Eroded volcanic plug in Agathala Peak, Narajo country, North East Arizona and north-east New Mexico. There are examples of exfoliation domes in both hot deserts and humid ­temperate regions. Excellent examples occur in granite masses of Yosemite National Park in the USA. Exfoliation domes also occur in the monsoon regions where temperatures and rainfall are high. At one time, exfoliation domes were thought to be the product of rock breakdown through temperature change. While temperature changes play an important part in rock breakdown, it is the process of unloading through pressure release that is responsible for sheeting which is the characteristic feature of exfolia­ tion domes.

3.6  Chapter 3

A Closer Look  ▼ A large mass of coarsely grained igneous rock can become dome-shaped by granular disintegration. This is the break-up of a granular rock into its separate grains by the absorption of water, which causes various minerals to expand and contract at different rates. Because the rock does not have well-developed joints, exfoliation plates do not form, and such a dome is not a true exfoliation dome. Sugar Loaf Mountain of Rio de Janeiro has been formed by granular disintegration. An eroded volcanic plug is depicted in Figure 3.6. Agathala Peak consists of volcanic breccia cut by dykes of an unusual igneous rock called minette.

By frost action When water freezes, its volume increases. Most rocks contain cracks (Figure  3.7), and some contain joints. When water enters these and freezes, a tremendous pres­ sure is applied to the sides of the cracks. Repeated freezing and thawing (melting) causes the cracks to get wider and deeper. In time, frost action breaks down rocky outcrops into angular blocks, which later break down into smaller fragments. These fragments pile up at the bottom of the slopes to form fan-shaped mounds, called screes. Figure 3.4 shows screes at the foot of a mountain in the Lake district of England. Frost action occurs in both arctic and cool temperate regions, but it is most marked in the latter. This is because repeated freezing and thawing is far more common in these regions than it is in arctic regions where water in the rocks tends to be frozen for many months of the year. Frost action also takes place in some regions in the trop­ ics, which have sufficient altitude, including hot deserts, and it is possible that it plays a part in the process of exfoliation. Some rocks, especially well-jointed rocks, break down into large rectangle-shaped blocks under the action of mechan­ ical weathering caused by alternate heating and cooling, aided perhaps by frost action. When rocks break down in FIGURE 3.6  Eroded Volcanic Plug in the Navajo this manner, it is called block disintegration (Figure 3.8). Country, Northeast Arizona and North East. By alternate wetting and drying All rocks absorb a certain amount of water, but some absorb more than others. The absorption of water by surface rocks causes them to swell. When the rocks dry out (and they do this quickly in tropical regions), the outer surface of the rocks shrinks. Water turns to ice which occupies a larger volume: the cracks are enlarged

Water collects in cracks

Temperature Falls below 0ºC FIGURE 3.7  Freeze– Thaw Action Operates in Rock Cracks.

Weathering of Slopes   3.7

The alternate wetting and drying weakens the rocks and they begin to crack. This type of physical weathering takes place along the coast, especially on coastal rocks, which are alternately wetted and dried with the rise and fall of the tide. Therefore, physical weathering is collectively caused by various factors including temperature—differential expansion and contraction of materials exfoliation and frost action; water—rain, running water, wave action and glacial formation; wind—rapid stormy winds.

Chemical weathering Some rocks decompose when they come into contact with water (H2O), or oxygen (O2) and carbon dioxide (CO2), two of the major gases that make up air. Some minerals in rocks undergo chemical change with water and air, and FIGURE 3.8  Block Disintegration. Joints are Opened when this happens, they may be removed from the rocks, both by Frost Action and by Expansion and which results in the rocks being reduced in size, and thus Contraction. weakened. Chemical reactions take place on the surface of exposed rocks but they tend to be greater below the surface. They can operate to depths of 200 m or more where water is able to enter via pores, joints and cracks. The upstanding granite masses of Dartmoor and Bodmin Moor, known as tors (Figure 3.9) have been formed in this way. The action of physical weathering is to break up the surface rocks, which results in an increase in the surface area, thus making it possible for chemical weathering to be more effective. When water enters the soil, it combines with various acids derived from decom­ posing organic matter in the soil. The amount of rock disintegration that soil water can affect depends on the composition and strength of the soil water; the tempera­ ture; the minerals dissolved in the soil water, and the presence of soil bacteria.

(b) Chemical Weathering Taking Place Along the Joints. Eventually Corestones will be Formed. (a) Well-Jointed Granite

Chemical Weathering Corestones Being Formed

(c) A Granite Tor—the Weathered Material has been Washed Away and the Rock has Mostly Broken up into Boulders (Corestones). Tor

Boulders (Corestones)

FIGURE 3.9  The Formation of a Granite Tor. Unweathered Rounded Lumps of Granite Called Corestones form a Conspicuous Feature of a Tor.

3.8  Chapter 3

(b) (a)

Clint Grike

A Few cm to Several m Joints FIGURE 3.10  (a) The Effects of Weathering on the Surface of Limestone Rock; (b) Limestone Pavement Showing Grikes (Grooves) and Clints (Ridges) Near Malham in Yorkshire.

Chemical weathering consists of five processes: 1. 2. 3. 4. 5.

solution, hydration, hydrolysis, oxidation and carbonation.

Solution Only a few minerals are directly soluble in water, but some, especially calcium car­ bonate, are freely soluble when carbon dioxide is dissolved in water. Rain dissolves both carbon dioxide and oxygen as it falls through the air, so that when it reaches the ground it consists of very weak acid, called carbonic acid. This acid helps to turn many insoluble minerals into minerals that are soluble in water, and which can then be carried away in solution. Sodium, potassium, calcium and magnesium are easily removed from rocks in dissolved state. In the chemical weathering of limestone rocks, solution causes the joints to become widened and deepened, and on the surface, deep grooves called grikes develop. These grikes are separated by flat-topped ridges, called clints (Figure 3.10(a) and 3.10(b)). Soil water in humid tropical regions often dissolves all minerals except the very stable ones such as iron and aluminium hydroxides. Aluminium hydroxides (bauxite) and iron hydroxides (laterite) get left behind in the top layers of the soil through the process of leaching (downward movement of water containing dissolved minerals in the soil). Laterites develop best in regions, which have a definite dry and wet season. Bauxite is the main source of aluminium. Hydration Some minerals absorb water and in doing so, they give rise to new compounds. For example, haematite, an iron oxide, combines with water to give limonite, another iron compound. Therefore, in this process, molecules of water become attached to particular rock material during the chemical reaction, for example

Weathering of Slopes   3.9

2 Fe2O3 + 3H2O → 2Fe2O3 Haematite

Limonite

· 3H2O

Another example is the absorption of water by calcium sulphate to give gypsum. Some hydrated minerals are soluble in water, whereas the minerals from which they are formed are insoluble. Sometimes hydration produces new compounds, which are of greater bulk; this again ­weakens the structure of the rocks. Hydrolysis In this process, hydrogen (from water) combines with cer­ tain metal ions (from minerals) to form different chemical compounds. Hydrolysis is therefore quite different to hydra­ FIGURE 3.11  Sectional View of a Granite Boulder tion, e.g., the hydrolysis of potassium feldspar ­produces Showing Spheroidal Weathering. kaolin. Hydrolysis causes some rocks to decay to as much as 100 m below the surface, especially in warm humid climates. The soluble products of hydrolysis are usually removed by water. Sometimes they may react with insoluble ones too and subsequently form clays. Hydroxides in the presence of CO2 change to carbonates and bicarbonates. In the ionized state, water act as a weak acid on siliceous matter. Look at the following examples depicted by equations. KAlSi3O8 + HOH → HAlSi3O8 + KOH 2HAlSi3O8 + 8HOH → Al2O3 + 3H2O + 6H2SiO3 Hydrolysis may also produce a type of exfoliation called spheroidal weathering (Figure 3.11) in fine-grained rocks such as basalt. Weathering attacks the rocks from all sides and under some climatic conditions, it occurs below ground level. In hot humid climates, granite weathering occurs to depths of 80 m through spheroidal weathering. In time, the granite is turned into a mass of rock particles, which contain unweathered, rounded lumps of granite called corestones (Figure  3.9). Eventually, these become exposed on the surface as the weathered rock mass is removed by erosion. Oxidation This happens when oxygen combines with a mineral. Oxidation takes place actively in rocks, which contain iron, when the oxygen combines with the iron to form iron oxides. The reaction produces oxide, which when dissolved in water weakens the rock and results in weathering.

4FeO

Ferrous oxide

+ O2 → 2Fe2O3

Ferric oxide

Hydrolysis often precedes, and accompanies, oxidation. The new minerals formed by oxidation are often easily attacked by other weathering processes. Iron, manganese oxide, sulphides and aluminium foil are easily oxidized. The structure of a rock in which iron and a silicate are joined, is completely broken down by oxi­ dation of the iron.

3.10  Chapter 3

Carbonation Usually two or more chemical weathering processes take place at the same time. Chemical weathering is most marked in hot, wet regions. In hot, arid climates, the growth of salt crystals, especially in porous rocks near the surface, caused by evaporation draws up water towards the surface and results in cracks in rocks becoming enlarged.

This process involves the combination of carbonate or bicarbonate ions with a min­ eral, which produces a soluble compound that is carried away in solution. For example, Ca(OH) 2

CO2

+ CO2 → CaCO3 + H2O → Ca(HCO3) 2

For example, calcium hydroxide reacts with carbonic acid to give calcium carbon­ ate, which is soluble, and water. Hydrolysis often accompanies carbonation. In this process, CO2 combines with water to produce carbonic acid, which is a weak acid

CO2 + H2O → H2CO3 (carbonic acid) For example, hydrolysis and carbonation break down feldspar into clay, soluble carbonate, and silica. Reduction It takes place in the deep force whose oxygen is absent. Reduction means removal of oxygen from minerals, e.g., 2Fe2 O3 → 4FeO + O2

Biotic Weathering The roots of plants, especially trees, can force apart joints and cracks in rocks as shown in Figure  3.10. In addition, the roots of some plants produce chemicals, which cause weathering of the rocks. This chemical weathering is sometimes more extensive than rock break-up caused by root pressure (Figure 3.12). Burrowing animals such as the earthworm and rabbit also bring about a consid­ erable break-up of surface rocks, while ­microorganisms such as bacteria cause both physical and chemical breakdown of rocks (Figure 3.13). The action of these organisms overlaps with chemical weathering because some organisms exude chemicals during their digestion or as they move about.

Soil

As the Root Grows the Joint is Opened Up

Joints

FIGURE 3.12  The Roots of Plants, Especially Trees, Can Sometimes Help to Open the Cracks and Joints in Rocks.

FIGURE 3.13  Many Animals, Such as These Piddocks With Shells, Bore into Rocks for Protection Either by Scraping Away the Grains or Secreting Acid to Dissolve the Rock.

Weathering of Slopes   3.11

Davis’ Cycle of Erosion Youth

Altitude

Uplift

Maturity Old Age Altitude of Main Valley Floors Altitude of Highest Divide Pediplain

Base Level

Altitude

Time Penck’s Cycle of Erosion Waxing Waning Development Development Altitude of Main Valley Floors Altitude of Highest Divide Endrumpf

Base Level

Time King’s Cycle of Erosion

Altitude

Uplift

Base Level

Youth

Maturity

Old Age

Altitude of Main Valley Floors

Altitude of Highest Divide Pediplain

Pediplain

Time

Geomorphic Cycles of Slope Development There are three Geomorphic models of slope development proposed by W. M. Davis, W. Penck and L. C. King. Their major theories are the Peneplanation or Slope Decline theory of Davis; Pendiplanation or Slope Replacement Theory of Penck, and pedimentation or Parallel Retreat Theory of King. According to Davis, erosion occurs only after upliftment is completed. Figure 3.15 describes the Geographical Cycle of Davis. Structure, time and pro­ cesses together encompass the Trio of Davis. As per Davis, erosion starts when upliftment stops in the youth stage. Table 3.2 illustrates stages of drainage devel­ opment and slope development. However, according to Penck, shape of landforms is the function of ratio of velocity of endogenous movement to intensity of erosion. In other words, surface is the result of competition between endogenetic and exogenetic forces; its shape and intensity depends on the rate of incision and removal of debris. Penck has also described three stages of slope development. First is waxing or accelerating stage (Aufsteigende). Erosion and upliftment go together and initial stages have rapid upliftment. Second is uniform development stage (Gleichformige) followed by the third stage, waning or development Stage (Absteigende). Table  3.3 describes the comparison between Davis and Penck cycles of slope development. Whereas, according to the Parallel Retreat Theory of King, there is a parallel retreat of scarps followed by constraint on down-wearing of scarp surface and uplift happening due to denudational unloading. L. C. King illustrates a slope profile

FIGURE 3.14  Geomorphic Cycles of Slope Development

3.12  Chapter 3

consisting of a waxing slope (Crest) occurring largely as a convex slope where due to the high slope, there is increase in transportation. Bedrock outcrop appears as a free face (Scarp). Next is a debris slope (Constant Slope) that retreats with free face. Third is a waning slope (Pediment) occurring largely on a concave slope with sheet wash. Youth

Height

B A

Maturity

Uplift Stops

Heigh

t of Div

Valle

y Flo Leve or l Base Level

Old Age

ide Su

mmits

Monadn

ocks

Time FIGURE 3.15  Davis’s Stages of Slope Development.



Table 3.2

Stages of cycle of Davis.

CYCLE

YOUTH

OLD

MATURITY

Cycle of Drainage Development

Major rivers

Rivers with floods

Flood plains

Cycle of Slope Development

Convex slope

Vertical downcut

Lateral erosion flatter slope

Table 3.3

Comparison between Davis and Penck Cycles of erosion.

DAVIS DESCRIPTION

PENCK DESCRIPTION

Erosion starts when upliftment stops

Erosion and upliftment go together; initial stages have rapid upliftment

Long crustal stability after initial uplift

Continuous upliftment which stops very late in the cycle

Compared to erosion, upliftment happens in very small period

Span of upliftment is more or less same as that of erosion

Structure, time, and process affect the erosion

Erosion depends on rate of upliftment and degradation

Time dependent

Time independent

Start on many different structural units

Starts on convex lowland known as primarrumpf

Peneplain is the end product

Endrumpf is the end product

Down wasting of slopes

Backwashing of slopes

Definite landform sequence

Landform sequence depends on rate & characteristics of upliftment & erosion

Backward Looking Theory

Forward Looking Theory

Weathering of Slopes   3.13

Mass wasting refers to the movement of regolith on slopes by creeping, flowing, sliding, slumping and falling, affected by the three factors named above. These are natural processes, which usually result in slow mass movement. However, the impact of human activities on slopes through deforestation, the building of settlements, farming and mining has increased the instability of slopes in many parts of the world, often with disastrous results. It is important to remember that in nature, there appears to be a state of balance, which until the dawn of early people was little affected by the flora and fauna. From the time of the growth of the first civilizations, the effects of human impact on the earth’s surface have steadily increased, first through farming and mining, and later through extensive industrial development and urbanization. Forests have been cut down, grasslands ploughed up, hills have been levelled and water-logged hollows filled in. These physical changes made to the earth’s surface have made it possible for a part of the world’s vast population to be fed, clothed and housed to a reasonable standard but the cost of doing so has been enormous. Vast areas of land have become derelict either through bad farming, which has destroyed the soil; or mining, which has cut up the surface into huge open pits or riddled the land with underground tunnels that have made the surface unsafe; or deforestation, especially in humid tropical regions, which has resulted in the frequent rains washing away the soil; and through the build­ ing of vast settlements often covering hundreds of square kilometres which has pre­ vented rain from entering the soil thereby adversely affecting the hydrological cycle. Some of the more important aspects of human impact upon the earth’s surface will be discussed in this chapter and in later chapters.

Rejuvenated and Polycyclic Landforms Rejuvenation extends the period of the cycle of erosion. It means accelera­ tion of erosive power of the fluvial process, i.e., rivers caused by various factors (Figure 3.16). Therefore, in a single landscape, a number of features representing different ages or stages exist, thus indicating a variety of incomplete geographical cycles which were interrupted (called ‘accidents’ by Davis) for various reasons. Some are mostly localized, dynamic reasons involving uplift or subsidence of land resulting in a change in base level. Whereas, there exist static reasons, e.g., a reduction in river load or an increase in volume (due to precipitation or deforestation) that may some­ times alter the rate of erosion. In addition, there may be eustatic reasons implying Knick Point Waterfalls Original Course of River Lyn

Breach in Coastal Geology

‘Valley of the Rocks’

Present - Day Village of Lyn mouth

FIGURE 3.16  An Example of Rejuvenated Landforms Caused by Erosive Forces.

3.14  Chapter 3

a worldwide change in sea level due to diastrophism, glaciation, or climatic reasons, such as aridity, glaciation, etc. Therefore, we can say that a landform is created by a number of geographi­ cal cycles occurring one after the other in order, leaving their distinct marks on the landscape called a polycyclic landform. Older alluvium terraces, rejuvenated landforms, synclinal ridges and anticlinal valleys are instances of polycyclic land­ forms. Scarped erosional surface of different ages, fault line scarp due to differen­ tial erosion, and uplifted peneplains also fall in this category. Polycyclic landforms are also formed under conditions, which do not exist now—largely referred to as Palaeomorphic landforms. These include both relict landforms like drainage systems of north Sahara in Africa and buried landforms like those formed by con­ tinental glaciers in the USA. The principal difference between a rejuvenated valley and a young valley (although they show similarity of features) is that the ‘initial’ surface in the former is an uplifted peneplain and in the latter, it is a former sea floor. With the drainage on uplifted peneplain already established, the streams are merely rejuvenated and they cut a deep ‘V-shaped’ valley into their old shallow courses and when the maturity of the second cycle is reached, the former peneplain is completely consumed, but its influ­ ence is seen generally in the accordant summits of hilltops over the region as a whole.

A Closer Look  ▼ Types of Rejuvenation Generally rejuvenation is categorized into three types. First is dynamic rejuvenation caused by tilting of land area, upliftment in the landmass, and lowering of outlet. Second is eustatic rejuvenation where changes in sea level are observed due to diastrophic events like subsidence of sea floor or rise of coastal land and glaciation. Third is static rejuvenation whereby either decrease in the river load is observed or there is increase in the volume of water and consequent stream discharge due to increased rain­fall, melt-water or due to river capture. Some examples of regional rejuvenation includes Chotanagpur highlands of Jharkhand which has experienced three phases of upliftment in response to three episodes of upliftment of the Himalayas during tertiary period and the Patlands of the Ranchi plateau and Palawan uplands, which were subjected to an upliftment of 305 m resulting in the interruption of fluvial cycle of erosion and rejuvenation of North Koel river and its tributaries. Therefore, topographic expressions of rejuvenation are diverse at places resulting in the formation of mosaic of poly- or multi-cyclic landforms, which include topographic discordance, ­ valley-in-valley or multi-storeyed valleys, uplifted peneplains, incised meanders, paired terraces, nick points, etc. The Damodar valley at Rajroppa in Hazaribagh is a typical example of polycyclic valley or topographic discordance, which is characterized by two-storeyed valleys. The Damodar River developed its broad and flat valley of senile stage before the onset of Tertiary upliftment.

Mass Wasting and Slope Processes The contours of a topographic map represent altitudes and therefore they tell us something about the slope of the land. Changes in the spacing of contours indicate changes in the angle of slope. This is the angle between the slope of the land and the horizontal. A slope may be straight, concave or convex (Figure 3.17).

Weathering of Slopes   3.15

ex Conv

(a)

Straigh

t

Conc a

ve

Soil Creep Causes Fences to Bulge

Soil Moved Slowly Down the Slope

(b) Horizontal

lope and Angle of S e of the L p Slo

FIGURE 3.17  (a) Three Types of Slope; (b) The Angle of Slope.

Soil Accumulates at the Bottom of the Slope

Soil Creep Causes Tree Trunks to Bend in the Direction of the Creep

FIGURE 3.18  Some of the Affects of Soil Creep.

The angle of slope, gravity and water together determine whether the regolith (broken down layers of unconsolidated rock particles) on the slope stays where it is or moves down the slope, i.e., whether it is in a stable or unstable state. All move­ ments on slopes are determined by these three factors and together they result in several slope processes.

Types of Slope Movement Let us understand the different types of slope movement in the following section.

Soil creep There is a continuous but slow downslope movement on all slopes. This move­ ment is called soil creep. Rainwater lubricates soil particles and enables them to slide over each other. Other factors which help soil creep are the heating and cooling of the soil, alternate wetting and drying of the soil, and the trampling of grazing animals (especially noticeable on sloping land in sheep-­rearing regions), and the burrowing of animals in the soil. Soil creep is a slow movement but it can be recognized by fences and trees that lean downslope and by bulging walls behind  which the soil mounds up. See Figure  3.18 to understand the affects of soil creep.

Flow A flow is usually faster than a creep and it generally takes place on slopes whose materi­ als consist of fine particles. Heavy and continuous rains that fall on such slopes can turn the surface materials into a semi-fluid state. Under certain conditions, this material then acts as a liquid and it ‘flows’ downslope as an earthflow. It was this type of flow that struck Aberfan. Aberfan—a tragedy that could have been avoided. The waste material from the coal mines near to Aberfan had been dumped year after year behind the village where it formed an ugly black tip, the size of a small hill. Small slides occurred in the tip in 1944 and 1963; so in 1965, a petition was signed by the townspeople and sent to the Merthyr Borough Council. It took no action. In late October 1966, after

3.16  Chapter 3

a week of heavy rain, the tip became unstable and it began to move. Suddenly it slid and poured over Aberfan’s primary school killing 116 children and 5 teachers (see  Figure  3.19). There followed a great public outcry and the Government and Coal Board were forced to introduce new measures to ensure that all coal and other waste material tips had to be rendered harmless to prevent similar catastrophies from occurring. Among other things, this led to the levelling of tips and covering them with soil so that grass and trees could be planted. Another type of flow is known as a mudflow (Figure 3.20). As the name suggests, it is composed of very fine rock particles. It is more rapid and less viscous than an earth­ flow. It can occur on desert slopes which are not protected by a cover of vegetation, e.g., during torrential storms when more rain falls than the soil can absorb. Such a mudflow soon dries out and it ceases to flow. A mudflow can also develop on the slopes of certain types of volcano when they are erupting. This can happen if heavy rain falls on the volcanic ash covering the slopes. Herculaneum, at the foot of Mount Vesuvius, was devastated by a mudflow at the same time as Pompeii was buried beneath volcanic ash during the eruption of the volcano in 79 CE. Mudflows also occur in tundra regions during the early summer when the frozen soil thaws and turns into a semi-liquid state thus enabling it to slide over the still fro­ zen subsoil. This is sometimes called solifluction. (a)

Coal Waste Tip

Aberfan

tion Direc ow of Fl

Water Issues from the Ground as a Spring Where the Water Table Meets the Surface

Rocks Saturated with Water

Water Table

(b)

FIGURE 3.19  Coal Waste Flow at Aberfan: (a) A Diagram to Show Why the Waste Tip was So Unstable.The Spring Beneath the Tip, the Rain that Soaked into the Tip, and the Steep Slope Resulted in the Flow; (b) Shows the Coal Waste After it Flowed Down the Slope Covering Part of Aberfan.

Weathering of Slopes   3.17

FIGURE 3.20  The Downhill Movement of a Mudflow Often Causes Vertical Cracks to Develop at the Head of the Mudflow. The Movement also Causes The Mudflow to Pile up into Ridge-Like Features at the Foot of the Slope.

Slumping Begins Here

(a)

Landslide Remnants of a Landslide

(b)

Direction of Slide Slide Plane

Cliff

Sea

FIGURE 3.21  Two Types of Landslide, (a) A Landslide of Loose Rocks Down a Steep Cliff Slope. The Blue Arrows Represent Lubrication and the Direction of Movement. (b) Slabs of Rock Slide Down the Slide Plane. This Diagram Illustrates the Meaning of Slide.

Landslide A landslide takes place when large quantities of loosened surface rocks and soil suddenly slide down a steep slope such as a cliff face (Figure 3.21(a)), a valley side, or an embankment. In some landslides, slabs of surface rocks slide down a slope, more or less intact as shown in Figure 3.21(b). Landslides are caused by the lubri­ cating action of water and the pull of gravity. A landslide may either take the form of sliding, or of slumping (Figure 3.22). Loose and wet rocks slump down under the pull of gravity along curved slip planes The undercutting of the base of a steep slope by a river, or by the sea, an earth­ quake, or prolonged heavy rain all help to produce a landslide.

The Tragedy of Yungay Mount Huascaran (7300 m) is in the high Andes in Peru. In May 1970, an earthquake dislodged a mass of rocks and ice from the sides of the mountain and produced a landslide of thousands of tonnes of rock and mud, which swept down the valley leading to Yungay, a remote mountain settlement. (Figure 3.23) The landslide contained boulders as big as houses and as it roared down the valley; the ice in it melted and turned the landslide into a huge ­mudflow. At times, it was estimated to have reached a speed of 150 to 200 km/h. It obliterated many villages and buried Yungay in mud to a depth of 5 m. About 20,000 p ­ eople died.

3.18  Chapter 3

FIGURE 3.22  This Landslide Has Been Caused by Slumping. Slumping of This Type Occurs on Steep Slopes Made of Clay. It Is Especially Common on Cliffs of Clay, Which Are Subject to Wave Attack.

Loose, Wet Rocks Slump Down Under the Pull of Gravity Along Curved Slip Planes

FIGURE 3.23  Landslide at Bahrabise Nepal, Date (17 March, 2018).

Concave Slope Although most landslides have natural causes such as the one that obliterated Yungay, some are triggered by human activities. Quarrying, the removal of vege­ tation and the construction of roads and railways on steep slopes can affect the stability of the slopes. Figure 3.25 shows how heavy rain on the slopes of a railway cutting can trigger a landslide.

Weathering of Slopes   3.19

Hong Kong—flats that should not have been built Hong Kong is a hilly island, which has a rapidly growing population. Many people live in highrise blocks of flats, some of which are built on the steep slopes of the island. Usually, the slopes are carefully surveyed to make sure that they are stable before permission is given for any buildings to be constructed, but sometimes, unknown to the government planners, buildings have been sited on unstable slopes. These are slopes on which soil creep and perhaps landslides may occur. Besides being hilly, Hong Kong receives over 2000 mm of rain a year. In June 1972, about 200 mm of rain fell in one day with similar amounts falling on the following days. The result was disastrous. A landslide developed on one of Hong Kong’s FIGURE 3.24  Removal of Debris at Chautara, slopes and within a few minutes, it had cut Sindhupalchok Nepal (15 March 2018). across two roads and demolished a high-rise block of flats causing more than 50 deaths. This landslide was triggered by the rapid convergence of rainwater in the soil on the upslope of the flats, causing the soil to become saturated and unstable. See Figure 3.26. A similar situation is depicted in higher altitude of Nepal (Figure 3.27) Landslides are always likely in urban areas on steep slopes in regions, which have heavy rainfall. In Hong Kong, attempts have been made to reduce the occurrence of landslides by planting trees on the slopes above the built-up areas and by covering other steep slopes with a kind of plaster. The trees bind the soil with their roots and they also absorb some of the rainwater, thus reducing the amount in the soil. The cement plaster helps to prevent the soil from becoming saturated by allowing the rainwater to run off the surface instead of soaking into it. Landslides in these areas can be prevented if the soil on the exposed slopes is prevented from getting saturated.

Concave Slope

FIGURE 3.25  Landslides Sometimes Take Place on The Sides of Railway and Road Cuttings Caused By Interference of the Topsoil Through Flow. The Excess of Water Increases The Weight of the Material and Increases Lubrication Partly by Loss of Soil Strength and Decreased Friction.

Rockfall When a mass of rock falls from a steep slope it is called a rockfall (Figure  3.28). A  rockfall only takes place after a rock is weakened, i.e., the strength of the rock has been overcome. This can be caused by alternate expansion and contraction

3.20  Chapter 3

caused by temperature variations and by ice action, which excavates joints or cracks in the rock. If a rockfall goes on repeatedly for a long time, the broken rocks collect at the bottom of the slope in a mound (talus).

Heavy Continuous Rain Falls on Steep Unstable Slope Which Has Little Vegetation; Soil Becomes Saturated and Begins to Flow Downslope Forming a Landslide Retaining Wall Flats Road

Retaining Wall is Breached and Weight of Soil and Rocks of Landslide Knocks Down Block of Flats

FIGURE 3.26  Block of flats destroyed by a landslide in Hong Kong: main causes of the landside.

Blocks of Rock Fall From the Cliff Face Broken Rocks Collect Here as Talus

FIGURE 3.27  Landslide Triggered by Rapid Slope Movement at Tatopani Nepal.

FIGURE 3.28  A Rockfall. The Larger Blocks Usually Collect at the Base of the Talus.

Weathering of Slopes   3.21

Other Types of Slope Movement If the soil on a slope is unable to absorb all of the rain that falls on the slope or if the water table is near to the surface, then some of the rain will flow over the surface as shown in Figure 3.29. When this flow forms a continuous layer, it is called a sheet flow or sheet wash, It is best developed in arid or semi-arid regions where there is little or no vegetation covering to bind the soil particles together.

Vegetation Protects the Slopes Raindrops have considerable power, especially those that fall in heavy storms. The splash formed by a raindrop hitting the ground can dislodge rock particles below the size of a grain of sand. Figure 3.30 shows what happens. The dislodged particles move downslope. The action of raindrops combined with surface flow can lead to the formation of gashes in the slopes. Such gashes are called gullies. Surface flow first produces narrow parallel channels called rills, some of which become deepened

Precipitation

d rlan Ove

Run-Off (Stream)

Infiltration

Flow

w roughflo

Th

Water Table

FIGURE 3.29  When Rainfall Exceeds the Amount of Water that can Enter the Soil (Infiltration) then the Excess Forms Overland Flow. The Overland Flow Plus the Through Flow (in the Soil) Together Form the Run-Off (Stream or River). An Extensive Overland Flow is Called a Sheet Flow. Rain Drop

FIGURE 3.30  The Effect of Raindrop Impact on a Slope. There is More Splash Down the Slope Than Up the Slope.

3.22  Chapter 3

and widened to form gullies. Some gullies develop to great sizes. Boulders lying on a slope protect the underlying soil from the action of raindrop splash. In time, pil­ lar-like features, called earth pillars may develop (Figure 3.31). Sheet flow, gullies, and earth pillars usually only develop when slopes have no covering of vegetation or when their vegetative covering has been removed or badly damaged. Rain falling over a forest or a grassland only comes into contact with the soil after the raindrops have trickled down the leaves and stems or trunks of the plants. It then soaks into the soil. Not only do plants protect the soil from the force of the rain but their roots bind together soil particles thereby giving the soil stability (Figure 3.32). Some of the most serious human impacts on slopes have taken place in heavily forested regions in both temperate and tropical regions. Tropical rainforests have suffered enormous damage. The destruction of the forests has threatened the sur­ vival of thousands of species of plants and animals (including insects), has resulted in huge areas of soil being washed away, and has caused the climate of the forestlands

(a)

(b)

(i) Early Stage

(ii) Later Stage

Boulder Protects Underlying Clay from Being Washed Away by Rain

Rain Wash has Removed Large Amounts of Clay Which Were not Protected by Boulders

Rain Wash Removes Soft Clay

Earth Pillar

FIGURE 3.31  (a) The Development of Earth Pillars; (b) Earth Pillars Near Gavusin in Turkey.

FIGURE 3.32  Vegetation Binding the Fragility of the Middle Himalaya During Soil Erosion.

Weathering of Slopes   3.23

to change. A forest regulates the transfer of water from the land to the atmosphere through the processes of transpiration and evaporation. Some idea of the amount of water transfer that a forest achieves can be obtained by noting that a single large tree may transfer as much as 900 L of water every day. With the removal of the forests, less water enters the soil and hence water transfer from the land to the atmosphere is reduced. In consequence, the air becomes drier. Also, many of the animals, birds and insects of the forests unwittingly performed functions, which were of use to peo­ ple; some pollinated flowers thereby enabling some species to go on reproducing; others such as bacteria consumed the remains of dead animals and plants and in the process added nitrogen to the soil.

Key facts ●● ●● ●● ●● ●●

●● ●● ●● ●● ●● ●● ●● ●●

●● ●●

Denudation refers to weathering, mass wasting, erosion and transportation. Mechanical weathering causes rocks to disintegrate; chemical weathering causes them to decompose. Physical weathering (frost action, temperature changes, and alternate drying and wetting) attacks rocks via bedding planes and joints. Exfoliation is effected on the outer layers of rock through large diurnal tem­ perature variations. Granular disintegration is the break-up of a rock grain by grain through the absorption of water causing certain minerals to expand and contract at dif­ ferent rates. Block disintegration is caused by the physical weathering of well-jointed rocks. Chemical weathering breaks down rocks by the processes of solution, hydra­ tion, hydrolysis, oxidation and carbonation. Hydration can result in spheroidal weathering, which is similar in form to exfoliation. Mass wasting refers to the movement of large quantities of weathered mate­ rial down a slope. Mass wasting involves the processes of creeping, sliding, slumping, flowing and falling. Slopes are either straight, concave or convex. Sheet flow refers to the movement of individual particles downslope. Strictly, this is not an aspect of mass movement. Rain action produces gullies and earth pillars when conditions are favour­ able, e.g., gullies form on sloping land free of vegetation when rainfall is fairly heavy; earth pillars form on slopes composed of clay and boulders where there is little or no vegetation. The end product of weathering is called regolith. Water in regolith moves upward by capillary action and downward by infiltration.

3.24  Chapter 3

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct answer. 1. One type of mechanical weathering is known as exfoliation. Exfoliation is most active (a) in regions where frost occurs fairly regularly. (b) in hot humid regions. (c) in arid or semi-arid regions which have a large diurnal temperature range. (d) in limestone areas. (e) at high altitudes. 2. Which of the following statements best explains the meaning of weathering? (a) The freezing of water in cracks in rocks. (b) The alternate heating and cooling of rocks. (c) The removal of minerals from surface rocks by soil water. (d) The excavation of cracks by plants and burrowing animals. (e) The slow break-up of rocks exposed at ground level by physical and chemical forces. 3. Which one of the following statements is incorrect? (a) Hydration and hydrolysis are two forms of chemical weathering. (b) The absorption of water by minerals is the basis of hydration. (c) In the process of hydrolysis, certain minerals combine with hydrogen. (d) In the process of hydration, certain minerals combine with oxygen. (e) Clay is formed by the hydrolysis of feldspar. 4. Iron in a mineral sometimes combines with oxygen to form a new mineral, and this process helps to break down rocks. The process is known as (a) chemical weathering. (b) oxidation. (c) carbonation. (d) solution. (e) spheroidal weathering. 5. Which one of the following combinations of processes makes up mass wasting? (i) soil creep (ii) mudflow (iv) landslide and rockfall (iii) granular disintegration (a) (i) and (ii) only. (b) (i), (ii), and (iii) only. (c) (i), (ii), and (iv) only. (d) (i), (iii), and (iv) only. (e) (ii) and (iv) only. 6. Which of the following is NOT considered as an agent of denudation? (a) Weathering (b) Mass wasting (c) Erosion (d) Folding 7. Which one(s) of the following is/are NOT correct statement(s)? (a) Frost action and pressure release are types of mechanical weathering. (b) A large mass of coarsely grained igneous rock can become dome-shaped by granular disintegration. (c) A slope may be straight, concave or convex. (d) Exfoliation occurs due to combination of mineral with oxygen.

Weathering of Slopes   3.25

8. Which one of the following is not a type of slope movement? (a) Flow (b) Solifluction (c) Transportation (d) Soil creep 9. Borrowing animals such as the rabbit and earthworm play major role in (a) biotic weathering (b) erosional weathering (c) mechanical weathering (d) mass wasting 10. Formation of caves in limestone areas occurs due to (a) hydrolysis (b) carbonation and frost action (c) carbonation (d) crystallization 11. Final stage of the cycle of erosion is termed as ………………. by W.M. Davis. (a) rejuvenation (b) peneplain (d) structural plain (c) riverine 12. ‘Any landforms is a functions of its Structure, Process, and Time,’ the statement is proposed by (a) W. Penck (b) W. M. Davis (d) Woodridge (c) Crickmay 13. The slow downslope movement on all slopes is called ……………. (a) flow (b) Aberfan (c) mudflows (d) soil creep 14. Sheet flow is best developed in (a) karst region (b) dense forest (c) arid or semi-arid region (d) None of these 15. Identify the correct order of the soil erosion from the following: (a) Sheet erosion, Splash erosion, Gully erosion, Rill erosion (b) Rill erosion, Gully erosion, Sheet erosion, Splash erosion (c) Splash erosion, Sheet erosion, Rill erosion, Gully erosion (d) Gully erosion, Rill erosion, Sheet erosion, Splash erosion

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Which of the following features is not a product of weathering? (a) earth pillar (b) fault (c) gully (d) grike (e) talus 2. Write brief notes on any two of the following sets of terms: (a) hydrolysis and hydration; (b) granular disintegration and spheroidal weathering; (c) oxidation and carbonation. 3. State the meaning of mass wasting. Name three features that it can produce. Illustrate with relevant diagrams.

3.26  Chapter 3

4. Briefly explain what could happen to the surfaces of steep slopes in a part of the British highlands: (a) when belts of trees are cut down; (b) after a prolonged period of frosty weather; (c) when a cutting is made for a road. 5. Carefully distinguish between the following: (a) weathering by temperature changes and weathering by frost action; (b) a scree and an exfoliation dome.

Answer key Exercise 1    1. (c)    6.  (d)   11. (b)

2.  (e) 3.  (d) 4.  (b) 5.  (c) 7.  (d) 8.  (c) 9.  (a) 10.  (c) 12.  (b) 13.  (d) 14.  (c) 15.  (c)

4

Water on the Surface

Learning Outcomes After completing this chapter, you will be able to: ● ● ● ●

Understand the concept of hydrological cycle. Explain the drainage basin as a natural system. Discuss water resource availability in different river system. Describe the process of river erosion and its landforms.

Keywords Atmosphere, Drainage, Irrigation, Flooding, River System, River Erosion.

1

4.2  Chapter 4

(a)

SOLAR

Introduction

ENERGY

Condensation

Condensation

Moist air

Precipitation

moves

onsho

re

Evaporation from vegetation Evaporation from soil

Forest Overla nd flow

Infiltration

Precipitation

Evaporation from lakes and rivers

Lake

Overland flow

Evaporation from ocean

Throughflo w to lakes, rivers and oceans

Ocean

(b)

The Water Cycle Water Storage in Ice and Snow Precipitation

Water Storage in the Atmosphere

Condensation

Transpiration

Evaporation Snowmelt Runoff to Streams

Surface Runoff

Ground Water Infiltration Ground Water Discharge Ground Water Storage

Freshwater Storage

Ground Water Infiltration

Water Storage in Oceans

Ground Water Discharge

FIGURE 4.1  The Hydrological Cycle. A Closed System Powered by Solar Energy. The Overland Flow Plus the Throughflow are Together Called the Run-Off.

Some of the rain that falls on the earth’s surface infiltrates the soil (soaks in) moving through the  soil as throughflow; some lies temporarily on the surface as ponds and lakes; some flows over the  surface (overland flow), and some returns to the atmosphere as water vapour through ­evapotranspiration. The extent of each of these is dependent on the nature of the soil, the s­ub-soil, and the parent rock, the steepness of slopes and the nature of the climate. Water also passes into the air from the oceans through evaporation where it is carried by winds to the land and where it is deposited as rain or snow when conditions are favourable. Once on the land, some of the water returns to the oceans via streams and rivers. This movement of water is called the hydrological cycle (Figure 4.1). It is a closed system in that there is a definite amount of water in the atmosphere and lithosphere and that there is neither loss nor gain from this total amount. The system, like many other systems is powered by solar energy. The surface of the earth is where the solids (the lithosphere), the gases (the atmosphere), and the liquids (the hydrosphere) are in perpetual interaction. The most important of these is water. It occurs in the air, on the surface, and in the rocks (Figure 4.2). Long-wave g Short-wave hort-w wa ave Radiation Radiati adiatio o on Radiation Atmosphere Atmo At mosp sphe here ere re re

Sun

(b)

(a)

Input – Solar Energy

Atmosphere

Hydrosphere

Biosphere Biospher ere e

Lithosphere

Geosphere

FIGURE 4.2  Interactions of Atmosphere, Hydrosphere, and Lithosphere.

Hydrosphere Hydr H y rosph herre he re

Water on the Surface   4.3

The input of this system is solar energy, which provides the power to sustain the constant interaction.

40 000 km3

Evaporation

Evaporation

100 000 km3 Precipitation 330 000 km3

290 000 km3 40 000 km3

Evaporation

Precipitation

Run-off

Sea

(b)

60 000 km3

Land

Atmosphere 13 000 km3

Ice Caps And Glaciers 29.2 Million km3 Surface Water and Ground Water 8.83 Million km3 Oceans 1322 Million km3

FIGURE 4.3  (a) Water Evaporated from the Oceans is Balanced by Water Added Through Precipitation and Run-Off; (b) Where the World’s Water is Stored and its Various Types of Transfers.

Evapo-transpiration

There is about 1360 million km3 of water in the world of which 1322 million km3 is in the oceans. Of the remaining 4 million km3, 76.8 per cent is locked up in the ice caps and glaciers, 21.6 per cent is in the ground water (water below the surface), 0.56 per cent is in surface water (streams, rivers, and lakes), and only 0.04 per cent is in the atmosphere in the form of water vapour. The amount of this water remains fairly constant, but there is a steady movement of water from the oceans to the land through evaporation and precipitation. At the same time, there is a movement of water from the land to the atmosphere through evapotranspiration (evaporation and transpiration). The transfer of water from one system to another and back again is called the hydrological cycle. The amounts of water involved on an annual basis are shown in Figure 4.3(a) and (b). It is important to note that of all the water in the world, only 0.001 per cent is in the atmosphere and that only 0.016 per cent is in the surface water. It must also be noted that the surface water is mainly responsible for the landforms that we see around us. Water exists in three states—vapour, liquid, and solid but continual interchange takes place between states. Water is also transported in several ways—by precipitation, infiltration, runoff, and convection. The slow breakdown of the earth’s surface does not begin with the development of a stream or a river system. The influence  of water, often in conjunction with some other agent such as temperature is effective throughout a whole drainage basin and not just upstream from the source of a river. Figure 4.4(a) is a contour map of an undulating surface which has surface drainage; Figure 4.4(b) represent a Contour Map of undulating surface with flow of drainage extracted from Advance Space borne Thermal Emission Global Digital Elevation Model (ASTER GDEM). Rain falling at points X and Y will follow downslope routes as shown. Other similar flows are shown from points A and B. The dotted line, which

(a)

Precipitation (Rain and Snow)

Global Water and the Atmosphere

4.4  Chapter 4

passes between points A and B, and between points X and Y ­separates the drainage of one area from that of a neighbouring area. This line is called a watershed and it follows the crests of the hills, ridges, and spurs. The area inside the watershed is called a drainage basin (marked D in Figure 4.4(a)). All the rain that falls over a drainage basin feeds the stream or river system of that basin. Each stream and river occupies a valley, the formation of which will be discussed later in this chapter. At the bottom of each valley, the stream or river flows in a trough-like depression called a channel. If a constant flow of water is supplied to a channel by the valley slopes, then the flow of water in the channel becomes permanent, but if the slope feed-ins occur only at times of heavy rain, then the channel flow is temporary. Streams and rivers get their water from the slopes above them. They do not begin to flow at the watershed. Water more than anything else sculptures the landscape. Other agents such as temperature changes, wind, waves, and ice are also at work, but their influences are much more localized. The activity of water can be regarded as an endless chain of inter-relationships.

(a)

(b)

20

0

200

240

240

0 24 0 26

A

260

260

28

0

260

X

220

Y B

240

240

D 260

Watershed

180

200 200

240 220

Streams

FIGURE 4.4  (a) The Shaded Area D is a Drainage Basin; (b) Contour Map of Undulating Surface with Flow of Drainage Extracted from ASTER – GDEM.

Water on the Surface   4.5

River Basin Drainage—An Open System

Precipitation (Rainfall and Snowfall)

Input

The action of water in producing landforms on the earth’s surface begins with the impact of rain hitting the surface. Once on the surface, some of the rain flows over it by following the steepest slopes available, i.e., flowing water crosses the contours at right angles. If the rain falls Infiltration Throughput on an undulating surface, there will be many individual water flows Throughflow each of which will join a stream at the bottom of the slope. The rain that falls over a drainage basin forms the input to the system. Infiltration and throughflow (the downslope flow of water in the soil) make up the throughput. Evaporation and transpiration (called evapotranspiration) and run-off make up the output. This is Evaporation shown in Figures 4.5. As we shall see later in this chapter, all rivers Output Transpiration have another input, the sediment, derived from river ­erosion and Run-off mass wasting of slopes. Rivers that flow through farming and urban FIGURE 4.5  River Basin is an Open landscapes have a third input, namely effluents. Figure 4.6 visualizes Mandakini watershed with many individ- System Because the Input is Later Lost ual water flows on an undulating surface in the Alaknanda Basin of Through Various Processes. Himalayas (extracted from ASTER GDEM). Sometimes, a region’s parent rock is limestone, Mandakini Watershed and if this lies near or at the surface, most of the water will infiltrate the ground to become ground water. Because of this, limestone regions have few surface streams. Rainwater that infiltrates the soil is called soil water and some of this passes through the soil into the sub-soil and the underlying rocks if conditions are favourable. Infiltration takes place in two ways: 1. via the spaces, called pore spaces, separating the individual grains of rock; 2. via joints and faults in a rock. A rock which has pore spaces into which water can infiltrate is called a porous rock, e.g., sandstone; one which has joints or faults into which water can infiltrate is called a pervious rock, e.g., granite. Some rocks allow water to flow through them; such rocks are said to be permeable, e.g., sandstone. Some rocks do not allow water to flow through them; such rocks are said to be impermeable, e.g., clay. Some rocks are both porous and permeable, e.g., sandstone, and some rocks are porous but impermeable, Relief and Drainage e.g.,  clay. When the pore spaces of a rock are filled High : 6251 with water, the rock is said to be saturated. Figure 4.7 shows how water enters a porous rock and a pervious Low : 622 River Stream rock. Water that passes through the soil and enters the underlying rocks is called groundwater. FIGURE 4.6  Mandakini Watershed, Himalayas (Extracted from ASTER GDEM).

4.6  Chapter 4

(a) Zone of Non-Saturation

Zone of Intermittent Saturation

(b)

Zone of Permanent Saturation River

Dry Season Water Table

Rain Falls on Ground Rock Particles Pore Space Water Table

Wet Season Water Table

(c)

Rain Falls on Ground

Saturated Rock

(d) Either Joint or Crack

Rock Particles Pore Spaces

Rock Water Table Joints/Cracks Filled with Water. This Part of the Rock is Saturated

(e) Rock Particles

Pore Spaces

Water Table

FIGURE 4.7  Water may Enter a Rock Via Pore Spaces, Joints, and Cracks: (a) Water Zones Below the Surface; (b) a Porous Rock; (c) a Pervious Rock; (d) a Porous and Impermeable Rock; (e) a Porous and Permeable Rock. In (d) the Rock Contains a Large Amount of Pore Space But the Connections Between the Pores are Small. In (e) the Soil Particles are of Irregular Shape and there is a Large Amount of Pore Space, Some of Which is in the Form of Long Passages, Which Makes the Soil Very Permeable. In (a) the Level of the Water Table Rises After Heavy Rain, and it Falls During Periods of Drought.

Water that enters surface rocks moves downward until it reaches a layer of impermeable rock when further downward movement ceases. Figure 4.7(a) shows the three water zones below the surface: 1. T  he zone of permanent saturation where the pore spaces in a rock are always filled with water. The upper surface of this layer is called the water-table. 2. The zone of intermittent saturation where the pore spaces in a rock contain water only after heavy rain. 3. The zone of non-saturation where the pore spaces may contain some water yet they are never saturated—they just allow the water to pass through them.

Water on the Surface   4.7

Some of the rain falling on vegetation evaporates back into the air before it can reach the ground. Some of the rain reaching the ground also evaporates. Evaporation also takes place from rocks, lakes and streams. As we saw in Figure 4.5, evaporation is a part of the output of a river basin system. Another aspect of this output is transpiration. This is the movement of water through a plant system from the soil to the leaves from which it is evaporated. Of the remaining rain that falls over a drainage basin, that which enters the streams and rivers and leaves the basin as stream flow is called the run-off. The relationship between precipitation and evapotranspiration and run-off is called the water balance.

Dynamics of Water Supply Most people in the developed world have easy accessibility to an abundant supply of water. They just turn the tap to get the water flowing. Most people know there is a connection between a water tap in a house and a reservoir somewhere outside the town, and that between the two there is a water plant, which purifies the water. But how many people are aware of the factors, which decide how much water will collect in the reservoirs, and of the need to balance the demand for water to the availability of water?

Severe Drought in Europe (1976) In 1976, a severe drought affected most of Europe, which resulted in a water famine. So severe was the drought that industry and households were rationed for water in an attempt to conserve water supplies. The low water supply of 1976 at last convinced government bodies in Great Britain that the country could no longer rely on having all the water it needed with its existing systems of collecting and storing water. Unusually low rainfall and unusually high summer temperatures were the causes of the 1976 water famine but as we shall see later, other factors operate to reduce water availability.

Supply and demand Water is required for all sorts of uses. It is needed in the home for drinking, for washing clothes and household utensils, and for use in the bathroom. It is needed in the industry for a variety of purposes. It is needed for cooling machinery for irrigating farmland. Water also has other uses: It is used for generating electricity, for the movement of goods, and for recreation. It is important that all of these demands be met but this can only be achieved by careful planning, which involves effective measures to collect and store sufficient water and to distribute it as and when it is required. At the same time, measures have to be taken to ensure that when a river has too much water, it does not overflow its banks and flood settlements and farmland. It is very difficult to produce plans, which solve water shortages and which prevent rivers from flooding. The demand for water is not constant. It fluctuates from region to region and from season to season. For example, holiday resorts require more water in the summer to meet the additional demands from the tourists. And rainfall is not constant. It fluctuates throughout the year. In 1952, disastrous floods hit the Devonshire village of Lynmouth caused by an estimated fall of 250 mm of rain in the River Lyn basin (area 100 km2) in a period of 24 hours. This produced a river discharge of over 500 m3/s (the volume of water in a river passing a given point in a given time period). The discharge of the River Thames reaches this level only once in 50 years. It is to be noted that the ‘Thames demonstrates a relatively stable relationship bet­ ween rainfall and run-off over the last 120 years, with variations in rainfall, particularly

The discharge of a river is rarely uniform throughout the year. Discharge increases when rainfall increases but there is always a time lag before this happens.

4.8  Chapter 4

variation between the four seasons, providing the dominant cause of variation in statistics of flow’. To add to it there are also shifts in stream channels of every river. This is illustrated in Figure 4.8 for Mandakini River using Landsat series data.

Mandakini River Main Channel

Human Impact on Drainage Basins To balance fluctuations in rainfall and fluctuations in demand for water, it is necessary to store water so that it can be released when it is needed, and also to prevent flooding.

Water storage

Legend Channel area (2016) Channel area (1980)

FIGURE 4.8  Shift in the Stream Channel Area Between 1980 and 2016 in Mandakini River Based on Landsat Series Data.

(a) Main River of Catchment Area Feeding Reservoir Reservoir

Artificial lakes called reservoirs are created by constructing dams (strong concrete walls) across a river valley where the flow of water is fairly uniform and of sufficient volume. Dams are usually constructed in highland areas because these are the areas, which ­frequently receive plenty of rain. When a river is dammed, the water that collects upriver of the dam forms the reservoir, the area and depth of which depend upon the configuration (shape) of the valley and the height of the dam. The creation of a reservoir always results in a large piece of land being inundated (‘drowned’) and before any work on the construction of the dam begins, very careful consideration has to be given to the effects that the ‘drowning’ of the valley will have on settlements and the communication pattern between settlements, and the uses of the land that will be inundated, A further consideration has to be made in respect to the safety of settlements and communications down-valley of the dam. Figure 4.9(a) and (b) is a diagram of a dam and reservoir.

(b)

Dam

FIGURE 4.9  (a) Diagram, to Show the Location of a Dam and Reservoir. The Dam is Usually Built Around a Narrow Part of a River Valley Near to the Place Where the River Flows from the Highland to the Lowland; (b) Satellite Imagery to Show the Location of Tehri Dam and Reservoir, Uttarakhand, 2018.

Water on the Surface   4.9

Great Britain is divided into a number of areas each of which has a water authority whose function is to maintain a regular supply of good water to houses, hotels, and factories. The authorities are faced with many problems on each of which a decision has to be taken. A major problem faces all the authorities in Great Britain—how to maintain a steady flow of water throughout the summer when demand is higher than it is in the winter but when the natural flow of water in the country is lower than it is in the winter. The volume of water in a river passing a given point in a given period of time is called the river’s discharge. The ­discharge of most British rivers is at its highest during the months of December to March inclusive and the discharge is nearly always in excess of demand. The creation of suitably placed reservoirs helps to store some of the excess discharge but by no means all of it. This is because most of Great Britain’s reservoirs are sited in the wetter areas of the highlands in the western part of the country but several of the larger rivers drain eastwards which means that the surface flow in the basins of these valleys on the down-valley side of the dams never enters the reservoirs. The answer might seem to lie in constructing the dams farther down the valleys but a glance at maps of Great Britain, which show the locations of settlements, communications, industrial land, and agricultural land soon shows that such a proposition is unacceptable. If a dam were constructed, for example near to Windsor across the River Thames, it would have to be a very long, low dam to reach from one side of the valley to the other, and it would result in the inundation of a vast land area containing several large and numerous small settlements, hundreds of kilometres of roads and railways, whole industrial estates and much more. Clearly, such a proposition would not be worth considering. Reservoirs have been built in almost every country. Some have been well sited; others have been inadequately sited and constructed with the result that the dams of some of these have cracked and broken, either because of the pressure of water in the reservoir during unusually wet periods or because of earthquakes. The sudden release of an immense volume of water can have disastrous effects on any settlements and farmland located in the valley below the dam. In addition to this potential danger, all reservoirs suffer from sedimentation, i.e., the deposition of sediments by the inflow of water (Figure 4.10). If the catchment area of a reservoir (that part of the river basin whose drainage flows into the reservoir) is not completely covered with vegetation and is located in a region, which has heavy but seasonal rainfall, then only a small part of the rain will soak into the soil with the majority flowing into the reservoir, taking with it large amounts of soil. Because of this, insufficient water soaks into the ground to keep the streams flowing during the dry season, and also an unusually large amount of sediment will enter the reservoir. A continuous covering of vegetation (especially trees) in the catchment area in regions of heavy rainfall is essential.

Almost every ­proposal to create reservoirs in Great Britain has met with opposition. The ­inhabitants of a village will protest sometimes by petition to the central government if their village is threatened with inundation; farmers’ unions will also protest if valuable farmland is threatened; societies for the preservation of the countryside will ­protest as will the Ministry of Transport if roads have to be re-routed around the proposed site of the reservoir. All these protests have to be ­carefully examined before a decision is taken.

Reservoir

Multi-purpose reservoirs Some reservoirs are multi-purpose, i.e., their water is used for generating electricity and for irrigation as well as for supply to towns. The water of the Colorado River in the United States is collected in a series of reservoirs, which have been constructed in the valley as shown in Figure 4.11. So much water is used up from the river that it no longer reaches the Gulf of California into which it once drained. Figure 4.12 shows the link between Mahanadi (Manibhadra) and Godavari (Dowlaiswaram) for water supply in states of Orissa and Andhra Pradesh including cities Cuttack, Bhubaneswar, Vishakhapatnam, and Rajahmundry, India.

Dam

Deposited Sediment

Inflowing Water Carrying Sediment

FIGURE 4.10  All Water Flowing into a Reservoir Carries Sediment, Which is Deposited on the Reservoir Bed. Sedimentation Increases During Heavy Rain and When the Catchment Area has Little Vegetation.

4.10  Chapter 4

A Closer Look  ▼ Water Resources Scenario in India In India, rivers are the major source of water. Here, utilizable annual surface water in rivers of the country is 690 km3. Groundwater is the other major source of water, which is recharged from the precipitation mostly in the m ­ onsoon season. The annual potential of natural groundwater recharge from rainfall is about 342.43 km3, which is 8.56 per cent of total annual rainfall of the country. Irrigation systems including canal irrigation also ­contribute to the recharging in the groundwater. The annual potential groundwater recharge augmentation from canal irritation system is about 89.46 km3. Water is very scarce and polluted in most of the Indian cities. Further, water supply is insufficient, low, or erratic. Rural areas too are facing low quality of water. (Table 4.1) shows average water flow and utilizable water in top 15 river basins in India. A large part of India is facing water scarcity mainly during the summer season, which includes, Rajasthan, part of Madhya Pradesh, Maharashtra, and Gujarat and the southern states. In the meantime, its large part is inundated during the monsoon season. It means that drought and flood prevail together in India. Mahanadi (Manibhadra) - Godavari (Dowlaiswaram) Link

R.

N Cuttack

E

an

ad

i

W

M

ah

S Bhubaneshwar

Manibhadra Dam

Legend

Colorado River

State Boundary Basin Boundary District H.Q River Reservior/Dam Link Canal / Tunnel Link Command Project Major Canal Command Area

ORISSA Berhampur

Rushikulya

Lake Powell Glen Canyon Dam

Lake Mead Hoover Dam

I

N

D

I

Xxxxx Xxxxx

Xxxxx

A Va m

sa

vali

dh

Naga

ra

Bay of Bengal

Grand Canyon Gorge Andhra Pradesh

Vishakhapatnam

N

Parker Dam

@ Government of India, Copyright 2004.

Gulf of California

0

200 km

FIGURE 4.11  The Colorado River is Used for Irrigation and for Supplying Water to Cities.

The external boundaries and coastlines of India agree with the Record/Master Copy certified by Survey of India.

iR av

ar

Rajahmundry

od

The territorial waters of India extend into the sea to a distance of tewelve nautical miles measured from the appropriate base line.

G

The Responsibility for the corrections of Internal details rests with the publisher.

.

Based upon Survey of India map with permission of the Surveyor General of India.

Dowlaiswaram Barrage

MINISTRY OF WATER RESOURCES NATIONAL WATER DEVELOPMENT AGENCY

FIGURE 4.12  The Link between Mahanadi (Manibhadra) and Godavari (Dowlaiswaram) for Supplying Water to Cities.

Water on the Surface   4.11



Table 4.1

Average water flow and utilizable water in top 15 river basins in India

RIVER BASINS Ganga– Brahmaputra– Meghna basin

AVERAGE ANNUAL WATER FLOW (IN km3/ YEAR

UTILIZABLE FLOW (IN km3/YEAR

% OF TOTAL AVERAGE ANNUAL WATER FLOW IN INDIA

% OF TOTAL UTILIZABLE WATER FLOW IN INDIA

1,202

274

61.6

40

West flowing rivers south of Tapi

201

36

10.3

5.2

Godavari

111

76

5.7

11

Indus

73

46

3.8

6.7

Krishna

70

58

3.6

8.4

Mahanadi

67

50

3.4

7.2

Narmada

46

35

2.3

5.0

Brahmni–Baitarani

28

18

1.5

2.7

East-flowing rivers between Mahanadi and Godavari

17

Un-assessed

0.9

Un-assessed

West-flowing rivers of Kachchh and Saurashtra including Luni

15

15

0.8

2.2

Tapi

15

15

0.8

2.1

Subarnarekha

12

6.8

0.6

1.0

Mahi

11

3.1

0.6

0.4

East-flowing rivers between and Cauvery

10

17

0.5

2.4

Rivers draining into Bangladesh

8.6

NA

0.4

NA

1887

649.42

96.62

94.12

Total

Total average annual water flow in all river basins (in km3/year): 1953 Total utilization water flow in all river basins (in km3/year): 690

4.12

Chapter 4

Irrigation Most of the early civilizations of the Mediterranean region, India, China, and other regions which developed on river lowlands, were based on an agricultural economy, which was intimately related to irrigation. Elaborate systems were used to get the water from the rivers onto the land bordering them so that as much land as possible could be watered. So efficient were the irrigation works that enough food was grown to allow a part of the population to devote their time to the development of the arts and science. Beautiful cities were built, intricate works of art were made by the craftsmen whilst (a) Irrigation Canal thought and discussion in the field of Irrigation Channel science produced the principles on Sluice Gate or which rests much of our basic science Row of Crops Mud Wall today. Of all the uses to which water is put today, irrigation uses more water than any one of them. Vast areas of irrigated land form the basis of much of India’s and China’s agriculture whilst in Egypt, Iran, and Pakistan, agriculture is dependent on irrigation. In most irrigation systems, water is allowed to flow along shallow furrows, which separate the rows of crops. The (b) furrows are connected to irrigation canals and the flow of water into the furrows is controlled by opening or Irrigated Market Garden closing sluice gates. Sometimes simple mud walls are used to close the flow of water. When the flow is to be resumed, the walls are broken down. This is shown in Figure 4.13. Irrigation is used in dry regions which have insufficient rainfall for agriculture but to which water can be taken, and in regions which have annual rainfall which is either too low for continuous cropping or too variable from year to year to allow proper crop cultivation. Water has to be colMain Channel lected and stored before irrigation can be practised. Irrigation is not confined to regions with low rainfall. A large part of the world’s rice is grown on irrigated land in regions where the annual FIGURE 4.13 (a) Layout of an Irrigation System Commonly Used rainfall rarely falls below 1524 mm (in Throughout Asia and Africa; (b) Drip Irrigation System with Crops Great Britain, the average annual rainfall is about 1000 mm). Arranged in Row.

Water on the Surface   4.13

The River Nile Schemes

ea

dS

Re

These schemes supply water to vast areas of cultivated land in the lower valley of Irrigated the Nile all the year round. This is called Areas Cairo perennial irrigation. Dams The Nile rises in the highlands of central Africa, which receive heavy but seaAswan sonal rainfall. Before the dams were built Scheme across the Nile, the river flooded every summer in the lower reaches where the floodwaters spread over the floodplain ­ Aswan depositing large quantities of fertile ­ silt. The ­floodwaters renewed soil fertility every year and they also watered the soil. But during the winter, the river’s discharge was too low to cause flooding and so various methods had to be used to get the river’s water to the fields. With the Gezira construction of several dams, large reserKhartoum Scheme voirs were created whose waters could be Jebel used throughout the year for irrigation Aulia (Figure 4.14). However, the Nile irrigaSennar tion scheme like other similar schemes ra has its problems. Lake Tana All irrigation schemes represent interN il e ference with the water cycle. Much of the sediment, which the Nile used to deposit on its floodplain and in its mouth, where it formed a delta, is now deposited 0 400 km behind the dams. The reservoir behind the Aswan High Dam is slowly filling with sediment. Also, the enormous decrease in the quantity of sediment deposited Lake Turkana in the river’s mouth has resulted in the Lake Albert Mediterranean flooding over 4,00,000 ha of fertile delta land. Before the construcOwen Falls Lake tion of the dams, sandbars were built up Edward in the river’s mouth, which helped to proLake Victoria tect  the low-lying delta land from inundation by the  sea. Further, the farmers FIGURE 4.14  The Aswan and Gezira Schemes. of lower Egypt no longer get their fields enriched by the river-deposited sediment. Instead, they have to buy expensive artificial fertilizers, which as we shall have their hazards. Another problem that arises with many of the large irrigation schemes involves the deposition of salts in the surface soil. When the land is exposed to large volumes of water, the water table rises. This means that water moves towards the surface. The water contains ­dissolved salts which when deposited by evaporation at the surface sometimes cause a crust of salts to form. Such a crust prevents plants from ­growing. Riv er

Ata

ba

Blue

e Nile hit W

4.14  Chapter 4

Figure 4.15 shows how extensive irrigation may pollute the soil. Figure 4.16 shows the irrigation system in Madhya Ganga Canal, India. Extensive irrigation schemes have been developed in many other countries. Figure 4.17 shows Canal irrigation schemes in India. Irrigation Water

Evaporation Salts Deposited by Rising Water in Soil

Soil Infiltration Throughflow

Water Table

Zone of Upward Water Movement

FIGURE 4.15  Irrigation in Regions Where Evaporation Exceeds Leaching, if Only for a Part of the Year, Usually Results in Salt Deposits Occurring in the Surface of Soils.

INDEX MAP OF MADHYA GANGA CANAL Ganga

N

hi

Bud

Narora Barrage

Lover Ganga Canal

Kali Nadi

r ive

aR

anal nga C

ya Ga

Madh

Anupshahr

Up

rG

pe

MR BR FE E

Kali Nadi

Parall Nat Br

rC

MEERUT

a

ee de

ng

Can a

on R Hind

Pa

rra

l

iver

ranch

Mat B

lF

Ga

an

al

R DE

NAGAR MUZAFFAR Kali Nadi

Up pe r

Branch

i

ga Barrage

ad

Madhya Gan

al

an

aC

g an

lle

arrage oda B ator II Stage Bhim G Head Regul

r. r B adi aha N h s up Nim r. An iB ot a kh La

Ka li N

ng

Ga

Hindo

n

una

Yam

iver

R

Krisna River

Ri v una Yam

er

iver

aR Yamun

CENTRAL WATER COMMISSION, NEW DELHI

FIGURE 4.16  The Irrigation System in Madhya Ganga Canal, India.

r

Rive

Legend:1. Canal 2. Barrage 3. River 4. Command Area 5. Railway Line 6. State Boundary

Water on the Surface   4.15

Ra

vi

ri D

Ba

Be as

b oa

Sirhind

Sutlej Bhakra-nangal Ram Ganga

Gurugaon

i M

Son Damodar Valley Hirakud

Narmada

Tawa

Ukai

Ghataprabha

Tapti Kakrapara Go dav ari Bh Ma im njra a Kr ish na

er

y

si Mayurakshi

Mahanadi R. Nizam Sagar Nagarjunasagar

abha Ma l a p r Tungabhadra

Ca uv

Ka

un a

ak

ah

nd

Ch am ba

Ra

Ga

a

i

rd

Ya m

Sa

a

jas th

ng

an

Ga

Sarju

Vamsadhara

Godavari Delta

Mettur

Schemes in Operation or in Advance Stage of Development Kallada FIGURE 4.17  Canal Irrigation Schemes in India.

Schemes in Early Stages of Development

4.16  Chapter 4

Water Availability in Great Britain The amount of water that is available for use in Great Britain, i.e., after allowing for evaporation from the ground and water used by plants, is shown in Figure 4.18. This map also shows the locations of major reservoirs and urban areas. You can see that most of the water available for use is located in the highland areas. These are sparsely populated. The map also shows the connections or linkages (water pipelines and rivers) between the reservoirs and some of the urban areas. For example, the urban centres of Liverpool and Birmingham obtain their water supplies from reservoirs in Wales while Manchester has its water supply piped all the way from the Lake District.

River Tyne

River Humber

River Severn River Thames

FIGURE 4.18  This Map Shows the Areas, Which Have the Most Available Water Supply, the Location of Some Major Urban Areas and Reservoirs, and the Water Connections Between the Two. Some Reservoirs Flow Direct Into Rivers, Which Carry the Water to The Urban Areas. Other Urban Areas are Supplied from Water Pipelines.

0

200 km

Over 1000 mm Rainfall

Reservoir

500–1000 mm Rainfall

River

Large Urban Areas

Water Pipeline

Water on the Surface   4.17

At the beginning of this chapter, we saw that some rainwater soaks into the underlying rocks to form underground water. Sometimes, a layer of permeable rock lies between layers of impermeable rocks with the layers forming a shallow syncline. If the ends of the permeable layer outcrop on the surface, then rain falling onto these infiltrates the rock. In time, the permeable layer may become saturated. The water cannot escape because it is enclosed by impermeable rock layers. The permeable layer is called an aquifer (Figure 4.19) and the water it contains is called groundwater. The water in an aquifer can be obtained by sinking wells. Some aquifers are extensive and they contain considerable amounts of water. Although underground water occurs throughout Great Britain, appreciable amounts that can be utilized are located in the southeastern part of the country as shown in Figure 4.20. Figure 4.21 shows the hydrogeological map of India highlighting the ground water potential of the region. This impacts the water flow of the area.Figure 4.21  The areas of Ground water potential in India.

Rain Rain

Water Table Artesian Well Impermeable Rocks

Aquifer FIGURE 4.19  The Structure of an Artesian Basin. If a Well is Sunk into the Aquifer, the Water Pressure Will Force Water out Forming an Artesian Well.

FIGURE 4.20  The Areas of Substantial Underground Water in Great Britain.

4.18  Chapter 4

Legend Ground Water Potential (Yield Litres/sec) >40 25–40 10–25 3000

Shale

Gneiss

Limestone

Intrusives

Granite

Unclassified

Schist

FIGURE 5.12  Acquifer Systems of India.

N

0

W

E S

250 kilometers

500

Underground Water and Limestone Features   5.9

Acquifers lie under parts of Great Britain. There is a large one under the Thames Basin. Figure  5.13 shows a part of the artesian basin of the Sahara Desert. Wind erosion sometimes exposes a part of an aquifer that bends up toward the surface. When this happens, a pool of water appears on the surface and this is called oasis.

Karst Cycle of Erosion

Oasis

Rain

Sand Dunes

Catchment Area

Limestone Mesas

Wadi

Aquifer of Sandstone

Sand

Impermeable W. M. Davis along with Siegfried Passarge and Jovan Limestone Rocks Cvijić provided a model—the cycle of erosion in arid conditions. But these cycles cannot be used FIGURE 5.13  A View of a Part of an Artesian Basin Under to describe the forms that evolve in a karst terrain. the Sahara Desert. It was only in 1918, when Jovan Cvijić provided an ordered description of successive changes that take place in the progress of the karst cycle. Karst topography develops mainly in limestone and dolomite regions because the response of these rocks to weathering is different from other rocks. This is mainly because of the solubility of calcium carbonate in natural waters. In a karst terrain, water circulates almost entirely underground. The basic assumption for the occurrence of all stages of evolution is a mass of pure limestone, which is soluble and is formed of strata inclined at a great angle but without disruptions from crushing or faulting. Another important criterion is the thickness of the limestone mass; it should not have a thick protective covering of vegetation and must have a developed system of streams. Cvijić also opined that there are three hydrographic zones in a well-developed karst landscape. These are mentioned as follows.

1. The zone immediately underneath the surface is compiled of channels and reservoirs, which transmit water in times of storm, but are generally dry. 2. The intermediate zone lies between the dry and wet zones where caverns and channels are generally flooded for considerable amounts of time, but not permanently. 3. The lowest zone lies immediately above the underlying impermeable strata; this zone has permanent streams and reservoirs, which are always full of water. He further states that there are four stages in the evolution of landforms in a karst terrain: youth, maturity, late maturity and old age (Figure 5.14) The youth stage begins with surface drainage on an initial limestone surface or one that has been laid bare and is marked by a progressive expansion of underground drainage. This stage is marked with imperfect underground drainage that leaves most of the rain on the ground surface although the cracks and crevices of the rock are filled with water. Wherever the limestone is exposed to the rain, the ground is now covered by a network of furrows eaten out of the rock that has been dissolved by water. These have been named differently such as Karren (German

5.10  Chapter 5

Felsenmeer

Immature Pavement

Juvenile Karst K-I

Relict Cave Suffosion Sinkhole Scree Small Caves Uniform Rockhead

Valley Stream Short Cave Spring

Youthful Karst K-II Pavement

Integrated Buried Cave System Rock Scars Suffosion Sinkholes Sinkhole Stream Dry Valley Rockhead Fissures Sink Sinks

Relict Cave

More Dissolution at Karst Margin Integrated Caves Buried Sinkhole

Mature Karst K-III Collapse Sinkhole Buried Cave Dissolution Doline Subsidence Sinkholes Fissured Rockhead Fissured Floor

Complex Karst K-IV

Fissured Outcrop Stoped Cave Roof

Collapse Sinkhole Foot Subsidence Sinkholes Cave Irregular Rockhead

Remanent Hill

Buried Sinkhole

Large Doine Cone hil in Cone Karst Extreme Karst K-V Large old Cave Pinnackes Tufa on Hill Large Stopped Cave Remanent Lower Stone Re-Actived Pinnacked Rockhead Leeth Buried Sinkhole Dropout Undercut Cliff Sinkhole

Dissolution Sinkhole

Old Fool Cave Tufa

FIGURE 5.14  The Karst Cycle of Erosion.

Underground Water and Limestone Features   5.11

equivalent) or rascles (in French Alps). Cvijić uses the term lapies for such landforms. When water penetrates through lines of weakness such as faults or bedding plane of joints, these deep and narrow chasms are eroded and form what Cvijić calls bogaz. Slowly, these furrows and chasms are deepened and underground channels are created and the surface streams disappear in the ground leaving their valleys dry—either wholly or in part. Such valleys are called blind valleys. Examples of such topography may be found in Tennessee, Kentucky and Virginia in the USA. In the mature stage, there is maximum development of underground drainage as the subterranean system is adequately developed to carry off all the surface water. Since now, all the surface water is carried immediately underground, no lakes exist unless the depressions are so deep that they dip below the upper level of the saturated zone. So the presence of intermittent lakes is common with presence of large number of caves and caverns. Surface drainage is limited to short sinking creeks, which end in swallow holes and blind valleys. This stage represents the maximum development of karst topography with all its characteristic landforms. Such topography is found to have developed in the Dinaric coastal strip of the Adriatic Sea. Late maturity in the Karst cycle begins with the decay and decline of karst topography as now the limestone has gradually stripped off the underlying impermeable strata. As this happens, the drainage can no longer flow underground and surface streams reappear. Various feature of the karst topography expand and coalesce to form uvalas. Poljes with their hums are to be seen on the surface. With the beginning of the old stage, there is return to surface drainage. Now, the caverns collapse, leaving open, flat-floored valleys. The process of unroofing of caverns and regressive erosion takes place at both the edges of the plateau and along the sides of the gorges where the streams cut back and capture the dolines and uvalas. As a result, the karst windows, natural tunnels and bridges, and other solution features disappear. Only isolated knolls remain as remnants of the former limestone surface, which are mere shells honeycombed with caverns. A normal system of surface streams is now in possession of the land, which was formerly dominated by limestone mass.

Limestone Landforms All types of rocks influence the type of landforms but perhaps limestone has the greatest influence. Limestone consists chiefly of insoluble calcium carbonate but rain, which absorbs carbon dioxide as it falls through air, turns calcium carbonate into soluble calcium hydrogen carbonate. This results in the joints and bedding planes of the limestone becoming enlarged by rain water. In time, the surface becomes broken and rugged, and most of the rain falling on to the surface disappears into the enlarged joints. Limestone surface features One of the most noticeable features of a limestone landscape is the almost complete absence of surface drainage. The permeability of limestone allows rain to soak into it very easily. Joints soon become widened and deepened, which results in the surface becoming criss-crossed with wide, irregular gullies called grikes. The separated blocks of limestone surface are called clints. Such a surface is known as a l­imestone pavement (Figure 5.15).

5.12  Chapter 5

Sinking Creek Spring Karst Window (Uvula) Lapies Caves

Sinkholes

Sinkhole Lake

Rivers entering a limestone region often disappear down a vertical hole in the limestone surface. They give rise to underground rivers. The vertical holes, which are called swallow holes (Figure 5.16) or sink holes, are enlarged Swallow Hole joints (Figure  5.17). Sometimes, several (Sink) swallow holes join together to form a large depression, called a doline. Similarly, dolines may join together to form a large depression called an uvala. Caverns Sometimes, a very large depression develops in association with faulting. These depressions usually have areas of Dripstone several hundred square kilometres. They (Stalactites/ Stalagmites) are called poljes (Figure 5.18).

Travertine Underground Stream FIGURE 5.15  Erosional and Depositional Karst Landforms.

Underground limestone landforms Rivers, which flow inside limestone beds develop underground caves (Figure  5.17) and caverns as they flow along joints and bedding planes. Some caves are very large, e.g., Cango Caves (Figure 5.19) in South Africa. Stalagmites and stalactites

(Figure 5.20) develop in caves, and sometimes they join together to form natural pillars. Water containing calcium hydrogen carbonate constantly drips from the roofs of caves. As the water evaporates, it leaves behind calcium carbonate. In time, the calcium carbonate forms long, slender, needle-shaped features, which hang down from the cave roofs. These features are called stalactites. Some of the drops of water fall on to the floors of caves where similar features develop. But these grow FIGURE 5.16  Gapping Gill Near Ingle Borough, Yorkshire. upward. They are called stalagmites. Sometimes, the roof of an underground cave collapses and a gorge, with almost vertical sides, develops. Cheddar Gorge in Somerset was formed in this way. Sometimes a gorge develops when a river cuts across, and down into, a limestone region. Rivers which go underground in a limestone region reappear on the surface again where the junction of the limestone and the underlying i­ mpermeable rocks meet the surface. Dry, gorge-like valleys often mark the formed courses of such rivers, and these occur between the point of disappearance and the point of re-emergence, as shown in Figure 5.17. The former courses of rivers in limestone

Underground Water and Limestone Features   5.13

Clay Plain

Limestone Platform

River which went underground reappears at the foot of the limestone cliff

Sandstone Hills (with Surface Drainage)

Flat Floor Limestone Surface

Steep Sides

Limestone gorge formed by the collapse of the roof of an underground cave River Disappears Down Swallow Hole

Dry Valley

Faults

Polje

FIGURE 5.18  Poljes Sometimes Occur in Heavily Faulted Limestone Regions. A Polje Often has a Flat Floor and Steep Sides; the Floor may be Covered with Residual Red Clay, Called Terra Rossa. This Forms an Impermeable Soil and Sometimes During Heavy Rains, Poljes are Flooded Because of this. Some Poljes Even have Small Lakes. Impermeable Underground Cave with Stalactites and Stalagmites Rocks FIGURE 5.17  The Main Features of a Limestone Region. Not all of the Features Shown Here will Necessarily Occur in a Particular Limestone Region.

regions of Europe were probably made possible by the frozen sub-soils that existed during the last Ice Age. Figure  5.21 explains how dry valleys may develop. Limestone landscape The surface of a limestone region is both stony and broke, and large areas consist of a bare rock. Any soil that occurs usually forms a thin layer which in most temperate regions supports fairly poor, grass or scrub-like vegetation. Limestone regions in the humid tropics often support trees and bushes, as well as coarse grass. Rugged escarpments, gorges with cliff-like sides, and depressions of varying sizes and depths, are common features in many limestone regions. A  limestone landscape is known as s karst landscape. Karst landscapes are well developed in Yugoslavia, Mexico (the Yucatan Peninsula), south-west China (Figure  5.22) and the Pennines. Figure 5.23 shows the stages in the development of a karst landscape. The karst of Yugoslavia has red earths called terra rossa, which represent weathered residues rich in insoluble hydroxides of iron.

FIGURE 5.19  The Interior of the Cango Caves, South Africa. Notice the Stalactites in Background of the Photograph and the Pillar in the Forefront.

Well-jointed Limestone Stalactite Cave

Pillar Stalagmite Underground River

FIGURE 5.20  An Interior View of a Limestone Cave Showing a Stalactite, a Stalagmite, a Pillar, and a Cave.

5.14  Chapter 5

(a)

Valley Followed by River

Limestone Frozen Sub-Soil

Impermeable Rocks

(b)

River Disappears Here Dry valley

River Here

Re-Emerges

A

B

300

250

B

225

200 175 150 125 100 75 50

(c)

A

The amount of vegetation in a karst landscape is usually limited. It often consists of shrubs, grasses and, in some regions, sweet-­smelling herbs. Trees only occur in the bottoms of larger valleys, which have been excavated, through erosion, to the rocks underlying the limestone. Although the limited plant life in limestone regions varies from one area to another, since it is dependent upon the nature of the climate, the general appearance of all karst landscapes is much the same. Settlements in karst regions are usually small and few, partly because the soils are poor and of limited extent, and partly because of inadequate water supply. In some regions, there is sufficient grass to support sheep and goats, and occasionally there are areas of fairly good soils, but these are usually confined to uvulas and poljes. Chalk landscape

FIGURE 5.21  The Formation of a Dry Valley: (a) Frozen Subsoil Prevented the River Going Underground During the Ice Age; (b) Sub-soil Thawed after the Ice Age and the River Went Underground; (c) Contour Map of the River’s Course. Compare the Contour Map with the Sectional Diagrams and Explain why there is no River on the Map between the Points A and B.

FIGURE 5.22  Steep-sided Hills of Limestone Tower Besides Xinping, in South West China.

Chalk is made of calcium carbonate but chalk is fine-grained and very pure. Its surface is not marked by outcrops of hard rock. Instead, it is undulating, its hills are rounded as in the downs of southern England and the Wolds of Yorkshire, and its valleys, which are usually without rivers, are open with gently sloping sides. Chalk is a porous rock and rain rapidly soaks into it. There is very little surface run-off, which means there are very few streams. Because the valleys are without rivers, they are called dry valleys or coombs. Chalk landscapes occur in parts of southeast England, and the thin soils are covered with grass, although there are patches of woodland. The region is used for sheep rearing and for growing crops.

Underground Water and Limestone Features   5.15

(a)

Dolines

(b)

Uvalas

Underlying Rocks Limestone

Cave (d)

(c)

Collapsed Cave

Residual Peaks of Limestone

Surface Drainage Reappears

Polje with Lake

Figure 5.23  Stages in the Development of a Karst Landscape. The Limestone Rocks are Eventually Removed Except for Residual Peaks Together with a Thin Layer Covering the Underlying Rocks.

Key facts ●● ●● ●● ●●

●● ●● ●● ●● ●● ●● ●●

●● ●●

When water sinks into the ground, it is called underground water. Water table refers to the upper surface of a saturated layer of rock below the ground. Water that flows out of the ground is called a spring. A spring is formed when the water table meets the surface of the ground. An artesian basin is a saucer-shaped structure formed of a permeable layer of rock lying between two impermeable layers with one or both ends of the structure exposed at the surface. The permeable rock layer of an artesian basin is called an aquifer when this contains water. A well is a hole sunk into the ground down to a saturated rock layer. If it is sunk into an artesian basin, it is called an artesian well. Solution of limestone produces clints and grikes. Infiltration in limestone regions produces such features as swallow holes, dolines, uvalas and poljes. Underground water in limestone regions produces a large variety of features, the most important of which are underground rivers and caves. A well-developed surface of a limestone region is said to have a karst topography. Chalk is similar to limestone in chemical composition but it is made of much finer rock grains. Chalk rocks produce a smooth, gently undulating landscape whereas limestone rocks produce a rugged broken landscape. Limestone weathers to give a reddish soil called terra rossa. Stalactites, stalagmites, and limestone pillars often develop in limestone caves.

5.16  Chapter 5

●● ●● ●● ●● ●●

When the roof of a limestone cave or limestone cavern collapses, a gorge is formed. Enlargement of joints by solution in a limestone surface gives rise to swallow (sink) holes. When swallow holes join together, dolines are formed. A Polje is a very large depression probably caused by solution and faulting. Underground rivers in limestone rocks produce caves and caverns mainly by the process of solution.

Underground Water and Limestone Features   5.17

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, Select the correct a­ nswer. 1. Limestone topography is often dry because (a) evaporation exceeds precipitation. (c) the soils are porous. (e) rain percolates downwards. 2. A well will always contain water if (a) it is sunk at the bottom of a hill.

3.

4.

5.

6.

7. 8. 9. 10. 11.

(b) there is very little rainfall. (d) it is always mountainous. (b) the bottom of the well is always below the water table. (d) it is loaded in a rainy region.

(c) it is sunk into sedimentary rocks. (e) it is on a spring line. Which one of the following features never occurs underground in a limestone region? (a) stalagmite (b) cavern (d) stalactite (c) grike (e) lake Which one of the following are characteristic of limestone region? (a) dry valleys (b) meandering streams (d) good vegetation cover (c) deep soils (e) salt marshes A karst landscape is most likely to develop in a region whose rocks are (a) porous. (b) impermeable. (c) chiefly made of calcium carbonate. (d) well jointed. (e) sedimentary. Sinkholes and caves are created when limestone formations are dissolved by (a) strong hydrochloric acid from mine runoff (b) weak carbonic acid contained in ground (c) strong hypochlorite from acid rain (d) weak sulphuric acid from animal remains What would NOT be shown on a karst topographic map? (a) Cavern (b) Underground water reservoir (c) Sinkhole (d) High-pressure system Rocks formed of gypsum can also develop karst features. (a) True (b) False Generally, karst will form less rapidly in areas with (a) steep relief (c) high vegetation (b) flat surfaces (d) housing development Which karst feature forms on the floors of caves? (a) Stalactites (b) Stalagmites (c) Cave stacks (d) All of the above _____ is the erosion of largely insoluble rocks or sediments by rapidly flowing water. (a) doline (b) scouring (c) thermokarst (d) epikarst

5.18  Chapter 5

12. A type of landscape in rainy regions where there is limestone near the surface, characterized by caverns, sinkholes and valleys. (a) Pavement karst (b) Phreatic zone (c) Karst landforms (d) Karst topography 13. Which of the following does not lead to formation of sinkholes? (a) Strong pumping of groundwater for crop irrigation in dry weather (b) Pumping groundwater for spraying on crops to prevent freezing (c) Construction of a heavy building over a limestone cavern (d) Pumping waste fluids into the ground for disposal (e) Drilling for groundwater 14. Caves and caverns most commonly develop in (a) granite (b) sandstone (c) shale (d) limestone 15. What is karst and how does it form? (a) Karst is the ragged surface dissolved on the surface of limestone bedrock formerly buried under soil. (b) Karst is windblown dust deposited in areas of dune-like hills. (c) Karst is the deposit of calcium carbonate that precipitates on rocks in soil when groundwater evaporates. (d) Karst is the cold wind that blows off the North Atlantic Ocean at times in late fall.

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. A limestone region that has been actively weathered and eroded by solution has a distinctive landscape know as a karst landscape. (a) Name three features common to the surface of a karst landscape. (b) For two of these features briefly state how each was formed using well-labelled ­diagrams to illustrate your answer. (c) Briefly explain how an underground cavern can be turned into a gorge. (d) Name two regions which have well-developed karst landscapes. 2. With the aid of diagrams, describe and explain the characteristics and mode of formation of any three of the following: (a) dry valley (b) artesian well (c) Stalactite (d) uvala (e) spring For each feature chosen, name one region in Great Britain where an example may be seen. 3. Draw well-labelled diagrams to show the differences between a spring and a well. Write brief notes on each and state in what type of region either, or both, may be found.

Underground Water and Limestone Features   5.19

4. Figure 5.24 shows a section across a valley. At which of the hollows labelled from A to E will there be water all the year? Winter Water Table A

B

C

D

E

Summer Water Table FIGURE 5.24  A Section Across a Valley

5. Elaborate karst topography and different stages of karst cycle of erosion.

Answer key Exercise 1 1.   (e) 6.   (b) 11.   (b)

2. (b) 7. (d) 12. (d)

3. (e) 8. (a) 13. (d)

4. (a) 9. (b) 14. (d)

5. (e) 10. (b) 15. (a)

Thispageisintentionallyleftblank

6

Glacial Processes

Learning Outcomes After completing this chapter, you will be able to: ●

● ● ●

Understand the extent of ice masses and regional distribution of glaciers and ice caps in the world Discuss about various type of glaciers and their features Explain different glacial processes: erosion, transportation and deposition Identify landforms produced by different glacier process

Keywords Glacier, Ice Age, Snow Line, Glacial Movement, Glacial Erosion, Rôche moutonnée, Permafrost

1

6.2  Chapter 6

Introduction Numerous changes have occurred in the climatic patterns of the earth during its long geological history. Scientific evidence indicates that the impacts of climate change and globalization are being experienced in all remote glacial environments. Although climate change is mostly caused by highly industrialized pasts of the world, the effects are taking their toll on the sensitive mountain areas too. This is a serious concern for the whole world, because glaciers hold approximately three-fourth of the total fresh water of the land. They are also responsible for the formation of several lakes and river systems. The scenery in parts of Great Britain, Europe, and North America reflects periods in the earth’s history when large areas were buried beneath thick sheets of ice and glaciers. Such periods are known as ice ages, and the last of these occurred in the Pleistocene Period. During this period, the northern parts of Eurasia and North America were ­buried beneath vast masses of ice. This ice mass has now melted, but the region around the North Pole, much of Greenland and Iceland, the entire Antarctica, and some high mountain areas are still buried beneath ice masses. Figure 6.1 shows the past and present extent of ice in the Northern Hemisphere. In this period, most of the world’s mountains had their tops buried under ice, which extended down the valleys in enormous tongues called glaciers. The Pleistocene Ice Age began about two million years ago. During the last 20,000 years, environmental conditions have become warmer, and the period in which we now live is called an inter-glacial period. From the beginning of the Pleistocene Ice Age, several climatic changes occurred that resulted in the ice advancing, which produced an ice age, and then ice retreating to give an inter-glacial period. It has been calculated that for these changes to occur, a temperature change of only 5°C was needed. In other words, a fall of 5°C would be sufficient to cause the ice to advance once again, including the glaciers in the mountains of East Africa.

Figure 6.1  The Extent of Ice Masses in the Northern Hemisphere in 1930 and 2018.

Glacial Processes  6.3



Table 6.1

An overview of the regional distribution of glaciers and ice caps in the world.

CONTINENTS/COUNTRIES

AREA (SQ. KM)

1. New Guinea

3

2. Africa

6

3. New Zealand

1600

4. Scandinavia

2940

5. Central Europe

3785

6. South America

25,500

7. Northern Asia

59,600

8. Antarctica (not including the main ice sheets)

77,000

9. Central Asia

114,800

10. North America

124,000

11. Arctic Islands (not including the Greenland ice sheet)

275,500

Total

684,734

Regional Distribution of Glaciers Glaciers are always formed above the snow line and in extreme conditions. Snow line is the level above which a permanent snow cover exists. The ideal conditions under which glaciers are formed occur in the North and South Poles. The weather is extremely cold on the large mountain ranges. Table 6.1 shows the distribution and area of glaciers in different countries/continents. Extensive glaciers are found in Antarctica, Alaska, Canada, Chile, Greenland, and Iceland. Antarctica and Greenland contain 98 per cent of all the glaciers and 70 per cent of freshwater in the world.

Accumulation of ice and the snow line When the temperature of the air falls below 0°C, some of its water vapour condenses and freezes into ice crystals that fall on the earth as snow. Although many regions in the high latitudes receive snow in the winter, the snow melts in the summer in most of these regions. If some of the snow does not melt, it results in a permanent snow covering.   This occurs in Greenland, Antarctica, and other similar regions, as well as on the tops of mountains that are at high altitudes. The level above which there is a permanent snow covering is known as the snow line, the height of which ranges from sea level around the poles to about 4800 m in the mountains of East Africa. Figure 6.2 shows the height of the snow line for selected latitudes.

6.4  Chapter 6

(a)

pan ) Ma tter 450 h 6 m orn (S wit zer lan Log d) a n 605 (Ro 0m cki For es) e l 386 (Gre 0 m enl and ) Sp itsb erg en

Fuj i 377 yama 6 m (Ja

Eve (Him rest 884 alaya 7 m s)

Kili m 589 anjar 5 m o (T anz ani a)

Altitude (Metres)

If the accumulation of snow in a region increases year after year, the snow gradually converts into ice by its own weight. 10 000 Snow consists of small ice crystals separated by air spaces. As the depth of snow increases, the pressure exerted on the bot7500 tom layers causes some of the crystals to melt, and the water thus formed trickles down into the air spaces where it freezes. 5000 This causes the snow to convert into a granular mass called névé or firn. In time, all the air spaces disappear, and eventu2500 ally, the névé is converted into a compact mass of ice known as glacier ice. When a continuous mass of ice covers a large land surface, it is called an ice sheet.. There are 0 22½º 45º 67½º 90ºN ice sheets in Greenland and Antarctica. In Figure 6.2  The Height of the Snow Line at Selected Latitudes. The both these regions, the ice sheets reach to Snow Line Lies at an Altitude of About 5000 m at the Equator, and the coasts and into the surrounding seas. it Reaches Down to Sea Level at the Poles. Large masses detach from the ice sheets and form icebergs. When a mass of ice occupies a valley, it is called a glacier. Glacier is a body of ice originating on land by recrystallization of snow or other forms of solid precipitation and showing evidence of past and present flow. In other words, we can also define glacier as a huge pile of snow that starts ­moving downwards under the influence of gravity. It occurs in winter when snowfall exceeds ice melting in summer. The largest glaciers today are in high mountain ranges such as the Himalayas, The term ‘Glacier’ is Andes, Rockies and Alps; some mountainous highlands in Africa also have gladerived from a French ciers. Figures  6.3(a) and 6.3(b) depict glaciers of Mount Everest in 2008 and word ‘glace’ meaning 2018, respectively. The comparison illustrates the impact of climate change on ‘ice’. It refers to a large the glaciers. mass of perennial ice that These are Mounts Kenya and Kilimanjaro and the Ruwenzori Mountains. Snow forms on land through the falls annually on these mountains and on the tops of the Atlas Mountains, the recrystallization of snow. Ethiopian Highlands, the Drakensberg, and the Cape Ranges.

(b)

Figure 6.3  (a) Mount Everest in 2008; (b) Mount Everest in 2018.

Glacial Processes  6.5

Classification of glaciers There are four main types of glaciers. These are as follows:

Ice sheet or ice cap This is a very extensive mass of ice that accumulates in mountainous regions, burying most if not all of the relief features. It spreads out to the surrounding lowlands where it often forms valley glaciers. During the Pleistocene epoch, vast ice sheets covered large parts of the lowlands in Great Britain, Eurasia, and North America. The most extensive ice sheet now covers Antarctica (Figure 6.4). This sheet often extends across the continent’s coast and forms ice shelves which may be extend up to 500-km wide. Almost 85 per cent of the world’s ice is locked up in the Antarctic ice sheet, with about 10 per cent in the ice sheet of Greenland.

Valley glacier This is a tongue of ice which usually extends from a mass of ice in the highland and which is confined to a valley. Figure  6.5 shows a valley glacier originating from a cirque glacier.

Cirque glacier This is a small mass of ice which accumulates in a rock hollow either at the head of a valley or on the side of a mountain. It feeds a valley glacier when it is located at the head of a valley. Figure 6.6 shows cirque glacier in Himalayas.

Piedmont glacier When valley glaciers extend to the plains, they sometimes join together to form vast lobes of ice called piedmont glaciers. (a)

(b)

Figure 6.4  (a) Thickness of Ice Sheet in Antarctica in 1930; (b) Thickness of Ice Sheet in Antarctica in 2013.

6.6  Chapter 6

Figure 6.5  A Valley Glacier Located in Siachen, Himalayas, India, Originating from a Cirque Glacier.

Figure 6.6  Cirque Glacier in Himalayas.

Glacial System A valley glacier develops from ice that has accumulated in the highlands, and as ice accumulates further, it will be added to the head of the glacier. At the other end of the glacier, which is usually called the snout, ice melts especially in the summer months. Ice also melts inside the glacier, underneath the glacier, and on its surface. The loss of ice through melting and through the growth of icebergs (pieces of a glacier that detach and float away where a glacier reaches the sea) is called ablation. The opposite process to ablation is accumulation. The difference between accumulation and ablation is the ice that remains in the glacier over a definite time period. This difference is called the net balance. In effect, the accumulation of ice in a glacier represents the input, while the loss of ice through ablation represents the output. The glacier itself is the storage part of the system.

Glacial Processes  6.7

Glacial movement The reason for the flow of ice from the glacier is not yet known, but it is thought to be in part caused by the thaw–freeze action. Pressure at the bottom and sides, and in the middle of a mass of ice is very high, and it causes some of the ice to melt. The water thus formed is again converted to ice almost immediately, but there is just sufficient time for it to trickle a little way down the slope. Throughout a mass of ice, bits of ice are continuously melting, trickling down the slope as water, and then freezing back into ice. This means that within a mass of ice, there is a gradual movement. It is possible that the thaw–freeze action develops along temporary lines of weakness, which causes large masses of ice in an ice mass to slip forward.

Surface features and moraines Transverse Crevasses Caused by Steep Floor

Crevasses and moraines are discussed below.

Movement of Glacier

The gradient and the thickness of the ice affect the speed at which ice flows. The movement of valley glaciers is greater than that of ice sheets. Also, the movement on the surface and in the centre of a valley glacier is greater than that at the sides and bottom. These different rates of movement lead to the formation of cracks called crevasses. They occur at right angles to the glacier (transverse crevasses), parallel to its sides (marginal crevasses), or oblique to its sides according to the direction of the forces of stress (see Figure  6.7) Sometimes, there is an abrupt steepening of the valley floor, and when a glacier moves over this valley floor, deep transverse crevasses develop that lead to ice falls, as shown in Figure 6.6.

Valley Side

Crevasses

Marginal Crevasse Caused by Ice Spreading Sideways

Oblique Crevasses Caused by Different Rates of Movement in the Ice Figure 6.7  The Different Types of Crevasses that can Develop in a Valley Glacier.

Moraines A glacier is capable of transporting a vast amount of rock waste called moraine. Glacial erosion on the floor and sides of a valley produces a part of this rock waste. The rest comes from the weathering, especially from frost action, of the slopes above the glacier. This waste includes material brought down the slopes by avalanches, scree slides, and rockfalls. There are five types of moraines (see Figure 6.8): 1. Subglacial moraine is transported along the valley floor by the glacier. Some of this is eroded from the floor by abrasion; some work its way from the sides of the valley and from the surface of the glacier. 2. Englacial moraine refers to the rock material inside the glacier. Some of this comes from the glacier surface and from its crevasses. 3. Lateral moraine collects along the edges of a glacier.

6.8  Chapter 6

(a)

Valley Glacier

Cirque Glacier

Frost Action Produces Screes

(b) Zone of Ablation Terminal Moraine

Crevasse

Snout of Glacier Glacier Movement

Medial Moraine

Lateral Moraine

Ground Moraine

(c)

Medial Moraine Glacier

Lateral Moraine Terminal Moraine

Medial

Englacial Moraine Subglacial Moraine

Lateral Moraine

Crevasse

Moraine

Glacier Tongue

Glacier

Glacial Lake

Terminal Moraine

Figure 6.8  (a) and (b) The Five Types of Moraine; (c) The Different Types of Moraine in Siachen Glacier, Himalayas. Flat Boulder Glacier Surface

Ice Pedestal

Figure 6.9  Glacier Table.

4. M  edial moraine refers to two lateral moraines, which have joined together. This occurs when a tributary glacier joins a main glacier. 5. Terminal moraine is the moraine that collects along the snout of a glacier. It is the sum total of the first four moraines mentioned above.

Glacial processes Glacier table: This is a large, flat boulder that sits on a pedestal of ice (Figure 6.9). The ice of the glacier surface around the pedestal has melted, but the boulder protects the ice beneath it, which melts much more slowly. This feature is short-lived.

Masses of ice that move over a landscape for great periods of time modify that landscape by erosion, transportation, and deposition. Glacial erosion, which predominates in the highlands, consists of three processes: 1. Sapping: the disintegration of rocks by alternate freezing and thawing of water at the bottom of cracks between a mass of ice and the side and floor of a valley, or the side of a mountain; 2. Plucking: the tearing away of blocks of rock which have become frozen into the sides or bottom of a glacier; 3. Abrasion: the wearing of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.

Glacial Processes  6.9

Landforms Produced by Glacial Erosion Valley glaciers produce more impressive features than ice sheets because they operate in valleys produced by rivers. These valleys already have relief, which is worked over by the glaciers. Ice sheets cover a much larger area, and their main erosive action is scouring. Erosional landforms are mainly found in the highlands. The main features of glacial erosion are overdeepened valleys, which are often called glacial troughs, truncated spurs, cirques, aretes, pyramidal peaks, hanging valleys, rock basins, rôche moutonnées, and crag and tails.

Glacial trough This has steep sides and a flat floor. The original valley was developed by a river, and any spurs that projected from the valley sides have been eroded to become truncated spurs. As a glacier develops in a river valley, the accumu/lation of ice at the head of the glacier, together with that from tributary glaciers, gives the glacier increased erosive power. This results in the glacier deepening, straightening, and widening the valley. The upper slopes of the valley lying above the glacier surface are left intact, i.e., they are not eroded by moving ice. After the glacier melts, they form benches or alps (see Figure  6.10). Figure  6.11 depicts the V-shaped valley and Pyramidal Peak Cirque with Lake Truncated Spur Alp or Bench

Hanging Valley

Snow and Ice

Arete

Interlocking Spurs

River

V-Shaped Valley

Valley Deepened and Widened by Glacier Moraine in Bottom of Glacier Pre-Glacial Valley

River Valley

Waterfall

Glacial Trough

Alp or Bench

Moraine Deposits Give Flat Floor Glacier Level

Moraine

250200 50 1 100

250 00 0 2 15 100

Contour Pattern — Glacial Trough

1000 900

0 60500

700

Truncated Spur

800 700 600 500

6 00

800

Normal Spur

600 500

1000

900 800 700

Contour Pattern — V-Shaped Valley

(a)

(b)

(c)

Figure 6.10  Stages in the Development of a Glacial Trough (a) River Valley System; (b) Same Valley Filled with a Glacier; (c) Same Valley after the Glacier has Melted. Compare the Shapes of the River and Glaciated Valleys, and also the Contour Maps that Represent these. (Examine the Contour Map and Diagram of a Truncated Spur. What Ice Action Causes a Spur to Become Truncated?)

6.10  Chapter 6

interlocking in Nepal Himalayas. Weathering and mass movement on the upper slopes since the ice melted have caused large amounts of scree to collect at the bases of the valley sides, thus making them less steep. Figure 6.12 illustrates a wide valley in the Himalayan Glacier.

Cirque

Figure 6.11  V-shaped Valley and Interlocking Spurs in Nepal Himalayas.

Figure 6.12  Valley Deepened and Widened by a Glacier in Himalayas.

A semi-circular, steep-sided basin cut into the side of a mountain or at the head of a valley. Such a feature is called a corrie in Scotland, a cwm in Wales, and a cirque in France. The accumulation of snow in a depression on a mountain side, or in a valley head, gradually leads to the formation of a glacier (Figure 6.13). As this grows in size, water trickles down between the glacier and the side of the depression. By the process of sapping, which is most pronounced at the base of the depression’s sides, the sides become steepened. Whilst this is occurring, the glacier pulls away from the sides and the back wall of the depression, thereby making the entry of water easier. At the same time, the action of plucking occurs, which steepens the sides, and especially the back wall, even further. Eventually, a deep crevasse known as a bergschrund (Figure  6.13(c)) develops between the back wall of the cirque and the cirque glacier. The depression is overdeepened at the same time by the movement of the glacier. The depression develops into the shape of an armchair with a distinctly concave floor. The edge of the floor on the downslope side forms a lip,

Glacial Processes  6.11

(a) Round-Topped Mountain

(b) Mountain Top Attacked by Frost Action

River Valley

Neve

Round-Topped Mountain

Neve

River Valley

(d) (c) Mountain Top Severely Attacked By Frost Action

Pyramidal Peak

Arete

Cirque Lip Glacier

Ice Falls

Valley Glacier

Bergschrund

Cirque Glacier in Valley Head

Plucking on Back Wall Moraine

Steep Back Wall of Cirque Sharp-Edged Mountain Top

Bergschrund Lip

Cirque Glacier

Ice Falls Crevasse

Lake Lip

Figure 6.13  Stages in the Development of a Cirque and Associated Features: (a) Before Glaciation; (b) Early Glaciation; (c) Late Glaciation; (d) a Block Diagram of the Valley Head, and a Cirque Lake.

as shown in Figure 6.13(c). Some cirques contain glaciers, but in other cirques, the glaciers have melted and they now contain lakes (sometimes called tarns).

6.12  Chapter 6

Arête

Pyramidal Peak

(a)

A steep-sided, knife-edge ridge separating two cirques (Figure 6.14(a, b)) and formed by the curtailing of the back walls of the cirques by plucking (see Figure 6.13(d))

Arete

Arete

Cirque Glacier

Cirque Glacier (b)

Pyramidal peak A jagged peak formed by the steepening of the back walls of several cirques which lie on the sides of a mountain. These peaks become sharpened by frost action. Figure  6.15 shows how a pyramidal peak and arêtes are formed. The most famous pyramidal peak is the Matterhorn in Switzerland.

Hanging valley A tributary valley of a U-shaped valley, which ends abruptly, high above the floor of the U-shaped valley and separated from it by an almost vertical slope. Vertical erosion of the main Figure 6.14  (a) Arêtes Radiating Out from a Pyramidal Peak valley by its glacier is greater than that (b) Arêtes Radiating Out in Himalayas. of a tributary valley, which contains either only a small glacier or no glacier at all. When the main valley glacier melts, the tributary is left hanging. If a river occupies the hanging valley, it plunges as a waterfall to the floor of the main valley, and sometimes, it builds an alluvial fan of coarse materials. These features are shown in Figure 6.16.

X

Cirque

Pyramidal Peak

X1

(a)

(b)

Cirque Glacier

Bergschrund

Aret

e

Arete

Cirque

e

et Ar

Y

Cirque Glacier

Pyramidal Peak Cirque Glacier

Y1

Cirque X

Section Along XY

Y

X1

Section Along X1 Y1

Figure 6.15  The Development of a Pyramidal Peak and Arêtes: (a) Three Cirques Containing Glaciers – the Arrows Indicate the Movements of the Glaciers; (b) The Position After a Prolonged Period of Erosion.

Y1

Glacial Processes  6.13

Rock basin An irregular depression in the floor of a U-shaped valley formed by unequal glacial erosion of the bed rock. This feature often develops when the thickness and weight of a glacier increase, e.g., at the junction of two glaciers. After the glacier melts, the rock basins become the sites of lakes. Sometimes, uneven vertical erosion caused by rocks of different strengths results in the formation of rock steps. Figure 6.l7 illustrates the formation and appearance of a rock basin and rock steps.

Rôche moutonnée It is an outcrop of resistant rock smoothed by ice on the upstream end into a gentle slope and plucked on the downstream end to give a steep, jagged slope. Rocks embedded in the base of the ice smooth down the outcrop, sometimes cutting deep grooves into its surface. These grooves are called striations. They can be seen on rock outcrops in both glaciated highlands and lowlands. Rôche moutonnée may be a small feature with a height of few metres or a large feature, more like a small hill (see Figure 6.18).

Crag and tail This is a knob of resistant rock protecting a less-resistant rock on the down slope of the glacier (see Figure 6.19). This should not be confused with a rôche moutonnée. Valley Glacier Tributary River

River Valley

Tributary Vallley Glacier (b)

(a)

Moraine

Thickness of Ice Increases Here, and Hence Vertical Erosive Power of Glacier Increases

Hanging Valley

Main Vallley Glacier Water Fall

Alluvial Fan (c)

Figure 6.16  The Various Features that Develop When a River Valley is Glaciated: (a) Before Glaciation; (b) During Glaciation, the River Valley is Straightened, Widened, and Deepened (Below Sea level) by the Glacier; (c) After Glaciation. Note the Position of the Alluvial Fans.

Trough End

Rock Basin with Lake

Rock Steps

Figure 6.17  Profile of a Glaciated Valley Showing a Rock Basin and Rock Steps.

6.14  Chapter 6

(a) Directio n of Ice Flow

(b)

Resistant Rock

Eroded Steep Side

Less-resistant Rock Gives the ‘Tail’ Sometimes Rock Debris is Deposited on Top of this by the Glacier

Movement of Ice Figure 6.18  Diagram of Rôche Moutonnée.

Figure 6.19  Diagram of a Crag and Tail: (a) During Glaciation; (b) After Glaciation.

A Closer Look  ▼ Glacial Transport The material used by a glacier or an ice sheet in the process of erosion comes in part from erosion caused by the glacier and in part from weathering and mass wasting of the slopes above the glacier surface. A glacier or an ice sheet is capable of transporting boulders over great distances. When the ice melts, the blocks are left stranded. Very often, they are deposited in regions whose rocks and structure differ from the regions from where the blocks originated. Blocks of rock transported in this manner are called erratics. Some idea of the distances involved is given by the location of erratics, which originated in the granitic mass of Ailsa Craig in the Firth of Clyde. Some of these erratics are found in Antrim, Northern Ireland, the Isle of Man, and South Wales. An erratic made of Silurian grit is found near Austwick in Yorkshire; it is perched on blocks of carboniferous limestone. Here, ice has retreated the limestone surface and lowered it by about 45 cm by chemical weathering, except for the limestone immediately beneath the erratic.

Landforms Produced by Glacial Deposition The eventual melting of glaciers and ice sheets has resulted in the deposition of vast quantities of rock debris, especially along the snouts of glaciers and the fronts of ice sheets. Landforms produced by glacial deposition are most common on the lowlands. Glacial deposits fall into three groups: boulder clay (sometimes called till) which is unsorted and not stratified; outwash deposits which are materials that have been transported and graded by glacial meltwater and are therefore sorted and stratified; and intermediate deposits, i.e., materials partly sorted and stratified by water action. Each group produces distinct landforms.

Glacial Processes  6.15

As the ice melted and ice sheets and glaciers retreated, thick layers of glacial till were left on the lowland surfaces, sometimes burying all pre-glacial minor landforms. The surface of the glacial till contain erratics, drumlins, eskers, kames, and moraines, but all of these rarely occur in one specific region.

Boulder clay deposits Boulder clay is the most extensive of all glacial deposits, and it has greatly influenced shaping of the landscapes of large parts of North America, Great Britain, and Europe, which were once covered by glaciers. It consists mainly of fine clay with scattered boulders, and it often completely buries the ice-eroded surface that lies beneath it. Boulder clay is often several metres thick. Boulder clay deposits occur on the floors of glaciated valleys as well as on neighbouring lowlands where sometimes they are so extensive that they form boulder clay plains. Some boulder clay surfaces are hummocky, with the individual hummocks forming long low mounds called drumlins. Drumlins vary in length from 100 to 800 m, and are between 25 and 100 m in height. The side facing the direction from which the ice came is steeper than the down-glacier side as shown in Figure 6.20. Another feature, ridge-like and often winding, is sometimes located along the edge of the boulder clay deposits on the downslope side. This is called a terminal moraine because it represents the greatest advance reached by the ice. It is composed of material that was under, inside, and on the ice, and it was deposited at the snout of the glacier. It is mainly boulder clay although some of its material may have been partially sorted by ice melt water. A terminal moraine is 20–50  m high and up to 100-m wide, and it may extend for many kilometres. A  well-developed terminal moraine indicates that the ice has remained stationary for a very long time. Sometimes, a terminal moraine interferes with drainage from the boulder clay on the upslope side, causing the formation of lakes (see Figure 6.20).

(a) Glacier Advanced in this Direction Terminal Moraine Outwash Plain

Drumlins Boulder Clay Plain

Rock Platform Scoured and Polished by Ice Erosion An Esker

Moraine Dammed Lake

(b)

Direction of Ice Advance

Drumlin Section

Sandy Deposits True Glacial Deposits (Boulders, Sand and Mud)

Drumlin Plan

Lakes Occupying Rock Hollows

An Erratic (a Large Boulder Transported and Deposited by a Glacier)

Swarm of Drumlins (Basket of Eggs Topography)

Figure 6.20  (a) Features Associated with a Boulder Clay-covered Region. An Outwash Plain is also Shown. Notice the Position of the Terminal Moraine in Relation to the Outwash Plain; (b) Drumlins Sometimes Occur in Closely Packed Groups or Swarms (Basket of Eggs Topography).

6.16  Chapter 6

Outwash deposits The melting of the ice at the end of the last ice age set free enormous quantities of water. Some of this water was collected in hollows or was retained by glacial deposits, where it formed lakes. The Great Lakes of North America were formed in this way. The melting of the ice gave rise to powerful rivers which carried and later deposited vast amounts of glacial deposits in regions outside those covered by the ice. These deposits formed plains known as outwash plains. Outwash plain: Having been deposited by water, the material of an outwash plain is both sorted and stratified. It is composed mainly of gravel and sand which were transported and deposited by meltwater streams away from the margin of an ice sheet or under the snout of a glacier. The coarsest deposits, i.e., the gravel, occur near the ice. The deposits become finer with distance from the ice. All outwash plains have a fairly steep gradient. Those plains formed in valleys are called valley trains.

Intermediate deposits Esker: A steep-sided ridge, about 40-m high that winds across a country (even over hills). It is made of gravel and sand. A river flowing inside a glacier is under tremendous pressure and is therefore able to carry a large load, most of which is ultimately deposited in the tunnel through which the river flows. Eskers develop best in ice that remains stationary for a long time. When the ice melts, the tunnel walls collapse, and the sorted river deposits become exposed on the surface. Some eskers are formed by rivers that emerge from a retreating ice front. This causes the deposits to form a mound that elongates as the ice retreats. Eskers are formed at right angles to terminal moraines, and unlike moraines, they are s­tratified, i.e., their material occurs in layers. Kame: An irregular-shaped mass of stratified material formed as a delta on the surface of a stationary glacier or at its margin (Figure 6.21). Some represent sorted material that are collected in crevasses.

(a) Rock Waste Collects in Lake Along Margin of Glacier

Rock Waste Collects in Small Lake on Surface of Glacier

(b)

Marginal Kame

Figure 6.21  The Development of a Kame: (a) During Glaciation; (b) After Glaciation.

Kame

Glacial Processes  6.17

Evidence that Glacial Activity Occurred in Great Britain In almost every part of Great Britain, we can see how rivers and streams are slowly producing changes on the surface over which they flow. This is particularly apparent when they are flooded because then we notice how dark and muddy their waters become, and on further examination, we find that the colour comes from minute particles of rock, which could only have come from the land. But now that the ice sheets and glaciers have disappeared from Great Britain, how do we know that they were ever here? In Great Britain, there is abundant evidence in both uplands and lowlands to prove that both ice sheets and glaciers once occupied large parts of the country (Figure 6.22). For example, erratics are found in various parts of the country, which do not match the rocks of the regions in which they occur. The sizes of some of these erratics are such that their locations cannot be attributed to either Ice-Free Area flowing water or the wind. The only agent that could have No Glaciation Here moved them is ice. Evidence also comes from corries, the Ice Centre armchair-like feature in the mountains of Scotland and Directions of Ice Flow Wales. Similar features are observed in the Alps, but many of these contain glaciers. By studying these corries and their Ice-Free Area glaciers, we can show how the glaciers plus frost action have produced the characteristic shape and form of the corries. Figure 6.22  The Main Centres of Ice in It is not unreasonable to show that almost identical features Great Britain During the Last Ice Age and in the mountains of Wales and Scotland were formed by the Main Directions of Ice Flow. Only the the same processes. Indeed practically, all the features that Southern Part of the Country was not can be seen in the Swiss Alps, namely the corries, pyramidal Covered by Ice. peaks, aretes, hanging valleys, glacial troughs, etc., can also be seen in the Scottish mountains. Evidence of glacial activity also comes from lowland Great Britain. It is evident in the form of coverings of boulder clay, moraines, drumlins, erratics, and eskers. By mapping the locations of these various features, especially the boulder clay, it is possible to show the most southerly extent of the ice sheets in Great Britain. The southern part of England, i.e., south of a line from the Thames estuary to Avon and north Somerset, contains no erosional or depositional features that can be said to have been produced by glacial action. It is therefore reasonable to assume that this part of England was never under ice sheets or glaciers, i.e., not during the Pleistocene Period. As the ice sheets and glaciers of Great Britain slowly melted, an enormous quantity of water must have been liberated, and there is evidence in parts of Great Britain that this meltwater gave rise to vast lakes and powerful rivers. What is this evidence?

Ice-Dammed Lakes and Overflows The Aletsch Glacier in Switzerland occupies a large main valley, and it has blocked one of its tributary valleys, thereby causing the drainage of that valley to be impounded. This has produced a lake called the Märjelensee (Figure 6.23). Lakes of this type are quite common in many mountainous regions, which have glaciers. If an ice barrier is sufficiently large and high, the impounded lake rises to the height of a pass or col at

6.18  Chapter 6

(a)

(b)

a

b c d

Märjelen See e

Figure 6.23  (a) Section of the Glacier Lake Called the Marjelen See; (b) The Ice-Dammed Lake of the Märjelensee. The Altesch Glacier, Switzerland, has Dammed a Tributary Valley, Which has Resulted in the Development of the Lake. Eiger and Monch are the Two Peaks Above the Glacier with Jugfrau Still Hidden Off.

(b)

(a)

Direction from Which Ice Advanced

Cleveland Hills

North Sea

North York Moors le Va of rk Yo

Yorkshire Wolds Ice Front Ice Movement

Yorksh World

(c)

Ice-dammed Lake Spillway (Overflow Channel)

River Esk Newton Dale

of k

r Yo

Riv Der er wen t

le Va

Figure 6.24  (a) Pre-Glacial Relief as it Probably was; (b) Ice-Dammed Lakes Formed Between the Uplands and the Ice, and Lake Pickering Developed. The Northern Lakes Drained into Lake Pickering, Which Drained South; (C) Post-Glacial Relief.

Kirkham Gorge

Vale of Pickering

Glacial Processes  6.19

the head of its valley or on its valley side, and the water escapes through an overflow channel into a neighbouring valley. Evidence suggests that a similar situation developed in the North York Moors during the melting of the ice sheets in Great Britain. The glacial lakes of Yorkshire: The Cleveland Hills and the Yorkshire Moors formed an obstruction to the southward-moving ice as shown in Figure 6.24(a). The ice appears to have worked its way southwards through the Vale of York to the west and across the North Sea to the east. The rivers draining northward from the largely ice-free hills and moors were dammed by the ice along the northern edge of the uplands, and a series of lakes were formed. It also appears that the ice blocked the eastern end of the Vale of Pickering, thus causing a large lake to form in that vale. As the water in the northern lake rose, it began to escape southward across low cols in the York Moors. The escaping water carved out overflow channels or spillways (Figure 6.24(b)). The best developed feature of these is Newton Dale which carried water into Lake Pickering. This dale was probably a pre-glacial river valley. Newton Dale is shown in Figure 6.25. A similar spillway was developed by water moving southward from Lake Pickering. This is called Kirkham Gorge (Figure 6.24(c)). Similar types of ice-dammed lakes and associated spillways were developed in Scotland.

Examples of Glaciated Landscapes The best examples of glaciated highlands occur in mountain ranges such as the Alps, the Rockies, the Andes, and the Himalayas. All these regions contain active glaciers where glacial features in the making can be seen. The best examples of glaciated lowlands occur in regions such as the lowlands around Hudson Bay in North America and the lowlands of northern Europe. Figure  6.26 shows the lowland area around the Baltic Sea. The lowlands experienced many southward advances of the continental ice sheets during the Pleistocene period. The melting of the ice sheets has left behind many landforms common to lowland glaciation. Numerous lakes and eskers occur in southern Finland, while drumlins are very common in southern Sweden. Terminal moraines extend from west to east across the north European lowlands. Outwash plains stretch southward from the terminal moraine. Similarly, the Imja Glacier (Figures 6.27(a), (b)) has shown glaring impacts of climate change in the Himalaya. The gigantic tongue of m ­ illennia-old glacial ice would be reduced to a lake within the next 50 years. The series of pictures taken by the American mountain geographer Alton Byers reveals not only the dramatic reduction in glacial ice in the Himalayas but also the effects of climate change on the people who live there. Today, the Imja Glacier, which is just 6 km from Mount Everest, continues to recede at a rate of 74 m a year – the fastest rate of all the Himalayan glaciers. A major UN Environment Programme report warned that at the current rates of global warming, the Himalayan glaciers could shrink from 500,000 sq. km to 100,000 sq. km by the 2030s. Imja is one of 27 glacial lakes in Nepal that are classified as potentially dangerous. If the moraines, which dam the lake are breached, thousands of lives in the most densely populated Sherpa valley in Nepal are at risk from flooding and landslides. We are aware about the coldest and driest continent Antarctica. It contains about 90 per cent of the total ice of the world. It provides the largest and the most beautiful ice sheet covering an area of 14,000,000 sq. km (77,000 sq. km not excluding the main ice sheet). The ice sheet in Antarctica is found at 3,000 m above the sea level.

6.20  Chapter 6

Devensian Ice Margin

Hartlepool

DURHAM

TEES VALLEY Middlesbrough s

e Te

Northellerton

ale

Sw

Ur

e

Thirsk

Guisborough

Whitby

N o r t h Y o r k Goathland Osmotherley M o o r s Scarborough

NORTH Pickering

Helmsley

Filey

YORKSHIRE

en

t

Malton

De

rw

Figure 6.25  A Map of the North York Moors Showing the Limits of the Devensian Glaciation.

um

lins

Direction of Ice Flow

Lakes, Eskers

Dr

a

ltic

Se

Ba

m Ter

ora lM ina

s ine

Outwash Plain 0 HIGHLAND

320 km

GLACIATION

Figure 6.26  A Map to Show the Region of Lowland Glaciation Around the Baltic Sea.

EAST RIDING OF YORKSHIRE

The average thickness of ice in the Antarctica region is 2 km. The whole region is divided into West and East Antarctica (Figure  6.28(a)). The East Antarctica region, also known as Great Antarctica, covers an area of about 66 per cent of the total area, whereas the remaining 33 per cent area is West Antarctica. The Lambert Glacier, the largest in the world, is situated in the East Antarctic ice sheet. It is formed in the South Pole. The other major glacier is Totten Glacier, which is formed on the Australian Antarctica territory. The West Antarctic ice sheet covers the Western Antarctic region and is situated on the Transantarctic Mountain (Figure  6.28(b)). It is a marine-based ice sheet and bounded by the Ross Ice Shelf, Ronne Ice Shelf, and Outlet glacier. Climate change is strongly affecting Antarctica too. Around the Antarctic Peninsula, temperatures are warming at a rate that is approximately six times the global average. Air temperatures increased by ~2.5°C from 1950 to 2000. Rapid warming in this region began in the 1930s. The annual mean air temperature −9°C isotherm has moved southwards, resulting in ice-shelf collapse and glacier recession. A recent ice core from James Ross Island shows that warming in this region began around 600 years ago and then accelerated over

Glacial Processes  6.21

(a)

(b)

Figure 6.27  (a) Visible Glaciated Cover in the Lmja Glacier in 2003, Nepal Himalayas; (b) Receding Glaciated Cover in the Lmja Glacier in 2018, Nepal Himalayas.

0° 70°

B

S

Antarctic Peninsula 80°

Ronne Ice Shelf

3 4

90° W

3

Ross Ice Shelf

ins nta ou M

2

Vostok

tic

Byrd

90° E

Tra ns an ta rc

2 1 Ice sheet Ice shelt Ice-free land Contour interval: 1 km

180° 0

Figure 6.28  (a) and (b) The Antarctica Glacier Region and its Ice Sheets. (Continued )

1000 km

6.22  Chapter 6

G HAKON VII H AV KON

T AN

AR

CT IC

WEDDELL SEN

PE NI SS UL A

DRONNING MAUD LAND

East Antarctic Ice Sheet

TP AN ELLSWORKTH LAND S

ROSS ICE SHELF

E OC

ROSS SEA

S AN LAND NT ORIA OU CT M VI

RN HE UT SO

AN

Figure 6.28  (Continued ) (a) and (b) The Antarctica Glacier Region and its Ice Sheets.

MARIE BYRD LAND

LAND PRINCESS ELIZABETH LAND WILHELMI LAND

AN TA R IC CT

West Antarctic Ice Sheet AMUNDSEN SEN

KEMP LAND

COATS LAND RONNE ICE SHELF

ENDERBY LAND

QUEEN MARY LAND WILKES LAND TEPRE ADELIE

QATES LAND

the last century. Higher air temperatures around the Antarctic Peninsula contribute to ice-shelf collapse by increasing the amount of meltwater ponding on the surface. When combined with ice shelves that are thinning due to melting from below following the incursion of warm ocean currents onto the continental shelf, you have a recipe for rapid ice-shelf disintegration. Further, Figure 6.28 illustrates the ice shelves around Pine Island Glacier that are thinning and receding. The thinning of these ice shelves may limit their ability to buttress the flow of ice from the interior of the ice sheet. There is increasing evidence that glaciers around the Antarctic Peninsula are shrinking and receding. Alison Cook found that 87 per cent of the glaciers around the Antarctic Peninsula are receding. Other researchers have found evidence of glacier recession and a measurable sea level contribution. There is evidence of widespread glacier recession around the northern Antarctic Peninsula. Land-terminating glaciers in this region are shrinking particularly rapidly, which is significant, as their mass balance is more directly controlled by temperature and precipitation when compared with marine-terminating glaciers, which respond non-linearly to climate forcing. The Himalayan glacier region is another important region as it is the source of the major rivers flowing in the Indian sub-­continent. Some of the glaciers here are Gangotri, Dokriani, and Kolhai. The false colour image shown in Figure 6.30 illustrates one of the largest glacier in the region. Snout of the Gangotri Glacier emerges from Gomukh at an elevation of 4,000 m. The depth of the glacier is about 200 m. Gangotri has been receding since 1780, although studies show that its retreat quickened after 1971. It should be noted that the blue contour lines drawn here to show the recession of the glacier’s terminus over time are approximate. According to NASA, Gangotri Glacier is retreating, with a recession rate of 76 m/year from 1996 to 1999 alone.

Glacial Processes

Q

E J

FI

V

6.23

N 17E

B 23E

R

26E

U

S BR

AP

A

F

Ronne P

Fig.4 BS

W

EAIS

WAIS

SH C

Bathymetery –450m

T

AS Ross –725m Fig.3 Ice-shelf ΔT/Δt 3 m yr–1 Grounded icesheet loss(Gt yr–1)

D CS

1

–3 m yr–1 –7 m yr–1

100

0

MU

500

B

N AV BM RE

H Temperature

DB CO

NI M 152E

1,000

1.0 °C 0.0 °C –1.0 °C –2.5 °C

km

FIgure 6.29 Pine Island Glacier.

1780 1935 1956 1964 1971 2001

tri

o ng Ga ier

ac Gl

Scale (km) 0

0.5

1

FIgure 6.30 Retreat of Gangotri Glacier, Himalayas.

6.24  Chapter 6

Rate of Retreat (in mt /year)

24

Rate of Retreat (in mt / year)

18

12

6

5) 01 –2 91 19

Years Figure 6.31  Rate of Gangotri Glacier Retreat.

6

(2

0) 19

71

–1

99

1

(2

6) 19

35

–1

97

1

(3

6) (4 5 93 –1 89

18

18

17

–1

88

9

(7

2)

0

Figure 6.31 illustrates the retreat of Gangotri Glacier. Earlier, the Gangotri Glacier appeared as a convex-shaped structure from atop Tapovan, the meadow at the base of Shivling Peak beyond Gomukh, but presently, the glacier appears to be caving in and is concave in shape, thus highlighting the retreat. The retreat points to lesser ice formation each year than its current rate of melting, and according to scientists at the National Institute of Hydrology, Roorkee, this process is continuing. So, the point to be noted is that the Earth’s climate is becoming warmer, and the signs of this change are everywhere, with the most important threat being warming of the glacial environment. When temperature rises and ice melts, more water flows to the seas from glaciers and ice caps. The oceanic water warms and expands in volume. It results in the loss of sea-based and land-based ice. As we know, glaciers have 80–90 per cent albedo. Another dimension of concern is Glacial Lake Outburst Floods (GLOFs). These are catastrophic overflow of water resulting primarily from melting of glaciers. In recent times, retreat of the glaciers has led to an enlargement of several glacial lakes. A GLOF is characterized by a sudden release of a huge amount of lake water that rushes along the stream channel downstream in the form of dangerous flood waves. These flood waves comprise water mixed with morainic materials and have devastating consequences for riparian communities, hydropower stations, and other infrastructures. With the retreat of glaciers, there will be less glaciated area for reflectivity. The result will be increase in global temperature. This has further resulted in the loss of habitat for animals as well as the effect of increasing sea level on low-lying regions of the world. This combination of effects has played the major role in raising the average global sea level between 4 and 8 in. (10 and 20 cm) in the past 100 years, according to the Intergovernmental Panel on Climate Change (IPCC, 2001). Increasing ultraviolet (UV) radiation due to the ozone hole also cause changes in phytoplankton communities and could have effects on the food chain.

Glacial Processes  6.25

Economic Value of Glaciated Landscapes Glacial landforms of specific value Glacial landforms of specific value are mentioned below in the list. ●● ●●

●● ●●

●● ●●

●●

●●

Boulder clay plains are sometimes very fertile, e.g., East Anglia in Great Britain and parts of the Dairy Belt of North America. Old glacial lakebeds are invariably fertile. Extensive areas of the Canadian Prairies producing vast amounts of wheat each year owe their prosperity to the rich alluvium, which once collected on the floors of glacial lakes. Some glacial lakes, e.g., the Great Lakes of North America, are of real value as natural routeways. Waterfalls issuing from hanging valleys are sometimes suitable for the development of hydroelectric power (HEP). Both Norway and Switzerland develop large amounts of HEP from such waterfalls. Glaciated mountain regions attract tourists, especially during the winter season when heavy snowfalls make skiing and other sports possible. Some glacial lakes have cut deep overflow channels where they have drained away. Some of these channels today form excellent routeways across a ­difficult country, e.g., the Hudson–Mohawk Gap, which leads down to New York. Many glaciated valleys have benches or ‘alps’ high up on their sides. During the summer, these ‘alps’ have good pasture, but during the winter, they are covered with snow. Cattle are grazed on the alpine pastures during the summer and are brought down to the sheltered valley bottom pastures during the winter. This movement of animals is called transhumance, and it is still being continued in Switzerland, Norway, and other mountainous countries. The glacial fringelands outside the ice-covered regions are dry, and during the Pleistocene period, outblowing winds carried fine sediment and deposited it in thick layers. This material is called loess. The reddish earth in East Anglia is loess. It was deposited in the Pleistocene period. Similar loess deposits occur in northern Europe and northern America.

Glacial Landforms of Little Value 1. Boulder clay deposits in some regions, e.g., Central Ireland, have produced a marshy landscape, which is of little or no value to agriculture. 2. Many outwash plains contain infertile sands, which form extensive areas of waste land. It is true that this is sometimes of recreational value, but from an agricultural standpoint, such regions are uncultivable. 3. Extensive areas of land are sometimes turned into myriads of lakes by morainic deposits. Such lake landscapes offer little scope for agricultural development.

6.26  Chapter 6

Melting Permafrost The lands surrounding the ice sheets in the northern hemisphere are almost permanently frozen. These lands are mainly in the northern lowlands of Canada and Russia, and they are called the tundra lands. The temperature from the surface to a few metres below it never rises above 0°C, and because of this, this top layer of the ground is perpetually frozen. It is called the permafrost. It is estimated that almost 20 per cent of the world’s land surface has permafrost. Most of this land is in north of the Arctic Circle. Permafrost covers a quarter of the entire northern hemiWinter Summer sphere, approximately 25  per cent of the land area. When Pool or Marsh the earth remains frozen for at least two consecutive years, it is termed as permafrost. In other words, any type of ground Permafrost ‘from soil to sediment to rock including ice or organic material’ that has been frozen continuously for a minimum of two Parent Rock years or for a longer period (as many as hundreds of thousands of years). It is formed when a puddle of water freezes during frigid winter nights and water that is trapped in sediFigure 6.32  Permafrost in Winter and Summer. ment, soil, and the cracks, crevices, and pores of rocks turns to ice when ground temperatures drop below 32°F (0°C). Permafrost can extend downward below the earth’s surface from a few feet to more than a kilometre that layer entire regions, e.g., Arctic tundra, or a single, isolated spot, e.g., a mountain top or alpine permafrost. If the ground freezes and thaws every year, it is considered ‘seasonally frozen.’ As mentioned below one-fourth of the entire northern hemisphere that is 23  million km² of the land area is under permafrost. It is found in Alaska (approximately 85 per cent of the state sits over a layer), Greenland, Canada, and Arctic region of Siberia. It is also found on the Tibetan plateau, in high altitude regions, for example the Rocky Mountains. In the southern hemisphere, it is found in mountainous region including New Zealand’s Southern Alps and South American Andes. It is also found below Antarctica. As per estimated geographic continuity in the landscape, the lowland permafrost regions are categorized in many zones but a typical classification recognizes three categories as depicted in Table 6.2 and Figure 6.33. The variation in the thickness of permafrost is slightly less than 1 m to greater than 1500 m. Most of the present permafrost has been formed during cold glacial periods. Climatic changes are visible in permafrost region. Scientists have observed that due to global warming, temperature is rising everywhere and Arctic region is warming twice fast as comparison with any other region. Therefore, due to rise in surface temperature ground temperature is also rising and resulting in thawing of permafrost and shrinking the footprint in these region. A recent study has estimated that with



Table 6.2 SR. NO.

Typical Classification of lowland permafrost. ZONES

LANDSCAPE (PER CENT)

1.

Continuous permafrost

90–100

2.

Discontinuous permafrost

50–90

3.

Sporadic permafrost

0–50

Glacial Processes  6.27

Continuous permafrost Discontinuous permafrost Sporadic permafrost Isolated patches

FIGURE 6.33  Permafrost Distribution.

every additional 1°C warming there is a disappearance of additional 1.5 m ­ illion km2 permafrost. Another study led by NASA funded scientists using NASA hyperspectral satellite images have found that even plant productivity is changing as permafrost thaws in different plant communities in the northernmost part of the United States. Current probability of near-surface permafrost in Alaska is depicted in Figure 6.34. Scientists have observed innumerable impacts of permafrost thaw like the loss of greenhouse gas stores with carbon and methane trapped in permafrost. A  recent study found that Arctic permafrost is a massive repository of natural mercury which is estimated to be 15 ­million gallons, nearly twice the amount of mercury found in the ocean, atmosphere, and all other soils combined. It is serious because this mercury is a potent neurotoxin and once released as per a scientific study, can spread through water or air into ecosystems and potentially even food supplies. Also, some studies have noticed that bacteria and viruses can lie dormant for thousands of years in

6.28  Chapter 6

FAIRBANKS

100 kilometers

FIGURE 6.34  Current Probability of Near-surface Permafrost in Alaska.

permafrost’s cold, dark confines before waking up when the ground warms. Similar studies relate 2016 anthrax outbreak in Siberia linked to a decades-old reindeer carcass infected with the bacteria when exposed to thawed permafrost. Further, it is feared that mining of precious metals and petroleum in the Arctic region is exposing human population to thawed, ancient, and possibly ‘zombie’ pathogens. At places, even infrastructures in the frozen regions are crumbling and landscapes are getting altered. In Alaska, one study puts ‘the cost of repairing public infrastructure such as roads, train lines, buildings, and airports damaged by thawing permafrost and other climate-related factors at as much as $5.5 billion by the end of this century.’ Further, thawing permafrost may create thermokarsts, areas of sagging ground, and shallow ponds that are often characterized by ‘drunken forests’ of askew trees making soil vulnerable to landslides and erosion, particularly along coasts. As this softened soil erodes, it can introduce new sediment to waterways, which may alter the flow of rivers and streams, degrade water quality (including by the introduction of carbon), and impact aquatic wildlife. The discovery and extraction of oil in Alaska has led to many problems. When the oil deposits were first being worked, the pipelines carrying the warm oil melted the permafrost, and many of them buckled and ruptured. In a similar way, newly constructed roads cracked and buckled and houses subsided, as shown in Figure 6.35. This diagram shows the precautions to be taken to prevent the permafrost from melting. Further problems arose in the construction of pipelines to carry the oil from the oil wells to the port of Valdez on the Gulf of Alaska. This port is ice-free throughout the year because of the North Pacific Warm Current. It was soon discovered that the

Glacial Processes  6.29

(a)

(b)

Insulated Stilts

Gravel Bank Provides Insulation Problem

Solution

Problem

Solution

FIGURE 6.35  Some of the Problems Caused by Permafrost to Develop Buildings and Roads, and How these Problems can be Solved.

pipelines had to be insulated and in some places had to be carried well above the ground to cross rivers and to allow free passage to herds of wild animals such as the elk, which wander extensively in search of food. Glacial management is incomplete without climate change mitigation programmes. Reducing the emission of green house gases (GHGs), which includes mainly carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and fluorinated gases, is the most important strategy for managing glacial environment. Therefore, inter-governmental cooperation and information exchange on glacial retreat at the global level is needed for managing the glacial environment.

Key facts ●● ●● ●● ●● ●● ●● ●● ●● ●●

●●

●●

The level above which there is a p ­ ermanent covering of snow is called the snow line. The steady accumulation of snow produces granular ice called ‘Névé’ or firn. As more snow accumulates, the neve compacts to form glacier ice. Ice masses are classified into glaciers, e.g., cirque, valley, and piedmont, and into ice caps (ice sheets). The glacial system refers to inputs (accumulation, storage (the ice mass), and outputs (ablation)). Accumulation refers to an increase in the ice mass; ablation refers to a decrease in the ice mass. Crevasses, namely transverse, longitudinal, and marginal crevasses, are caused by stresses within the ice. Rock debris that accumulates on and in the ice is called moraine. Types of moraine are lateral, medial, terminal, englacial, and subglacial. Abrasion and plucking are the two main processes of glacial erosion. The main landforms produced by glacial erosion are glacial troughs, hanging valleys, truncated spurs, aretes, pyramidal peaks (also partly caused by frost action), and cirques. The main landforms produced by glacial deposition are boulder clay, drumlins, and moraines (unsorted and unstratified), outwash deposits (well sorted and stratified), and eskers and kames (partially sorted and stratified). Ice-dammed lakes sometimes overflow and cut overflow channels or spillways.

6.30  Chapter 6

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. Direction for questions from 1 to 6: Each question has one or more correct option/s. Identify which of the options are correct and select the answer as per following. For each of the questions 1 to 6, one or more of the responses given is/are correct. Decide which of the responses is/are correct and then choose. (a) if 1 only is correct. (b) if 1 and 2 only are correct. (c) if 2 and 3 only are correct. (d) if 1, 2, and 3 are all correct. (e) if 3 only is correct. 1. The glacial processes of sapping and plucking play an important part in the formation of a (a) rôche moutonnée. (b) glacial trough. (c) cirque. 2. Arêtes and pyramidal peaks are the products mainly of (a) frost action. (b) glacial abrasion. (c) cirque development. 3. Glacial deposits which are not sorted and not stratified are often called boulder clay. Examples of these deposits are (a) drumlins. (b) moraines. (c) kames. 4. Ablation refers to glacial processes which result in (a) an increase in a mass of ice. (b) the evaporation of ice. (c) a decrease in a mass of ice. 5. The best areas of Great Britain to examine for evidence of past glacial activity are (a) coastal regions. (b) high mountains. (c) northeast Yorkshire. 6. An erratic refers to a large stone or boulder which owes its present position to (a) plucking. (b) sapping. (c) glacial transport. 7. Crevasses, which are cracks on the surface of a glacier, are caused by (a) the melting of ice in the glacier. (b) the thickness of the ice. (c) the action of rain on the surface of the glacier. (d) boulders falling on to the surface of the glacier. (e) differential rates of movement of the ice in the glacier. 8. Which one of the following best accounts for the origin and shape of a cirque in a glaciated highland? (a) Accumulation of ice at the head of a valley (b) Movement of a glacier downhill (c) Freeze-thaw action resulting in sapping and plucking (d) Melting of ice in ice-scoured hollows (e) Frost action. 9. Which one of the following features occurs in a glaciated lowland region? (a) Arête (b) Pyramidal peak (c) Cirque (d) Esker (e) Hanging valley

Glacial Processes  6.31

ii, iv, and v only

10. Which one of the following features is evidence that ice can move upslope? (a) Rock step (b) Alp (c) Ice fall (d) Hanging valley (e) Lip of a cirque 11. “A winding low ridge composed of gravel, sand, and clay, often several kilometres long, stretches across a poorly drained lowland dotted with lakes.” This is a description of a feature called (a) a terminal moraine. (b) an outwash plain. (c) an esker. (d) a medial moraine. (e) a kame. 12. Which of the following combinations are characteristics of a rôche moutonnée? (i) Boulder clay tail (ii) Jagged downstream slope (iii) Composed of sand and gravel (iv) Made of rock different from that of its surroundings (v) Striations on the upstream slope (a) (i) and (iii) only (b) (i) and (iv) only (c) (i), (ii), and (v) only (e) (ii), (iv), and (v)only (d) (ii) and (v) only 13. Which one of the following features is formed by, and is evidence of, the smoothing and plucking action of ice? (a) Drumlin (b) Kame (c) Rôche moutonnée (e) Moraine (d) Esker 14. Which one of the following features is not produced by glacial erosion? (a) Arête (b) Rock basin (c) pyramidal peak (d) Esker 15. Of the several features produced by the action of ice, some are the products of both erosion and deposition. Which one of the following features is of this type? (a) Arete (b) Hanging valley (c) Erratic (d) Crag and tail (e) Alp

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Using well-labelled diagrams, explain the main differences between three of the following pairs of features. (a) Truncated spurs and interlocking spurs. (b) Eskers and kames. (c) Crevasses and ice falls. (d) Crag and tails, and rôche moutonnées. (e) Study the features marked A and B in Figure 6.35. Name these features and give a brief account of the formation of each. (f) Figure 6.36 shows a lowland area covered with ice. Name and briefly account for the features that would be found at ‘X’ and ‘Y’ after the ice has melted.

6.32  Chapter 6

B

600 580 560 540 500

640 620

660

680

660

700 680

640

620

600

580 560 540 520 500

Lakes

A 0

1 km

FIGURE 6.35

Ice

Ice

River

Z

600 400

800

Land

Y

1200

1300 120 0

Deposition X

800

ce

Ice Blocks

1000 900

of I

0 110 0 100 900

ge

X

70 6000 500 400

Ed

Y

FIGURE 6.36  Lowland Area Covered with Ice.

Figure 6.37  Contour Map of Part of a U-shaped Valley.

Glacial Processes  6.33

2. The diagram in Figure 6.37 is a contour map of part of a U-shaped valley and the surrounding landscape. (a) Name the features, which occur at the points “X,” “Y,” and “Z.” (b) Briefly explain the processes which lead to the formation of each of the features at these three points. 3. The following features often occur in glaciated regions: cirque (corrie), moraine, hanging valley, pyramidal peak, arete. Choose three of these features and for each, (a) briefly explain how it may have originated; (b) show its appearance by means of a well-labelled diagram; (c) name one region where an example may be found. 4. With the aid of diagrams, describe three of the following and explain how they may have been formed: esker, drumlin, terminal moraine, crag and tail, rôche moutonnée. (a) Outline the main differences between continental ice sheets and valley glaciers. (b) Briefly explain the main differences between ice action in mountain regions and ice action in lowland regions and name the characteristic physical features produced in each region. 5. Briefly explain three of the following: (a) The sides of a valley glacier move more slowly than its middle; (b) Boulder clay differs from an outwash plain in that its material is unsorted; (c) Glaciated valleys are distinctly U-shaped; (d) Floor of a corrie is usually concave; (e) Tributary valleys usually join a glaciated valley high above the floor of the glaciated valley. 6. Choose two of the following landforms: a rift valley, an atoll, a glaciated valley, a fiord, and for each of the two chosen: (a) describe its characteristic features; (b) briefly explain how it has originated; (c) show the landform by using an annotated diagram; (d) name one region where the landform may be located.

Answer key Exercise 1 1.   (e) 6.   (e) 11.   (c)

2. (e) 7. (e) 12. (d)

3. (b) 8. (a) 13. (c)

4. (c) 9. (d) 14. (c)

5. (c) 10. (a) 15. (c)

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7

Desert Processes

Learning Outcomes After completing this chapter, you will be able to: ●● ●● ●● ●●

Understand the features of deserts and involved processes Discuss the significant characteristics and extent of deserts of the world Explain different landforms created by various desert processes Identify features produced by water in deserts region

Keywords Desert, Abrasion, Deflation, Saltation, Wind Erosion, Inland Drainage, Desertification

1

7.2  Chapter 7

INTRODUCTION One-third land surface area of the earth is covered by ­deserts. They are unique and extremely diverse. The dunes of the Saharan Desert, the Pacific coastline of the Atacama and the icy tundra of Antarctica are all scientifically classified as deserts despite their differences. However, they all receive less than 10 in. or 250 mm of rainfall a year and this makes them a desert. Figure 7.1 visualizes on how the general atmospheric circulation affects desert processes and brings dry, subtropical air into mid latitudes. It is to be noted that the Sahara desert extends from the Atlantic to the Red Sea, a distance of almost 5000 km, and covers an area of about 7.5 m ­ illion km2 (about one-quarter the size of Africa); it is the largest tropical desert in the world. There are two other deserts in Africa, both south of the equator—the Kalahari Desert, which is semi-arid and which has an area of about 500,000 km2, and the Namib Desert, which is arid.

Descending Cool, Dry Air

North Pole High Polar Easterlies lar fron t Po

Rising Warm, Moist Air s

terlie

Wes

Descending Cool, Dry Air

Subtropical High Pressure High

High

Northeast Trades Equator

Equatorial Low pressure Southeast Trades

Southeast Trades Subtropical High High Pressure

High Descending Cool, Dry Air

Pola ront r F polar Low Easterlies

Rising Warm, Moist Air

High FIGURE 7.1  Effects Atmospheric Circulation on Desert Processes.

South Pole After Marker et al. (1997)

Descending Cool, Dry Air

Desert Processes  7.3

Why Deserts Occur Aridity, which is caused by low rainfall and high evaporation, is the dominant characteristic of a desert. Arid deserts rarely receive more than an average rainfall of 250 mm a year, but semi-arid deserts, such as the Kalahari, receive at least 500 mm of rainfall a year. The rock structure of a desert has nothing to do with its being a desert.

Desert Locations The largest arid and semi-arid deserts occur between 15°N and 45°N, and 15°S and 30°S, and most of these are located on the western sides of the continents, in the trade wind belt where the winds are offshore, as shown in Figure 7.2 Such winds are dry, having lost most of their moisture in their journey across the eastern sides of the continents. Onshore local winds do blow across the west coasts but they rarely bring rain. This is because in crossing the cool ocean currents that parallel the west coasts of these latitudes, the wind moisture condenses into mist, or fog, or light rain before the winds reach the coasts. The Sahara Desert extends from the Atlantic to the Red Sea, and the desert continues eastwards as the Arabian Desert and into south-west Asia as the Iranian and Thar Deserts. Deserts occur in southwest Asia because dry, land winds blow over them for most of the year. The locations of the main trade wind deserts are shown in Figure 7.2. A second type of desert occurs in the continental interiors of North America and Asia, between 30°N and 45°N. They are far from the nearest ocean which

Arctic Circle

Turkestan 45˚N

30˚N

Californian Current

Gobi

Great Basin Canaries Current

Mohave

45˚N

30˚N

Thar

Tropic of Cancer 15˚N

15˚N Sahara

Iranian

Equator

Arabian

Atacama

15˚S

15˚S

Kalahari

Australian

Tropic of Capricorn 30˚S 45˚S

Namib Benguela Current

Peruvian Current

30˚S

West Australian Current

45˚S

Patagonian Hot Desert

Temperate Desert

Cold Current

FIGURE 7.2  Map Showing Important Ocean Currents Which Influence the Climate of Some Desert Areas.

7.4  Chapter 7

Some deserts are increasing in area. It is estimated that the world’s deserts increase in area by 5,000,000 ha. each year. The Sahara Desert is steadily extending southwards and it has been doing so for a very long time. As deserts advance, they sometimes overrun settlements, often completely burying them with vast sheets of sand.



Table 7.1

explains, in part, why they are deserts. Some of these deserts are located in basins, or depressions, and are surrounded, or nearly so, by mountains or highland. The Gobi Desert and the deserts of the Tarim Basin and Dzungaria, all in central Asia, are in part surrounded by highlands, and they are in the rain shadow of these. The Great Basin of Nevada, in the United States, is separated from the Pacific by the Sierra Nevada and from the eastern lowlands by the Rockies. Another characteristic feature of these deserts is their seasonal climate—very hot summers and very cold winters. The only remaining mid-latitude desert is the Patagonian Desert in southern Argentina. It lies in the rain shadow of the Andes with the prevailing winds blowing from the west. Also, a cool offshore current prevents local onshore winds from bringing rain. Whereas Table 7.1 highlights the significant characteristics of s­ ubtropical deserts of the world, Tables 7.2–7.4 illustrate the major characteristics of coastal deserts, cold deserts, and polar deserts, respectively.

Significant characteristics of subtropical deserts of the world.

DESERTS

LOCATION Most of northeastern Africa which is about 10% of the continent.

Sahara Deserts

Arabian Desert

It extends from the Atlantic Ocean in the west to the Mediterranean in the north to the Red Sea in the east.

Third largest desert after Antarctica and the Arctic. The average temperature in the Sahara is a comfortable 86°F (29°C) but can reach as high as 120°F (49°C) in hottest months of the year.

There are 10 countries in Africa that have some part of their nation in the Sahara Desert.

It has some of the largest sand dunes in the world.

It makes up nearly all of the Arabian Peninsula.

Earth’s largest area of unbroken sand called Rub’al-Khali found in its centre.

The Arabian Desert borders the Nile River Valley on its west side and the Gulf of Suez on the eastern end.

The remaining Part of Desert is made up of gravel plains and rocky hills.

The countries include of Qatar, Yemen, Oman, United Arab Emirates, Kuwait, and Saudi Arabia.

Kalahari Desert

CHARACTERISTICS

It covers most of the African nations of Namibia, Botswana, and portions of South Africa.

The terrain of Sahara is actually comprised of sand dunes.

It includes natural occurrences of the notorious and lethal “quicksand.”

It receives nearly double the rainfall (5–9 in. annually) of the Arabian and Sahara desert, yet portions of the Kalahari still meet the definition of a desert in terms of rainfall. Central Kalahari Game Reserve (the second largest protected area for wildlife in the world) is located here.

Desert Processes  7.5

DESERTS

Mojave Desert

LOCATION

The southwestern United States covering portions of southern Nevada and southeastern California.

CHARACTERISTICS It contains the infamous Death Valley, known for being the lowest point, driest location, and hottest area on the North American continent. It is characterized by extreme heat and cold. The highest temperature recorded is 134°F (57°C) and lowest is 15°F (–10°C), here.

Sonoran Desert

It is located in the southwestern part of the United States reaching into northwestern Mexico.

It is the hottest desert in the North American continent with air temperatures consistently rising above 118°F (48°C). It is the only place in the world where a naturally occurring Saguaro Cacti is found, which can live for over 200 years. It accounts for approximately 1.5% of the land area on the North American continent.

Chihuahuan Desert

Thar Desert

Gibson Desert

It reaches into Texas, Arizona, and New Mexico in the United States and Chihuahua, Durango, Coahuila, Zacatecas, and Nuevo Leon in Mexico.

It is known as the Great Indian Desert and lies in the northwestern part of India—predominantly in the Royal Rajasthan states. It is in the central part of Western Australia.

Great Sandy Desert

It is located in the northern section of Western Australia, just south of the coastline.

Great Victoria Desert

The Great Victorian Desert is surrounded by the Gibson Desert to the North, the Little Sandy Desert to the northwest, the Simpson and Sturt Stony Desert to the east, and plains to the south.

It is the largest desert in the North American continent. It is considered a rain shadow desert. This area is renowned for its high diversity of cacti with over 300 species found in within its boundaries. It is the seventh largest subtropical desert and the most sparsely populated region in India. The desert is uniquely home to five salt water lakes that mark the landscape. People of this region are part of the aboriginal tribes of Australia. The Great Sandy Desert is home to the Kata Tjuta—Uluru, otherwise known as Ayers – a world heritage site. Desert Bloodwood—a mid-size tree that has blood-like sap and leathery leaves is found here. It is the third largest subtropical desert in the world after the Sahara and Arabian. It one of the least populated areas in the southwestern part of Australia.

7.6  Chapter 7

DESERTS





LOCATION

CHARACTERISTICS It is rich in natural Resources, heavily used by the mining industry.

Tanami Desert

It is located in the northern territory in the central part of Australia.

Sturt Stony Desert

It is located in the northeastern part of South Australia, surrounded by the Simpson Desert to its west, the Strzelecki Desert to the southeast, and the Tirari Desert to the southwest.

It is characterized by its “gibber desert.” A gibber desert is one that is covered by a crowded surface of stones with shiny tops.

Simpson Desert

It is located in central Australia in the eastern half of the continent.

The fat-tailed marsupial mouse is extremely rare and largely found in this area of Australia.

Table 7.2

Significant characteristics of coastal desert.

DESERTS

LOCATION

Namib Desert

It is located in the southern part of Africa.

Atacama Desert

It is located on the continent of South America and is largely inside the country of Chile.

Table 7.3

It is home to many endangered plants and wildlife such as the Wallaby and Mulgara.

CHARACTERISTICS It has had a similar arid climate for more than 50 million years. The Namib features gravel plains, mountains, and sand dunes. Its unique geographical profile creates this arid environment from the rain shadow effect. The Andes mountains block moisture on one side and the Chilean Coast range of mountains block it on the other.

Significant characteristics of cold desert.

DESERTS

Great Basin Desert

Colorado Plateau Desert

LOCATION It covers much of the southwestern USA, it borders the Sierra Nevada Range on the west and the Rocky Mountains to the east.

It is located in southwestern part of United States and covered 337,000 km2.

CHARACTERISTICS It is a result of the great mountains that surround it, which cool the air and strip it of its moisture. It is a cold desert mainly as a result of its elevation and latitude. Its 7–12 in. of annual precipitation primarily fall as snow—even in the warm summers. The plateau is a geologic enigma, remaining intact while punctuated by high mountain ranges and cut by low river canyons. The most famous feature of the plateau is the Grand Canyon, which has had a large hand in shaping the landscape of the “Red Rock Country” region.

Desert Processes  7.7

DESERTS

Patagonian Desert

LOCATION

It spans much of the country of Argentina with small parts residing in Chile.

CHARACTERISTICS It is truly a cold winter desert experiencing about 7 months of winter and with temperatures that rarely exceed 53.6°F (12°C) and average only 37°F (3°C). Much of the desert’s landscape is continuously sculpted (and re-sculpted) by the force of the wind, which also makes it the largest source of dust over the south Atlantic Ocean. The desert climate is marked by long, hot, and dry summers and unpredictable, cold winters.

Karakum Desert

Kyzylkum Desert

Taklamakan Desert

Gobi Desert

It spreads Turkmenistan in Central Asia, east of the Caspian Sea.

It is also home to the Darvaza gas crater, affectionately known to the locals as the Door to Hell, which is a natural gas deposit that has been burning continuously since it was lit by Soviet petroleum engineers in 1971.

It is Located in Central Asia between the Amu Darya and Syr Darya rivers. Kyzylkum is divided between the countries of Kazakhstan and Uzbekistan, with a small portion making an appearance in Turkmenistan (the home to the slightly larger Karakum desert).

Kyzylkum is Turkic for “red sand,” describing the slightly reddish hue of the sand that comprises the desert’s expansive sand dunes and ridges. Temperatures can be very high during the summer months, the annual precipitation is low and the winters are cold ranging from an average of 16°F to 32°F (–9°C to 0°C) in January.

It covers northwest China and is the second largest shifting sand desert with about 85 per cent of its area characterized by shifting sand dunes

Extreme lows can be recorded in winter, at times well below – 4°F (–2°C), which in part can be attributed to the frigid air masses in neighbouring Siberia.

It covers portions of northern and northwestern China and southern Mongolia.

It is the fifth largest desert in the world boasting five of its own ecoregions. And due to a process known as desertification, the Gobi continues to expand at an alarming rate. Though the Gobi is home to sand dunes that can be covered in frost or even snow and due to winds that whip across the plateau, much of its landscape is actually comprised of exposed bare rock.

7.8  Chapter 7



Table 7.4

Significant characteristics of polar deserts of the world.

DESERTS

LOCATION

CHARACTERISTICS

Arctic Desert

Arctic desert is located north of Arctic Circle. It is a polar region located in the northernmost part of Earth consisting of parts of United States (Alaska), Canada, Finland, Denmark (Greenland), Iceland, Norway, Sweden, and Russia

The region is comprised of vast ocean, seasonal ice cover, and permafrost (soil that for two or more years is consistently at or below the freezing point of water). Here the average temperature for the warmest month of the year is still below 50°F (10°C) and average winter temperatures can fall as low as –40°F –40°C), with record lows of up to –90°F (–68°C). Annual precipitation is low (less than 20 in. or 50 cm).

Antarctic Desert

Antarctica contains the geographic South Pole. It lies almost entirely south of the Antarctic Circle and is surrounded by the Southern Ocean.

Considered a desert climate, Antarctica has an annual precipitation of only 8 in. (200 mm) near the coast and much less when venturing further inland. Antarctica is extremely cold—the coldest continent on Earth—with temperatures reaching as low as –129°F (–89°C)!

Action of Winds in a Desert Features produced by either erosion or deposition occur in a desert and all of these are formed mainly by the action of weathering, wind, and water. Mechanical weathering (mainly exfoliation), is more important than chemical weathering in desert regions. Weathering by the action of frost takes place in some of the uplands in the tropical deserts and in most of the interior continental deserts, in the winter season. Dew and Temperature Wind Abrasion Develops Furrows in the Less– Although wind erosion is effective in the deserts, Changes Initiate Joint Resistant Rocks Opening transport and deposition are far more significant. The wind transports large quantities of fine particles of rock waste over considerable distances, while the coarser particles are bounced over the surface. This bouncing movement is called saltation. Wind erosion Zeugen consists of the following three processes:

Height of 3–30 m Less resistant Rock Resistant Rock Forms Block-like Ridges Called Zeugens FIGURE 7.3  Zeugens Formed by Wind Abrasion.

1. Abrasion, by which small particles of rock are hurled by the wind against rock surfaces helping to produce such features as rock pedestals, zeugens, and yardangs (Figure 7.3); Blackwelder has classified three different impacts of abrasion: (a) polishing and pitting, (b) grooving, and (c) shaping and cutting smooth surface on the windward side of the rock, also called faceting. As wind blows from different direction, polished flat surfaces c­ reate a ventifact. (Figures 7.4).

Desert Processes  7.9

FIGURE 7.4  Ventifact Formed by Wind Abrasion.

2. Deflation, by which the wind blows away rock waste and in doing so, lowers the desert surface producing depressions; Deflation transports sand only to a limited distance. Deflation concentrates the coarser grained particles at the surface, eventually resulting in a surface composed only of the coarser grained fragments that cannot be transported by the  wind. Such a surface is called desert pavement (Figure 7.5). The process of deflation leaves hollowed features in its wake. Dust storms are another evidence of wind deflation (Figure 7.6). 3. Attrition. It is the process by which rock particles rub against each other and wear away. In spite of low annual rainfall in deserts, water plays a significant part in shaping the surface features, both by erosion and by deposition, especially in desert uplands.

Deflation Deflation

Desert Pavement

FIGURE 7.5  Process of Deflation and Creation of Desert Pavement.

According to Richard Huggett, particles larger than 100 μm diameter cannot be lifted and carried unless the wind is especially strong.

7.10

Chapter 7

FIGURE 7.6 A Dust Storm Approaches Stratford Texas on 18 April 1935, 4 Days After Black Sunday. (Image: Noaa George e. Marsh album).

Wind deposition produces mounds of rock waste, which are called dunes, which are either crescent-shaped or ridge-shaped.

The combined action of wind and water produces four main types of desert landscape.

Erg or sandy desert This consists of an undulating plain of sand the surface of which is blown into ripples and sand dunes. Figure 7.7 shows a part of the sand desert of the Sahara, and Figure 7.8 shows the location of Erg Chigaga and Erg Chebbi. The Sand Sea of Egypt and Libya is a good example of an erg.

(b) (a)

FIGURE 7.7 A Part of Sand Desert, or (a) Erg in the Sahara; (b) Erg in the Chigaga.

Desert Processes  7.11

FIGURE 7.8  Location of Erg Chebbi and Erg Chigaga.

Reg or stony desert The surface is covered with boulders, angular pebbles, and gravels which have been produced by daily temperature changes. In Libya and Egypt they are called serir (Figure 7.9).

Hamada or rocky desert This consists of extensive areas of bare rocks from which all fine materials have been removed by deflation, while abrasion polishes and smooths the rock surfaces. One of the largest hamada is the Hamada el Hamra in the Sahara of Libya.

Badlands This type of desert is quite different from the three already examined in that it develops in semi-desert regions which experience sudden violent rainstorms. The land is broken by extensive gullies separated by steep-sided ridges. One of the best examples of this type of desert landscape (Figure 7.10) extends from Alberta to Arizona and into the Dakotas in the United States.

7.12  Chapter 7

FIGURE 7.9  A view of a Stony Desert, or Reg, Near to Ain Salah, in Algeria. The Reg is in the Foreground of the Photograph.

FIGURE 7.10  The Badlands of the Dakotas, in the USA; Notice how the Sides of the Hills are Deeply Gullied.

Desert Processes  7.13

Features produced by wind erosion Wind abrasion attacks rock masses and sculptures them into strange shapes. Some of these, because of their shape, are called rock pedestals.

Rock pedestal

LessResistant Rock Resistant Rock

Resistant Rock is Worm Away More Slowly

Rocky Mass Formed of Alternate Layers of Resistant and Less Resistant Rock

Abrasion is Greatest Near to Ground Level

The weaker strata in a mass of rocks are FIGURE 7.11  Formation of Rock Pedestal. shaped by wind abrasion and weathering to give tower-like structures of various Layers of Sedimentary Rock Typical ‘Mushroom’ Cap shapes (Figures 7.11 and 7.12). They are very common in the Tibesti Mountains of the central Sahara. There are also rock More pedestals of extraordinary shape in Saudi Sand Particles Resistant Carried by Arabia, as Figure 7.13 shows. Abrasion Rock the Wind

Zeugen Wind abrasion turns a desert surface which has a layer of resistant rock underlain by a layer of weak rock into a “ridge and furrow” landscape. The ridges are called zeugens and may be as high as 30 m. Eventually, they are undercut and worn away. Figure 7.14 shows how zeugens develop.

Occurs

Zone of Maximum Erosion up to 1 m

Softest Rock

FIGURE 7.12  A Rock Pedestal.

Yardang When bands of resistant and weak rocks lie parallel to the prevailing winds, wind abrasion produces another type of “ridge and furrow” landscape. The belts of resistant rock stand up as sloping ridges, varying in height from 5 m to 15 m but having lengths of up to 1,000 m. These ridges are called yardangs. Undercutting occurs due to wind abrasion (Figure 7.15). Good examples of yardangs occur near In Salah (central Algeria) and near to Kom Ombo (Egypt).

FIGURE 7.13  Rock Pedestals in the Gara Mountains in Saudi Arabia.

Deflation hollows Some hollows produced by deflation reach down to the water-­bearing rocks. When this happens, a swamp or an oasis develops. The Qattara Depression southwest of

7.14  Chapter 7

(c)

(a)

A Zeugen Varies in Height From 3 to 30 m

Dew and Temperature Changes Together Open the Joints

Zeugen Less Resistant Rock Resistant Rock FIGURE 7.14  Stages in the Resistant Development of Zeugens: Rock Forms Block(a) Weathering Opens up the Joints; Like Ridges Called (b) Wind Abrasion Continues the Work Zeugens of Weathering; (c) Wind Abrasion Slowly Lowers the Zeugens and Widens the Furrows.

(b) Wind Abrasion Develops Furrows in the Less Resistant Rocks

(a) (i)

(ii) Trough

Less Resistant Rock

Yardang

Undercut by Wind Abrasion

Direction of the Prevailing Winds Resistant Rock

(b)

Resistant Rock

Less-Resistant Rock

Undercut Side

500 to 1000 km FIGURE 7.15  Stages in the Development of Yardangs: (a [i]) Wind Abrasion Turns the Belts of Soft Rocks into Troughs; (ii) Hard Rocks are Undercut and They Stand up as Narrow Ridges Called Yardangs. (b) Shows a Sectional View of a Yardang. Undercutting Occurs Due to Wind Abrasion.

Desert Processes  7.15

Alexandria (Egypt) is more than 120 m below sea level (Figure 7.17). It has salt marshes, and the sand excavated from it forms a belt of dunes on the leeside. Some deflation hollows are probably produced in part by faulted rocks as shown in Figure 7.18.

Inselberg In some deserts, erosion has removed the entire original surface except for isolated pieces that stand up as round-topped masses called inselbergs. Some of these may be the remains of plateau edges which have been cut back by weathering followed by the removal of the weathered debris by sheet wash. Others may be the result of wind erosion, or the combined action of wind FIGURE 7.16  Major Landforms Caused by Wind and water erosion. Erosion (Compare Each of them). Prevailing Wind

Features produced by wind deposition The winds that cross desert surfaces sometimes carry vast quantities of desert dust (very fine rock particles), from one part to another part of a desert or from a desert to a neighbouring region. Very strong winds produce dust storms which result in the sudden movement of vast amounts of desert dust. Slight movements of air, called eddies, cause sand grains to move forward in a series of leaps. The process effecting this is called saltation. Saltation gives rise to gentle ripples or larger features known as dunes. Ripples occur on the slopes of dunes as well on sandy surfaces. Dunes vary in size from a few metres to over 100 m in height. There are two main types: 1. B  archans: Crescent-shaped and lying at right angles to the prevailing wind with the horns pointing downwind, a barchan usually develops from the accumulation of sand caused by a small obstruction such as a rock or piece of vegetation. As the mound grows larger, its two edges are slowly carried forward, i.e., downwind, and the typical crescent shape develops.

Sand Removed From The Depression By The Wind Is Deposited As Dunes

Depression Produced By Deflation

Mediterranean Sea Alexandria

Qattara Depression

The Great Sand Sea

Sand

Aquifer

Port Said

Cairo River Nile Prevailing NE Trades

Water Seeps Out of Aquifer And Forms Swamps or an Oasis

FIGURE 7.17  A Sectional View of the Qattara Depression and its Location in Egypt. The Qattara Depression is 122 m below Sea Level. It has Salt Marshes and the Sand Excavated from it Forms a Zone of Dunes on the Leeside. Prevailing Wind (a) (b) Sand Removed By Deflation Resistant Rock

Fault

Less Resistent Rock

FIGURE 7.18  A Depression Caused in Part by Faulting and Part by Wind Deflation: (a) Initial Depression Caused by Faulting; (b) Wind Erosion Opens up the Fault Lines and Attacks the Less Resistant Underlying Rocks.

7.16  Chapter 7

(a) Horns

Steep Concave Leeward Slope

Gentle Windward Slope

Barchan

Prevail in Eddy

g Wind

(b)

Prevailing Wind FIGURE 7.19  (A) The Structure of a Barchan. Barchans are Not Always as Clearly Defined as They are in this Diagram; (B) A Group of Barchans. Barchans Slowly Advance at the Rate of a Few Metres Per Year in the Direction of the Prevailing Wind.

The windward face of the dune is gently sloping but the leeward side is steep and slightly concave. This is caused by wind eddies set up by the prevailing wind (as shown in Figures 7.19 and 7.20). A barchan moves forward as grains of sand are carried up the windward face which then slip down the leeward side. A barchan ranges in height from a few metres to 30 m and may be as wide as 400 m. They occur singly and in groups. Formations of barchans occur in the Erg du Djourab, south of the Tibesti Mountains; and there are groups of barchans near the Djado Plateau, in northern Nigeria. The Arabian Desert has numerous barchans. Figure 7.21 shows barchans in the Namib Desert and Figure 7.22 shows isolated barchans on Mars. 2. Seifs: ridge-shaped with steep sides and lying parallel to the prevailing wind, and parallel to each other (Figure  7.23). The crest of a seif is sharp, and may be over 100-m high and 150-km long. Seifs are separated by flat corridors, 25–400 m wide, and the corridors are swept clear of sand by the prevailing wind. Eddies blow up against the sides of the dunes and it is these that drop the sand grains which

Slip face Wind direction Lip Windward slope

Horn

34˚

Sand slipage

Horn Dune movement FIGURE 7.20  Parts of a Barchan Dune.

Desert Processes  7.17

are added to the dunes. A seif usually develops from a small sand ridge, and as it forms, it slowly moves forward in the direction of the prevailing wind. Some of the best examples of seif dunes occur in the Great Sand Sea of Egypt and Libya, in Algeria near to El Oued and in the Namib Desert between Walvis Bay and Luderitz. Seifs also occur in eastern Nigeria, in the Grand Erg de Bilma.

Steep Concave leeward Slope Gental Windward Slope

Horns

Barchan

Seif is therefore formed when gentle winds collect sand to form a normal, crescent-shaped dune, which is intermittently FIGURE 7.21  Barchans in the Namib Desert. Heavy Rain has disturbed by strong winds, and the shape Slightly Changed the form of these Barchans, but the Crescent and mass of initial dune is modified. The two Shape can Still be Seen. winds take turns to develop a seif as shown in Figure 7.24. (a)

Dune

Wind-blown deposits of desert origin.

Eddies

The wind blows fine rock particles out of the deserts each year. Some are blown into the sea and some are blown on to the land where they accumulate to form loess. Loess is friable and easily eroded by rivers, which carve deep gorges into it. Caves, to serve as houses, can be cut into the sides of gorges, as shown in Figure 7.25. There are extensive loess deposits in northern China which are formed from dust

Dominant Wind Dune From a Few to Several Kilometres (b)

Seif

Eddy Prevailing Wind

FIGURE 7.22  Isolated Barchans on Mars.

FIGURE 7.23  Seif Dunes or Sand Ridges: (a) Corridors Between the Dunes are Swept Clean of Sand by the Wind; (b) the Shape of Seif Dunes.

7.18  Chapter 7

a

a'

b

b'

1

2

3

c'

a' b" 4

5

Steady Gentle Wind Sand Bearing Strong Seasonal WInd

FIGURE 7.24  Formation of Seifs

blown out of the Gobi Desert to the west as shown in Figure 7.26. Similar, deposits occur in central Europe. These were probably deposited in the last Ice Age when out-blowing winds carried fine glacial dust from the ice-sheets of northern Europe. The loess deposits of northern China form a plateau in the Hwang-Ho basin, and these have been eroded by rivers to give a “badland” landscape. The loess deposits have an area of about 600,000 km2, and a depth of 50–150 m. The loess is so soft that roads soon become sunk below the general loess surface (Figure 7.27).

Features produced by water in desert regions FIGURE 7.25  Loess Cave Dwellings in Shansi Province of Northern China. The Dwellings are Warm in the Winter and Cool in the Summer. However, they are Unstable and Even Slight Earthquakes can Cause Serious Landslides, with Consequent Loss of Life. Gobi Desert

Hw

an

In most deserts there is no rain fall for several years and then an unexpected downpour of 100–250 mm occurs. These rare but heavy rainstorms give rise to rushing torrents on steep slopes, and to sheet floods on gentle slopes. The run-off on steep slopes is usually via rills (shallow grooves) which connect with gullies. These in turn lead into deep, steep-sided valleys whose rocky

Shantung Peninsula

g-

ho

0

Tsinling Monutains Wind-deposited Loess

320 km

River-deposited Loess

Winter Winds Which Bring The Loess

FIGURE 7.26  The Great Loess Deposits in Northern China are located in and Around the Valley of the Hwang-Ho River.

FIGURE 7.27  The Loess plateau Surface in Northern China.

Desert Processes  7.19

walls rise from almost flat floors. These valleys are called wadis (in Algeria they are known as chebka). During sudden rainstorms, flood waters rush down the wadis as flash floods, carrying large quantities of materials, which soon turn them into mudflows. Figure 7.28 shows a wadi in the Somali Republic of East Africa, with the main features marked. Streams connecting with a wadi via gullies also carry a large amount of material which is deposited where they join the wadi. The deposited material is fan-shaped and it is called an alluvial fan. Figure  7.29 shows the ­location of various features which arise in the formation of a wadi. The Saharan uplands are all dissected by wadis, which are thought to have formed in a past period when rainfall was heavier than it is now.

Basin of inland drainage

Rock Wast Forming Incipient Alluvia Fan Steep Wadi Side

Flat bliuvial floor

FIGURE 7.28  A Wadi in the Somali Republic. Study this Photograph and Notice the Steep Sides and Flat Floor of the Wadi. Most of the Rock Waste in the Wadi has been Carried and Deposited there by Occasional, but Torrential, Short-Lived River Floods. Wadi

Gully

Rill

Because there are no permanent drainage patterns in deserts, any rain that falls evaporates, infiltrates into the surface, or runs off and drains into basins (depressions). After heavy falls of rain, temporary streams and rivers develop, which may eventually drain into a basin where they produce lakes, but the lakes soon dry up through evaporation and turn into salt flats. There are temporary lakes and salt flats in the Sahara called sebkhas (playas in America). Flat Floor Alluvial Fan of Wadi Some basins are rimmed by uplands and temporary rivers emerging from the uplands build up FIGURE 7.29  Various Features Associated with a Wadi. alluvial cones (similar to fans but made of coarser material) at the foot of the uplands. These eventually join together to form a continuous feature known as a bajada. Between the sebkha and the bajada is a gently sloping platform called a pediment, which is formed as the edge of the uplands is pushed back by erosion and weathering. Sometimes alluvial deposits overlie the edge of the pediment surrounding the sebkha. This part of the pediment is known as the peripediment. All of these features are shown in Figure 7.30. Basin landscapes of this type frequently occur in intermontane basins, e.g., the Tarim Basin (central Asia).

7.20  Chapter 7

Temporary River Alluvial Cones Bajada

Temporary Lake Peripediment

Bedrock

Pediment

Sebka Pediment

Bajada

FIGURE 7.30  A Basin of Inland Drainage Showing Some of the Features that Usually Occur.

A Closer Look  ▼ Deserts and Rivers Most deserts are regions of inland drainage, i.e., their rivers and streams never reach the sea. Very few rivers persist throughout the year in deserts but there are significant exceptions. The Nile, in northern Africa, the Tigris–Euphrates in southwest Asia, and the Colorado in the USA are three of the best examples of rivers which cross desert areas and which are permanent rivers. This is because these rivers originate in regions of rain which falls throughout the year and which is sufficient to sustain a permanent flow of water across the desert areas.

Are Deserts Expanding? People live in the semi-arid areas bordering the arid deserts as shown in Figure 7.31. The Arabs call these areas the Sahel—the region of poor semi-arid grass, low bushes, and scattered trees. It is sometimes called the dry scrub region. Rainfall in the African Sahel is seasonal and unreliable and not always sufficient to support cultivation. The growth of areas of desertification are apparent in this satellite view, which reveals the extent of a global process of desertification not confined to Africa’s Sahara, but already progressed across quite large regions of both North and South America as well, on account of rapidly accelerating changes in micro-climates worldwide: “Although desertification can include the encroachment of sand dunes on land, it doesn’t refer to the advance of deserts. Rather, it is the persistent degradation of dryland ecosystems by human activities—including unsustainable farming, mining, overgrazing and clear-cutting of land—and by climate change” UNCCD (United Nations Convention to Combat Desertification).

Desert Processes  7.21

45˚N Gobi Araban

Tropic of Cancer

Sahara Sahel

Thar

Equator Tropic of Capricorn

Atacama Namib

45˚S Patagonian

Australian Kalahari

Desert Subject to Desertification

Trade Winds

FIGURE 7.31  The Main Areas of the World Which are Subject to Desertification.

Most of the inhabitants are nomadic herders and they with their sheep, cattle, and goats migrate seasonally in search of pasture for their livestock. Some of the inhabitants are shifting cultivators who clear small patches of scrub by burning. After 2 or 3 years, the patches are abandoned because the soil fertility falls and crops can no longer be grown. In the better areas, the abandoned patches lie fallow for perhaps 3 years before they can be cultivated again. In the areas of poorer soils, they may lie fallow for many more years. The population of Africa is growing very fast and each year the amount of food required increases. This has resulted in fallow land being put back into cultivation long before the soil has regained its fertility. This in turn means that the soils become increasingly infertile. The mineral content of the soil is no longer balanced, its structure breaks down and soil erosion by rain, surface wash, and wind begins. Infertile soils have little if any covering of vegetation and therefore no protection from rain and wind. The nomadic herder’s livestock in search of food consume whatever vegetation is available and this too lays bare the soil to erosion. Over-cultivation and over-grazing in the semi-arid regions of the world are turning vast areas of these regions into deserts. The process by which land which is suitable for farming, albeit to a limited extent, is turned into desert is called desertification. It is estimated that almost 30 million km2 have become desert through this process. Arid regions have a hostile environment, i.e., an environment which has few if any of the factors that are necessary to enable people to live and survive on a permanent basis. A hostile environment may have insufficient water, e.g., a hot arid region, or it may be too cold for any plant life to exist, e.g., a cold desert. Thus hostile environments occur mainly in the hot and cold deserts. The map shown in Figure  7.32 made by the United States Department of Agriculture—NRCS, Soil Science Division, reveals the dangers of expanding desertification at an extremely fine grain. Expanses threatened by increased desertification are illustrated in the worldwide satellite view, whose regions ringed in red highlight the areas (Figure 7.33) of a dramatic increase of desertification and an apparently unstoppable cascade of deep environmental change and release of carbon gasses.

7.22  Chapter 7

VULNERBILITY

OTHER REGIONS

Low

Dry

Moderate

Cold

High Very High

Humid:/Not vulnerable Ice/glacier

Miller Projection SCALE 1:100, 000, 000 500 1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

KILOMETERS

FIGURE 7.32  Desertification Vulnerability at Global Level

FIGURE 7.33  Worldwide Satellite View; those Regions Ringed in Red Highlight the Areas of a Dramatic Increase of Desertification and an Apparently Unstoppable Cascade of Deep Environmental Change and Release of Carbon Gasses.

Desert Processes  7.23

A Closer Look  ▼ Impact of desertification: ●● ●● ●● ●● ●●

About 2.6 billion people depend directly on agriculture, but 52% of the land used for agriculture is moderately or severely affected by soil degradation. Land degradation affects 1.5 billion people globally. Loss of arable land is estimated at 30–35 times the historical rate. Due to drought and desertification each year, 12 million hectares are lost (23 ha./ minute!), where 20 million tonnes of grain could have been grown. 74% of the poor (42% of the very and 32% of the moderately poor) are directly affected by land degradation globally.

Key facts ●● ●● ●● ●● ●● ●●

●● ●● ●● ●● ●● ●●

Most of the largest deserts are located between 15°N and 45°N, and 15°S and 30°S, and most of these that are within the tropics lie in the trade wind belt. Wind transport and deposition are the main agents of denudation in deserts; together they produce features such as barchans and seifs. Wind erosion is also important and it produces features such as rock pedestals, yardangs, zeugens, deflation hollows, and inselbergs. Water action in deserts may produce wadis, bajadas, pediments, and sebkha. The main desert landscapes are erg (sandy); reg (stony); hamada (rock platform), and badlands (restricted to semi-arid regions). Basins of inland drainage occur in most arid and semi-arid regions. Water collects in the depressions producing temporary lakes but very infrequently (usually once in several years). Rapid evaporation of water from the temporary lakes turns them into salt flats called sebkhas in the Sahara and playas in America. A bajada is a gently sloping platform of coarse rock particles formed by the joining together of alluvial fans. A pediment is a gently sloping rock platform that lies between the bajada and the sebkha. Wind transports material by saltation and it erodes rocks by abrasion. Sahel refers to semi-arid regions in Africa. Similar landscapes occur in Australia, Asia, and North and South America. The process by which land that is suitable for farming is turned into desert is called desertification.

7.24  Chapter 7

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct answer. 1. Wind erosion is caused by the process of (a) solution (b) abrasion (c) exfoliation (e) hydrolysis (d) corrasion 2. Which one of the following statements is not true? (a) Low rainfall and relative high evaporation are characteristics of tropical and temperate deserts. (b) Deflation can lower a desert surface to below sea level. (c) Floods never occur in hot deserts. (d) Sand dunes are not confined to desert landscapes. (e) Inselbergs and rock pedestals are in part the result of wind abrasion. 3. Sand grains are to dune as alluvial cone is to (a) Pediment (b) Salt flat (c) Mud flow (e) Playa (d) Bajada 4. Which one of the following features is not formed by wind erosion? (a) Zeugen (b) Barchan (c) Yardang (d) Hamada (e) Rock pedestal 5. Two types of plains frequently occur between the borders of desert basins and the surrounding highlands. They are called bajadas and pediments. Bajadas are (a) located at the entrance to a wadi. (b) sloping rock platforms produced by wind erosion. (c) the steep slopes of the surrounding highlands. (d) alluvial fans which have joined together to form a sloping plain around a desert basin. (e) salt lakes. 6. Which one of the following is not a characteristic of a barchan? (a) It is made of sand. (b) Its windward slope is concave. (c) It has a crescent shape. (d) Its horns point in a downwind direction. (e) It moves in the direction of the wind. 7. Desert is defined as the area where (a) annual rain occurs less than 25 cm. (b) there is prominence of sand burrows. (c) the temperature remains more than 42°C. (d) no plants are found. 8. Consider the following statements: 1. In India, Eastern Himalayan region receives more rain from northeastern winds. 2. Tropical deserts in the world are found in the trade wind bands of western ends of continents. Which among the above statements is/are/correct? (a) Only 1 (b) Only 2 (c) Both 1 and 2 (d) Neither 1 nor 2.

Desert Processes  7.25

9. Which one of the following features is described as knife-shaped ridge of sand or longitudinal dune. (a) Yardang (b) Sand dunes (c) Seif (d) None of these 10. Zeugen is termed as (a) Rock mushrooms (b) Dreikanter (c) Ripple mark (d) None of these 11. Basins of inland drainage occur in (a) only arid region (b) only semi-arid region (c) most arid and semi-arid region (d) None of these 12. Wind transports material by saltation and it erodes rocks by (a) Yardangs (b) Abrasion (d) Corrosion (c) Simulation 13. The runoff on steep slopes is usually by rills which connects with (a) Gullies (b) Alluvial Fan (c) Wadi (d) Loess 14. The process by which rock particles rub against each other and wear away is referred to as (a) Deflation (b) Attrition (d) All of the above (c) Abrasion 15. In Libya and Egypt, reg or stony deserts are called (a) Hamada (b) Dreg (c) Erg (d) Serir

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Explain the meanings of any two of the following and describe how they change the appearance of the landscape in which they operate. (a) wind deflation (b) wind abrasion (c) flash flood (d) attrition 2. Explain the differences and similarities of any two of the following, illustrating your answer with relevant diagrams. (a) Barchan and Seif (b) Pediment and Bajada (c) Zeugen and Yardang (d) Erg and Hamada

7.26  Chapter 7

3. Figure 7.31 shows the main areas in the world which are subject to desertification. (a) Give three causes of desertification. (b) Choose one of the causes given in (a) and with reference to an area that you have studied explain the ways in which it has contributed to the spread of desert in that area. (c) There are several ways of reducing and controlling slight desertification, especially around settlements. Name two of these ways and explain how they work. 4. Are deserts expanding? Illustrate your answer with examples. 5. Describe significant characteristics of subtropical deserts of the world.

Answer key Exercise 1 1. (b) 6.   (b) 11.   (c)

2. (d) 7. (a) 12. (b)

3. (d) 8. (b) 13. (a)

4. (b) 9. (c) 14. (b)

5. (b) 10. (a) 15. (d)

8

Coastal Processes

Learning Outcomes After completing this chapter, you will be able to: ●● ●● ●● ●●

Understand the interactions between the ocean and the land Describe coastal processes Classify waves Explain various landforms produced by the action of wave

Keywords Coastal Profile, Tide, Shore, Waves, Beach, Lulworth Cove region, Coastal Dune.

1

8.2  Chapter 8

Introduction There is constant interaction between the atmosphere and the oceans. Solar radiation transmits enormous amounts of energy to the oceans through the planetary wind systems, and this energy is represented by the movements of the ocean waters — the currents and the waves. Ocean currents result in the mass movement of surface waters, while waves result in the movement of energy through surface waters. As we shall see, there is no forward movement of water in a deep-water wave. There is also a constant interaction between the oceans and the land, and this interaction can be observed along every coast (Figure 8.1). Atmosphere Solar Energy

Lithosphere Sediment Rivers Glaciers Slopes

Sediment

Precipitation and Evapotranspiration

Solar Energy Hydrosphere

Coast

Precipitation and Evapotranspiration

Sediment

Wave Energy

Oceans

Lakes

Seas

Rivers

FIGURE 8.1  Relationships in the Coastal System.

Coasts Coasts are constantly changing. Wave deposition causes some coasts to retreat while others to advance. Some coasts are high and some are low; some have steep slopes, others have gentle slopes; and some coasts are rocky, while others are sandy. The nature of a coast is the product of many factors. Several processes operating on the land, which we have studied, together with the processes of the sea and the types of rocks forming the coast are responsible for the majority of coastal landforms. The nature of some coasts is further influenced by human activities such as the construction of groynes and sea walls to facilitate leisure activities or to protect parts of a coast from erosion. The action of rivers and glaciers and the mass wasting on slopes results in a large amount of sediment being carried and deposited in coastal waters. The abundant energy that the sea possesses uses this sediment as its principal tool, pushing and dragging it backward and forward across the coast, eroding and depositing. FIGURE 8.2  Continuous Interaction between the Ocean and the Figure  8.2 shows the relationships in the coastal system. Land Near Dagara, Odisha, India.

Coastal Processes   8.3

Terms related to Coastal Geography The level of the sea is not constant. It falls at low tide and rises at high tide. During very severe storms, storm waves can cause the level of the sea in coastal waters to rise even higher, with the highest levels being reached when storm waves coincide with high tide. The line along the land reached by the highest level of the sea is called the coastline. The line reached by the lowest water level, that is, the level of the low water breakers is known as the shoreline. The strip of land that extends from the coastline to the shoreline is called the shore. The foreshore extends from the low tide level to the high tide level, while the backshore extends from the high tide level to the coastline. Outside the low water breaker line is the offshore. This is usually a wave-built terrace consisting of sediment and sand carried from the shore by the waves and deposited on the sloping bedrock. The beach refers to a zone of sediment and sand deposited between the coastline and the shoreline. All these features are shown in Figure 8.3, which represents a coastal profile. The coastal profile is by no means constant. It varies according to the nature of the material forming the coast and the energy changes of the sea. For example, storm waves contain vastly more energy than ordinary waves, and when they break along the coast they can hurl enormous amounts of sediments and huge boulders against the land. Offshore

Foreshore

Beach

Low Tide Level

Cliff

High Tide Level

Coastline

Back Shore

Low Water Breaker Line

Shore

Wave-cut Platform

Figure 8.3  Profile of a Coast Showing Zonation of the Coast.

Formation of waves The water of the oceans is in constant motion. The gravitational pull of the sun and moon oscillates the surface of the oceans twice a day, while the wind agitates it into waves. Figure 8.5 shows how waves are caused by winds. The surface of the sea exerts frictional drag on the bottom layer of a wind blowing over it, and this layer exerts a frictional drag on the layer above it, and so on. The top layer has the least frictional drag

High Tide Level Low Tide Level

8.4  Chapter 8

(a)

exerted on it; which means that the layers of air move forward at different speeds. The air tumbles forward and finally develops a circular motion. This motion causes a downward pressure (DP) on the surface at its front and an upward pressure (UP) at its rear, and this causes the surface to take on the form of a wave. Figure 8.5 shows the wind pressing on the back of a developing wave, which makes it steeper. The back of the wave tumbles forward, but it moves back later and slows the forward movement at the front of the wave. The wave now grows bigger. The four parts of the movement in a wave are shown in Figure 8.6. Any particle of water at point A moves to A1, then to A2, A3, and back to A. It is therefore the waveform, and not the water, that moves.

Wind

Layers of Air

Surface of Sea (b)

d

Win

UP

DP

UP

DP

The nature of waves

The highest point of a wave is the crest and the lowest point is the trough. The difference between the two is known as the wave height (Figure 8.7). The height of a wave increases if the wind blows strongly for several hours, and waves of consider(a) able height develop in open oceans if a strong wind blows for several days. The size of a wave depends upon the strength ck k a c B of the wind, the duration of the wind, and the fetch (the disa B tance of open water over which the wind blows). The stron(b) (c) Fully-formed Wave ger the wind, the longer it blows, and the greater the fetch, the more powerful is the wave. Storm waves are particularly d powerful. The limiting factor in all wave development is the n i W length of the fetch. Seas such as the Irish, Baltic, and North Sea have short lengths of fetch, and the waves of these seas are never as large as those of open oceans. Very powerful waves move in from the long fetch of the Atlantic and operate along Figure 8.5  A Later Stage in the Formation the coasts of southwest England and Ireland. The height of of a Wave. these waves is further increased by the prevailing westerly winds whose direction coincides with the direction of fetch. Winds generate waves, and each wave Wind has a circular movement. When the wind is very strong, the wave steepens, i.e., its height Upwards Forwards increases, but the wave length remains the same. A A1 When the wind abates, the height of the wave A decreases and the wave length increases. The same phenomenon occurs when steep waves enter calmer water. As the height decreases, the Downwards Backwards wave becomes more undulating and regular in size and direction. This type of wave movement is called a swell. When a wave enters shallow water, frictional drag at the bottom of the wave A3 A2 causes the top of the wave to move forward, i.e., the wave breaks (Figure 8.8). Figure 8.6  The Movement of Water Particles within a Wave. Ba

t

on Fr

ck

Figure 8.4  The Initial Stages in the Formation of a Wave.

Coastal Processes   8.5

Shore Wind

Tro ugh

Crest of Wave

Figure 8.7  The Trough and Crest of a Wave. The Wind Throws the Water Surface into Undulations Which Grow into Waves Under Wind Pressure. There is No Forward Movement of Water in a Deep Water Wave. When a Wave Enters Shallow Water, It Breaks. The Top of the Wave is Thrown Forward. Thus, for the First Time, There is Forward Movement of Water. Water From the Breaking Wave Runs Up the Shore as Swash and Runs Down the Shore as Backwash.

Trough of Wave Length of Wave

Breaking Wave Height of Wave

Wave Breaks

Direction of Wind Calmer Water Wind-generated Wave

Water Shallows; Waves Get Steeper; Wave Length Decreases

Swell

w

w

Waves Have a Circular Movement

w

w

Zone of Swash

w

Frictional Drag Causes Wave From to Become Elliptical

W—Wave Length is Long in Swell Waves but Short in Inshore Waves

Types of waves Various types of waves are described below

Deep water waves Wind-generated waves are called sea waves. They usually comprise a number of waves of different lengths superimposed on one another. As we have seen, these waves become more regular as they enter calmer water. Swell waves are straight and long, and they travel great distances across the oceans while maintaining most of their power.

Figure 8.8  Wind-Generated Waves, Swell Waves, and Inshore Waves.

8.6  Chapter 8

Wave Breaks

Inshore waves

Swash

When a wave enters water whose depth is less than the length of the wave, its velocity decreases. This causes the wave length to decrease, which in turn results in an increase in the height of the wave. The wave steepens, becomes unstable, and eventually breaks as shown in Figure 8.8. The water thrown up the beach is the swash and that which drains down the beach under gravity is the backwash (Figure 8.9).

h

as Back W

Land Figure 8.9  A Breaking Wave Produces Swash and Backwash.

Constructive waves The circular movement of water in a wave is shown in Figure 8.10. Notice how the water moves forward on the crest and backward in the trough. When a wave moves towards the shore, the circular form becomes elliptical. When waves of long wave length and low height approach a gently sloping beach, the ellipse becomes horizontal Figure 8.11(a). When the waves break, the swash sweeps up the beach as a sheet of water often reaching the upper beach (Figure 8.8). Most of the swash soaks into the beach, which means that there is very little backwash. Waves of this type are called constructive or spilling waves.

Destructive waves High waves of short wave length have an ellipse which is vertical (Figure 8.11(b)). When these waves break on a steeply sloping beach, the water plunges forward into the trough. The steepness of the slope prevents a good development of swash, but the backwash is very powerful. Backwash carries material down the beach. Waves of this type are called destructive or plunging waves. It should be noted that small waves breaking on a steeply sloping beach tend to spill rather than plunge.

Wave Length A 9

A

A

A 8

7

6

5

A A

A Trough

3

4

A

2

1

A

Direction of Wave

Figure 8.10  Circular Movement in a Wave. As the Wave Shape Moves from Right to Left, Point a Moves in an Anticlockwise Direction from the Crest of the Wave (Position 1) to the Trough of the Wave (Position 5) and Back to the Crest (Position 9).

Coastal Processes   8.7

(a) Wave Direction

Swash as Backw

h

(b) Wave Direction

Swash h Backwas

Figure 8.11  (a) Constructive Wave; (b) Destructive Wave.

Refracted waves Waves travel in shallow water in the offshore zone as they approach the shore. We have already seen that frictional drag by the sea bed retards the bottom of a wave. Usually, the sea bed in the offshore zone is not uniform, i.e., the depth of the water varies from one place to another. This results in the waves in the shallower water being retarded more than those in the deeper water. This causes the wave crest to become curved as shown in Figure 8.12. This is known as wave refraction. Along a shore which consist of headlands and bays, the approaching waves develop a pattern similar to the configuration of the coast (Figure 8.13). This results in wave energy being concentrated around the headlands and being distributed in the bays. This explains why headlands have cliffs, while bays have beaches at their heads.

8.8  Chapter 8

G

E

C

A Wave Direction O t res eC av W

Shore

P

A X

Crest Wave Y

B

Q B

W av S H

Headland

Headland Bay

F

D

e C re st

T

Figure 8.13 Wave Refraction Along a Bay And Headland Coast.  The Wave AB Approaches the Shore at Right Angles Figure 8.12  All Parts of the Wave Along the Line AB to it. Its Crest Line is Straight. Segments X and Y are of the Move with the Same Velocity. The Wave Crest is Straight Same Width and have the Same Amount of Energy. Both are and Oblique to the Shore. The Wave Now Reaches Line in Deep Water in Position AB.As they Move Towards the Shore, CD. That Part of the Line CO is Now in Shallower Water Segment X Reaches Shallower Water First, and it Slows Down. and is Retarded. As the Wave Approaches the Shore, an Segment Y, Which is Approaching the Bay, is Not Slowed Increasing Amount of the Wave is Retarded. In Position Down. The Wave Length of X is Reduced, and its Height is EF, The Portion EP is Retarded. In Position GH, the Portion Increased.At Position ST, Segment X Breaks on the Headland. GQ is Retarded. The Process of Wave Refraction Results You Can See that the Energy of Segment X is Concentrated in the Wave Crest Becoming Curved. Over a Smaller Length of Coast than is that of Segment Y. Direction of decreasing depth of water

A Closer Look  ▼ Action of Waves The interaction between a wave and the beach largely determines whether a wave erodes or deposits. Usually, steep waves are destructive, i.e., they erode, while less steep waves are constructive, i.e., they deposit. The wave period, which is the time taken by successive wave crests to pass a given point, directly influences the nature of the backwash. For example, if a wave breaks onto the backwash of a preceding wave, then the swash has only limited action while the backwash becomes dominant and affects erosion, but if the wave breaks after the backwash of the preceding wave has died down, then the swash reaches well up the beach and deposition by the swash becomes dominant. It should also be noted that an onshore wind helps waves to erode.

Coastal Processes   8.9

Wave Erosion The water from a breaking wave affects coastal erosion, and this process occurs in four ways: 1. Abrasion: This is caused by boulders, pebbles, and sand being hurled against the base of a cliff by breaking waves, which results in undercutting and rock break-up. When a cliff consists of layers of rocks of differing resistance, the less resistant rocks are eroded more rapidly. They are turned into hollows which later become caves at the sea level, while the more resistant rocks project seaward as ledges. 2. Hydraulic action: When water is thrown against a cliff by breaking waves, it often compresses the air in cracks and crevices in the rocks. When the waves retreat, the air expands, sometimes explosively. This causes rocks to shatter as the cracks are enlarged and extended. 3. Attrition: As boulders and rocks are hurled against the shore and against each other by breaking waves, they gradually disintegrate into smaller pieces. 4. Solution: The minerals in some rocks react chemically with sea water. This causes the rocks to become less resistant to erosion.

Landforms produced by wave erosion Cliff and wave-cut platform Imagine a newly drowned land surface. Marine erosion will begin to cut a notch in the land where the sea meets it. This will be at about high tide level. As erosion proceeds, the notch is further developed, and the first signs of a cliff appear. Further landward recession of the notch results in further development of the cliff. The cliff base is steepened by wave erosion, but while this is in progress, the cliff face above high tide level is attacked by weathering processes, and mass wasting becomes dominant. This causes the cliff face to become less steep as shown in Figure 8.14. In front, i.e., seaward of a cliff of this type, there is a broken surface of bare rock, which is called a wave-cut platform. The platform develops as the cliff recedes and as rock debris, in part from marine erosion and in part from mass wasting, is swept backwards and forwards by breaking waves. Some of the debris settles on the platform, thus forming a continuous cover. The rest of the debris is either carried into the deeper waters of the offshore zone or is carried along the shore to areas where less active waves deposit it as a beach. As the cliff retreats further, the wave-cut platform becomes wider, and if the process continues, it becomes sufficiently wide to prevent breaking waves from reaching the cliff base, because the water over the platform is too shallow to allow the waves to ­proceed to the coast (see Figure 8.14).

Cliff profiles The form a cliff profile takes depends very much on the nature of the rocks and whether the rock strata dip or arc horizontal or are vertical. Steep cliffs develop in rocks whose strata are vertical or horizontal. The steepness is achieved by the vertical collapse of rocks in the former (Figure 8.15(a)) and by undercutting in the latter

Most wave erosion occurs between low and high tide levels (see Figure 8.13). The force of breaking storm waves may be 20 tonnes or more per square metre.

8.10  Chapter 8

(a) Notch Develops Here HT LT

(b)

Cliff Produced by Undercutting HT LT

Wave-cut Platform Develops (c) Mass Wasting Lowers Cliff HT LT Wave-cut Platform Increases in Width (d)

Wave Erosion Decreases as Water Becomes Shallow

(e)

HT LT

Rock Debris Covers Wave-cut Platform HT LT

HT High Tide

LT Low Tide

Figure 8.14  Stages in the Development of a Cliff and a Wave-Cut Platform.

(Figure 8.15(b)). A particularly fine example of a vertical cliff is shown in Figure 8.16. Much more gentle cliffs develop in rocks whose strata dip, either seaward or landward. This type of cliff is shown in Figure 8.15(c) and 8.15(d). The interplay between wave erosion at the base of a cliff and mass movement on the cliff face helps to determine the profile of the cliff. Where cliff base erosion is dominant, rock fall on the cliff face results in the cliff remaining vertical. Where mass wasting is dominant, the cliff becomes less steep.

Coastal Processes   8.11

Cliff forms

(a)

Vertical Cliffs

Structure and the differing resistance of rocks to erosion also influence cliff form. When resistant rocks alternate with less resistant rocks along a coast which is under wave attack, an indented cliff-line develops. The resistant rocks form headlands and the less resistant rocks form bags as shown in Figure 8.17. In this diagram, the rock structures are at right angles to the coast, but this is not an essential condition. The coast between Seaward Cliffs Chesil Beach and Christchurch in southern (c) England has some of the finest bay structure, but the rock structure here is parallel to the coast. Figure 8.18 shows the rock types and their layout for the coastal stretch containing Lulworth Cove. The sea has broken through the more resistant rock parallel and near to the coast and has eroded the less resistant rock inland of this, which, through wave refraction has been converted into an almost circular bay called Lulworth Cove (Figure 8.19). Figure 8.15  Cliff Profiles. Bays and headlands can also develop in a single rock structure, say limestone, which has lines of weakness such as joints or faults. The sea has excavated the joints in the chalk structure at Birchington, Kent, and has developed almost vertical cliffs which are a mass of small headlands and bays (Figure 8.20).

(b)

Horizontal Cliffs Resistant

Less Resistant Undercutting Here (d)

Landward Cliffs

Cave, arch, and stack Wave attack on the sides of a headland sometimes leads to the formation of caves, especially where there are vertical faults or joints. Sometimes, a cave in a headland or similar feature is eroded right through to form a natural arch (Figure 8.21). With the progression in time, the roof of the arch collapses and a stack is formed. Figure 8.22 shows the stages in the development of a stack, while Figure 8.23 is a photograph of stacks near John o’Groats, Caithness.

Figure 8.16  Vertical Cliffs Cut into Horizontally Bedded Strata in Bunda Cliffs, Australia.

Blow hole and geo Some caves develop in well-jointed rocks. Joints and bedding planes are opened under wave attack by both abrasion and hydraulic action. The force of breaking waves exerts enormous pressure on the rocks at the cliff base, and as we have seen, breaking waves compress air trapped in cracks and crevices, but when the waves retreat, the pressure is released and the air expands. Sometimes, a joint extends from the roof of the cave to the top of the cliff, and this becomes enlarged by the

8.12  Chapter 8

Christchurche Poole Harbour

Poole Bay

Swanage

Stack Chalk

Sand and Clay Bay Swanage

Headland

Figure 8.17  Differing Rock Strengths can Result In the Development of Bays and Headlands. Swanage Bay has Developed in Sand and Day Rocks that are Less Resistant than Chalk, Which Forms the Headlands.

Poole Harbour Lulworth Cove

St Alban’s Head Chalk

LessResistant Rock

Lulworth Cove

Figure 8.18  Plan of the Geology of the Lulworth Cove Region.

Resistant Rock

N

Coastal Processes   8.13

Figure 8.19  Lulworth Cave, Uk (Almost a Circular Bay).

Figure 8.20  Small Scale Headlands at Birchington, Kent.

Figure 8.21  A Natural Arch, Benagil Sea Cave, Portugal.

8.14  Chapter 8

(b)

(a) Headland

Arch

Bay Cave

Cave

Cave

Wave Attack Continues

Caves Developing Opposite to Each Other Waves Curve Round a Headland and attack it on All Sides

The Two Caves Ultimately Join up and an Arch Forms

Headland

Headland

Arch

(c) Arch Stack

Figure 8.22  Stages in the Development of a Stack. The Diagram also Shows Headlands and Bays.

Arch

Wave Attack Continues

Stack

The Arch Collapses and a Stack Results

Headland

alternating compression and expansion of air in the joint. With the progression in time, the joint may be opened right up to the top of the cliff. When this occurs, a blow hole is formed, the development of which is shown in Figure 8.24. A photograph of a blow hole is given in Figure 8.25. The roof of the cave ultimately collapses, and a narrow long inlet called a geo is formed (Figure 8.24).

Coastal Processes   8.15

Figure 8.23  The Stacks of Duncansby, Near John O’ Groats, Scotland.

Initial Opening Develops Here in Zone of Weakness

Cliff Made of Resistant Rocks

Blow Hole Cave

HT

Cave Grows Longer HT

Blow Hole

Geo

Wide Cave Entrance HT Roof of Cave has Collapsed and a Narrow Inlet is Formed Figure 8.24  Stages in the Development of a Cave. These Diagrams Show how Waves Extend the Initial Opening Made at the Base of the Cliff. After a Cave has Fully Developed, Further Erosion and the Pull of Gravity Sometimes Cause the Roof to Collapse. When this Occurs, an Inlet or Geo is Formed.

8.16  Chapter 8

Figure 8.25  Collapse of the Roof of a Sea Cave has Produced this Blow Hole at Holborn Head in Caithness. Relate this Photograph to the Diagram in Figure 8.24.

Beach

Direction of Longshore Drift Backwash Sw

as h

t

es

r eC

av Direction of Wave Advance

W

Figure 8.26  Longshore Drift. (Note that the Swash and Backwash are not Uniform in Development)

Materials Transported by Waves Some of the load transported by waves comes from rivers entering the sea, some from mass movements of cliff slopes, and the rest from wave erosion. The load consists of mud, sand, and shingle. When waves break obliquely to the shore, the swash moves obliquely up the beach as indicated in Figure 8.26. The backwash runs back down the steepest slope, which is usually at right angles to the shore. Waves which develop a good swash push material up the beach diagonally, and the backwash

Coastal Processes   8.17

of larger waves drags some of this material back down the beach. The swash from the next breaking wave pushes some of the material back up the beach, and this process continues. These two movements result in material drifting along the shore. This movement is called longshore drift. The diagonal movement of the swash also produces a movement of water along the shore as a longshore current. This current is capable of moving material in suspension. The movement of material along the shore by longshore drift and longshore current continues so long as the direction of the wave advance is about 45° to the short (Figure 8.27(a)). If this angle falls, say below 30°, not all the material will be transported, i.e., some will be deposited. This occurs when the coast changes direction (Figure 8.27(b)). (b)

(a)

longshore Drift

Longshore Drift

30º

Sediment

45º

es t

D Wa irect ve ion Ad of va nc e

av

e

Cr

Direction of Wave Advance

W

W

av

e

C

re

st

Sediment

Figure 8.27  (a) Sediment is Moved Along the Shore by Longshore Drift Provided the Direction of Wave Advance is not Less than 45° to the Coast; (b) A Change in Direction of the Coast Results in the Direction of Wave Advance being Only 30° to the Coast. Sediment is Still Moved by Longshore Drift, But Where the Coast Changes Direction, the Sediment is Carried into the Sea as Shown. This Gives Rise to a Spit.

Landforms Produced by Wave Deposition The best known and the most common depositional landform is the beach.

Beach This may be made of sand, shingle, boulders, or mud. Brim of most beaches are made of sand or shingle. The material which forms the tip of beaches comes from eroded headlands, rivers, and other beaches through the action of longshore drift, and all of it is sorted by the action of swash and backwash. The strong swash of a constructive or spilling wave usually pushes the coarsest material up the beach. The backwash is less strong (some of the swash soaks into the beach).

Coast

Shore High Tide Level

le hing

S

ch

Bea

Low Tide Level

ch

ea

B nd

Sa

Figure 8.28  Beaches are Sometimes Formed between Low and High Tide Levels, the Beach is Well Developed and Consists of Shingle and Sand.

8.18  Chapter 8

Beach Face

Break Point Surf Zone

Berm

Deposition Figure 8.29  Beach Profile. Breaking of Steep Waves Which have a Strong Backwash Results in Material Being Dragged Down the Beach and Deposited Near the Break Point. This Develops into an Offshore Bar.

Bay Head Beach

Stack

Headland

The coarsest material is often at the limit of wave action along the landward edge of the beach. The finer material usually occurs along the seaward edge of the beach (Figure 8.28). The profile of a typical beach is shown in Figure 8.29. Names are given to the various parts as follows: Berm — the top area of a beach reached only by storm waves; Beach face — the upper part of a beach reached by waves at high tide; Surf one — that part of a beach between the zone where the waves break and the point reached by the swash; Break point — the point at which waves break.

Bayhead beach This is the simplest of beaches. It results from wave refraction in bays, and it is usually concave to the sea. Figure 8.30 shows a bayhead beach.

Barrier beach

Figure 8.30  Some of the Features that can Develop Along Some Coasts. Bays between Headlands Develop Bayhead Beaches. The Shallow Water in Bays Results in the Waves Building Up Rather than Wearing Away the Coast. The Water Around Headlands is Deep, and Erosion Occurs. Note: As the Headlands are Worn Back and the Bays are Filled in, the Coast Becomes Straightened.

A barrier beach develops from an underwater offshore bar as the latter moves towards the land. It is called a barrier beach when its surface appears above the water. It is a long ridge of either shingle or sand parallel to the coast and separated from it by a lagoon. Barrier beaches develop best along coasts which have exposure to swells. Loe Bar in Cornwall is a good example of a shingle barrier beach (see Figure 8.38). Very extensive barrier beaches of sand have developed off the coast of the USA from Texas to Virginia. Some of these represent beaches which have been driven shorewards by rising sea level or falling land level. Refer to Figure 8.33.

Spit A spit is a low, narrow ridge of pebbles or sand joined to the land (mainland or island) at one end, with the other end terminating in the sea. It is formed by longshore drift. There are many spits around the coast of Great Britain; indeed, spits are common major depositional features. A very good example is Spurn Head (Figure 8.34). This diagram shows a photograph and a locational plan of the spit.

Coastal Processes   8.19

Figure 8.31  Bays between Headlands, Tamil Nadu, India.

Figure 8.32  Headland, Vivekananda Rock Memorial, Kanyakumari, India.

(b)

(a) Along Very Gently Sloping Coasts the Waves Break Well Offshore. a l Pl sta Coa

Shoreline

Breaking Wave N

Washington

in

Spit This Wave Scoops up Sand Which is Thrown Forward Where it Accumulates as an Offshore Bar.

Richmond

Ri

erR oano ke

v

The Offshore Bar has Become Wider and Higher. The Water Between The Bar and the Shoreline is Called

Lagoon

Swamp

Offshore Bar

lain

lP sta Coa

Swamp

n

o go

Cape Hatteras

La

Off-shore Bars In Time the Lagoon Becomes Filled in with Sediment Forming Swamp or Marsh and Finally Dry Land.

0

100 km

Figure 8.33  (a) The Development of an Offshore Bar; (b) Some of the Best Developed Offshore Bars Lie Off the Usa Coast from Texas to Virginia.

8.20  Chapter 8

(a)

River A i de

N

Lo ng rift

eD or

sh

Hu

m

be

r

Orford Est

uar

y

Spurn Head

FIGURE 8.34a  Illustration of Spurn Head (Location of the Spun Head in Humber Estuary)

Spit 0

5 KM

Figure 8.35  The Spit Across the Mouth of River Alde has Deflected the River’s Outlet to the South.

(b)

If longshore drift operates across the mouth of a river, an area of slack water develops between longshore drift and the river, which results in the deposition of material by longshore drift. This material builds a spit which may extend across the river’s mouth, thereby causing it to be diverted. The spit at the mouth of River Aide in Suffolk has diverted the river almost 12 km to the south (Figure 8.35). When a spit is formed in deep water, the tip of the spit often turns towards the land. This results in the spit becoming hooked as that in Hurst Castle spit in the Solent (Figures 8.37 and 8.38). FIGURE 8.34b  Spurn Head, East Riding of Yorkshire, England.

Bar

This is a ridge of sediment which is parallel to the coast. It may be exposed at both high and low tides or only at low tides or not at all. A bar often develops into a barrier. Loe Bar is a shingle barrier inland with a freshwater lagoon called the Loe (Figure 8.39).

Tombolo When a bar joins an island to the mainland, it is called a t­ ombolo. The best example of a tombolo in Great Britain is Chesil Beach (Figure 8.40).

Coastal Processes   8.21

Figure 8.36  The Spit across the Mouth of River Alde is Clearly Visible in Satellite Data.

Figure 8.37  Hurst Castle Spit Turning Towards the Land.

Mud Banks With Grass

al ion s a c s Oc ind W

P W reva in ili ds ng

N

0

1 km Figure 8.38  Hurst Castle Spit.

8.22  Chapter 8

Helston

Isle of Portland

N

Cl

iff s The Loe

Weymouth

Loe Bar

0

2 km

Chesil Beach

The Fleet

Gunwalloe Fishing Cove

0

5 km

N Figure 8.39  Loe Bar, Cornwall.

Figure 8.40  Chesil Beach is a Tombolo – Location Sketch Map and Photograph.

Figure 8.41  Kovalam Beach, Kerala. Cliff Zone of Swash

Tides

Coast Line

Beach

Figure 8.42  Different Coastal Features are Marked in an Imagery, Varkala Beach, Kerala.

Coastal Processes   8.23

Other examples as depicted by satellite data are Kovalam Beach (Figure 8.41) and Varkala Beach (Figure 8.42) in Kerala, whose different coastal features  are marked in the imagery (Figure 8.42). reli n

e

Hythe

Salt marshes In addition to shingle and sand, waves and sea water transport and deposit much finer particles of matter called silt or mud; whilst shingle and sand are deposited by moving water, mud settles to the bottom only when the water is almost motionless. This is why mud accumulates in sheltered areas. Various types of grasses and seaweed grow in the mud in areas which are exposed at low tide, and in these areas, the growing grasses cause more mud to be deposited. The level of the mud therefore is raised. As more grasses grow, the mudflats, as they are sometimes called, become salt marshes (Figure 8.44). There are many examples of salt marshes along the coast of Great Britain. One of the best examples occurs at Blakeney Point on the north coast of Norfolk. The salt marshes here have developed in the shallow water behind the shingle spit and sandy ridges (Figure 8.45). The mudflats, which are crossed with creeks or channels through which the tidal water flows, are exposed at low tide. Another area of salt marshes occurs in the estuary of River Dee (Figure 8.46). Chester was a port during the

Old S

This is usually formed from the steady accumulation of sand or shingle where two curved spits meet, i.e., the spits enclose a triangular-shaped lagoon which becomes the site for deposition. The best example of this landform is the Dungeness Foreland or Romney Marsh. The actual origin of this feature is complex and not completely known, but as shown in Figure 8.43, the development of this feature has caused the coast to be pushed seaward. In Roman era and up to the Middle Ages, towns such as Rye were used as ports.

ho

Cuspate foreland

Rye

Romney Marsh Shingle Ridges

Winchelsea

English Channel Figure 8.43  Romney Marsh – A Cuspate Foreland Containing Many Old Shingle Ridges. Grasses Grasses and Sea Weeds High Tide Level Low Tide Level Mud Flat

Mud

Figure 8.44  Section across a Typical Salt Marsh. Seaweeds Dominate the Vegetation in the Wetter Seaward Areas; Grasses Predominate in the Landward Areas.

Low Water Mark Salt Marsh

Blakeney Point Spit

Fresh Water Marsh

Direction of Longshore Drift

N

0

2 km

Figure 8.45  Location Map Showing Salt Marshes Near Blakeney Point.

8.24  Chapter 8

N

Bootle

Birkenhead

Liverpool

Ri

ve

This Area is Mainly Dry at Low Water

Chester

Riv 0

r M ersey

er D o e

10 km

Figure 8.46  Dee Estuary Still has Large Areas of Salt Marshes. Humber Estuary, England and Poovar Estuary, Kerala

Roman occupation of Great Britain, but today, it is far from the sea. The deposition of mud over the centuries has converted the river’s estuary into an extensive area of mudflats and salt marshes (Figures 8.47 and 8.48). As the level of a salt marsh is raised by the deposition of mud, the entry of seawater is gradually restricted until it enters perhaps only once or twice a year during the highest spring tides. When this stage is reached, it then becomes possible to reclaim the salt marshes fairly easily by building embankments and digging drainage channels. This is known as marsh reclamation. Since 1850, large areas of salt marsh have been reclaimed from the estuary of River Ribble (see Figure 8.49). This reclaimed marshland has been converted into agricultural land by adding lime and fertilizers to the soil. Reclamation of the greatest area of sea-covered lowland has been performed in the Netherlands

Figure 8.47  Humber Estuary Showing Spurn Head, East Riding of Yorkshire, England.

Figure 8.48  Poovar Estuary, Kerala, India.

Coastal Processes   8.25

N Blackpool

Preston

R

le

ve

Ri

Lytham St Andries

e ibbl

bb

i rR

ary

Salt Marshes

Estu

Reclaimed Salt Marshes

Figure 8.49  Ribble Estuary Location Map Showing Reclaimed Marshland and Present Day Marshes.

0

Southport

5 km

through the Zuider Zee Scheme. The extent and ambitions of this scheme are so great that it is beyond the scope of this book to examine them.

Coastal Dunes Sand dunes form on the landward side of some beaches. They develop if sand beaches dry out and onshore winds are sufficiently strong and persistent to blow the finer sand particles inland. In some coastal regions, sand dunes have advanced several kilometres inland, burying farmland, woodland, and sometimes houses. The largest area of sand dunes in Great Britain is on the southern shore of the Moray Firth, Scotland. It is  called  the Culbin Sands (see Figure 8.50). Sand grains are bounced along by the process of saltation, as discussed in Chapter 7, until they meet an obstacle such as wet ground or a water surface, a low mound or a pile of stones, or tufts of grass. The bouncing sand grains absorb a little moisture each time they land on a wet ground surface. This tends to make the grains stick together, thus stopping further movement. When the wind meets a mound or pile of stones, eddy currents are set up, which

Figure 8.50  Old Photograph of Culbin Sands, Moray, Scotland.

Wind

sit

epo

dD San

Eddy Current

Pile of Stones form Obstacle

Figure 8.51  Eddy Currents are Caused When Wind Blows over an Obstacle such as Pile of Stones. Sand is Deposited and a Dune May Develop.

8.26  Chapter 8

Marram Grass

Figure 8.52  Blades of Marram Grass.

Figure 8.53  Coastal Dunes Flora in the Thoothukudi District, Tamil Nadu, India.

cause the grains to be dropped as shown in Figure  8.51. When the wind meets tufts of grass, the air that hits the blades of grass slows down, and the sand grains are dropped. The sand grains deposited by a mound or a pile of stones builds up a sand dune, which is elongated in the direction of the wind. The grains deposited around a tuft of grass may also give rise to a dune, but if there are many tufts of grass closely grouped together, an irregular sand deposit is likely to develop. Sand dunes can be anchored, i.e., prevented from moving, by planting certain types of marram grass on them (Figure 8.52). Coastal dunes flora in the Thoothu­kudi District (Tamil Nadu) are shown in Figure 8.53. Dunes with a poor grass cover are unstable in that when gale force winds blow, the bare sand areas of the dunes are removed by saltation and blow outs occur. However, not all blow outs have natural causes. Some are caused by human activities such as the breakdown of the grass covering by camp fires, by parking of cars, and by too many people walking over the dunes.

Types of Coasts Neither the level of the land nor the level of the sea remains unchanged for long periods of time. There is abundant evidence in coastal areas around the world that land and sea levels have changed relative to one another. There are raised beaches with backing cliffs both now out of reach of the sea, and drowned river valleys and submerged forests.

Changing Sea Levels There is evidence in many countries which suggests that one part of the crust has been pushed up relative to another part. In western Scotland and Norway, there are sea beaches which now lie several metres above sea level. This indicates that either the land has been pushed up or the level of the sea has fallen.

Coastal Processes   8.27

It has been calculated that the Pleistocene glaciation resulted in sea level throughout the world falling between 80 and 100 m. With the melting of the ice as the climate became warmer causing the Pleistocene ice sheets to retreat to the polar regions and the glaciers to the upper valleys in high mountains, the level of the sea began to rise again. Until about 50,000 years ago, the fall in the level of the oceans was such that Great Britain was joined to Europe. During this period, the lower sea level caused pre-­ glacial rivers to be rejuvenated, and many of them cut deep gorges into their flood plains. River Thames once flowed through a gorge of this origin. With the melting of the ice, rivers, e.g., Thames, filled in the gorges with sediment as sea level rose. As sea level rose further, the lower parts of the valleys were inundated or “drowned”. River valleys which were “drowned” are called rias; “drowned” glaciated valleys are called fiords. As the ice melted and sea level rose, those land surfaces that had been depressed by the heavy weight of the ice sheets also rose. The slow rise of land in coastal regions has caused sea beaches and backing cliffs to be uplifted above the sea level of today. Such beaches are called raised beaches. In some regions, e.g., the Baltic, the land is still rising. Mooring rings which 80 years ago were at sea level are now out of reach of the boats. The land in the Baltic is rising about 20 mm per year. Some raised beaches are backed by sea cliffs. You can see from this that changes in the relative levels of land and sea are complicated. The withdrawal of water from the oceans and the accumulation of vast weights of ice on the land during the Pleistocene glaciation caused sea level to fall and some land surfaces to be depressed with a time lag between the two. The subsequent melting of the ice resulted in a rise in sea level and an uplift of the land surfaces which had been depressed, but again with a time lag between the two. Carefully controlled experiments showed that the northern part of Great Britain is rising, while the southeastern part is falling relative to sea level. Ria is the lower part of a river’s valley which is not filled with sediment but which has been submerged by the sea (see Figure 8.54). The valleys of the rivers Fal, Dart, and Yealm are examples of rias. The banks of rias are fairly steep compared with those of estuaries. This indicates that the rivers were well incised in their valleys prior to the rise in sea level.

A Closer Look  ▼ Example of an Estuary An estuary is the lower part of a river’s valley which has been filled with sediment and subsequently submerged by the sea. It is usually funnel shaped. An estuary’s banks are much more gently sloping than those of a ria. It is this characteristic together with its shape that distinguishes it from a ria. The lower Thames is a good example of an estuary, but there are many estuaries along Great Britain’s coasts. The ports of Glasgow, Liverpool, Southampton, and Hull are all sited on estuaries.

8.28  Chapter 8

(a)

X X

(i) River Valleys

River (ii) A

Sea

B

Y Sea

X

Y River

Y (b)

River

A

B

X

Lower Parts of Valleys go Under the Sea, they Become Rias Y Sea

(i)

X River

(ii)

River Sea

A

B

End of Headland Becomes an Island

X

Y Sea

Y

A

B

Figure 8.54  The Formation of a Ria Coast. These Diagrams Show the Appearance of the Coast (a) Before and (b) After Submergence. The Dotted Lines in (A) (I) Represents the Position of the Coast Line After Submergence. Compare the Long Profiles and the Cross-Sections of the River Valley before and after Submergence. Furthermore, Note that the Tips of the Headlands are Sometimes Converted into Islands as a Result of Submergence.

Fiord. When the coast of a glaciated highland is submerged, the lower part of a glaciated trough is “drowned” to form a fiord. The sides of a fiord are very steep, and the depth of water in it may exceed 1000 m. Unlike rias, fiords are not solely the result of “drowning”. Their great depths resulted from glacial erosion, often to below sea level. Even if sea level had not risen, these glaciated troughs would have been “drowned” when the ice melted. The water depth at the seaward end of a fiord is shallower than it is inside the fiord, usually less than 100 m. This end of a fiord is called the threshold. The origin of the threshold may be due to several causes. It may be the result of the end of the glacier floating, thereby reducing the glacier’s power to erode; it may also be the result of deposition of morainic sediment, or it may indicate decreased glacial erosion because the glacier here was thinner. Fiords are usually developed on the western coasts of temperate regions which are backed by high mountains. Such regions occur in western Scotland, British Columbia, Norway, the South Island of New Zealand, and southern Chile. Figure 8.55 illustrates how a fiord develops.

Coastal Processes   8.29

(a)

Interlocking River Valley Spurs

(b) Valley Glacier

Sea

(c) Steep Sides

Sea

Fiord

Threshold

Figure 8.55  Stages in the Development of a Fiord: (a) Before Glaciation; (b) During Glaciation Interlocking, Spurs are Removed by the Valley Glaciers. The River Valley is Straightened, Widened, and Deepened (Below Sea Level) by the Glacier; (c) After Glaciation, the Valley becomes Submerged Partly by Overdeepening and Partly by a Rise in Sea Level. A Rise in Sea Level Causes the Lower Parts of Glaciated Valleys to Become Deep Water Submerged. Compare this Type of Feature With a Ria in the Fiord (Figure 8.54).

A Closer Look  ▼ Protection of Coastal Areas We have seen how waves can take away the land through the process of erosion and add to it through the process of deposition. We have also seen that beach sediment is moved along the coast by the process of longshore drift. It is possible to prevent beach material being removed from a coast by longshore drift by building groynes out from the coast as shown in Figure 8.56. A groyne is a wall made either of concrete or wooden posts and planks, and it extends from the coastline to low water level. The groyne acts as a sediment trap with the sediment piling up on the side facing the direction from which longshore drift advances. Usually, a series of groynes is built and the build-up of beaches is as shown in Figure  8.56. Groynes are successful in preventing the removal of a beach front, but they cause problems to beaches on the downdrift side. As shown in Figure 8.57, Beach A has groynes and longshore drift from west to east. This piles up beach sediment on the western sides of the groynes. Beach B does not have groynes, and longshore drift steadily removes beach material from it.

8.30  Chapter 8

Coasts which experience strong wave attack can be protected from erosion by the building of sea walls. Some of these have a concave vertical face, which helps to deflect the force of the waves. Some have a long sloping ramp which helps to dissipate the energy of the waves. These features are shown in Figure 8.58. But neither groynes nor sea walls can prevent serious erosion, deposition, or flooding from occurring when waves are exceptionally high and strong. In 1953, a very deep depression moved southward across the North Sea. Its pressure was so low that the level of the sea rose by just over 0.5 m. Gale force winds thrust water from the Atlantic into the North Sea where it developed an anticlockwise circulation. The surge of water could not get through the Straits of Dover into the English Channel, and this resulted in water pouring into the low-lying regions of Holland and eastern England. The surge of water backed by a high tide prevented the rivers from emptying their waters into the North Sea which poured onto adjacent low-lying land instead. Sea walls were battered and breached, cliffs were eroded, and beaches were lowered in various parts of Lincolnshire.

Direction of Longshore Drift

(a)

Beach Groyne Direction of Wave Advance

Figure 8.56  (a) Groynes Prevent the Beach from Drifting; (b) Sectional View Shows that the Beach Depth Decreases from the Updrift Side of a Groyne to the Downdrift Side of the Next Groyne.

(b)

West

Figure 8.57  Diagram to Show the Effects of Groynes on the Deposition of Sediment Along a Coast.

Groyne

Longshore Drift Beach A

East Beach B Beach Sediment Here is Removed by Longshore Drift

Direction of Wave Advance

Coastal Processes   8.31

(a)

Sea Wall

Sea

Land

Sea Wall With Ramp

(b)

Sea

Figure 8.58  (a) Sea Wall with Concave Face Deflects the Force of the Waves; (b) Sea Wall with Sloping Ramp Dissipates Wave Energy.

Key facts ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●●

●●

Solar radiation transmits energy to oceans through the prevailing winds. This energy gives rise to ocean currents and waves. There is a constant interaction between the oceans and the land. Sediment used by the sea to erode the land comes from rivers, glaciers, and the mass wasting of slopes. The level of the sea changes; it rises at high tide and falls at low tide. The coastline is the highest level that the sea reaches on land. The shore is the land between the coastline and the shoreline (the lowest level reached by the sea). The foreshore lies between low and high tide levels; the backshore lies between the coastline and high tide level. Sediment deposited between the coastline and the shoreline is called a beach. The surface of the oceans is oscillated by the pull of the sun and moon twice a day; the wind agitates the surface and creates waves. Waves are caused by winds. The height of a wave is determined by the strength and duration of the wind and the length of the fetch. Fetch refers to the distance of open water over which a wind blows. When the wind velocity falls, wave height decreases and wave length increases; waves become regular in size and direction and they form a swell. A wave breaks when it enters shallow water. Swash is the water thrown up a beach by a breaking wave; backwash is the water that drains back to the sea. A constructive wave is one that deposits sediment; a destructive wave is one that removes sediment. The crestline of waves approaching an indented coast takes the configuration of the coast through wave refraction. Wave refraction causes wave erosion to be concentrated around headlands. Waves cause erosion by abrasion, hydraulic action, and attrition. Landforms produced by wave erosion include cliffs, wave-cut platforms, caves, stacks, natural arches, and geos. Landforms produced by wave deposition include beaches, spits, bars, tombolos, and forelands. Sediment is moved along a coast by longshore drift.

8.32  Chapter 8

●●

●●

●● ●● ●● ●●

The return of water to the oceans after the last ice age caused sea level to rise and “drown” the lower parts of some valleys. A “drowned” river valley is called a ria; a “drowned” glaciated valley is called a fiord. The melting of much of the ice which covered the land caused some land areas to rise as the crust underwent isostatic re-adjustment. Raised beaches are evidence of this phenomenon. An estuary is a sediment-filled lower river valley that has been “drowned”. A ria is a sediment-free lower river valley which has been “drowned”. A groyne is a wall built from the coastline to low water level to prevent beach removal by longshore drift. Strong onshore winds blow fine sand from dry sand beaches inland and deposit it to form sand dunes. Sand dunes can be fixed by growing marram grass.

Coastal Processes   8.33

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. 1. Which of the following combinations of features could develop when a headland is under attack from destructive waves? (i) Arch (ii) Cave (iv) Lagoon (iii) Stack (a) (i) and (ii) only (b) (i), (ii), and (iii) only (c) (i), (ii), and (iii) only (e) (ii) and (iii) only (d) (i), (ii), and (iv) only 2. A wave-cut platform forms (a) as longshore drift becomes well established. (b) when headlands develop. (c) as deposition of rock waste fills in a bay. (d) when constructive waves are dominant. (e) when a cliff line retreats because of wave erosion. 3. Which one of the following features is not produced by wave erosion? (a) Headland (b) Stack (c) Beach (d) Cliff (e) Blow hole 4. The base of a cliff is undercut by rocks and sand being hurled at it by breaking waves. This process is called (a) attrition. (b) corrasion. (c) solution. (d) hydraulic action. (e) abrasion. 5. All of the following features are produced by wave deposition except (a) sand bar. (b) beach. (c) stack. (d) spit. (e) mud flat. Directions for question from 6 to 10: Each question has one or more correct option/s. Identify which of the options are correct and select the answer as per following. (a) if 1 only is correct. (b) if 1 and 2 only are correct. (c) if 1, 2, and 3 are all correct. (d) if 2 and 3 only are correct. (e) if 3 only is correct. 6. A sea cave is formed as a result of (a) a weakness in the rock. (b) wave erosion. (c) longshore drift. 7. A sea wave (a) represents a mass movement of water. (b) is produced by a wind. (c) represents a movement of energy. 8. Wave refraction (a) causes wave energy to be concentrated in bays. (b) occurs when a wave breaks. (c) occurs when a wave approaches a headland and bay coast.

8.34  Chapter 8

9. Wave erosion is most marked (a) below low tide level. (b) at high tide level. (c) between low and high tide levels. 10. A steep cliff is likely to form when (a) rock strata are horizontal. (b) rocks have well-developed joints. (c) rock strata dip landward. 11. Which of the following are marine agents of erosion? 1. Waves 2. Beach materials 3. Hydraulic action 4. Solvent action Select the correct answer from the following codes (a) Only 1 and 3 (b) 1, 2 and 4 (c) Only 3 and 4 (d) 1, 2 ,3 and 4 12. A flat area at the base of a cliff which was formed as the cliff retreated is called a/an: (a) Arch (b) Beach (c) Bar (d) Wave-cut platform 13. Select the response which lists the features in the order that they are formed. (a) Arch, cave, stack (b) Cave, arch, stack (c) Cave, stack, arch (d) Stack, arch, cave

EXERCISE 2 Long Answer Questions Direction: Answer the following questions in 150 words. 1. Figure 8.59 is a cross-section of a coastal area. Use this figure to answer questions 1 and 2. (a) What are the five features labelled from A to E in this diagram? Choose from the following: spit, cave, cliff, beach, natural arch, wave-cut platform, A B C D E stack, raised beach. (b) Which of the following could have been used to prepare this cross-section: cuspate foreland, bay, headland? (c) Choose any two of the features selected by you in (a) and for each briefly describe its origin. FIGURE 8.59  Cross-Section of a Coastal Area (d) Name the type of waves which operate along this coast and explain how they develop. (e) Name one coastal region in Great Britain which has features of this type. 2. Study Figure 8.60 which shows a coastline of bays and headland. (a) Name the five features labelled from A to E making your selection from this list: cuspate foreland, lagoon, headland, bayhead beach, bar, barrier beach, spit. (b) Name the feature at B and explain how such a feature is formed. (c) What is a groyne? (d) Explain why groynes have been built at D.

Coastal Processes   8.35

3. All these terms refer to processes operating along coastE Longshore Drift lines: swash, backwash, longshore drift, constructive Sea D A B wave, destructive wave. (a) Briefly explain the meanings of any three of these. (b) For each of the remaining two terms, draw a C Land Groyne well-labelled diagram to illustrate its main characteristics and name the type of coast where it FIGURE 8.60  Coastline of Bays and Headland develops.    4.    (a)  By using diagrams, state the main differences between (i) ria coast (ii) fiord coast (b) For each type of coast, name one region where an example of it may be seen. 5. Wave-cut platforms, caves, and geos are features formed by the work of the sea. Explain how each is formed. Draw well-labelled diagrams to illustrate your answers and give an example of each from a coastline in Great Britain which you have studied. 6. Write a short account of the relationships in the coastal system.

Answer key Exercise 1 1. (c) 5.   (c) 9.   (e) 13.   (b)

2. (e) 6. (b) 10. (a)

3. (c) 7. (c) 11. (d)

4. (e) 8. (a) 12. (d)

Thispageisintentionallyleftblank

9

The Oceans

Learning Outcomes After completing this chapter, you will be able to: ● ● ● ●

Comprehend the vastness of oceans and sea Examine the action of waves in shaping earth’s land surfaces Understand the difference in surface water temperature and ocean currents Develop the understanding of living oceans, coral reefs, and human impact on the ocean

Keywords Ocean, Ocean Currents, Tides, Coral Reef, Tsunami

1

9.2  Chapter 9

Introduction The world’s ocean is the most prominent feature on the earth. Oceans cover 70.8 per cent of the earth’s surface. The origin and development of life on earth are connected to the ocean. The oceans have a long history on earth. Earth has one ocean which is divided into four principle oceans—Pacific Ocean, Atlantic Ocean, Indian Ocean and Arctic Ocean. Specific characteristics of Pacific Ocean, Atlantic Ocean, Indian Ocean and Arctic Ocean are given in Table 9.1 and their location is shown in Figure 9.1. Pacific ocean is known as the world’s deepest ocean and earth’s largest geographic feature named in 1520 by Ferdinand Magellan. Atlantic Ocean is half the size of the Pacific Ocean, shallower than the Pacific Ocean, and it separates the Old

Pacific Ocean is the World’s largest ocean that accounts for more than half of Earth’s ocean space.

ARCTIC OCEAN

ES AN KJ GE Y D RE RI

L RA H KU ENC TR

-A RI TLA DG NT E IC

NORTH AMERICA

ASIA EUROPE

PER U-

MANIHIKI PLATEAU

E IC RIS

BRAZIL BASIN

EAST

ARGENTINE BASIN

ANGOLA BASIN

CHAGOSLACCADIVE PLATEAU MAS CAR PLAT ENE EAU AN DI IN D- GE MI RID

PACIF

SOUTH AMERICA

TR ILE ENCH

KER MA TRE DEC NCH

C

H

MID-ATLANTIC RIDGE

PACIFIC OCEAN

SOUTHWEST PACIFIC BASIN

AFRICA

ATLANTIC OCEAN

NINETYEAST RIDGE

MIDPACIFIC MTS.

M A TR RIA EN N CH A

M

ID

JA TR PAN EN CH

TIAN ALEU H C TREN

T ES W GE TH RID INDIAN OCEAN U SO IAN D ATLANTICIN KERGUELEN E INDIAN RIDG PLATEAU ENDERBY WEDDELL PLAIN PLAIN

ANTARCTICA

PACIFIC OCEAN

MELANESIAN BASIN

AUSTRALIA

PACIFICANTARCTIC RIDGE 0

1600 mi

0

2500 km

FIGURE 9.1  Distribution of Oceans.



Table 9.1

Surface area, water volume, and depth of different oceans.

OCEAN

SURFACE AREA (106 km2)

WATER VOLUME (106 km3)

AVERAGE DEPTH (km)

MAXIMUM DEPTH (km)

Pacific

180

700

4.0

11.0

Atlantic

93

335

3.6

9.2

Indian

77

285

3.7

7.5

Arctic

15

17

1.1

5.2

The Oceans    9.3

World from the New World. Indian Ocean is smaller than the Atlantic Ocean and similar in depth to the Atlantic Ocean. It is primarily in the Southern Hemisphere. The Arctic Ocean is 7 per cent of the size of the Pacific Ocean. It is the shallowest world ocean with permanent layer of sea ice a few meters thick.

Oceanic Zones In Chapter 8, we examined the action of waves in shaping the earth’s land surfaces which may lead us to believe that wave erosion and deposition are the principal activities of the sea. This is not so. Nearly 71 per cent of the earth’s surface covered by the seas (Figure 9.2), most of which forms the Atlantic and Pacific Oceans (Figure 9.3) with the deep seas occupying almost 55 per cent of the total water surface. The oceanic zone is the region with 65 per cent of the ocean’s completely open water. In other words, it includes the ocean lying beyond the edge of the continental shelf. Operationally, the term oceanic zone is often used from the point where the water depths drop to below 200 m. This is generally referred to as seaward, originating from the coast to the open ocean. Figure 9.4 illustrates the oceanic division at different depths including, the epipelagic, mesopelagic and bathypelagic zones (Figure 9.4). Table 9.2 details important characteristics of these three zones. The surfaces of all the seas are in motion through the generation of waves by the frictional effect of the winds but below the surface there is a continuous movement of water which is not wind generated and which results in the transfer of vast amounts of energy, all of which is derived from the Sun. Because of their surface area, the seas receive almost 71 per cent of all incoming solar energy.

90 per cent of the ocean called deep portion lies in the bathypelagic (aphotic) zone into which no light penetrates.

Pacific Ocean 165 Million km²

Water Surface 360 Million km2

Land Surface 150 Million km2

FIGURE 9.2  The Approximate Areas of Water and Land Surfaces Forming the Earth’s Surface.

Arctic Ocean 14 Million km² Seas 25 Million km²

Atlantic Ocean 82 Million km² Indian Ocean 74 km²

FIGURE 9.3  The Relative Areas of the World’s Oceans and Seas.

9.4  Chapter 9

High water

Pelagic Neritic

Oceanic Epipelagic

Photic

200 m

Low water Sublittorial or shelf

Mesopelagic 10 ºC

Littoral

700 to 1 000 m

l hya Bat

Bathypelagic 4 ºC

2 000 to 4 000 m

hic

nt

Be

Aphotic

Ab

Abyssalpelagic

ys

sa

l

Oceanic divisions

6 000 m

Hadal

Hadalpelagic

10 000 m FIGURE 9.4  Oceanic Divisions at Different Depth.

Table 9.2

Detailed characteristics of oceanic zones.

DIVISIONS

TEMPERATURE

CONDITION OF LIGHT

CHARACTERISTIC

Epipelagic (euphotic) zone

Ranges from 40°C to −3°C (104 °F to 27 °F)

Sunlit zone covered with enough sunlight

Enough sunlight to support photosynthesis

Mesopelagic (disphotic) zone

Ranges from 5°C to 4°C (41 °F to 39 °F)

Twilight zone covered with scarce amount of sunlight

The pressure is higher here, it can be up to 1470 lb./in.2 and increases with depth.

Bathypelagic (aphotic) zone

Ranges from 0°C to 6°C (32 °F to 43 °F)

Midnight zone with complete lack of sunlight where only light source is bioluminescence.

Water pressure is very intense and due to absence of sunlight photosynthesis cannot occur here.

Ocean Current The amount of the Sun’s energy absorbed by a sea surface depends on the way the Sun rays meet the surface. When the rays are vertical, all of the Sun’s energy is absorbed; when they are oblique, most of the Sun’s energy is reflected as shown in Figure 9.5. The energy absorbed by the seas is confined to the surface layers, which are warmed by

The Oceans    9.5

it, and which become less dense (lighter) as Oblique Sun Rays Vertical Sun Give Maximum a result. This means that the heated surface Rays Give Little Reflection layers remain above the colder, denser layers Reflection beneath. The Sun’s rays are vertical or nearly vertical for several hours almost every day of the year in equatorial latitudes whereas in polar latitudes, they are always very oblique. Rays This means that the seas are much warmer Enter Sea in the equatorial latitudes than in the polar Water latitudes. These differences in surface water temperature produce a movement of water, which is called an ocean current. Ocean currents are convection currents FIGURE 9.5  Warming Effects on a Sea Surface by Vertical and and they are similar to air currents in the Oblique Sun Rays. atmosphere. Both result from differences in temperature and both give rise to a transfer of heat energy. Ocean currents, which transfer heat are called warm ocean currents and they are surface currents. The movement of these currents produces a worldwide water circulation that affects all the oceans in which there is a returning current of cold water for each warm current. A flow of cold water in this circulation is called a cold ocean current. In general, warm ocean currents flow more quickly than cold ocean currents. The world’s major ocean currents are shown in Figure 9.6.

Ocean currents and winds Whilst it is true that differences in temperature in an ocean cause the flow of water, which is called a current, it must not be assumed that all of the ocean currents shown in Figure 9.6 are caused by temperature differences. In general, warm ocean currents bring warm water from equatorial regions to the polar regions and cold ocean currents bring cold water from polar regions to the equatorial regions. If you look at Figure 9.6 again, you will notice that the directions of the prevailing winds are given.

North Westerlies

nt

t West Wind Drif

Falkland Current

ro Ku

wo

Si

o Siw

23½°N

A B

t r e n ali a t n

h Equatorial Sout rrent Cu

Be ng C u ru ela re nt

South - East Trades

u G No rth Equ atorial C urr e n t

North Equatorial Current Counter Curre n t

Pe C u ruvia n rre n t

North - East Trades

S

a Oy

Ca Cu na rre ries nt

ro

Ku

o iw

c nti tla A t rth Drif No am tre fl S

l Brazi Curre

South Westerlies

or ad br nt La urre C

Prevailing Winds

es W u st A rr Cu

23½°S

Upwelling Water

FIGURE 9.6  The Main Ocean Currents in Relation to The Prevailing Wind Systems. Blue Lines Stand for Cold Currents; Black Lines for Warm Currents. The Reversal of the Monsoon Wind System Causes a Reversal of Ocean Currents in the Arabian Sea (A) And the Bay of Bengal (B).`

9.6  Chapter 9

In addition to surface ocean currents, there is known to be a large movement of water well below the surface, often in directions opposite to the surface currents.

When a wind blows over an open water surface, it has a dragging effect upon the surface, which causes the water to move in the direction of the wind. This effect is particularly well developed when a wind blows continuously across a wide water surface such as that of an ocean. Figure 9.6 shows that a definite relationship exists between the trade winds and the Atlantic Ocean’s equatorial currents, in regard to direction, and similarly between the westerly winds and the Gulf Stream. The trade winds blow continuously and with such regular strength that they drag surface water from Africa and pile it up in the Caribbean and the Gulf of Mexico, causing the level of the water in the latter to rise. This pile up of warm water is one of the causes of the Gulf Stream. Similarly, the waters of the Gulf Stream are dragged along by the westerly winds towards the coasts of Great Britain and Europe. Other correlations between winds and ocean currents can be recognized in the Pacific Ocean, the Arabian Sea, and the Bay of Bengal. The main effect of the prevailing winds on ocean current flow occurs in an east-west direction. The main effect of temperature differences occurs in the north-south flows. Another effect of prevailing winds on ocean current flow occurs along some leeward coasts, i.e., coasts where the winds are offshore. This can be seen along the west coast of Africa. As the winds blow water away from the coast, sub-surface cold water wells up offshore, which forms the cold currents known as the Canaries and Benguela currents. Another point to remember is that the Coriolis force caused by the rotation of the earth results in ocean currents following curving paths. As we know, the ocean is not still but moving. The complexity of deep ocean currents due to variations in density as a result of differences in salinity and temperature is continuously replacing ocean water with colder water in the deep ocean. The density of ocean water is higher in cold water as compared with warm water. Water gets colder with depth because cold, salty ocean water sinks to the bottom of the ocean basins below the less dense warmer water near the surface. The sinking and transport of cold, salty water at depth combined with the wind-driven flow of warm water at the surface creates a complex pattern of ocean circulation called the “global conveyor belt” (Figure 9.7). NORTH AMERICA

ASIA EUROPE

NORTH AMERICA

AFRICA

SOUTH AMERICA

Pacific Ocean

Indian Ocean

Atlantic Ocean

rm Wa

Su

low

eF

c rfa

AUSTRALIA

Cool Subsurface Flow

FIGURE 9.7  The Great Ocean Conveyor Moves Water Around the Globe. Cold, Salty Water is Dense and Sinks to the Bottom of the Ocean While Warm Water is Less Dense and Remains on the Surface.

The Oceans    9.7

Living Oceans The oceans are teeming with life, from microscopic Sea Surface organisms to animals such as the whale, which is larger than any animal living on the land surface. There is sufficient sunlight in the upper layers of oceans to allow A l nta green algae to live using the process of photosynthesis, ine t n Co which enables them to make carbohydrates in their lf She chloroplasts from water and oxygen in the presence of sunlight. The algae plus the tiny crustaceans, which B feed on them, are called planktons. This drifts with the ocean currents and becomes the food for many species of fish. Plankton and the fish that feed on it tend to be concentrated in coastal waters, which is why many fishing grounds are located in these waters. At lower levels, sometimes called the twilight zone, there are many types of organisms including fish, C which obtain their food either from marine detritus (remains of dead plant and animal organisms), which is continuously sinking down from the upper layers, or from eating other animal organisms. No plants live in the twilight zone because sunlight cannot penetrate Ocean Floor to the depth of this zone but there is an abundance of bacteria, which are the principal decomposers. In the FIGURE 9.8  The Three Layers Each deepest waters, there is yet another form of marine life. Containing Different Types of Marine Life. This consists of many species all of which have become highly adapted to the total darkness and high pressures A: Upper Layer; B: Twilight Layer; C: Deep of these deep waters. These organisms live either on Layer. detritus or on detritus-­eating organisms. Figure 9.8 shows the locations of these three levels. All the time, a constant stream of minute particles of sediment slowly descends from the upper levels and settle on the ocean floor where it forms a layer of mud, which in time becomes sedimentary rock. We have seen that many important fishing grounds tend to be concentrated in shallow coastal waters where there is plenty of plankton. However, fishing grounds develop wherever there is sufficient plankton to support a large fish population. Mention has already been made of the Grand Banks fishing grounds. The mixing of the waters of the two ocean currents draws up the nutrients from the continental shelf and these become food for surface organisms and in turn for the fish that feed on them. Nutrients are also brought to the surface by upwelling water along the west coast of parts of Africa and South America thereby enabling important fishing industries to develop.

Coral Reefs Coral reefs are fruitful marine ecosystems, large underwater ­structures composed of the skeletons of coral-marine invertebrate animals. H ­ erma­typic or “hard” corals are the species that help in building the coral reefs; by extracting calcium carbonate from seawater (salt water), they create a hard, durable exoskeleton that acts as a protection for their soft, sac-like bodies. Corals are associated with the phylum Cnidaria—a diverse group that includes species like hydroids, jellyfish, and sea anemones. Every individual coral is called a polyp and new coral polyps

9.8  Chapter 9

live on the calcium carbonate exoskeletons of their ancestors that add their own exoskeleton to the existing coral structure. With passing times, since hundreds of years, the coral reef gradually grew and evolved one tiny exoskeleton at a time, and subsequently took the shape of massive features of the undersea environment. Individual polyps have a diameter of 1–3 mm and each polyp is connected with another through a very thin layer of tissue. The connection of each polyp helps in sharing of nutrients. Corals are colonial organisms and so diverse that they are termed as rainforests of the sea. Reefs provide food and shelter for approximately one-quarter of all marine species but reefs cover only a tiny fraction; less than two per cent of the ocean bottom and less than one per cent of the earth’s surface. Reefs occupy coastline along more than 1,50,000 km in more than 100 countries and territories that covers globally less than 0.2 per cent of the seabed including the world’s oceans (Figure 9.9), from the Aleutian Islands off the coast of Alaska to warm tropical waters of the Caribbean Sea. The biggest coral reefs are found in the clear, shallow ocean waters of the tropics and subtropics where they grow quickly. The largest of these coral reef systems—the Great Barrier Reef of Australia—is more than 2400 km in length.

Major reef types The coral reef classification, first proposed by Charles Darwin is still relevant today. Charles Darwin explored the Indo-Pacific region in his expedition around the coral reefs and identified three categories of them on the basis of geological evolution of Pacific oceanic islands. Darwin’s general “reef evolution” theory was finally verified for Indo-Pacific reefs in the early 1950s after the analyses of the results of deep core drilling at Bikini and Eniwetok Atolls. Darwin hypothesized that there is growth of fringing reefs near the shorelines of new islands due to ideal ecological conditions for hard coral growth. Then, as the island began to gradually subside into the sea, and the coral was able to keep pace in terms of growth, it remained in place at the

FIGURE 9.9  Distribution of Shallow Water Coral Reefs.

The Oceans    9.9

sea surface. Eventually away from shore, corals take the shape of a barrier reef with presence of only the ring of coral encircling the central lagoon and disappearance of the island and thereby forming an atoll (Figure 9.10). Therefore, each of the three major types of coral reefs, i.e., fringing reef, barrier reef, and atoll are often formed by quite different geomorphic processes. So these reefs are differentiated on the basis of reef morphology—their shape, size, and their relation with adjacent land.

Fringing Reef

Fringing reefs Also known as shore reefs, are the most common category of reef that grow seaward directly from the shore without true lagoon-deep water channel between the nearby lands and reef. They form borders with surrounding islands and along the shoreline. These are more sensitive to human impact because of increasing population in coastal area, intensive agriculture, and accompanying increases in coastal development, and therefore presently causing the destruction of fringing reef. Some of the causes of decimation of fringing reef include the fresh water runoff, pollution, and sedimentation.

Barrier Reef

Barrier reefs Are far less common than fringing reefs or atolls, although examples can be found in the tropical Atlantic as well as the Pacific. These are extensive linear reef complexes that are parallel to a shore and are separated from it by lagoons, referring to a comparatively wide band of water that lies between the shore and the main area of reef development and contains at least some deep portions. Largest in this category is the Great Barrier Reef found in Australia. However, it is not actually a single reef as the name implies, but rather a very large complex consisting of many reefs. The second largest Indo-Pacific barrier reef lies off New Caledonia’s northeast coast.

Atoll FIGURE 9.10  The Evolution of Main Types of Coral Reefs, as First Proposed by Charles Darwin.

Atoll In contrast, an atoll is a roughly circular (annular) oceanic reef system surrounding a large (and often deep) central lagoon. They are present in mid-ocean in the South Pacific in the Caroline and Marshall Islands, and the Cook Islands, French Polynesia and Micronesia. Numerous atoll formations are also present in the Indian Ocean located in Maldives and Chagos island groups, the Seychelles, and in the Cocos Island group. In contrast, atolls are relatively rare in the Caribbean. The reason for far greater number of atolls in the Indo-Pacific region of coral reef development—as opposed to the Greater Caribbean region—can be mainly attributed to the far greater size of the former region along with its unique geomorphology, which is far more conducive to volcanic island formation and subsequent subsidence. Coral reefs are also very important to people, providing food, jobs based on tourism, medicines, and protection of shorelines. However, the greatest threats to reefs are ocean acidification and rising water temperatures, linked to rising carbon dioxide levels. High water temperatures cause corals to lose the microscopic algae that

9.10  Chapter 9

produce the food needed by corals—a condition known as coral bleaching. “Coral bleaching” occurs when coral polyps lose their symbiotic algae, the zooxanthellae, and without the presence of these algae the living tissues are nearly transparent with visible stony skeleton. Many different kinds of stressors can cause coral bleaching— water that is too cold or too hot, too much or too little light, or the dilution of seawater by lots of fresh water can all cause coral bleaching. The biggest cause of bleaching however today has been rising temperatures caused by global warming. Temperatures more than 1°C above the normal seasonal maximum–minimum can cause bleaching. Other threats to coral reefs and their habitats therefore are climate change, destructive fishing practices, overfishing, careless tourism, pollution, sedimentation, and coral mining.

Nature of Tides When a tidal wave produced by a high tide enters an estuary, the height of the wave increases as the estuary becomes increasingly narrow and shallow. Eventually, the wave breaks and forms a “wall” of foaming water, which often surges forward at several kilometres an hour. Such tidal waves can push the water of a river back up its channel for several kilometres. This occurs in the Severn estuary. The increased height of a tidal wave caused by a narrowing of an estuary is called a bore. The Severn bore is made higher by the blocking of the outflow of the River Severn. The gravitational pull of the moon and, to a lesser extent the Sun, on the earth’s surface causes a rising and falling motion to develop in the waters of the larger oceans. This up and down movement of the surface water produces a tide, and the tide-­ producing forces cause the surface of the L1 (a) Moon’s Orbit water to oscillate. This means that the water rocks bodily, rising and falling near its edges, i.e., along the surrounding coasts. H2 Figure 9.11 helps to explain how moon causes H1 Moon tides. When the moon is overhead (position H2 in Moon’s Figure 9.11(a)), the water at H2 is pulled towards Pull Earth the moon more than the earth, and it piles up to Rotation L2 form a high tide. At the same time, the earth is pulled towards the moon more than the water at (b) H1, i.e., the water at H1 lags behind and piles up to Moon Moon form a high tide. The water, which piles up at H1 (i) (ii) and H2 forming high tides, has been pulled away A from L1 and L2 where low tides occur. A2 A1 24 Since the earth rotates once in 24 hours, it 28 days hrs would appear from Figure 9.11(a) that every part Earth Earth of the earth comes into the position of two high tides and two low tides every day. But this is not quite true. Look at Figure 9.11(b). This diagram Earth Takes 1/28 Day (52 minutes) to Rotate shows that the earth rotates once in 24  hours whilst the moon revolves around the earth once from Position A1 to in 28 days. The starting position (i) shows point A Position A2 on the earth directly beneath the moon. TwentyFIGURE 9.11  (a) The “Pull” of the Moon Causes the Tides; four hours later (position (ii)), point A has made (b) Explains Why Tides Arrive 52 Minutes Later Each Day. one complete rotation and arrives at position A1.

The Oceans    9.11

Meanwhile, the moon has travelled exactly 1/28 of its distance around the earth, which means that Earth to Sun = 149,785,000 km it will take the earth 1/28 of a day, i.e., 52 minutes to “catch up” with the moon by reaching Sun 27 position A2. And when the earth is in position 19 22×10 Metric Tons Moon = 7.3x10 A2, it will experience high tides again. In other Metric Tons words, it takes the earth 24 hours, 52 minutes to move from high tide position A1 to the next high Earth to Moon = 384,835 km tide position A2. This explains why high tide is 52 minutes later each day. Mass Tide-Generating = ∞ Relationship of tide-generating force 3 (Distance) Force between the Earth, Moon, and Sun is illustrated in Figure 9.12. There is an inverse relationship Tide-Generating = ∞ Sun Mass 3 Force of the Sun between tide-generating forces and the cube of (Sun’s Distance to Earth) the distance from the tide-­ generating object. NOTE: The Sun has 27 Million Times More Mass than the Consequently, the Sun’s tide-generating force Moon and is 390 Times Farther Away from the is about half that of the moon, and the moon Earth than the Moon. is the dominant force affecting the earth’s tides 3 27 Million So... and the currents they produce. = 0.46 or 46% (390) =59,000,000 59 Million When the Sun, earth, and moon are in a Therefore the Sun has 46% of the Tide-Generating Force straight line as they are 1 or 2 days after full of the Moon. moon and new moon each month, the gravFIGURE 9.12  Relationship of Tide Generating Force itational pull is at its greatest because the Sun Between the Earth, Moon and Sun. and the moon pull in the same or in opposite directions as shown in Figure  9.12. These two Earth Sun positions give very high tides called spring tides. Moon When the Sun and the moon are at right angles to each other (Figure  9.14), the difference between high and low tides is at its lowest. These Full Moon tides are called neap tides. In the open oceans, the difference between Earth Sun high and low tides is only about 80 cm whereas Moon along coastlines, it may be much higher. The spring tides around Great Britain sometimes reach heights of 300—400 cm. The difference New Moon between high tide and low tide is called the tidal range (Figure 9.15). FIGURE 9.13  The Pull of the Moon and Sun Produces Spring Tides.

Tidal influences

Half Moon

Some ports have a high tidal range, which allows large ships to use the ports only at high tide. Sun London, Le Havre, and Rotterdam all have a high tidal range. However, it is sometimes possible to excavate deep basins in the port areas, which permit large ships to load and unload at any time, even at low tide. The basins are called Earth wet docks and a large number of these have FIGURE 9.14  Neap Tides are Caused by the Pull of the Moon been built in the Port of London. and Sun Operating at Right Angles.

9.12  Chapter 9

Wet Docks As ships became larger, deeper and larger wet docks were needed, and these have been built further down the Thames estuary as shown in Figure 9.16. This diagram shows that the port is moving down the estuary. The older parts of the port, which are farther inland, are being redeveloped for other uses.

Other ports, such as those in the Mediterranean, have a low tidal range, which means that ships can enter and leave at any time. It should be remembered that ports, which can only be used at high tides, increase the operational costs of ships, which use those ports. Ships that have to wait several hours for the high tide so that they can enter or leave these ports are earning no money whilst they are waiting. Also, waiting may add to harbour charges which are payable by the ship owners in respect of the use made of the ports. Spring Tidal Range

High Tide Tidal Range Low Tide

Neap Tidal Range High Tide Low Tide

Tidal Range

FIGURE 9.15  Tidal Ranges of Spring and Neap Tides.

River Tha me s

A

B

0

5 km

Tha me s

City of London

y tuar Es

Wet dock FIGURE 9.16  London Dockland. As Ships Increased in Size, the Docks in Zone a Could Not Handle Them. All the Docks in this Zone are now Closed. New Docks were Built in Zone B (Wet dock).

The Oceans    9.13

Energy from the Oceans The oceans have tremendous energy, which is reflected in waves, currents, and tides, and several schemes have been devised to harness some of this energy. One scheme involves the turning of tidal energy into electricity. This can be done in places where there is a high tidal range and where there is a bay or estuary across which a dam can be constructed. The way this scheme would work is shown in Figure 9.17. A scheme of this type is operating in the Rance estuary in Brittany, France. Other schemes involve using (1) wave energy and (2) temperature differences between warm surface water and cold deeper water in tropical regions. The latter would use the temperature difference to alternately vaporize and condense an agent such as ammonia with the pressure built up on vaporization being harnessed to drive a turbine generator. (a)

Dam

Reservoir

Sea

(b)

Turbine Generator FIGURE 9.17  At High Tide (a) Water Enters Reservoir; at Low Tide (b) Water Leaves Reservoir. Each Flow of Water Drives the Turbine Generator.

Natural Hazards of Oceans From time to time, unusually large waves develop as a result of an earthquake or a volcanic eruption either underwater or on land near to the sea. These sudden disturbances, transmit shock waves to the surface of the water, which produce waves quite different to those caused by the wind. The wavelength of these waves, may be 150 km or more compared with that of an ordinary wave, which is between 100 and 250 m. They travel up to 700 km/h whereas ordinary waves travel at less than 100 km/h. These huge waves are called tsunamis (Figure 9.18). They have enormous energy, which is released when they break in shallowing water causing a wall of water to surge forward over and well beyond the beach area. A tsunami consists of several waves, the time lapse between the waves varying from 15 to 50 minutes. The height of the waves builds up dramatically with the first wave being no more than a well-developed swell. The largest waves are usually the fourth to eighth waves.

9.14  Chapter 9

(a) Stuck Overriding Plate Subdu cting plat e

Tsunami Waves Spread

orition S l o w Dist

Earthquake Starts Tsunami

Stuck Area Ruptures, Releasing Energy in an Earthquake

(b)

(c)

FIGURE 9.18  Tsunamis (a) Effects of Earthquake on Tsunamis, (b) Tsunami Along East Coast of India (c) Tsunami-Affected Areas.

Figure  9.19 visualizes the devastation occurred during the tsunami in Indian Ocean through a comparison of two imageries before and after Tsunami. Another type of unusually large wave is called a storm surge. Relationship between storm surge and storm tides is illustrated in Figure  9.20. Storm surge occurs when an abnormal rise of water is generated by a storm, over and above the predicted astronomical tides; storm tides are defined as the water level rise due to the combination of storm surge and the astronomical tide. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 ft. or more in some cases.

The Oceans    9.15

Storm surges consist of high waves caused by very strong winds blowing across an extensive ocean surface, the strength of the winds often being increased by low atmospheric pressure. Storm surges occur in tropical regions, which experience cyclones and the height of the waves may reach 15 m where onshore winds pile up the water in restricted coastal regions such as gulfs and bays (Figure 9.21).

15 ft Surge

17 ft Storm Tide 2 ft Normal High Tide

Mean Sea Level

NOAA/The COMET Progra

FIGURE 9.20  Relationship between Storm Surge and Storm Tide.

FIGURE 9.19  Devastation Visualized through Imageries (Tsunami, 2004), Indian Ocean.

BANGLADESH Hatla

48

Dakhin Shahbazpur Patuakhali Morelganj Calcutta Sagar Island Contai Balasore

INDIA

32

Taungya Taung

Paradip Puri

16 0 Wave Surge Height in Feet

Sandwip MYANMAR Chittagong Cox’s Bazar

Gopalpur Kaling Apatna Visakhapatna Kakinada

Machilipatnam Ongole Kavali Nellore Madras Pondicherry Nagapattinam

y Ba

l nga Be f o

Andaman Islands (India)

FIGURE 9.21  A Storm Surge Graphic of the Great Boha Cyclone of November 1970. The Bars Indicate How High in Feet the Storm Surge was at Various Locations Along the Coasts of India and Bangladesh. Each Bar is “48 Ft.” Long and the Green Shading Inside the Bars Shows How Many Feet (Out of 48’) the Surge Reached. The Greatest Height Was 40 ft. at Hatia, Bangladesh. Graphic from Extreme Weather: A Guide and Record Book Based Upon Data Supplied by the Indian Meteorological Department.

9.16  Chapter 9

Beneficial Influences of the Oceans The oceans provide the landmasses with their water supply through the interaction of their surface waters with the atmosphere. Water enters the atmosphere through the process of evaporation, is carried over the land surfaces by winds and is precipitated as rain and snow through the process of condensation. Some of the solar energy absorbed by the oceans is passed on to land surfaces through warm ocean currents and some winds. The South Westerlies, which blow over Great Britain, plus the warming influence of the North Atlantic Drift, enable the west coast of Scotland to support sub-tropical plants in sheltered inlets. The oceans are ideal for moving goods and bulky materials in large quantities easily and cheaply between continents.

A Closer Look  ▼ Human Impact on the Oceans The pollution of surface water and sub-surface water by chemicals released through industrial activities either directly into rivers or into the air, and by fertilizers used in agriculture, has led to the pollution of coastal waters via river systems. Nitrogen phosphates in sewage and nitrates used in fertilizers result in extensive algal growth, which rapidly lowers the oxygen content of the water. This causes a decrease in the fish population. Other harmful chemicals such as mercury and chlorinated hydrocarbons such as DDT insecticides have polluted some sea areas sufficiently to cause harm to many species of fish. This has been particularly serious in the Baltic. The rapid expansion of the oil industry over the past 30 years has led to an increase in coastal water pollution. Although it is now illegal to dump ballast water mixed with oil in to the sea, it still goes on in some waters. And with the movement of most of the world’s oil requirements by very large oil tankers, the probability of extensive oil spillage through collision is fairly high.

Most of the countries in the developed world produce varying amounts of waste chemical products such as radioactive waste, which are considered to be too dangerous to dispose of on the land, and so they are dumped in the sea. Although the wastes are put into strong water-tight containers, which in theory should remain intact for a very long time, nobody can be sure for how long this will be. The optimists claim that even if the containers break down, spilling the waste on the seabed, it is not likely to cause serious damage because of the depth at which the containers are dumped. But it is now known that currents operate in the oceans even in the deepest parts and as we get more information on the movements of these ­currents, we may find that the chemicals from the dumped waste reach the upper levels of the oceans faster than was once realized. A great deal of discussion at regional, national, and international levels is taking place on the extent to which pollutants are damaging not only the oceans but the whole hydrosphere. Nobody denies that damage is being done; disagreement appears to centre on the severity of the damage and on ways of reducing pollution.

The Oceans    9.17

Key facts ●●

●●

●● ●● ●●

●● ●● ●● ●● ●●

Salinity of the ocean is dependent on ➤➤ temperature ➤➤ amount of fresh water added ➤➤ ocean currents There are two types of movement in the oceans: ➤➤ vertical—governed by temperature and salinity; ➤➤ horizontal—governed by wind (direction influenced by shape of land and earth rotation); this movement produces ocean currents (warm and cold). Ocean currents influence temperature, especially in coastal areas where winds are onshore. The moon, and to a lesser extent the Sun, exert a gravitational pull on the earth, which causes the tides. The highest and lowest tides occur when the Sun, earth, and moon are in a straight line, i.e., at full moon and new moon. These tides are called spring tides. High and low tides are at their lowest when the Sun and moon are pulling at right angles, i.e., at half moon. These tides are called neap tides. A tidal wave entering an estuary causes a surge of water to push against the out-flowing river. This water surge is called a bore. Landmasses get most of their water supply from the oceans. Upwelling of bottom water and the meeting of cold and warm currents give rise to rich fishing grounds. A tsunami is a powerful wave produced by submarine earthquakes and sometimes submarine volcanic eruptions.

9.18  Chapter 9

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct answer. 1. All of the following play a part in the development of ocean current systems; but which one plays the most important part? (a) differences in salinity of the water (b) planetary wind systems (c) differences in temperature of the water (d) rotation of the earth (e) shapes of landmasses 2. Ocean currents flow in an anticlockwise direction in the southern hemisphere but they flow in a clockwise direction in the northern hemisphere. This is because (a) most of the currents in the northern hemisphere are warm. (b) the oceans of the southern hemisphere are larger. (c) land in the southern hemisphere has a smaller east to west extent. (d) the earth rotates from west to east. (e) there are strong and more persistent winds in the southern hemisphere. 3. Most warm ocean currents flow (a) in tropical seas. (b) on the eastern sides of continents. (c) towards the equator. (d) in high latitudes. (e) towards the high latitudes. 4. Consider the following factors (i) Rotation of the Earth (ii) Air pressure and wind (iii) Density of ocean water (iv) Revolution of the earth Which of the above factors influence the ocean currents? (a) (i) and (ii) (b) (i), (ii), and (iii). (c) (i) and (iv) (d) (ii), (iii), and (iv) 5. Consider the following statements: (i) Ocean currents are the slow surface movement of water in the ocean. (ii) Ocean currents are set in motion primarily by prevailing winds. (iii) Ocean currents are affected by the configuration of the ocean. (iv) Ocean currents assist in maintaining the earth’s heat balance. Which of these are correct? (a) (i) and (ii) (b) (ii), (iii), and (iv) (c) (i), (iii), and (iv) (d) (i), (ii), (iii), (iv) 6. The average depth of Indian Ocean is (a) around 1.1 km. (b) around 3.7 km. (c) around 4.0 km. (d) around 3.6 km. 7. The zone in the ocean, which completely lacks sunlight and where the only light source is bioluminescence. (a) Epipelagic (b) Mesopelagic (c) Bathypelagic (d) Abyssopelagic

The Oceans    9.19

8. Storm Surge is an example of (a) unusually large wave (b) land surface wind (c) local small devastating wave (d) None of these 9. …………………. influence/s the temperature in coastal area where winds are onshore. (a) Sea waves (b) Ocean salinity (c) Tidal wave (d) Ocean currents 10. Which one of the following best describes a tsunami? (a) A human induced stormy surge. (b) A powerful earthquake which is generated inside of the continental crust and v­ olcanic eruptions. (c) A powerful wave produced by submarine earthquakes and sometimes submarine volcanic eruptions. (d) Both (a) and (b) 11. Which one of the following denotes water salinity gradient? (a) Thermocline (b) Halocline (d) Chemocline (c) Pycnocline 12. Tides occur in oceans and seas due to which of the following? (1) Gravitational force of Sun (2) Gravitational force of moon (3) Centrifugal force of earth Select the correct answer using the code given below: (a) 1 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3 13. Which one of the following oceanic currents is not associated with the Pacific Ocean? (a) Peruvian (Humboldt) current (b) California current (d) Kuroshio current (c) Canaries current 14. Which of the following is not matched correctly? (a) Peruvian current—North Pacific Ocean (b) Brazil current—South Atlantic Ocean (c) Agulhas current—Indian Ocean (d) Gulf stream—North Atlantic Ocean 15. What explains the eastward flow of the equatorial counter current? (a) Convergence of the two equatorial currents (b) Difference in salinity of water (c) Occurrence of the belt of calm near the equator (d) The earth’s rotation on its axis

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Briefly explain, with the aid of diagrams, any two of the following. (a) The Benguela Current is cold but the Mozambique Current is warm. (b) The connection between the North Atlantic Equatorial Current and the Gulf Stream. (c) The ocean currents of the North Indian Ocean change direction twice a year. (d) Salinity within an ocean varies from one part of the ocean to another.

9.20  Chapter 9

2. Write a brief explanation, illustrated with relevant diagrams and sketch maps where appropriate, for any two of the following: (a) Both horizontal and vertical currents occur in the oceans. (b) Winds are the main cause of the surface ocean currents. (c) Important fishing grounds are usually located in the waters covering continental shelves. (d) The salinity of polar oceans is lower than that of tropical oceans. 3. With the aid of sketch-maps and diagrams, write a brief account on three of the following: (a) the Benguela Current; (b) the high degree of salinity in the Dead Sea; (c) the seasonal change in direction of some ocean currents; (d) the continental shelf. 4. Define the coral reef. Explain the development and types of coral reef with an example. 5. Illustrate the relationship between the main ocean current and prevailing wind system. 6. Describe the human impact on the ocean with reference to coral bleaching .

Answer key Exercise 1 1. (c) 6.   (b) 11.   (b)

2.   (d) 7.   (c) 12.   (d)

3.   (c) 8.   (a) 13.   (c)

4.   (b) 9.   (d) 14.   (a)

5.   (d) 10.   (c) 15.   (d)

10

Atmosphere: Temperature

Learning Outcomes After completing this chapter, you will be able to: ●● ●●

●●

●●

Describe the structure of the atmosphere Understand the exchange of energy between earth and atmosphere through various processes Explain the various factors that affect the amount of heating received by any area of the earth’s surface Explain the temperature pattern at global level

Keywords Atmosphere, Greenhouse effect, Insolation, Heat Budget, Six’s Thermometer

1

10.2  Chapter 10

Introduction The air that surrounds the earth is called the atmosphere. It extends for approximately 950 km from the earth’s surface, and it comprises a mixture of gases, mainly nitrogen (approximately 78 per cent) and oxygen (approximately 21 per cent). The remaining gases are argon and carbon dioxide. Water vapour forms a small but vitally important component of the atmosphere. It is confined mainly to the lower atmospheric layer, which is called the troposphere. This layer extends from the earth’s surface to a height of approximately 9 km at the poles and to approximately 17 km at the equator. The other two principal atmospheric layers are the stratosphere and the ionosphere as shown in Figure 10.1.

Structure of Atmosphere The molecules of these atmospheric gases are not distributed evenly throughout the atmosphere. The greatest concentration of the gas molecules is in the troposphere. The distribution of gas molecules thins out with the increasing distance from the earth’s surface, i.e., the air becomes increasingly rarefied. The upper atmosphere acts as a protective barrier against a constant bombardment of charged particles, harmful rays, and meteorites. Ultraviolet light from the sun forms an gets absorbed by the ozone layer which is present in the lower part of the stratosphere, and this layer absorbs the harmful radiation from the outer space. Beyond the upper limit of the ionosphere, hydrogen and helium are the predominant gases, whilst with the further distance, hydrogen replaces helium as the predominant gas. The atmosphere has weight, i.e., it exerts a pressure on the earth’s surface. At the sea level, this pressure is approximately 1.03 kg/cm2. The pressure decreases with altitude, and at the top of the stratosphere, it is only approximately 1/10,000th part of the pressure at the sea level. This aspect will be examined in more detail in Chapter 11. It has been estimated that the troposphere carries almost 75 per cent of the weight of the atmosphere; this perhaps gives a better idea of the fall in atmospheric pressure with increasing altitude. Temperature (ºF) –148 –112 –76 –40 –4 100

320km (195.6mi)

80km (49.7mi)

Mesosphere

50km (21.1mi) Stratosphere

12km (7.5mi) Troposphere

80

Mesopause

70

50

Mesosphere

60 50

68 62

Thermosphere 56

90

Thermosphere

Altitude (km)

Figure 10.1  The Structure of the Atmosphere up to a Height of 300 km. Temperature Decreases from the Earth’s Surface to the Top of the Troposphere; this is Called a Tropopause. The Temperature then Increases to the Stratopause, the Top of the Stratosphere. It then Again Decreases.

32

Stratopause

44 37 31

40

25

30 Stratosphere

19

20

12

10

Tropopause Troposphere

0 –100 –80 –60 –40 –20 0 Temperature (ºC)

6 0 20

Altitude (miles)

Exosphere

Atmosphere: Temperature  10.3

Troposphere When we say it is hot or wet or cloudy, we are saying something about the weather. Weather refers to the state of the troposphere gets absorbed by the — its temperature, pressure, wind direction and speed, and humidity for a particular place over a short period of time. The troposphere contains substances that are vital for the growth of plants and animals. It contains nearly all of the earth’s water vapour. Although by total volume, water vapour is less than 2 per cent of the atmosphere, it enables the atmosphere to act as an efficient heat carrier. Water enters the atmosphere as water vapour through the process of evaporation from the oceans, rivers, vegetation, and lakes. For this purpose, it takes heat from the earth’s surface and the atmosphere. This heat is stored as latent heat in the water vapour, and it is carried by winds to different parts of the earth as well as to different heights in the atmosphere. When the water vapour condenses into rain or hail or snow, the heat is released to the atmosphere. We can regard the atmosphere as a vast energy transport system that operates between cold polar and hot tropical regions. This energy transport is achieved by the three global wind systems: the polar winds, the trade winds, and the westerly winds. These winds not only carry water (as vapour) but also latent heat (energy) from regions that have abundance of both vapour and energy to regions that have little of either.

Atmospheric System The interaction among pressure, temperature, and water vapour in the atmosphere is ­complex. Changes in pressure produce corresponding changes in temperature and vice versa. For example, the temperature of the air at the top of a mountain several thousand metres high is lower than that at the bottom of the mountain, while the air pressure at the mountain top is lower than that at its base. The temperature of the air is also affected by the amount of water vapour in the air. For example, on a cloudy night, the heat that the earth’s surface would normally lose to the atmosphere is radiated back by the cloud cover. In contrast, on a clear night, there is a steady loss of heat from the earth’s surface. The earth’s surface and the atmosphere receive all their heat from the sun. This heat is derived from solar energy, sometimes called solar radiation. Solar energy received by the earth is called insolation. We Components Inputs examined the fate of solar radiation in Chapter 1. of the System You will recall that approximately 6 per cent of solar Solar Energy radiation is reflected by the atmosphere and 27 per cent is reflected by the cloud cover, while the remainTemperature ing 67 per cent of solar radiation is absorbed by the earth and the atmosphere and is re-radiated. Pressure The atmospheric system (Figure  10.2) shows Evapotranspiration the inter-relationships of pressure, temperaHumidity ture, and humidity, which form the components of the system. To understand it as a system, it is necessary to examine each of the components in Re-radiation greater detail. This is done in this chapter and in Figure 10.2  The Atmospheric System. Chapters 11 and 12.

Outputs Precipitation

Winds

10.4  Chapter 10

Heating of Atmosphere The short-wave solar radiation absorbed by the earth’s surface is c­ onverted into heat energy, and it is this heat energy that heats the atmosphere (Figure 10.3) through the processes of radiation, c­ onduction, convection, evaporation, condensation, and compression heating (Figure 10.4).

Radiation On absorption by the ground, short-wave radiation is c­ onverted into long-wave energy, which is readily absorbed by the atmosphere, especially by water vapour. Some of the

Heat is Transferred up the Layers of Air

Sun Heats up the Earth’s Surface Layers of Air

Heated Air Figure 10.3  Illustration for the Heating of Atmosphere.

Earth’s Surface

Sun

Incoming Solar Radiation Some Heat Passes out into Space Reflection (Some of the Incoming Radiation is Reflected by Earth’s Surface and the Atmosphere Back out of Space)

Figure 10.4  Process of Atmosphere Heating.

Absorbtion (Most Radiation is Absorbed by the Earth’s Surface and Warms it)

Earth

Most Heat is Absorbed and Re-emitted by Greenhouse Gas Molecules, Further Warming Earth

Atmosphere: Temperature  10.5

heat absorbed by the atmosphere is re-radiated back to the ground and some back into the outer space. This process occurs continuously, i.e., from day to night. The water vapour acts as an insulator, i.e., the greater the water vapour content, the less is the loss of heat to the outer space; conversely, the lower is the water vapour content, the greater is the loss of heat to the outer space. This explains why the diurnal temperature range (the difference between maximum and minimum daily temperatures) is lower when the sky is cloudy and higher when it is clear. This insulating effect is called the greenhouse effect (Figure 10.5) because glass allows short wave solar radiations to enter but prevents their escape when they are converted to long waves on absorption.

Conduction Some heat passes into the air by the process of conduction. If the ground is warmer than the air above it, then heat passes from the ground into the air. However, because air is a poor conductor of heat, very little heat passes from one mass of air to another air mass.

A

T

M

O

S

Net Incoming Solar Radiation: 240 Watt per m2

G

R

E

E

Solar Radiation Passes through the Clear Atmosphere. Incoming Solar Radiation: 343 Watt per m2

N

H

O

P

H

E

R

Some Solar Radiation is Reflected by the Atmosphere and Earth’s Surface Outgoing Solar Radiation: 103 Watt per m2

U

S

E

G

E Some of the Infrared Radiation Passes through the Atmosphere and is Lost in Space Net Outgoing Intrared Radiation: 240 Watt per m3

A

S

E

S

Some of the Infrared Radiation is Absorbed and Re-emitted by the Greenhouse Gas Molecules. The Direct Effects is the Warming of the Earth’s Surface and the Troposphere. Surface Gains more Heat and Infrared Radiation is Emitted again

Solar Energy is Absorbed by the ...and is Converted into Heat Causing Eart’s Surface and Warms it... the Emission of Longwave (Infrared) 168 Watt per m2 Radiation back to the Atmosphere

E Figure 10.5  Process of Greenhouse Effect in Atmosphere Heating.

A

R

T

H

10.6  Chapter 10

Convection When air is heated, it expands and becomes less dense. This causes it to rise and become cooler; subsequently, more dense air descends to take its place. In this manner, heat is transferred from one part to another part.

Evaporation and condensation A large amount of heat received by the earth’s surface is consumed in converting water into water vapour. This is affected by the process of evaporation. The heat used in this process is retained by the water vapour and is called latent heat. When environmental conditions are favourable and water vapour is converted back into water (rain) or snow, this heat is released. This is affected by the process of condensation.

Compression heating When air contracts, its temperature increases as it descends. This means that when air moves from a higher altitude to a lower altitude, it is moving from an area of lower pressure to an area of higher pressure. This process compresses the air, i.e., it contracts and its temperature rises. These processes are shown diagrammatically in Figure 10.6. The temperature of the earth remains fairly constant, i.e., it becomes neither hotter nor colder. The 65 per cent of the short wave radiation absorbed by the atmosphere and the earth is balanced by the re-­radiation of an equal amount of long-wave radiation to the outer space. This is called heat balance or heat budget (Figure 10.7).

How the Earth is Heated Convection

When Air is Heated, it Expands

The Earth is Heated by (1) Direct Solar Radiation as

Heat to Atmosphere

Cloud

Compression Heating Condensation Heat Transferred to Atmosphere

Water Vapour Rain

Descending Air

Convection Short-Wave Radiation

Long-Wave Radiation

Short-Wave Radiation Absorbed by Earth and Turned to Heat

Heat Re-radiated to Earth

Greenhouse Effect

Conduction

Earth’s Surface

Figure 10.6  Exchange of Energy Between the Earth and the Atmosphere.

Parcel of Air

Atmosphere: Temperature  10.7

(a)

(b) SOLAR RADIATION 100% 100%

17%

Reflected by

8% Atmospheric Gases

Reflected by Clouds

and Dust

23%

Absorbed by Atmosphere 4% Clouds, 19% Water Vapour, Gases and Dust 52%

46%

Reflected 25%

Absorbed 22% 6%

Reflected by Earth’s Surface

Top of Atmosphere

Reflected 3% Up 7%

Scattered

Direct Radiation 33%

Down 10%

Absorbed by Earth’s Surface (Land and Water)

Figure 10.7  (a) and (b) Balance of Heat between the Earth and the Atmosphere.

Heating of the Earth (a) Atmosphere Long Distance Sun Rays

La

Earth Short A Distance

Latitude In general, tropical latitudes where the sun’s altitude is always high are hotter than temperate latitudes where the sun’s altitude is generally lower. Likewise, temperate latitudes are warmer than polar latitudes. Figure  10.8 helps to explain this phenomenon. The bands marked X contain equal amounts of insolation, but because area A is smaller than area B, the temperature at A is higher than that at B. Furthermore, the sun’s rays pass through a more thicker layer of air at B than at A; this means that the amount of insolation absorbed by dust and water vapour in the air is higher at B than at A.

B a Are rge

Small Area

The earth is heated by (1) direct solar radiation as already discussed and by (2) indirect radiation, which has also been discussed ­previously. The amount of heating received by any area of the earth’s surface depends on some or all of these factors: latitude; altitude; nature of the surface; distance from the sea; winds; cloud cover and humidity; aspect; length of day, and ocean currents.

Equator (b)

Mid-day Sun

Early Morning Late Evening Sun

Figure 10.8  (a) Insolation Covers a Smaller Area when the Sun’s Rays are Vertical (See A) than when they are Oblique (See B). Furthermore, Oblique Rays Pass Through a More Thicker Layer of Air than Do Vertical Rays; (b) The Sun’s Rays at Midday Warm a Smaller Surface Area than they Do at Sunrise or Sunset.

10.8  Chapter 10

n

Direction

Pressure Falls in this

latio Inso

rm Surf ac Wa e

3000 m

Warm Surface

Low Pressure Causes Rarefied Air

latio

Inso

12°C

n

Heat Escapes from the Surface More Slowly Because Dense Air Contains Dust

Altitude

Heat Rapidly Escapes from the Surface because Rarefied Air Contains Little Water Vapour and Dust

High Pressure Causes Dense Air

30°C

Figure 10.9  A Diagram to Show the Effect of Altitude on Temperature. (What would the Temperature be at the Top of a 4500-m High Mountain if the Temperature at the Sea Level was 30°C?)

Water vapour and dust in the air prevent heat formed at the surface of the earth from rapidly transferring back into the outer space. But at high a­ ltitudes, e.g., on the tops of high mountains, the air is rarefied, and it contains very little dust or water vapour. The heat from the earth’s surface therefore rapidly escapes, and the air remains cold. The temperature of stationary air falls by an average of 6°C for every 1000 m of ascent. In other words, if a balloon carrying a thermometer rose from the earth’s surface, the thermometer would record an average fall of 6°C for every 1000 m it ascended. This decrease in temperature is called the environmental lapse rate (ELR). If a mountain is 3,000 m high, it will have a temperature of approximately 12°C at its top if the temperature at its base, say at the sea level, is 30°C (Figure 10.9).

Nature of the surface We read in Chapter 1 that every type of surface, whether land or water, has different heating properties. A coniferous tree surface absorbs approximately 90 per cent of the solar energy reaching it, whilst a snow surface absorbs approximately 20 per cent. A grass-covered surface absorbs between 60 and 80 per cent of the energy. You should note that most of the energy absorbed by a vegetated area is used by the process of evaporation; this explains why these areas do not become very hot.

Distance from the sea Land surfaces heat and cool more quickly than water surfaces. This means that during the night, when radiation is cut off, a water surface tends to retain its heat; this results in the temperature over an adjacent land surface being a little higher than it would otherwise be. In other words, both diurnal and seasonal temperature ranges over a water surface (provided this is large and deep) are lower than they are over a land surface. Solar radiation can penetrate water to a considerable depth, which results in the heat energy being more equally distributed. In comparison, heat can penetrate a land surface to only a few centimetres. Furthermore, heat transfer in a body of water is affected by currents, tides, waves, and convection systems, which enable the heat to be distributed throughout the body of water. Hence, the surface water takes a long time to heat. Similarly, when a body of water is cooling, say during the winter or at night, the loss of heat from its surface is slowed down because of the convection currents circulating the water throughout the water body. Land surfaces are quite different in this aspect. Heat can only be transferred by conduction which takes much longer than the

Atmosphere: Temperature  10.9

transfer of heat by convection currents, etc. With the heat localized in the top few centimetres of a land surface, its loss by conduction to the atmosphere is quite rapid when solar radiation is cut off. These characteristics have a marked influence on temperature, especially in temperate latitudes where the sea warms coastal regions in the winter but cools them in the summer (Figure 10.10(a)) The warming influence is confined to a narrow coastal belt because the sea air rapidly loses its warmth to the colder land. Air temperatures decrease from the coast inland (Figure 10.10(b)). In the summer, land surfaces are warmer than sea surfaces, and the air over the land, therefore, is warmer than that over the sea. Coastal regions tend to be cooler than inland regions. Climates whose temperatures are influenced greatly by the sea are called maritime or oceanic or insular climates. These climates occur in coastal regions which lie under prevailing onshore winds. Climates whose ­temperatures are greatly influenced by remoteness from the sea are called ­­continental climates. These climates occur most ­frequently in the interiors of temperate continents.

(a) Incoming Heat From the Sun Coastal Region

10ºC Cool

Cooled by Sea Air

15ºC Warm Land Absorbs Heat Quickly

Sea Absorbs Heat Slowly

(b) Outgoing Heat from the Earth Coastal Region

5ºC Cool Sea Loses Heat Slowly

Warmed by Sea Air

0º Cold Land Loses Heat Rapidly

Figure 10.10  Distance from the Sea Affects Temperature: (a) Mid-summer Temperature Conditions in a Temperate Latitude; (b) Mid-winter Temperature Conditions in a Temperate Latitude. Temperatures Shown are Approximate.

Winds In temperate latitudes, prevailing winds from the land lower the winter temperatures but raise the summer temperature. In contrast, prevailing winds from the sea raise the winter temperatures but lower the summer temperatures. In tropical latitudes, onshore winds modify the temperatures of coastal regions because they have blown over cooler ocean surfaces. Local winds sometimes produce rapid upward or downward temperature changes.

Cloud cover and humidity Clouds reduce the amount of solar radiation reaching the earth’s surface and the amount of earth radiation leaving the earth’s surface. When there are no clouds, both types of radiation are at a maximum. Humid equatorial regions have a fairly low temperature range partly because the heavy cloud cover prevents day temperatures from going much higher than 30°C, and partly because of the loss of heat as latent heat during evaporation (Figure 10.11) In the hot deserts (Figure 10.12), the absence of clouds, and hence the presence of dry air, results in very high day temperatures of over 38°C and much lower night temperatures of 21°C or below.

10.10  Chapter 10

(a)

Incoming Solar Radiation

Loss of Earth Radiation is Checked by Cloud Out-going Earth Radiation

Hot

> 26.7°C

Cloud Absorbs and Absence of Clouds Reflects a Good Results in the Deal of Solar Maximum of Solar Radiation Radiation Reaching the Earth’s Surface High Rate of Evaporation from Earth’s Surface Cloud Prevents Excessive Loss of (b) Earth Radiation — it Acts as a Blanket

(b)

Out-going Earth Radiation

Hot

Earth’s Surface

Cool 20°C

South-facing Surface

Earth’s Surface Figure 10.12  The Absence of Cloud Cover in the Hot Deserts Results in a Large Daily Range of Temperature: (a) by day; (b) by night.

Level Surface

90º

45º

Additional Beam Spreading Surface Area for a Sun Altitude of 45º

Very Hot 38°C

Clear Sky Allows the Earth’s Heat to Escape Freely

> 21°C

Figure 10.11  The Influence of Cloud Covers on (a) Day and (b) Night Temperatures at the Equator. This Together with the Loss of Heat as Latent Heat During Evaporation Causes a Small Diurnal Range of Temperature.

North-facing Surface

Out-going Earth Radiation

(a)

All of the Beam Spreading Surface Area for a Sun Altitude of 90º and most of the Area for a Sun Altitude of 45º

Figure 10.13  The Influence of Aspect on Insolation at the Earth’s Surface in the Northern Hemisphere.

Very humid air absorbs heat during the day and retains it during the night. It also helps to prevent loss of heat from the lower layers of the air. Therefore, in the humid tropics, the air remains warm at night, even on days when there is little or no cloud.

Aspect The influence of aspect on temperature is only noticeable in temperate latitudes. South-facing slopes at north of the Tropic of Cancer and north-facing slopes at south of the Tropic of Capricorn always face the midday sun. Between the Tropic of Cancer and the Equator, south-­ facing slopes face the midday sun for a longer part of the year than do north-facing slopes. The reverse is true for places between the Tropic of Capricorn and the Equator (Figure 10.13). The altitude of the sun often influences the spacing between tall buildings, such as blocks of flats. In high latitudes, the midday sun has a low altitude in winter, and flats are usually built

Atmosphere: Temperature  10.11

Flats

Ocean currents

f eo

gl

An

Length of day The length of day increases as latitude increases in the Northern Hemisphere during the northern summer and increases in the same manner in the Southern Hemisphere during the southern summer. Clearly, the average daily temperature of a place having 18 hours of daylight is likely to be higher than a place having, say, only 10 hours of daylight.

Su n

(b)

the

id-

M

n

Su

Flats Flats

Shadow

N

S N

eM

y da

idd

ay

(a)

An gle of th

far apart to allow all the flats to receive some sunshine, but in low latitudes, flats can be built closer to each other because the sun’s altitude is much higher (Figure  10.14). However, because of the higher temperatures in the tropics, a compromise has to be reached between close spacing to block the sun’s rays and wider spacing to allow good flow of air between the buildings.

Flats

S

Figure 10.14  (a) Tall Buildings are Often Widely Spaced in High Latitudes to Allow the Sun’s Rays to Reach all Floors of the Buildings. The Aspect is Very Important in Making Full Use of Sunlight in Housing Developments; (B) In Low Latitudes, Dense Urban Development Presents Fewer Problems of Light, but Aspect is Still Important in Housing Developments.

Warm and cold currents often raise or lower the temperatures of coastlands, ­especially if the winds are onshore.

Effects of warm currents Warm currents moving polewards carry tropic warmth into high latitudes, and this warming influence is very marked between latitudes 40° and 65° on the west sides of continents, especially along the seaboard of western Europe (Figure 10.15). The warmth of the North Atlantic Drift is carried to the land by the prevailing westerly winds — mainly in the winter. The warming influence decreases inland from the coast as shown in Figure 10.16. A similar warming influence experienced along the coast of British Columbia is caused by the North Pacific Drift. Figure 10.17 illustrates similar warming in Indian Ocean during summer.

Effects of cold currents Cold currents have less effect on temperatures of coastlands because they are mainly located in areas where the winds are offshore. There are, however, exceptions to this phenomenon, e.g., during the summer, winds along the east coast of Canada are onshore, and they are cold because they have crossed the very cold Labrador Current. When they blow, they lower the summer temperature. Compare the summer temperature of Nain, which is on the Labrador coast, with that of Sitka, which is on the Alaskan coast (Table 10.1). Note that both towns have almost the same latitude. Summer temperatures are similarly lowered along the coasts of California and Peru where onshore winds cross the cold Californian and Peruvian Currents, respectively. Similarly, the cooling influence of the southward-flowing cold Canaries Current can be experienced as far south as Cape Verde on the west coast of Africa. This current gives rise to thick fog along the coast of Senegal. Further south, a stream of cold water, which is probably an off-shoot from the cold Benguela Current, lowers

56

½

°N

10.12  Chapter 10

Labrador Cu rrent

Cold

Nain –21.6°C

New York –1.1°C

Glasgow 3.9°C

41

th

r No

ic nt

°N

Oporto 8.3°C

t

la At

Cool

if Dr

North Atlantic Drift

Warm Current

Cold Current

Prevailing Westerly Winds

Temperatures are the Means for January

Figure 10.15  The Influence of the Warm North Atlantic Drift on the Winter Temperatures of Coastal Western Europe.

Valencia Jan 7°C July 15°C

Oceanic

–51°N

In flu en ce

Atlantic Ocean

ence Influ l a t nen nti o C

Warm

Winter Sea Influence

Cool

Summer Sea Influence

River Ob Barnaul 53°N Jan –19°C Jully 25°C

Winter Temperatures Decrease Summer Temperatures Increase Figure 10.16  The Warming Influence of the North Atlantic Drift Rapidly Decreases Inland.

the summer temperatures of coastal Ghana. This cooling effect is experienced most strongly between June and August. Both sea and air temperatures are lowered, and this may be responsible for causing the dry weather in eastern coastal Ghana. Some examples of ocean current influence Figure  10.15 shows how large is the difference in winter temperatures between the coastlands of western Europe and the coastlands of eastern North America. The former lies under onshore winds which blow over the warm North Atlantic Drift, while the latter lies under the cold winds that blow out from the interior of North America. Table 10.2 makes a similar comparison between the west and east coasts of the USA in summer. The difference in temperature between Nain and Sitka in the summer is fairly large. In the winter, this difference is even larger (Table 10.1).

Atmosphere: Temperature  10.13

The factors affecting the heating of the earth’s surface, which we have earlier discussed, explain why there is a large range of temperature in respect of the  ­horizontal temperature pattern of the earth’s surface. We must now examine the temperature changes that occurs in a vertical direction in the earth’s atmosphere.

South West Monsoon Drift

re n

t

Equatorial Counter Current

Auglhas

str West Au

Mo

za

alian Cu

mb

iqu

rrent

eC

ur

South Equatorial Current

Current

West Wind Drift Warm Current

Cold Current

Figure 10.17  Ocean Currents in the Indian Ocean during Summer.

Table 10.1

Onshore winds blowing over a cold current can lower summer temperature, while onshore winds blowing over a warm current can raise winter temperatures.

PLACE

LOCATION

OFFSHORE CURRENT

WIND

TEMPERATURE WINTER SUMMER

Nain

56°N

cold

onshore

–217°C

8.3°C

east coast

Labrador

–0.16°C

I3.3°C

Current Sitka

57°N

warm

onshore

west coast

North

(summer)

Pacific

offshore

Drift

(winter)

10.14  Chapter 10

Table 10.2

A comparison of the summer temperatures of San Diego and Charleston. Notice that the summer temperature of San Diego is lowered by the cold Californian Current.

PLACE

LOCATION

OFFSHORE CURRENT

WIND

TEMPERATURE

San Diego

35°30’N west coast

cold

onshore

19.6°C

onshore

27.4°C

Californian Current

Charleston

38°30’N

warm

east coast

Gulf Stream

Temperature “Inversion” An Atmospheric Layer where the Temperature Decrease with Height is much Less than Normal

Height

Normal Decrease of Temperature with Height

Temperature Inversion

Temperature

Elevation

Typical Weather Profile

Surface

Temperature Increases with Height (Stable Atmosphere Near Surface)

Temperature Decreases with Height (Unstable Atmosphere)

Surface Temperature of 50ºF

Freezing Level Temperature Profile

Temperature Inversion

Inversion Layer

Surface Temperature of 25ºF

Temperature

Figure 10.18  (a) Graph of Temperature Inversion (Blue) (b) Typical Weather Profile and Temperature Conversion at Surface Temperature of 50°F and 25°F.

Temperature Changes within the Atmosphere

The temperature of stationary air decreases with height at an average rate of 6°C per 1000  m. This decrease in temperature is called the ELR. The ELR varies from place to place and from one part of the year to another. Sometimes, the ELR is reversed, i.e., the temperature in a stationary mass of air rises as height increases. This phenomenon occurs in winter when warm westerly winds blow over cold easterly winds at approximately 600–700 m above the surface. Consequently, the cold air on the ground is stable, and pollutants such as hydrocarbons and oxides of nitrogen from motor vehicles and smoke from industrial plants as well as water vapour become trapped beneath the inversion. These pollutants can cause very poor visibility as well as pose a health hazard. A temperature inversion may also develop when there is a rapid loss of heat from the ground by nocturnal radiation which is common in winter. Figure  10.18 shows a temperature inversion in graphical format. A reversed ELR is called a temperature inversion. When a mass of air rises, its temperature decreases in a different manner. As the air mass rises, it expands, and in doing so, it consumes some of its heat energy. If the rising air is unsaturated, its temperature falls 10°C for every 1000  m of ascent, but if the rising air is saturated, the decrease in temperature is

Atmosphere: Temperature  10.15

2000 SALR 1500

Metres

about 6°C per 1000 m of ascent. This rate is lower because condensation occurs when saturated air cools, and during this process, heat is given out by the condensing water vapour. The rate at which the temperature falls in a rising unsaturated mass of air is called the dry adiabatic lapse rate or DALR. The fall in temperature in a rising mass of saturated air is called the saturated adiabatic lapse rate or SALR. When a mass of air descends, it will be warmed at one of the adiabatic rates, i.e., at the SALR whilst the air remains saturated and at the DALR when it becomes unsaturated. The ELR, DALR, and SALR are shown in Figure 10.19.

Maximum thermometer

1000

ELR

500

DALR

Measurement of temperature Variations in temperature represent responses to differences in insolation. Temperature is one of the main factors that determine the type of weather and climate of a particular region. Temperature is measured by an instrument called a thermometer. A continuous temperature reading is given by a thermograph.

Condensation Level and Rising Body of Air Becomes Saturated

0

5

10

15

Capillary 50 45 40 35 30 25 20 15 10 5 0 –5 –10 –15 –20

Metal Index Figure 10.20  A Maximum Thermometer. Glass Tube

Mercury

25

30°C

Figure 10.19  Diagram to Show the Relationships of the Three Lapse Rates. In this Example, the Ground Air Temperature is 20°C and the ELR is 8°C Per 1000 m. A Body of Warm Unsaturated Air of Temperature 22°C, Say a Wind, is Introduced at the Ground Level. This Air Rises Because it is Warmer than the Surrounding Air. As it Rises, it Cools at the DALR, Until it Reaches 1000 m When it Becomes Saturated. It Now Becomes Warmer and Continues Rising, but this Time Cooling at the SALR.

When the temperature rises, the mercury expands and pushes the index along the tube (Figure  10.20). In contrast, when the temperature drops, the mercury contracts and the index remains at the same position (Figure 10.21). The maximum temperature is obtained by reading the scale at the end of the index which was in contact with the mercury. In Figure  10.20, this is shown as 30°C. The index is then drawn back to the mercury by a magnet. This thermometer is used to register the maximum temperature reached during 24 hours.

Bulb

20

°C

Vacuum

30 25 20 15 10 5 Figure 10.21  The Position of the Marker after the Mercury has Contracted due to a Fall in Temperature.

10.16  Chapter 10

The sum of the daily mean temperature for 1 month divided by the number of days for that month gives the mean monthly temperature. The sum of the mean monthly temperatures divided by 12 gives the mean annual temperature.

Minimum

ºC −25

Metal Index

Figure 10.22  A Minimum Thermometer.

Alcohol Index

Figure 10.23  The Index of a Minimum Thermometer.

Minimum thermometer Maximum

Alcohol

°C

Meniscus

ºC 40

−20 −15

40 35 30 25 20 15 10 5 0 –5 –10 –15 –20

Alcohol

35 Metal Index

30

When the temperature falls, the alcohol contracts, and its ­meniscus pulls the index along the tube. When the temperature rises, the alcohol expands. The index does not move but remains in the position to which it was pulled. The minimum temperature is obtained by reading the scale at the end of the index which is nearer to the meniscus. In Figure 10.22, this is shown as 15°C. By raising the bulb of the thermometer, the index is returned to the meniscus, as shown in Figure 10.23. This thermometer is used to register the minimum temperature reached during 24 hours. The daily readings of the maximum and minimum ­thermometers are used to work out the average or mean temperature for one day (called the mean daily temperature) and the temperature range for one day (called the daily or diurnal temperature range).

−10

25

Mean daily temperature

−5

20

The maximum and minimum temperatures for one day are added together and then halved, e.g.,

0

Mercury

15

5

10

10

5

15

0

20

−5

25

Metal Index

30

−10 −15

35

−20

40

−25

Figure 10.24  A Six’s Thermometer.

maximum temperature minimum temperature mean daily temperature =

31°C 29°C 31 + 29 2 30°C

Daily or diurnal temperature range the maximum temperature minus the minimum temperature for any one day gives the daily temperature range, e.g., maximum temperature minimum temperature daily temperature range

31°C 29°C 2°C

The highest mean monthly temperature minus the lowest mean monthly temperature gives the mean annual temperature range, e.g.,  Lagos has a mean maximum temperature of 27.5°C (March) and a mean minimum temperature of 24.5°C (August). Its mean annual temperature range is therefore 3°C.

Atmosphere: Temperature  10.17

Six’s thermometer This thermometer can be used to measure maximum and minimum temperatures at the same time (Figure 10.24). When the temperature rises, the alcohol in the left-hand limb expands and pushes the mercury down the left-hand limb and up the right-hand limb. The alcohol in this limb also heats up and part of it is vaporized and occupies the space in the bulb. The maximum temperature is read from the scale on the right-hand limb. When the temperature falls, the alcohol in the left-hand limb contracts, and some of the alcohol vapour in the conical bulb liquefies. This causes the mercury to flow in the reverse ­direction. A metal index in each limb marks the temperature reached.

How to Describe the Temperature of the Air? When we talk about the weather or the climate, we often use words such as hot, cool, warm, cold, etc. These words should indicate temperature values. As a guide, we could use the values shown in Table 10.3 to refer to daily, monthly, and annual temperatures. For example, a month with a mean temperature of 25°C could be called a hot month. Table 10.4 gives words and temperatures for use with the mean annual temperature range.

Table 10.3

Words related to daily, monthly, and annual mean temperatures.

TEMPERATURE °C

DESCRIBING WORDS

Below −10°

very cold

−10° to 0°

cold

0° to 10°

cool

10° to 21°

warm

21° to 30°

hot

Over 30°

very hot

Table 10.4

Words related to annual mean temperature ranges.

ANNUAL TEMPERATURE °C

DESCRIBING WORDS

Below 3°

negligible

3° to 8°

small

8° to 19°

moderate

19° to 30°

large

Over 30°

very large

10.18  Chapter 10

(a)

24

24

B

25

How Temperature is Shown on a Map?

24 25 26 26

24 26

A

26 (b)

24º 25º Isotherm will Pass Midway between Points 24º and 26º

25º

26º Figure 10.25  (a) Part of a Map Showing Isotherms; (b) Interpolation.

The positions of all weather stations are first plotted by dots on the map of the region for which the temperature pattern is to be shown. The temperature for each station is adjusted to what it would be if that station were at the sea level. This is done by adding 6°C for every 1000 m of height for the station. The adjusted temperature values are then written alongside the dots, and all places with the same temperature are joined by a smooth line. This line is called an isotherm. Isotherms rarely pass through a station, and they must be ­interpolated, which is based on proportion. Figure  10.25 shows (a) how isotherms appear on a map and (b) what interpolation means. Actual readings are plotted. If there are sufficient readings for 24°C, 25°C, 26°C, and so on, then isotherms for these temperatures may be drawn. If there is not a sufficient number for, say 25°C, then the position of the 25°C ­isotherm has to be estimated (see points A and B).

Note: Temperatures on a map are usually given as sea level temperatures, that is, the temperatures are adjusted for height.

A Closer Look  ▼ The Temperature Pattern of Great Britain During the winter, the isotherms over Great Britain have a general north to south direction, whilst in the summer, their direction is generally west to east. Why is this so? (see Figure 10.26). The trend of the winter isotherms indicates that in general, temperatures decrease from west to east across the country, i.e., the ­temperatures fall in an easterly direction away from the Atlantic Ocean. We have already seen that the prevailing westerly winds carry the warmth of the North Atlantic Drift towards the land. As these winds blow over the country, heat is steadily transferred to the air over the land and to the surface of the land. Furthermore, the westerly winds are both moist and warm. As they move across the country, they first have to cross the mountains of Wales, the Lake District and Scotland, and the moors of the southwest peninsula. This results in condensation and precipitation, which release latent heat as already described. But as they move

Atmosphere: Temperature  10.19

4º 14º

14º 6º

4º

16º

16º

4º

18º 4º

16º 6º July ºC January ºC

Reduced to Mean Sea Level

Figure 10.26  January and July Isotherms for Great Britain. eastwards, they meet with cooler, drier winds which blow out from central Europe during the winter. The mixing of the two wind systems causes the heat energy to be distributed throughout a larger mass of air. Condensation and the resultant release of latent heat still occur though often to a smaller extent. It must also be remembered that during the winter, insolation over Great Britain and Europe is at a minimum. Much of the warmth that Great Britain receives during this season comes from the transfer of heat from the Atlantic to the land. During the summer, insolation over Great Britain is at its maximum, and the country receives most of its heat from this phenomenon. Because of the different rates of heating between the Atlantic and the land, air temperatures over the land are higher than they are over the sea. In general, at this time of year, temperatures decrease from south to north as we noted in Chapter 1. We can say that during the summer, Great Britain’s temperatures are mainly determined by its latitude, whereas  in  the winter, they are determined by its close proximity to the Atlantic Ocean.

10.20  Chapter 10

World Distribution of Temperature Figure 10.27 shows the mean monthly temperatures of the world for January and July. Study these maps and try to account for the following statements.

(a)

Very Cold Below −9°C −9°C −9°C 0°C

0°C

10°C 10°C

21°C

Tropic of Cancer 21°C Equator

Cold −9°C to 0°C

Cool 0°C to 10°C Warm 10°C to 21°C

Tropic of Capricorn 21°C

Hot 21°C to 29°C

21°C 10°C 10°C

Very Hot Over 29°C

(b)

Cool 0°C to 10°C 10°C 21.1°C

Warm 10°C to 21°C

Tropic of Cancer Equator

Hot 21°C to 29°C

21°C Tropic of Capricorn

10°C

Figure 10.27  Mean Surface Temperatures for the World for (a) January, and (b) July.

Very Hot Over 29°C

Atmosphere: Temperature  10.21

Following observations can be made by observing the graphs shown above. ●● ●● ●● ●● ●● ●● ●●

There is a northward “movement” of all isotherms from January to July. This “movement” of isotherms is greater over the land than over the sea. The highest temperatures for both January and July are over the continents. The lowest January temperatures are over the northern continents (Asia and North America). In January, the isotherms bend polewards over the oceans but equatorwards over the continents. In July, the isotherms bend equatorwards over the oceans but polewards over the continents. Seasonal changes are less over the southern continents than over the northern continents.

Figure 10.28 shows the mean annual temperature range for the world. It indicates the importance of maritime and continental influences. Study this map and try to account for the following four statements. 1. In general, the range of temperature increases from the equator to the poles. 2. The greatest annual range of temperature occurs over Asia and North America in latitude 60°N, and not at the poles. 3. Coastal regions have a smaller range (seasonal and annual) of temperature than continental interiors. 4. The range of temperature on the eastern coasts of Asia and North America is greater than that on the western coasts in the same latitude.

30°C

44°C

30°C

Below 3°C 55°C

44°C 3°C to 8°C

19°C 8°C

8°C to 19°C Tropic of Cancer 3°C Equator 19°C to 30°C

3°C Tropic of Capricorn

Over 30°C 8°C

8°C 8°C

Figure 10.28  Mean Annual Temperature Ranges for the World.

10.22  Chapter 10

Key facts ●● ●●

●● ●● ●● ●●

●● ●● ●● ●●

●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●● ●●

Solar radiation is a short-wave radiation; re-radiation from the earth is a longwave radiation. About 67 per cent of solar radiation is absorbed by the atmosphere and the earth’s surface. The same amount is re-radiated back to the outer space from the earth. This process is called heat balance or heat budget, which means energy gained equals energy lost. The atmosphere is heated by radiation, conduction, and convection from the earth’s surface and by evaporation and compression heating. The earth is heated by direct and indirect radiation. Land heats and cools more quickly than water. The temperature of a region is determined by some or all of the factors: latitude; altitude; nature of the surface; distance from the sea; winds; cloud cover; humidity; aspect; length of day; ocean currents. Climates whose temperatures are influenced by remoteness from the sea are called continental climates. Climates whose temperatures are influenced by the sea are called maritime, insular, or oceanic climates. The decrease in temperature with altitude in stationary air is called the environmental lapse rate. The temperature of a rising/subsiding mass of air rises/falls by 10°C per 1000 m when the air is not saturated (DALR) and by 6°C per 1000 m when the air is saturated (SALR). Mean daily temperature refers to the average of the daily maximum and ­minimum temperatures. Mean monthly temperature refers to the average of the daily mean temperatures. Mean annual temperature refers to the average of the monthly mean temperatures. Diurnal or daily temperature range is the difference between the daily ­maximum and minimum temperatures. Annual temperature range is the difference between the highest and lowest monthly mean temperatures. Temperature is measured by a thermometer. A Six’s thermometer measures minimum and maximum temperatures. A line of equal temperatures across different locations on a map is called an isotherm. Winter temperatures of Great Britain are largely determined by the warming influence of the Atlantic Ocean and its North Atlantic Drift. The largest annual range of temperature in the world occurs over the interiors of Asia and North America. Continental interiors have a larger annual range of temperature than coastal regions. The range of temperatures on the eastern sides of continents is greater than that on the western sides in temperate latitudes.

Atmosphere: Temperature  10.23

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. 1. There are four towns which have the same latitude and altitude, but their January mean temperatures are all different. Which town is farthest from the sea? (a) 8°C (b) −12°C (c) 10°C (d) −6°C (e) 3°C 2. The DALR refers to (a) a fall in temperature of 6°C per 1000 m of ascent. (b) a decreasing temperature. (c) the fall in temperature of a rising mass of saturated air. (d) temperature changes in the troposphere. (e) the rate at which unsaturated air cools when it rises. 3. The ELR is best described as (a) a fall in temperature. (b) the temperature changes that occurs in the stratosphere. (c) a characteristic of the troposphere. (d) a fall in air temperature caused by the ascent of a mass of air. (e) a fall in temperature of approximately 6°C per 1000 m in stationary air. 4. The mean annual range of temperature for a place is obtained by (a) adding the monthly mean temperatures for 1 year and dividing by 12. (b) subtracting the lowest temperature from the highest temperature for 1 year. (c) subtracting the lowest mean monthly temperature from the highest mean monthly temperature for 1 year. (d) adding the mean daily temperature for 1 year and dividing by 365. (e) subtracting the lowest mean daily temperature from the highest mean daily temperature for 1 year. 5. The atmosphere is a mixture of several gases. Near the earth surface it contains mainly: (a) nitrogen and oxygen (b) nitrogen and carbon dioxide (c) oxygen and carbon dioxide (d) ethane and oxygen 6. The correct sequence of different layers of the atmosphere from the surface of the Earth upwards is (a) Troposphere, stratosphere, ionosphere, mesosphere (b) Stratosphere, troposphere, mesosphere, ionosphere (c) Stratosphere, troposphere, ionosphere, mesosphere (d) Troposphere, stratosphere, mesosphere, ionosphere 7. Most weather activity occurs in which atmospheric layer? (a) Ozonosphere (b) Ionosphere (c) Troposphere (d) Exosphere 8. The stratosphere is said to be ideal for flying jet aircrafts; this is because (a) this layer is rich in ozone, which reduces fuel consumption (b) the temperature is constant and ideal for aircraft engine efficiency (c) this layer is out of the firing range of antiaircraft guns (d) the clouds and other weather phenomena are absent

10.24  Chapter 10

9. Which of the following protects life on earth from the harmful radiations of the sun? (a) Troposphere (b) Ionosphere (c) Ozone layer (d) Mist (e) All these. 10. Which of the following statements is not true? (a) Presence of water vapour is highly variable in the lower atmosphere. (b) Frigid zones are located in both the hemisphere between the polar circles and the poles. (c) The zone of maximum temperature is located in both the hemispheres between the polar circles and the poles. (d) Jet streams are high altitude winds affecting the surface weather conditions. 11. Normally, temperature decreases with the increase in height from the earth’s surface because (1) The atmosphere can be heated upwards only from the earth’s surface. (2) There is more moisture in the upper atmosphere. (3) The air is less dense in the upper atmosphere. Select the correct answer using the code given below: (a) Only 1 (b) 2 and 3 (c) 1 and 3 (d) 1, 2, and 3 12. Consider the following statements: (1) The annual range of temperature is greater in the Northern Hemisphere than that in the Southern Hemisphere. (2) The annual range of temperature is greater in the Pacific Ocean than that in the Atlantic Ocean. Which of the following statements given above is/are correct? (a) 1 only (b) 2 only (d) Neither 1 and 2 (c) Both 1 and 2 13. Re-radiation from the earth’s energy is best described as (a) Short-wave radiation (b) Long-wave radiation (c) Short-long wave radiation (d) None of the above 14. The decrease in temperature with altitude in stationary air is termed as (a) Atmospheric lapse rate (b) Temperature inversion (d) Environmental lapse rate (c) Adiabatic lapse rate 15. The adjusted temperature values written alongside the dots and all places having the same temperature are joined by a smooth line which is called (a) Isohyet (b) Isotope (c) Isotherm (d) Isobar

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. (a) The minimum temperature at IGI Airport on 28 January was 4°C. What ­instrument was used to record the temperature? (b) Describe how this instrument works. (c) What other reading would be required to calculate the diurnal range of temperature? (d) Briefly state how you would calculate the diurnal range of temperature.

Atmosphere: Temperature  10.25

2. (a) Only approximately 50 per cent of solar radiation is absorbed by the earth’s surface. Briefly state what happens to the other 50 per cent of the solar radiation that reaches the atmosphere. (b) Name three ways by which the atmosphere is heated. (c) Explain why the west coast of Great Britain is warmer than the east coast in winter. (d) When solar radiation is cut off, a land surface loses its heat more rapidly than a water surface. Using this information, what action could you take to protect the blossom of fruit trees if they were threatened by frost? 3. (a) Study Table 10.5 which shows the average January and July temperatures for five places located near to 50°N latitude. Which place is most likely to experience transport problems in winter? (b) Which place has the highest annual range of temperature? (c) The temperatures of St John’s and Valencia are the same in summer but quite d ­ ifferent in winter. Briefly explain the reasons for this. (d) Why does Moscow have a lower winter temperature than Berlin? Table 10.5

Average January and July temperatures for five places VANCOUVER

ST JOHN’S

VALENCIA

BERLIN

MOSCOW

January °C

2

−5

7

−1

−10

July °C

17

16

16

18

19

4. Carefully explain, with the aid of labelled diagrams, the following statements: (a) equatorial regions receive rain throughout the year; (b) the highest daily temperatures are recorded outside equatorial latitudes; (c) the hearts of the northern continents receive little rain. 5. How is the atmosphere heated? Illustrate your answer with suitable examples.

Answer key Exercise 1 1.   (a) 6.   (d) 11.   (c)

2. (c) 7. (c) 12. (a)

3. (e) 8. (d) 13. (b)

4. (a) 9. (c) 14. (d)

5. (a) 10. (c) 15. (c)

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11

Atmosphere: Pressure and Wind

Learning Outcomes After completing this chapter, you will be able to: ● ● ● ●

Understand the relationship between air temperature and air pressure Describe the influence of various factors on air pressure Explain the basic type of air mass, fronts, and wind system Describe the distribution air pressure and planetary wind during different seasons

Keywords Pressure, Barometer, Isobar, Winds, Anemometer, ITCZ, Tropical Cyclone, Tornado, Anti-cyclone

11.2  Chapter 11

Introduction We read in Chapter 10 that insolation determines the temperature which in turn determines evaporation. We also read that a heat balance exists, which results in energy gain equalling energy loss. What is it that allows this balance to exist? The answer lies in pressure and winds. Changes in air temperature produce corresponding changes in air pressure. This weakens or strengthens the speed of the wind or even changes its direction. It is this movement of air, which is called a wind, that balances changes in air temperature and pressure from one part of the atmosphere to another part. If there were no movements, i.e., no winds, all of the earth’s condensation and rainfall would be over the oceans—there would be no transfer of water to the land. Air pressure increases when air descends

Origin of pressure

Low Pressure

Low Pressure

Molecules Get Closer Together and Outward Pressure Increases

Molecules Get Further Apart and Outward Pressure Decreases

Influence of altitude on pressure

High Pressure

High Pressure Very Little Air Above this Height

18 km Less Dense Air Atmospheric Pressure 505 mb

This Layer Accounts for Half the Total Weight of the Atmosphere

Dense Air Atmospheric Pressure 1010 mb

Figure 11.1  Effect of Altitude on Air Pressure.

Density Decreases in this Direction

This Layer Accounts for Half the Total Weight of the Atmosphere

Air has weight, and therefore, it exerts pressure called atmospheric pressure on the earth’s surface. This pressure is not the same for all regions, nor is it always the same for any one region all the time, i.e., in some regions, the pressure is higher for one part of the year than it is for another part of the year. Atmospheric pressure is affected by altitude, temperature, and earth rotation.

5 1/2 km

Sea Level

Air pressure at sea level is higher than it is at the top of a mountain. This is because at sea level, air has to support a greater weight of itself than does air on the top of a mountain. The molecules of the air at sea level push outwards with a force equal to that exerted by the air above it, whereas the molecules of the air at the top of a mountain push outwards with much less force because the weight of the air above it is less. When air descends, its volume decreases, but the number of molecules in it remains the same. The outward pressure of these molecules is spread over a smaller area. Similarly, when air rises, its volume increases, and the outward pressure of its molecules is spread over a larger area; consequently, its pressure decreases. This is explained in Figure 11.1.

Atmosphere: Pressure and Wind   11.3

Influence of temperature on pressure Effects of temperature on pressure are listed below. 1. When air sinks, its pressure increases because it becomes ­compressed. When air is compressed, its molecules move more quickly and heat is produced. The temperature of air rises when its pressure rises. 2. When air rises, its pressure decreases because it expands. When air expands, its molecules move more slowly and heat is consumed. The temperature of air falls when its pressure falls. 3. When air is heated, it expands, and when this occurs, the outward pressure of its molecules is spread over a larger area. This means that the pressure of the air decreases. The pressure of the air falls when its temperature rises. 4. When air is cooled, it contracts, and when this occurs, the outward pressure of its molecules is spread over a smaller area. This means that the pressure of the air increases. The pressure of the air rises when its temperature falls. Figure  11.2 explains, in simplified terms, Cold Air Sinks at the the effects of temperature on pressure on North Pole—High Pole Pressure Develops Pole a global scale; this will be discussed further High Pressure later in this chapter. Cold If only temperature affected pressure, Sun rays Hot Air Rises at the pressure pattern of the atmosphere the Equator—Low Hot Low Pressure would be something like that shown in Pressure Develops Figure  11.2. There would be a belt of low Cold pressure around the earth at the equator, High Pressure Cold Air Sinks at and two belts of high pressure, one over Pole the South Pole—High each pole. But because altitude, temperaPressure Develops ture, and earth rotation all affect pressure, the pressure pattern is not so simple. Figure 11.2  Effect of Temperature on Pressure.

Influence of rotation on pressure The rotation of the earth causes the air at the poles to be pushed towards the equator. In theory, this should result in air piling up along the equator to produce a belt of high pressure, whilst at the poles, low pressure should develop as shown in Figure 11.3. But what actually occurs is much more complicated, and we must examine how temperature and rotation together affect the pressure pattern.

Slow Turning Fast Turning

Slow Turning

Pole

E a rth R o t a ti o n

Pole

Air is Thrown Away from the North Pole—Low Pressure Develops Air Piles up Along the Equator— High Pressure Develops Air is Thrown Away from the South Pole—Low Pressure Develops

Figure 11.3  Effect of Rotation on Pressure.

Pole

Low Pressure High Pressure

Low Pole Pressure

11.4  Chapter 11

Temperature Low temperatures at the poles cause the air to contract—high pressure develops. High temperatures along the equator cause the air to expand—low pressure called the doldrum low pressure develops. Rotation Air moving away from the poles crosses parallels that are getting longer, and it spreads out to occupy greater space, i.e., it expands and its pressure falls. These low pressure belts are noticeable along 60°N and 60°S. They are known as the temperate low pressure belts. As the air moves away from the poles, more air moves in from higher levels to take its place. Some of this comes from the rising low pressure air along 60°N and 60°S. Air rising at the equator spreads out and moves towards the poles. As it does so, it crosses parallels that are getting shorter, and it has to occupy less space. It contracts and its pressure rises. This occurs near 30°N and 30°S, and in these latitudes, the air begins to sink where it builds up sub-tropical high pressure belts, sometimes called the horse latitudes. Some of the high pressure air in latitudes 30°N and 30°S moves over the surface towards the equator, and some of it moves towards the poles. The air that moves towards the equator replaces the air that rises there. The air moving towards the poles reaches latitudes 60°N and 60°S where it replaces the air that rises there. Figure 11.4 summarizes these movements and developments. If the earth had a uniform surface, i.e., if it were all land or all water, the pressure pattern would be like that shown in Figure 11.5. Winds are shown because they are caused by differences in pressure. This diagram shows that winds blow over the surface from high pressure to low pressure. At high levels, they blow from the low pressure systems to the high pressure systems. You will see that in each hemisphere, there are three pressure systems: polar high pressure, temperate low pressure, and subtropical high pressure. The equatorial low pressure is common to both hemispheres. Zone of Rising Air

er

de

High (Sub-Tropical) High Pressure Caused by Temperature and Earth Rotation

s

60ºS

South Pole

W

lar

Fro n

SE T

ra

es

Low (Doldrums)

High (Sub-Tropical)

Low Pressure Caused by High Temperatures

High Pressure Caused by Temperature and Earth Rotation

Po

s

li e s

High Pressure Low Pressure Caused by Low Caused by Warm Westerlies Temperatures Rising Over Cold Polar Air

T

nt

Fro

Low (Temperate)

NE

lar

High (Polar)

t

Intertropical Front

Po

Pol a r

W es

ra

30ºS

t

Equator

30ºN

de

Zone of Descending Air North Pole 60ºN

Zone of Rising Air Zone of Rising Air Zone of Descending Air Zone of Descending Air Zone of Descending Air

t e r li e

s

Polar

Low (Temperate) Low Pressure Caused by Warm Westerlies Rising Over Cold Polar Air

High (Polar) High Pressure Caused by Low Temperatures

Figure 11.4  The Pressure and Wind Patterns Over the Earth’s Surface From the North Pole to the South Pole.

Atmosphere: Pressure and Wind   11.5

Altitude (km)

Tropical tropopause

15 Polar front

10

North Pole

mid-latitude tropopause

60º N

Polar tropopause

5

30º N

Horse Latitudes (Sub-Tropical HP)

0º

Doldrums (Equatorial LP) Horse Latitudes (Sub-Tropical HP)

30º S 60º S

0 North pole High pressure

60°N

30°N

Low pressure

High pressure

High Pressure

Equator

South Pole

Low pressure

Figure 11.5  Pressure Pattern on Earth if it had a Uniform Surface.

Figure 11.6  Three Pressure Systems in Each Hemisphere.

Actual pressure systems The earth’s surface is not uniform. Some of it is land, and some of it is water. Further, the earth’s axis is tilted at an angle of 66½°.

World pressure pattern in July (Figure 11.7) Following observations can be made by studying the pressure system pattern.

23½° N

0°

–23½° S

High Pressure

Low Pressure

Figure 11.7  Pressure Systems for the World for July.

Low Pressure

Very Low Pressure

11.6  Chapter 11

●

●●

●●

It is summer in the Northern Hemisphere with the sun overhead along the Tropic of Cancer on 21 June. The doldrums extend well into the Northern Hemisphere and link up with the low pressure areas over the Asian land mass and southwest USA. The sub-tropical high pressure of the Northern Hemisphere exists as separate areas over the oceans. In the Southern Hemisphere (where it is winter), this high pressure area is almost continuous around the earth. The temperate low pressure areas over the North Atlantic and North Pacific Oceans are poorly developed.

World pressure pattern in January (Figure 11.8) Following observations can be made by studying the pressure system pattern. ●●

●●

●●

It is summer in the Southern Hemisphere with the sun overhead along the Tropic of Capricorn on 22 December. The doldrums extend into the Southern Hemisphere and are particularly well developed over Australia and central Africa. The sub-tropical high pressure of the Southern Hemisphere is no longer continuous. It forms separate areas over the oceans. In the Northern Hemisphere, it is continuous, having linked up with the high pressure areas over the Aslan and North American land masses. The temperate low pressure areas over the North Atlantic and Pacific Oceans are well developed.

23½° N

00° 23½° S –23½° S

High Pressure

Very High Pressure

Figure 11.8  Pressure Systems for the World for January.

Low Pressure

Atmosphere: Pressure and Wind   11.7

Measurement of Air Pressure Because air has weight, it exerts a pressure on the earth’s surface. At sea level, the pressure is about 1.03 kg/cm2. Pressure varies with temperature and altitude; the instrument which measures pressure is called a barometer. There are three types of barometers: mercury barometer, aneroid barometer, and barograph. Air pressure is usually measured in terms of a millibar (mb). At latitude 45° and at a temperature of 15°C, air pressure forces mercury in a mercury barometer to rise to a height of 760 mm. This is called the normal air pressure. The relationship of millibars to millimetres is approximately 1 mb equivalent to 0.75 mm. Therefore, 760 mm is equivalent to 1013.3 mb, and this is the normal air pressure at 15°C at latitude 45°.

Mercury barometer This consists of a hollow tube from which the air is extracted before the open end is placed in a bath of mercury. Mercury is forced up the tube by the pressure of the atmosphere on the mercury in the bath. When the pressure of the mercury in the tube balances the pressure of the air on the exposed mercury surface, the mercury stops rising in the tube. The height of the column of mercury changes as air pressure changes, i.e., it rises when air pressure increases and falls when air pressure decreases. A view of this barometer is shown in Figure 11.9.

Changes in air temperature cause corresponding changes in atmospheric pressure, e.g., when the temperature falls, the level of mercury falls, and when the temperature rises, the level of mercury also rises. Because of this relationship between temperature and pressure, corrections have to be made to mercury barometer readings.

Vacuum

Glass Tube Pressure of the Atmosphere

Aneroid barometer This is an instrument which consists of a small metal cylinder which is a vacuum ­chamber. A strong metal spring prevents the ­chamber from collapsing. The spring contracts and expands with changes in atmospheric ­pressure. These changes are magnified by a series of levers, and they are conveyed to a pointer which moves across a calibrated scale (Figure 11.10).

Barograph This is actually an aneroid barometer which continuously records air pressure for 1 week. Changes in pressure are recorded by a flexible arm which traces an ink line on a rotating paper-covered drum. The paper is divided by vertical lines at 2-hour intervals. This is shown in Figure 11.11.

760 mm

Container Mercury Figure 11.9  A Mercury Barometer. The Pressure of the Air on the Mercury in the Container Supports a Column of Mercury About 760 Mm High. This Amount of Mercury has the Same Weight as a Column of Air About 18 Km High Having the Same Cross-Sectional Area as the Mercury Column.

11.8  Chapter 11

(c)

(a)

(b) Pointer

Spring

Lever Mechanism Collapsible Metal Box

Case

Pen

Figure 11.11  A Barograph.

Figure 11.10  (a) An Aneroid Barometer; (b) Sectional View; (c) Face of the Barometer.

Rotating Drum with Chart

Atmosphere: Pressure and Wind   11.9

How Pressure is Shown on a Map? The atmospheric pressure is recorded at numerous weather stations for a region, and these recordings are plotted on a map of the region. Before plotting, the pressures are ‘reduced’ to sea level, i.e., they are adjusted to what they would be if the stations were located at sea level. The pressures are also adjusted for temperature and the effect of gravity. The pressures are plotted on a map, i.e., each pressure is shown at the point of location for the station where the pressure was read. Lines are then drawn through points whose pressure reading is the same. These lines are called isobars. An isobar is a line on a map that passes through all points having the same pressure as shown in Figure 11.12(a).

Winds The earth is neither gaining nor losing heat, and it is neither gaining nor losing air pressure. The winds smooth out the global differences in both temperature and air pressure, i.e., they redistribute these differences by taking from places with an excess to places with a ­deficiency. The winds can be regarded as transport agents.

The origin of winds Because isobars are lines of equal pressure, it follows that there must be a difference in pressure between adjacent isobars. Figure  11.12(a) shows this aspect. The four isobars are labelled A, B, C, and D. The difference in pressure between isobar A and isobar B is 4 mb. The same difference exists between B and C, and between C and D. This difference in pressure between adjacent isobars is called the pressure ­gradient. (b) X

2

1 10

Y

08

10

w Lo 4 98

04

10

988

(a)

992

998 1002

996

998 1006

A

1010

1002 1006

1002 B

C

1010

1000

D

Figure 11.12  (a) Part of an Isobar Map; (b) Pressure Gradients (Y – Gentle; X – Steep).

04

10

08

10

11.10  Chapter 11

Cold air

Warm air

Figure 11.13  Differences in Atmospheric Pressure are Balanced by Winds. Polar High Temperate Low Sub-Tropical High Doldrums Low Sub-Tropical High

This is always greatest at right angles to the isobars in the same way that the greatest gradient of a surface slope is always at right angles to the contours. Further, the distance between isobars reflects the steepness of the pressure gradient. The closer the isobars, the steeper is the gradient. The pressure gradient represents a horizontal flange in pressure from an area of high pressure to an area of low pressure, and as with surface slopes, the gradient may be steep or gentle (Figure 11.12). The velocity of a wind is directly related to the steepness of the pressure gradient. The steeper the gradient, the greater is the velocity. A wind tries to balance the difference in pressure between a high and a low pressure area. Air diverges in the high pressure area, while it converges in the low pressure area (Figure 11.13). If the earth’s surface were uniform, i.e., all land or all sea, and if the surface of land had no irregularities, the winds would have a laminar flow similar to that of a stream whose bed is uniform and regular. But the earth’s surface is not uniform and winds move in a turbulent manner. If the earth were stationary, i.e., if it did not rotate, then winds would blow more or less along the pressure gradient. But as we read in Chapter 1, the earth rotates, and this causes the winds to be deflected so that they cross the isobars obliquely. In the Northern Hemisphere, winds are deflected to the right of the direction in which they blow, while in the Southern Hemisphere, they are deflected to the left of this direction. Figure 11.14 shows what the world wind pattern would be like on a ­non-rotating earth, while Figure 11.15(b) shows how the earth’s rotation deflects the major winds.

Temperate Low Polar High Figure 11.14  Illustration of the Pressure and Wind Patterns if the Earth did not Rotate and if its Surface were Uniform.

It is possible to estimate the speed of the wind without using an anemometer. This is done by observing the way certain objects are moved by the wind and by using the Beaufort scale (Figure 11.18).

Measurement of wind direction and velocity

A wind vane is used to indicate wind direction. It consists of a horizontal rotating arm pivoted on a vertical shaft. The rotating arm has a tail at one end and a pointer at the other. When the wind blows, the arm swings until the pointer faces the wind. The directions, namely north, east, south, and west, are marked on arms that are rigidly fixed to the shaft. In Figure 11.16, the wind is blowing from southwest. The speed of the wind is measured by an anemometer (Figure 11.17), which consists of three or four metal cups fixed to metal arms that rotate freely on a vertical shaft. When there is a wind, the cups rotate. The stronger the wind, the faster is the rotation. The number of rotations are recorded on a meter to give the speed of the wind in kilometres per hour.

Atmosphere: Pressure and Wind   11.11

How winds and wind velocity are shown on a map? Winds are shown by arrows on a weather map. The shaft of an arrow shows wind direction and the feathers on the shaft indicate wind velocity, as shown in Figure 11.19. Combinations of these can thus be used to show any velocity. Arrows are inserted on a weather map at the positions of the weather stations. The tip of the arrow away from the feathers points to the direction in which the wind is blowing. (b) Global wind circulation

Sinking winds (dry)

pattern impacts on regional climates

90º Polar Easteries

60º

(a)

Subpolar Low Zone

Cool air sinks, warms up and 30º dries out (deserts)

Prevailing Westerlies

Temperate Low Sub-Tropical High

Sinking winds (dry)

Subtropical High Zone Tropical Easterlies

Polar High Warm, moist air rises and 0º produces rain

Rising winds (wet)

Intertropical Convergence Zone

Rising winds (wet)

Tropical Easterlies

Doldrums Low

Subtropical High Zone

(deserts) 30º

Sub-Tropical High 60º

Temperate Low

Sinking winds (dry)

Prevailing Westerlies Subpolar Low Zone Polar Easteries 90º

Rising winds (wet)

Sinking winds (dry)

Polar High

FIGURE 11.15  (a) The Blowing Pattern of the Main Wind Systems on a Rotating Earth; (b) Deflection of Major Winds due to Earth’s Rotation.

Freely Rotating

W

N

S

E

Figure 11.16  A Wind Vane.

Figure 11.17  An Anemometer.

11.12  Chapter 11

Figure 11.18  The Beaufort Scale. SCALE NO

SPEED (KNOTS)

DESCRIPTION OF WIND

EFFECTS OF THE WIND (FEATURES OBSERVED)

0

0

Calm

Smoke rises vertically

1

1–3

Light air

Direction is shown by the way smoke drifts

2

4–6

Light

Leaves rustle, wind is felt on the face, the vanes of the wind vane move

Breeze 3

7–11

Gentle Breeze

4

12–16

Moderate

Light flags blow out in the wind; leaves and twigs are in constant motion Dust and loose paper blow about; small branches are moved

Breeze 5

17–21

Fresh Breeze

6

22–26

Strong Breeze

7

27–32

Moderate

Small trees with leaves begin to sway, crested waves form on inland waters Whistling is heard in telegraph wires, large branches are set in motion, difficult to open umbrellas Difficult to walk against the wind; whole trees set in motion

Gale 8

33–39

Fresh gale

Twigs are broken off trees

9

40–46

Strong gale

Slight structural damage to buildings occurs

10

47–55

Whole gale

Trees are uprooted and considerable structural damage to buildings occurs

11

56–63

Storm

Widespread damage occurs

12

over

Hurricane

Widespread devastation occurs in some tropical regions

64

WIND SPEED IN KNOTS

SYMBOL USED

WIND SPEED IN KNOTS

0 (calm)

33–37

1–2

38–42

3–7

43–48

8–13

49–53

14–18

54–58

19–22

59–62

23–28

63–68

29–32

69–72

SYMBOL USED

Figure 11.19  The Symbols Used for Indicating Wind Strength on Weather Maps.

Atmosphere: Pressure and Wind   11.13

Wind direction for a specific place can be shown on a wind rose (Figure 11.20). It is made up of a circle from which rectangles radiate. The directions of the rectangles represent the points of the compass. The lengths of the rectangles are determined by the number of days the wind blows from that direction. Usually, this is completed for 1 month. The number of days when there is no wind is recorded in the centre of the rose. The wind rose shown here is a simple one. It has many variations.

N

6

Planetary Winds Planetary winds are sometimes called prevailing winds because they blow more frequently than other winds. Winds are usually named after the direction from which they blow. There are three major global wind systems, all created by pressure differences. These are polar easterly winds, mid-latitude westerly winds, and tropical easterly winds (trade winds). These winds occur in both hemispheres. The zone of greatest heat moves south and north as the earth revolves around the sun. The pressure systems move likewise.

Figure 11.20  A Wind Rose to Show Direction for a Specific Place. The Number in the Centre Represents the Number of WindFree Days.

Polar easterly winds The high pressure system over the poles gives rise to diverging air which blows towards the temperate low pressure system around latitude 60°. Mid-latitude westerly winds Winds blow down the pressure gradient from the sub-tropical high pressure system around latitude 30° towards the temperate low pressure system. Earth’s rotation deflects them and gives them a west to east direction, and they are often called the prevailing westerlies. Tropical easterly winds These winds blow from the sub-tropical high pressure, down the pressure gradient towards the equatorial low pressure system. They, too, are deflected by earth’s rotation, and they blow in a general easterly direction. They are called the trade winds. The trade winds of both hemispheres converge on the same region. The area where they meet is called the doldrums or the Intertropical Convergence Zone (ITCZ). The three global wind systems are shown in Figures 11.21 and 11.22. The seasonal movement of the equatorial low pressure system causes a similar movement of the ITCZ. This results in the trade winds crossing the equator, and they subsequently become deflected by earth’s rotation, e.g., the southeast trade winds cross the equator and change direction to the southwest (Figure 11.23). These winds bring heavy rain to southern Asia. These winds change direction seasonally and are called monsoon winds.

11.14  Chapter 11

Monsoon is derived from the Arabic word mausim, which means season, and this name is given to winds whose direction is completely reversed from one season to the next season. The reversal is caused by a reversal in pressure systems. Monsoon winds are best developed in Asia (Japan, China, southeast Asia, and the Indian subcontinent) and to a lesser extent along the coast of West Africa and over northern Australia. General synoptic pattern at the onset of southwest monsoon is shown in Figure 11.24. AF

A

(a) AF

Pc

Pc PF

PF

Tc

23½°N

Tm

Tm

E

E

0°

Z

ITC

E

Tm

23½°S

Tc

Tm

PF

PF

PF

PF-Polar Front

PF

ITCZ Intertropical Convergence Zone

(b)

A

AF

PF

Pc

AF

23½°N

AF Arctic Front

Pc

PF

Tm

Tm

ITC

Tc E

E

0°

Z

E Tm

23½°S Tm

Tm Tropical Maritime

Pf

Polar Continental

A: Arctic

Tc

PF

Tm

E

Equatorial

F

PF

Tc

Tropical Continental

P

Figure 11.21  The Main Wind Patterns of the World for (a) January and (b) July. Examine the Seasonal Changes to the Wind Patterns of Asia and North America. (How many Winds Can You Name?)

Atmosphere: Pressure and Wind   11.15

(a) Sea-Level Pressure and Surface Winds

Jan

L L

L

H H

H

H L

H

H

L

L

L

L

H

L

L

995 1000 1005 1010 1015 1020 1025 mb - 2 - 1 4 8 Data: NCEP/NCAR Reanalysis Project, 1959–1997 Climatologies

16

32 m/sec

(b) Sea-Level Pressure and Surface Winds

Jul L

L

L

H

H

H

L

H

L

L

L

L L

H

H

H

H

995 1000 1005 1010 1015 1020 1025 mb - 1 - 2 4 8 Data: NCEP/NCAR Reanalysis Project, 1959–1997 Climatologies

16

32 m/sec

Figure 11.22  The Relationship Between Sea Level Pressure and Surface Winds in World for (a) January and (b) July.

11.16  Chapter 11

DECEMBER and JANUARY

JUNE and JULY

rtropical Conve Inte rge nce Z o ne Kozhikode

Kozhikode cal Convergenc e Z Darwin opi r t r one Inte

A. Winter

Darwin

B. Summer ITCZ Southwest monsoon

Northeast monsoon Arabia

Arabia

India

Africa

Africa

Equator Figure 11.23  Movement of Intertropical Convergence Zone (ITCZ) in Southwest Asia in Winter and Summer.

ical trop Inter

e zon ce gen r e v con

Equator

India Warm rising air

General Synoptic Pattern at onset of Southwest Monsoon Polar Jet

L

Subtropical Jet

H

Mei-yu Tropical Easterly Jet ITCZ

Figure 11.24  General Synoptic Pattern at the Onset of Southwest Monsoon.

Cross Equatorial Jet

Changma Mei-yu/Baiu Pre-Mieyu

Atmosphere: Pressure and Wind   11.17

Monsoon winds of the Asian region The July pattern (Figure 11.25) We have already noted that high temperatures give rise to low pressure. By July, the temperatures over central Asia are very high relative to the temperatures over the oceans to the east and south of Asia.

CZ Him

Low Pressure

SW Monsoon

IT

alay

as

Overhead Sun SW Monsoon

1.  A  n intense low pressure develops over Asia, which is separated by the Equator Himalayas from a smaller but just as intense a low pressure over the northern lowlands of the Indian subcontiSE Trades nent, called the Punjab. SE Monsoon 2.  A high pressure develops over northHigh Pressure (Horse Latitudes) ern Australia, where it is winter. 3.  Winds blow from the Australian high pressure to the more intense Asian low pressure. The winds blow from the southeast, but on crossing the equator, they are deflected to the right and Figure 11.25  The Monsoon Wind Pattern of Asia and Australia for July. blow as southwest winds. As they move north, they enter the anti-clockwise circulation of the Asian low pressure and blow as southeast winds across Japan and China. 4.  Winds blow from the sub-­tropical high High High Pressure NW Monsoon pressure, from the southeast, across Pressure the doldrums towards the more intense Him Punjab low pressure. After crossing the ala y as equator, they are deflected to the right and blow as southwest winds across Sri Lanka and India. Over the Bay of Bengal, the winds now enter the anti-clockwise circulation of the NE Monsoon Punjab low pressure and blow as southeast Equator winds across the Ganges lowlands.

The January pattern (Figure 11.26) By mid-winter, temperatures over central Asia are very low, resulting in high pressure. The pressure over the oceans to the east and south of Asia is low because the air over these oceans is relatively warm.

ITCZ

Overhead Sun

NW Monsoon SE Trades

Low Pressure

Higher Pressure Figure 11.26  The monsoon wind pattern of Asia and Australia for January.

11.18  Chapter 11

1.     An intense high pressure develops over central Asia, and another one over the Punjab. They are separated by the Himalayas. 2.    An  intense low pressure develops over northern Australia, where it is summer. 3.    W  inds blow from the Asian high pressure to the more intense Australian low pressure. Over Japan and northern China, the winds blow from the northwest; over southern China and southeast Asia, they blow from the northeast because they are deflected to the right. After crossing the equator, they are deflected to the left, i.e., they blow from the northwest. 4.    W  inds blow from the Punjab high pressure, from the northwest over the Ganges lowlands, but as they continue southwards, they are deflected to the right and arrive at the doldrums as northeast winds where they meet the southeast trade winds ­blowing from the sub-tropical high pressure of the Southern Hemisphere.

Local Winds These are winds that occur regularly or periodically. The areas only and blow for short periods of time. Figure  11.27 visualizes local winds of India. They are small scale winds, but they have distinctive characteristics and produce important effects of the weather on the regions over which they blow.

Bardoli Chheerha Loo

Kalbaisakhi

Bardoli Chheerha Mango Shower

Kalbaisakhi Loo

Blossom Shower Figure 11.27  Local Winds of India.

Mango Shower Blossom Shower

Atmosphere: Pressure and Wind   11.19

Land and sea breezes We have seen that when air becomes warmer, it expands and its density decreases, i.e., a rise in temperature produces a fall in pressure at sea level. Further, when air becomes cooler, it contracts and its density increases, i.e., a fall in temperature produces a rise in pressure at sea level. On a warm summer’s day, coastal areas become hot, i.e., the temperature of the air over them rises. While this is occurring, the sea and the air over it are becoming warmer at a slower rate. As the day progresses, the pressure of the air over the land becomes lower relative to the pressure of the air over the sea, and a wind blows from the sea to the land. It is cool and is called a sea breeze. During the night, the reverse occurs. The land cools more quickly than the sea, which results in the air over the land having a lower temperature and therefore a higher pressure than that over the sea. A wind now blows from the land to the sea. It is called a land freeze (see Figure 11.28). Land and sea breezes are similar to monsoon winds in their mode of formation, but the latter affect large parts of continents. Monsoon winds are large scale winds.

Descending winds In Chapter 10, we discussed the ELR, DALR, and SALR. This information is now required to understand the temperature changes that occur in descending winds. Figure 11.29 shows the temperature changes in a warm, moist but unsaturated wind that rises over a mountain range of height 2000 m and descends on the other side to plains that are 500 m above sea level. The temperature of the wind at the foot of the mountain range is 22°C. It is unsaturated, and it therefore cools at the DALR (10°C per 1000 m) as it rises. When it reaches 1000 m, its temperature is 12°C. We will assume that at this temperature, the air becomes saturated. As the wind continues to rise, it now cools at the SALR (6°C per 1000 m). At the top of the mountain

environmental lapse rate (ELR) dry adiabatic lapse rate (DALR) saturated adiabatic lapse rate (SALR)

Day Descending Cool Air Rising i W Warm Air Low Pressure

Sea Breeze

Land

Higher Pressure Sea

Night Rising Warm Air

Descending Cool Air Higher Pressure Land

Land Breeze

Low Pressure Sea

Figure 11.28  Land and Sea Breezes.

11.20  Chapter 11

6º C 2000 Unsaturated SA

LR DA

LR

Saturated

12º C

Figure 11.29  Temperature Changes in a Warm, Moist but Unsaturated Wind.

1000

a U n s atur

ted D

R AL

17º C

22º C

Chinook (Winter) Desert

North America

Santa Ana (Winter) Nov–Feb

Warm Depression Wind

21º C

0

(2000  m), the wind’s temperature will be 6°C. Although the wind is still saturated, it does not have as much water vapour as it had before, some of it having been precipitated as rain in its ascent from 1,000 m to 2,000 m. As it now descends the mountain, it will warm up at the DALR. When it reaches the plains, its temperature will now be 21°C, i.e., the wind is 4°C warmer than it was at the same height on the windward side of the mountain. Descending warm winds operate in the Alps where these are called föhn, in the Rockies where they are known as c­ hinook (Figure  11.30), in Iran where they are called samun, and off the plateau of South Africa where they are called berg.

Valley breeze and mountain breeze

South America Warm Descending Wind

500

Zonda (Summer) Pampero (Summer) Nov–Feb

Cold Depression Wind Figure 11.30  The Descending Winds and the Winds of Depressions of North and South America.

It is important to understand about Anabatic (Valley Breeze) and Katabatic (Mountain Breeze). Both are termed as diurnal periodic winds. Figures  11.31(a) and 11.31(b) show that valley winds during day blow from valley to mountain, whereas in the evening, the mountain winds create a high pressure which causes winds to blow down the mountains towards valley floor. Generally, mountain winds are stronger than the valley winds.

Atmosphere: Pressure and Wind   11.21

(a)

(b)

isobar

cool

L

H warm

L

L

H

cool

isobar

H

warm

Figure 11.31  (a) Valley Breeze; (b) Mountain Breeze.

Convection winds In some hot deserts, there are violent convection currents caused by intense heating, and these produce convection winds which give rise to dust and sand storms. The dust devils of the Sahara, which also affect parts of West Africa, are caused by convection winds, as are the simoon, also of the Sahara, which are stronger and of larger development.

Depression winds These are associated with warm, moist tropical air, and cold, fairly dry polar air. Figure  11.32 illustrates different types of local winds in different continents, including (a) Asia, (b) Africa, (c) Australia, (d) Europe, (e) South America, and (f) North America. Because of this, depression winds may be warm or cold winds. Warm winds As a depression moves eastwards across the Mediterranean Sea, its front draws in air from the Sahara. This air is very dry, hot, and dusty, and the winds associated with it are called the sirocco in North Africa, the khamsin in Egypt, the chili in Tunisia, and the leveche in Spain (Figure  11.33). Similar winds occur in other parts of the world, e.g., the brickfielder in Victoria, Australia (Figure 11.34), and the zonda in Argentina. The Santa Ana (Figure 11.30) is a similar wind. During October to March, northeasterly winds blow out from the Sahara towards the coast of West Africa where air pressure is lower. These winds are warm and extremely dry. They dry up the nose, throat, and eyes and cause fingernails and skin to crack. The winds are called the harmattan. They also transport vast amounts of very fine dust particles. Cold winds These are associated with the rear of a depression, and they originate in the polar air stream. In the Mediter­ranean region, they are called the mistral (Figure 11.33) over southern France and the bora over the Adriatic. In the Southern Hemisphere, these winds are the pampero of Argentina (Figure  11.30) and the southerly burster (Figure 11.34) of New South Wales in Australia.

H

L

11.22  Chapter 11

Local Winds (Asia)

Local Winds (Africa)

Sirocco

Buran Karaburan

Khamsin

Harmattan Haboob (Non-Directional)

Berg

Local Winds (Europe)

Local Winds (Australia)

Helm Brickfielder

Bora Southerly

Mistral Fohn Levant

Etesians

Local Winds (North America)

Local Winds (South America)

Chinook

Blizzards

Norther

Pampero

Figure 11.32  Local winds of different continents.

Norte

Atmosphere: Pressure and Wind   11.23

Air Masses and Fronts

Mistral (Winter)

Föhn (Winter)

ce ee Gr

Bora A large volume of air, whose temperature and Spain (Winter) humidity are fairly uniform and which covIta ly ers an extensive surface area, is called an air mass. An air mass only develops over an area which is very large and which is uniform in build and shape, e.g., a desert area such as the Sahara Desert or an ocean surface. The charMediterranean Sea acteristics of an air mass are derived from the Sirocco Leveche region over which it formed (Figure  11.35). (Spring) (Spring) Chili Generally, these characteristics are retained (Spring) by the air mass when it moves away even to Warm Wind considerable distances from its source of Khamsin origin. Cold Wind (Spring) Some air masses are moist and warm, some are moist and cold, some are dry and warm, and some are dry and cold. There are four Figure 11.33  Local Winds Common to the Mediterranean basic types of air mass: Region. The Cold Winds are Strong and Gusty. The Warm

1.    Equatorial which forms over equatorial Winds, Especially those Originating in the Sahara, are Hot, Dry, and Dusty, but they Become Very Humid after Crossing the oceans and is warm and unstable; 2.    T  ropical which forms near the belts of Mediterranean Sea. sub-tropical high pressure; 3.    Polar which forms near to the sub-polar low pressure; 4.    A  rctic and antarctic which form, respectively, over the ice sheets of Greenland and Antarctica. Air masses (3) and (4) are cold and stable. Australia

Each type of air mass can be sub-divided according to whether it forms over the sea, Southerly Burster where it is called maritime, or over the land, where it is called continental. We therefore have (Summer) Brickfielder air masses such as polar maritime (Pm), polar Nov–Feb (Summer) continental (Pc), tropical maritime (Tm), Nov–Feb tropical continental (Tc), arctic ­maritime (Am), etc. phere, Tm air In the Northern Hemis­ originates over the warm waters of the Warm Wind Caribbean and central Atlantic, while Pm air originates over the cold waters of the Cold Wind Atlantic around Iceland and Greenland. Am air comes from the cold seas around the Figure 11.34  Warm and Cold Local Winds of Australia. The arctic ice mass, while Tc air originates over Cold Winds are Strong and Gusty; the Warm Winds Originating the hot sands of north Africa. in the Heart of Australia are Dry.

11.24  Chapter 11

When air masses of differing characteristics meet, a front develops between them. A front Low is a zone of transition. The front that develops between polar and tropical air masses is called a polar front, and the one that develops between polar maritime and polar continental air 50000* 50 masses is called an arctic front. Another important front develSystem moves 40 40 ops between the converging hot air of the northern and southern trade wind belts. This is called 30 30 the ITCZ. These fronts are shown in Figure 11.21. COLD WARM It is important to emphasize AIR 20 20 COLD AIR that very little mixing occurs in AIR the zone of transition. The zone 0ºC 10 10 is usually marked by changes in Warm sector 0ºC 0ºC temperature, pressure, winds, Shallow clouds A B and precipitation. These three fronts are the major fronts in 400 300 200 100 0 0 100 200 300 400 500 600 Long period that they cover very large areas as Narrow rain belt steady rain can be seen in Figure 11.21. Two other fronts develop which are Figure 11.35  Formation of an Air Mass. localized but have a profound effect on the weather, especially 1000 Cool of Great Britain and western Europe, and other west Air coast regions in the temperate latitudes. These fronts 996 Cold 992 form with the development of a depression. They are Air called the cold front and the warm front, shown on synB optic charts by triangles and semi-circles, respectively (Figure 11.36). At this point, the strong and regular winds that blow in the upper atmosphere about 10 km above the surface A Warm Front need to be mentioned. In the upper atmosphere, a large 1004 Cold Front area of low pressure lies over the polar regions, while a Warm Air Precipitation continuous belt of higher pressure covers the tropics. The above-mentioned winds blow between the two pressure systems in a west to east direction. These winds form Figure 11.36  Cold Front and Warm Front a belt of air which flows from between 100 and 300 km/h. Shown Symbolically by Triangles and Semicircles, The winds are called jet streams (Figure 11.37). The jet Respectively. streams operate between 30° and 60°, and they form giant waves. The waves develop as the jet streams swing to the right and left as they pass the large areas of cold and warm air lying over the oceans and continents. They also rise and fall as they cross the Rocky Mountains.

Atmosphere: Pressure and Wind   11.25

A peculiar land temperature anomaly was observed between 20 and 27 July 2010 over Europe and Asia (Figure 11.38). In the given period, a blocking event froze the meanders of the jet stream over Europe and Asia. The pattern led to extreme weather across the continents. The eastward-moving jet streams greatly influence the movement of the high and low pressure systems in the lower levels of the atmosphere, which were discussed at the beginning of this chapter. This is why the depressions, which influence the weather of the mid-latitudes so greatly, also move in an easterly direction. The lows and highs of the lower levels also influence movement of the jet streams, e.g., an extensive low pressure near the surface can cause a jet stream to bend so much that a part of it detaches, resulting in the cold air of the broken bend to move towards the equator. A pool of cold air of this type then brings cool wet weather to places which are usually warm and dry. The interaction between the upper and lower layers of air also results in the formation of depressions and anti-cyclones.

Permanent Low Pressure

Arctic

C irc l e

HP LP

Jet Stream

Permanent High Pressure Equator

Figure 11.37  The Jet Streams form Waves Which Shift Left and Right as Well as Up and Down Around the Poles.

COLD AIR breaks European warm spell COLD, LOW PRESSURE AIR encourages rain over mountains to the north of Pakistan

JET S TR EA

M

HOT, WET AIR from Africa dumps moisture over eastern Europe, becoming HOT DRY AIR, which causes the heatwave in Moscow

20–27 July 2010 Land surface temperature anomaly (°C) (compared to temperatures for the same dates from 2000 to 2008) –12

0

12

Figure11.38  Land Surface Temperature Anomaly.

11.26  Chapter 11

Depression It is well known that Great Britain’s weather is highly changeable. This is because Great Britain is situated in the zone where two very different types of air mass meet. The meeting of these two air masses gives rise to depressions which are areas of low pressure whose shape on a map is oval or circular. The isobars of a depression are closed, and the lowest pressure is at the centre as shown in Figure 11.39. The air of a depression in the Northern Hemisphere circulates in an anticlockwise direction, while in the Southern Hemisphere, it circulates in a clockwise direction. The air of a depression moves from the outside where the pressure is higher to the inside where it is lower. Consequently, it is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere by the earth’s rotation, as already discussed. Depressions are rarely stationary. Some are small, while others are large; however, all move in an easterly direction and are associated with unsettled weather usually with overcast skies and periods of continuous rain. The rain is caused by the uplifting of the warm, moist tropical air by the cold, drier polar air (see Figure 11.40). Although depressions can affect Great Britain’s weather throughout the year, they are more frequent during the winter when the meeting zone between the two air masses is over or near to Great Britain.

Development of a depression Cold air moves in a general westerly direction along the polar front, while warm tropical air moves in a general easterly direction. The frictional effects of the two air flows cause the polar front to develop undulations, most of which grow larger forming waves at the centres of which there is low pressure (Figure 11.40). As a wave bulges into the colder air, it gets larger and pressure falls at the tip of the wave, and an anti-clockwise circulation of winds blows around the front as shown in Figure 11.40(b). As the bulge develops, the warm air gradually displaces the cold air that lies ahead of it. The front where this occurs is called the warm front. At the rear of the bulge, the cold air gradually displaces the warm air ahead of it. The front where this occurs is called the cold front. The warm air between the two fronts is called the warm sector (see Figure 11.40(c)). Both fronts slope upwards from the surface, but the warm front is more gentle. As a

(a)

General Movement of Air

(b) 995 MB

General Movement of Air

998 MB Low Pressure

Winds are Deflected to the Right

1001 MB

Low Pressure

Winds are Deflected to the Left

Figure 11.39  Air Circulation in a Depression: (a) Northern Hemisphere; (b) Southern Hemisphere.

Atmosphere: Pressure and Wind   11.27

depression moves, the cold front moves faster than the warm front, and consequently, the warm air is pushed upwards out of the depression by the cold air. When this is achieved, an occluded front is formed. When the warm air has disappeared, the depression no longer exists. This is shown in Figure 11.40(d).

Weather associated with a depression Figure  11.41(a) is a sectional view of a depression. It shows the two fronts and the types of clouds that form. It also shows where rain occurs. Imagine this depression approaches and travels over a place A. The type of weather that will be experienced at A can be answered by examining Figures 11.41(b)–11.41(d) which shows the positions of the depression before it reaches A, when it is centred over A, and after it has passed A.

(a)

(b)

(c) 1018 1016

Cold Air Fr

on

t

Cold, Dry Heavy Air from the North Pole

1016

Pressure Falls War

Co

mF

1018

Po

la

r

t

ron

W Se arm ct or

ld F

Warm, Moist Light Air from the Tropics

ront

Warm Air

(d) of (i)

(ii)

CF

CF

WF

WF

(iii)

CF CF

Cold Front (CF)

WF

WF

CF CF

(v)

(iv)

WF

CF WF

Warm Front (WF)

Cold Air

Warm Air WF CF

WF

Occluded Front (OF)

Figure 11.40  (a)–(c) Show the Development of a Temperate Depression. The Width of the Depression is 320– 800 km. In the Northern Hemisphere, the Wind Blows in an Anticlockwise Direction, (d) Shows the Formation of an Occluded Front (v). The Diagrams Below Show the Cold Front Catching up with the Warm Front. In Diagram (v), the Warm Air is Lifted Away From the Surface and is Replaced by Cold Air. The Depression has Disappeared, and there is no Longer any Wave in the Front Separating the Cold and Warm Air.

11.28  Chapter 11

Direction in which the Weather Moves

d Col t n Fro

(a)

Cirrus Cumulus Warm Sector

Cold Sector

Rain

Rain

ont Cold Sector m Fr War Movement of Depression

960 km Movement of a Temperate Depression Path of a Tempetate Depression’s Centre (b)

(c)

t ld

on

Fr

Fr on

nd

m

Co

SW

Wi

r Wa

Warm Sector

NW

A

(d)

A

A

Wi

nd

nd

t

SW

Wi

Figure 11.41  Sectional View of a Temperate Depression (a) and the Passage of this Depression Across a Centre A. (a) The Sky Will be Clear Except for a Little High Cirrus Cloud. The Wind Will Blow From the Southeast. After Some Time, a Definite Cloud Cover Develops, and Light Showers of Rain Occur Which Become Heavier With the Fall in Air Pressure. The Warm Front Passes Centre A; (c) The Rain Stops and the Wind Veers From Southeast to Southwest. Temperatures Rise and the Air is Humid Because the Warm Sector Lies Over A; (d) As the Cold Front Passes, the Weather Changes Very Rapidly. The Wind now Blows From the Northwest, the Temperature Falls, and the Air Pressure Rises. With the Passage of the Depression, the Sky Clears and it Remains Cool.

Weather and depressions Figure  11.42 shows how warm and cold fronts appear on a weather map, while Figure  11.43 shows a depression in its initial stage of development. It shows cold air flowing in from the northeast and meeting moist, warm air flowing in from the southwest. As they meet, the cold air forms a wedge beneath the warm air. In a few days, this young depression develops into a mature depression which on a weather map appears as shown in Figure 11.44. As a depression grows, the winds blowing in an anti-clockwise d ­ irection become firmly established around the low pressure centre. This centre develops as the warm air is drawn in and rises up over the cold air. As the circulation is established, more air is drawn in and pressure falls further. The pressure at the centre may fall to 980 mb. Because the average pressure at sea level is 1013 mb, it is not surprising that gale force winds are often a characteristic of depressions. A mature depression often covers a very extensive area, sometimes extending from Iceland to Great Britain. As was stated earlier, depressions are very common over Great Britain, especially in winter. They usually develop in series.

Atmosphere: Pressure and Wind   11.29

A Warm Front Moving Eastwards Across the British Isles

A Cold Front Moving South-Eastwards Across the British Isles

Showers Fine

Rain Cloudy Cloudy

Rain Fine but High Cloud Increasing

Figure 11.42  How Cold and Warm Fronts are Shown on a Weather Map.

X r Ai

r Ai

X Y Figure 11.43  The Initial Stage of Development of a Depression.

Warm Air 900 km (About 560 miles)

Y

11.30  Chapter 11

(c) 16 10 12 10

10

10 04

–5

992 988

996

t on Fr

2

101

ld

2 4

t

Co

4

Warm Fron

Warm Air

100 8 1004 1000 -

Cold Air

2 2

2 2 0

0 LOW3

1

2

3

CLOUD Symbol

Figure 11.44  (a) A Mature Depression; (b) Sectional View Across the Depression; (c) A Weather Map Gives Information on all Aspects of the Weather. This Type of Map is Called a Synoptic Chart.

–21

–9 –10

–4

–12

–1 5

4

0

1

–1

–3

–3

–6

–3

–2

12

14

11

Cold Air

–6

–10

8 –6 –1 –5 3 972 0 –2 976 9 8 –5 98 80 7 –9 9 4 9 HIGH 0 0 9 88 4 –1 –1 99 92 –9 6 10 –4 –2 9 0 10 10 0 5 2 11 10 04 0 –2 –2 08 11 9 1 5 11 6 2 6 6 7 –2 1 8 11 7 012 12 5 7 11 8 11

1016

(b)

Warm Air

–5

3 0

13

Cold Air

–24 –26 –25 –21 –17

2

1

–24

–28

–2

–7 –9 –3 –8 –8 –1 12 3 0 –2 –1

1

D

–12

–1

–3

(a)

Low

–19

–2

–2

–4

el rav of T Cold Air n tio irec

–6

–2

1

984 980 976

1000

–6

1

1004

–1

1008

–6 0

996 –10 –19

–6

08 2

Cloud amount (oktas) 0 1 or less 2 3 4

WEATHER Symbol - Weather Mist Fog Drizzle Rain and drizzle Rain Rain and snow Snow Rain shower

5 6 7 or more 8 Sky obscured Missing or doubtful data

k

Rain and snow shower Snow shower Hail shower Thunderstorm

WIND Symbol

Wind speed (knots) Calm 1–2 3–7 8–12 13–17

For each additional half-feather add 5 knots

Atmosphere: Pressure and Wind   11.31

Tropical Cyclone A tropical cyclone is an area of intense low pressure. The isobars that represent a cyclone on a map are closely spaced, and they form a circular shape. Strong winds spiral towards the centre (anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), rushing upwards with great force in the area called the vortex which surrounds the centre or eye of the cyclone. The rapidly rising air gives rise to torrential rains, and the strong winds cause considerable damage to buildings and vegetation. Figure 11.45 shows the distribution of tropical cyclones. Figure 11.45 is a view across cyclones, and Figure 11.46 describes the anatomy of tropical cyclone. Tropical cyclones are called typhoons in Asia (Figure 11.50), hurricanes in the West Indies, and willy willies in northwest Australia (Figure 11.50) This diagram also shows the paths taken by tropical cyclones and temperate depressions. They frequently occur in August and September in the Southern Hemisphere.

Hurricanes

Hurricanes

Typhoons Cyclones

Equator

Figure 11.45  Distribution of Tropical Cyclones.

Hot and Calm, High Humidity

Dense Clouds, Violent Winds, Thunderstroms

Calm

Dense Clouds, Violent Winds, Thunderstroms

Gusty Winds

Gusty Winds Sea

Low Swell

Hot and Calm, High Humidity

Very Rough Vortex 32 to 64 KM

Very Rough Eye 16 to 32 KM

Figure 11.46  A Sectional View of a Tropical Cyclone.

Vortex 32 to 64 KM

Low Swell

Sea

11.32  Chapter 11

Eye

Outflow cloud shield

Eye wall Outflow cloud shield Spiral rainbands (thunderstorms)

Spiral rainbands (thunderstorms)

Top view Clockwise winds Descending air

Eye

Eye wall

0 miles 0 km 100

100 200

Eye wall

200

Spriraling Rotation of wings cyclone

300

Figure 11.47  Anatomy of Tropical Cyclone.

Figure 11.48  Satellite Imagery Showing a Cyclone Over Southern India.

Atmosphere: Pressure and Wind   11.33

2.23 The formation of a tropical cyclone 15 000 m

Eye–calm, clear sky

Winds spiral outwards Cirrus canopy Eye wall Cumulonimbus clouds

Up to 250 km from centre

Up to 250km from centre

Rainbands

0 Winds get stronger towards the eye wall Ocean temperature above 26.5ºC

Strong surface winds increase sea level in centre of cyclone Causing a storm surge

Warm moist air drawn into centre

Figure 11.49  Formation of a Tropical Cyclone.

23½° N Typhoons

Hurricanes

WillyWillies

0° Cyclons 23½° S

Tropical Cyclones and Paths

Temperate Depressions and Paths

Figure 11.50  This Map Shows the Paths Usually Followed by Depressions and Tropical Cyclones.

Development of a tropical cyclone Tropical cyclones develop where the air masses brought by the ­northerly and southerly trade winds meet along the intertropical front (Figure 11.49).

11.34  Chapter 11

Air Flows Outwards

Air Flows Outwards Rapidly RIsing Air Produces Clouds And Heavy Rain

Warm, Moist Air

Warm, Moist Air

Warm Sea Surface of Temperature 27°C Figure 11.51  Structure of a Tropical Cyclone. Inner Zone

Rapidly Rising Moist Air Vortex

Descending Air

Eye

Outer Zone

Rapidly Rising Moist Air Vortex

They form over oceans because the air masses that have met travelled over the oceans and have warm, moist lower layers. However, the upper layers of the air masses are cooler and drier. When two such air masses meet, one is lifted up over the other. The rising air cools, and its moisture condenses to produce heavy rainfall. Latent heat is set free by condensation, and it is this energy that enables the cyclone to rotate. Tropical cyclones move in a general westerly direction because their supply of warm, moist air is cut off. Three conditions are necessary for a tropical cyclone to develop.

1. A  n abundant source of warm, moist air of temperature about 27°C near to the sea surface. 2. T  he air must be blowing inwards and rising rapidly to great heights to give clouds of great vertical extent capable of producing torrential rainfall. 3. T  here must be an outward flow of air at the upper level. Figure 11.51 shows how a tropical cyclone forms. Figure 11.52 shows that the air on the outside rises rapidly, whilst the air on the inside is fairly calm. The cyclone is funnel-shaped.

Figure 11.52  A Section Through a Tropical Cyclone.

Weather associated with a tropical cyclone Weather conditions associated with a tropical cyclone are listed below. ●    B  efore the tropical cyclone arrives, the air becomes very still, and temperature and humidity are high. ●    A  s the front of the vortex arrives,gusty winds develop and thick clouds appear. ●    W  hen the vortex arrives, the winds become violent (upward surges), and they often reach speeds of 240 km or more per hour. Dense clouds and torrential rain reduce visibility to a few metres. ●● Calm conditions return when the eye of the cyclone arrives. ●● The arrival of the rear of the vortex brings in violent winds, dense clouds, and heavy rain. The wind now blows from a direction opposite to that of the front of the vortex.

Atmosphere: Pressure and Wind   11.35

Tornado A tornado differs from a tropical cyclone in that it forms over land. It is more destructive than a cyclone because its winds often exceed 320  km per hour. Fortunately, tornadoes are only a few hundred metres across in width. Figure 11.53 is a photograph of a tornado. If a tornado passes over a sea or lake, water is sucked up towards its centre, and a ­waterspout is formed (see Figure 11.54).

Cumulus or cumulonimbus clouds

Base of cloud Water spout

Strong up-rising air

Figure 11.53  A Tornado Passing across Texas, USA. The Uprising Funnel of Air is a Characteristic Feature of a Tornado. Although a Tornado is Only a Few Hundred Metres Wide at the Most, it Can Cause Tremendous Damage by its ‘Vacuum’ Effect.

Strong up-rising air

Surface of sea

Figure 11.54  A Diagram to Show the Formation of a Waterspout.

11.36  Chapter 11

Tropical Cyclones — a Natural Hazard Typhoon Rose A tropical cyclone, named Typhoon Rose, formed over the west Pacific Ocean on 10 August 1971. By 15 August, it was about 400 km from Hong Kong. The typhoon was then centred about 160 km from Hong Kong. Early on the morning of 17 August, the eye of the typhoon passed close to the west of Hong Kong. The air pressure decreased sharply from 1,007 mb at about 9.00 am on 16 August to 963 mb in the early hours of 17 August. It then rose sharply to 1,008 mb at noon on the same day as the eye passed by. During the passage of the typhoon, winds of 105 knots were recorded, and 288 mm of rainfall was recorded on 17 August. Gale force winds and heavy rainfall caused extensive flooding and numerous landslides. Ships were capsized, and some were lifted up by the huge waves and dropped on the rocks in coastal waters. More than 100 people were killed.

Figure 11.55  Double Eye-Wall Structure of Typhoon Dujuan.

Typhoon Dujuan

The image (Figure  11.55) indicates Typhoon Dujuan exhibiting a double eye-wall structure while approaching the eastern coast of Guangdong in early September 2003. The diameters of the inner and outer eyes were about 20 km and 100 km, respectively. The double eye-wall structure in tropical cyclones is not rare in the west Pacific. It had been observed in Typhoon Kujira (2003), Typhoon Kirogi, and Typhoon Kai-Tak (2000). Associated with strong typhoons, double eye walls were also observed by the Observatory’s weather radar during the passage of Typhoon Elsie in 1975 and Typhoon Hope in 1979.

Hurricane David Towards the end of August 1979, one of the worst and most powerful hurricanes, named Hurricane David, swept across the Atlantic from Cape Verde for a distance of about 5000 km, through the islands of the eastern Caribbean Sea, and into the Atlantic states of the USA. The width of the hurricane was about 450 km, with an eye 40 km wide.

Atmosphere: Pressure and Wind   11.37

Canada

New York

USA Atlantic Ocean

Dominican Republic

Cuba

Puerto Rico Dominica

Jamaica

Haiti Martinique

Caribbean Sea

0

800 km

Figure 11.56  The Path Taken by Hurricane David in August 1979. The full force of the 240 km per hour winds struck Dominica, completely destroying the banana crop, the main crop of the island, and devastating over half of the island’s buildings. The damage caused 60,000 people to become homeless. But the greatest damage occurred in the Dominican Republic where more than 600 people were killed, over 160,000 people were rendered homeless, and a total damage of more than US$1 billion incurred. The hurricane also caused severe damage on the islands of Guadeloupe and Martinique. Altogether, Hurricane David took 1100 lives. Figure 11.56 shows the path of the hurricane through the Caribbean region

11.38  Chapter 11

Anti-cyclone An anti-cyclone develops in a region of descending air and is therefore an area of high pressure. Air moving polewards in the upper atmosphere from equatorial regions descends forming the sub-­tropical high pressures. The one in the Northern Hemisphere is well developed and extensive. It often reaches sufficiently far northward to affect Great Britain. The high pressure area is called an anti-­cyclone. It forms an oval or circular shape of closed isobars. The highest pressure is at the centre. The winds associated with an anti-cyclone blow outward from its centre in a clockwise direction in the Northern Hemisphere and in an anti-clockwise direction in the Southern Hemisphere. As previously explained, the direction of the winds is caused by the rotation of the earth. Figure 11.57 shows the wind circulation in an anti-cyclone. An anti-cyclone is a uniform air mass of great area, often as much as 3000 km across. Because the air in an anti-cyclone is descending, it becomes warmer, and therefore, the formation of cloud and rain is unlikely. Moreover, the pressure gradient over large distances is small, and therefore, winds in an anti-cyclone are generally light. An anti-cyclone usually gives fair weather, especially in the summer, but in the winter, it may produce dull and cloudy weather.

Weather maps and anti-cyclones Winter anti-cyclones from Scandi­navia or central Europe often extend eastward as far as Great Britain. When this occurs, cold winds blow from the east frequently causing temperatures to fall below freezing point. As these winds blow over the North Sea, they carry sufficient moisture to precipitate snow in eastern England. Anti-cyclonic air can be very still, and on clear winter nights, the rapid loss of heat from the ground by radiation may give rise to fog. If an anti-cyclone affects the weather in Great Britain in the summer, it is marked by very fine conditions. It gives clear skies and virtually no rain, and temperatures can reach as high as 30°C. Very strong anti-cyclonic weather affected Great Britain in the summer of 1976 and again, though to a lesser extent, in 1984, causing drought conditions to develop. Figure 11.58 shows an anti-cyclone over Great Britain in the spring. (a)

(b)

ral ne nt e G me ve Mo air of

ral ne nt e G me ve Mo air of

High Pressure

High Pressure Figure 11.57  Air Circulation in an AntiCyclone: (a) Northern Hemisphere; (b) Southern Hemisphere.

1030 mb 1028 mb 1026 mb Winds are Deflected to the Right

Winds are Deflected to the Left

Atmosphere: Pressure and Wind   11.39

20W

10W Low

0

10E

20E

1

9 0 00

mb

4 00

7

10

9

1

mb

1

1

11

10

9

b 8m

100

m 012

11 10

b

b

12

6m

101

15

11

13 16

20

10

4 02

mb

12

mb

1

13

14 15

b

10

2 03

13

14

13

m 28

m

14

13

15

15

13 15 13

b

1

50

11

1

60

15 15

15 17 13

19

High

19 18 17

22

b

0m

102

18 19 27

20 26 18

19

21 Low

10W

0

40

Figure 11.58  AntiCyclone over Great Britain and Northwest Europe in the Spring.

11.40  Chapter 11

Key facts ●● ●● ●● ●●

●● ●● ●● ●● ●●

●●

●●

●●

●● ●●

The pressure of the air for any region depends upon altitude, temperature, and the rotation of the earth. When air descends, its pressure and temperature rise; when it ascends, its pressure and temperature fall. When the temperature of the air increases, its pressure decreases; conversely, when the temperature decreases its pressure increases. There are two types of air pressure — low pressure and high pressure. Areas of low pressure are called depressions (in temperate latitudes) and cyclones or hurricanes or typhoons (in tropical latitudes). Areas of high pressure are called anti-cyclones. Atmospheric pressure is measured by a barometer. The difference in pressure between adjacent isobars is a pressure gradient. Pressure is shown on a map by lines called isobars — these are lines of equal pressure. Temperature and earth rotation together produce specific pressure systems which have fairly definite locations. There are three main pressure systems in each hemisphere — polar high, temperate low, and sub-tropical high, with a belt of low pressure (doldrums) that extends around the earth over the equator. The positions of the main pressure systems change as the seasons change, i.e., they move northwards in the northern summer and southwards in the northern winter. The pressure systems over North America, Asia, and Australia change from high pressure (in their summer season) to low pressure (in their winter season). A difference in air pressure between two regions results in a flow of air from the higher pressure to the lower pressure. This air flow is called a wind. The rotation of the earth deflects winds to their, right, in the Northern Hemisphere, and to their left, in the Southern Hemisphere. Planetary or prevailing winds blow more frequently than any other wind, and they operate over extensive areas. Local winds can be classified according to their origin. DEPRESSION WINDS Hot winds

Cold winds

Sirocco

Santa Ana

Mistral

Harmattan

Chili

Bora

Leveche

Brickfielder

Pampero

Khamsin

Zonda

Southerly burster

Descending winds

Convection winds

Föhn

Dust-devils

Chinook

Simoon

Samun Berg

Atmosphere: Pressure and Wind   11.41

●● ●● ●●

●●

●●

●● ●●

Wind direction is determined by a wind vane, and wind velocity is given by an anemometer. A wind rose is used to record wind direction for a specific place each day. There are four basic types of air mass: tropical, polar, continental, and maritime. Variations in these are tropical continental, tropical maritime, polar continental, etc. The zone separating two air masses is called a front. The two main fronts are the polar front and the intertropical front. Specific weather conditions are associated with fronts. Great Britain’s weather is largely determined by the frequent passage of depressions from the west, especially in the winter. These depressions are caused by the polar front. Winds enable a global balance of temperature and pressure to be maintained in time and space. The larger part of the destruction caused by tropical cyclones is confined to coastal regions.

11.42  Chapter 11

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. 1. Which one of the following statements is not true? (a) When air sinks, its pressure increases. (b) When the temperature of air increases, the pressure of the air decreases. (c) When the temperature of air decreases, the pressure of the air increases. (d) When the temperature of air falls, the outward pressure of its molecules is spread over a larger area. (e) The outward pressure of the molecules in air is spread over a smaller area when air pressure increases. 2. A belt of low pressure, called the doldrums, occurs (a) between the westerlies and the trade winds. (b) within equatorial latitudes. (c) in regions where the air is rising. (d) on the poleward side of the tropics. (e) in the area of the westerly winds. 3. The main effect of the high pressure system which develops over central Asia in the winter is (a) the heavy rainfall in Indo-China. (b) the migration of cattle to the lowlands in central Africa. (c) the lower air temperatures over most of China. (d) to bring heavy falls of snow to northern China. (e) the movement of strong air currents towards the east, southeast, and south. 4. All the following winds are associated with depressions. Which pair consists of a warm wind and a cold wind? (a) Leveche and sirocco (b) Pampero and brickfielder (c) Bora and mistral (d) Pampero and southerly burster (e) Brickfielder and zonda 5. All these statements are true about depressions (temperate and tropical) in general, but only one is true with reference to the Northern Hemisphere. Which is that statement? (a) Pressure increases from the centre outwards. (b) The air moves in a circular manner. (c) They generally form over the oceans. (d) Their winds often bring rain. (e) The air circulation is anti-clockwise. 6. Which of the following is not a cyclone? (a) Hurricane (b) Zonda (c) Willy willy (d) Typhoon (e) Tornado

Atmosphere: Pressure and Wind   11.43

Direction for questions from 7 to 11: Each question has one or more correct option/s. Identify which of the options are correct and select the answer as per following. (a) if 1 only is correct, (b) if 1 and 2 only are correct, (c) if 1, 2, and 3 are all correct, (d) if 2 and 3 only are correct, (e) if 3 only is correct. 7. The temperature and pressure balance in the atmosphere is maintained by (1) gravity. (2) humidity. (3) winds. 8. When a unit of air rises, its pressure decreases. This is because (1) its molecules occupy a larger volume. (2) its temperature falls. (3) its volume decreases. 9. The reading of a mercury barometer has to be adjusted for (1) wind velocity. (2) gravity. (3) temperature. 10. A line on a weather map joining places having the same atmospheric pressure is called an (1) isotherm. (2) isohyet. (3) isobar. 11. The most common atmospheric disturbance affecting the weather of Great Britain is (1) the depression. (2) the anti-cyclone. (3) the tropical maritime air mass. 12. Assertion (a) 60°–65° latitudes in both the hemispheres have a low-pressure belt instead of high pressure. Reason (b) the low-pressure areas are stable over oceans rather than on land. Code: (a) Both (a) and (b) are true and (b) is the correct explanation of (a). (b) Both (a) and (b) are true and (b) is not the correct explanation of (a). (c) (a) is true but (b) is false. (d) (a) is true but (b) is true. 13. Consider the following statements: (1) Horse latitudes are low-pressure belts. (2) Either of the two belts over the oceans at about 30°–35°N and S latitudes is known as Horse latitude. Which of the statement/s given above is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 14. Air pressure is lowest in (a) summer season (b) autumn season (c) winter season (d) spring season

11.44

15.

Chapter 11

The difference in pressure between adjacent isobars is called (a) isotope (b) pressure bar (c) pressure effect (d) pressure gradient

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1.

Figure 11.59 shows the locations of the main low pressure (LP) and high pressure (HP) areas of the world in July. The black areas represent very low pressure. Explain why pressure is low over Asia. Briefly explain the origin and nature of the winds marked ‘A’, and account for the change in direction of these winds as shown in the diagram. Draw an annotated sketch-map to show the pressure and wind patterns over Asia and Australasia in January.

LP

A

A

HP

FIgure 11.59 Location of the Main Low Pressure and High Pressure Areas of the World in July.

2.

Figure 11.60 shows a sea breeze occurring over part of the southeast coast of England in the summer. (a) Make a copy of this diagram and complete it by

(i) drawing arrows to complete the probable air-flow cycle; (ii) marking distances on the scale line to give some idea of scale.

(b) In which part of the day are sea breezes most likely to occur? Give reasons for your answer. (c) In Great Britain, sea breezes are usually local and temporary. Under what type of weather conditions do they mostly occur? (d) Are the activities of a coastal resort affected by sea breezes, and if so, in what way?

Atmosphere: Pressure and Wind   11.45

Sea

Land Scale

Figure 11.60  Depiction of the Sea Breeze Occurring Over Part of the Southeast Coast of England in Summer.

3. Many parts of the world experience local winds. Some bring unseasonal warmth, and some are exceptionally cold. The names of some of these winds are chinook, sirocco, föhn, southerly burster, harmattan, and brickfielder. (a) Name and locate two such winds and state the effect that each may have on human activity in the area it affects. (b) For one of these winds, explain when and why it occurs. 4.   (a)  What is meant by a ‘prevailing wind’? (b) Describe how the weather is influenced by prevailing winds in your home area or another local area which you know. (c) Name one large area of. the world where the rainfall pattern is affected by the direction of the wind. Give the reasons for both the pattern of air flow and its effects upon the rainfall. 5. Explain the monsoon wind pattern of Asia for July and January with reference to ITCZ shift.

Answer key Exercise 1 1.   (d) 6.   (b) 11.   (a)

2. (b) 7. (e) 12. (c)

3. (e) 8. (b) 13. (b)

4. (b) 9. (e) 14. (a)

5. (e) 10. (e) 15. (d)

Thispageisintentionallyleftblank

12

Atmosphere: Water

Learning Outcomes After completing this chapter, you will be able to: ● ● ● ●

Understand the vastness of water in the oceans and sea Explain the concept of humidity, condensation, and dew point Classify the various types of precipitation according to their origin Classify clouds as per the shapes and origin and world pattern of rainfall distribution

Keywords Humidity, Condensation, Advection, Smog, Clouds, Precipitation, Isohyet

1

12.2  Chapter 12

Introduction The total volume of water in the oceans and seas is constant. This is because all the water that evaporates from the earth’s water bodies is eventually returned to them directly by the processes of condensation and precipitation, and indirectly by stream and overland flow from the land surfaces. As a part of the earth’s climatic system, water vapour is the most critical component in the atmosphere. It constitutes more than 99  per  cent of the atmospheric moisture. Water vapour is referred to as the earth’s primary greenhouse gas. It traps more heat than c­ arbon dioxide (CO2), and approximately half of the heat is transported through this medium from the tropics to the poles, thereby affecting the precipitation received in these regions. Under certain conditions, water vapour takes the form of tiny droplets of water (or ice if the dew point temperature is below the freezing point). These droplets appear as dew, fog, and frost, collectively called condensation, and rain, drizzle, sleet, snow, and hail, collectively called precipitation.

Wet

Dry

ºC 43

ºC 43

38

38

32

32

27

27

21

21

16

16

10

10

4

4

–1

–1

–7

–7

–12

–12

–18

–18

Humidity Air absorbs water through the process of evaporation, which results in water changing from the liquid state to the gaseous state. The gaseous state is called water vapour. The amount of water vapour in the air is called humidity. The humidity of the air depends upon the temperature, e.g., if the temperature rises, then air can hold more water vapour. When air can hold no more water vapour, it is said to be saturated. When the temperature of air falls, it cannot hold as much water vapour as it could at higher temperatures. The actual amount of water vapour in a given volume of air at a particular temperature is called the absolute humidity. It is, however, more important to know the ratio between the absolute humidity of a given mass of air and the maximum amount of water vapour that it could hold at the same temperature. This ratio is called the relative humidity. For example, if the temperature of a mass of air is 30°C and if its relative humidity is 75 per cent, then it could hold another 25 per cent of water vapour at the same temperature before it became saturated. Changes in humidity account for the production of precipitation. Sometimes, the temperature of the air falls causing the relative humidity to be 100 per cent (saturated). If the temperature continues to fall, then some of the water vapour condenses (converts into the liquid  state), thereby causing tiny water droplets to form as either cloud, rain, mist, fog, or dew.

Measurement of humidity Muslin Container Water Figure 12.1  A Simple Hygrometer.

Two ordinary thermometers are used to measure the humidity, and both are kept in the Stevenson Screen. The bulb of one thermometer is wrapped in thin ­muslin, which dips into a small bath containing water, thereby keeping the muslin moist. This thermometer is known as the wet bulb thermometer. The other thermometer has no muslin, and it is known as the dry bulb thermometer. The two thermometers (Figure 12.1) are known as a hygrometer.

Atmosphere: Water  12.3

If the air is not saturated, water evaporates from the muslin, and this cools the bulb of the thermometer, thereby causing its mercury to contract. The bulb of the dry bulb thermometer is not affected in the same way, and so, the two thermometers show different readings, i.e., the wet bulb thermometer shows a lower reading. The difference between the readings is used to calculate the relative humidity by using a set of tables similar to that shown in Table 12.1. If the air is saturated, then there is no evaporation, and both thermometers show the same readings. This means that the relative humidity is very high, i.e., 100 percent or nearly so. Modern electronic devices are digital and use the dew point, that is, temperature of condensation or variation in electrical capacitance or resistance to measure humidity ­differences (Figure 12.2). Since 1987, water vapour measurements are conducted through v­ arious satellite instruments including Special Sensor Microwave Imager (SSM/I), Special Sensor Microwave Imager Sounder (SSMIS), Tropical Rainfall Measuring Mission

Table 12.1

Following inferences can be made on the basis of observation of the two thermometer readings: no difference—air is saturated small difference—humidity is high large difference—humidity is low

Relative humidity table.

DRY BULB °C

DEPRESSION OF THE WET BULB (DEGREES CELSIUS ) 1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

L·8

1·9

2·0

17·0

90

89

88

87

86

85

84

83

82

81

80

16·5

90

89

88

86

85

84

83

82

81

80

79

16·0

89

88

87

86

85

84

83

82

81

80

79

15·5

89

88

87

86

85

84

83

82

81

80

79

15·0

89

88

87

86

85

84

83

82

81

79

78

14·5

89

88

87

86

85

83

82

81

80

79

78

14·0

89

88

86

85

84

83

82

81

80

79

78

13·5

88

87

86

85

84

83

82

81

80

78

77

13·0

88

87

86

85

84

83

81

80

79

78

77

12·5

88

87

86

85

83

82

81

80

79

78

77

12·0

88

87

86

84

83

82

81

80

78

77

76

11·5

88

86

85

84

83

82

80

79

78

77

76

11·0

87

86

85

84

83

81

80

79

78

76

75

10·5

87

86

85

83

82

81

80

79

77

76

75

10·0

87

86

84

83

82

81

79

78

77

76

74

9·5

87

85

84

83

82

80

79

78

76

75

74

12.4  Chapter 12

(TMI), Advanced Microwave Scanning Radiometer (AMSR-E), WindSat, and GPM Microwave Imager (GMI) (Table 12.2). These are very useful for climate studies, and the data generated through them have enabled climatology maps, a global trend map, and a latitude-time plot. Data derived from these instruments include atmospheric water vapour, surface wind speed, rain rate, and cloud liquid water. TMI also measures sea surface temperatures (SST), ocean surface wind speeds, and columnar water vapour. Tropical Rainfall Measuring Mission (TRMM) was a joint programme between NASA and the Japan Aerospace Exploration Agency (JAXA). The AMSR-E has operated on three satellites: (1) AMSR-2 on JAXA’s GCOM-W1 spacecraft; (2) AMSR-E on NASA’s EOS Aqua spacecraft; and (3) AMSR on JAXA’s ADEOS-II spacecraft. The Wind Sat Polarimetric Radiometer

Figure 12.2  Modern Electronic Hygrometer.

Table 12.2

Different satellite instruments for water vapour measurements.

INSTRUMENT

DATA RANGE

VERSION

SSM/I—Special Sensor Microwave Imager

1987–present

V7

SSMIS—Special Sensor Microwave Imager Sounder

2003–present

V7

TMI—Tropical Rainfall Measuring Mission

1997–2015

V7.1

AMSR-E—Advanced Microwave Scanning Radiometer

2002–2011

V7

WindSat—Wind Sat Polarimetric Radiometer

2003–present

V7.0.1

GMI—Global Precipitation Measurement (GPM) Microwave Imager

2014–present

V8.1

Atmosphere: Water  12.5

was developed to measure the ocean surface wind vector from space (Figure  12.3). Prior to launch, the only instrument capable of measuring ocean wind ­vectors were scatterometers (active microwave sensors). The WindSat instrument can also measure ice and snow characteristics, SST, and soil moisture. GMI is a microwave radiometer onboard of The Global Precipitation Measurement (GPM) satellite. All these instruments have provided data of great value, which is being used in many areas of research, s­ pecially related to climate variability and agriculture studies. Figure 12.3  WindSat Satellite.

Condensation Condensation occurs when the following three conditions are satisfied: (1) there must be sufficient water vapour molecules in the air; (2) there must be nuclei (minute particles of matter such as salt and smoke); and (3) the air temperature must fall below the dew point temperature (the temperature at which air becomes saturated). Water vapour content Water is converted into water vapour by the process of evaporation when the temperature is sufficiently high and when water is available. The amount of evaporation depends on the temperature and the amount of water vapour already in the air. Nuclei Minute particles of matter, which are hygroscopic, i.e., having an affinity for water, form the nuclei around which condensation occurs. The two most common hygroscopic nuclei are salt and smoke. The former is very common in air over or near to the oceans; the latter is common in the air of most urban and industrial environments. Cooling For condensation to occur followed by large-scale precipitation, air has to rise and cool adiabatically. Cooling can, and does, occur to below the dew point by radiation of heat from the earth’s surface, but this is limited to the lower layers of the atmosphere.

Types of condensation Following are the types of condensation phenomena:

Dew Dew develops at night when there are no clouds and no wind and when the air has a high water vapour content. As the ground loses its heat by radiation, the temperature of the air nearest to it may fall to the dew point. If it falls below this, tiny water droplets form on the ground, forming a layer of dew.

12.6  Chapter 12

Heat Lost from Surface by Radiation

Fog

Cold Air

Cold Air Valley Bottom

Figure 12.4  Loss of Heat by Radiation Cools the Bottom Layer of Air, Sometimes Inducing Condensation. This Produces Radiation Fog, which Often Drains Downslope and Collects in Valley Bottoms. The Downslope Flow of Cold Air Often Causes a Temperature Inversion. (a) Southern Africa

Warm Sea

Fog

Hot Desert

Sea Breeze Cold Benguela Current

(b) Onshore Moist Winds

Hot Surface

Fog Cold Water

Figure 12.5  (a) Fog Frequently Develops off the Coast of Namibia when Warm, Moist Onshore Winds Blow towards the Coast and Cross the Cold Benguela Current; (b) Diagram to show how Advection Fog Forms.

Frost If the dew point temperature is below the freezing point, water vapour turns directly into its solid form through the process of condensation. Frost consists of tiny ice crystals.

Fog If the temperature of the air falls below the dew point because the air is in contact with a cold surface, and provided the air is still and contains hygroscopic nuclei, then a fog develops. This is called radiation fog (Figure 12.4). This form of condensation is similar to dew and frost, except that it occurs in large layers of air and that the water droplets remain suspended in the air. Radiation fogs occur mainly in cool temperate latitudes. When a radiation fog develops in a hilly country, the fog slowly drains downhill and collects in the hollows, often leaving the hills standing out as ‘islands’. In other words, radiation fogs are formed by gravitation. A fog often forms when a warm moist air current is cooled as it passes over a cold sea or land surface and mixes with the cold air above this surface. Because the movement of air is in a horizontal direction, this type of fog is called advection fog. An advection fog may also develop when cold air moves horizontally across a warmer surface and mixes with the warmer air over this surface. Advection fogs develop extensively where warm moist air flows over a cold ocean current, e.g., off the west coasts of California, northern Chile, Australia, and Namibia (see Figure 12.5). The west coasts of all these regions are backed by hot deserts. Similarly, advection fogs develop where moist warm air flows over a cold current. One of the best known and most extensive fogs of this type occurs near the Grand Banks of Newfoundland, where moist air that has crossed the warm Gulf Stream moves over the cold Labrador Current (Figure 12.6). Another type of fog is called smog (mixture of fog and smoke). Smog used to be

Atmosphere: Water  12.7

We st

erli es

tL

aw ren ce Riv

quite common in industrial areas, which experience normal fog, but Cold since many cities have now banned North Labrador the use of coal for heating, smog America Current of this type has greatly decreased N in extent and occurrence. The last disastrous smog in Great Britain er occurred in 1952 in London when nearly 4000 people died from respiFog ratory diseases aggravated by the smog. Another type of smog, which is Warm S becoming more common is caused by Gulf Stream the accumulation of chemicals in car exhaust fumes which are converted into harmful substances by sunlight. This type of smog develops in large cities which have sunny weather and where the air is fairly still as in the Los Figure 12.6  The Meeting of Warm and Cold Currents off the Mouth of Angeles district in California. the St Lawrence River, North America, gives Rise to Widespread Fog.

Clouds Clouds are another form of condensation. They consist of minute droplets of water or particles of ice which are in suspension. The shape, height, and movements of clouds can indicate the type of weather that is likely to occur, and because of this, they are carefully studied by meteorologists who prepare weather forecasts. Clouds are classified according to their appearance, form, and height. There are four groups, as shown in Figure 12.7. TYPE OF CLOUD

HEIGHT

High clouds

6000 –12,000 m

Middle clouds

2100 –6000 m

Low clouds below

2100 m

Clouds of great vertical extent

1500 –9000 m

The different types of cloud are given Latin names, which are all combinations of the following words: 1. Cirrus means looking like a feather. It is used to describe the very high clouds. 2. Cumulus means looking like a heap. It is used to describe clouds, which have flat bases and rounded tops. There are patches of blue sky between the clouds. 3. Stratus means lying in level sheets. It is used for layer-type clouds. 4. Nimbus means rain cloud. 5. Alto means high. Despite a tremendous number of cloud forms, it is possible to recognize 10 basic types (Figure 12.7) each of which will now be discussed.

A fog and a cloud are similar in that they consist of water droplets in suspension, but they differ in that a fog results from air being cooled below the dew point through contact with a cold surface, whereas a cloud results from air being cooled by rising.

12.8  Chapter 12

Cirrostratus Cirrus

Cumulonimbus

Cirrocumulus 20,000 AGL

High Clouds

Altostratus Altocumulus

Clouds with Vertical Development

Middle Clouds 6,500 AGL Low Clouds Nimbostratus

Stratus

Figure 12.7  Four Main Groups of Clouds.

Cold Front Warm Front Stationary Occluded

Cumulus

Stratocumulus

High clouds These clouds are 6000 –12,000 m above sea level. Cirrus These are composed of small ice crystals; white, wispy, fibrous or feather-like in appearance; in bands or patches (Figure 12.8). Cirrocumulus

Figure 12.8  Cirrus Clouds; Notice the Feathery Appearance of the Clouds.

These are also composed of ice crystals, but they are globular or rippled (like ripples in the sand on a sea shore) in appearance, forming a thin cloud (Figure 12.9).

Cirrostratus Looks like a thin white, almost ­transparent sheet, which causes the sun and moon to have ‘halos’. This is because the ice crystals, which form the clouds act as prisms which split the sunlight into its primary colours. In this way, a ­‘rainbow’ is formed around the sun (Figure 12.10).

Middle clouds Altocumulus These are composed of water droplets in layers or patches, globular or bumpy-­ looking with flattened bases arranged in lines or waves (Figure 12.11). Alto means

Atmosphere: Water  12.9

Figure 12.9  Rows of Globular Cirrocumulus Clouds.

Figure 12.11  Altocumulus Clouds

Figure 12.10  Cirrostratus Clouds.

Figure 12.12  Altostratus Clouds.

in latin, ‘cumulus’ means heaped. Altocumulus is a ­middle-altitude cloud characterized by globular masses or rolls in layers or patches. Altostratus Composed of water droplets; forming sheets of grey or watery-looking clouds, partly or totally covering the sky (Figure 12.12).

Low clouds Stratocumulus Large globular masses; bumpy-looking; soft and grey Figure 12.13  Stratocumulus Clouds. in appearance and forming a pronounced regular and sometimes wavy pattern (Figure 12.13).

12.10  Chapter 12

Nimbostratus Dark grey and rainy-looking; dense and shapeless; often produce continuous rain (Figure 12.14). Stratus These clouds are low, grey, and layered and have almost ­fog-like in appearance; they bring dull weather and are often accompanied by drizzle (Figure 12.15).

Clouds of great vertical extent Cumulus Round-topped and flat-based forming a whitish-grey globular mass; they consists of individual cloud units as shown in Figure 12.16. Cumulonimbus

Figure 12.14  Nimbostratus Clouds.

This is a special type of cumulus cloud of great vertical extent; they are white or black globular masses whose rounded tops often spread out in the form of an anvil; their summits rise like towers and mountains; and they often indicate convectional rain, lightning, and thunder (Figure 12.17). In today’s time, clouds are visualized using high-­ resolution satellite data for accurate research. The structure of clouds in the satellite image can inform the meteorologist a lot about the weather, and animations inform him about the movement of weather systems. Figure 12.18 visualizes cumulonimbus clouds (thunderstorms) with overshooting tops. The satellite also measures the temperature of the clouds and the surface of the earth with an infrared sensor. Infrared data visualizes the warm clouds as grey, the cool clouds as white, and the very cold clouds as bright white. Meteorologists may also colour code infrared imagery in order to more easily interpret the data (Figure 12.19). CloudSat was launched in 2006 to improve our understanding of the role clouds play in our climate system (Figure 12.18).

Formation of clouds The formation of different types of clouds actually depends on the height at which dew point temperature occurs and the way in which the air rises vertically. For example, air which rises steeply in columns will tend to produce cumulus-type clouds regardless of whether the rising of the air results from differences in atmospheric heating or from high mountains or from the effect of fronts. The height reached by these clouds depends on the amount of further rising after dew point has been reached. (Figures 12.23 show the different causes of cumulus cloud formation.)

Figure 12.15  Stratus Clouds.

Atmosphere: Water  12.11

Figure 12.16  Cumulus Clouds.

Figure 12.17  Cumulonimbus Clouds. These are Typical Thunderclouds. Notice the Anvil Shape of these Clouds.

Figure 12.18  Visible Satellite Image of Cumulonimbus Clouds (Thunderstorms) with Overshooting Tops.

Figure 12.19  Colourized Infrared Imagery. Grey is Relatively Warm, Blue is Cooler, and Red Indicates Clouds that are the Coldest, Tallest, and Most Likely to Produce Rain.

Figure 12.20  Artist’s Illustration of NASA’s CloudSat Satellite.

12.12  Chapter 12

Cirrus Clouds

Cirrostratus Cloud Altostratus Cloud

Stratus Cloud

Cold Air

Figure 12.21  Layers of Clouds Form when Ascent is Slow and Cooling is Gradual.

When large layers of air, as opposed to columns of air, rise slowly, their cooling is gradual. When dew point is 0 Clear Sky reached, clouds form which are in layers or sheets of the 1 One-eighth Cover stratus type. Further gradual rising may give rise to the 2 Two-eighths Cover formation of cirrus clouds as shown in Figure 12.21. The amount of cloud cover for a region is estimated in 3 Three-eighths Cover oktas. One okta represents approximately one-eighth of 4 Half of Sky Covered the sky covered with cloud. The symbols used on weather 5 Five Eighths Cover maps are given in Figure 12.22. Three-quarters Cover 6 Some clouds are caused by temperature inversion Seven Eighths Cover 7 The earth’s surface loses heat through radiation at night, Completed Cloud Cover 8 and this causes the air in contact with the surface to cool. Sometimes, the cooling is pronounced, and when this Sky Obscured, e.g, Fog 9 occurs, it results in the temperature increasing with altiFigure 12.22  Symbols Used on Weather Maps to tude. This is known as a temperature inversion, which is Show the Amounts of Cloud. the reverse of the normal lapse rate. If the cooling occurs on a land surface that slopes down to a hollow, e.g., a valley, the cool air moves down the slope and collects in the hollow (see Figure 12.4). The warmer air is pushed upwards so that it lies on top of the cold air. When the temperature of the cold air falls below dew point, it causes formation of stratus cloud. The cloud cannot rise through the temperature inversion because of the warmer air above. A cloud that forms in this way is usually quickly dispersed when the sun warms the air, thus causing the temperature inversion to disappear. Code No

Symbol

Description of Cloud

Precipitation When water droplets or ice particles become too heavy to remain in suspension in clouds, they fall to the ground by gravitation. When water droplets are heavy enough to fall to the ground, they are called raindrops. We have already noted that the formation of water droplets require the presence of hygroscopic nuclei. Not all clouds give rise to precipitation. What are then the conditions for water droplets to turn into raindrops? This is achieved by small droplets joining together to form

Atmosphere: Water  12.13

large ones which are too heavy to remain in 12 200 m Tropopause suspension. For this to occur, both water and ice particles need to be present in the same cloud. When currents of rising air reach the freezing level, the water droplets evapIce Crystals orate and condense around the ice particles. As this continues, the particles become 9150 m –40ºC sufficiently heavy to fall to the ground. In Mixture of Ice Crystals the humid tropics where large amounts of and Super-Cooled warm, moist air rise rapidly, the raindrops Water Droplets falling from the top of the clouds often 6100 m collide with water droplets at lower levels, –10ºC thereby increasing the size of the raindrops. Raindrops that are formed in cumuloSuper-Cooled Water Droplets nimbus clouds can be very large. This is 3050 m 0ºC (Freezing Point) the type of rain that falls over most equaWater Droplets torial regions. Figure  12.23 shows what happens in a Dew-Point cumulonimbus cloud. Such a cloud can Convection Convection usually be divided into four sections. Currents Currents Notice that the first sector is made up of Earth’s Surface water droplets, the second of super-cooled Figure 12.23  The Four Sectors in a Large Tower Cloud water droplets (they are called super-cooled (Cumulonimbus Cloud). Notice that the Water Droplets do not because although the temperature is below Freeze at 0°C. It is only When the Temperature Reached is About 0°C, the droplets have not turned into −40°C that all Condensation is in the form of Ice Crystals. ice), the third sector is a mixture of supercooled water droplets and ice crystals, and 3000 m 0° the fourth sector is made up of ice crystals. In such a cloud, the raindrop may rise and fall several times in the convection currents 2000 m before finally becoming heavy enough to Layer Cloud fall to the ground. When it is rising and falling in the tower cloud, the raindrop will 1000 m Dew-Point attract more and more water droplets and will grow bigger and bigger, and eventually Drizzle give heavy rainfall. You will also see that it is Earth’s Surface possible for a raindrop to start its life as an ice crystal. If an ice crystal from the top of Figure 12.24  Formation of Drizzle. the cloud becomes heavy enough, it will fall to the ground, but because of the higher temperatures at the base of the cloud, it may melt to form a raindrop. Sometimes, it may also happen that the ice crystals become quite large. They then fall very quickly through the cloud and reach the ground before melting. This form of ­precipitation is known as hail.

Types of precipitation Rain This is the most common type of p ­ recipitation. Raindrops are small when air rises slowly and large when air rises rapidly.

12.14  Chapter 12

Drizzle Like rain, it is formed of ­liquid drops, but those of drizzle are much smaller. Drizzle usually forms from stratus clouds,which may develop when air rises slowly. The droplets of some drizzle evaporate as they fall to the ground. This gives rise to mist (see Figure 12.24).

Snow When water vapour condenses directly into ice crystals, which join together, they produce snowflakes. When snowflakes fall to the earth’s surface, they sometimes partially melt which destroys their crystal structure.

Hail Rapid up and down movements of air in a thundercloud carry raindrops continually above and below the freezing level. The drops freeze and grow ­bigger. Eventually, they become too heavy for upward air currents to carry them, and they fall to the ground as spherical pieces of ice called hail (see Figure 12.25).

Sleet Raindrops may freeze as they fall through a layer whose ­temperature is below freezing. This produces sleet, which is actually frozen rain. Sleet usually forms only when there is a temperature inversion.

a)

Cumulonimbus Cloud

12 200 m

b)

Ice Crystals 9150 m Hailstone Rising and Falling Within the Cloud

–30˚c

Mixture of Ice Crystals and Super-Cooled Water Droplets

6100 m

–20˚c –10˚c

–10°C Super-Cooled Water Droplets

3050 m

Water Droplets Convection Currents

–40˚c

–40°C

0˚c

Water Droplets Freezing Spontaneously Supercooled Water Droplets Come in Contact with Ice Nuclei and Freeze Ice Nuclei are Present Supercooled Water Droplets Water Droplets

–0°C Dew-Point

Hailstone Sea level

Figure 12.25  (a) Formation of Hail; (b) Formation of Hail.

Ice Crystals Ice Nuclei Supercooled Water Droplets Water Droplets

Atmosphere: Water  12.15

How air is cooled Air is cooled in two main ways. These are Being made to rise. This is how most of the world’s rain is caused. It occurs when 1.  hot air rises by convection currents (Figure 12.26); 2.  a wind flows over a mountainous region (Figure 12.27); 3.  warm air rises over cold air (Figure 12.28). Flowing over a cold surface. Most of the world’s fogs and mists result from this type of cooling. This occurs when 1. a warm wind flows across a cold current (refer to Figure 12.5); 2. a warm wind flows across a cold land. Further Ascent Causes More Expansion and More Cooling; Rain Takes Place

Cumulus Cloud

The Rising Air Expands and Cools; Condensation Takes Place

Cool Air Descends and Replaces the Warm Air

Ri

si n

g

W a Air rm

Figure 12.26  A Heated Surface Causes Air to Rise, and this Often gives Rise to Clouds and Convection Rain.

Rain

Earth’s Hot Surface Heats the Air Above it – the Heated Air Expands and Becomes Lighter than the Surrounding Air, and it Rises

d d Win Warm, Humi

Tropical Sea Warm Sea Air Absorbs a lot of Water Vapour

Highlands

The Highlands Cause the Humid Air to Rise — it Cools And Very Heavy Orographic Rain Falls

Figure 12.27  When Moist Air is Forced to Rise Over a Mountain Range, Clouds and Rain, Often Heavy, Occur.

12.16  Chapter 12

Warm Air Rises Over Cold Air; it Expands and Cools, Condensation, Clouds and Rain Form

Cumulus Cloud

This Line Represents the Plane Separating Warm Air from Cold Air

Rain

Warm Air

Cold Air Figure 12.28  Warm Air is Forced to Rise when it is Undercut by Colder Air; Clouds and Sometimes Rain Occur.

Types of Rain Different types of rain based on their characteristic features are described below. Cumulonimbus Cloud ++++++++

–– – – – – –– – – –– – ––

+++++++++++++++ Movement of Storm Ascending Air Lightning

Ground Level

+ Positive Charge – Negative Charge

Figure 12.29  A Generalized Section through a Thunderstorm. As the Drops of Rain Fall, the Energy they Contain Produces a Positive Charge of Electricity at the Top of the Cloud and a Negative Charge at the Bottom. Flashes of Electricity – Lightning – Pass Between the Top and Bottom of the Cloud and between the Bottom of the Cloud and the Ground (which is Positively Charged).

Convection rain This is the main type of rain in the humid tropics and in the interiors of continents in the summer. In tropical latitudes, the surfaces and the air above them become very hot, and powerful convection currents are set up in the air. Because the air is hot, it absorbs very large amounts of moisture, and when the warm, moist air is forced to rise rapidly by convection currents, torrential rains develop. Enormous amounts of energy are released when condensation occurs, and this provides energy to power the thunderstorms that are frequently associated with this type of rain. The maximum heating of the land occurs in the afternoon, and it is at this time that convectional rain tends to fall. See Figure 12.26. Most of the rain that falls in West and Central Africa is convectional rain. This is also true for Indonesia, Malaysia, the Amazon Basin, and parts of Central America. Thunderstorms can occur whenever land surfaces are greatly overheated. They are common in humid tropical regions, e.g., Central Africa. They usually occur in the afternoon, and they are especially frequent during the period of heavy convectional rains. The thunder is caused by the rapid expansion and ­ contraction of the air resulting from electrical discharges that generate intense heat. Figure 12.29 shows the structure of a typical thunderstorm.

Atmosphere: Water  12.17

Depression or cyclonic or frontal rain This occurs when air masses of different characteristics meet and their air mixes. The warm moist air of a maritime tropical air mass is forced to rise over the cold of a polar air mass when the two meet, and this gives rise to rain. Depression rain, in temperate latitudes, is usually lighter than convectional rain, and it is of much greater duration, up to several days. Cyclonic rain also occurs throughout the doldrums where the trade winds meet.

Relief or orographic rain Convection rain only occurs in regions, whose surfaces are greatly heated by the sun, and depression rain only occurs where air masses of different characteristics meet; however, relief rain occurs in all latitudes. It is most common where onshore winds rise up over hilly or mountainous regions that are at right angles to the direction of the winds. Figure 12.30(a) explains the formation orographic rain in more detail, and Figure 12.30(b) shows its formation in Himalayas. Distribution of orographic rain is visualized through satellite data in Figure 12.31. It should be mentioned here that thunderstorms represent violent but localized disturbances in the atmosphere. Thunderstorms are of frequent occurrence in the humid tropical regions, but they also occur in Great Britain, usually in late summer.

Cyclonic rain also occurs throughout the doldrums where the trade winds meet.

Thunderstorms When the land surface is heated by the sun, convection currents are set up in the air in contact with it. If the heating is intense, the convection currents will be strong. This in turn will form tower clouds such as cumulonimbus clouds, which may stretch right up to the tropopause. Figure 12.29 shows a thunderstorm cloud. The summit of the cloud is spreading out in the shape of an anvil. This is because the convection currents cannot rise into the troposphere. Inside the cloud because of the strong convection currents, the raindrops and hailstones are rising and falling. This continual rising and falling of the raindrops and hailstones causes a separation of the electric charges present in the cloud. The positive charges collect at the top of the cloud (see Figure 12.29) and the negative ones at the bottom. In addition, the ground under the cloud has positive charges. In time, the electrical pressure (b)

ASIAN SUMMER MONSOON

Rain from Expansion and Cooling

TIBET/LADAKH

Air

HIMALAYA

Evaporation from Compression and Warming

,M oi m

Figure 12.30  (a) The Formation of Orographic Rain; (b) Formation of Orographic Rain in Himalayas.

W ar

Sea

st

Ai

r

Dry Lit tle W or No ind Ra in

NEPAL/BHUTAN

ry

Moist Wind

Region of Rain Shadow

,D

Rain

When Air Descends the Pressure on it Increases; the Air Contracts and Warms

arm W

When Air Rises the Pressure on it Decreases; the Air Expands and Cools

Pressure Increases

Pressure Decreases

(a)

Rain on this Side of Range Produces Lush, Moist Climate.

Lack of Rain on this Side of Range is Due to a Rain Shadow, Creates a Warmer, Dryer Climate.

12.18  Chapter 12

Figure 12.31  Distribution of Orographic Rain in India.

will reach such a force that enormous sparks will flash between the negative and positive charges within the cloud and between the negative charges at the bottom of the cloud and the positive charges in the ground below. These enormous sparks are called lightning. The lightning flash heats the air in contact with it. This causes the air to expand quickly. Cooling soon occurs and the air contracts. It is this rapid expansion and contraction of the air, which causes thunder. We can see the lightning almost immediately, but the thunder takes about 3 seconds to travel a kilometre. Therefore, by timing the interval between the lightning and the thunder, you can estimate how distant the thunderstorm is. Because of the great vertical extent of the thundercloud, these storms bring very heavy rain in temperate latitudes and intense ­hailstorms at higher latitudes. Thunderstorms can occur whenever land surfaces become greatly heated. In humid tropical regions such as Indonesia, Malaysia, Central and West Africa, the Amazon Basin, and Central America, thunderstorms are very common. They usually occur in the afternoon and are especially frequent in the season of heavy convection rains.

A Closer Look  ▼ Ocean Currents can Influence Rainfall Onshore winds absorb moisture when they blow across a warm current, and they give rise to heavy rain, especially if they are made to rise by mountains. For example, heavy rains occur over the highlands of western Great Britain and Norway, which lie under the influence of prevailing westerly winds that blow over the warm North Atlantic Drift. Onshore winds tend to lose moisture when they blow over a cold current. When they cross the land during the summer, they are warmed by the land, and they tend to absorb moisture rather than yield it. A good example of this is provided by the onshore winds across southern California.

Atmosphere: Water  12.19

Measurement of Rainfall

An instrument called a rain gauge is used to measure rainfall. It consists of a cylindrical copper container, in which there is a copper collecting can containing a glass jar, and a copper funnel that fits on to the top of the container, as shown in Figure 12.32. The gauge is sunk into the ground so that the top of the funnel is about 30 cm above the ground level. Rain falling over the funnel collects in the glass jar. This is emptied, usually every 24 hours, and measured in a tapered glass measure, graduated in millimetres. The tapered end of the jar enables very small amounts of rain to be measured accurately. The gauge is placed in an open space so that no run-off from trees or buildings or other objects can enter the funnel. Further, the outer case is sunk in the ground to prevent the sun’s heat from evaporating any of the rain collected in the glass jar. It projects about 30 cm above the ground level to prevent any raindrop from splashing up from the ground into the funnel. (a)

(b) Measuring Cylinder

Rain Guage

mm 5.0

2030

12.8 cm

20

01 40 50 60 70 80 90 100 11

Funnel

4.0 3.0 2.0

40 mm 35

30 cm Container

1.0 0.5

Can

.05 Ground

40 35

30

30

25

25

20

20

15

15

10

10

5

5

Jar

The rainfall recorded for a place, either for a day, a week, a month, or a year can be shown on a map. This is done by using lines called isohyets. An isohyet is a line on a map, which passes through all places having the same rainfall (Figure 12.33). Average annual rainfall in India is 300–650 mm (11.8–25.6 in.) Figure  12.34. Rainfall is very unreliable because India is diverse in physiography and shows atmospheric variation. The southwest monsoon accounts for most precipitation in India.

17

How Rain is Shown on a Map

5

Figure 12.32  (a) A Typical Rain Gauge Instrument; (b) A Rain Gauge Instrument.

250 225

200

Figure 12.33  Isohyet Map. A Scale of Colours or Line Shading is Sometimes Used to Indicate Rainfall Values.

12.20  Chapter 12

INDIA ANNUAL RAINFALL PAKISTAN N

CHINA (TIBET) N

I

N

D

E

P

A

I

L

BHUTAN

A

BANGLADESH Tropic of Cancer

MYANMAR ARABIAN SEA

BAYOF BENGAL RAINFALL IN Cms Above 400 200 – 400 100 – 200 60 – 100 40 – 60 20 – 40 0 – 20 ANDAMAN & NICOBAR ISLANDS (INDIA)

LAKSHADWEEP (INDIA) INDIAN

SRI LANKA

Figure 12.34  Average Annual Rainfall in India.

OCEAN

Atmosphere: Water  12.21

Rainfall Over Great Britain Most of Great Britain’s rain is brought by ­depressions, which move in from the Atlantic; this means that the western side of the country experiences the depression first. This explains in part why western Great Britain is wetter than eastern Great Britain, but it must be remembered that most of the highland, mountains, and hills are in the western zone. Depressions yield frontal or cyclonic rain, and this type of rain affects most of Manchester the country, though in varying amounts and at different times of the year. Orographic rain occurs most frequently, often in heavy falls, over the western and northern highlands as shown in Valencia Figure  12.35. This map, which shows the annual rainfall, indicates that the London west is wetter than the east and that the highland areas are wetter than the lowland areas. Rainfall is fairly evenly distributed mm throughout the year as shown by the rainfall graph for London (Figure 12.36). Over 2500 1125 To 1250 It was mentioned earlier in this chapter that the west receives more rain in the 1250 to 2500 Less than 1125 winter than in the summer, and this is supported by the rainfall graph for Figure 12.35  Average Annual Rainfall. Valencia (Figure 12.37), which is located in southwest Ireland. Although the amount of rain that a Mm region receives annually is important, the frequency of rainfall, the duration 175 of each fall of rain, and the amount 150 of each rainfall are of much greater 125 importance. For example, Manchester has the reputation of having long Mm 100 periods of rainfall, yet its average 75 75 annual rainfall is not much higher than that of the seaside resort of Lancashire. 50 50 Manchester certainly has many hours 25 25 of drizzle, which gives rise to personal discomfort, but the city does not 0 0 J FMAM J J A SOND J FMAM J J A SO N D receive very much more rain than the coastal resorts. Figure 12.36  Average Figure 12.37  Average Figure  12.38 shows the average Monthly Rainfall for Monthly Rainfall for number of rainy days occurring in

London.

Valencia.

12.22  Chapter 12

Rainy Days Over 2500 200 to 250 150 to 200 Figure 12.38  Number of Rainy Days Each Year. Great Britain. Comparing this map with that of Figure  12.35, it can be seen that the Southern Uplands of Scotland have fewer rainy days than the west coast of Ireland, but the latter has a higher average annual rainfall. The same applies to the northern coastal area of East Anglia and the Lake District.

World Patterns for Rainfall Distribution We discussed in Chapter 11 how the revolution of the earth and the tilt of its ‘axis’ result in a movement of some pressure belts northward and southward, and a change in others, and how this in turn causes the global prevailing wind systems to move in synchrony with them. We also examined the effects of seasonal changes in the pressure systems of Asia on the wind pattern of this region. We will examine first the seasonal rainfall patterns on a global scale. Figure 12.39 visualizes the total precipitation projected onto a gridded equal population cartogram.

Atmosphere: Water  12.23

Precipitation in mm

0–50 50.1–100 100.1–200 200.1–300 300.1–400 400.1–500 500.1–750 750.1–1000 1000.1–1250 1250.1–1500 1500.1–1750 1750.1–2000 2000.1–3000 3000.1–4000 > 4000

Global pattern for May to October The sun is overhead in the Northern Hemisphere during this period. The equatorial rain belt lies in this hemisphere, and more rain falls here than in the Southern Hemisphere. Southern and eastern Asia, and the eastern parts of North and South America receive heavy rainfall from onshore winds. Large parts of southwest Asia, northern Africa, northern and western Australia, southern Chile, northern Peru, the southeastern states of North America, and the Namib Desert receive little or no rain because they lie under the offshore trade winds. The Arctic lowlands receive very little rainfall because the low temperatures reduce evaporation to a minimum. This is shown in Figure 12.40.

Global pattern for November – April The sun is overhead in the Southern Hemisphere; the equatorial rain belt is centred in this hemisphere, and most of the rainfall occurs in this hemisphere. Northern and eastern Australia, southeastern Africa and Brazil, and eastern Argentina receive rain from onshore trade winds. You should note that in northern Australia, rainfall comes mainly from monsoon winds, which are actually modified northeast trade winds. Large areas of southwest Asia, northern Africa, central and western Australia, southern Peru, northern Chile, and southwest Africa receive very little rainfall because they lie under offshore trade winds. The Arctic lowlands receive very little rain for reasons already given. Now examine Figure 12.41.

Effects of overhead sun migration on global rainfall patterns The migration of the overhead sun has marked effects on some rainfall patterns. This can be seen by examining Figures 12.40 and 12.42 from which we can deduce the following inferences: Regions, which are little affected

● ●

regions lying under onshore winds throughout the year; regions which lie permanently in the doldrum belt.

Figure 12.39  Visualization of the Total Precipitation Projected on to a Gridded Equal Population Cartogram.

12.24  Chapter 12

Arctic Circle

Tropic of Cancer

Equator

Tropic of Capricorn

mm Over 1500

500–1000

1000–1500

250–500

Under 250

Figure 12.40  World Rainfall Distribution for the Period May to October. Name the Regions of the World that Receive Heavy Rainfall from Onshore Winds and the Regions that Receive Little or No Rain because they Lie Under offshore Winds (Overhead Sun Mainly in the Northern Hemisphere). Arctic Circle

Tropic of Cancer

Equator

Tropic of Capricorn

mm Over 1500

500–1000

1000–1500

250–500

Under 250

Figure 12.41  Rainfall Distribution for November to April. Which Regions Receive Rain from Onshore Trade Winds and which Regions Receive Little or No Rain because they Lie Under Offshore Winds? Compare the Winds over Asia and the Northern Indian Ocean with those in Figure 12.33, and Name the Climate for which this Wind Pattern is Characteristic (Overhead Sun Mainly in the Southern Hemisphere).

Atmosphere: Water  12.25

Regions which are greatly affected

●

regions lying between two belts of prevailing winds; the interiors of Asia and North America; ● regions bordering the doldrum belt; ● monsoon regions, but note in some regions, winds are onshore throughout the year. ●

Seasonal Distribution of Rainfall In Figure 12.42, the world has been divided into four types of rainfall regions. These are 1. 2. 3. 4.

regions which receive rain throughout the year; regions which receive a maximum of rain between May and October; regions which receive a maximum of rain between November and April; regions which receive little or no rain.

Arctic Circle

Tropic of Cancer

Equator

Tropic of Capricorn

Rain Throughout the Year Rain Mainly Between May and October

Rain Mainly Between November and April Little or no Rain

Figure 12.42  The Seasonal Distribution of Rainfall for the World. Compare this Map with Figures 12.40 and 12.41, and Name the Regions where Rain throughout the Year is Caused Mainly by Onshore Winds. Are All of these Regions within the Tropics?

12.26  Chapter 12

Seasonal Rainfall and Type of Rainfall Figure  12.43 shows the seasonal distribution of winds and rain, together with the type of rainfall, in Eurasia and north Africa. The winds on the left-hand side operate over the western part of the region, and those on the right-hand side operate over the eastern part of the region. Examine this diagram and pay particular attention to the following:

45º

N.E. Trades

30º

30º

15º 7º

Doldrums Rain all the Year

Convection Rain

0º Rain in Summer Only

Chiefly Convection Rain, but Also Relief Rain

Southerly Monsoon Winds

45º

Convection and Relief Rain

Depression and Relief Rain

66º

Chiefly Convection Rain

60º

Summer

Winter

Eurasia and North Africa

Northerly Monsoon Winds

Winter

Winds

661/2º North East Trades South Westerlies

S Westerlies

Summer

Winds

7º 0º Rain in Winter Only

Doldrums Little or No Rain

Figure 12.43  The Relationship Between Seasonal Rainfall Distribution and Winds.

1. T  he north-to-south shift of some wind belts and the change in wind direction of other belts from season to season. 2. The location of onshore winds (which usually bring rain) and of offshore winds (usually rainless). 3. The distribution of rainfall in relation to the lines of latitude which are given.

Atmosphere: Water  12.27

Annual Global Rainfall The average annual rainfall for the world is shown in Figure  12.44. This map is included for reference purpose only. For reasons already given, the average annual rainfall patterns as such are not of very great value. Data collection has been enriched over the years through improvement of the NVAP Global Water Data set for climate hydrological & weather studies and ­measures the water vapour data set that is collecting earth system data reords for use in research. Figure  12.45 shows the Aqua Satellite with six onboard instruments including the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU-A), the Humidity Sounder for Brazil (HSB), the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), the ModerateResolution Imaging Spectroradiometer (MODIS), and the Clouds and the Earth’s Radiant Energy System (CERES). Each instrument has unique characteristics and capabilities, and all the six instruments serve together to form a powerful package for Earth observations. With change in both clouds and water vapour content in the atmosphere, it has become relevant to track climate variability.

Arctic Circle

Tropic of Cancer

Equator

Tropic of Capricorn

mm Below 250

500 – 1000

2000 – 5000

250 – 500

1000 – 2000

Over 5000

Figure 12.44  Annual Rainfall Distribution. Name the Regions that Receive Over 1000 Mm of Rain. Which of these Lie Under Onshore Westerly Winds? Name the Regions, which Receive Less than 250 Mm Rainfall. Which of these Regions Lie Under Offshore Winds?

12.28  Chapter 12

AMSR-E

–Yv/c

+Xv/c

(VELOCITY)

MODIS

CERES (2) AMSU-A1

Figure 12.45  Aqua Satellite Carries Six Stateof-the-Art Instruments in a Near-Polar Low-Earth Orbit.

AIRS

AMSU-A2 HSB

+Zv/c

(NADIR)

X-band Antenna

Key facts ●●

●● ●● ●●

●● ●●

●● ●● ●● ●● ●●

Condensation occurs when (i) air contains sufficient water vapour; (ii) there are hygroscopic nuclei in the air; and (iii) the air temperature falls below the dew point. Dew, frost, fog, and clouds are forms of condensation. Rain, drizzle, snow, hail, and sleet are forms of precipitation. Air is cooled by rising (by convection currents or by being forced over a mountain or by being forced over a colder body of air); passing over a colder surface (e.g., warm air over a cold current or over a cold land surface). Most of the world’s rain is caused by air being made to rise. There are three types of rain according to origin: convection rain (common in the humid tropics and in the interiors of continents in the summer); cyclonic rain (occurs when air masses of different characteristics meet); orographic rain (occurs when moist air rises over high land). Thunderstorms are associated with strong convection currents rising in moist air. The ratio of absolute humidity to the amount of water vapour that the air could hold at the temperature of the air is called the relative humidity. Humidity is measured using an instrument called a hygrometer. Rainfall is measured using a rain gauge. Rainfall is shown on a weather map by isohyets. An isohyet is a line, which joins places having the same rainfall.

Atmosphere: Water  12.29

●● ●● ●●

Most of Great Britain’s rainfall is brought by depressions, which develop when cold polar air mixes with warm moist sub-tropical air. Depressions move from west to east across Great Britain. They are most frequent in the winter. The migration of the ‘overhead sun’ between the tropics causes a migration of pressure belts and associated winds, and a reversal of pressure belts and winds in monsoon regions, which result in seasonal changes to the world’s rainfall pattern. That is, some regions have rain throughout the year; some have rain only in the summer; some have rain only in the winter; and some have little or no rain in most years.

12.30  Chapter 12

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. Direction for questions from 1 to 5: Each question has one or more correct option/s. Identify which of the options are correct and select the answer as per following. (a) if 1, 2, and 3 are all correct. (b) if 1 and 3 only are correct. (d) if 2 and 3 only are correct, (c) if 2 only is correct. (e) if 3 only is correct. 1. Most of the rainfall of Great Britain is a result of (a) prevailing westerly winds. (c) temperate depressions.

(b) thunderstorms.

2. The decrease in the temperature of a rising body of air whose relative humidity is less than 100 per cent is called (a) ELR. (b) SALR. (c) DALR. 3. Condensation occurs in the atmosphere when (a) air rises. (b) air contains hygroscopic nuclei. (c) the temperature falls below the dew point temperature. 4. A line on a weather map which joins places having the same rainfall is called an (a) isotherm. (b) isobar. (c) isohyet. 5. A local wind differs from a prevailing wind in that it (a) affects only a small area. (b) operates only at night. (c) is usually of short duration. 6. Which one of the following best describes the type of rainfall that occurs over most of Central Africa? (a) Orographic (b) Cyclonic (c) Frontal (d) Anti-cyclonic (e) Convection 7. Which one of the following statements is not true? (a) Fogs rarely occur over land surfaces. (b) Convection rains occur in the interior of Asia in the summer. (c) The leeward slopes of mountains lying in the path of onshore winds receive less rain than the windward slopes. (d) Cyclonic rainfall in the tropics is often accompanied by thunderstorms. (e) Convection rain is common in humid equatorial regions. 8. Figure 12.46 shows the distribution of rainfall over a relief section. Which of the five relief diagrams given is most probably the correct one?

Atmosphere: Water  12.31

Wind

100 mm

30 mm

40 mm 20 mm

8

1 2 3 4

5

Figure 12.46:  Distribution of Rainfall Over a Relief Section.

9. Which one of the following is mainly the cause of the seasonal characteristics of the rainfall over Asia? (a) High summer temperatures (b) the great extent of latitude (c) Mountainous nature of interior Asia (d) Numerous islands of southern Asia lying astride the equator (e) Seasonal directions of the prevailing winds between sea and land 10. Which one of the following conditions is not necessary for precipitation to occur? (a) Air must contain water vapour (b) Air must have a high temperature (c) There must be hygroscopic nuclei in the air (d) Temperature of the air must fall below dew point (e) Air must be saturated 11. Which of the following conditions is most likely to cause advection fog to develop? (a) A warm wind crossing a hot desert (b) A rapid loss of heat from the earth’s surface on a clear, calm night (c) A wind descending from the Alps on to the snow-covered lowlands (d) Hot, moist air rising over a hill (e) Warm, moist air moving over a cold surface

12.32  Chapter 12

12. All the following statements are true except one. Which is it? (a) Convection rain is very common in humid equatorial regions. (b) Cyclonic rain in the tropics is often accompanied by thunderstorms. (c) The leeward sides of mountains receive less rain than the windward sides. (d) Convection rain often occurs in the heart of Asia in the summer. (e) Fog rarely develops over water. 13. The aridity of central and northwestern Australia is not caused by (a) dry trade winds. (b) the permanent existence of high pressure over central Australia. (c) its great distance from the sea. (d) cold ocean currents moving towards a warm land surface. (e) onshore winds blowing over a cold sea surface. 14. Assertion (1) the amount of moisture in the atmosphere is associated with latitude. Reason (2) the ability to keep the moisture in the form of water vapour is related to temperature. Code: (a) Both (1) and (2) are true and (2) is the correct explanation of (1). (b) Both (1) and (2) are true but (2) is not the correct explanation of (1). (c) (1) is true but (2) is false. (d) (1) is false but (2) is true. 15. Consider the following climatic and geographical phenomena: (1) Condensation (2) High temperature and humidity (3) Orography (4) Vertical wind Thunder clouds development is due to which of these phenomena (a) 1 and 2 (b) 2, 3, and 4 (c) 1, 2, and 4 (d) 1, 2, 3, and 4

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. Directions for questions from 1 to 3: Study Figures 12.47 and 12.48, and answer the questions based on them. Figure 12.47 shows the location of Delhi. Figure 12.48 gives information on the rainfall, t­ emperature, and hours of sunshine for Delhi. 1. (a)    In which country is Delhi? (b) Approximately how far is Delhi from Bali? 4000 km 5500 km 7000 km (c) Name the winds that the arrows represent. (d) In what part of the year do these winds blow?

Atmosphere: Water  12.33

Japan China Delhi Burma

Sri Lanka Sumatra 0

1500 km Bali

Figure 12.47  Location of Delhi City.

2. (a)     Which month in Delhi has both the lowest rainfall and the most sunshine? (b) How much rain falls in the month named in (a)? (c) What is the amount of daily sunshine in the month named in (a)? (d) Give two reasons why July and August would not be popular with tourists. 3. (a)     Name the month that has the highest monthly temperature. (b) Name the month that receives the most rainfall. (c) Briefly explain why the hottest month comes before the wettest month. (d) Name two countries shown on this map, excluding India, which have a monsoon climate. 4. Choose any three of the following and explain why they occur. (a) Tropical thunderstorms usually occur in the late afternoons. (b) Rainfall is usually unevenly distributed over mountain barriers. (c) Advection fog often develops over the Canaries Current. (d) The Mediterranean lowlands receive rain only in the winter. (e) More rain falls in West Africa between 1 May and 31 October than between 1 November and 30 April.

12.34  Chapter 12

(a)

(b)

Mm

°c

200 150

30

100

20

50

10

0

0 (c) 10 Daily Sunshine in Hours 8 6 4 2 0

Figure 12.48  Graphics Presenting Information on (a) Rain Fall, (b) Temperature and (c) Hours of Sunshine.

5. Explain any two of the following, using relevant diagrams and sketch-maps (a) The Sahara Desert receives little or no rain. (b) Regions bordering the equatorial zone of Africa receive most of their rain in the hot season. (c) Western Europe receives rain throughout the year.

Atmosphere: Water  12.35

Answer key Exercise 1 1.   (e) 6.   (e) 11.   (e)

2.   (e) 7.   (d) 12.   (b)

3.   (a) 8.   (b) 13.   (d)

4.   (e) 9.   (a) 14.   (a)

5.   (b) 10.   (b) 15.   (c)

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13

The Weather Station and Weather Maps

Learning Outcomes After completing this chapter, you will be able to: ●● ●● ●●

Discuss about various instruments that measure weather condition Understand the functions and utilities of different weather instrument List different satellites and instruments of NASA and NRSC that enhance w ­ eatherrelated information

Keywords Stevenson Screen, Weather, Visibility, National Remote Sensing Centre

1

13.2  Chapter 13

INTRODUCTION Almost every activity that we do in our daily life is connected directly or indirectly with the weather conditions prevailing in that point of time. From planning our schedule outdoors to making lifestyle changes, we are dependent on the weather conditions. Moreover, in daily newspapers and mobile phone applications, we look for the forecast to see how the weather looks like for each day. In this chapter, we will look at various methods and equipment that are used to observe and record weather and climate conditions.

Weather Station A weather station is a place where the elements of weather such as t­emperature, humidity, pressure, wind direction and velocity, cloud cover, and ­sunshine are measured and recorded as accurately as possible. The weather station consists of an open piece of land and it ­contains the ­following instruments: thermometers (all kept in a Stevenson screen, Figure 13.1); a rain gauge; a wind vane; an anemometer; a sunshine recorder; and barometers.

Stevenson Screen This screen is a wooden box standing on four legs of height about 120  cm. The screen is built so that the shade temperature of the air can be measured. The sides of the box are louvred to allow free entry of air, and the roof is made of double boarding to prevent the Sun’s heat from reaching the inside of the screen. Insulation is further improved by painting the outside of the screen white. The screen is usually placed on a grass-covered surface thereby reducing the radiation of heat from the ground to a minimum.

(a)

Airspace Between Roof Layers Forms a Bad Conductor of Heat

Louvres Air Enters and Leaves via the Louvres

(b)

Louvres — Air Enters and Leaves by Them

(c)

60 cm 40 cm

Insulated Roof Maximum Minimum

65 cm

Dry Bulb

Sides are Made of Double Wooden Louvres; One Side is Hinged and it Acts as a Door

Wet Bulb 121 cm

Stand

FIGURE 13.1  Stevenson Screen: (a) Sectional View; (b) Showing the Four Thermometers; (c) Giving the Dimensions

The Weather Station and Weather Maps   13.3

Instruments kept inside the Stevenson screen Following are the instruments that are kept inside a Stevenson screen. 1. 2. 3. 4.

Maximum and minimum thermometers; Wet and dry bulb thermometers (called a hygrometer), Thermograph (measures temperature continuously); Hydrograph (measures atmospheric humidity). These four instruments are shown in Figure 13.3.

Instruments kept outside the Stevenson screen There are five important instruments, which are kept in the weather station but not inside the screen. These are listed as follows. 1. 2. 3. 4. 5.

FIGURE 13.2  A Picture of Stevenson Screen

A rain gauge; A wind vane; An anemometer; A sunshine recorder; An aneroid barometer and a barograph.

Rain gauge. All that need be noted is its position. It must be kept well clear of trees and buildings so that only raindrops enter the funnel of the gauge. Wind vane and anemometer.

FIGURE 13.3  Instruments in a Stevenson Screen.

These are placed well away from any building or trees that may interfere in any way with the free movement of air. Buildings may either increase the flow of air by channeling it through narrow passages between two buildings or decrease the flow of air by blocking its path. Trees have a similar effect. + Sunshine recorder Sphere support Glass sphere Card support Lower support clamp Lower support screw Check nut

Base plate Sub-base

FIGURE 13.4  (a) A Sunshine Recorder; (b) Sunshine Recorder Instrument.

Sunshine card

Leveling screw Lock nut

13.4  Chapter 13

Sunshine recorder. The number of hours and minutes of sunshine received each day by a place is measured and recorded by a sunshine recorder. This is a glass sphere partially surrounded by a metal frame (Figure 13.4a,b) on the inside of which is a strip of sensitized card. The card is graduated in hours and minutes. When the sun shines, the glass sphere focuses the sun’s rays on the card and as the sun moves across the sky, the rays burn a trace on the card. This happens only when the sun is shining. At the end of the day, the card is taken out and the length of the trace is turned into hours and minutes which represents the total amount of sunshine for the day. (a)

Aneroid barometer and barograph.

East

Stevenson Screen North

South

Sunshine Recorder

Rain Gauge

Wind Vane and Anemometer (b) Anemometer

E S

N W

Wind Vane

These Form Part of a Weather Station

Stevenson Screen

FIGURE 13.5  (a) Layout of a Weather Station; (b) Close-up of a Stevenson Screen, Wind Vane, and Anemometer.

Since atmospheric pressure is the same inside a building as it is outside the building, there is no need to keep these pressure-measuring instruments in the open. They are kept in a building, which protects them from rain and other weather elements, which could damage them. The layout of a typical weather station is shown in Figure  13.5. There are thousands of weather stations all over the country. Some are attached to universities, colleges, and schools; some to defence centres; some to airports, and some are specifically operated by the meteorological office. Many of these send their daily reports to collecting centres where the information is used for preparing synoptic charts and weather forecasts. In today’s time, there is a facility for internet-based weather stations readily available on most mobile phones (Figure 13.6). Another aspect of weather, which is recorded is visibility which is discussed ahead.

Visibility The distance that an observer with normal eyesight can see at a particular time for a particular place is called visibility. Of course, this refers to a clear line of sight where there are no obstacles such as trees or buildings.

FIGURE 13.6  Internet-Based Weather Station.

The Weather Station and Weather Maps   13.5

Table 13.1

Visibility scale.

CODE FOR OBJECT

DISTANCE (METRES)

A

20

B

40

C

100

D

200

E

400

F

1000 (kilometres)

G

2

H

4

1

7

J

10

K

20

L

30

M

40

Air always contains particles of matter such as dust, smoke, and water vapour, and these affect the visibility. If a lot of particles are present, then visibility is reduced; if not many are present, the air is clear and visibility is increased. Visibility is measured by using a standard scale as shown in Table 13.1. This consists of a set of letters each of which represents a specific distance. When determining the visibility at a weather station, the observer does so by estimating the distance of the farthest object that can be seen in relation to known distances of objects visible from the weather station.

Recording Weather The instruments for measuring weather elements have been studied, and we must now examine how their measurements are recorded. The weather recording sheet, a copy of which is shown in Table 13.2 contains sufficient spaces for daily recordings for one month for the place where the recordings are made. The sheet contains columns for rainfall, temperature, humidity, wind, cloud and sunshine measurements. Recordings for one day are given on this sample weather sheet. You should note the following: 1. Readings are taken in the early morning, i.e., 7.00 a.m. to 8.00 a.m. 2. These readings were taken on the 16th day. 3. The maximum temperature (31°C) is for the period from the morning of the 15th to morning of the 16th. This obviously occurred on the 15th.

MM

MAX C°

MIN C°

MEAN C°

TEMPERATURE RANGE C°

DRY C°

WET C°

DIFFERENCE C°

RELATIVE HUMIDITY %

DIRECTION

WINDS SPEED

TYPES

COVER

CLOUDS

A typical weather chart used for recording one month’s weather readings. When readings are taken early in the morning, the maximum temperature reading will have occurred in the preceding day whereas the minimum temperature reading will have occurred in the very early morning of the day when the readings are taken. The rainfall readings also refer to the previous day.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 11 31 16 23 27 8 25 23 2 84 SW ____// Cumulus _____// 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 total means Summary of weather for the month of ………..  year ……….. Highest maximum ………..………    on  ………..………..   Highest minimum ………..………    on  Heaviest daily rainfall ………..………..  on  ………..………. Lowest maximum ………..………..    on  ………..………..   Lowest minimum ………..………..    on  Number of days on which rain fell ………..……….. Note: The procedure outlined above is for amateur weather recorders the type of procedure followed by many schools. Professional weather recorders follow a more sophisticated procedure.

DATE

RAIN

Table 13.2

The Weather Station and Weather Maps   13.7

4. The minimum temperature (23°C) was for the same period. This obviously occurred on the 16th. 5. Humidity reading is for the actual time of reading. 6. Rainfall is for the whole 24-hour period. 7. Wind direction and speed, and cloud type and cover are for the actual time of observation.

Gathering Information Weather recordings are used for making weather forecasts, which are of great importance to many vital economic activities such as farming, shipping, and aviation. But in addition to this, most people are interested in knowing what type of weather is likely to occur tomorrow or the next day, or even next Temperature week. This is especially true of people who (8˚C) plan to take their holiday at a seaside resort. Cloud Cover 8 Atmospheric Pressure (3/8 of Sky) The study of weather conditions and 175 175 = 1017.5 mb the changes that take place in the atmoVisibility (3 km) 30 Rain sphere is called meteorology and a person who engages in this science is called 6 Dee-Point (6˚C) a meteorologist. Altogether there are about 10,000 weather stations in the Rain Shower world containing the most up-to-date equipment, which are manned by highly Wind Direction (SW) trained professional meteorologists who and Speed (25/45 km/h) cooperate in making detailed observations regularly and at specific times. It is FIGURE 13.7  International Code Used for Sending worth noting that this is just one exam- Weather Information. ple of several in which international +6 cooperation takes place very efficiently. Instruments are read and recordings Low made by the meteorologists every 3 hours 985 +6 +7 in respect of ­humidity, wind direction and 10 00 speed, ­ temperature, and atmospheric pressure. Maximum and minimum tem- 10 05 peratures and precipitation are recorded every six hours. All this information is 101 +11 0 then recorded in international code form 1020 10 such as that shown in Figure 13.7, which is 15 High transmitted by a teleprinter to the various 1 02 0 local weather centres. Each centre receives about thirty pieces of such information, + 17 which are then transmitted to the national 1020 centre. This centre selects a representation High sample from its input and it is this sample, which is transmitted to other countries. By this process, any weather centre is able 1025 to obtain weather information for a large + 25 part of the world in a very short time. The symbols related to various weather parameters detailed in Table 13.3 (a) and FIGURE 13.8  Part of a Synoptic Chart. 13.3 (b).

13.8  Chapter 13

Table 13.3

Symbols related to various weather parameters

WEATHER SYMBOLS

Partly to mainly (variably) cloudy with moderate amount of rain and possibility of lightning

Clear, daily sunny

Cloudy with small amount of rain and snow

Partly to mainly (variably) cloudy with considerable amount of rain and possibility of lightning

Mainly clear, daily sunny

Partly to mainly (variably) cloudy with small amount of rain and snow

Partly cloudy

Partly to mainly (variably) cloudy with moderate amount of rain and snow

Mainly cloudy

Partly to mainly (variably) cloudy with considerable amount of rain and snow

Cloudy, but lucid

Cloudy with moderate amount of rain and snow Cloudy with considerable amount of rain and snow

Cloudy with small amount of snow

Cloudy

Partly to mainly (variably) cloudy with small amount of snow

Cloudy and gray

Partly to mainly (variably) cloudy with moderate amount of snow

Fog, clear

Partly to mainly (variably) cloudy with considerable amount of snow

Cloudy with considerable amount of snow

Fog, mainly clear or partly cloudy

Partly to mainly (variably) cloudy with snow and possibility of lightning

Fog, then partly to mainly (variably) cloudy with rain

Fog

Cloudy with small amount of rain

Misty and cloudy (overcast)

Cloudy with moderate amount of rain

Partly to mainly (variably) cloudy with small amount of rain

Cloudy with considerable amount of rain

Partly to mainly (variably) cloudy with moderate amount of rain

Cloudy with possibility of lightning

Partly to mainly (variably) cloudy with considerable amount of rain

Cloudy with small amount of rain and possibility of lightning

Partly to mainly (variably) cloudy with possibility of lightning

Cloudy with moderate amount of rain and possibility of lightning

Partly to mainly (variably) cloudy with small amount of rain and possibility of lightning

Cloudy with considerable amount of rain and possibility of lightning

WIND SYMBOLS

21

Strong southwest (SW)

22

Weak west (W)

Strong east (E)

23

Moderate west (W)

04

Weak north (E)

24

Strong west (W)

05

Moderate north (E)

06

Strong North (N)

25

Calm (C)

07

Weak northeast (NE)

08

Moderate northeast (NE)

09

Strong northeast (NE)

Wind direction East E North N

10

Weak northwest (NW)

NE

Northeast

11

Moderate northwest (NW)

NW

Northwest

12

Strong northwest (NW)

13

Weak south (S)

S SE

South Southeast

14

Moderate south (S)

SW

Southwest

15

Strong south (S)

W

West

16

Weak southeast (SE)

17

Moderate southeast (SE)

Wind speed Weak [ 2 m/s – 5m/s ]

18

Strong southeast (SE)

Moderate

[ 5 m/s – 9.9 m/s ]

Strong

[ > 9.9 m/s ]

01

Weak east (E)

02

Moderate east (E)

03

19

Weak southwest (SW)

20

Moderate southwest (SW)

Cloudy with moderate amount of snow

Fog, then partly to mainly (variably) cloudy with snow Misty and cloudy (overcast) with possibility of snow

Misty and cloudy (overcast) with possibility of rain

Cloud cover (in eighths of sky) Rain

0

Continuous slight rain

1

Continuous heavy rain

2

Sleet

3

Snow

4

Thunderstorm

5

Shower

6

Hail

7

Fog

8

The Weather Station and Weather Maps   13.9

Observations from perhaps as many as 1500 centres are plotted on a synoptic chart by using a computer. This is done by turning the coded information into international symbols. The synoptic chart shows the weather state for a particular area (see Figure 13.7). There are three World Meteorological Centres, in Moscow, Melbourne, and Washington, DC and each of these prepares maps covering the whole world. The information contained on these maps is then transmitted by radio signals, which can be received by meteorological offices throughout the world and turned back into maps so that a picture of the weather for a specific region can be obtained within a few hours from the time the original observations were made.

NASA and Weather Information The National Aeronautics and Space Administration (NASA) collects information related to weather for two reasons, one for improving and enhancing our knowledge of the weather phenomena and processes and two for meeting the needs of various operational agencies supplying information related to meteorology as for example the National Oceanic and Atmospheric Administration (NOAA); the Federal Aviation Administration (FAA); the U.S. Department of Defense (DoD) and other agencies. Satellite-based profiles are regularly collected by NASA Sensors for measuring temperature and moisture (Figures 13.9 and 13.10). Related Earth Science Division Missions of NASA including data Sets and instruments are shown in Table 13.4. Calipso Satellite (Fig 13.8) and others support Terra Observation from space (Fig 13.9) enrich weather information in current times.

FIGURE 13.9  A View of Calipso Satellite.

Connecting systems with tera observations

Examples of each instrument’s capabilities (clockwise): land composition (ASTER), reflected energy (CERES), vegetation snow and ice (MODIS), aerosols (MISR), carbon monoxide (MOPITT) FIGURE 13.10  Terra Observation. It Explores the Connections Between Earth’s Atmosphere, Land, Snow and Ice, Ocean, and Energy Balance to Understand Earth’s Climate and Climate Change and to Map the Impact of Human Activity and Natural Disasters on Communities and Ecosystems.

13.10  Chapter 13

Table 13.4

Earth Science Division Missions of NASA Including Data Sets and Instruments.

PHASE

NAME

OPERATING Satellite Missions

Aqua CALIPSO CloudSat GPM Suomi NPP Terra SMAP CYGNSS

Suborbital Investigations: Instruments

MTS UAVSAR LASE RainCube MASC TWiLiTE DAWN OAWL, GrOAWL

Suborbital Investigations: Ground-based Observations

X-BADGER

PAST Satellite Missions

TRMM

Suborbital Missions

CalWater2 HS3

Aircraft Missions

PECAN CPEX

The Weather Station and Weather Maps   13.11

National Remote Sensing Centre (NRSC) and Atmospheric Observations

The National Remote Sensing Centre (NRSC) collects information related to weather for two reasons, one for improving and enhancing our knowledge of weather phenomena and processes, and two to support the research related to atmosphere at NRSC and populate National Information System for Climate and Environment Studies (NICES) with short- and long-term quality data on various atmospheric parameter to be used by the civil society. In situ measurements at NRSC for various atmospheric parameters include the ones related to solar energy, i.e., both incoming and outgoing radiation (shortwave and longwave) with a pyranometer (Figure 13.11), aerosols with Sunphotometer (CIMEL), multiwavelength radiometer (Figure 13.12), aethelometer (Figure 13.13), and nephelometer (Figure 13.14), surface-level trace gases, O3, NOx, analyzer (Figure  13.15) including Greenhouse Gas Analyzer and CO2 with precise trace gas analyzers measuring data on a 24 x 7 basis at the NRSC Campus (Atmospheric Science Laboratory) located at Shadnagar, Telangana India.

FIGURE 13.11  A Pyranometer.

FIGURE 13.13  An Aethelometer.

FIGURE 13.12  A Multiwavelength Radiometer.

13.12  Chapter 13

FIGURE 13.14  A Nephelometer.

FIGURE 13.15  An O3, NOx, Analyzer.

Key facts ●● ●●

●● ●● ●● ●●

A Stevenson screen contains: maximum and minimum t­hermometers, wet and dry bulb thermometers (hygrometer). A Stevenson screen is located in a weather station. Other instruments used in a weather station are rain gauge; anemometer; wind vane; sunshine recorder; barometer (kept inside a building). A sunshine recorder gives a continuous reading of the amount of sunshine for a place for 24 hours. Observations made at a weather station are used for compiling a weather map (synoptic chart). The study of atmospheric changes with reference to weather patterns is called meteorology. Weather observations at local centres are linked throughout the world via national weather centres which pass on weather information from the local centres to the World Meteorological Centres in Melbourne, Moscow and Washington DC.

The Weather Station and Weather Maps   13.13

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. 1. Where should an anemometer be sited to obtain accurate readings? (a) High above the ground in an open space (b) On a level piece of ground (c) In a well-ventilated room (d) Out of the direct rays of the sun (e) In direct sunlight 2. A hygrometer is an instrument used for finding out the (a) Amount of rainfall that has fallen in 24 hours. (b) Average temperature. (c) Speed of the wind. (d) Humidity of the air. (e) Amount of sunshine a region has. 3. Which of these instruments should not be kept in a Stevenson screen? (a) Hydrograph (b) Hygrometer (c) Maximum and minimum thermometers (d) Barograph (e) Thermograph 4. Which of these instruments is kept in the weather station but not inside the screen? (a) Barometer (b) Barograph (c) Anemometer (d) Anemograph 5. Rainfall is measured by (a) Sunshine gauge (b) Rain Gauge (c) Rain graph (d) Hydrograph 6. Which recorder gives a continuous reading for 24 hours of the amount of sunshine for a place? (a) Weather recorder (b) Solar plate (d) Internet recorder (c) Sunshine recorder 7. The study of weather conditions and changes that take place in the atmosphere is called (a) Meteorology (b) Pedagogy (c) Limnology (d) None of these 8. For which reason/s does NRSC collect information related to weather? (1) For improving and enhancing our knowledge of weather phenomena and processes (2) To support the research related to atmosphere Code: (a) Only 1 (b) Only 2 (c) Both 1 and 2 (d) None of these 9. Which one of the following types of instrument measures North, South, East or West? (a) Wind vane (b) Barometer (c) Rain gauge (d) Anemometer 10. Which one of the following two instruments measure in centimetres or inches? (a) Wind vane and anemometer (b) Barometer and thermometer (c) Barometer and rain gauge (d) Anemometer and rain gauge

13.14  Chapter 13

11. In what direction does a wind vane point? (a) Where the wind is blowing to (b) Where the wind is coming from (c) Where the wind was yesterday (d) Where the wind will be tomorrow 12. Which one the following types of instrument measures in degrees celsius? (a) Wind vane (b) Barometer (c) Rain gauge (d) Thermometer 13. Which one of the following satellite missions is not related to NASA? (a) Cloudsat (b) CYGNSS (c) Aqua (d) MetOp-SG 14. Which one of the following types of instrument measures in degrees Fahrenheit? (a) Wind vane (b) Barometer (d) Thermometer (c) Rain gauge 15. Which one of the following types of instrument measures in miles per hour? (a) Wind vane (b) Rain gauge (d) Anemometer (c) Barometer

EXERCISE 2 Long Answer Type Questions Direction: Answer the following questions in 150 words. 1. Explain the meanings of any two of the following: (a) A Stevenson screen must allow free entry of air. (b) The top of a rain gauge must project about 30 cm above ground level. (c) A wind vane should be placed well above the ground in an open space. (d) A synoptic chart can be used to forecast weather changes. 2. Figure 13.16 shows a piece of equipment used at a weather station. (a) What is the name of the equipment? (b) Briefly describe how it operates. (c) What is the equipment used for?

3. Figure 13.17 is a weather map for a day in September. (a) Name the feature represented by the line symbol. (b) What is the temperature, the wind direction and the wind speed at A? (c) What weather changes are likely to occur in London in the next 24 hours? (d) At noon on a day in June, there was a cloudless sky and no wind at B (Figure 13.18). How would you show this, information on the map?

1 1 20

FIGURE 13.16

The Weather Station and Weather Maps   13.15

FIGURE 13.17

13.16  Chapter 13

B

FIGURE 13.18

4. (a)  Name one region in the world where there is a large annual ­temperature range. (b) Why is there such a big difference between the summer and ­winter average temperatures in this region? (c) How is the work of farmers in your named area influenced by the ­temperature differences through the year? 5. (a) Name one region in the world where rain comes mainly in one season with the rest of the year being dry. (b) Why does rain fall during only that part of the year in the region you named? (c) How is the work of farmers in the region named influenced by the rainfall distribution throughout the year?

The Weather Station and Weather Maps   13.17

Answer key Exercise 1 1.   (a) 6.   (c) 11.   (b)    

2. (d) 7. (a) 12. (d)

3. (d) 8. (c) 13. (d)

4. (c) 9. (a) 14. (d)

5. (b) 10. (c) 15. (d)

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14

Climate, Weather, and Natural Environment

Learning Outcomes After completing this chapter, you will be able to: ● ● ● ●

Explain the relationship between climate and weather systems Understand the various factors affecting climate and climatic types Describe the linkages and interactions between climate, vegetation, and soil Assess the human impact on the ecosystem and suggested measure for conservation

Keywords Weather, Climate, Ice Age, Monsoon, Continentality, Habitat, Photosynthese, Respiration, Soil, Leaching.

1

14.2  Chapter 14

Introduction ‘Scientific evidence for warming of the climate system is unequivocal,’ reported by Intergovernmental Panel on Climate Change (IPCC), formed in 1988 by United Nations. Historically, in the last 6,50,000 years, there have been seven cycles of ­glacial advance and retreat with the abrupt end of the last Ice Age about 7000 years ago, which marked the beginning of the modern climate era and of human ­civilization. The current global warming trends are worth noting because it is highly ­probable that most of them are caused by anthropogenic interventions since the mid-twentieth century and they are continuing at a rate that is unparalleled.

Weather and Climate Climatic activities in the tropopause, the lower part of the atmosphere that forms a part of the biosphere, have a profound effect on all forms of life. Most of the nitrogen, carbon dioxide, oxygen and water vapour of the atmosphere are confined to the tropopause, and it is in this zone that changes in temperature and wind systems operate. Climatic activities in the tropopause are largely determined by temperature and pressure changes, which produce winds and affect the relative values of the rates of evaporation, condensation and precipitation. The environmental condition of the tropopause for a specific region at a specific time, i.e., the nature of the temperature, pressure, winds, cloud cover and precipitation for a region at a specific time, is known as the weather. Weather is constantly changing; in other words, it is dynamic. It changes from hour to hour and from week to week. Weather is a real phenomenon; it happens and can be experienced in real time. Climate refers to something that is quite different. It refers to the average state of the weather over a long period, perhaps 30 years. For example, it is possible to determine the rainfall for a place for a 24-hour period, which ended at, say 0800 hours this morning. We could also determine whether that place for that period had any sunshine and if so, for how many hours; what were the minimum and maximum temperatures; and whether a wind was blowing and if so, from what direction and at what strength. All this information tells us something about the weather for that place for that time period. Climate is derived from a study of all weather recordings for rainfall, temperature, winds, humidity, etc., for a large number of weather stations covering a large region. Averages for these weather elements are then calculated for several years.

Factors Affecting Climate and Climatic Types There are five major factors that affect the climate of a place. These  are listed as follows. 1. 2. 3. 4. 5.

Latitude (this affects the amount of solar energy received) Pressure systems and the winds to which they give rise Distance from the sea (continentality) Altitude Ocean currents (these can affect temperature and humidity)

Climate, Weather, and Natural Environment   14.3

Figure 14.1 summarizes the major pressure and wind systems from which climatic types can be derived. The diagram shows the main areas of descending and rising air, and the areas where depressions predominate. Two basic climatic types can be recognized. The first type is characterized by fairly uniform temperature and rainfall patterns; the second type is characterized by temperature and rainfall patterns that change on a seasonal basis. The global distribution of the basic climatic types is given in Figure 14.2.

Fairly uniform climates Regions with a fairly uniform atmospheric pressure throughout the year tend to have fairly uniform temperature and rainfall patterns. High pressure resulting in descending air covers the polar and sub-tropical regions all the year, while low pressure resulting in rising air covers the equatorial and cool temperate regions. This gives rise to four climatic types.

Equatorial climate Rising warm moist air produces clouds of great vertical extent which, as they rise, yield heavy falls of thunderstorm rain (see Figures 14.2 and 14.3).

North-East Polar Winds

South-East Winds North-East Trades

Pressure

Zone

Climate

HP

Descending Air

Arctic Tundra

LP

Depressions

Mediterranean HP

Descending Air

Hot desert Savanna

LP

Rising Air

Equatorail Savanna

South-East Trades North-East Winds

Cool, Temperate Maritime

HP

Descending Air

Hot Desert Mediterranean

LP

Depressions

Cool, Temperate Maritime Tundra

South-East Polar Winds

HP

Descending Air

Arctic

Figure 14.1  Diagram to Show the Major Pressure Belts and Wind Systems and the Climatic Types which they Help to Determine.

14.4  Chapter 14

Barrow

Bulun Cambridge

Yakutsk

Valencia Paris

Algiers

23½ºN

Odessa

Moscow

Aswan

Delhi

Kayes

Bombay

0º Kisangani 23½ºS Uniform Climates Equatorial

Mediterranean Savanna

Hot Desert

Monsoon Other Climates Cool Temperate Continental

Arctic Cool Temperate (West Seasonal Climates Margin) Tundra

Cold Continental

Temperate Desert Warm Interior Cool Temperate (East Margin) Cold East Margin Mountain

Figure 14.2  The Global Distribution of the Basic Types of Climate.

mm 1000

Kisangani 0°33´N

°C

Annual Range 2.5°C

900

25

800 700 Figure 14.3  The Climate Graph for Kisangani Summarizes the Main Features of Climate for Tropical Rainforests. The Position of Kisangani is Shown in Figure 14.2. This Map also Shows the Main Areas of Equatorial Climate.

20

600

300

Annual Rainfall 1780 mm

10 6°C 5

200

0

100 0

High pressure with descending air is a permanent feature of the climate. The air is both dry and warm, and hence, rainfall occurs infrequently and often as short thunder showers of limited duration. ­ Skies are cloudless for most of the year. Figure  14.4 summarizes the temperature and rainfall characteristics.

15

500 400

30

Hot desert climate

J FMAM J J 0 A S OD

Arctic climate Cold air over the polar regions produces high pressure and descending air which flows away from the poles as northeast and southeast polar winds in the Northern and Southern Hemispheres, respectively. The descending air is generally dry. Precipitation as snow occurs when depressions move into the zone of descending air.

Climate, Weather, and Natural Environment   14.5

Cool temperate maritime climate Some of the descending air of the sub-tropical high pressure systems flows toward the polar regions as southwest and northwest winds in the Northern and Southern Hemispheres, respectively. Depressions form where these winds meet the cold air of the polar winds and produce frontal rain. Depressions with their changeable weather characterize this climate. Great Britain and most of western Europe have this type of climate as shown in Figure 14.2.

Aswan 24°N

mm

°C

Annual Range 21°C

40 30 20

6°C

100

10 0

60

Annual Rainfall 33 mm

20

J FMAM J J A SOND

A Closer Look  ▼

Figure 14.4  Climatic Graph for a Hot Desert Station. The Location of Aswan is also Shown in Figure 14.2.

Climate of Great Britain Although the weather changes daily, the annual temperature range is moderate and rainfall is fairly evenly distributed throughout the year. Figure  14.5 shows two important isotherms and the paths taken by depressions. The short but sometimes cold winter is responsible for most of the trees being deciduous. These are tall broad-leaf trees such as beech, oak, birch, elm and ash. Willow and alder grow on damper soils, while coniferous trees are dominant on poorer, sandy soils. In parts, the tree vegetation is mixed with deciduous and coniferous woodland. Figure 14.6 shows a typical forest scene in a temperate deciduous forest. Westerly winds crossing the Atlantic Ocean gain warmth from the North Atlantic Drift, especially in winter, and water vapour from the ocean. Figure  14.7 shows the January isotherm pattern, which clearly reflects the warming influence from the westerly winds. This diagram also shows that the sea is warmer than the land during this season. The isotherm pattern for the summer is quite different as shown in Figure  14.8. The isotherms now have a west to east orientation; this shows that the Atlantic Ocean does not have the same influence on temperature. The main factor influencing the temperature during this season is latitude. You will remember that during this season, the midday sun reaches its highest inclination for places in the Northern Hemisphere. It should be noted that the isotherms bend northward over the land. This is because the land is warmer than the sea during the summer Great Britain’s average annual rainfall is shown in Figure 14.9.This map clearly shows that rainfall decreases from west to east and that the western regions are considerably wetter than the eastern regions. You will remember this aspect from the study of rain. Regional climatic variations occur in every basic climatic type. This is shown by Figure 14.10, which illustrate climatic graphs for Valencia, Cambridge, and Paris. These towns are shown in Figure 14.2.

14.6  Chapter 14

(a)

(b) Summer

Colder

tic

an Atl h t r No Drift

V

Warmer

15°C (Summer) 0°C (Winter)

C

P

Low Pressure

Colder

er rm Wa High Pressure

(c) Winter

Figure 14.5  (a) Isotherms 15°C (Summer) and 0°C (Winter) Divide Western Europe into Four Zones, Each of which has its Own Climatic Characteristics; (b) and (c) Show the Pressure Systems Affecting Europe and the Paths Taken by Depressions During the Summer and Winter. These Two Maps Show the fairly Uniform Nature of Precipitation Remembering that Most of Western Europe’s Rain is Brought by Depressions.

Figure 14.6  Temperate Deciduous Forest in Norfolk.

High Pressure

Path of Depressions (

)

Climate, Weather, and Natural Environment   14.7

Seasonal climates Outside the equatorial zone, all places receive less heat in the winter than they do in the summer (caused by the earth’s revolution). This affects the main pressure belts and their associated wind systems, causing them to move north and south within a belt of about 10°. This results in the areas on either side of the major pressure belts coming under the influence of the pressure belt first on the one side and then on the other side on a seasonal basis as shown in Figure 14.11. The climates of the areas between the major pressure belts are characterized by seasonal change. There are four climatic types as given below.

Tundra climate

5°C

4°C Westerly Winds

6°C

7°C 4°C

Depressions formed in the southwest wind system bring light rainfall in the summer. In the winter, all the major pressure and wind systems have moved south, and the polar high pressure with its cold winds is the dominant feature. There are regional variations even in this climate (see Figure 14.12).

5°C 6°C 7°C Figure 14.7  Average Temperatures for January.

Mediterranean climate.

The sub-tropical high pressure over the hot desert regions extends northward to cover the Mediterranean region in the summer. There is no rain in this season. In the winter,

13°C

13°C

Over 1500 mm

14°C 15°C 14°C

15°C

Below 750 mm

16°C 15°C 17°C

Below 750 mm 750 to 1500 mm

16°C Figure 14.8  Average Temperatures for July.

Figure 14.9  Annual Average Rainfall.

14.8  Chapter 14

mm

Valencia 51°55´N

°C 25 mm

800 700

Annual Temp. Range 11°C

20

Annual Range 8.1°C

Cambridge 52°N

°C Paris 46°N

20

Annual Range 15°C

15 15

600

6°C

20

10

15

5

10

500

°C

10

0 5

400

5

6°C 300

0

0

Annual Rainfall 1415 mm

Annual Rainfall 570 mm

Annual Rainfall 550 mm

200

–5 –10

50 100 0

–15

25 J F MAM J J A S O N D

0

J F M A M J J A S O N D

J F M A M J J A S O N D

–20

J-January; F-February;MM-March; J-June; J-July; A-August; S-September; O-October; N-November; S – September N – November M – May J – July J – January – March A-April; M-May; D-December. O – October D – December J – June A – August F – February A – April Figure 14.10  Climate Graphs for Valencia, Cambridge, and Paris Show the Regional Variations within this Climatic Type (see Figure 14.2 for the Global Distribution of the Climate and for the Station Locations).

Prevailing Winds

Climate (Uniform)

N.E,P.W.

Arctic

S.W.W.

Temperate Maritime

Prevailing Winds

Climate (Seasonal)

N.E,P.W. Tundre

S.E.T.

N.W.W.

S.E.P.W.

Hot Desert Equatorial Hot Desert

S.W.W.

N.E.T. Rising Air

N.E.T. Rising Air

Figure 14.11  Diagram to Show the Latitudinal Shift of the Major Pressure Belts and Wind Systems, which Determine the Seasonal Nature of Climates of the Zones where the Overlaps Occur. The ‘Seasonal’ Climates are also Shown.

S.E.T.

Temperate Maritime

N.W.W.

Arctic

S.E.P.W.

Summer in Northern Hemisphere

Mediterranean Savanna Savanna Mediterranean Tundre

Winter in Northern Hemisphere

Climate, Weather, and Natural Environment   14.9

the depressions of the southwest wind system bring rains to the Mediterranean region (see Figure 14.13). The global distribution of this climate is shown in Figure 14.2.

Savanna climate The equatorial low pressure belt covers the savanna region in the summer bringing humid air and heavy rainfall. In the winter, the hot desert high pressure belt extends over the region bringing dry outblowing warm winds. There is no rain in this season (Figure 14.14). High temperatures and no rain make the winter a season of drought. You should note that ‘winter’ is used here to mean the northern winter. Tropical climates do not have a winter season, as we understand it. Figure 14.2 shows the global distribution of the savanna climate.

Monsoon climate The seasonal changes in the pressure systems over central Asia and Australia result in a seasonal reversal of winds between the two continents. Onshore winds, which cross a wide ocean surface bring copious rainfall in either or both seasons. Figure 14.15 shows the temperature and rainfall.

The eight types of climate examined above can be regarded as basic types. Variations in some of these types produce similar climates but with distinctive characteristics.

Continentality The influence of continentality (distance from the sea) is at its greatest in the interior regions of North America and Asia. These interior regions have hot summers and very cold winters (the sea is too distant to have any cooling and warming influence, respectively). This results in a large annual temperature range.

Barrow 71° 16´N Annual Range 31 °C

Bulun 70° 45´N

°C 15 10 5 0 –5 –10 –15 –20 –25 –30

Annual Range 51.5 °C

–35 –40 –45

–35 –40 –45 mm 100

Annual Rainfall 183 mm J FMAMJ J A SOND

°C 15 10 5 0 –5 –10 –15 –20 –25 –30

mm 100

Annual Rainfall 270 mm J FMAMJ J A SOND

Figure 14.12  Climatic Graphs for Tundra Climate. Figure 14.2 Gives the Locations of the Stations and the Global Distribution of the Climate.

mm 700 600

Algiers 36º 50´N Annual Range 14.1ºC

ºC 30 25

500

20

400

15

300

10

200 100 0

6ºC Annual Range 688 mm

5 0

J F M A M J J A S ON D

Figure 14.13  Climatic Graph of a Mediterranean Climate in the Northern Hemisphere. Name the Months of the Growing Season Using Rainfall as the Basis (see Figure 14.2 for the Location of Algiers and the Global Distribution of Climate).

14.10  Chapter 14

mm

Kayes 14º 24´N

ºC

Annual Range 9.6ºC

Delhi 28º 40´N mm

Bombay 18º 56´N ºC

Annual Range 19ºC

mm

ºC Annual Range 56ºC

40

1000

1000

35

900

30

900

900

30

800

25

800

25

800

700

20

700

20

700

15

600

15

600

15

10

500

10

500

10

5

400

5

400

5

0

300

0

600 500 400

Annual Rainfall 760 mm 6ºC

0

300 200 100 0

J F M A M J J A S ON D

300 200

200

100

100

0

Figure 14.14  Climatic Graph for Kayes. Figure 14.2 Shows the Global Distribution of the Savanna Climate and the Location of Kayes.

Annual Rainfall 670.3 mm

35 1000

J FMAM J J A SO N D

0

35 30

Annual Rainfall 2018 mm

25 20

J FMAM J J A SO ND

Figure 14.15  Climatic Graphs for Tropical Monsoon Climate (see Figure 14.2 for the Locations of the Two Stations and the Global Distribution of Monsoon Climate).

The climates of these regions are called continental climates; these are of two basic types according to the latitude. One occurs in cold temperate latitudes, and the other in cool temperate latitudes. The latter is known as the cool temperate continental climate.

Cool temperate continental climate This climate is controlled by two air masses —polar continental and tropical continental, both of which contain dry air. This explains why this climate has a low rainfall. During the summer, the land warms fairly quickly producing a low-pressure system into which moist air flows. This results in convection rain usually accompanied by thunderstorms, but the annual fall ranges from 250 mm in eastern parts to 700 mm in the western parts as compared to Asia. Temperatures fall to below 0°C in the winter. Depressions are formed only occasionally, causing snowfall as the p ­ ressure is high. Figure 14.16 compares climatic graphs for a cool temperate maritime climate and a cool temperate continental climate.

Cold continental climate This is similar to the cool temperate continental climate, but it differs by having much lower winter temperatures and hence a larger annual temperature range. Temperatures rise above 0°C for approximately 5 months only. For most part of the year, the climate is determined by the cold continental air mass. Annual precipitation is approximately 250 mm, most of which falls as rain in the summer (Figure 14.17).

Climate, Weather, and Natural Environment   14.11

mm

Moscow 56ºN Annual Range 28.9ºC

ºC

mm

Odessa 47ºN Annual Range 26ºC

mm

ºC

20

800

700

15

700

15

600

10

600

10

500

5

500

5

400

0

400

300

–5

300

–10

200

–15

100

–20

0

200 100 0

Annual Rainfall 534 mm J F M AM J J A S O N D

Annual Rainfall 410 mm

J F MAMJ J A SOND

–5

°C 20 15 10 5 0 –5

0

–10 700

–15

–10

600

–20

–15

500

–25

–20

Figure 14.16  Climatic Graphs for Cool Maritime (Moscow) and Continental (Odessa) Climates (see Figure 14.2 for the Locations of the Two Stations and the Global Distribution of the Climate).

Ecosystem

Annual Range 62°C

20

800

Yakutsk 62°N

400 300 200 100

–30 Annual Rainfall 350 mm

0 J F MAM J J A S ON D Figure 14.17  This Graph Represents the Climatic Conditions for Coniferous Forest Regions. Yakutsk’s Position is Given in Figure 14.2.

Much of the work studied in earlier chapters of this book is related to various aspects of the physical environment. In this chapter, we shall examine the relationships between the physical environment and living organisms, i.e., ­ between non-living and living parts of the natural environment. These two parts are called elements of the environment and the system in which they jointly operate is called an ecosystem. A tropical rainforest is an ecosystem because it represents the product of plants and animals (living or biotic components) and soil, slope, temperature, and rainfall (non-­living or abiotic components). There are many different ecosystems, but they all occur in a thin zone consisting of the top layer of the lithosphere (the soil and rocks immediately below the soil) and the tropopause or bottom layer of the atmosphere in contact with the lithosphere. This zone is called the biosphere (Figure 14.18). All living organisms live in the biosphere, and each is dependent for its survival on having a suitable habitat or home whose conditions enable it to grow and reproduce. All animals, including humans, depend for survival either directly or indirectly upon green plants which are called the producers because they produce carbohydrates (a food) from carbon dioxide (taken from the air) and water (taken from the soil) using the pigment chlorophyll (which gives green plants their colour) in the presence of sunlight. This process is called photosynthesis (Figure 14.19). Green plants are the sole food for some animals called herbivores (plant eaters); other ­animals eat both plants and other animals (these are called omnivores). Animals, which eat only meat are called carnivores. All these animals are called the consumers.

–35 –40 –45 –50

14.12  Chapter 14

Climate

Vegetation

Vegetation

Soil

Soil

Biosphere

Figure 14.18  Diagram to Show the Interactions Between Climate, Vegetation, Soils, and Rocks (Black Arrows). The Blue Arrows Represent Water Movements.

Sun’s Energy

Lithosphere

Climate

Atmosphere

Sun

Rocks

Linkages and interactions in an ecosystem Sunlight

Water Vapour Enters Air Through Transpiration

Carbon Dioxide

Oxygen Water Vapour (Evaporation) Rain

Ground Water

Evaporation Rain

Soil Water

Root Hair

Figure 14.19  The Process of Photosynthesis. Soil Water Carrying Mineral Salts Enters the Roots Through the Root Hairs and Moves Through the Plant Stems into the Leaves where Photosynthesis Occurs. Transpiration Refers to the Evaporation of Water Through the Stomata of the Leaves. It is Caused Partly by the Drawing-up Effect Resulting from Evaporation of Water from the Leaves.

All life processes are intimately associated with the atmosphere by two cycles — the carbon and oxygen cycles. There is a flow and transfer of energy, nutrients, and water in both these cycles. The water is the transport agent, which enables the nutrients to enter a plant from the soil and the food substances manufactured by a plant to be transported to the growing and storage organs of the plant. This flow and transfer are enabled by the linkages and interactions between the various components.

Carbon cycle Carbon is an essential element in all organic compounds, and because there is only a limited amount, it must be recycled continuously. This recycling process occurs in the biosphere. Atmospheric carbon (­carbon dioxide) is fixed in green plants by photosynthesis. The carbon in the carbon food compounds so produced is later released as these compounds are utilized, and it returns to the atmosphere through the process of respiration. This process involves intake of oxygen, (from the air) which is used to degrade the food compounds into other compounds and energy, both of which are required for growth. The by-products of respiration are carbon dioxide and water, which are returned to the air. Respiration occurs in both plants and animals. The carbon cycle is completed by ­bacteria and fungi, collectively called the decomposers, which degrade dead plant and animal tissues; this p ­ rocess again releases carbon, some of which enters the air and

Carbon Dioxide Storage in the Atmosphere

CO2 Dissolves in Rain Water

Respiration Returns CO2 to the Atmosphere Producers Consumers (Green Plants) (Animal) Turn Eaten By Plant Tissuse into Contain CO2 in Animals Animal Tissue Forms of Organic Which Contains Compounds Organic Compounds Dead Plant Tissue Under Pressure Results in Geological Formation of Carbon Fossil Fuels (Coal and Oil) Contain CO2

On Death

Decomposers (Bacteria and Fungi) Breakdown of Dead Tissue and Release of CO2

Water Bodies CO2 in Water in Form of Soluble Hydrogen Carbonate Deposition

CO2 in Calcium Carbonate in Rocks, Shells and Animals Returns CO2 to the Atmosphere

Combustion of Fossil Fuels Releases CO2

CO2 Returned to the Atmosphere Through Decomposition

Climate, Weather, and Natural Environment   14.13

Respiration

Figure 14.20  The Carbon Cycle. The Process of Respiration is the Reverse of Photosynthesis in Respect of the Movements of Carbon Dioxide and Oxygen. The Amount of Atmospheric Carbon and Oxygen Used by Plants, Animals, and Bacteria is Balanced by their Return Through the Process of Photosynthesis, Respiration, and Decomposition.

some enters water in the soil. All producers and consumers do not undergo complete decomposition after their death. The organic matter of some of them is stored for millions of years. This organic matter is preserved in Light fossil fuels such as petroleum and coal. The carbon cycle is shown in Figure 14.20. Maximum Effect

Oxygen cycle All living organisms receive oxygen from the air through the process of respiration, which they use to degrade organic matter, and energy is released in this process. As we have seen, carbon dioxide is returned to the air by respiration, which tends to balance the oxygen used in photosynthesis. Both these processes are powered by solar energy. The relationship between photosynthesis and respiration in a forest is shown in Figure 14.21. In addition to carbon and oxygen, plants need nitrogen for making protein, which is necessary for plant growth. Nitrogen is obtained mainly from the soil. Like carbon and oxygen, nitrogen consumption and utilization occur in a cycle. This cycle is called the nitrogen cycle.

Oxy

gen

Day Photosynthesis Minimum Effect

Respiration n Dioxide Carbo

Figure 14.21  Photosynthesis Increases the Level of Oxygen and Decreases the Level of Carbon Dioxide During the Day in the Air Around a Forest. Respiration Occurs During Both Day and Night, but the Carbon Dioxide Level is Increased at Night when Photosynthesis Ceases.

14.14  Chapter 14

Nitrogen cycle The soil contains nitrates, and these compounds enter plants through the soil water that moves into plants via the roots through the process of transpiration. This is a movement of water, which extends from the soil through the roots and stem or trunk to the leaves where it is needed for photosynthesis. The nitrogen in the nitrates is an essential element for the manufacturing of tissue building blocks called proteins. Proteins, which are present in seeds such as peas and beans, pass into animals when the plants are eaten. Some of the nitrogen taken in by animals is used for renewing tissues (growth), and some is excreted. Most of the nitrogen excreted by people is lost to the soil because most human sewage is deposited in the sea. The nitrogen in animal manure is returned to the soil when animals are kept in fields. The nitrogen in the tissues of dead plants and animals is returned to the soil when bacteria convert it into nitrates through the process of decomposition. This circulation of nitrogen from soil through plants and animals, back to the soil is called the nitrogen cycle (Figure 14.22). In a natural ecosystem such as the remote interior of a rainforest where human activities have minimal effect or there are no human activities at all, the nitrogen cycle is balanced, i.e., the amount of nitrogen circulating in the cycle is fixed. A number of years ago, the world’s population was very small and was scattered in small groups or tribes over only a few parts of the earth’s surface. The nitrogen

Nitrates in Solution

Consumed

Plants

Leaf fall Litter

Death

Consumed

Decaying Tissue

Manures (Nitrites)

Animals

Consumed

Humankind

Death

Sewage (Nitrates)

Death Decomposition by Bacteria

Nitrates in Soil

Loss of Nitrates

Decaying Tissue

Decomposition by Bacteria Nitrates Added

Nitrates Washed Out of Soil

Artificial Fertilizers and Leguminous Plants

Sea Loss of Nitrates

Figure 14.22  The Nitrogen Cycle. Nitrogen is Lost to the Sea Through Sewage, While Nitrates are Added to the Soil by Growing Leguminous Plants, e.g., Clover, which Convert Atmospheric Nitrogen into Nitrates, and by Addition of Artificial Fertilizers.

Climate, Weather, and Natural Environment   14.15

cycle at that time was balanced. The growth of the world’s population led to the destruction of large areas of many natural ecosystems and their replacement by new ecosystems, some designed for producing food, some for the manufacturing of a great variety of goods, and some for accommodating the huge concentrations of population. These activities disturbed the natural balance of not only the nitrogen cycle but also many other cycles such as the hydrological cycle.

Adaptation in plants Plants produce their food by using water, carbon dioxide, sunlight, and mineral salts, especially nitrates. They perform this activity by using their roots, stems, and leaves. The produced food enables the plants to grow and reproduce, the latter often being enabled by seeds which they produce. The four basic requirements of plants listed above are not uniformly distributed over the earth’s surface. Some regions are wet, while others are dry; some regions are hot throughout the year, some are cold throughout the year, and some regions are alternately hot and cold on a seasonal basis. Furthermore, some regions have deep rich soils, some have shallow poor soils, and some regions lack soil. But practically in all regions, there is some form of plant life. The differences in the physical characteristics of the natural ecosystems over immense periods of time have resulted in the evolution of a tremendous number of different types of plants. A few examples will show how plants have become adapted to the environments in which they live. By using the adaptation process, they are able to survive.

Influence of temperature and water on plants Plant growth occurs when the temperature reaches 6°C. The growth becomes continuous when the temperature rises and remains above 6°C, provided there is sufficient water available to meet the plant’s requirements. The availability of water depends upon the amount and regularity of rainfall, the evaporation rate of water from the soil, the slope and extent of surface run-off, and the permeability of the soil. Some plants, especially trees, adapt to cool climates by losing all their leaves during the winter. They are called deciduous trees and have broad leaves (see Figure  14.23(a)). It should be noted that some sub-tropical and tropical regions have a definite dry season, e.g., the monsoon regions of southeast Asia. Many trees in these regions are also deciduous, losing their leaves in the dry season. Some trees do not shed all their leaves at once; they lose a few at a time throughout the year. They are called evergreen trees. Some have broad leaves, while others have rolled, needle-like leaves which slow down the transpiration rate. They are called coniferous trees because they produce seeds in cones (see Figure 14.23). Plants, which are adapted to arid and semi-arid conditions are called xerophytes. Some have small, fleshy leaves, which have a waxy skin. This prevents the loss of water by transpiration. Some xerophytes are trees, e.g., the baobab. This tree has a swollen trunk in which water is stored (see Figure 14.24).

14.16  Chapter 14

(a)

Broad Leaf

Soil

Soil

(b)

Coniferous Leaf (Needle)

Soil

Soil Shallow Spreading Roots

Deep Roots Summer (Tree in Leaf)

Winter (Same Tree Without Leaves)

Summer (Tree in Leaf)

Winter (Same Tree in Leaf)

Figure 14.23  (a) Deciduous Tree; (b) Coniferous Tree. Note the Difference in Root Structure Between the Two Trees.

Main Types of Vegetation The world’s natural vegetation consists of forest, grass, scrub, tundra, and desert plants. The global distribution of this vegetation is shown in Figure 14.25. Trees, excluding conifers, require a large intake of water daily, and they are located mainly in regions, which have abundant rainfall that is fairly evenly distributed throughout the year. Grasses require a much lower intake of water daily, and their water intake does not have to be evenly distributed throughout the year. Wherever forests thin out because of inadeBaobab Tree Stores quate rainfall (or the equivalent in available Water in its Trunk soil water), grasses tend to form the plant community. One type of vegetation usually blends Cactus Plants with Thick Skins into the next type across a broad transition zone where plants of one type gradually give 3–5m way to those of the next type as shown in Figure 14.26. 4 – 8 m 0.5 – 1 m When a vegetation covering is dominated by a group of plants, which form a stable plant community, this community is called the climax vegetation. The earth’s surface, except for the ice-covered polar regions and the tops of high mountains, was once covered with vegetation, mainly forest and grass with each region Figure 14.24  Some Examples of Plants that can withstand having its own climax vegetation. Tropical rainforest, areas of which still exist, is perhaps the Drought.

Climate, Weather, and Natural Environment   14.17

Tropic of Cancer

Equator

Tropic of Capricorn Forest Grassland Scrub, Tundra and Desert Figure 14.25  The Distribution of the World’s Main Areas of Forest, Grassland, Scrub, Tundra, and Desert.

Transition Zone Tropical Rainforest

Transition Zone

Savanna Grassland

Semi-Desert

Desert

Acacia Tree

Increasing Rainfall; Decreasing Length of Dry Period

Decreasing Rainfall; Increasing Evaporation

Figure 14.26  Transition Zones Between Savanna Grassland and Dry Desert, and Between Savanna Grassland and Tropical Rainforest.

14.18  Chapter 14

most extensive and the best example of a climax vegetation. Figure 14.27 shows the global distribution of the main types of climax vegetation. It is important to understand the relationship between climax vegetation and climate. The three essential requirements for plant growth are sunlight, water, and gases (carbon dioxide and oxygen). Two of these, namely sunlight and water, form two of the elements of climate, and the influence of these requirements on the type and nature of vegetation is profound, so much so we can say that a climax vegetation is broadly determined by climate. In other words, each type of climate tends to have a specific climax vegetation. However, it should be remembered that soil and soil drainage, as well as slope, also influence the nature of a climax vegetation, though on a very limited scale in terms of area. A climax vegetation does not consist of only one species of plant. Each climax vegetation contains a large number of different plant s­ pecies. Tropical rainforest is regarded by many geographers as being the last great climax vegetation, and a closer look at this vegetation makes an interesting reading.

1

66 2 ˚N

1

23 2 ˚N

0˚ 1

23 2 ˚S

Desert, Semi-Desert and Scrub Evergreen Forest

Mountain

Mediterranean

Tundra

Deciduous Forest

Grassland

Rainforest (Broad Leaf)

Monsoon (Broad Leaf)

Tropical (Savanna)

Temperate (Broad Leaf)

Temperate (Broad Leaf)

Temperate (Prairie and Steppe)

Coniferous Figure 14.27  The Main Types of Climax Vegetation.

Climate, Weather, and Natural Environment   14.19

Tropical rainforest Giant buttress-rooted trees, mostly over 40 m in height, dominate the tropical rainforest as shown in Figure 14.28. These are known as the big trees, and their tops form an almost complete canopy that blocks much of the sunlight from the forest floor, thereby making it dark and gloomy. Below the big trees, smaller trees that require less sunlight grow, and below these smaller trees, plants that require shade, abundant moisture, and even temperatures grow on the forest floor. Many of these plants are saprophytes, i.e., they live on the decaying tissue of dead plants. The saprophytes contrast with another group of plants called epiphytes, which survive by using the trees, mainly the big trees, for support in their search for sunlight. Many of the epiphytes are giant climbing plants (Figure 14.28). The tropical rainforest has more species of plants than any other climax vegetation. The rainforest of the Amazon Basin has over 2,400 species of trees and between 30,000 and 50,000 species of other plants. In addition, there are as many species of animals, insects, and other similar organisms. The inter-relationships between the plant and animal species and between these species and the physical environment make the tropical rainforest the most fascinating and complex ecosystem in the world. In spite of this rich diversity, more than 200,000 km2 of tropical rainforest are being cut down every year. This is approximately the area of Great Britain. At this rate, there will be no tropical rainforest left 150 years from now. It must be remembered that most of the tree felling is quite ruthless in that it is not selective, i.e., to obtain the trees that are wanted, trees of the whole area are felled, of which approximately 40 per cent trees are wasted. It must also be remembered that it takes between 200 and 600 years for tropical rainforest to re-establish itself after it has been devastated. This systematic destruction of the rainforests is occurring in South America, Africa, and southeast Asia. Deliberate attempts to conserve the rainforest have been made only in isolated parts, e.g., in Costa Rica and in other parts of central America.

m 40 30

Leaves of Giant Trees Form Complete Canopy

20

Hanging Epipytes

10

Climbing Epiphytes

0

Forest Floor with Few Plants Thin Soil

3 to 4 m

Sub-Soil Buttress Root Figure 14.28  Diagram Showing the Make-up of a Tropical Rainforest, which is a Good Example of Climax Vegetation.

14.20  Chapter 14

Tropical rainforests mainly have evergreen trees, but there are also some deciduous trees. In addition, they have climbing plants, herbaceous plants, mosses, ferns, and fungi. All these plants form a plant community, i.e., a collection of plants among which some are dominant, but all of which live in an interdependent balanced association. The rainforest, like any other natural vegetation, has evolved with time in a particular place. In the beginning, plant communities were very simple, but gradually, they became more complex, eventually reaching their present state of being fairly stable, highly complex plant communities. The process by which a simple plant community becomes a complex plant community is called succession. This process can be observed in a newly formed surface of volcanic ash, an area of burnt-over forest, or an area of cleared land that is abandoned. New plants soon become established in the area but are soon followed by other plants until a stable community is reached. Such a vegetation covering is called a climax vegetation. Let us now see what happens when the balance of a climax vegetation is disturbed. If one or two giant trees are felled, they will bring down several of the smaller trees surrounding them as they crash to the ground. This will enable sunlight to reach down to the forest floor, and plants soon begin to grow in the open space on the floor. Small plants, which require sunlight will be the first to grow, followed by larger bushes. With the progression in time, a dense undergrowth appears that gradually crowds out the first colonizers. Some of the seeds of the giant trees which fall to the ground eventually germinate in the damp shade provided by the undergrowth, and gradually the young saplings from these seeds push their way through the undergrowth. This usually takes several years, but eventually, the saplings grow above the level of the undergrowth, and provided their growth remains undisturbed, the opened patch of forest once again becomes a part of the climax vegetation. The balance of the tropical rainforest is thus restored.

Soil The formation of soil The processes of weathering are responsible for the formation of regolith, which gradually gives rise to the formation of soil. As weathering degrades the surface of the parent rocks, air and water enters the spaces between the rock particles, which leads to chemical changes and the production of chemical substances. The passage of water in the regolith, upwards through capillary action and downwards through infiltration, is an essential component of the soil-forming system. Bacteria and plants soon appear. When plants die, their partially decomposed tissue forms a black sticky substance called humus. As humus forms, organic acids are produced which play a vital role in degrading soil minerals, thereby enabling chemicals such as potassium (K), ammonium (NH4), sodium (Na), magnesium (Mg), and calcium (Ca) to become available to plants which absorb them with the soil water through the process of transpiration. Humus combines with clay particles in soil to form molecules, which have a negative electrical charge. These molecules attract ions or groups of atoms such as sodium, ammonium, magnesium, calcium, and potassium, which are positively charged, and thus, the molecules of humus and clay contain some of the chemicals essential for plant growth. The soil-forming system

Climate, Weather, and Natural Environment   14.21

illustrates the linkages between the components and the flow of energy and nutrients that occurs. The amount of humus in a soil depends on the amount of vegetation, which in turn depends on the climate. In hot humid climates, plant growth is luxuriant, but the action of bacteria is so rapid that most of the dead plant tissue is consumed and very little humus is formed. Because arid and semi-arid soils have limited amounts of vegetation, their soils contain very little humus. Some bacteria can convert nitrogen gas, which enters soil pore spaces with air, into compounds of nitrogen such as nitrates. It is from these compounds that plants obtain their nitrogen requirement. These bacteria inhabit nodules on the roots of plants such as clover and beans. Hence, these plants are grown to improve the quality of soil. Bacteria help to degrade dead plant and animal tissues, but they need air for this process. If air is limited, as it is in waterlogged soils, then the number of bacteria decreases, and the process of humus degradation is slowed.

Water movement in the soil Water in the soil is called soil water. Some of this forms a film around soil particles. This water is called hygroscopic water. Clay particles, because of their very small size, have a very large surface area, and the hygroscopic water content of clay soils is therefore high. Sand particles are large, and their surface area is smaller than that of clay. The hygroscopic water content of sand soils is low (see Figure 14.29(a)). The movement of water in a soil affects the quality of (b) (c) the soil. When the movement is mainly from the surface (a) downwards, humus and mineral salts such as manganese, potassium, and calcium are removed from the top layer of the soil and are deposited in lower layers. This process is known as leaching (Figure  14.29(b)). Sometimes, water moves upwards towards the surface, and the reverse process occurs, i.e., mineral salts are deposited near the surface. In some regions, heavy rainfall throughout the year results in leaching being the dominant process. This mainly occurs in equatorial regions. In other regions such as hot deserts, low rainfall and a high rate of evaporation result in an upward water movement being the dominant process. Deposition of salts near the surface is called calcification. Between these two regions, there are regions Figure 14.29  Three Types of Soil Water are as Follows: where leaching occurs in one season and calcification in (a) Hydroscopic; (b) Gravity; and (c) Capillary. Soil the other season. Upward water movement occurs by capil- Grains are Stippled, and the Water is Shown by the Blue lary action (Figure 14.29(c)). Margins. The Pore Spaces that Contain Air are Left White.

Soil profile Most soils consist of three layers called horizons. The top layer, the A horizon, contains the finest soil particles. The humus in a soil is usually in the upper part of the A horizon, and this part is sometimes called the A1 horizon. On top of this layer, there

14.22  Chapter 14

Humus

A

B

C

Figure 14.30  A Section Through a Soil Showing the Three Layers or Horizons: A — Soil; B — Subsoil; C — Weathered Parent Rock.

may be a layer of decomposing vegetation. This forms the A0 horizon. The layer below the A horizon is called the B horizon. This is the subsoil which is halfway in its formation between the A horizon and the C horizon, which is the parent rock and which lies below the B horizon as shown in Figure 14.30. The depths of these soil horizons vary according to the type of climate under which a soil has developed. In tropical humid regions, a soil may be up to 10-m deep, whereas in a humid temperate region such as Great Britain, it is rarely more than 2 m. The parent rock influences soil type mainly in respect of the size of soil grains. Climate and vegetation also influence soil type, e.g., the amount of precipitation and whether it is regular or irregular and seasonal or evenly distributed in relation to the rate of evaporation determine whether soil water movement is predominantly upward or downward. Climate also plays an important role in determining the type of vegetation in a region.

A Closer Look  ▼ Human impact on the Soil From the dawn of humankind, forests have been considered to have little value except for their timber, which has been used as a fuel, for building ships and houses, and more recently for making paper. Forests were seen as obstacles to the expansion of farmland, the building of settlements, and the construction of roads and railways, and they were therefore cut down or destroyed in many parts of the world to allow the ‘progress’ of people to continue. Vast areas of natural vegetation have been replaced by farmland, settlements, and factories; and this has resulted in a breakdown of the natural association of processes such as weathering, soil formation, and plant growth. This association enabled the acids from decaying vegetation to fasten rock weathering which is necessary for plants to obtain a steady supply of nutrients essential to their growth. It also enabled plants to influence the type of soils that developed. The natural environment no longer influences soil formation and plant growth.Today, people are the largest and most powerful single factor influencing soils and plants. A large part of the natural vegetation that has been removed has been replaced by another type of vegetation, i.e., the crops, which provide food to people and their domestic animals. Over countless centuries, a balance had been established in the relationships between natural vegetation and soils. The replacement of the natural vegetation by cultivated crops means that changes will occur in the soils.To ensure that these changes are not detrimental to people, it is essential for us to understand the nature of the relationships between plants and soils. The changes made to the plant life of the natural ecosystems (Figure 14.31) have given rise to new ecosystems (Figure  14.32) such as cropland, pastureland, woodland, and moorland, all of which have been developed within the last 2,000–3,000 years, but most of which are the products of the last 200 years. These new ecosystems lack the stability of the natural ecosystems, which means that changes may occur, sometimes with disastrous results

Climate, Weather, and Natural Environment   14.23

Figure 14.31  Part of the Tundra Ecosystem in Alaska.

Figure 14.32  Farmland in Gloucestershire. The Landscape is an Example of an Ecosystem Created by Humans.

Great Britain’s Lost Landscape Several years ago, large parts of Great Britain were covered with forests of mixed deciduous and coniferous trees, the latter being more common in the wetter, colder regions.Today, these natural forests have disappeared. This did not happen because the trees failed to reproduce; it happened because the growing population, in need of more food and timber for fuel and building, deliberately, century after century, attacked the forests

14.24  Chapter 14

with fire and axe. The use of fire enabled only a few people to destroy vast areas of forest. With the progression in time, the forest lands of Great Britain were converted into fields for crops and pasture (the latter being hedged to prevent the cattle from wandering). In recent years, the use of larger, more powerful farming equipment for sowing the crops and reaping the harvests has resulted in the removal of hedges between fields in many areas. This action has produced further changes in the farmland ecosystems by denying birds and insects their breeding grounds, which are the hedges.This has further resulted in the breakdown of many relationships in the ecosystem, some of which have had adverse effects on the growing of some crops, e.g., the spread of greenfly which attacks green leafy vegetables. Although the destruction of forests by fire is no longer allowed in most temperate countries, it is still used by forest dwellers in the tropical forests in Indonesia, Malaysia, Papua New Guinea, Brazil, and many other countries in South America and Africa.

Destruction of tropical rainforests Figure 14.33 shows a part of a rainforest being cleared by fire by shifting cultivators. A shifting cultivator is a person who clears a piece of forest, usually by burning, so that food crops can be planted for use by the cultivator’s family. Crops grown by a family or a small community for its own use are called subsistence crops. The great height of trees, immense luxuriance of plant growth, and the vast number of plant species of a rainforest might suggest that the soils are deep and fertile. But, this is not the case. The forest floor is covered with a thin layer of dead plant tissue called litter, which is rapidly consumed (oxidized) by bacteria, resulting in the formation of very little humus in the soil. This means that organic acids, which are formed by humus are also lacking. It should be remembered that about 95 per cent of rainforest nutrients are present in the forest vegetation. There is hardly any soil on the forest floor; thus, when a forest is

Figure 14.33  A Patch of Forest in the Amazon Basin in Brazil Burnt by Shifting Cultivators.

Climate, Weather, and Natural Environment   14.25

cut down and the land is neglected, the leftover soil rapidly deteriorates and erosion soon sets in. It should further be remembered that about 1,000 hectares of tropical rainforest are being destroyed by commercial activities every hour in the world.

Soil productivity When a soil contains all the nutrients that a healthy growing plant requires, it is said to be fertile. This does not mean that the soils in a natural environment contain sufficient nutrients to allow cultivated crops to grow effectively, i.e., to produce the amount and type of food material that the farmer expects. This can be looked at from another point of view. The luxuriant tree growth of a tropical rainforest may suggest that the soils of the forest are both fertile and deep. But, this is not true. The fallen leaves decompose fairly quickly and form litter. The nutrients derived from litter are absorbed by the trees. Very little nutrients enter and remain in the soil. This is an example of recycling, and in a tropical rainforest, it is a continuous process. In other words, the demands made by the trees on the soil are fairly small. A similar situation exists in all natural environments. When a natural forest is burnt, some of the nutrients in the trees remain in the wood ashes and some are burnt away in the fire. The cultivation of crops in the burnt-over forest soils soon depletes most of the nutrients, and because the crops are removed from the land when they are harvested, the nutrients, which they contain are never returned to the soil. The soil loses its fertility and in tropical regions becomes ‘dead’ after a few years. This is why the farmers in tropical forests who burn the trees so that they can grow crops and then move after a few years to another part of the forest (which they burn so that they can grow crops again) are called shifting cultivators. The abandoned forest patch is left to recuperate. Some farmers in temperate regions follow a similar practice of leaving a part of the cropland fallow, i.e., unused for 3 to 4 years. However, research on cultivated crop requirements has shown that different types of crops require different nutrients, and this has enabled farmers to cultivate the same piece of land by rotating the crops grown. For example, wheat may be grown in 1 year, followed by clover (which adds nitrogen to the soil, thus compensating for that absorbed by wheat the year before) in the next year, and perhaps a root crop or pasture in the subsequent year. Different plants require differing amounts of nutrients such as calcium, phosphorus, iron, etc. this means that if the same plant is grown on the same piece of land year after year, the soil of that land will become deficient in one or more of the essential nutrients. To overcome this, crop rotation was introduced in the eighteenth century in Great Britain and in other parts of Europe. This was done to allow the nitrogen content of the soil to be replenished by growing clover and to allow other nutrients to be returned to the soil by growing pasture on which the farmers’ cattle grazed, wherein the nutrients were returned to soil through the droppings of the cattle. In more recent years, ­artificial fertilizers containing various nutrients have been produced and extensively used. These fertilizers have increased the output of food crops enormously, but sometimes with disastrous side effects.

Soil Erosion Some soil erosion occurs naturally, e.g., when lightning sets fire to a forest, when a forest is destroyed by volcanic eruption, and when an earthquake triggers a landslide. In each of these cases, the natural catastrophe may result in soil erosion. But

14.26  Chapter 14

by far, the largest amount of soil erosion has resulted from poor farming practices; the deliberate destruction of natural vegetation, especially the exploitation of the forests; and by unplanned, uncontrolled mining of minerals. Extensive soil erosion results from the removal of the natural vegetation, both forest and grass. So long as the vegetation cover remains, there can be little, if any, erosion because the roots of the plants bind the soil particles together, and the vegetation itself protects the soil from the action of rain and wind. The rapid expansion of farmland in the nineteenth and twentieth centuries resulted in some of the worst soil erosion. Soil erosion is caused by water (overland flow) and wind. Erosion caused by overland flow is most dominant on slopes whose vegetation has been removed. On gentle slopes, erosion takes the form of sheet erosion, while on steep slopes, the form is gully erosion.

By water Sheet erosion This affects large areas, and it occurs when rain falls on a gentle slope which is bare of vegetation. This type of erosion results in the removal of a uniform depth of soil. Figure 14.34 shows the nature of sheet erosion in a woodland. You can see that the land is bare of vegetation in this example. Gully erosion This is more localized, and it occurs when heavy rainfall rushes down a steep slope, cutting deep grooves into the land. The grooves become deepened and widened to form gullies, which eventually cut up the land to give badlands. Figure 14.35 shows gully erosion in Kenya. Gully erosion is especially frequent in semi-arid regions. Figure 14.36 illustrates the distribution of sheet and gully erosion in South Africa.

Figure 14.34  Sheet Erosion in Southern Africa.

Climate, Weather, and Natural Environment   14.27

Figure 14.35  Gully Erosion in Kenya.

ZIMBABWE N E

W

MOZAMBIQUE

BOTSWANA

SWAZILAND

NAMIBIA

S

Legend Towns International Boundaries Provinces Water Bodies

LESOTHO

Soil loss Very Low Low Moderate

L AT

High

TI

AN

Very High

C

Extremely High

OC N EA

0 50 100

200

INDIAN OCEAN

Figure 14.36  Distribution of Sheet and Gully Erosion in South Africa.

300

400

km

14.28  Chapter 14

By wind Regions with a low rainfall, or a definite dry season, are liable to have their soils reduced to dust and blown away by the wind if the land is bare of vegetation. We have already seen how the wind removes fine particles of material from desert regions by the process of deflation. The same process also occurs in marginal zones of some of the temperate grasslands whose natural vegetation of grass has been replaced by crops such as wheat or which have been overgrazed by cattle. In these marginal zones, rainfall is not only lower than in the grasslands proper, but it is also less reliable. Sufficient rain may fall for a few years to enable the crops to thrive, but this is always followed by a series of drier years when the crops fail and the land has to be abandoned.

The danger of too much artificial fertilizer We read about soil structure and humus earlier in this chapter. The latter is organic matter derived from the decay of plant and animal tissues. It contains valuable nutrients, and it helps to maintain soil structure and retain the moisture content of soils. If the humus is removed from a soil by leaching or by overland water flow, then the soil degrades, i.e., its particles disintegrate. The soil pores become clogged, which makes soil drainage difficult, and overland flow becomes more persistent. Soil erosion may now become an active process. During dry seasons, the soil particles can easily be removed by strong winds. Artificial fertilizers do not contain humus. If organic fertilizers are not used in crop farming, the continual use of artificial fertilizers could well lead to widespread soil erosion, especially in regions of heavy rainfall or regions with a pronounced windy dry season. A further risk of overuse of artificial fertilizers is that the excess fertilizer in the soil is washed out into the streams, thereby threatening the aquatic life.

Soil Conservation Soil erosion has made millions of hectares of land unproductive. As the world’s population increases year by year, an increasing amount of food has to be produced to eliminate famine and diseases and to enable all people to get an adequate and balanced diet. The governments of most countries have for a long time realized that soil erosion is a great enemy to people, and measures are now being taken to reduce erosion to a minimum and to reclaim land that has already been eroded. International organizations such as the Food and Agricultural Organization (FAO) have for many years arranged for lectures on soil conservation to be delivered to students pursuing agricultural courses. The organization also conducted a programme on soil conservation that involved the introduction of sound methods of farming.

Types of soil conservation Following are the techniques/practices used for conservation of soil.

Contour ploughing In contour ploughing, the furrows in which the crops are planted follow the contours. If the furrows go up and down the slopes, gullying is likely to occur. Figure 14.37 is an aerial view of contour ploughing in Texas, USA.

Climate, Weather, and Natural Environment   14.29

Terracing Steep slopes can be cut into a series of wide steps on which the crops can then be grown. These steps are called ­terraces. Terracing is very common in those parts of Asia where rice is grown. The rice terraces are flooded during the growing season, with the water passing from one terrace to the one below it (see Figure 14.38). The flooding of terraces is a very good indication of the ability of the terraces to prevent soil erosion.

Figure 14.37  Contour Ploughing in the USA.

Figure 14.38  Terrace Cultivation in Northern China.

14.30  Chapter 14

Planting of shelter belts Belts of trees are often planted across a flat region which is liable to experience wind erosion. The trees disrupt the force of the wind and thus protect the strips of land between the belts from being eroded (see Figure 14.39).

Prevailing Winds Crops

Strip cultivation and crop rotation Belt of Trees Belt of Trees Figure 14.39  Belts of Trees Separate Broad Zones where Crops are Grown. The Trees Break the Force of the Winds and Reduce the Possibility of Wind Erosion.

Figure 14.40  Strip Farming in the Mid-West of the USA.

Other farming methods include the cultivation of alternate strips at right angles to the prevailing winds, so that when one strip is laid bare for ploughing, the adjacent strip is under grass or is growing a crop, as shown in Figure  14.40. If the wind blows soil off the bare strip, it will be caught and anchored by the vegetated strip. Crop rotation and the use of mulch, i.e., organic fertilizer, ensure that the soil remains fertile and does not lose its structure. It will therefore stick together better and be less likely to blow away.

Climate, Weather, and Natural Environment   14.31

A Closer Look  ▼ Climate Change in Recent Times ‘There has been a 20-fold increase in the number of global climate change laws since 1997, according to the most comprehensive database of relevant policy and legislation’(Grantham Research Institute on Climate Change and the Environment and the Sabin Center on Climate Change Law). Today, there are 164 countries, which have 1,200 relevant related policies in place. Most of them (95 per cent) account for greenhouse gas emissions. Figure 14.41 indicates the increase in climate change laws (legislative and executive) from 1996 to 2016. Many scientific evidence reveals that current warming is occurring ­approximately 10  times faster than the average rate of ice age-recovery warming. The evidence for rapid climate change is experienced through global temperature rise (Figures 14.42(a) and 14.42(b)), warming oceans, shrinking ice sheets (Figure  14.43), glacial retreat (Figure 14.44), sea level rise (Figure 14.45), decreased snow cover, declining Arctic Sea ice, and ocean acidification. The planet’s average surface temperature has risen about 2°F (1.1°C) since the late nineteenth century. Most of the warming occurred in the past 35 years, with 16 of the 17 warmest years on record occurring since 2001. The year 2016 was not only the warmest year on record, but 8 of the 12 months of that year – from January through September, except June — were also the warmest. The oceans have absorbed much of this increased heat, with the top 700 m (about 2300 ft.) of ocean showing warming of 0.302°F since 1969. The mass of Greenland and Antarctic ice sheets has decreased. Data from NASA’s Gravity Recovery and Climate Experiment show that Greenland lost 150–250 km3 (36–60 miles) of ice per year between 2002 and 2006, while Antarctica lost about 152  cubic km (36 cubic mi3) of ice between 2002 and 2005. Glaciers are retreating almost globally — including in the Alps, Himalayas, Andes, Rockies, Alaska, and Africa. Forty-six gigatons of ice from Alaskan glaciers was lost on average each year from 2003 to 2010.This is according to data from NASA’s GRACE satellite, as analysed by a team of scientists from the University of Colorado at Boulder. Further observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and that the snow is melting earlier. Global sea level rose about 8 in. in the last century. The rate in the last two decades, however, is nearly double that of the last century. Both the extent and thickness of Arctic Sea ice have declined rapidly over the last several decades. Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by approximately 30 per cent. This increase is the result of humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the oceans. The amount of carbon dioxide absorbed by the upper layer of the oceans is increasing by about 2 billion tonnes per year (Figure 14.46). Climate change has also increased the length of the fire season, the size of the area burned, and the number of wildfires (Figure 14.47).

14.32  Chapter 14

Figure 14.41  Increase in Climate Change Laws (Legislative and Executive) from 1996 to 2016 Around the World.

Total Laws and Executive Policies

1,400 1,200 1,000 Total Legislative

800

Total Executive 600 400 200

d

94

an

0 e or 996 998 000 002 004 006 008 010 012 014 016 f 2 1 2 1 2 2 2 2 2 2 2 be

19

Figure 14.42  (a) Annual Surface Temperature Compared to the 1981–2010 Average from Multiple, Independent Research Groups and NOAABased data (2016). (b) Global Temperature Anomaly (1870–2020).

Difference From Average (°C)

1.0

0.5

0.0

1981-2010 Average

–0.5

–1.0 –1.5 1900

1920

1940

1960

1980

2000

1.0

Global Temperature Anomaly (c) Compared to 1901-2000

0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 1870 1880

1890 1900

1910 1920 1930

1940 1950 1960 Year

1970 1980 1990 2000 2010 2020

Climate, Weather, and Natural Environment   14.33

Figure 14.43  Melt Streams on the Greenland Ice Sheet on 19 July 2015. Ice Loss from the Greenland and Antarctic Ice Sheets as Well as Alpine Glaciers has Accelerated in Recent Decades. Ice Loss from the Greenland Ice Sheet Increased Six Fold, from 34 Gigatons/ Year Between 1992 and 2001 to 215 Gigatons/ Year Between 2002 and 2011. NASA Photo by Maria-José Viñas.

1780 1935 1956 1964 1971 2001 o ng

Ga tri ier ac

Gl

Scale (km) 0

0.5

1

Figure 14.44  This False-Colour Satellite Image Shows the Gangotri Glacier, Situated in the Uttarkashi District of Garhwal Himalaya. Currently, 30.2 km (19 mi) Long and Between 0.5 and 2.5 km (0.31 to 1.5 mi) Wide, Gangotri Glacier is One of the Largest Glaciers in the Himalayas.

14.34  Chapter 14

Sea Height Variation (mm)

SATELLITE DATA: 1993-PRESENT Data Source: Satellite Sea Level Observations. Credit: NASA Goddard Space Flight Center

Rate of Change 3.2 Millimeters per year

80 60 40 20 0

1995

2000

2005 Time

2015

2010

Carbon dioxide Level (parts per million)

Figure 14.45  Core Samples; Tide Gauge Readings; and, Most Recently, Satellite Measurements Inform us that Over the Past Century, the Global Mean Sea Level (GMSL) has Risen by 4–8 in. (10 to 20 cm). However, the Annual Rate of Rise Over the Past 20 Years has been 0.13 in. (3.2 mm) a Year, Approximately Twice the Average Speed of the Preceding 80 Years.

500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160

Currect Level For Centuries, Atmospheric Carbon dioxide had Never been Above this line

400,000

350,000

300,000

250,000

200,000

150,000

100,000

50,000

1950 Level

0

Years Before Today (0 = 1950)

Figure 14.46  This Graph, Based on the Comparison of Atmospheric Samples Contained in Ice Cores and More Recent Direct Measurements, Provides Evidence that Atmospheric CO2 has Increased Since the Industrial Revolution. (Credit: Vostok Ice Core Data/J.R. Petit et al.; NOAA Mauna Loa CO2 Record).

Climate, Weather, and Natural Environment   14.35

Figure 14.47  Climate Change has Increased the Length of the Fire Season, the Size of the Area Burned, and the Number of Wildfires.

2019 ushers in extreme Weather conditions: Serious climate change is unfolding before our eyes and we should not be in the slightest surprised that we are seeing very serious heatwaves and associated impacts in many parts of the world. Extreme weather has struck across Europe, from the Arctic Circle to Greece, and across the world, from North America to Japan. The UN weather agency, WMO, reported on 01 February 2019, with ‘dangerous and extreme cold in North America, record high heat and wildfires in Australia, heavy rains in parts of South America, and heavy snow on the Alps and Himalayas. ●●

●●

The WMO assessment of January’s 2019 weather, describes it as ‘a month of extremes’, with large parts of North America gripped by bitterly cold temperatures, caused by the influence of the Polar Vortex. ➤➤ In southern Minnesota, reports the UN weather agency, the wind chill factor pushed readings down to minus 65°F (−53.9°C) on 30 January. The national low temperature record was measured at minus 56°F (−48.9°C). ➤➤ Parts of the European Alps saw record snowfalls earlier in January 2019. In Hochfilzen in the Tirol region of Austria, more than 451 centimetres of snow fell in the first 15 days, an event statistically only expected once a century. ➤➤ The Indian Meteorological Department issued warnings on 21 January 2019 of heavy or very heavy rain and snow for Jammu and Kashmir and Himachal Pradesh, prompting warnings of avalanches amid an intense cold front. While the eastern US and parts of Canada are seeing record-breaking cold temperatures, Alaska and large parts of Australia and South America have been warmer than average.

14.36  Chapter 14

➤➤

Australia saw an unusual extended period of heatwaves which began in early December 2018 and continued into January 2019. The city of Adelaide reached a new record high of 46.6°C on 24 January. Australia’s annual mean temperature has warmed by just over 1 °C since 1910, and summer has warmed by a similar amount. Australia’s annual warming trend is consistent with that observed for the globe, according to the Bureau of Meteorology. Heatwaves are becoming more intense, extended and frequent as a result of climate change and this trend is expected to continue. Elsewhere in the southern hemisphere, heat records tumbled in Chile. A weather station in the capital Santiago set a new record of 38.3°C on 26 January 2019. In other parts of central Chile, temperatures topped 40°C.

➤➤

Argentina has also been gripped by a heatwave, prompting a number of alerts about high temperatures. Northeast Argentina, and the adjacent parts of Paraguay, Uruguay and Brazil have been hit with extensive flooding, with well above the long-term expected average rainfall. On January 8,2019 the Argentine city of Resistencia recorded 224mm of rainfall, setting a new 24-hour rainfall record, much higher than the previous highest of 206mm, recorded in January 1994, according to the national meteorological service, SMN Argentina.

➤➤

It is not too late to make the significant cuts needed in greenhouse gas emissions because the impacts progressively worsen as global warming increases.

Climate Change Performance Index (CCPI) 2018 Climate Change Performance Index (CCPI) keeps track of countries’ efforts in combating climate change. It is issued by Germanwatch, the New Climate Institute and the Climate Action Network. On the basis of standardized criteria, the index evaluates and compares the climate protection performance of 56 countries and European Union that together are responsible for about 90% of global energy-related CO2 emissions. CCPI 2018 report was released on November 15, 2017 on the sidelines of the UN Climate Change negotiations (COP23) in Bonn, Germany. The report has taken into consideration commitments by nations to reduce greenhouse gas emissions by at least 40 per cent and increase energy efficiency and renewables by at least 27 per cent by year 2030 ●● ●●

●● ●● ●●

China, which is one of the highest GHG emitting country, ranks 41st. Similar to last year’s rankings, top three rankings are still unoccupied as no country has adhered to commitments made in 2015 Paris Climate Accord that aims to keep the average global temperature rise below two degrees Celsius and as close as possible to 1.5 degrees Celsius. Following five ranks , that is rank 4 to rank 8 on CCPI,2018 include Sweden, Lithuania, Morocco, Norway and United Kingdom. India has ranked 14th out of 56 nations and European Union on Climate Change Performance Index (CCPI) 2018 and this marks an improvement from its 20th rank in CCPI 2017. The bottom three countries in the index are Korea (58), Iran (59) and Saudi Arabia (rank 60). These countries have not made any progress in this direction of reducing emission levels, nor have shown any ambition to do so.

Climate, Weather, and Natural Environment   14.37

Climate Change Performance Index (CCPI, 2018) – Ranks of 60 countries

RANK

COUNTRY

RANK

COUNTRY

RANK

COUNTRY

RANK

COUNTRY

1

Unoccupied

16

Italy

31

Slovenia

46

Argentina

2

Unoccupied

17

Denmark

32

Belgium

47

Turkey

3

Unoccupied

18

Portugal

33

New Zealand

48

South Africa

4

Sweden

19

Brazil

34

Netherlands

49

Iceland

5

Lithuania

20

Ukraine

35

Austria

50

Japan

6

Morocco

21

European Union (28)

36

Thailand

51

Canada

7

Norway

22

Germany

37

Indonesia

52

Malaysia

8

United Kingdom

23

Belarus

38

Spain

53

Russian Federation

9

Finland

24

Slovak Republic

39

Greece

54

Chinese Taipei

10

Latvia

25

Luxembourg

40

Poland

55

Kazakhstan

11

Malta

26

Romania

41

China

56

United States

12

Switzerland

27

Mexico

42

Bulgaria

57

Australia

13

Croatia

28

Egypt

43

Czech Republic

58

Republic of Korea

14

India

29

Cyprus

44

Hungary

59

Islamic Republic of Iran

15

France

30

Estonia

45

Algeria

60

Saudi Arabia

Source: Based on Germanwatch (2018), the New Climate Institute and the Climate Action Network, Report, 2018, p.11

Key facts ●● ●● ●● ●●

Weather refers to the actual state of the atmosphere for a place at a specific time. Climate is the average of weather conditions for a region for a period of at least 30 years. Plants cease growing when the temperature falls below 6°C for periods of several weeks or months. The major pressure belts of high and low pressure result in the development of climates, which have fairly uniform pressure, temperature, and rainfall values throughout the year.

14.38  Chapter 14

●● ●● ●●

●● ●● ●● ●●

●●

●●

●●

●● ●● ●● ●●

●● ●● ●● ●● ●● ●● ●● ●●

●●

Westerly winds with depressions operate for most of the year over western Europe and British Columbia. Seasonal climates develop in regions, which come under the influence of adjacent pressure belts on a seasonal basis. Continentality is used to describe a climate characterized by a large annual temperature range caused by a region distant from the sea, thereby being unaffected by maritime influence. An ecosystem is characterized by the relationships between living organisms and the physical environment. All ecosystems operate in the biosphere. The environment in which living organisms dwell is called a habitat. Green plants produce carbohydrates from carbon dioxide (from the air) and water (from the soil) in the chlorophyll (in  the leaves) in the presence of sunlight. This process is called photosynthesis. Respiration is the process by which oxygen (from the air) is used to degrade food by a living organism to produce energy and substances required for growth. This process produces carbon dioxide which is returned to the air. The carbon cycle involves the processes of photosynthesis and respiration. Carbon, an essential element to the building of organic tissue, features prominently in the cycle, passing into the cycle from the air and being returned to it on cycle completion. The living organisms of the biosphere fall into three groups: producers (food-producing green plants), consumers (herbivores, carnivores, and omnivores), and decomposers (bacteria and fungi). All three are essential to the carbon and nitrogen cycles. The circulation of nitrogen from the soil through plants and animals and back into the soil is called the nitrogen cycle. There are three types of trees: broad-leaf evergreen; broad-leaf deciduous, and needle-leaf coniferous. Evergreen trees have no seasonal leaf fall — there is a steady fall of a few leaves throughout the year, resulting in the trees always having leaves. Deciduous trees lose all their leaves during one season when there is insufficient soil moisture, e.g., a hot dry season or a cold season when low temperatures may freeze soil moisture. Plants adapted to hot dry conditions are called xerophytes. When several species of plants live together in a stable balanced relationship, the group of species constitutes a plant community. When a vegetation covering has become stabilized through the succession of plant communities, it is called the climax vegetation. Tropical rainforests have an immense number of plant species. A soil contains mineral matter, organic matter, water, air, and bacteria. Litter and dead plant tissue in tropical rainforests are rapidly consumed by bacteria. This is why very little humus forms. The nitrogen content of the soil can be increased by adding artificial fertilizers to the soil or by growing leguminous plants such as clover. The most frequent causes of soil erosion are crop cultivation in regions where rainfall is inadequate, ploughing at right angles to the contours on sloping land, shifting cultivation, growing the same crop repeatedly on the same land, removal of the forest covering, and overgrazing. Soil can be conserved by contour ploughing terracing of steep slopes, interplanting with shelter belts, strip cultivation, and crop rotation.

Climate, Weather, and Natural Environment  14.39

EXERCISE 1 Multiple Choice Questions Direction: For each of the following questions four/five options are provided, select the correct a­ nswer. 1. Which of the following refers to ‘continentality’? (a) Rainfall of less than 300 mm a year, most of which occurs in the summer. (b) High pressure, which produces descending air resulting in strong outblowing winds. (c) Remote location from the sea resulting in lack of maritime influence. (d) Large diurnal range of temperature throughout the year. (e) Alternating high and low pressure on a seasonal basis resulting in seasonal wind reversal. 2. The relationship between pressure systems and climatic characteristics is well developed in some types of climates. In respect of this, which of the following climates differs from the other four? (a) Hot desert climate (b) Arctic climate (c) Tundra climate (d) Equatorial climate (e) Cool temperate maritime climate 3. ‘Rain falls all the year with maximum in the winter; annual temperature range of about 10°C with mild winters.’ These climatic characteristics are liable to be experienced in a (a) temperate deciduous forest environment. (b) monsoon forest environment. (c) Mediterranean environment. (d) savanna environment. (e) tropical rainforest environment. 4. Which of the following does not form an essential part of the process of photosynthesis? (a) Sunlight (b) Carbon dioxide (c) Nitrogen (d) Water (e) Chlorophyll 5. The oxygen and carbon contents of the atmosphere are maintained at fairly stable levels by the processes of photosynthesis and respiration which occur in producers in the (a) stems. (b) seeds. (c) leaves. (d) roots. (e) flowers. Direction for questions from 6 to 9: Each question has one or more correct option/s. Identify which of the options are correct and select the answer as per following. (a) if 1, 2, and 3 are all correct. (b) if 1 and 2 only are correct. (c) if 2 and 3 only are correct. (d) if 1 only is correct. (e) if 3 only is correct. 6. Respiration is a process which occurs in (a) herbivores. (b) plants. (c) the carbon cycle.

14.40  Chapter 14

7. Decomposers play an important part in the operation of energy flow in a natural ecosystem. An important product of their action is the release of (a) water. (b) oxygen. (c) nitrogen. (d) carbon-dioxide. 8. Deciduous trees lose their leaves seasonally to slow down the process of evapotranspiration. This occurs when (a) there is insufficient soil moisture for several months. (b) the soil becomes waterlogged. (c) the temperature increases above 6°C for a few months only. (d) during autumn as cold weather approaches. 9. The nitrogen content of the soil can be increased by (a) growing leguminous plants. (b) mixing animal manure with the soil. (c) adding artificial fertilizers. 10. Which one of the following does not facilitate the formation of soil? (a) Organic matter (b) Soil water (c) Uniformly low temperatures (d) Bacteria (e) Air 11. One of the following is not usually associated with soils. Which one is it? (a) Living organisms (b) Capillary action (c) Condensation (d) Hygroscopic water (e) Leaching 12. Tropical rainforest soils contain very little humus. This is because (a) leaf fall is not seasonal. (b) there is very little undergrowth. (c) organic acids are poorly developed. (d) bacteria rapidly oxidize the litter. (e) there are a large number of plant species. 13. Soil erosion is caused in several ways. Which one of the following is not a major cause of soil erosion? (a) Overcropping (b) Weathering (c) Overgrazing (d) Deforestation (e) Deflation 14. Soil erosion in humid tropical regions can be checked and corrected by (a) leaving the land fallow. (b) removing the forests. (c) burning the grasslands. (d) rearing goats. (e) terracing. 15. Contour bunding is a method of soil conservation used in (a) desert margins, liable to strong wind action (b) low flat plains, close to stream courses. Liable to flooding (c) scrublands, liable to spread of weed growth (d) None of the above

Climate, Weather, Climate, Weather, and Natural and Natural Environment   Environment   14.41 14.41

EXERCISE 2 Long Answer Questions Direction: Answer the following questions in 150 words. 1. (a)  Explain why some climates are called uniform climates. (b) What are the main causes of a seasonal climate? (c) What is meant by ‘continentality’? 2. A temperate deciduous forest environment is associated with western Europe and other regions of similar location. (a) Why is the climate of these regions said to be uniform? (b) If temperatures were influenced by latitude only, these regions would have lower winter temperatures than they do have. What factor influences winter temperatures? (c) In what direction do winter temperatures decrease and why? (d) What is the length of the growing season in these regions and what influences its length? (e) Explain why most of the trees are deciduous. 3. The nitrogen cycle is an important system of the biosphere. (a) Briefly explain the part played by the decomposers in this system. (b) Name one human activity that harms the decomposers and state how this could adversely affect the nitrogen cycle. 4. The structure and function of a plant suggests that it has become adapted to the physical environment in which it lives. (a) What name would you give to the physical environment of the plant? (b) Explain why plants make very special adaptations to a hot, dry environment. (c) Name one type of adaptation and explain how this enables the plant to survive. (d) Explain why hot desert plants do not form a continuous cover on the ground. 5. Soil is one of the most important natural resources since people obtain their food requirements by making use of soil. (a) Write a short account on how soil is formed. (b) Explain what action has to be taken to ensure that the fertility of soil is properly maintained. (c) Describe the nature of shifting cultivation. 6. Study the two sets of photographs in Figures 14.48 and 14.49. (a) For each photograph marked (a): (i) Describe the type of erosion that has affected the surface; (ii) Name two possible causes of the erosion. (b) For each photograph marked (b): (i) Describe what changes have been made to the landscape; (ii) Name a method of soil conservation which may have been used.

14.42  Chapter 14

(a)   

(a)

(b)

(b)

Figure 14.48  (a) illustration of North Carolina, USA; (b) The Same Piece of Land 20 Years Later.

Figure 14.49  (a) Illustration of Nebraska, USA; (b) The Same Piece of Land after Several Years Later.

Answer key Exercise 1 Ans. (c)

1. (c) 6.   (a) 11.   (c)

2. (d) 7. (d) 12. (a)

3. (a) 8. (d) 13. (e)

4. (c) 9. (a) 14. (b)

5. (c) 10. (e)

Photo Credits Chapter 1: The Solar System: Positions and Time p.  1.1: Chapter opener—Triff. Shutterstock; p.  1.2: Figure 1.1— Christos Georghiou. Shutterstock; p.  1.7: Figure 1.8—Triff. Shutterstock

Chapter 2: Plate Tectonics: The Earth’s Structure and Landforms p. 2.1: Chapter opener—Tom Bean/Alamy Stock Photo; p. 2.3: Figure 2.3—KARSTEN SCHNEIDER/SCIENCE PHOTO LIBRARY p.  2.4: Figure 2.4—Arvind Singh Negi/Red Reef Design Studio. Pearson India Education Services Pvt. Ltd; p. 2.10: Figure 2.11b - Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  2.13: Figure 2.14—Plate tectonics—Science Learning Hub, publ. 21 July 2007, https://www.sciencelearn.org.nz/, accessed 28 March 2018, 2pm IST; p. 2.14: Figure 2.15—Plate tectonics—Science Learning Hub, publ. 21 July 2007, https:// www.sciencelearn.org.nz/, accessed 28 March 2018, 2pm IST; p. 2.15: Figure 2.18—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 2.16: Figure 2.20—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  2.28: Figure 2.30—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 2.28: Figure 2.31—United States Geological Survey (USGS), US Department of the Interior; p. 2.29: Figure 2.33—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 2.30: Figure 2.34—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 2.30: Figure 2.35a, b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  2.31: Figure 2.37—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  2.33: Figure 2.41b—Alan Curtis / Alamy Stock Photo; p.  2.35: Figure 2.46—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 2.35: Figure 2.48—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 2.36: Figure 2.50— Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/ Copernicus PGC/NASA, U.S. Geological Survey; p. 2.37: Figure 2.54—A Buldygina.shutterstock.com; p. 2.44: Figure 2.66—Kip Evans/Alamy Stock Photo; p.  2.48: Figure 2.72—dpa picture alliance/Alamy Stock Photo; p. 2.49: Figure 2.73—ZUMA Press, Inc./Alamy Stock Photo; p. 2.50: Figure 2.76—Paul Maguire. Shutterstock; p. 2.51: Figure 2.77—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  2.53: Figure 2.82—ian woolcock. shutterstock.com; p. 2.53: Figure 2.83—chinahbzyg.shutterstock.com; p. 2.53: Figure 2.84—Pecold. Shutterstock; p. 2.55: Figure 2.87/Image 1—Richard Bradford. Shutterstock; p. 2.55: Figure 2.87/Image 2—Tom Uhlman/ Alamy Stock Photo; p. 2.58: Figure 2.92—Gail Johnson. Shutterstock; p. 2.61: Figure 2.98—Johnny Greig/Alamy PC.1

PC.2  Photo Credits

Stock Photo; p. 2.61: Figure 2.99—Nikolai Link.shutterstock.com; p. 2.64: Figure 2.103—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/ NASA, U.S. Geological Survey; p. 2.66: Figure 2.107—trevor kittelty.shutterstock.com

Chapter 3: Weathering of Slopes p. 3.1: Chapter opener—Bill Bachman/Alamy Stock Photo; p. 3.4: Figure 3.1—VCF Photos. Shutterstock; p. 3.4: Figure 3.2—AfriPics.com/Alamy Stock Photo; p.  3.5: Figure 3.5—Art Directors & TRIP/Alamy Stock Photo; p. 3.6: Figure 3.6—Jez Campbell.shutterstock.com; p. 3.8: Figure 3.10b-Matt Gibson. Shutterstock; p. 3.9: Figure 3.11—imageBROKER/Alamy Stock Photo; p. 3.10: Figure 3.13—joophoek. 123rf.com; p. 3.16: Figure 3.19a— Network Photographers/Alamy Stock Photo; p. 3.16: Figure 3.19b—Network Photographers/Alamy Stock Photo; p. 3.22: Figure 3.31b—Gearsolid RT.shutterstock.com

Chapter 4: Water on the Surface p. 4.1: Chapter opener—Andrey_Kuzmin. Shutterstock; p. 4.4: Figure 4.4—Courtesy NASA/JPL-Caltech, ASTERGDEM; p. 4.5: Figure 4.6—Courtesy NASA/JPL-Caltech, ASTER-GDEM; p. 4.8: Figure 4.9b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/ NASA, U.S. Geological Survey; p.  4.10: Figure 4.12—Water Resource Information System of India (WRIS), National Water Development Agency, Ministry of Water Resources, Govt. of India; p. 4.11: Table 4.1--© 2019 Central Water Commission, Ministry of Water Resources, RD & GR, Government of India; p. 4.19: Figure 4.22— Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/ Copernicus PGC/NASA, U.S. Geological Survey; p.  4.20: Figure 4.25—ASTER-GDEM, NASA, Govt. of USA; p.  4.24: Figure 4.31—AfriPics.com/Alamy Stock Photo; p.  4.25: Figure 4.33b—Google Earth Pro, ©  Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 4.29: Figure 4.40—Paul Maguire/Alamy Stock Photo; p. 4.33: Figure 4.50—Mariusz Olszak. shutterstock.com; p.  4.35: Figure 4.54—David Wall/Alamy Stock Photo; p.  4.35: Figure 4.55—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  4.37: Figure 4.60—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 4.43: Figure 4.74a—Sumikophoto. Shutterstock; p. 4.43: Figure 4.74b—Courtesy NASA/JPL-Caltech; p. 4.45: Figure 4.78—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p.  4.46: Figure 4.79b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/ NASA, U.S. Geological Survey; p.  4.46: Figure 4.80b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 4.47: Figure 4.81b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey

Chapter 5: Underground Water and Limestone Features p. 5.1: Chapter opener—dubassy. 123rf.com; p. 5.12: Figure 5.16—Chris Howes/Wild Places Photography/ Alamy Stock Photo; p. 5.13: Figure 5.19—lasse nyborg linnes.shutterstock.com; p. 5.14: Figure 5.22—View Stock/ Alamy Stock Photo

Photo Credits  PC.3

Chapter 6: Glacial Process p. 6.1: Chapter opener—deserttrends. 123rf.com; p. 6.2: Figure 6.1—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.3: Table 6.1- Adapted from ‘Global distribution of glaciers and ice caps’, Fig. 3.7 Regional overview of the distribution of glaciers and ice caps, Pg. 17, © United Nations Environment Programme (UNEP), United Nations; p. 6.4: Figure 6.3a—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.4: Figure 6.3b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.5: Figure 6.4a—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.5: Figure 6.4b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.6: Figure 6.5—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.6: Figure 6.6—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.8: Figure 6.8a, b— stihii.shutterstock.com; p. 6.8: Figure 6.8c—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.10: Figure 6.11—robertharding / Alamy Stock Photo; p. 6.10: Figure 6.12—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.12: Figure 6.14b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.14: Figure 6.18—Chris Craggs / Alamy Stock Photo; p. 6.18: Figure 6.23b—Federica Violin.shutterstock.com; p. 6.21: Figure 6.27a—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.21: Figure 6.27b—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.23: Figure 6.30—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 6.27: Figure 6.33—© International Permafrost Association; p. 6.28: Figure 6.34—United States Geological Survey (USGS), United States Department of the Interior, Govt. of USA

Chapter 7: Desert Processes p. 7.1: Chapter opener—Massimo Mei/Alamy Stock Photo; p. 7.9: Figure 7.4—Siim Sepp. Shutterstock; p. 7.10: Figure 7.6—Science History Images/Alamy Stock Photo; p. 7.11: Figure 7.8—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 7.12: Figure 7.10—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 7.13: Figure 7.13—abc7. Shutterstock; p. 7.14: Figure 7.14—imageBROKER/Alamy Stock Photo; p. 7.14 Figure 7.15—Blue Jean Images/Alamy Stock Photo; p. 7.17: Figure 7.21—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 7.22: Figure 7.32—The United States Department of Agriculture (USDA); p.  7.22: Figure 7.33—Global Desertification, NASA Goddard Space Flight Center, Govt. of USA

PC.4  Photo Credits

Chapter 8: Coastal Processes p. 8.1: Chapter opener—dibrova. Shutterstock; p. 8.2: Figure 8.2—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.11: Figure 8.16—Ingo Oeland / Alamy Stock Photo; p. 8.13: Figure 8.19—Google Earth Pro, ©  Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.13: Figure 8.20—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.13: Figure 8.21— Lucian Pavel.shutterstock.com; p. 8.15: Figure 8.23—duchy. Shutterstock; p. 8.16: Figure 8.25—Vincent Lowe / Alamy Stock Photo; p. 8.19: Figure 8.31—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.19: Figure 8.32— Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/ Copernicus PGC/NASA, U.S. Geological Survey; p. 8.20: Figure  8.34b—Mr. Nut/Alamy Stock Photo; p. 8.21: Figure 8.36—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.21: Figure  8.37—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/ NASA, U.S. Geological Survey; p. 8.22: Figure 8.41—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.22: Figure 8.42—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.24: Figure  8.47—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.24: Figure 8.48—Google Earth Pro, © Google Inc., Data Courtesy: SIO, NOAA, U.S. Navy, NGA, GEBCO IBCAO INEGI Landsat/Copernicus PGC/NASA, U.S. Geological Survey; p. 8.25: Figure 8.50— JOHN BRACEGIRDLE/Alamy Stock Photo

Chapter 9:  The Oceans p. 9.1: Chapter opener—Willyam Bradberry. Shutterstock; p. 9.15: Figure 9.19—Science History Images/Alamy Stock Photo

Chapter 10: Atmosphere: Temperature p. 10.1: Chapter opener—Stocktrek Images, Inc./Alamy Stock Photo

Chapter 11: Atmosphere: Pressure and Wind p. 11.1: Chapter opener—NASA Archive/Alamy Stock Photo; p. 11.11: Figure 11.17—matthiasengelien.com/ Alamy Stock Photo; p. 11.32: Figure 11.48—“Satellite imagery showing Cyclone over southern India”, National Remote Sensing Centre, ISRO, Government of India, Hyderabad, India; p. 11.35: Figure 11.53—A. T. Willett/ Alamy Stock Photo; p. 11.36: Figure—NASA Image Collection/Alamy Stock Photo

Photo Credits  PC.5

Chapter 12: Atmosphere: Water p. 12.1: Chapter opener—Masaki Norton. Shutterstock; p. 12.5: Figure 12.3—Courtesy: US Navy; p. 12.8: Figure 12.8—Martchan. Shutterstock; p. 12.9: Figure 12.9—komkrit Preechachanwate.shutterstock.com; p. 12.9: Figure 12.10—Gianni Muratore/Alamy Stock Photo; p. 12.9: Figure 12.11—YuRi Photolife. Shutterstock; p. 12.9: Figure 12.12—phungatanee.shutterstock.com; p. 12.9: Figure 12.13—Grasstrax/Alamy Stock Photo; p.  12.10: Figure 12.14—RAFAL FABRYKIEWICZ. Shutterstock; p. 12.10: Figure 12.15—Cristian Zamfir.shutterstock. com; p. 12.11: Figure 12.16—Korionov. Shutterstock; p. 12.11: Figure 12.17—yauhenka. Shutterstock; p. 12.11: Figure 12.18—© National Aeronautics and Space Administration (NASA), Govt. of USA; p. 12.11: Figure 12.19—© National Aeronautics and Space Administration (NASA), Govt. of USA; p. 12.11: Figure  12.20—© National Aeronautics and Space Administration (NASA), Govt. of USA; p. 12.23: Figure 12.39—© National Aeronautics and Space Administration (NASA), Govt. of USA; p. 12.28: Figure 12.45—© National Aeronautics and Space Administration (NASA), Govt. of USA

Chapter 13: The Weather Station and Weather Maps p. 13.1: Chapter opener—dmac/Alamy Stock Photo; p. 13.3: Figure 13.4b—CambridgeBayWeather, released under Creative Commons Public Domain terms; p. 13.4: Figure 13.6—marketlan.shutterstock.com; p. 13.9: Figure 13.9—© National Aeronautics and Space Administration (NASA), Govt. of USA; p. 13.9: Figure 13.10—© National Aeronautics and Space Administration (NASA), Govt. of USA

Chapter 14: Climate, Weather, and the Natural Environment p. 14.1: Chapter opener—Sean Pavone/Alamy Stock Photo; p. 14.6: Figure 14.6—Jeff Holcombe. Shutterstock; p. 14.23: Figure 14.31—Anna Walsh/Alamy Stock Photo; p. 14.23: Figure 14.32—trevor woodville.shutterstock. com; p. 14.24: Figure 14.33—guentermanaus.shutterstock.com; p. 14.26: Figure 14.34—erichon. Shutterstock; p. 14.27: Figure 14.35—Bert de Ruiter/Alamy Stock Photo; p. 14.29: Figure 14.37—Aerial Archives/Alamy Stock Photo; p. 14.29: Figure 14.38—Sean Pavone/Alamy Stock Photo; p. 14.30: Figure 14.40—David R. Frazier Photolibrary, Inc./Alamy Stock Photo; p. 14.33: Figure 14.43—© National Aeronautics and Space Administration (NASA), Govt. of USA; p. 14.33: Figure 14.44—The National Aeronautics and Space Administration (NASA), image by Jesse Allen, Earth Observatory; based on data provided by the ASTER Science Team. Glacier retreat boundaries courtesy the Land Processes Distributed Active Archive Center; p. 14.34: Figure 14.45—© Copyright CSIRO Australia; p. 14.34: Figure 14.46—Courtesy NASA/JPL-Caltech, This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO2 has increased since the Industrial Revolution. (Credit: Vostok ice core data/J.R. Petit et al.; NOAA Mauna Loa CO2 record.)

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Index ablation 214 abrasion 154, 216, 250–251, 277 absolute humidity 398 accumulation 214 of ice and snow line 211–212 actual pressure systems 355–356 Advanced Microwave Scanning Radiometer (AMSR-E) 400 Advance Space borne Thermal Emission Global Digital Elevation Model (ASTER GDEM) 133 advection fog 402 aethelometer 442 African Plate 41 air mass 373–375 Airy Model of Isostasy 33, 34 alluvial fan 220, 261 Alpine Orogeny 49 alps 217 altitude 332 altocumulus clouds 404–405 altostratus clouds 405 Am. See arctic maritime American plate 41 AMSR-E. See Advanced Microwave Scanning Radiometer andesitic chain of volcanoes 59 anemometer 360, 361, 434 aneroid barometer 357, 358 annual global rainfall 423–424 Antarctic Circle 20 Antarctic plate 41 antecedent drainage 172 anticline 82 anti-cyclones 375, 388–389 aquiclude 193, 194 aquifers 147, 190, 192 global distribution of 195–196 aquifuge 193, 194 aquitards 193–194 Arabian plate 41 arch 279, 281 Arctic Circle 20 arctic climate 452 arctic front 374 arctic maritime (Am) 373 Arctic Ocean 307 arête 220 aridity 245 artesian basins 147, 192–197

artesian well 147 artificial fertilizers 473, 476 aspect, on temperature 334–335 ‘As per Resolution 5A of IAU’ 12 ASTER GDEM. See Advance Space borne Thermal Emission Global Digital Elevation Model Atlantic Ocean 306 atmosphere 16–18, 30, 132, 326 global water and 133–134 heating of 328–331 structure of 326–327 temperature changes within 338–341 atmospheric circulation, on desert processes 244 atmospheric pressure 352 altitude on 352 global pattern of 355–356 on map 359 measurement of, air pressure 357–358 rotation on 353–355 temperature on 353 atmospheric system 327–331 atoll 313–314 attrition 154, 251, 277

backshore 271 backwash 274 badlands 253–254, 474 bajada 261 bar 288, 290 barchans 257–259 barograph 357, 358, 434, 435 barometer aneroid 357, 358, 434, 435 mercury 357 barrier beach 286, 287 barrier reefs 313 barysphere 30 basalt 59 basins 83, 87, 92 artesian 192–197 of inland drainage 261–262 intermontane 261 rock 221 Batholith 61 bauxite 112 bayhead beach 286 bays 279, 280, 287

beach 285–286 barrier 286 bayhead 286 face 286 raised 295 Beaufort scale 362 bedding plane 47, 62 benches 217 bergschrund 218 berm 286 biosphere 459 biotic weathering 114–115 block disintegration 110 block faulting 87 block mountains 87–91 blow hole 279, 282–283 bluff 161 body waves 75 bore 314 boulder clay 47, 94 deposits 223–224 braided river 166, 167 break point 286 Buldir caldera 63 butte 93

calcification 469 caldera 63–65 Caledonian Mountains 48 Calipso Satellite 440 capillary action 468, 469 capillary fringe 191 carbonation 114 carbon cycle 460–461 carbonic acid 112 Caroline plate 41 cave 279 development stages of 282, 283 CCPI. See Climate Change Performance Index chalk landscape 202 channel 134, 151–153 channel flow 152–153 channel form 151 channel shape 152 chebka 261 chemical weathering 111–114 cinder cones 63, 64 cirque 218–219 cirque glaciers 213, 214 cirrocumulus clouds 404, 405

cirrostratus clouds 404, 405 cirrus clouds 404 cliff 277, 278 forms 279 profiles 277–279 river 161 climate 333, 450 factors affecting 450 of Great Britain 453 types of 451–459 climate change 479–483 Climate Change Performance Index (CCPI) 484, 485 climax vegetation 468 clint 112 clouds 333–334 formation of 406–408 types of 403–406 coastal areas protection 297–299 coastal desert characteristics of 248 coastal dunes 293–294 coastal profile 271 coastline 271 coasts 270 types of 294 Cocos plate 41 cold continental climate 458–459 cold currents 335–338 cold desert characteristics of 248–249 cold front 374 cold ocean current 309 cold winds 371, 373 collision of plates 39 Commentariolus 12 composite cone 64–66 compression heating 330–331 compression theory 92 concave slope 122–124 condensation 330, 401–403 condensational water 190 conduction 329 cone 62–66 conelet 64 confined aquifers 194 connate water 190 consequent river 168 constructive waves 274, 275 contemporary observations 12 continental climates 333, 458 continental-continental convergence 73–74

I.2  Index

continental drift 35–38, 36, 37 continental in nature 41 continentality 457–458 cold continental climate 458–459 cool temperate continental climate 458 continental plate fractures 44 Contour Map 133 contour ploughing 476 convection 330 rain 412 winds 371 convergent plate boundary 40 cool temperate continental climate 458 cool temperate maritime climate 453 coral bleaching 314 coral polyps 311–312 coral reefs 311–314 core 30 corestone 113 corrie 218 cracks 81 crag and tail 221–222 crater 62–65 crest 272, 273 crevasses 215 crop rotation 478 crust 30, 31, 36 volcanic features formed in 61–62 crustal plate 32 crystalline rocks 46 Culbin Sands 293 cumulonimbus clouds 406, 407 cumulus clouds 406, 407 cuspate foreland 291 cut-off 165–166 Cvijiæ Jovan 197, 199 cwm 218 cyclonic rain 413

DALR. See dry adiabatic lapse rate dam 138 Darwin Charles 312–313 deccan lava plateau 68 decomposer 460 deep-focus earthquakes 74 deep water waves 273 deferred junction 166–168 deferred tributary 168 deflation 95, 251 hollows 255, 257 delta 94, 158, 174–177 types of 174–177 dendritic drainage pattern 168 density difference 59

denudation 83, 106–107 deposition adjustment to 157 by glacier 222–224 by river 155–156 by waves 285–293 by wind 257–260 depositional plain 94 depression rain 413 depressions 375 development of 376–377 weather and 377–380 descending winds 369–370 desert action of winds 250–262 dust 95, 257 locations 245–250 desertification 262–265 destructive plate boundary 40 destructive waves 274–275 dew 401 dew point 401 dip 70 dissected plateau 93–94 distributary 174 divergence zones 57 doldrums 363 dome-shaped inselberg 89 downfold 82 drainage basin 134, 138, 169 drainage pattern 168–170 types of 170–172 drizzle 409 drumlins 223 dry adiabatic lapse rate (DALR) 339, 369 dry scrub region 262 dunes 244, 257 coastal 293–294 dwarf planet 12 dyke 62, 191

earth heating of 331–338 important dimensions of 14 shape of 14–16 solar energy budget 18–20 structure of 30–32 surface depends on the nature of the surface 17 system sun as an input into 16–18 earthflow 119–120 earth movements behind landforms 83 physical and organic events 53 Earth-Orbiting Solar and Heliospheric Observatory (SOHO) Satellite 17

earth pillar 126 earthquakes 31, 44, 71 effects of 76–77 nature of 73–76 occurrence of 71–73 East African Rift Valley Zone 57 ecosystem 459 linkages and interactions 460–463 plant adaptation 463 temperature and water on plants 463–464 eddies 257 effluent 135, 177 elbow of capture 173 ELR. See environmental lapse rate emergent coastal plain 95 englacial moraine 215 environmental lapse rate (ELR) 332, 338, 369 epicentre 73, 74, 75 epiphytes 467 equator 14 equatorial climate 451 erg 252–253 erosion 106 adjustment to 157 by glacier 217–222 gully 474–475 karst cycle of 197–199 by river 154–155 sheet 474 soil 473–476 by wave 277–284 by wind 250–251, 255–257 erosional plain 95 erratics 222 escarpment 70, 77, 170 eskers 224 estuary 295 Eurasia plate 41 evaporation 127, 133, 135, 330 evapotranspiration 132, 133, 135 exfoliation 108 exfoliation domes 108, 109 extinct 66 eye (of cyclone) 381

fairly uniform climates arctic climate 452 cool temperate maritime climate 453 equatorial climate 451 hot desert climate 452 faith scarps 77 fault(s) 40, 44, 77–80 in Great Britain 80 normal 77–79

parts of 77 reverse 77–78 scarp 88 tear 77–78 thrust 82 feldspar 113 fetch 272 Fiji plate 41 fiord 297–298 firn 212 fissure 62 fissure eçruptions 66–68 flooding 148–150 floodplain 164–165 flow 119–121 fluid lava cone 64 focus (of earthquake) 72–74 fog 402 advection 402 radiation 402 smog 402, 403 fold 81–83 fold mountains 84–87 fossil fuel 17 freshwater aquifers 195 fringing reefs 313 front arctic 374 cold 374 occluded 377 polar 374 warm 374 frontal rain 413 frost 402 action 109–110

Gangotri Glacier 230, 232 geo 279, 282–283 Geographical Cycle of Davis 115, 116 geoid 14, 15 geological era 54 geomorphic cycles of slope development 115–118 geosyncline 84 geothermal energy 69 geyser 68–69 glacial activity in Great Britain 225 glacial deposition landforms produced by 222–224 Glacial Lake Outburst Floods (GLOFs) 232 glacial lakes of Yorkshire 227 glacial movement 215 glacial processes 216 glacial trough 217–218 glaciated landscapes economic value of 233 examples of 227–232

Index  I.3

glacier(s) 133, 159, 160, 210 cirque 213, 214 classification of 213–214 defined 212 ice 212 piedmont 213, 214 regional distribution of 211–214 table 216 valley 213 global conveyor belt 310 global energy balance 17 Global Precipitation Measurement (GPM) satellite 401 global water and atmosphere 133–134 Gondwanaland 38 gorge 161, 171, 173 grade 157 Grand Canyon 173 granite 51, 113 granite tor 111 granular disintegration 110 gravity field and steady-state Ocean Circulation Explorer (GOCE) 14 gravity map 14, 15 Great Artesian Basin 195 Great Britain distribution and types of rocks of 50 faults in 80 glacial activity in 225 temperature pattern of 342–343 water availability in 146–148 great glen fault of Scotland 80 Great Rift Valley 44, 73, 91–92 greenhouse effect 329 Greenwich Mean Time (GMT) 20, 21 Greenwich Meridian 19–21, 20 grike 112 groundwater sources of 190–192 groynes 297, 298 Gulf Stream 310 gully 261 gully erosion 474–475

habitat 459 hail 409 hamada 253 hanging valley 220 Hawaiian Volcanoes 58 headlands 279, 280, 287 headward erosion 155 heat balance 330, 331 heat budget 330, 331 Helgafell eruption 59

Helgafell volcano 59 hemisphere 18 hermatypic corals 311 high clouds 404 Himalayan glaciers 227, 230 horizons 469 horse latitudes 354 horst 70, 87 hot desert climate 452 hot spring 68 humidity 333–334, 398 absolute 398 measurement of 398–401 relative 398 humus 468 Hurricane David 386–387 hurricanes 381 hydration 112–113 hydraulic action 154, 277 hydrograph 434 hydrological cycle 132–133, 463 hydrolysis 113 hydrosphere 30, 132 hygrometer 398, 434 hygroscopic water 469

ice ages 210 icebergs 212 ice cap 133, 213 ice-dammed lakes 225–227 ice masses in Northern Hemisphere 210 ice sheet 212, 213 igneous rocks 46, 51 Imja Glacier 227, 229 India-Australia-New Zealand plate 41 Indian Ocean 307, 318–319 infiltration 125, 135, 468 input 135, 151, 214 inselberg 87, 257 inshore waves 274 insolation 327, 343 insular climates 333 interaction 84, 106 inter-conversion of rocks 46 inter-glacial period 210 interlocking spurs 161, 162 intermediate deposits 224 intermediate zone 191 internal earth movements 70 International Astronomical Organization (IAU) 12 International Date Line 21–22, 22 Intertropical Convergence Zone (ITCZ) 363, 366 intra-plate oceanic volcanism 58 ionosphere 326 irrigation 142–148

isobars 359 isohyets 415 Isostasy 32–39 isostatic equilibrium 33 isotherm 342

jet streams 374 joints 81 juvenile water 190

kame 224 kaolin 113 karst cycle of erosion 197–199 karst landscape 201–203 Kirkham Gorge 226, 227 knickpoint 172 Krakatoa eruption 66

Lacroix in 1908, 60 lake ice-dammed 225–227 oxbow 165–166 Lambert Glacier 228 laminar flow 152 land breeze 369 landforms 70–84 earth movements behind 83 evolution of 197–199 limestone 199–203 mountains 84–91 plains and related landforms 94–95 plateaus and related landforms 92–94 rift valley 91–92 vulcanicity and 56–69 landslide 76, 121–122, 124 land surfaces 332–333 Lanzarote Island of Canary Islands 63 latent heat 327, 330 lateral erosion 155 lateral moraine 215 laterite 112 latitude 331 line of 18–20 Laurasia 38 lava 30, 46, 62–64, 66 cones 63 plateaus 67, 93 leaching 112, 469 length of day 335 levee 165, 167–168 lightning 414 limb 82 limestone 199 chalk 202 landscape 201–203

pavement 112 stalagmites and stalactites 200–202 surface features 199–200 underground 200, 201 lithosphere 30, 40, 132 lithospheric plate 42 litter 472 load 151, 154–155 local winds 333 convection winds 371 depression winds 371–372 descending winds 369–370 land breeze 369 mountain breeze 370–371 sea breeze 369 valley breeze 370–371 loess 47, 94, 233, 259–260 longitude line of 19–20 long profile 156 longshore current 285 longshore drift 284–285, 288 low clouds 405–406 lowland permafrost classification 234 Lulworth Cove 279–281

magma 30, 46, 57, 62 magmatic water. see juvenile water mantle 30 marine detritus 311 maritime 373 maritime climates 333 Märjelensee 225, 226 marsh reclamation 292 mass wasting 106, 117–124, 118–124 mean daily temperature 340 meanders 161, 163 incised 173 meander terraces 165–166 mechanical weathering 109 medial moraine 216 Mediterranean climate 455–457 mercury barometer 357 meridian 19–22, 20 mesa 93–94 mesosphere 30 metamorphic rocks 47–48, 51 meteoric water 190 meteorologist 438 meteorology 438 Mid-Atlantic Ridge 43, 57, 59 middle clouds 404–405 misfit 173 monsoon 364 monsoon climate 457 monsoon winds 363 pattern Asian region 367–368 Mont Pelée eruption 64

I.4  Index

moon 13, 314–315 phases of 16 moraines 47, 215–216 englacial 215 lateral 215 medial 216 subglacial 215 terminal 216, 223 mountain(s) of accumulation 87 block 70, 87–91 breeze 370–371 fold 70, 84–87 residual 87, 89–91 synclinal 85–86 Mount Etna 58 mudflats 291 mudflow 120–121 mulch 478 multi-purpose reservoirs 139 multiwavelength radiometer 442

National Aeronautics and Space Administration (NASA) 12, 440–441 National Information System for Climate and Environment Studies (NICES) 442 National Remote Sensing Centre (NRSC) 442–443 Nazca plate 41 neap tides 315, 316 nephelometer 442, 443 net balance 214 névé 212 Newton Dale 227, 228 Nicolaus Copernicus 12 nimbostratus clouds 406 nitrogen cycle 462–463 normal and reverse faults 77–78 North Atlantic Drift 335, 336 North Pole 18, 19

oasis 255 ocean currents 308–310, 335–338 oceanic climates 333 oceanic division 307–308 oceanic zones 307–308 ocean ridges 40 oceans 306–307, 311 beneficial influences of 320 distribution of 306 energy from 317 human impact on 320 natural hazards of 317–319 ocean winds 309–310 oktas 408 O3, NOx, analyzer 443

output 135, 151, 214 outwash deposits 222, 224 overflows 225–227 overfold 82 overland flow 125 oxbow lake 165 oxidation 113 oxygen cycle 461 ozone layer 326

Pacific Ocean 306 Pacific plate 40–41 Pacific Ring of Fire 59, 60 Pangea 38 parallel 18–20 Parallel Retreat Theory of King 115 pediment 90–91, 261 peneplain 87, 95 peripediment 261 permafrost 234–237 permeable rock 191, 192 Phanerozoic Period 52 phases of moon 16 Philippine plate 41 photosynthesis 311, 460, 461 phreatic aquifers 194 physical weathering 108–111 piedmont glaciers 213, 214 Pine Island Glacier 230, 231 pipe 62 plain 90, 94–95 coastal 95 depositional 94 erosional 95 flood 94, 158, 161 lava 62 outwash 224 and related landforms 94–95 river 94, 166 planet 12, 13 planetary winds mid-latitude westerly winds 363 monsoon wind pattern Asian region 367–368 polar easterly winds 363 tropical easterly winds 363–366 planktons 311 plant 463 community 464, 468 plate 30–32 boundaries types of 40, 42 collision of 39 continental 39–40 distribution of major and minor 40–42 movement 48–50, 70

on the ocean floor 42–44 oceanic 39 plateaus 70, 90, 92–94 dissected 93 intermontane 92 lava 62, 67, 93 and related landforms 92–94 tectonic 92 plate tectonics 32, 57 global pattern through 48–55 Pleistocene Ice Age 210 plucking 216 plunge pool 159 plunging waves 274 polar continental (Pc) 373 polar deserts characteristics of 250 polar front 374 polar maritime (Pm) 373 polar winds 327 pollutant 178 pollution 177–178, 320 polycyclic landforms 117–118, 118 polyps 311–312 pool 161 position of place on earth’s surface 18–20 pothole 154 Pratt’s concept of isostasy 34 Precambrian Period 52 Precambrian rocks 50 precipitation 133, 398, 408–409 cold and warm air 411–412 types of 409–410 pressure gradient 359 pressure release 109 primary planets in solar system 13 principle of “buoyancy” 35 pyramidal peak 220 pyranometer 442

Qattara Depression 255, 257

radial earth movement 70 radial forces 70 radial pattern 169, 170 radiation 328–329 radiation fog 402 rain 409 types of 412–414 rainfall annual global 423–424 global patterns distribution 418–421 Great Britain 417–418 on map 415–418

measurement of 415 seasonal distribution 421, 422 seasonal rainfall and type of 422 rain gauge 415, 434 rapid 158–161 rarefied air 332 reach 157 graded 157 reclamation 292–293 recycling 473 reef evolution theory 312 refracted waves 275, 276 reg 95, 253, 254 regolith 106, 117, 468 rejuvenation 117–118, 172–173 relative humidity 398 relief/orographic rain 413 reservoir 137–139 residual mountains 87, 89–91 respiration 460 rias 295, 296 Richter Scale 76 riffle 163–164 rift valley 44, 70, 91–92 rills 125, 260 ring-dykes 62 ripples 257 river 150–168 braided 166 capture 173–177 consequent 168–170 deposition 155–156 discharge 137, 149 erosion 154–155 rejuvenated 172 subsequent 170 transport 154–156 river basin drainage an open system 135–138 river Nile schemes 143 river’s discharge 139 river system 150 river valley development of 156–177 Rôche moutonnée 221, 222 rockfall 123–124 rocks 45–48 basin 221 classification and description of 45 igneous 44–46 impermeable 135, 136 metamorphic 47–48 pedestal 255 permeable 135, 136 pervious 135–136 porous 135–136 saturated 135, 136 sedimentary 46–47

Index  I.5

rock system in Great Britain 48–49, 50–51 in India 51–55 rocky desert 253 rotation 20–22 run-off 135, 137, 149

Sahel 262 SALR. See saturated adiabatic lapse rate saltation 154, 250, 257 salt flats 261 salt marshes 291–293 San Andreas fault 79 sand dunes 293, 294 sand grains 293–294 sandy desert 252–253 sapping 216 satellites 13 saturated adiabatic lapse rate (SALR) 339, 369 saturated zone 190 saturation zone of intermittent 136 zone of non-saturation 136 zone of permanent 136 Savanna climate 457 scarp 90 scarpland 51 scouring 217 scree 109–110 sea breeze 369 seafloor spreading 40, 57 sea levels 294–299 seasonal climates Mediterranean climate 455–457 monsoon climate 457 Savanna climate 457 tundra climate 455 sea walls 298, 299 seaward 307 sea waves 273 sebkhas 261 sediment 135, 143 sedimentary rocks 46–47, 51 sedimentation 84 seifs 258–260 seismograph 76 serir 253 shallow- and intermediate-focus earthquakes 74 shape of the earth 14–16 sheet floods 260 sheet flow (sheet wash) 125 sheeting 108 sheet-like intrusive body 61 shelter belts planting 478

shield 48 shifting cultivators 472, 473 shore 271 shoreline 271 shore reefs. see fringing reefs sill 61–62 silt 155, 174 Six’s thermometer 340, 341 sleet 409 slip-off slope 161 slope 115 slope development, geomorphic cycles of 115–118 slope movement types of 119–122, 125–127 slope processes 118–124 slumping 121–122 smog 402, 403 snout 214 snow 409 snow line 211, 212 soil creep 119 fertile 472 formation of 468–469 horizon 470 human impact on 470 productivity 473 profile 469–470 tropical rainforests destruction 472–473 water movement in 469 soil conservation contour ploughing 476 crop rotation 478 shelter belts planting 478 strip cultivation 478 terracing 477 soil erosion 473–474 by water 474–475 by wind 476 soil water 469 solar radiation 16, 327, 332 solar system 12–13 solifluction 120 solution 112, 154–155, 277 South Pole 18 Special Sensor Microwave Imager (SSM/I) 399 Special Sensor Microwave Imager Sounder (SSMIS) 399 spilling waves 274 spillways 227 spit 286, 288, 289 spreading 44 spreading boundary 41 spreading zones 32 spring sapping 155 spring tides 315–316

spur 161 interlocking 161 truncated 217 stack 279, 282, 283 stalactites 200–202 stalagmites 200–202 standard time 22 Stevenson Screen 433–435 stony desert 253, 254 storm surge 318–319 storm tides 318–319 stratified rocks 47 stratocumulus clouds 405 stratosphere 326 stratovolcanoes 65–66 stratus clouds 406 stream order 150 stream system 151–153 striations 221 strike 70 strip cultivation 478 subduction zones in circumpacific belt 59–60 subglacial moraine 215 submarine earthquakes 77 submarine ridges 32 subsistence crops 472 subsurface water 190 subtropical desert, characteristics of 246–248 succession 468 sun 12–13 as input into the earth’s system 16–18 sunshine recorder 434, 435 superimposed drainage 170–172 surface nature of 332 sea distance from 332–333 waves 75 surface features glacial system 215–216 surf one 286 suspension 154 swash 274 swell 272 swell waves 273 syncline 82 system atmospheric 327–331 coastal 270 glacial 214–216 open 135–137 river basin drainage 135–137

talus 65, 109, 124 tangential earth movement 70 tangential forces 70

tarns 219 tectonic plate 40 tectonic plateaus 92 temperature of air 341 distribution of 344–345 factors affecting 331–338 inversion 338, 408 on map 342–343 measurement of 339 pattern of Great Britain 342–343 terminal moraine 216, 223 terrace 165–166, 172 terracing 477 Terra Observation 440 terrestrial planets 12 thermograph 339, 434 thermometer 339, 434 maximum 339 minimum 340 Six’s 340, 341 threshold 296 throughflow 135 thunder 414 thunderstorms 413–414 tidal ranges 315–316 tides moon causes of 314 nature of 314–316 neap 315, 316 spring 315–316 tide-generating forces 315 till 222 tilt block 87–88 time 20–22 local (sun) 20–21 standard 22 zones 22 Tm. See tropical maritime tombolo 288, 290–291 topographic map 118 tornado 385 tors 111 Totten Glacier 228 traction 154 trade winds 327, 363 transform faults 40 transhumance 233 transpiration 133, 135, 462 transport(ation) 106 glacial 222 by river 154–156 by waves 284–285 trellis pattern 168–170 trench 31, 39, 40, 85 tributaries 168 TRMM. See Tropical Rainfall Measuring Mission tropical continental (Tc) 373

I.6  Index

tropical cyclone 381 anatomy of 382 development of 383–384 tornado 385 weather and 384 tropical maritime (Tm) 373 Tropical Rainfall Measuring Mission (TRMM) 400 tropical rainforest 126, 467–468, 472–473 Tropic of Cancer 20 Tropic of Capricorn 20 tropopause 459 troposphere 326, 327 trough 272, 273 tsunami 77, 317–318 tundra climate 455 tundra lands 234 turbulent flow 152 twilight zone 311 Typhoon Dujuan 386 Typhoon Rose 386 typhoons 381

unconfined aquifers 194 underground limestone 200, 201 underground water 190 unloading 108 unsaturated zone 190, 191 upfold 82 uplift 87 U-shaped valley 220

vadose zone 191 valley 134 breeze 370–371 glaciated 221 glaciers 213 hanging 220

river 156–179 trains 224 U-shaped 220 vegetation 125–127 types of 464–468 vent 62 vertical erosion 155 virgin water 190 viscous lava 64 visibility 435–436 volcanic activity distribution of 57–60 other forms of 68 volcanic bombs 67 volcanic eruptions 48 volcanic features formed in crust 61 formed on surface 62 volcanic plug 65, 109–110 volcanoes 57–62 active 66 distribution of 57–60 dormant 59, 66 eruptions of 31, 57–60, 62, 68 types of 62 vortex 381 vulcanicity 60 and landforms 56–69

wadis 261 warm currents 335–337 warm front 374 warm ocean currents 309 warm winds 371, 373 water availability in Great Britain 146–148 balance 137 conservation of 179 demand for 137

in desert region 260–262 ground 133, 139, 140 hygroscopic 469 recycling 179 soil 135, 469 storage of 138–141 surface 133 waterfall 158–162, 220 watershed 133–134, 134, 168 waterspout 385 water supply dynamics of 137–138 water surface 332 water table 136, 190–191 water vapour 326–329, 398 wave(s) action 276 deposition by 285–293 erosion by 277–284 formation 271–276 height 272 nature 272–273 refraction 275 transport by 284–285 types 273–276 wave-built terrace 271 wave-cut platform 277, 278 weather 327, 450 recordings 436–440 weathering 106–107, 250 biotic 114–115 chemical 111–114 physical 108–111 spheroidal 113 types of 107–108 weather station 433–435 well-jointed rocks 191 wells 192–197 westerly winds 327 wet docks 315, 316 wetted perimeter 151–153

willy willies 381 wind 309–310, 333, 352 convection 371 depression 371–372 descending 369–370 direction 360 local 333, 368–372 mid-latitude westerly 363 origin of 359–360 polar 327 polar easterly winds 363 prevailing 363 trade 310, 327, 363 westerly 327 wind-blown deposits 259–260 wind erosion 250–251 features produced 255–257 wind gap 173 wind-generated waves 272, 273 wind rose 363 WindSat Satellite 401 wind vane 360, 361, 434 wind velocity 360 on map 361–363 World’s Time Zones 23

Xinjiang fault 79, 80

yardang 255, 256

zeugen 255, 256 zone of destruction 39 zone of intermittent 136 zone of non-saturation 136 zone of permanent 136 zone of soil moisture 191 zone of subduction 39 zone of transition 374