Earth science [14th ed] 9780321928092, 1292061316, 9781292061313, 0321928091

Citation preview

FOURTEENTH EDITION

GLOBAL EDITION

Earth Science Edward J. Tarbuck Frederick K. Lutgens Illustrated by

Dennis Tasa

Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Munich Paris Montréal Toronto Delhi Mexico City São Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo

Acquisitions Editor: Andrew Dunaway Head of Learning Asset Acquisition, Global Edition: Laura Dent Senior Marketing Manager: Maureen McLaughlin Project Manager: Crissy Dudonis Project Management Team Lead: Gina M. Cheselka Executive Development Editor: Jonathan Cheney Director of Development: Jennifer Hart Content Producer: Timothy Hainley Assistant Acquisitions Editor, Global Edition: Murchana Borthakur Associate Project Editor, Global Edition: Binita Roy Project Manager, Instructor Media: Eddie Lee Editorial Assistant: Sarah Shefveland

Senior Marketing Assistant: Nicola Houston Senior Manufacturing Controller, Production, Global Edition: Trudy Kimber Project Manager, Full Service: Heidi Allgair Photo Manager: Maya Melenchuk Photo Researcher: Kristin Piljay Text Permissions Manager: Alison Bruckner Design Manager: Derek Bacchus Interior Design: Elise Lansdon Design Cover Design: Lumina Datamatics Photo and Illustration Support: International Mapping Operations Specialist: Christy Hall Cover Image: © Wojciech WÃÂ3jcik/123RF

Credits and acknowledgments borrowed from other sources and reproduced, with permission, in this textbook appear on the appropriate page within text or are listed below. Page 29: From J. Bronowski, The Common Sense of Science, p. 148. © 1953 Harvard University Press. Page 32: From L. Pasteur, Lecture, University of Lille (7 December 1854). Page 235: From R.T. Chamberlain, “Some of the Objections to Wegener’s Theory,” In: THEORY OF CONTINENTAL DRIFT: A SYMPOSIUM, University of Chicago Press, pp. 83-87, 1928. Page 284: W. Mooney, USGS Seismologist. Page 369: From J. Hutton, Theory of Earth, 1700; From J. Hutton, Transactions of the Royal Society of Edinburgh, 1788. Page 508: From A.J. Herbertson, “Outlines of Physiography,” 1901. Page 586: Sir Francis Bacon. Page 664: Copernicus, De Revolutionibus, Orbium Coelestium (On the Revolution of the Heavenly Spheres). Page 668: Joseph Louis Lagrange, Oeuvres de Lagrange, 1867.

Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: www.pearsonglobaleditions.com © Pearson Education Limited 2015 The rights of Edward J. Tarbuck, Frederick K. Lutgens, and Dennis Tasa to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. Authorized adaptation from the United States edition, entitled Earth Science, 14th edition, ISBN 978-0-321-92809-2, by Edward J. Tarbuck, Frederick K. Lutgens; illustrated by Dennis Tasa, published by Pearson Education © 2015. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a license permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. ISBN 10: 1-292-06131-6 ISBN 13: 978-1-292-06131-3 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Typeset in Times LT Std by Cenveo® Publisher Services. Printed and bound by CTPS in China.

BRIEF CONTENTS 1

Introduction to Earth Science

UNIT ONE | EARTH MATERIALS 2 3

52

Matter and Minerals

53

Rocks: Materials of the Solid Earth

79

UNIT TWO 4 5 6

23

|

SCULPTING EARTH’S SURFACE

114 115

Running Water and Groundwater

151

Glaciers, Deserts, and Wind

191

UNIT THREE | FORCES WITHIN

228

8 9 10

Plate Tectonics: A Scientific Revolution Unfolds

229

Earthquakes and Earth’s Interior

265

Volcanoes and Other Igneous Activity

297

Crustal Deformation and Mountain Building

337

UNIT FOUR 11 12

|

DECIPHERING EARTH’S HISTORY

13 14 15

16 17 18 19 20

429

Ocean Water and Ocean Life

453

The Dynamic Ocean

473

366

Geologic Time

367

Earth’s Evolution Through Geologic Time

393

DYNAMIC | EARTH’S ATMOSPHERE

24

504

The Atmosphere: Composition, Structure, and Temperature

505

Moisture, Clouds, and Precipitation

537

Air Pressure and Wind

571

Weather Patterns and Severe Storms

597

World Climates and Global Climate Change

627

UNIT SEVEN 21 22 23

428

The Ocean Floor

UNIT SIX

Weathering, Soil, and Mass Wasting

7

UNIT FIVE | THE GLOBAL OCEAN

EARTH’S PLACE IN | THE UNIVERSE

658

Origins of Modern Astronomy

659

Touring Our Solar System

683

Light, Astronomical Observations, and the Sun

715

Beyond Our Solar System

739

3

FIND SMART FIGURES AND MOBILE FIELD TRIP FIGURES In addition to the many informative and colorful illustrations and photos throughout this text, you will find two kinds of special figures that offer additional learning opportunities. These figures contain QR codes which the student can scan with a smart phone to explore exciting expanded online learning materials. Find SmartFigures where you see this icon. Find Mobile Field Trip Figures where you see this icon.

Chapter 1 1.1 1.6 1.8 1.15 1.21

Internal and External Processes (p. 24) Magnitude of Geologic Time (p. 28) Nebular Theory (p. 33) Earth’s Layers (p. 39) The Continents (p. 44)

Chapter 2 2.3 2.12 2.15 2.16 2.18

Most Rocks Are Aggregates of Minerals (p. 55) Color Variations in Minerals (p. 63) Common Crystal Habits (p. 64) Hardness Scales (p. 64) Cleavage Directions Exhibited by Minerals (p. 65)

Chapter 3 3.1 3.4 3.5 3.7 3.14 3.20 3.21 3.25 3.27 3.35

The Rock Cycle (p. 81) Composition of Common Igneous Rocks (p. 84) Igneous Rock Textures (p. 85) Classifications of Igneous Rocks, Based on Their Mineral Composition and Texture (p. 87) Sedimentary Rocks Exposed in Capitol Reef National Park, Utah (p. 91) Bonneville Salt Flats (p. 95) From Plants to Coal (p. 96) Metamorphic Rocks in the Adirondacks, New York. (p. 98) Confining Pressure and Differential Stress (p. 100) Common Oil Traps (p. 108)

Chapter 4 4.1 4.3 4.5 4.6 4.8 4.9 4.11 4.32 4.34

Excavating the Grand Canyon (p. 116) Mechanical Weathering Increases Surface Area (p. 119) Ice Breaks Rock (p. 119) Unloading Leads to Sheeting (p. 120) The Formation of Rounded Boulders (p. 123) Rock Types Influences Weathering (p. 124) Monuments to Weathering (p. 125) Gros Vestre Rockslide (p. 141) Creep (p. 143)

Chapter 5 5.2 The Hydrologic Cycle (p. 153) 5.4 Mississippi River Drainage Basin (p. 154)

4

5.9 5.13 5.20 5.25 5.32 5.33

Channel Changes from Head to Mouth (p. 157) Formation of Cut Banks and Point Bars (p. 162) Incised Meanders (p. 166) Broken Levee (p. 171) Cone of Depression (p. 177) Artesian Systems (p. 177)

Chapter 6 6.4 6.7 6.11 6.12 6.20 6.27 6.30 6.32 6.38 6.39 6.40

Movement of a Glacier (p. 195) Zones of a Glacier (p. 198) Glacial Abrasion (p. 200) Erosional Landforms Created by Alpine Glaciers (p. 201) Common Depositional Landforms (p. 206) Orbital Variations (p. 211) Dry Climates (p. 213) Landscape Evolution in the Basin and Range Region (p. 215) White Sands National Monument (p. 219) Cross Bedding (p. 220) Types of Sand Dunes (p. 221)

Chapter 7 7.2 7.10 7.15 7.17 7.21 7.23 7.31

Reconstructions of Pangaea (p. 231) Rigid Lithosphere Overlies the Weak Asthenosphere (p. 236) Continental Rifting (p. 240) Three Types of Convergent Plate Boundaries (p. 242) Transform Plate Boundaries (p. 245) Movement along the San Andreas Fault (p. 246) Time Scale of Magnetic Reversals (p. 253)

Chapter 8 8.5 8.10 8.23 8.31

Elastic Rebound (p. 268) Body Waves (P and S waves) versus Surface Waves (p. 271) Turnagain Heights Slide (p. 278) Seismic Gaps: Tools for Forecasting Earthquakes (p. 285)

Chapter 9 9.10 9.12 9.20 9.25 9.33 9.34

Anatomy of a Volcano (p. 306) Cinder Cone (p. 310) Super-Eruptions at Yellowstone (p. 318) Sill Exposed in Utah’s Sinbad Country (p. 322) Earth’s Zones of Volcanism (p. 328) Subduction of the Juan Fuca Plate Produced the Cascade Volcanoes (p. 330)

Chapter 10 10.1 10.6 10.7 10.8 10.15 10.16 10.26

Deformed Sedimentary Strata (p. 338) Common Types of Folds (p. 342) Sheep Mountain Wyoming (p. 343) Domes Versus Basins (p. 343) Normal Dip-Slip Fault (p. 346) Normal Faulting in the Basin and Range Province (p. 346) Collision and Accretion of Small Crustal Fragments to a Continental Margin (p. 353)

FIND SMART FIGURES AND MOBILE FIELD TRIP FIGURES

10.29 India’s Continued Northward Migration Severely Deformed Much of China and Southeast Asia (p. 355) 10.30 Formation of the Appalachian Mountains (p. 356) 10.31 The Valley and Ridge Province (p. 357) 10.33 The Effects of Isostatic Adjustment and Erosion on Mountainous Topography (p. 360)

Chapter 11 11.7 11.8 11.13 11.18 11.21

Inclusions (p. 372) Formation of an Angular Unconformity (p. 372) Applying Principles (p. 375) Fossil Assemblage (p. 380) Radioactive Decay Curve (p. 382)

Chapter 12 12.4 Major Events That Led to the Formation of Early Earth (p. 398) 12.10 Growth of Large Continental Masses Through the Collision and Accretion of Smaller Crustal Fragments (p. 403) 12.12 The Major Geologic Provinces of North America and Their Ages in Billions of Years (Ga) (p. 404) 12.16 Connection Between Oceans Circulation and the Climate in Antarctica (p. 406) 12.28 Relationships of Vertebrate Groups and Their Divergence from Lobefin Fish (p. 415)

Chapter 13 13.2 13.6 13.12 13.19

Distribution of Land and Water (p. 431) Satellite Altimeter (p. 433) Active Continental Margins (p. 438) Examples of Hydrogenous Sediment (p. 445)

Chapter 14 14.2 Variations in Surface Temperature and Salinity with Latitude (p. 455) 14.8 Variations in Ocean-Water Density with Depth for Low- and High-Latitude Regions (p. 458) 14.12 Benthos (p. 461) 14.16 Productivity in Temperate Oceans (Northern Hemisphere) (p. 466) 14.19 Ecosystem Energy Flow and Efficiency (p. 468)

Chapter 15 15.2 15.5 15.12 15.16 15.17 15.21 15.31 15.35

Major Surface-Ocean Currents (p. 475) Coastal Upwelling (p. 477) Passage of a Wave (p. 482) Wave Refraction (p. 484) The Longshore Transport System (p. 485) Some Depositional Features (p. 487) East Coast Estuaries (p. 496) Tidal Patterns (p. 498)

Chapter 16 16.5 16.7 16.12 16.16 16.19 16.21 16.24 16.26 16.28

Monthly CO2 Concentrations (p. 509) Antarctic Ozone Hole (p. 510) The Changing Sun Angle (p. 515) Characteristics of the Solstices and Equinoxes (p. 517) The Three Mechanisms of Heat Transfer (p. 520) Paths Taken by Solar Radiation (p. 523) The Greenhouse Effect (p. 524) Isotherms (p. 526) Monthly Mean Temperatures for Vancouver, British Columbia, and Winnipeg Manitoba (p. 527) 16.32 The Daily Cycle of Temperature at Peoria, Illinois, for Two July Days (p. 529) 16.34 World Mean Sea-Level Temperatures in July, in Celsius (°C) and Fahrenheit (°F) (p. 531)

5

Chapter 17 17.2 17.8 17.13 17.17 17.20 17.25 17.30

Changes of State Involve an Exchange of Heat (p. 539) Map Showing Dew-Point Temperatures on a Typical September Day (p. 543) Surface Convergence Enhances Cloud Development (p. 547) Atmospheric Conditions That Result in Absolute Stability (p. 549) Classification of Clouds, Based on Height and Form (p. 552) Map Showing the Average Number of Days per Year with Heavy Fog (p. 557) Formation of Hailstones (p. 561)

Chapter 18 18.2 18.7 18.8 18.17

Inches and Millibars (p. 573) Isobars on a Weather Map (p. 575) Coriolis Effect (p. 576) Idealized Global Circulation Proposed for the Three-Cell Circulation Model of a Rotating Earth (p. 581) 18.19 Sea and Land Breezes (p. 583)

Chapter 19 19.4 19.8 19.11 19.19 19.23

Snowfall Map (p. 600) Cold Front (p. 602) Idealized Structure of a Large, Mature Midlatitude Cyclone (p. 604) Thunderstorm Development (p. 609) The Formation of a Mesocyclone Often Precedes Tornado Formation (p. 611)

Chapter 20 20.6 20.16 20.17 20.28

Tropical Rain Forest (p. 633) Examples of E Climates (p. 641) Highland Climate (p. 642) Slope of the Shoreline (p. 652)

Chapter 21 21.3 Orientation of the Sun’s Rays at Syene (Aswan) and Alexandria, Egypt on June 21 (p. 661) 21.6 Ptolemy’s Explanation of Retrograde Motion (p. 663) 21.15 Using a Telescope, Galileo Discovered That Versus Has Phases Like Earth’s Moon (p. 667) 21.17 Orbital Motion of Earth and Other Planets (p. 669) 21.20 Locating the North Star (Polaris) from the Pointer Stars in the Big Dipper (p. 673) 21.23 Precession of Earth’s Axis (p. 675) 21.25 Phases of the Moon (p. 676) 21.27 Lunar Eclipse (p. 678)

Chapter 22 22.1 22.3 22.7 22.14 22.33

Orbits of the Planets (p. 684) Bodies with Atmospheres Versus Airless Bodies (p. 687) Formation and Filling of Large Impact Basins (p. 690) Olympus Mons (p. 695) Meteor Crater, Near Winslow, Arizona (p. 708)

Chapter 23 23.3 23.6 23.11 23.20

Formation of the Three Types of Spectra (p. 717) The Doppler Effect (p. 720) Reflecting Telescope (p. 722) Diagram of the Sun’s Structure (p. 728)

Chapter 24 24.8 24.10 24.16 24.22

Hertzsprung–Russell Diagram (p. 745) Evolutionary Stages of Stars Having Various Masses (p. 748) Spiral Galaxies (p. 754) Raisin Bread Analogy for an Expanding Universe (p. 757)

CONTENTS

1

Introduction to Earth Science 23

2

FOCUS ON CONCEPTS 23

What Is Earth Science? 24 Geology 24 Oceanography 25 Meteorology 25 Astronomy 25 Earth Science Is Environmental Science 25 Scales of Space and Time in Earth Science 27 The Nature of Scientific Inquiry 29 Hypothesis 29

GEO GRAPHICS |

EYE ON EARTH

31

35

37

Geosphere 38 A Closer Look at the Geosphere 38 Earth’s Internal Structure 38 The Mobile Geosphere 40 The Face of Earth 42 Major Features of the Continents 43 Major Features of the Ocean Basins 45 EYE ON EARTH

30

GEO GRAPHICS |

Gold

EYE ON EARTH

Hydrosphere 36 Atmosphere 36 Biosphere 37 EYE ON EARTH

Matter and Minerals 53

58

Why Atoms Bond 60 The Octet Rule and Chemical Bonds 60 Ionic Bonds: Electrons Transferred 60 Covalent Bonds: Electron Sharing 60 Metallic Bonds: Electrons Free to Move 61

30

Solar System: Size and Scale

EARTH MATERIALS 52

Minerals: Building Blocks of Rock 54 Defining a Mineral 54 What Is a Rock? 55 Atoms: Building Blocks of Minerals 56 Properties of Protons, Neutrons, and Electrons 56 Elements: Defined by Their Number of Protons 56

Early Evolution of Earth 32 Origin of Planet Earth 32 The Inner Planets Form 33 The Outer Planets Develop 34 Earth’s Spheres 34

GEO GRAPHICS |

|

FOCUS ON CONCEPTS 53

World Population Passes 7 Billion

Theory 30 Scientific Methods

UNIT ONE

45

Earth as a System 46 Earth System Science 46 The Earth System 47 Concepts in Review 48 | Give It Some Thought 49 |

62

Properties of a Mineral 62 Optical Properties 62 Crystal Shape, or Habit 63 Mineral Strength 64 Density and Specific Gravity 66 Other Properties of Minerals 66 Mineral Groups 66 Silicate Minerals 67 EYE ON EARTH

67

Important Nonsilicate Minerals 70 Natural Resources 72 Renewable Versus Nonrenewable Resources Mineral Resources 72

GEO GRAPHICS |

Gemstones

72

74

Concepts in Review 75 | Give It Some Thought 76 |

3

Rocks: Materials of the Solid Earth 79

FOCUS ON CONCEPTS 79

Earth as a System: The Rock Cycle 80 The Basic Cycle 80 Alternative Paths 80 Igneous Rocks: “Formed by Fire” 82 From Magma to Crystalline Rock 82 Igneous Compositions 83 What Can Igneous Textures Tell Us? 84 Common Igneous Rocks 86 How Different Igneous Rocks Form 89 EYE ON EARTH

89

Sedimentary Rocks: Compacted and Cemented Sediment Classifying Sedimentary Rocks 92 Lithification of Sediment 95 Features of Sedimentary Rocks 96 Metamorphic Rocks: New Rock from Old 98 What Drives Metamorphism? 98

91

CONTENTS EYE ON EARTH

99

Metamorphic Textures 101 Common Metamorphic Rocks 101 Resources from Rocks and Minerals Metallic Mineral Resources 103

GEO GRAPHICS |

Marble

Rapid Forms of Mass Wasting 140 Slump 140 Rockslide 141 Debris Flow 141 Earthflow 142 Slow Forms of Mass Wasting 142 Creep 142 Solifluction 143 Concepts in Review 144 | Give It Some Thought 147 |

103

104

Nonmetallic Mineral Resources 107 Energy Resources: Fossil Fuels 107 EYE ON EARTH

109

Concepts in Review 110 | Give It Some Thought 112 |

UNIT TWO

4

|

5

FOCUS ON CONCEPTS 151

SCULPTING EARTH’S SURFACE 114

Weathering, Soil, and Mass Wasting 115

FOCUS ON CONCEPTS 115

Earth’s External Processes Weathering 117

GEO GRAPHICS |

116

Some Everyday Examples of Weathering

GEO GRAPHICS |

GEO GRAPHICS |

121

The Old Man of the Mountain

Rates of Weathering 124 Rock Characteristics 124 Climate 124 Differential Weathering 124 Soil 125 An Interface in the Earth System What Is Soil? 126 Soil Texture and Structure 126 Controls of Soil Formation 127 Parent Material 127 Time 127 Climate 128 Plants and Animals 128 Topography 128 The Soil Profile 129 Classifying Soils 130 EYE ON EARTH

125

Controls and Triggers of Mass Wasting 136 The Role of Water 136 Oversteepened Slopes 137 Removal of Vegetation 137 Earthquakes as Triggers 138 Classifying Mass-Wasting Processes 138 Type of Motion 138 Rate of Movement 139 EYE ON EARTH

139

What Are the Largest Rivers?

EYE ON EARTH

167

Natural Levees 168 Alluvial Fans 169 Floods and Flood Control Causes of Floods 169 Flood Control 169

GEO GRAPHICS |

131

Landslides as Natural Disasters

158

Transportation of Sediment 160 Deposition of Sediment 161 Stream Channels 161 Bedrock Channels 162 Alluvial Channels 162 Shaping Stream Valleys 164 Base Level and Stream Erosion 164 Valley Deepening 164 Valley Widening 165 Changing Base Level and Incised Meanders Depositional Landforms 167 Deltas 167

122

Soil Erosion: Losing a Vital Resource 132 Mass Wasting: The Work of Gravity 134 Landslides as Geologic Hazards 134 The Role of Mass Wasting in Landform Development Slopes Change Through Time 134

GEO GRAPHICS |

118

Earth as a System: The Hydrologic Cycle 152 Earth’s Water 152 Water’s Paths 152 Storage in Glaciers 153 Water Balance 153 Running Water 153 Drainage Basins 154 River Systems 154 Drainage Patterns 155 Streamflow 156 Factors Affecting Flow Velocity 156 Changes from Upstream to Downstream 157 The Work of Running Water 158 Stream Erosion 158 EYE ON EARTH

Mechanical Weathering 119 Chemical Weathering 121 EYE ON EARTH

Running Water and Groundwater 151

134

135

169

Flash Floods

170

165

159

7

8

CONTENTS

Groundwater: Water Beneath the Surface The Importance of Groundwater 172 Groundwater’s Geologic Roles 172 Distribution of Groundwater 172 EYE ON EARTH

172

173

Factors Influencing the Storage and Movement of Groundwater Groundwater Movement 175 Springs, Wells, and Artesian Systems 175 Springs 175 Artesian Systems 177 EYE ON EARTH

178

Environmental Problems of Groundwater 179 Treating Groundwater as a Nonrenewable Resource 179 Land Subsidence Caused by Groundwater Withdrawal 179 Groundwater Contamination 180 The Geologic Work of Groundwater 182 Caverns 182 Karst Topography 183 Concepts in Review 185 | Give It Some Thought 187 |

6

Glaciers, Deserts, and Wind 191

FOCUS ON CONCEPTS 191

Antarctica Fact File

EYE ON EARTH

203

Moraines, Outwash Plains, and Kettles 204 Drumlins, Eskers, and Kames 206 Other Effects of Ice Age Glaciers 207 Changing Rivers 207 Crustal Subsidence and Rebound 207 Proglacial Lakes Created by Ice Dams 208 Sea-Level Changes 208 Pluvial Lakes 209 Extent of Ice Age Glaciation 209 Causes of Ice Ages 210

216

EYE ON EARTH

217

221

Concepts in Review 222 | Give It Some Thought 225 |

UNIT THREE

|

FORCES WITHIN 228

Plate Tectonics: A Scientific Revolution Unfolds 229

FOCUS ON CONCEPTS 229

196

Budget of a Glacier: Accumulation Versus Wastage Glacial Erosion 199 How Glaciers Erode 200 Landforms Created by Glacial Erosion 200 Glacial Deposits 203 Types of Glacial Drift 203

EYE ON EARTH

Wind Erosion 217 Deflation, Blowouts, and Desert Pavement Wind Abrasion 218 Wind Deposits 218 Loess 219 Sand Dunes 219 Types of Sand Dunes 220

7

Glaciers and the Earth System 192 Glaciers: A Part of Two Basic Cycles 192 Valley (Alpine) Glaciers 192 Ice Sheets 192 Other Types of Glaciers 194 How Glaciers Move 195 Observing and Measuring Movement 195

GEO GRAPHICS |

174

Plate Tectonics 210 Variations in Earth’s Orbit 211 Other Factors 211 Deserts 212 Distribution and Causes of Dry Lands 213 Geologic Processes in Arid Climates 214 Basin and Range: The Evolution of a Mountainous Desert Landscape 215

198

From Continental Drift to Plate Tectonics 230 Continental Drift: An Idea Before Its Time 231 Evidence: The Continental Jigsaw Puzzle 231 Evidence: Fossils Matching Across the Seas 232 Evidence: Rock Types and Geologic Features 233 Evidence: Ancient Climates 234 The Great Debate 235 Rejection of the Drift Hypothesis 235 The Theory of Plate Tectonics 236 Rigid Lithosphere Overlies Weak Asthenosphere 236 Earth’s Major Plates 237 Plate Boundaries 237 Divergent Plate Boundaries and Seafloor Spreading 238 Oceanic Ridges and Seafloor Spreading 239 Continental Rifting 240 Convergent Plate Boundaries and Subduction 241 Oceanic–Continental Convergence 242 Oceanic–Oceanic Convergence 243 Continental–Continental Convergence 244 Transform Plate Boundaries 245 How Do Plates and Plate Boundaries Change? 247 The Breakup of Pangaea 247 EYE ON EARTH

247

Plate Tectonics in the Future 248 Testing the Plate Tectonics Model 249 Evidence: Ocean Drilling 249 Evidence: Mantle Plumes and Hot Spots 250 Evidence: Paleomagnetism 251 How Is Plate Motion Measured 254 Geologic Evidence for Plate Motion 254 Measuring Plate Motion from Space 256 What Drives Plate Motions? 256 Forces That Drive Plate Motion 256 Models of Plate–Mantle Convection 257 EYE ON EARTH

258

Concepts in Review 259 | Give It Some Thought 262 |

CONTENTS

8

Earthquakes and Earth’s Interior 265

GEO GRAPHICS |

FOCUS ON CONCEPTS 265

What Is an Earthquake? 266 Discovering the Causes of Earthquakes Aftershocks and Foreshocks 268 Faults and Large Earthquakes 269 EYE ON EARTH

269

270

275

278

|

281

Historic Earthquakes East of the Rockies

Can Earthquakes Be Predicted? Short-Range Predictions 284 Long-Range Forecasts 284

GEO GRAPHICS

Seismic Risks on the San Andreas Fault System 286

FOCUS ON CONCEPTS 297

Mount St. Helens Versus Kilauea 298 The Nature of Volcanic Eruptions 299 Factors Affecting Viscosity 299 Quiescent Versus Explosive Eruptions 300 Materials Extruded During an Eruption 301 Lava Flows 301 Gases 303 Pyroclastic Materials 303 Anatomy of a Volcano 304

Shield Volcanoes 306 Mauna Loa: Earth’s Largest Shield Volcano 306 Kilauea, Hawaii: Eruption of a Shield Volcano 307

Cinder Cones

Kilauea’s East Rift Zone Eruption

310

FOCUS ON CONCEPTS 337

Crustal Deformation 338 What Causes Rocks to Deform? 338 Types of Deformation 339 Factors That Affect Rock Strength 340 Folds: Rock Structures Formed by Ductile Deformation Anticlines and Synclines 341 EYE ON EARTH

EYE ON EARTH

of Three Types of Volcanic | Comparison Cones 305

GEO GRAPHICS |

10

Crustal Deformation and Mountain Building 337

341

341

Domes and Basins 342 Monoclines 343 Faults and Joints: Rock Structures Formed by Brittle Deformation 345 Dip-Slip Faults 345 Strike-Slip Faults 346 Joints 347 Mountain Building 349 Subduction and Mountain Building 350 Island Arc–Type Mountain Building 350 Andean-Type Mountain Building 350

Volcanoes and Other Igneous Activity 297

GEO GRAPHICS

282

321

Tabular Intrusive Bodies: Dikes and Sills 322 Massive Intrusive Bodies: Batholiths, Stocks, and Laccoliths Partial Melting and the Origin of Magma 324 Partial Melting 324 Generating Magma from Solid Rock 324 Decrease in Pressure: Decompression Melting 325 Plate Tectonics and Volcanic Activity 326 Volcanism at Convergent Plate Boundaries 326 Volcanism at Divergent Plate Boundaries 327 Intraplate Volcanism 327 Concepts in Review 331 | Give It Some Thought 333 |

284

Earth’s Interior 289 Formation of Earth’s Layered Structure 289 Probing Earth’s Interior: “Seeing” Seismic Waves 289 Earth’s Layers 290 Crust 290 Mantle 291 Core 291 Concepts in Review 292 | Give It Some Thought 294 |

9

319

NATURE ON EARTH

What Is a Tsunami? 279 Earthquake Belts and Plate Boundaries

312

Volcanic Necks and Pipes 320 Intrusive Igneous Activity 321 Nature of Intrusive Bodies 321

Earthquake Destruction 276 Destruction from Seismic Vibrations 276 Landslides and Ground Subsidence 278 Fire 278

GEO GRAPHICS |

Eruption of Mount Vesuvius, AD 79

EYE ON EARTH

Finding the Epicenter of an Earthquake

EYE ON EARTH

310

Volcanic Hazards 313 Pyroclastic Flow: A Deadly Force of Nature 314 Lahars: Mudflows on Active and Inactive Cones 315 Other Volcanic Hazards 315 Other Volcanic Landforms 317 Calderas 317 Fissure Eruptions and Basalt Plateaus 319

267

Seismology: The Study of Earthquake Waves Instruments That Record Earthquakes 270 Seismic Waves 271 Determining the Size of Earthquakes 272 Intensity Scales 272 Magnitude Scales 273

GEO GRAPHICS |

Parícutin: Life of a Garden-Variety Cinder Cone Composite Volcanoes 311

GEO GRAPHICS | 308

351

Sierra Nevada, Coast Ranges, and Great Valley 352 Collisional Mountain Belts 352 Cordilleran-Type Mountain Building 352 Alpine-Type Mountain Building: Continental Collisions 354 The Himalayas 354 The Appalachians 355 What Causes Earth’s Varied Topography? 357

The Laramide Rockies

358

The Principle of Isostasy 360 How High Is Too High? 360 Concepts in Review 361 | Give It Some Thought 363 |

323

9

10

CONTENTS

UNIT FOUR

11

|

DECIPHERING EARTH’S HISTORY 366

FOCUS ON CONCEPTS 393

Is Earth Unique? 394 The Right Planet 394 The Right Location 395 The Right Time 395 Viewing Earth’s History 395 Birth of a Planet 397 From the Big Bang to Heavy Elements 397 From Planetesimals to Protoplanets 397 Earth’s Early Evolution 397 Origin and Evolution of the Atmosphere and Oceans 399 Earth’s Primitive Atmosphere 399 Oxygen in the Atmosphere 399 Evolution of the Oceans 400 Precambrian History: The Formation of Earth’s Continents 401 Earth’s First Continents 401

Geologic Time 367

FOCUS ON CONCEPTS

367

A Brief History of Geology 368 Catastrophism 368 The Birth of Modern Geology 368 Geology Today 369 Creating a Time Scale: Relative Dating Principles The Importance of a Time Scale 369 Numerical and Relative Dates 369 Principle of Superposition 370 Principle of Original Horizontality 370 Principle of Lateral Continuity 371 Principle of Cross-Cutting Relationships 371 EYE ON EARTH

EYE ON EARTH

374

375

is Paleontology Different from | How Archaeology? 377

Conditions Favoring Preservation 378 Correlation of Rock Layers 378 Correlation Within Limited Areas 378 Fossils and Correlation 378 Dating with Radioactivity 380 Reviewing Basic Atomic Structure 381 Radioactivity 381 Half-Life 382 Using Various Isotopes 382 Dating with Carbon-14 383 The Geologic Time Scale 384 Structure of the Time Scale 385 Precambrian Time 385 EYE ON EARTH

EYE ON EARTH

GEO GRAPHICS

|

GEO GRAPHICS |

Evolution of Life Through Geologic Time

EYE ON EARTH

GEO GRAPHICS |

386

387

Did Humans and Dinosaurs Ever Coexist?

388

412

413

Vertebrates Move to Land 414 Reptiles: The First True Terrestrial Vertebrates The Great Permian Extinction 415

385

Terminology and the Geologic Time Scale 386 Determining Numerical Dates for Sedimentary Strata

401

The Making of North America 404 Supercontinents of the Precambrian 404 Geologic History of the Phanerozoic: The Formation of Earth’s Modern Continents 406 Paleozoic History 406 Mesozoic History 407 Cenozoic History 409 Earth’s First Life 410 Origin of Life 410 Earth’s First Life: Prokaryotes 410 Paleozoic Era: Life Explodes 411 Early Paleozoic Life-Forms 411

374

Fossils: Evidence of Past Life Types of Fossils 376

GEO GRAPHICS

369

371

Principle of Inclusions 372 Unconformities 372 Applying Relative Dating Principles EYE ON EARTH

12

Earth’s Evolution Through Geologic Time 393

Demise of the Dinosaurs

414

416

Mesozoic Era: Age of the Dinosaurs 418 Gymnosperms: The Dominant Mesozoic Trees 418 Reptiles: Dominating the Land, Sea, and Sky 418 Cenozoic Era: Age of Mammals 420 From Reptiles to Mammals 420 Marsupial and Placental Mammals 420 Humans: Mammals with Large Brains and Bipedal Locomotion Large Mammals and Extinction 421 Concepts in Review 423 | Give It Some Thought 425 |

421

Concepts in Review 389 | Give It Some Thought 410

UNIT FIVE

13

|

THE GLOBAL OCEAN 428

The Ocean Floor 429

FOCUS ON CONCEPTS 429

The Vast World Ocean 430 Geography of the Oceans 430 Comparing the Oceans to the Continents 431 An Emerging Picture of the Ocean Floor 431 Mapping the Seafloor 431 Provinces of the Ocean Floor 434 Continental Margins 436 Passive Continental Margins 436

CONTENTS EYE ON EARTH

437

EYE ON EARTH

Active Continental Margins 439 Features of Deep-Ocean Basins 439 Deep-Ocean Trenches 439

GEO GRAPHICS |

Explaining Coral Atolls: Darwin’s Hypothesis

Abyssal Plains 442 Volcanic Structures on the Ocean Floor 442 The Oceanic Ridge 443 Anatomy of the Oceanic Ridge 443 Why Is the Oceanic Ridge Elevated? 443 Seafloor Sediments 444 Types of Seafloor Sediments 444 Seafloor Sediment—A Storehouse of Climate Data Resources from the Seafloor 446 Energy Resources 446 Other Resources 447 EYE ON EARTH

445

440

EYE ON EARTH

447

EYE ON EARTH

GEO GRAPHICS |

Ocean Water and Ocean Life 453

FOCUS ON CONCEPTS 453

Composition of Seawater 454 Salinity 454 Sources of Sea Salts 454 Processes Affecting Seawater Salinity 455 Recent Increase in Ocean Acidity 456 Variations in Temperature and Density with Depth Temperature Variations 457 Density Variations 457 EYE ON EARTH

GEO GRAPHICS |

462

464

Ocean Productivity 465 Productivity in Polar Oceans 465 Productivity in Tropical Oceans 465 Productivity in Midlatitude Oceans 466 Oceanic Feeding Relationships 467 Trophic Levels 467 Transfer Efficiency 467 Food Chains and Food Webs 467 Concepts in Review 469 | Give It Some Thought 470 |

15

The Dynamic Ocean 473

FOCUS ON CONCEPTS 473

The Ocean’s Surface Circulation 474 The Pattern of Ocean Currents 474 Upwelling and Deep-Ocean Circulation 477 Coastal Upwelling 477 Deep-Ocean Circulation 477 The Shoreline: A Dynamic Interface 478 The Coastal Zone 479 Basic Features 479 Beaches 480 Ocean Waves 481 Wave Characteristics 481

494

Tides 496 Causes of Tides 496 Monthly Tidal Cycle 497 Tidal Patterns 498 Tidal Currents 498 Concepts in Review 499 | Give It Some Thought 502 |

|

UNIT SIX

16

459

Deep-Sea Hydrothermal Vents

EYE ON EARTH

456

493

A Brief Tour of America’s Coasts

457

Ocean Layering 458 The Diversity of Ocean Life 459 Classification of Marine Organisms Marine Life Zones 461

491

Contrasting America’s Coasts 492 Atlantic and Gulf Coasts 492 Pacific Coast 492 Coastal Classification 493

Concepts in Review 448 | Give It Some Thought 450 |

14

481

Circular Orbital Motion 482 Waves in the Surf Zone 482 The Work of Waves 483 Wave Erosion 483 Sand Movement on the Beach 483 Shoreline Features 486 Erosional Features 486 Depositional Features 486 The Evolving Shore 487 Stabilizing the Shore 488 Hard Stabilization 489 Alternatives to Hard Stabilization 490

EARTH’S DYNAMIC ATMOSPHERE 504

The Atmosphere: Composition, on, Structure, and Temperature 50 505

FOCUS ON CONCEPTS

505

Focus on the Atmosphere 506 Weather in the United States 506 Weather and Climate 506 EYE ON EARTH

507

Composition of the Atmosphere 508 Major Components 508 Carbon Dioxide (CO2) 508 Variable Components 509 Ozone Depletion: A Global Issue 510

GEO GRAPHICS |

Acid Precipitation

511

11

12

CONTENTS

Vertical Structure of the Atmosphere Pressure Changes 512 Temperature Changes 513 Earth–Sun Relationships 514 Earth’s Motions 515 What Causes the Seasons? 515 Earth’s Orientation 516 Solstices and Equinoxes 516 EYE ON EARTH

EYE ON EARTH

519

Energy, Heat, and Temperature 520 Mechanism of Heat Transfer: Conduction EYE ON EARTH

The Weathermaker: Atmospheric Stability 548 Types of Stability 548 Stability and Daily Weather 550 Condensation and Cloud Formation 551 Types of Clouds 552

512

520

520

Mechanism of Heat Transfer: Convection 521 Mechanism of Heat Transfer: Radiation 521 Heating the Atmosphere 522 What Happens to Incoming Solar Radiation? 522 Reflection and Scattering 522 Absorption 523 Heating the Atmosphere: The Greenhouse Effect 524 For the Record: Air Temperature Data 525 Why Temperatures Vary: The Controls of Temperature 526 Land and Water 526 Altitude 528 Geographic Position 528 Cloud Cover and Albedo 528 EYE ON EARTH

529

World Distribution of Temperature 530 Concepts in Review 531 | Give It Some Thought 534 |

17

Moisture, Clouds, and Precipitation 537

EYE ON EARTH

Our Water Supply

562

Rime 564 Measuring Precipitation 564 Measuring Snowfall 564 Precipitation Measurement by Weather Radar 564 Concepts in Review 565 | Give It Some Thought 568 |

18

Air Pressure and Wind 571

Understanding Air Pressure 572 Visualizing Air Pressure 572 Measuring Air Pressure 573 Factors Affecting Wind 574 Pressure Gradient Force 574 Coriolis Effect 575 Friction with Earth’s Surface 576 Highs and Lows 578 Cyclonic and Anticyclonic Winds 578 Weather Generalizations About Highs and Lows General Circulation of the Atmosphere 580 Circulation on a Nonrotating Earth 580 Idealized Global Circulation 580 Influence of Continents 580

538

539

Humidity: Water Vapor in the Air 540 Saturation 540 Mixing Ratio 541 Relative Humidity 541 Dew-Point Temperature 542 Measuring Humidity 543 The Basis of Cloud Formation: Adiabatic Cooling Fog and Dew Versus Cloud Formation 544 Adiabatic Temperature Changes 545 Adiabatic Cooling and Condensation 545 Processes That Lift Air 546 Orographic Lifting 546 Frontal Wedging 546 Convergence 547 Localized Convective Lifting 547

GEO GRAPHICS |

FOCUS ON CONCEPTS 571

FOCUS ON CONCEPTS 537

Water’s Changes of State 538 Ice, Liquid Water, and Water Vapor Latent Heat 538

553

Fog 555 Fogs Caused by Cooling 556 Evaporation Fogs 557 How Precipitation Forms 558 Precipitation from Cold Clouds: The Bergeron Process 558 Precipitation from Warm Clouds: The Collision–Coalescence Process 559 Forms of Precipitation 559 Rain 560 Snow 560 Sleet and Glaze 560 Hail 561

544

EYE ON EARTH

578

580

The Westerlies 582 Local Winds 583 Land and Sea Breezes 583 Mountain and Valley Breezes 583 Chinook and Santa Ana Winds 584 Measuring Wind 585 EYE ON EARTH

585

El Niño and La Niña and the Southern Oscillation Impact of El Niño 586 Impact of La Niña 587

GEO GRAPHICS |

The 1930s Dust Bowl

Southern Oscillation 590 Global Distribution of Precipitation 590 The Influence of Pressure and Wind Belts Other Factors 591 EYE ON EARTH

586

589

590

591

Concepts in Review 592 | Give It Some Thought 594 |

CONTENTS

13

Humid Tropical (A) Climates 632 The Wet Tropics 632 Tropical Wet and Dry 634 Dry (B) Climates 635 Low-Latitude Deserts and Steppes 635 Middle-Latitude Deserts and Steppes 636 EYE ON EARTH

636

Humid Middle-Latitude Climates (C and D Climates) 637 Humid Middle-Latitude Climates with Mild Winters (C Climates) 637 Humid Middle-Latitude Climates with Severe Winters (D Climates) 638 Polar (E) Climates 640 Highland Climates 641 Human Impact on Global Climate 643 Rising CO2 Levels 643 EYE ON EARTH

643

The Atmosphere’s Response 644 The Role of Trace Gases 645

19

Weather Patterns and Severe Storms 597

FOCUS ON CONCEPTS 597

Air Masses 598 What Is an Air Mass? 598 Source Regions 599 Weather Associated with Air Masses EYE ON EARTH

599

600

Fronts 601 Warm Fronts 602 Cold Fronts 602 Stationary Fronts and Occluded Fronts 603 Midlatitude Cyclones 604 Idealized Weather of a Midlatitude Cyclone 604 The Role of Airflow Aloft 606 EYE ON EARTH

21

Hurricane Katrina from Space

617

Hurricane Formation and Decay 618 Hurricane Destruction 618 Tracking Hurricanes 620 Concepts in Review 621 | Give It Some Thought 623 |

20

World Climates and Global Climate Change 627

FOCUS ON CONCEPTS 627

The Climate System 628 World Climates 629 EYE ON EARTH

629

Climate Classification 630 The Köppen Classification 630

|

649

EARTH’S PLACE IN THE UNIVERSE 658

Origins of Modern Astronomyy 659

FOCUS ON CONCEPTS 659

612

646

Climate-Feedback Mechanisms 648 Types of Feedback Mechanisms 648 Computer Models of Climate: Important yet Imperfect Tools How Aerosols Influence Climate 649 Some Possible Consequences of Global Warming 650 Sea-Level Rise 651 The Changing Arctic 652 The Potential for “Surprises” 653 Concepts in Review 653 | Give It Some Thought 656 |

606

Tornado Forecasting 613 Hurricanes 615 Profile of a Hurricane 615

GEO GRAPHICS |

Greenhouse Gas (GHG) Emissions

UNIT SEVEN

Thunderstorms 607 What’s in a Name? 607 Thunderstorm Occurrence 608 Stages of Thunderstorm Development 608 Tornadoes 610 Tornado Occurrence and Development 610 Tornado Destruction and Loss of Life 612 EYE ON EARTH

GEO GRAPHICS |

Ancient Astronomy 660 The Golden Age of Astronomy 660 Ptolemy’s Model 662 The Birth of Modern Astronomy 663 Nicolaus Copernicus 663 Tycho Brahe 664 Johannes Kepler 665 Galileo Galilei 666 Sir Isaac Newton 668 Positions in the Sky 669 Constellations 669

GEO GRAPHICS |

Orion the Hunter

670

The Equatorial System 672 The Motions of Earth 673 Rotation 673 Revolution 674 EYE ON THE UNIVERSE

674

Precession 675 Motions of the Earth–Moon System 675 Lunar Motions 675 Phases of the Moon 677 Eclipses of the Sun and Moon 677 Concepts in Review 679 | Give It Some Thought 680 |

14

CONTENTS

22

Touring Our Solar System 683

FOCUS ON CONCEPTS 683

EYE ON THE UNIVERSE

Our Solar System: An Overview 684 Nebular Theory: Formation of the Solar System 685 The Planets: Internal Structures and Atmospheres 686 Planetary Impacts 687 Earth’s Moon: A Chip Off the Old Block 689 How Did the Moon Form? 689 EYE ON THE UNIVERSE

|

Hubble Space Telescope

Mars Exploration

Beyond Our Solar System 739

FOCUS ON CONCEPTS 739

696

Jovian Planets 699 Jupiter: Lord of the Heavens 699 Saturn: The Elegant Planet 701 Uranus and Neptune: Twins 703 Small Solar System Bodies 705 Asteroids: Leftover Planetesimals 705 Comets: Dirty Snowballs 706 Meteoroids: Visitors to Earth 707 Dwarf Planets 709 Concepts in Review 710 | Give It Some Thought 712 |

The Universe 740 How Large Is It? 740 A Brief History of the Universe 741 Interstellar Matter: Nursery of the Stars 742 Bright Nebulae 742 Dark Nebulae 744 Classifying Stars: Hertzsprung–Russell Diagrams (H-R Diagrams) 744 Stellar Evolution 746 Stellar Birth 746 Protostar Stage 747 Main-Sequence Stage 747 Red Giant Stage 747 EYE ON THE UNIVERSE

23

732

The Source of Solar Energy 734 Concepts in Review 735 | Give It Some Thought 737 |

24

692

692

Venus: The Veiled Planet 693 Mars: The Red Planet 694

GEO GRAPHICS

GEO GRAPHICS |

731

689

Terrestrial Planets 692 Mercury: The Innermost Planet EYE ON THE UNIVERSE

The Active Sun 729 Sunspots 729 Prominences 731 Solar Flares 731

Light, Astronomical Observations, and the Sun 715

FOCUS ON CONCEPTS 715

Signals from Space 716 Nature of Light 716 Light as Evidence of Events and Processes 718 Spectroscopy 718 Continuous Spectrum 718 Dark-Line Spectrum 719 Bright-Line Spectrum 719 The Doppler Effect 719 Collecting Light Using Optical Telescopes 720 Refracting Telescopes 720 Reflecting Telescopes 720 Light Collection 722 Radio- and Space-Based Astronomy 724 Radio Telescopes 724 Orbiting Observatories 725 The Sun 726 Photosphere 727 Chromosphere 728 Corona 728

747

Burnout and Death 748 Stellar Remnants 749 White Dwarfs 749 Neutron Stars 750 Black Holes 750 Galaxies and Galactic Clusters

GEO GRAPHICS |

The Milky Way

751

752

Types of Galaxies 754 Galactic Clusters 755 Galactic Collisions 756 The Big Bang Theory 756 Evidence for an Expanding Universe 756 Predictions of the Big Bang Theory 757 What Is the Fate of the Universe? 757 Concepts in Review 759 | Give It Some Thought 761 |

APPENDIX A

Metric and English Units Compared 763

APPENDIX B

Relative Humidity and Dew-Point Tables 764

APPENDIX C

Stellar Properties 765

GLOSSARY 768 INDEX 781

PREFACE Earth Science, 14th edition, is a college-level text designed for an introductory course in Earth science. It consists of seven units that emphasize broad and up-to-date coverage of basic topics and principles in geology, oceanography, meteorology, and astronomy. The textbook is intended to be a meaningful, nontechnical survey for undergraduate students who have little background in science. Usually these students are taking an Earth science class to meet a portion of their college’s or university’s general requirements. In addition to being informative and up-to-date, Earth Science, 14th edition, strives to meet the need of beginning students for a readable and user-friendly text and a highly usable tool for learning basic Earth science principles and concepts.

r


r


r


NEW TO THIS EDITION r
SmartFigures—art that teaches. Inside every chapter are several SmartFigures. Earth Science, 14th edition, has more than 100 of these figures. Just use your mobile device to scan the Quick Response (QR) code next to a SmartFigure, and the art comes alive. Each 3- to 5-minute feature, prepared and narrated by Professor Callan Bentley, is a mini-lesson that examines and explains the concepts illustrated by the figure. It is truly art that teaches. r
Mobile Field Trips. Scattered through this new edition of Earth Science are thirteen Mobile Field Trips. On each trip, you will accompany geologist–pilot–photographer Michael Collier in the air and on the ground to see and learn about landscapes that relate to discussions in the chapter. These extraordinary field trips are accessed in the same way as SmartFigures. You will scan a QR code that accompanies a figure in the chapter—usually one of Michael’s outstanding photos. r
New and expanded active learning path. Earth Science, 14th edition, is designed for learning. Every chapter begins with Focus on Concepts. Each numbered learning objective corresponds to a major section in the chapter. The statements identify the knowledge and skills students should master by the end of the chapter, helping students prioritize key concepts. Within the chapter, each major section concludes with Concept Checks that allow students to check their understanding and comprehension of important ideas and terms before moving on to the next section. Chapters conclude with sections called Give It Some Thought and Examining the Earth System. The questions and problems in these sections challenge learners by involving them in activities that require higher-order thinking skills such as application, analysis, and synthesis of material in the chapter. The questions and problems in Examining the Earth System are intended to develop an awareness of and appreciation for some of the Earth system’s many interrelationships. r
Concepts in Review. This all-new end-of-chapter feature is an important part of the text’s revised active learning path. Each review is coordinated with the Focus on Concepts at the beginning of the chapter and with the numbered sections within the chapter. It is a readable and concise overview of key ideas, which makes it a valuable review

r


tool for students. Photos, diagrams, and questions also help students focus on important ideas and test their understanding. Eye on Earth. Within every chapter are two or three images, often aerial or satellite views, that challenge students to apply their understanding of basic facts and principles. A brief explanation of each image is followed by questions that help focus students on visual analysis and critical thinking. GEOgraphics. As you turn the pages of each chapter, you will encounter striking visual features that we call GEOgraphics. They are engaging magazine-style “geo-essays” that explore topics that promote greater understanding and add interest to the story each chapter is telling. An unparalleled visual program. In addition to more than 200 new, high-quality photos and satellite images, dozens of figures are new or have been redrawn by renowned geoscience illustrator Dennis Tasa. Maps and diagrams are frequently paired with photographs for greater effectiveness. Further, many new and revised figures have additional labels that narrate the process being illustrated and guide students as they examine the figures. The result is a visual program that is clear and easy to understand. Significant updating and revision of content. A basic function of a college science text book is to provide clear, understandable presentations that are accurate, engaging, and up-to-date. Our number-one goal is to keep Earth Science current, relevant, and highly readable for beginning students. Every part of this text has been examined carefully with this goal in mind. Many discussions, case studies, and examples have been revised. This 14th edition represents perhaps the most extensive and thorough revision in the long history of this textbook.

DISTINGUISHING FEATURES Readability The language of this textbook is straightforward and written to be understood. Clear, readable discussions with a minimum of technical language are the rule. The frequent headings and subheadings help students follow discussions and identify the important ideas presented in each chapter. In this 14th edition, we have continued to improve readability by examining chapter organization and flow and by writing in a more personal style. Significant portions of several chapters have been substantially rewritten in an effort to make the material easier to understand.

Focus on Basic Principles Although many topical issues are treated in this 14th edition of Earth Science, it should be emphasized that the main focus of this new edition remains the same as the focus of each of its predecessors: to promote student understanding of basic Earth science principles. As much as possible, we have attempted to provide the reader with a sense of the observational techniques and reasoning processes that constitute the Earth sciences.

15

16

PREFACE

A Strong Visual Component Earth science is highly visual, and art and photographs play a critical role in an introductory textbook. As in all previous editions, Dennis Tasa, a gifted artist and respected geoscience illustrator, has worked closely with the authors to plan and produce the diagrams, maps, graphs, and sketches that are so basic to student understanding. The result is art that is clearer and easier to understand than ever before. Our aim is to get maximum effectiveness from the visual component of the text. Michael Collier, an award-winning geologist–photographer aided greatly in this quest. As you read through this text, you will see dozens of his extraordinary aerial photographs. His contribution truly helps bring geology alive for the reader.

FOR THE INSTRUCTOR PowerPoints® Two PowerPoint files for each chapter are available at www.pearsonglobaleditions.com/Tarbuck to cut down on your preparation time, no matter what your lecture needs: r
"SU All the line art, tables, and photos from the text have been preloaded into PowerPoint slides for easy integration into your presentations. r
-FDUVSF
PVUMJOF This set averages 35 slides per chapter and includes customizable lecture outlines with supporting art.

Animations and “Images of Earth” The Pearson Prentice Hall Geoscience Animation Library includes more than 100 animations illustrating many difficult-to-visualize topics in Earth science. Created through a unique collaboration among five of Pearson Prentice Hall’s leading geoscience authors, these animations represent a significant step forward in lecture presentation aids. They are preloaded into PowerPoint slides. “Images of Earth” allows you to supplement your personal and textspecific slides with an amazing collection of more than 300 geologic photos contributed by Marli Miller (University of Oregon) and other professionals in the field. The photos are available at www.pearsonglobaleditions.com/Tarbuck.

Instructor’s Manual with Test Bank The Instructor’s Manual contains learning objectives, chapter outlines, answers to end-of-chapter questions, and suggested short demonstrations to spice up your lecture. The Test Bank incorporates art and averages 75 multiplechoice, true/false, short-answer, and critical thinking questions per chapter. These are available at www.pearsonglobaleditions.com/Tarbuck.

TestGen TestGen® (www.pearsoned.com/testgen) enables instructors to build, edit, print, and administer tests using a computerized bank of questions developed to cover all the objectives of the text. TestGen is algorithmically based, allowing instructors to create multiple, but equivalent, versions of the same question or test with the click of a button. Instructors can also modify test bank questions or add new questions. The software and test bank are available for download from Pearson Education’s online catalog.

FOR THE LABORATORY Applications and Investigations in Earth Science, 8th edition, was written by Ed Tarbuck, Fred Lutgens, and Ken Pinzke. This full-color laboratory manual contains 23 exercises that provide students with hands-on experience in geology, oceanography, meteorology, astronomy, and Earth science skills. If you wish to purchase the lab manual, please contact your local Pearson representative for more details.

ACKNOWLEDGMENTS Writing a college textbook requires the talents and cooperation of many people. It is truly a team effort, and the authors are fortunate to be part of an extraordinary team at Pearson Education. In addition to being great people to work with, all are committed to producing the best textbooks possible. Special thanks to our geology editor, Andy Dunaway, who invested a great deal of time, energy, and effort in this project. We appreciate his enthusiasm, hard work, and quest for excellence. We also appreciate our conscientious project manager, Crissy Dudonis, whose job it was to keep track of all that was going on—and a lot was going on. The text’s new design resulted from the creative talents of Derek Bacchus and his team. We think it is a job well done. As always, our marketing manager, Maureen McLaughlin, provided helpful advice and many good ideas. Earth Science, 14th edition, was truly improved with the help of our developmental editor, Jonathan Cheney. Many thanks. The production team was led by Gina Cheselka at Pearson Education and by Heidi Allgair at Cenveo® Publisher Services. It was their job to make this text into a finished product. The talents of copy editor Kitty Wilson, compositor Annamarie Boley, and photo researcher Kristin Piljay were an important part of the production process. We think they all did a great job. They are true professionals, with whom we are very fortunate to be associated. The authors owe a special thanks to three people who were a very important part of this project: r
8PSLJOH
XJUI
%FOOJT
5BTB
XIP
JT
SFTQPOTJCMF
GPS
BMM
PG
UIF
UFYUT
PVUstanding illustrations, is always special for us. He has been a part of our team for more than 30 years. We not only value his artistic talents, hard work, patience, and imagination but his friendship as well. r
"T
ZPV
SFBE
UIJT
UFYU
ZPV
XJMM
TFF
EP[FOT
PG
FYUSBPSEJOBSZ
QIPUPHSBQIT
by Michael Collier, an award-winning geologist, author, and photographer. Most are aerial shots taken from his nearly 60-year-old Cessna 180. Michael was also responsible for preparing the remarkable Mobile Field Trips that are scattered through the text. Among his many awards is the American Geological Institute Award for Outstanding Contribution to the Public Understanding of the Geosciences. We think that Michael’s photographs and field trips are the next best thing to being there. We were very fortunate to have had Michael’s assistance on Earth Science, 14th edition. Thanks, Michael. r
$BMMBO
#FOUMFZ
IBT
CFFO
BO
JNQPSUBOU
BEEJUJPO
UP
UIF
Earth Science team. Callan is an assistant professor of geology at Northern Virginia Community College in Annandale, where he has been honored many times as an outstanding teacher. He is a frequent contributor to Earth magazine and is author of the popular geology blog Mountain Beltway. Callan was responsible for preparing the SmartFigures that appear throughout Earth Science’s 24 chapters. As you take advantage of these outstanding learning aids, you will hear his voice explaining the ideas. Callan also helped with the preparation of the Concepts in

PREFACE

Review feature found at the end of each chapter. We appreciate Callan’s contributions to this new edition of Earth Science. Great thanks also go to our colleagues who prepared in-depth reviews. Their critical comments and thoughtful input helped guide our work and clearly strengthened the text. Special thanks to: Patricia Anderson, California State University—San Marcos J. Bret Bennington, Hofstra University Nahid Brown, Northeastern Illinois University Brett Burkett, Collin College Barry Cameron, University of Wisconsin—Milwaukee Haluk Cetin, Murray State University Natasha Cleveland, Frederick Community College Adam Davis, Vincennes University Anne Egger, Central Washington University Joseph Galewski, The University of New Mexico Leslie Kanat, Johnson State College Mustapha Kane, Florida Gateway College at Lake City, FL Alyson Lighthart, Portland Community College Rob Martin, Florida State College at Jacksonville Ron Metzger, Southwestern Oregon Community College Sadredin (Dean) Moosavi, Rochester Community and Technical College Carol Mueller, Harford Community College

17

Jessica Olney, Hillsborough Community College David Pitts, University of Houston—Clear Lake Steven Schimmrich, SUNY Ulster Xiaoming Zhai, College of Lake County Last, but certainly not least, we gratefully acknowledge the support and encouragement of our wives, Joanne Bannon and Nancy Lutgens. Preparation of Earth Science, 14th edition, would have been far more difficult without their patience and understanding. Ed Tarbuck Fred Lutgens Pearson wishes to thank and acknowledge the following people for their work on the Global Edition:

Contributor Vincent Strak, Monash University

Reviewers Sandeep Sigh, Indian Institute of Technology Roorkee Vikram Vishal, Indian Institute of Technology Roorkee Sagarika Mukhopadhyay, Indian Institute of Technology Roorkee Arun Singh, Indian Institute of Technology Kharagpur

New learning path helps students master the concepts The new edition is designed to support a new four-part learning path, an innovative structure which facilitates active learning and easily allows students to focus on important ideas as they pause to assess their progress at frequent intervals. The chapter-opening Focus on Concepts lists the learning objectives for each chapter. Each section of the chapter is tied to a specific learning objective, providing students with a clear learning path to the chapter content.

UNIT THREE | FORCES WITHIN

7

Plate Tectonics: A Scientific Revolution Unfolds

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

7.1

Discuss the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

7.2

List and explain the evidence Wegener presented to support his continental drift hypothesis.

7.3

Discuss the two main objections to the continental drift hypothesis.

7.4

List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.

7.5

Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

7.6

Compare and contrast the three types of convergent plate boundaries and name a location where each type can be found.

7.7

Describe the relative motion along a transform plate boundary and locate several examples on a plate boundary map.

7.8

Explain why plates such as the African and Antarctic plates are getting larger, while the Pacific plate is getting smaller.

7.9

List and explain the evidence used to support the plate tectonics theory.

7.10

Describe two methods researchers use to measure relative plate motion.

7.11

Summarize what is meant by plate–mantle convection and explain two of the primary driving forces of plate motion.

Climber ascending Chang Zheng Peak near Mount Everest. (Photo by Stock Connection/SuperStock)

Each chapter section concludes with Concept Checks, a feature that lists questions tied to the section’s learning objective, allowing students to monitor their grasp of significant facts and ideas.

7.4 CONCEPT CHECKS 1 What major ocean floor feature did oceanographers discover after World War II?

2 Compare and contrast the lithosphere and the asthenosphere. 3 List the seven largest lithospheric plates. 4 List the three types of plate boundaries and describe the relative motion at each of them.

18

FOCUS ON CONCEPTS

209

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

7.1

Discuss the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

7.2

List and explain the evidence Wegener presented to support his continental drift hypothesis.

7.3

Discuss the two main objections to the continental drift hypothesis.

7.4

List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.

7.5

Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

76

Concepts in Review, a fresh approach to the typical end-of-chapter material, provides students with a structured and highly visual review of the chapter. Key Terms

Section Title

7

CONCEPTS IN REVIEW

Learning Objective

Review Statements

Tectonics: A Scientific | Plate Revolution Unfolds

Consistent with the Focus on Concepts and Concept Checks, the Concepts in Review is structured around the section title and the corresponding learning objective for each section.

7.1 FROM CONTINENTAL DRIFT TO PLATE TECTONICS Discuss the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents. ■

Fifty years ago, most geologists thought that ocean basins were very old and that continents were fixed in place. Those ideas were discarded with a scientific revolution that revitalized geology: the theory of plate tectonics. Supported by multiple kinds of evidence, plate tectonics is the foundation of modern Earth science.

7.2 CONTINENTAL DRIFT: AN IDEA BEFORE ITS TIME

0 0

10 cm 5 inches

7.3 THE GREAT DEBATE

List and explain the evidence Wegener presented to support his continental drift hypothesis.

Discuss the two main objections to the continental drift hypothesis.

KEY TERM S continental drift, supercontinent, Pangaea ■

■

■

■

German meteorologist Alfred Wegener formulated the idea of continental drift in 1917. He suggested that Earth’s continents are not fixed in place but have moved slowly over geologic time. Wegener reconstructed a super-continent called Pangaea that existed about 200 million years ago, during the late Paleozoic and early Mesozoic. Wegener’s evidence that Pangaea existed but later broke into pieces that drifted apart included (1) the shape of the continents, (2) continental fossil organisms that matched across oceans, (3) matching rock types and modern mountain belts, and (4) sedimentary rocks that recorded ancient climates, including glaciers on the southern portion of Pangaea.

GIVE IT SOME THOUGHT Q Today, we know that the early

1. After referring to the section in the Introduction titled “The Nature of Scientific Inquiry,” answer the following:

twentieth-century scientists who

a. rejected What observation led Alfred Wegener to develop his continental drift hypothesis? the idea of continental b. Why did the majority of the scientific community reject the continental drift hypothesis? drift were wrong. Were they therec. Do you think Wegener followed the basic principles of scientific inquiry? Support your answer.

Q Why did Wegener choose organisms such as Glossopteris and Mesosaurus as evidence for continental drift, as opposed to other fossil organisms such as sharks or jellyfish?

Wegener’s hypothesis suffered from two flaws: It proposed tidal forces as the mechanism for the motion of continents, and it implied that the continents would have plowed their way through weaker oceanic crust, like a boat cutting through a thin layer of sea ice. Geologists rejected the idea of continental drift when Wegener proposed it, and it wasn’t resurrected for another 50 years.

Give It Some Thought (GIST) is found at the end of each chapter and consists of questions and problems asking students to analyze, synthesize, and think critically about Earth science. GIST questions relate back to the chapter’s learning objectives, and can easily be assigned using MasteringGeology™

fore bad scientists? Why or why

John Cancalosi/AGE Fotostock ck

7.4 THE THEORY OF PLATE TECTONICS

2. Referring to the accompanying diagrams that illustrate the three types of convergent plate boundaries, complete the not? following: a. b. c. d.

Identify each type of convergent boundary. On what type of crust do volcanic island arcs develop? Why are volcanoes largely absent where two continental blocks collide? Describe two ways that oceanic–oceanic convergent boundaries are different from oceanic–continental boundaries. How are they similar?

List the major differences between Earth’s lithosphere and asthenosphere and explain the he e importance of each in the plate tectonics theory. KEY TERM S ocean ridge system, theory of plate tectonics, lithosphere, asthenosphere, lithospheric plate (plate) tee) ■

■

■

Research conducted during World War II led to new insights that helped revive Wegener’s hypothesis of continental drift. Exploration of the seafloor revealed previously unknown features, including an extremely long mid-ocean ridge system. Sampling off the oceanic crust revealed that it was quite young relative to the continents. The lithosphere is Earth’s outermost rocky layer that is broken into plates. It is relatively stiff and deforms ms by breaking or bending. Beneath the lithosphere How do you explain that theand oldest large-scale oceanic crust found on ust (either3.oceanic is the asthenosphere, a relatively weak layer that deforms by flowing. The lithosphere consists both of crust or continental) underlying Earth dates back to about 180 million years, whereas the oldest contiupper mantle. nental crust dates to about 4.4 billion years? There are seven large plates, another seven intermediate-size plates, and numerous relatively small microplates. Plates meetback along boundaries 4. Refer to the accompanying hypothetical plate map to answer the followr), or transform (moving laterally past that may either be divergent (moving apart from each other), convergent (moving toward each other), ing questions: each other).

6. Imagine that you are studying seafloor spreading along two different oceanic ridges. Using data from a magnetometer, you produced the two accompanying maps. From these maps, what can you determine about the relative rates of seafloor spreading along these two ridges? Explain.

a. How many portions of plates are shown? b. Are continents A, B, and C moving toward or away from each other? How did you determine your answer? c. Explain why active volcanoes are more likely to be found on continents A and B than on continent C. d. Provide at least one scenario in which volcanic activity might be triggered on continent C.

5. Volcanoes, such as the Hawaiian chain, that form over mantle plumes are some of the largest shield volcanoes on Earth. However, several shield volcanoes on Mars are gigantic compared to those on Earth. What does this difference tell us about the role of plate motion in shaping the Martian surface?

7. The velocity of the Pacific plate is about 8 centimeters per year toward the west-northwest. The velocity of both the Nazca and the Cocos plates is about 6 centimeters per year toward the east. Is it globally consistent with the average spreading velocity of the East Pacific Rise (see figure 7.35)? 8. Density is a key component in the behavior of Earth materials and is especially important in understanding key aspects of plate tectonics. Describe three different ways that density and/or density differences play a role in plate tectonics.

19

Dynamic visual program integrates text and technology Carefully selected art and photos aid understanding, add realism, and heighten student interest. NEW! SmartFigures bring key chapter illustrations to life! Found throughout the book, SmartFigures are sophisticated, annotated illustrations that are also narrated videos. The SmartFigure videos are accessible on mobile devices via scannable Quick Response (QR) codes printed in the text and through the Study Area in MasteringGeology. See the Preface for more detailed information on SmartFigures.

286

Bombs

Ve ent nt n Lava

Crater e

Pa Par P ara asi s tic si tic coo e con

Pyroclastic material

Callan Bentley, SmartFigure author, is an assistant professor of geology at Northern Virginia Community College (NOVA) in Annandale, Virginia. Trained as a structural geologist, Callan teaches introductory level geology at NOVA, including field-based and hybrid courses. Callan writes a popular geology blog called Mountain Beltway, contributes cartoons, travel articles, and book reviews to EARTH Magazine, and is a leader in the two-year college geoscience community.

Con ondui duit dui

Mag M ag gma ma chham c cha mbe be r be

SmartFigure 9.10 Anatomy of a Volcano Compare the structure of a “typical” composite cone to that of a shield volcano (Figure 9.11) and a cinder cone (Figure 9.12).

Mobile Field Trips Scattered through this new edition of Earth Science are thirteen video field trips. On each trip, you will accompany geologist-pilot-photographer Michael Collier in the air and on the ground to see and learn about landscapes that relate to discussions in the chapter. These extraordinary field trips are accessed in the same way as SmartFigures. You will scan a QR code that accompanies a figure in the chapter—usually one of Michael’s outstanding photos.

Sill

Mobile Field Trip 9.25 Sill Exposed in Utah’s Sinbad Country The dark, essentially horizontal bands are sills of basaltic composition that intruded horizontal layers of sedimentary rock. (Photo by Michael Collier)

20

As you turn the pages of this book, you will see dozens of extraordinary photographs by Michael Collier. Most are aerial shots taken from his nearly 60-year-old Cessna 180. Michael is an awardwinning geologist, author, and photographer. Michael’s photographs are the next best thing to being there. We were fortunate to have had Michael’s assistance on Earth Science, Fourteenth edition.

GEOGRAPHICS

NEW! GEOgraphics use contemporary, compelling visual representations to illustrate complex concepts, enhancing students’ ability to synthesize and recall information.

Acid Precipitation

A Human Impact on the Earth System As a consequence of burning large quantities of coal and petroleum, tens of millions of tons of sulfur dioxide and nitrogen oxides enter the atmosphere each year. Through a series of complex chemical reactions, these pollutants are converted into acids that eventually fall to Earth’s surface. The map shows precipitation pH values for 2008. 14

4.9

5.0 5.9 4.7

4.8

The pH scale measures the degree of acidity or alkalinity of a solution. Each whole number indicates a tenfold difference. Unpolluted rain has a pH of about 5. In the United States, acid rain is most serious in the Northeast.

4.8

5.7 6.2

4.6 5.2 4.7 5.1

LESS ACIDIC

4.8

4.8

4.4

pH VALUE

5.5

4.7

4.8 4.8 5.3 5.6

4.6

4.9

5.1 4.8

MORE ACIDIC

>5.3 5.2 - 5.3 5.1 - 5.2 5.0 - 5.1 4.9 - 5.0 4.8 - 4.9 4.7 - 4.8 4.6 - 4.7 4.5 - 4.6 4.4 - 4.5 4.3 - 4.4 1/2 wavelength

Velocity decreases (wave height increases)

15.5 The Work of Waves

at a water depth equal to its wave base. Such depths interfere with water movement at the base of the wave and slow its advance (see Figure 15.13, center). As a wave advances toward the shore, the slightly faster waves farther out to sea catch up, decreasing the wavelength. As the speed and length of the wave diminish, the wave steadily grows higher. Finally, a critical point is reached when the wave is too steep to support itself, and the wave front collapses, or breaks (see Figure 15.13, right), causing water to advance up the shore. The turbulent water created by breaking waves is called surf. On the landward margin of the surf zone, the turbulent sheet of water from collapsing breakers, called swash, moves

|

483

up the slope of the beach. When the energy of the swash has been expended, the water flows back down the beach toward the surf zone as backwash.

15.4 CONCEPT CHECKS 1 List three factors that determine the height, length, and period of a wave.

2 Describe the motion of a floating object as a wave passes. 3 How do the speed, length, and height of a wave change as the wave moves into shallow water and breaks?

15.5 THE WORK OF WAVES Describe how waves erode and move sediment along the shore.

During calm weather, wave action is minimal. However, just as streams do most of their work during floods, waves accomplish most of their work during storms. The impact of high, storm-induced waves against the shore can be awesome in its violence (FIGURE 15.14 ).

Wave Erosion Each breaking wave may hurl thousands of tons of water against the land, sometimes causing the ground to literally tremble. The pressures exerted by Atlantic waves in wintertime, for example, average nearly 10,000 kilograms per square meter (more than 2000 pounds per square foot). The force during storms is even greater. It is no wonder that cracks and crevices are quickly opened in cliffs, seawalls, breakwaters, and anything else that is subjected to these enormous shocks. Water is forced into every opening, causing air in the cracks to become highly compressed by the thrust of crashing waves. When the wave subsides, the air expands rapidly, dislodging rock fragments and enlarging and extending fractures. In addition to the erosion caused by wave impact and pressure, abrasion—the sawing and grinding action of the water armed with rock fragments—is also important. In fact, abrasion is probably more intense in the surf zone than in any other environment. Smooth, rounded stones and pebbles along the shore are obvious reminders of the relentless grinding action of rock against rock in the surf zone (FIGURE 15.15A ). Further, the waves use such fragments as “tools” as they cut horizontally into the land (FIGURE 15.15B ).

Sand Movement on the Beach Beaches are sometimes called “rivers of sand.” The reason is that the energy from breaking waves often causes large quantities of sand to move along the beach face and in the surf zone roughly parallel to the shoreline. Wave energy also causes sand to move perpendicular to (toward and away from) the shoreline.

Movement Perpendicular to the Shoreline If you stand ankle deep in water at the beach, you will see that swash and backwash move sand toward and away from the

shoreline. Whether there is a net loss or addition of sand depends on the level of wave activity. When wave activity is relatively light (less energetic waves), much of the swash soaks into the beach, which reduces the backwash. Consequently, the swash dominates and causes a net movement of sand up the beach face toward the berm. When high-energy waves prevail, the beach is saturated from previous waves, so much less of the swash soaks in. As a result, the berm erodes because backwash is strong and causes a net movement of sand down the beach face. Along many beaches, light wave activity is the rule during the summer. Therefore, a wide sand berm gradually develops. During winter, when storms are frequent and more powerful, strong wave activity erodes and narrows the berm. A wide berm that may have taken months to build can be dramatically narrowed in just a few hours by the high-energy waves created by a strong winter storm.

Wave Refraction The bending of waves, called wave refraction, plays an important part in shoreline processes (FIGURE 15.16 ). It affects the distribution of energy along the shore and thus strongly influences where and to what degree erosion, sediment transport, and deposition will take place. Waves seldom approach the shore straight on. Rather, most waves move toward the shore at an angle. However,

FIGURE 15.14 Storm Waves When large waves break against the shore, the force of the water can be powerful, and the erosional work that is accomplished can be great. These storm waves are breaking along the coast of Wales. (The Photolibrary Wales/Alamy)

484

CHAPTER 15

FIGURE 15.15 Abrasion—Sawing and Grinding Breaking waves

The Dynamic Ocean

A.

B.

armed with rock debris can do a great deal of erosional work. (Photo A by Michael Collier; photo B by Fletcher & Baylis/ Science Source)

Smooth, rounded rocks along the shore are an obvious reminder that abrasion can be intense in the surf zone.

This sandstone cliff at Gabriola Island, British Columbia, was undercut by wave action.

when they reach the shallow water of a smoothly sloping bottom, they are bent and tend to become parallel to the shore. Such bending occurs because the part of the wave nearest the shore reaches shallow water and slows first, whereas the end that is still in deep water continues forward at its full speed. The net result is a wave front that may approach nearly parallel to the shore, regardless of the original direction of the wave. Because of refraction, wave impact is concentrated against the sides and ends of headlands that project into the water, As these waves approach nearly straight on, refraction causes the wave energy to be concentrated at headlands (resulting in erosion) and dispersed in bays (resulting in deposition). Beach deposits

whereas wave attack is weakened in bays. This differential wave attack along irregular coastlines is illustrated in Figure 15.16. As the waves reach the shallow water in front of the headland sooner than they do in adjacent bays, they are bent more nearly parallel to the protruding land and strike it from all three sides. By contrast, refraction in the bays causes waves to diverge and expend less energy. In these zones of weakened wave activity, sediments can accumulate and form sandy beaches. Over a long period, erosion of the headlands and deposition in the bays will straighten an irregular shoreline.

Waves travel at original speed in deep water

Waves “feel bottom” and slow down in surf zone

Headland

Shoreline

Result: waves bend so that they strike the shore more directly Wave refraction at Rincon Point, California

SmartFigure 15.16 Wave Refraction As waves first touch bottom in the shallows along an irregular coast, they are slowed, causing them to bend (refract) and align nearly parallel to the shoreline. (Photo by Rich Reid/Getty Images, Inc.)

Path of sand particles

15.5 The Work of Waves

Beach drift

Net movement of sand grains

rrent

currents, however, move in a zigzag pattern, whereas rivers flow mostly in a turbulent, swirling fashion. In addition, the direction of flow of longshore currents along a shoreline can change, whereas rivers flow in the same direction (downhill). Longshore currents change direction because the direction that waves approach the beach changes seasonally. Nevertheless, longshore currents generally flow southward along both the Atlantic and Pacific shores of the United States.

re cu

ho Longs

Beach drift occurs as incoming waves carry sand at an angle up the beach, while the water from spent waves carries it directly down the slope of the beach. Similar movements occur offshore in the surf zone to create the longshore current.

485

Rip Currents Concentrated Longsh

movements of water that flow in the opposite direction of breaking waves are called rip curThese waves approaching the beach at a slight angle near Oceanside, California, produce a rents. (Sometimes rip currents longshore current moving from left to right. are incorrectly called rip tides, although they are unrelated to SSmartFigure 15.17 The Longshore Transport System The two tidal phenomena.) Most of the components of this sediment-moving system, beach drift and longshore backwash from spent waves currents, are created by breaking waves that approach the shoreline at an angle. finds its way back to the open These processes move large quantities of material along the beach and in the ocean as an unconfined flow surf zone. (Photo by University of Washington Libraries, Special Collections, John Shelton Collection, KC14461) across the ocean bottom called sheet flow. However, sometimes Longshore Transport Although waves are refracted, a portion of the returning water moves seaward in the form of most still reach the shore at some angle, however slight. Con- surface rip currents. These currents do not travel far beyond the sequently, the uprush of water from each breaking wave (the surf zone before breaking up and can be recognized by the way swash) is at an oblique angle to the shoreline. However, the they interfere with incoming waves or by the sediment that is backwash is straight down the slope of the beach. The effect of often suspended within the rip current (FIGURE 15.18 ). They this pattern of water movement is to transport sediment in a zig- can be hazardous to swimmers, who, if caught in them, can be zag pattern along the beach face (FIGURE 15.17 ). This move- carried out away from shore. The best strategy for exiting a rip ment is called beach drift, and it can transport sand and pebbles current is to swim parallel to the shore for a few tens of meters. hundreds or even thousands of meters each day. However, a more typical rate is 5 to 10 meters (16 to 33 feet) per day. 15.5 CONCEPT CHECKS Waves that approach the shore at an angle also produce 1 Describe two ways in which waves cause erosion. currents within the surf zone that flow parallel to the shore and 2 Why do waves that are approaching the shoreline often bend? move substantially more sediment than beach drift. Because 3 What is the effect of wave refraction along an irregular coastline? the water here is turbulent, these longshore currents easily move the fine suspended sand and roll larger sand and gravel 4 Describe the two processes that contribute to longshore transport. along the bottom. When the sediment transported by longshore currents is added to the quantity moved by beach drift, the total amount can be very large. At Sandy Rip current extends outward from shore Hook, New Jersey, for example, the and interferes with incoming waves. quantity of sand transported along the shore over a 48-year period averaged almost 750,000 tons annually. For a 10-year period in Oxnard, California, more than 1.5 million tons of sediment moved along the shore each year. Both rivers and coastal zones move water and sediment from one area (upstream) to another (downstream). This is why the beach has often been characterized as a “river of sand.” Beach drift and longshore ore cu

rrent

FIGURE 15.18 Rip Current These concentrated movements of water flow opposite the direction of breaking waves. (Photo by A.P. Trujillo/APT Photos)

486

CHAPTER 15

The Dynamic Ocean

|

15.6 SHORELINE FEATURES Describe the features typically created by wave erosion and those resulting from sediment deposited by longshore transport processes.

A fascinating assortment of shoreline features can be observed along the world’s coastal regions. Although the same processes cause change along every coast, not all coasts respond in the same way. Interactions among different processes and the relative importance of each process depend on local factors. The factors include (1) the proximity of a coast to sediment-laden rivers, (2) the degree of tectonic activity, (3) the topography and composition of the land, (4) prevailing winds and weather patterns, and (5) the configuration of the coastline and nearshore areas. Features that originate primarily because of erosion are called erosional features, whereas accumulations of sediment produce depositional features.

Erosional Features Many coastal landforms owe their origin to erosional processes. Such erosional features are common along the rugged and irregular New England coast and along the steep shorelines of the West coast of the United States.

Wave-Cut Cliffs, Wave-Cut Platforms, and Marine Terraces As the name implies, wave-cut cliffs originate in the cutting action of the surf against the base of coastal land. As erosion progresses, rocks overhanging the notch at the base of the cliff crumble into the surf, and the cliff retreats. A relatively flat, benchlike surface, called a wave-cut platform, is left behind by the receding cliff (FIGURE 15.19, left). The platform broadens as wave attack continues. Some debris produced by the breaking waves may remain along the water’s edge as sediment on the beach, and FIGURE 15.19 Wave-Cut Platform and Marine Terrace This wave-cut platform is exposed at low tide along the California coast at Bolinas Point near San Francisco. A wave-cut platform was uplifted to create the marine terrace. (Photo by University of Washington Libraries, Special Collections, John Shelton Collection, KC5902)

the remainder is transported farther seaward. If a wave-cut platform is uplifted above sea level by tectonic forces, it becomes a marine terrace (Figure 15.19, right). Marine terraces are easily recognized by their gentle seaward-sloping shape and are often desirable sites for coastal roads, buildings, or agriculture.

Sea Arches and Sea Stacks Because of refraction, waves vigorously attack headlands that extend into the sea. The surf erodes the rock selectively, wearing away the softer or more highly fractured rock at the fastest rate. At first, sea caves may form. When caves on opposite sides of a headland unite, a sea arch results (FIGURE 15.20 ). Eventually, the arch falls in, leaving an isolated remnant, or sea stack, on the wave-cut platform (see Figure 15.20). In time, it too will be consumed by the action of the waves.

Depositional Features Sediment eroded from the beach is transported along the shore and deposited in areas where wave energy is low. Such processes produce a variety of depositional features.

Spits, Bars, and Tombolos Where beach drift and longshore currents are active, several features related to the movement of sediment along the shore may develop. A spit (spit 5 spine) is an elongated ridge of sand that projects from the land into the mouth of an adjacent bay. Often the end in the water hooks landward in response to the dominant direction of the longshore current. Both images in FIGURE 15.21 show spits. The term baymouth bar is applied FIGURE 15.20 Sea Arch and Sea Stack These features at the tip of Mexico’s Baja Peninsula resulted from the vigorous wave attack of a headland. (Photo by Lew Robertson/Getty Images)

Sea stack

Wave-cut platform

Marine terrace Sea arch

15.6 Shoreline Features

Baymouth bar

Spit Tidal delta

when turbulent waters in the line of breakers heaped up sand that had been scoured from the bottom. Finally, some barrier islands may be former sand dune ridges that originated along the shore during the last glacial period, when sea level was lower. As the ice sheets melted, sea level rose and flooded the area behind the beach–dune complex.

The Evolving Shore A shoreline continually undergoes modification, regardless of its initial configuration. At first most coastlines are irregular, although the degree of and reason for the irregularity may differ considerably from place to place. Along a coastline that is characterized by varied geology, the pounding surf may initially increase its irregularity because the waves will erode the weaker rocks more easily than the stronger ones. However, if a shoreline remains stable, marine erosion and deposition will eventually produce a straighter, more regular coast. FIGURE 15.23 illustrates the evolution of an initially irregular coast that remains relatively stable and shows many of the coastal features discussed in the previous section. As

Mobile Field Trip 15.21 Some Depositional Features A. High-altitude image of a welldeveloped spit and baymouth Provincetown bar along the coast of Martha’s Vineyard, Massachusetts. (Image B. courtesy of ASCS/USDA) B. This photograph, taken from the International Space Station, shows Provincetown Spit at the tip of Cape Cod. (NASA Earth Observing

FIGURE 15.22 Barrier Islands Nearly 300 barrier islands

Spit

line the Gulf and Atlantic coasts. The islands along the coast of North Carolina are excellent examples. (Photo by Michael Collier)

Albemarle Sound

VIRGINIA NORTH CAROLINA

NC

System)

nd

co mli

Pa

to a sandbar that completely crosses a bay, sealing it off from the open ocean (see Figure 15.21A). Such a feature tends to form across bays where currents are weak, allowing a spit to extend to the other side. A tombolo (tombolo 5 mound), a ridge of sand that connects an island to the mainland or to another island, forms in much the same manner as a spit.

Hatteras Island Cape Lookout

Barrier Islands The Atlantic and Gulf Coastal Plains are relatively flat and slope gently seaward. The shore zone is characterized by barrier islands. These low ridges of sand parallel the coast at distances from 3 to 30 kilometers (2 to 19 miles) offshore. From Cape Cod, Massachusetts, to Padre Island, Texas, nearly 300 barrier islands rim the coast. The chapter-opening photo and FIGURE 15.22 show examples from North Carolina. Most barrier islands are 1 to 5 kilometers (0.6–3 miles) wide and between 15 and 30 kilometers (9–18 miles) long. The tallest features are sand dunes, which usually reach heights of 5 to 10 meters (16–33 feet); in a few areas, unvegetated dunes are more than 30 meters (100 feet) high. The lagoons separating these narrow islands from the shore are zones of relatively quiet water that allow small craft traveling between New York and northern Florida to avoid the rough waters of the North Atlantic. Barrier islands probably formed in several ways. Some originated as spits that were subsequently severed from the mainland by wave erosion or by the general rise in sea level following the last episode of glaciation. Others were created

Sou

Hatteras Island ATLANTIC OCEAN

Pamlico Sound

Road

Dunes

ATLANTIC OCEAN

487

488

CHAPTER 15

The Dynamic Ocean

FIGURE 15.23 The Evolving Shore These

Bay

diagrams illustrate changes that can take place through time along an initially irregular coastline that remains tectonically stable. The diagrams also serve to illustrate many of the features described in the section on shoreline features. (Top and

Sea arch Island

Sea arch

bottom photos by E. J. Tarbuck;

Wave-cut cliff

middle photo by Michael Collier)

Spit Sea stack Tombolo

Tombolo

T I M E

Spit Baymouth bar

Beach deposits

Wave-cut cliff

nt

re curre

Longsho

Spit

Wave-cut platform

headlands are eroded and erosional features such as wave-cut cliffs and wave-cut platforms are created, sediment is produced that is carried along the shore by beach drift and longshore currents. Some material is deposited in the bays, while other debris is formed into depositional features such as spits and baymouth bars. At the same time, rivers fill the bays with sediment. Ultimately, a smooth coast results.

|

15.6 CONCEPT CHECKS 1 How is a marine terrace related to a wave-cut platform? 2 Describe the formation of the features labeled in Figure 15.20 and Figure 15.21.

3 List three ways that barrier islands may form.

15.7 STABILIZING THE SHORE Summarize the ways in which people deal with shoreline erosion problems.

The coastal zone teems with human activity. Unfortunately, people often treat the shoreline as if it were a stable platform on which structures can be built safely. This approach jeopardizes both people and the shoreline because many coastal landforms are relatively fragile, short-lived features that are easily damaged by development. As anyone who has endured a tsunami or a strong coastal storm knows, the shoreline is not always a safe place to live. Figure 15.8 illustrates this point.

Compared with natural hazards, such as earthquakes, volcanic eruptions, and landslides, shoreline erosion appears to be a more continuous and predictable process that causes relatively modest damage to limited areas. In reality, the shoreline is one of Earth’s most dynamic places that changes rapidly in response to natural forces. Storms, for example, are capable of eroding beaches and cliffs at rates that far exceed the long-term average. Such bursts of accelerated erosion not

Jetties interrupt the movement of sand causing deposition on the upcurrent side.

15.7 Stabilizing the Shore

only have a significant impact on the natural evolution of a coast but can also have a profound Erosion by sand-starved currents occurs downcurrent impact on people who reside from these structures. in the coastal zone. Erosion along the coast causes significant property damage. Huge Jetties nt curre sums are spent annually not only to hore s g n Lo repair damage but also in an attempt to prevent or control erosion. Already a problem at many sites, shoreline erosion is certain to become increasingly serious as extensive coastal development continues. During the past 100 years, growing affluence and increasing demands for recreation have brought unprecedented development to many coastal areas. As both the number and the value of buildings have increased, so too have efforts to protect property from storm waves by stabilizing Jetties the shore. Also, controlling the natural migration of sand is an ongoing struggle in many coastal areas. Such interference can result in unwanted changes that are difficult and expensive to becomes sand-starved. As a result, the current erodes sand correct. from the beach on the downstream side of the groin. To offset this effect, property owners downstream from Hard Stabilization the structure may erect a groin on their property. In this manStructures built to protect a coast from erosion or to prevent ner, the number of groins multiplies, resulting in a groin field the movement of sand along a beach are known as hard sta- (FIGURE 15.25 ). An example of such proliferation is the bilization. Hard stabilization can take many forms and often shoreline of New Jersey, where hundreds of these structures results in predictable yet unwanted outcomes. Hard stabiliza- have been built. Because it has been shown that groins often do not provide a satisfactory solution, they are no longer the tion includes jetties, groins, breakwaters, and seawalls. preferred method of keeping beach erosion in check. Jetties Since relatively early in America’s history, a principal goal in coastal areas has been the development and Breakwaters and Seawalls Hard stabilization maintenance of harbors. In many cases, this has involved can also be built parallel to the shoreline. One such structhe construction of jetty systems. Jetties are usually built in ture is a breakwater, the purpose of which is to protect pairs and extend into the ocean at the entrances to rivers and boats from the force of large breaking waves by creating harbors. With the flow of water confined to a narrow zone, a quiet water zone near the shore. However, when this is the ebb and flow caused by the rise and fall of the tides keep done, the reduced wave activity along the shore behind the the sand in motion and prevent deposition in the channel. structure may allow sand to accumulate. If this happens, However, as illustrated in FIGURE 15.24 , a jetty may act as a the boat anchorage will eventually fill with sand, while the dam against which the longshore current and beach drift deposit sand. At the same time, wave activity removes sand on the other side. Because the other side is not receiving any new sand, there is soon no beach at all.

Groins To maintain or widen beaches that are losing sand, groins are sometimes constructed. A groin (groin 5 ground) is a barrier built at a right angle to the beach to trap sand that is moving parallel to the shore. Groins are usually constructed of large rocks but may also be composed of wood. These structures often do their job so effectively that the longshore current beyond the groin

489

FIGURE 15.24 Jetties These structures are built at entrances to rivers and harbors and are intended to prevent deposition in the navigation channel. The photo is an aerial view at Santa Cruz Harbor, California. (Photo by U.S. Army Corps of Engineers, Washington)

FIGURE 15.25 Groins These wall-like structures trap sand that is moving parallel to the shore. This series of groins is along the shoreline near Chichester, Sussex, England. (Photo by Sandy Stockwell/London Aerial Photo Library/Corbis)

490

CHAPTER 15

FIGURE 15.26 Breakwater Aerial view of a breakwater at Santa Monica, California. The structure appears as a line in the water behind which many boats are anchored. The construction of the breakwater disrupted longshore transport and caused the seaward growth of the beach. (Photo by

The Dynamic Ocean

Boat anchorage (quiet water) Breakwater

University of Washington Libraries, Special Collections, John Shelton Collection, KC8275)

Longshore transport

Longshore transport

Disruption of longshore transport causes seaward growth of beach

takes such a toll on the seawall that it will fail and a larger, more expensive structure must be built to take its place. The wisdom of building temporary protective structures along shorelines is increasingly questioned. The opinions of many coastal scientists and engineers is that halting an eroding shoreline with protective structures benefits only a few and seriously degrades or destroys the natural beach and the value it holds for the majority. Protective structures divert the ocean’s energy temporarily from private properties but usually refocus that energy on the adjacent beaches. Many structures interrupt the natural sand flow in coastal currents, robbing affected beaches of vital sand replacement.

Alternatives to Hard Stabilization downstream beach erodes and retreats. At Santa Monica, California, where the building of a breakwater has created such a problem, the city uses a dredge to remove sand from the protected quiet water zone and deposit it farther downstream, where longshore currents continue to move the sand down the coast (FIGURE 15.26 ). Another type of hard stabilization built parallel to the shore is a seawall, which is designed to armor the coast and defend property from the force of breaking waves. Waves expend much of their energy as they move across an open beach. Seawalls cut this process short by reflecting the force of unspent waves seaward. As a consequence, the beach to the seaward side of the seawall experiences significant erosion and may, in some instances, be eliminated entirely (FIGURE 15.27 ). Once the width of the beach is reduced, the seawall is subjected to even greater pounding by the waves. Eventually this battering

Armoring the coast with hard stabilization has several potential drawbacks, including the cost of the structure and the loss of sand on the beach. Alternatives to hard stabilization include beach nourishment and relocation.

Beach Nourishment One approach to stabilizing shoreline sands without hard stabilization is beach nourishment. As the term implies, this practice involves adding large quantities of sand to the beach system (FIGURE 15.28 ). Extending beaches seaward makes buildings along the shoreline less vulnerable to destruction by storm waves and enhances recreational uses. The process of beach nourishment is straightforward. Sand is pumped by dredges from offshore or trucked from inland locations. The “new” beach, however, will not be the same as the former beach. Because replenishment sand is from somewhere else, typically not another beach, it is new to the beach environment. The new sand is usually different

FIGURE 15.27 Seawall Seabright in northern New Jersey once had a broad, sandy beach. A seawall 5 to 6 meters (16 to 18 feet) high and 8 kilometers (5 miles) long was built to protect the town and the railroad that brought tourists to the beach. After the wall was built, the beach narrowed dramatically. (Photo by Rafael Macia/Science Source)

Seawall

15.7 Stabilizing the Shore

in size, shape, sorting, and composition. These differences pose problems in terms of erodibility and the kinds of life the new sand will support. Beach nourishment is not a permanent solution to the problem of shrinking beaches. The same processes that removed the sand in the first place will eventually remove the replacement sand as well. Nevertheless, the number of nourishment projects has increased in recent years, and many beaches, especially along the Atlantic coast, have had their sand replenished many times. Virginia Beach, Virginia, has been nourished more than 50 times. Beach nourishment is costly. For example, a modest project might involve 50,000 cubic yards of sand distributed across a half mile of shoreline. A good-sized dump truck holds about 10 cubic yards of sand. So this small project would require about 5000 dump-truck loads. Many projects extend for many miles. Nourishing beaches typically costs millions of dollars per mile.

Relocation Instead of building structures such as groins and seawalls to hold the beach in place or adding sand to replenish eroding beaches, another option is available. Many coastal scientists and planners are calling for a policy shift from defending and rebuilding beaches and coastal property in high-hazard areas to relocating storm-damaged buildings in those places and letting nature reclaim the beach. This approach is similar to an approach the federal government adopted for river floodplains following the devastating 1993 Mississippi River floods, in which vulnerable structures were abandoned and relocated on higher, safer ground.

Dredge

Such proposals, of course, are controversial. People with significant nearshore investments want to rebuild and defend coastal developments from the erosional wrath of the sea. Others, however, argue that with sea level rising, the impact of coastal storms will get worse in the decades to come, and oft-damaged structures should be abandoned or relocated to improve personal safety and reduce costs. Such ideas will no doubt be the focus of much study and debate as states and communities evaluate and revise coastal land-use policies.

15.7 CONCEPT CHECKS 1 List three examples of hard stabilization and describe what each is intended to do. How does each affect sand distribution on a beach?

2 What are two alternatives to hard stabilization, and what potential problems are associated with each?

EYE ON EY

EARTH E

This structure along the eastern shore of Lake T M Michigan was built at the entrance to Port Shelton, Michigan. to QU E S T I ON 1 What is the purpose of the artificial QUE structure pictured here?

Michael Collier

QU ESTIO N 2 What term is applied to structures such as this? QU ESTIO N 3 Suggest a reason there is a greater accumulation of sand on one side of the structure than the other.

FIGURE 15.28 Beach Nourishment If you visit a beach along the Atlantic coast, it is more and more likely that you will walk into the surf zone atop an artificial beach. In this image, an offshore dredge is pumping sand to a beach. (Photo by Michael Weber/Alamy)

Offshore sand pouring onto the beach

491

492

CHAPTER 15

The Dynamic Ocean

|

15.8 CONTRASTING AMERICA’S COASTS Contrast the erosion problems faced along different parts of America’s coasts. Distinguish between emergent and submergent coasts.

The shoreline along the Pacific coast of the United States is strikingly different from that of the Atlantic and Gulf coast regions. Some of the differences are related to plate tectonics. The West coast represents the leading edge of the North American plate; therefore, it experiences active uplift and deformation. By contrast, the East coast is a tectonically quiet region that is far from any active plate margin. Because of this basic geologic difference, the nature of shoreline erosion problems along America’s opposite coasts is different.

reinforces this point. This process and the dilemma that results have been described as follows: Waves may move sand from the beach to offshore areas or, conversely, into the dunes; they may erode the dunes, depositing sand onto the beach or carrying it out to sea; or they may carry sand from the beach and the dunes into the marshes behind the barrier, a process known as overwash. The common factor is movement. Just as a flexible reed may survive a wind that destroys an oak tree, so the barriers survive hurricanes and nor’easters not through unyielding strength but by giving before the storm. This picture changes when a barrier is developed for homes or a resort. Storm waves that previously rushed harmlessly through gaps between the dunes now encounter buildings and roadways. Moreover, since the dynamic nature of the barriers is readily perceived only during storms, homeowners tend to attribute damage to a particular storm, rather than to the basic mobility of coastal barriers. With their homes or investments at stake, local residents are more likely to seek to hold the sand in place and the waves at bay than to admit that development was improperly placed to begin with.3

Atlantic and Gulf Coasts

FIGURE 15.29 Relocating the Cape Hatteras Lighthouse Despite a number of efforts to protect this 21-story lighthouse, the nation’s tallest, from being destroyed due to a receding shoreline, the structure finally had to be moved. (Photos by USGS National Center and AP Photo/Virginian–Pilot, DREW C. WILSON)

Much of the coastal development along the Atlantic and Gulf coasts has occurred on barrier islands. Typically, a barrier island consists of a wide beach that is backed by dunes and separated from the mainland by marshy lagoons. The broad expanses of sand and exposure to the ocean have made barrier islands exceedingly attractive sites for development. Unfortunately, development has grown more rapidly than has our understanding of barrier island dynamics. Because barrier islands face the open ocean, they receive the full force of major storms that strike the coast. When a storm occurs, the barriers absorb the energy of the waves primarily through the movement of sand. FIGURE 15.29 , which shows changes at Cape Hatteras National Seashore,

Various attempts to protect the lighthouse failed. They included building groins and beach nourishment. By 1999, when this photo was taken, the lighthouse was only 36 meters (120 ft.) from the water.

Former location of lighthouse

88

4

me

ter

s(

29

0

0 To save the famous candy-striped ft. ) landmark, the National Park Service authorized moving the structure. After the $12 million move, it is expected to be safe for 50 years or more.

Pacific Coast In contrast to the broad, gently sloping coastal plains of the Atlantic and Gulf coasts, much of the Pacific coast is characterized by relatively narrow beaches that are backed by steep cliffs and mountain ranges (FIGURE 15.30 ). Recall that America’s western margin is a more rugged and tectonically active region than the eastern margin. Because uplift continues, the rise in sea level in the West is not so readily apparent. Nevertheless, like the shoreline erosion problems facing the East’s barrier islands, West coast difficulties also stem largely from the alteration of natural systems by people. A major problem facing the Pacific shoreline—particularly along southern California—is a significant narrowing of many beaches. The bulk of the sand on many of these beaches is supplied by rivers that transport it from the mountainous regions to the coast. Over the years, this natural flow of material to the coast has been interrupted by dams built for irrigation and flood control. The reservoirs effectively trap the sand that would otherwise nourish the beach environment. When the beaches were wider, 3

Frank Lowenstein, “Beaches or Bedrooms—The Choice as Sea Level Rises,” Oceanus 28 (No. 3, Fall 1985): 22. Copyright © 1985 Woods Hole Oceanographic Institution. Reprinted with permission.

15.8 Contrasting America’s Coasts

they protected the cliffs behind them from the force of storm waves. Now, however, the waves move across the narrowed beaches without losing much energy and cause more rapid erosion of the sea cliffs. Although the retreat of the cliffs provides material to replace some of the sand impounded behind dams, it also endangers homes and roads built on the bluffs. In addition, development atop the cliffs aggravates the problem. Urbanization increases runoff, which, if not carefully controlled, can result in serious bluff erosion. Watering lawns and gardens adds significant quantities of water to the slope. This water percolates downward toward the base of the cliff, where it may emerge in small seeps. This action reduces the slope’s stability and facilitates mass wasting. Shoreline erosion along the Pacific coast varies considerably from one year to the next, largely because of the sporadic occurrence of storms. As a result, when the infrequent but serious episodes of erosion occur, the damage is often blamed on the unusual storms and not on coastal development or the sediment-trapping dams that may be great distances away. If, as predicted, sea level experiences a significant rise in the years to come because of global climate change, increased shoreline erosion and sea-cliff retreat should be expected along many parts of the Pacific coast.

FIGURE 15.30 Pacific Coast Wave refraction along the steep cliffs of the California coast south of Shelter Cove. (Photo by Michael Collier)

Coastal Classification The great variety of shorelines demonstrates their complexity. Indeed, to understand any particular coastal area, many factors must be considered, including rock types, size and direction of waves, frequency of storms, tidal range, and offshore topography. In addition, practically all coastal areas were affected by the worldwide rise in sea level that accompanied the melting of Ice Age glaciers following the Last Glacial Maximum. Finally, tectonic events that uplift or downdrop the land or change the volume of ocean basins must be taken into account. The large number of factors that influence coastal areas makes shoreline classification difficult.

EYE ON EY

EARTH E This is an aerial view of a small portion of a T c coastal area in the United States.

QUE S T I ON 1 Is this an emergent coast or a QU subm submergent coast? QU ESTIO N 2 Provide an easily seen line of evidence to support your answer to Question 1.

Michael Collier

QU ESTIO N 3 Is the location more likely along the coast of North Carolina or California? Explain.

493

Many geologists classify coasts based on changes that have occurred with respect to sea level. This commonly used classification system divides coasts into two general categories: emergent and submergent. Emergent coasts develop either because an area experiences uplift or as a result of a drop in sea level. Conversely, submergent coasts are created when sea level rises or the land adjacent to the sea subsides.

GEOGRAPHICS

494

CHAPTER 15

The Dynamic Ocean

A Brief Tour of s Coasts*

6 2 1

4 3

7 1 A small portion of the Cape Cod coast shows the entrance to Nauset Bay. Depending on the whims of recent storms and the strength of coastal currents, the opening into the bay may only be a few hundred feet wide. Tidal currents have created an underwater sandbar just inside the harbor.

10

9

5 8

11

10 Sea ice hugs Alaska's north slope near Barrow. The Arctic shore is locked in ice for much of the year.

7 This highway clings to the California Coast south of Big Sur. Uplift is occurring in this area near the boundary separating the Pacific and North American plates.

3 North Carolina's Outer Banks. Oregon Inlet Bridge connecting Bodie Island (foreground) and Hatteras Island. These narrow wisps of sand are part of an extensive barrier island system. The beach and dunes on the left face the Atlantic Ocean. On the right are the quieter waters of Pamlico Sound. * Prepared with the assistance of Michael Collier. All photos by Michael Collier.

15.8 Contrasting America’s Coasts

495

Coasts are among Earth’s most dynamic landscapes. Waves, tides, and currents continuously shape this interface between land and sea. Coasts may also exhibit the effects of mountain building, sea level changes, rivers, glaciers, and people. Here is a very small glimpse at the diversity and beauty of America’s coasts.

9 A small glacier flows down a steep mountain slope 5 The delta of the Mississippi River is a major feature in the Gulf of Mexico. This low-lying coastal zone is a maze of low, soggy islands that are barely above sea level with a myriad of natural distributaries and artificial channels.

into the Cook Inlet southwest of Anchorage, Alaska. The total length of Alaska's irregular coastline is nearly 71,000 kilometers (44,000 miles).

6 Waves have aggressively attacked the coast north of La Push Harbor, Washington. In places, remnants of the cliff remain as sea stacks.

4 Mobjack Bay near Gloucester, Virginia, is

2 Spruce urtle urtl ur tlt eand fir cover Turtle Island, part of Maine's Acadia National Park. This region was sculpted by Ice Age glaciers, then flooded when sea level rose as the ice sheets melted.

part of the much larger Chesapeake Bay These bays, called estuaries, are actually river valleys that were drowned when sea level rose at the end of the Ice Age.

.

8 Cliffs and waterfalls along the north coast of Hawaii's Big Island. This portion of the island receives abundant rainfall. There are still active volcanoes elsewhere on the island. Paddrre Island Pa Isllaand Is nd is is a barrier barrrriieer island ba bar isllaanndd is 11 Padre that that th at protects pro rotec tect te cts the the coast th ccooas oas ast of of southern soouuth ther ern rn Texas Texxaas from Te ffrrrom oom m storms. sttoorm rms. s. Winds Win inddss off offf the thhee Gulf GGuul ullff of of Mexico Mexxiiccoo create Me cre reat ate te dunes, dduune ness,, which whhiicchh offer offe of fer a temporary tem te mppoorraarry footing ffoooottin tiinng for foor vegetation vveeggeeta tatitioonn that thhaat is is occasionally occccas asio sio ionnaalllly stripped strriipp st ipppped ed away awa way by by hurricanes. huurrrriiccaane ane ness..

496

CHAPTER 15

SmartFigure 15.31

PENNSYLVANIA

NEW JERSEY

la De

MARYLAND

re wa y Ba

EEastt Coast C Estuaries The lower portions of many river valleys were flooded by the rise in sea level that followed the end of the Quaternary Ice Age, creating large estuaries such as the Chesapeake and Delaware Bays.

The Dynamic Ocean

seven episodes of uplift. The ever-persistent sea is now cutting a new platform at the base of the cliff. If uplift follows, it, too, will become an elevated marine terrace. Other examples of emergent shores include regions that were once buried beneath great ice sheets. When glaciers were present, their weight depressed the crust, and when the ice melted, the crust began gradually to spring back. Consequently, prehistoric shoreline features today are found high above sea level. The Hudson Bay region of Canada is one such area; portions of it are still rising at a rate of more than 1 centimeter (0.4 inch) per year.

DELAWARE

Submergent Coasts In contrast to the preceding

ay Chesapeake B

VIRGINIA

Atlantic Ocean

NORTH CAROLINA

examples, other coastal areas show definite signs of submergence. Shorelines that have been submerged in the relatively recent past are often highly irregular because the sea typically floods the lower reaches of river valleys flowing into the ocean. The ridges separating the valleys, however, remain above sea level and project into the sea as headlands. These drowned river mouths, which are called estuaries, characterize many coasts today. Along the Atlantic coastline, Chesapeake and Delaware Bays are examples of estuaries created by submergence (FIGURE 15.31 ). The picturesque coast of Maine, particularly in the vicinity of Acadia National Park, is another excellent example of an area that was flooded by the postglacial rise in sea level and transformed into a highly irregular submerged coastline. Keep in mind that most coasts have a complicated geologic history. With respect to sea level, many coasts have at various times emerged and then submerged again. Each time, they retain some of the features created during the previous event.

15.8 CONCEPT CHECKS Emergent Coasts In some areas, the coast is clearly emergent because rising land or falling water levels expose wave-cut cliffs and marine terraces above sea level. Excellent examples include portions of coastal California where uplift has occurred in the recent geologic past. The elevated marine terrace in Figure 15.19 illustrates this situation. In the case of the Palos Verdes Hills, south of Los Angeles, California, seven different terrace levels exist, indicating at least

|

1 Briefly describe what happens when storm waves strike an undeveloped barrier island.

2 How might building a dam on a river that flows to the sea affect a beach?

3 What is an observable feature that would lead you to classify a coastal area as emergent?

4 Are estuaries associated with submergent or emergent coasts? Explain.

15.9 TIDES Explain the cause of tides, their monthly cycles, and patterns. Describe the horizontal flow of water that accompanies the rise and fall of tides.

Tides are daily changes in the elevation of the ocean surface. Their rhythmic rise and fall along coastlines have been known since antiquity. Other than waves, they are the easiest ocean movements to observe (FIGURE 15.32 ). Although known for centuries, tides were not explained satisfactorily until Sir Isaac Newton applied the law of gravitation to them. Newton showed that there is a mutual attractive force between two bodies, as between Earth and the Moon. Because both the atmosphere and the ocean are fluids

and are free to move, both are deformed by this force. Hence, ocean tides result from the gravitational attraction exerted upon Earth by the Moon and, to a lesser extent, by the Sun.

Causes of Tides To illustrate how tides are produced, consider an idealized case in which Earth is a rotating sphere covered to a uniform depth with water (FIGURE 15.33 ). Furthermore, ignore the

15.9 Tides

effect of the Sun for now. It is easy to see how the Moon’s High tide gravitational force can cause the water to bulge on the side of Earth nearer the Moon. In addition, however, an equally large tidal bulge is produced on the side of Earth directly opposite the Moon. Both tidal bulges are caused, as Newton discovered, by the pull of gravity. Gravity is inversely proportional to the square of the distance between two objects, meaning simply that it quickly weakens with distance. In this case, Low tide the two objects are the Moon and Earth. Because the force of gravity decreases with disMinas Basin tance, the Moon’s gravitational NEW BRUNSWICK pull on Earth is slightly greater on the near side of Earth than IA MAINE OT SC on the far side. The result of y VA d O N un Tidal flat this differential pulling is to fF AT LAN TIC yo a O CEAN B stretch (elongate) the “solid” Earth very slightly. In contrast, the world ocean, which is mobile, is deformed quite dramatically by this effect, pro- (FIGURE 15.34A ). The combined gravity of these two tideproducing bodies causes larger tidal bulges (higher high ducing the two opposing tidal bulges. Because the position of the Moon changes only moder- tides) and larger tidal troughs (lower low tides), producing ately in a single day, the tidal bulges remain in place while a large tidal range. These are called the spring (springen 5 Earth rotates “through” them. For this reason, if you stand on to rise up) tides, which have no connection with the spring the seashore for 24 hours, Earth will rotate you through alter- season but occur twice a month, during the time when the nating areas of higher and lower water. As you are carried Earth–Moon–Sun system is aligned. Conversely, at about the into each tidal bulge, the tide rises, and as you are carried time of the first and third quarters of the Moon, the gravinto the intervening troughs between the tidal bulges, the tide itational forces of the Moon and Sun act on Earth at right falls. Therefore, most places on Earth experience two high angles, and each partially offsets the influence of the other (FIGURE 15.34B ). As a result, the daily tidal range is less. tides and two low tides each day. In addition, the tidal bulges migrate as the Moon revolves These are called neap (nep 5 scarcely or barely touching) around Earth about every 29 days. As a result, the tides, like tides, and they also occur twice each month. Each month, the time of moonrise, shift about 50 minutes later each day. then, there are two spring tides and two neap tides, each In essence, the tidal bulges exist in fixed positions relative to about 1 week apart. the Moon, which slowly moves progressively eastward as it orbits Earth. After 29 days the cycle is complete, and a new one begins. Lower N Higher high tide Many locations may show an inequality between the high high tide tides during a given day. Depending on the Moon’s position, the tidal bulges may be inclined to the equator, as in Figure 15.33. This figure illustrates that one high tide experienced by an observer in the Northern Hemisphere is considerably higher than the high tide half a day later. In contrast, a Southern Hemisphere observer would experience the opposite effect. To Moon

ion

at Rot

Monthly Tidal Cycle The primary body that influences the tides is the Moon, which makes one complete revolution around Earth every 29 days. The Sun, however, also influences the tides. It is far larger than the Moon, but because it is much farther away, its effect is considerably less. In fact, the Sun’s tide-generating effect is only about 46 percent that of the Moon’s. Near the times of new and full moons, the Sun and Moon are aligned, and their forces are added together

Tidal bulge

Tidal bulge S

497

FIGURE 15.32 Bay of Fundy Tides High tide and low tide on Nova Scotia’s Minas Basin in the Bay of Fundy. Tidal flats are exposed during low tide. (Photos courtesy of left: Ray Coleman/Science Source; right: Jeffrey Greenberg/ Science Source)

FIGURE 15.33 Idealized Tidal Bulges Caused by the Moon If Earth were covered to a uniform depth with water, there would be two tidal bulges: one on the side of Earth facing the Moon (right) and the other on the opposite side of Earth (left). Depending on the Moon’s position, tidal bulges may be inclined relative to Earth’s equator. In this situation, Earth’s rotation causes an observer to experience two unequal high tides in a day.

The Dynamic Ocean

FIGURE 15.34 Spring and Neap Tides Earth–Moon–

of actual tides at a particular place. Many factors—including the shape of the coastline, the configuration of ocean basins, the Coriolis effect, and water depth—greatly influence the tides. Consequently, tides at various locations respond differently to the tide-producing forces. Thus, the nature of the tide at any coastal location can be determined most accurately by actual observation. The predictions in tidal tables and tidal data on nautical charts are based on such observations. Three main tidal patterns exist worldwide. A diurnal (diurnal 5 daily) tidal pattern is characterized by a single high tide and a single low tide each tidal day (FIGURE 15.35 ). Tides of this type occur along the northern shore of the Gulf of Mexico, among other locations. A semidiurnal (semi 5 twice, diurnal 5 daily) tidal pattern exhibits two high tides and two low tides each tidal day, with the two highs about the same height and the two lows about the same height (see Figure 15.35). This type of tidal pattern is common along the Atlantic coast of the United States. A mixed tidal pattern is similar to a semidiurnal pattern except that it is characterized by a large inequality in high water heights, low water heights, or both (see Figure 15.34). In this case, there are usually two high and two low tides each day, with high tides of different heights and low tides of different heights. Such tides are prevalent along the Pacific coast of the United States and in many other parts of the world.

Solar tide

Sun positions influence the tides.

Lunar tide

To Sun

Full moon

New moon

A. Spring Tide When the Moon is in the full or new position, the tidal bulges created by the Sun and Moon are aligned and there is a large tidal range.

First quarter moon Solar tide To Sun

Tidal Currents

Lunar tide Third quarter moon B. Neap Tide When the Moon is in the first-or third-quarter position, the tidal bulges produced by the Moon are at right angles to the bulges created by the Sun and the tidal range is smaller.

Tidal Patterns The basic causes and types of tides have been explained. Keep in mind, however, that these theoretical considerations cannot be used to predict either the height or the time

Tidal current is the term used to describe the horizontal flow of water accompanying the rise and fall of the tides. These water movements induced by tidal forces can be important in some coastal areas. Tidal currents that advance into the coastal zone as the tide rises are called flood currents. As the tide falls, seaward-moving water generates ebb currents. Periods of little or no current, called slack water, separate flood and ebb. The areas affected by these alternating tidal currents are called tidal flats (see Figure 15.32). Depending on the nature of the coastal zone, tidal flats vary Two high tides and two low tides of approximately equal heights during each tidal day

SmartFigure 15.35 tidal pattern (lower right) shows one high tide and one low tide each tidal day. A semidiurnal pattern (upper right) shows two high tides and two low tides of approximately equal heights during each tidal day. A mixed tidal pattern (left) shows two high tides and two low tides of unequal heights during each tidal day.

Two high tides and two low tides of unequal heights during each tidal day

NORTH AMERICA

1.5

7.0

0.5 0

2.8 2.2

–0.5

Gulf of California 1.2

–1.0 0

12

24 Hours

12

MIXED TIDAL PATTERN

1.0

2.0

0.5

5.5

2.5

1.0 Height (m)

Tidal Tid l Patterns A diurnal

24 PACIFIC OCEAN

Diurnal Mixed

0.7

Bay of 2.0 Fundy

0.7

1.5 2.0 ATLANTIC OCEAN

Gulf of Mexico

0.7

0.6 0.5 0.5

0

15.0

0.9

3.7 5.7

2.7

–0.5 0

12

24 Hours

12

Height (m)

CHAPTER 15

–1.0 24

SEMIDIURNAL TIDAL PATTERN One high tide and one low tide each tidal day 1.0

SOUTH AMERICA

0.5 0

Semidiurnal

–0.5

2.7 Spring tidal range (m) 0

12

24 Hours

12

–1.0 24

DIURNAL TIDAL PATTERN

Height (m)

498

Concepts in Review

from narrow strips seaward of the beach to zones that may extend for several kilometers. Although tidal currents are not important in the open sea, they can be rapid in bays, river estuaries, straits, and other narrow places. Off the coast of Brittany in France, for example, tidal currents that accompany a high tide of 12 meters (40 feet) may attain a speed of 20 kilometers (12 miles) per hour. Tidal currents are not generally considered to be major agents of erosion and sediment transport, but notable exceptions occur where tides move through narrow inlets. Here they scour the narrow entrances to many harbors that would otherwise be blocked. Sometimes deposits called tidal deltas are created by tidal currents (FIGURE 15.36 ). They may develop either as flood deltas landward of an inlet or as ebb deltas on the seaward side of an inlet. Because wave activity and longshore currents are reduced on the sheltered landward side, flood deltas are more common and are actually more prominent (see Figure 15.21A). They form after the tidal current moves rapidly through an inlet. As the current emerges into more open waters from the narrow passage, it slows and deposits its load of sediment.

15

CONCEPTS IN REVIEW

Tidal flats

Because this tidal delta has developed on the landward side of the inlet, it is called a flood delta.

Barrier island

tidal current (flood current) moves through a barrier island’s inlet into the quiet waters of the lagoon, the current slows and deposits sediment, creating a tidal delta. Because this tidal delta has developed on the landward side of the inlet, it is called a flood delta. Such a tidal delta is shown in Figure 15.21A.

15.9 CONCEPT CHECKS 1 Explain why an observer can experience two unequal high tides during one day.

2 Distinguish between neap tides and spring tides. 3 How is a mixed tidal pattern different from a semidiurnal tidal pattern?

4 Contrast flood current and ebb current.

| The Dynamic Ocean

Discuss the factors that create and influence ocean currents and describe the affect ocean currents have on climate.

15.2 UPWELLING AND DEEP-OCEAN CIRCULATION Explain the processes that produce coastal upwelling and the ocean’s deep circulation.

KEY TERMS: gyre, Coriolis effect

■

FIGURE 15.36 Tidal Deltas As a rapidly moving

Lagoon

15.1 THE OCEAN’S SURFACE CIRCULATION

■

499

The ocean’s surface currents follow the general pattern of the world’s major wind belts. Surface currents are parts of huge, slowly moving loops of water called gyres that are centered in the subtropics of each ocean basin. The positions of the continents and the Coriolis effect also influence the movement of ocean water within gyres. Because of the Coriolis effect, subtropical gyres move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Generally, four main currents comprise each subtropical gyre. Ocean currents can have a significant effect on climate. Poleward-moving warm ocean currents moderate winter temperatures in the middle latitudes. Cold currents exert their greatest influence during summer in middle latitudes and yearround in the tropics. In addition to cooler temperatures, cold curB rents are associated with greater fog frequency and drought. A

Q Assume that arrow A represents prevailing winds in a Northern Hemisphere ocean. Which arrow, A, B, or C, best represents the surface-ocean current in this region? Explain.

C

K E Y T E R M S : upwelling, thermohaline circulation ■

■

Upwelling, the rising of colder water from deeper layers, is a wind-induced movement that brings cold, nutrient-rich water to the surface. Coastal upwelling is most characteristic along the west coasts of continents. In contrast to surface currents, deep-ocean circulation is governed by gravity and driven by density differences. The two factors that are most significant in creating a dense mass of water are temperature and salinity, so the movement of deep-ocean water is often termed thermohaline circulation. Most water involved in thermohaline circulation begins in high latitudes at the surface, when the salinity of the cold water increases as a result of sea ice formation. This dense water sinks, initiating deep-ocean currents.

500

Concepts in Review

15.3 THE SHORELINE: A DYNAMIC INTERFACE Explain why the shoreline is considered a dynamic interface and identify the basic parts of the coastal zone. K EY TERMS: shoreline, shore, coast, coastline, foreshore, backshore, nearshore zone, offshore

zone, beach, berm, beach face ■

■

The shore is the area extending between the lowest tide level and the highest elevation on land that is affected by storm waves. The coast extends inland from the shore as far as oceanrelated features can be found. The shore is divided into the foreshore and backshore. Seaward of the foreshore are the nearshore and offshore zones. A beach is an accumulation of sediment found along the landward margin of the ocean or a lake. Among its parts are one or more berms and the beach face. Beaches are composed of whatever material is locally abundant and should be thought of as material in transit along the shore.

Q Assume that you are the photographer who took this photo and that the photo was taken at high tide. On which part of the shore are you standing? Shutterstock

15.4 OCEAN WAVES List and discuss the factors that influence the height, length, and period of a wave and describe the motion of water within a wave. K EY TERMS: wave height, wavelength, wave period, fetch, circular orbital motion, wave base, surf ■

■

Waves are moving energy, and most ocean waves are initiated by wind. The three factors that influence the height, wavelength, and period of a wave are (1) wind speed, (2) length of time the wind has blown, and (3) fetch, the distance that the wind has traveled across open water. Once waves leave a storm area, they are termed swells, which are symmetrical, longer-wavelength waves. As waves travel, water particles transmit energy by circular orbital motion, which extends to a depth equal to one-half the wavelength (the wave base). When a wave enters water that is shallower than the wave base, it slows down, which allows waves farther from shore to catch up. As a result, wavelength decreases and wave height increases. Eventually the wave breaks, creating turbulent surf in which water rushes toward the shore.

15.5 SHORELINE PROCESSES Describe how waves erode and move sediment along the shore. K EY TERMS: abrasion, wave refraction, beach drift, longshore current, rip current

■

■

Wind-generated waves provide most of the energy that modifies shorelines. Each time a wave hits, it can impart tremendous force. The impact of waves, coupled with abrasion from the grinding action of rock particles, erodes material exposed along the shoreline. Wave refraction is a consequence of a wave encountering shallower water as it approaches shore. The shallowest part of the wave (closest to shore) slows the most, allowing the faster part (still in deeper water) to catch up. This modifies a wave’s trajectory so that the wave front becomes almost parallel to the shore by the time it hits. Wave refraction concentrates impacting energy on headlands and dissipates that energy in bays, which become sites of sediment accumulation. Beach drift describes the movement of sediment in a zigzag pattern along a beach face. The swash of incoming waves pushes the sediment up the beach at an oblique angle, but the backwash transports it directly downhill. Net movement along the beach can be many meters per day. Longshore currents are a similar phenomenon in the surf zone, capable of transporting very large quantities of sediment parallel to a shoreline.

Q What process is causing wave energy to be concentrated on the headland? Predict how this area will appear in the future.

Less energy = deposition Wa ve

pa th

More energy = erosion

Wave front

■

Michael Collier

501

Concepts in Review

15.6 SHORELINE FEATURES

B.

C.

D.

A.

Describe the features typically created by wave erosion and those resulting from sediment deposited by longshore transport processes. K EY TER MS: wave-cut cliff, wave-cut platform, marine terrace, sea arch, sea stack, spit,

baymouth bar, tombolo, barrier island ■

■

Erosional features include wave-cut cliffs (which originate from the cutting action of the surf against the base of coastal land), wave-cut platforms (relatively flat, bench-like surfaces left behind by receding cliffs), and marine terraces (uplifted wave-cut platforms). Erosional features also include sea arches (formed when a headland is eroded and two sea caves from opposite sides unite) and sea stacks (formed when the roof of a sea arch collapses). Some of the depositional features that form when sediment is moved by beach drift and longshore currents are spits (elongated ridges of sand that project from the land into the mouth of an adjacent bay), baymouth bars (sandbars that completely cross a bay), and tombolos (ridges of sand that connect an island to the mainland or to another island). Along the Atlantic and Gulf coastal plains, the coastal region is characterized by offshore barrier islands, which are low ridges of sand that parallel the coast.

Q Identify the lettered features in this diagram.

15.7 STABILIZING THE SHORE Summarize the ways in which people deal with shoreline erosion problems. K EY TER MS: hard stabilization, jetty, groin, breakwater, seawall, beach nourishment ■

■

Hard stabilization is a term that refers to any structures built along the coastline to prevent movement of sand. Jetties project out from the coast, with the goal of keeping inlets open. Groins are also oriented perpendicular to the coast, but with the goal of slowing beach erosion by longshore currents. Breakwaters are parallel to the coast but located some distance offshore. Their goal is to blunt the force of incoming ocean waves, often to protect boats. Like breakwaters, seawalls are parallel to the coast, but they are built on the shoreline itself. Often the installation of hard stabilization results in increased erosion elsewhere. Beach nourishment is an expensive alternative to hard stabilization. Sand is pumped onto a beach from some other area, temporarily replenishing the sediment supply. Another possibility is relocating buildings away from high-risk areas and leaving the beach to be shaped by natural processes.

D A

C B

Q Based on their position and orientation, identify the four kinds of hard stabilization illustrated in this diagram.

15.8 CONTRASTING AMERICA’S COASTS Contrast the erosion problems faced along different parts of America’s coasts. Distinguish between emergent and submergent coasts. K EY TER MS: emergent coast, submergent coast, estuary ■

■

■

The Atlantic and Gulf coasts of the United States are markedly different from the Pacific coast. The Atlantic and Gulf coasts are lined in many places by barrier islands—dynamic expanses of sand that see a lot of change during storm events. Many of these low and narrow islands have also been prime sites for real estate development. The Pacific coast’s big issue is the thinning of beaches due to sediment starvation. Rivers that drain to the coast (bringing it sand) have been dammed, resulting in reservoirs that trap sand before it can make it to the coast. Thinner beaches offer less resistance to incoming waves, often leading to erosion of bluffs behind the beach. Coasts may be classified by their changes relative to sea level. Emergent coasts are sites of either land uplift or sea-level fall. Marine terraces are features of emergent coasts. Submergent coasts are sites of land subsidence or sea-level rise. One characteristic of submergent coasts is drowned river valleys called estuaries.

Q What term is applied to the masses of rock protruding from the water in this photo? How did they form? Is the location more likely along the Gulf coast or the coast of California? Explain.

Michael Collier

502

Concepts in Review

15.9 TIDES

■

Explain the cause of tides, their monthly cycles, and patterns. Describe the horizontal flow of water that accompanies the rise and fall of tides. ■

K EY TERMS: tide, spring tide, neap tide, diurnal tidal pattern, semidiurnal

tidal pattern, mixed tidal pattern, tidal current, tidal flat, tidal delta ■

Tides are daily changes in ocean surface elevation. They are caused by gravitational pull on ocean water by the Moon and, to a lesser extent, the Sun. When the Sun, Earth, and Moon all line up about every 2 weeks (full moon or new moon), the tides are most exaggerated. When a quarter moon is in the sky, the Moon is pulling on Earth’s water at a right angle relative to the Sun, and the daily tidal range is minimized as the two forces partially counteract one another.

Tides are strongly influenced by local conditions, including the shape of the local coastline and the depth of the ocean basin. Tidal patterns may be diurnal (one high tide per day), semidiurnal (two high tides per day), or mixed (similar to semidiurnal but with significant inequality between high tides). A flood current is the landward movement of water during the shift between low tide and high tide. When high tide transitions to low tide again, the movement of water away from the land is an ebb current. Ebb currents may expose tidal flats to the air. If a tide passes through an inlet, the current may carry sediment that gets deposited as a tidal delta.

Q Would spring tides and neap tides occur on an Earth-like planet that had no moon? Explain.

GIVE IT SOME THOUGHT

2. During a visit to the beach, you get in a small rubber raft and paddle out beyond the surf zone. Tiring, you stop and take a rest. Describe the movement of your raft during your rest. How does this movement differ, if at all, from what you would have experienced if you had stopped paddling while in the surf zone? 3. You found out from your friend in New York that the city experienced a very harsh winter last year. Going through news reports, you realized that New York receives more snowfall in winter than Paris does. Despite the fact that Paris is located further north of New York, the European city is not as cold as the U.S. one. Can you provide an explanation for the lesser snowfall in Paris? What is the reason behind the lower temperatures in New York? 4. Examine the accompanying aerial photo that shows a portion of the New Jersey shoreline. What term is applied to the wall-like structures that extend into the water? What is their purpose? In what direction are beach drift and longshore currents moving sand? Is sand moving toward the top or toward the bottom of the photo?

John S. Shelton/University of Washington Libraries

1. In this chapter you learned that global winds are the force that drive surface ocean currents. A glance at the accompanying map, however, shows a surface current that does not exactly coincide with the prevailing wind. Provide an explanation.

5. What structures could prevent a tourist beach from losing its sand when faced with high longshore currents? 6. The force of gravity plays a critical role in creating ocean tides. The more massive an object, the stronger the pull of gravity. Explain why the Sun’s influence is only half that of the Moon, even though the Sun is much more massive than the Moon. 7. This photo shows a portion of the Maine coast. The brown muddy area in the foreground is influenced by tidal currents. What term is applied to this muddy area? Name the type of tidal current this area will experience in the hours to come.

Marli Miller

Examining the Earth System

503

EXAMINING THE EARTH SYSTEM

Paul Souders/Corbis

2. If wastage (melting and calving) of the Greenland Ice Sheet were to dramatically increase, how would the salinity of the adjacent North Atlantic be affected? How might this influence thermohaline circulation?

3. Describe the interactions between the hydrosphere, the atmosphere, and the biosphere which occur close to a shoreline where strong upwellings occur. 4. This dam in the San Gabriel Mountains near Los Angeles was built on a river that flows into the Pacific Ocean. What impact might this artificial structure have on coastal beaches?

Dam

Michael Collier

South West Images Scotland / Alamy

1. Palm trees in Scotland? Yes, in the 1850s and 1860s, amateur gardeners planted palm trees on the western shore of Scotland. The latitude here is 57° north, about the same as the northern portion of Labrador across the Atlantic in Canada. Surprisingly, these exotic plants flourished. Suggest a possible explanation for how these palms can survive at such a high latitude.

UNIT SIX | EARTH’S DYNAMIC ATMOSPHERE

16

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

16.1

Distinguish between weather and climate and name the basic elements of weather and climate.

16.2

List the major gases composing Earth’s atmosphere and identify the components that are most important to understanding weather and climate.

16.3

Interpret a graph that shows changes in air pressure from Earth’s surface to the top of the atmosphere. Sketch and label a graph that shows atmospheric layers based on temperature.

16.4

Explain what causes the Sun angle and length of daylight to change during the year and describe how these changes produce the seasons.

16.5

Distinguish between heat and temperature. List and describe the three mechanisms of heat transfer.

16.6

Sketch and label a diagram that shows the paths taken by incoming solar radiation. Summarize the greenhouse effect.

16.7

Calculate five commonly used types of temperature data and interpret a map that depicts temperature data using isotherms.

16.8

Discuss the principal controls of temperature and use examples to describe their effects.

16.9

Interpret the patterns depicted on world maps of January and July temperatures.

This power plant in Andalucia, Spain, produces clean thermoelectric power from the Sun. Solar radiation provides practically all the energy that heats Earth’s surface and atmosphere. (Photo by Kevin Foy/Alamy)

505

506

CHAPTER 16

The Atmosphere: Composition, Structure, and Temperature

arth’s atmosphere is unique. No other planet in our solar system has an atmosphere with the exact mixture of gases or the heat and moisture conditions necessary to sustain life as we know it. The gases that make up Earth’s atmosphere and the controls to which they are subject are vital to our existence. In this chapter we begin our

E

|

examination of the ocean of air in which we all must live. We try to answer a number of basic questions: What is the composition of the atmosphere? Where does the atmosphere end, and where does outer space begin? What causes the seasons? How is air heated? What factors control temperature variations around the globe?

16.1 FOCUS ON THE ATMOSPHERE Distinguish between weather and climate and name the basic elements of weather and climate.

Weather influences our everyday activities, our jobs, and our health and comfort. Many of us pay little attention to the weather unless we are inconvenienced by it or when it adds to our enjoyment outdoors. Nevertheless, there are few other aspects of our physical environment that affect our lives more than the phenomena we collectively call the weather.

FIGURE 16.1 Memorable Weather Events Few aspects of our physical environment influence our daily lives more than the weather. The image on the right shows hundreds of cars stranded on Chicago’s Lake Shore Drive during a blizzard on February 2, 2011. (Kiichiro Sato/AP Photo) The top photo shows the aftermath of the tornado that devastated Moore, Oklahoma, on May 20, 2013. (Photo by Julie Dermansky/Corbis)

Weather in the United States The United States occupies an area that stretches from the tropics of Hawaii to beyond the Arctic Circle in Alaska. It has thousands of miles of coastline and extensive regions that are far from the influence of the ocean. Some landscapes are mountainous, and others are dominated by plains. It is a place where Pacific storms strike the West coast, and the East is sometimes influenced by events in the Atlantic and the Gulf of Mexico. Those in the center of the country commonly experience weather events triggered when frigid southward-bound Canadian air masses clash with northward-moving ones from the Gulf of Mexico. Stories about weather are a routine part of the daily news. Articles and items about the effects of heat, cold, floods, drought, fog, snow, ice, and strong winds are commonplace. Of course, storms of all kinds are frequently front-page news (FIGURE 16.1 ). Beyond its direct impact on the lives of individuals, the weather has a strong effect on the world economy, influencing agriculture, energy use, water resources, transportation, and industry. Weather clearly influences our lives a great deal. Yet, it is important to realize that people influence the atmosphere and its behavior as well (FIGURE 16.2 ). There are, and will continue to be, significant political and scientific decisions that must be made involving these impacts. Important examples are air pollution control and the effects of human activities on global climate and the atmosphere’s protective ozone layer. So there is a need for increased awareness and understanding of our atmosphere and its behavior.

Weather and Climate Acted on by the combined effects of Earth’s motions and energy from the Sun, our planet’s formless and invisible envelope of air reacts by producing an infinite variety of weather, which in turn creates the basic pattern of global climates. Although not identical, weather and climate have much in common.

16.1 Focus on the Atmosphere

Weather is constantly changing, sometimes from hour to hour and at other times from day to day. It is a term that refers to the state of the atmosphere at a given time and place. Whereas changes in the weather are continuous and sometimes seemingly erratic, it is nevertheless possible to arrive at a generalization of these variations. Such a description of aggregate weather conditions is termed climate. It is based on observations that have been accumulated over many years. Climate is often defined simply as “average weather,” but this is an inadequate definition. To more accurately portray the character of an area, variations and extremes must also be included, as well as the probabilities that such departures will take place. For example, farmers need to know the average rainfall during the growing season, and they also need to know the frequency of extremely wet and extremely dry years. Thus, climate is the sum of all statistical weather information that helps describe a place or region. Suppose you were planning a vacation trip to an unfamiliar place. You would probably want to know what kind of weather to expect. Such information would help as you selected clothes to pack and could influence decisions regarding activities you might engage in during your stay. Unfortunately, weather forecasts that go beyond a few days are not very dependable. Therefore, you might ask someone who is familiar with the area about what kind of weather to expect. “Are thunderstorms common?” “Does it get cold at night?” “Are the afternoons sunny?” What you are seeking is information about the climate, the conditions that are typical for that place. Another useful source of such information is the great variety of climate tables, maps, and graphs that are available. For example, the graph in FIGURE 16.3 shows average daily high and low temperatures for each month, as well as extremes for New York City. Such information could no doubt help as you planned your trip. But it is important to realize that climate data cannot predict the weather. Although a place may usually

EYE ON EY

EARTH E

This is a scene on a summer day in a portion T o southern Utah called Sinbad Country. Hebes of Mountain is in the center of the image. (Photo by M Michael Collier) Mic QU ESTIO N 1 Write two brief statements about this place—one that clearly relates to weather and another that might be part of a description of climate. QU ESTIO N 2 Explain the reasoning associated with each statement.

FIGURE 16.2 People Influence the Atmosphere Smoke bellows from a coal-fired electricity-generating plant in New Delhi, India, in June 2008. (Gurinder Osan/AP Photo)

507

CHAPTER 16

The Atmosphere: Composition, Structure, and Temperature

FIGURE 16.3 Graphs Can Display Climate Data This

48

graph shows daily temperature data for New York City. In addition to the average daily maximum and minimum temperatures for each month, extremes are also shown. As this graph shows, there can be significant departures from the average.

Record daily highs

44

110

40 100

36 Average daily highs

32

90

28

80

24 70

Temperature (˚C)

20 Average daily lows

16

60

12 50

Record daily lows

8 4

40

0

30

Temperature (˚F)

508

–4 20

–8 –12

(climatically) be warm, sunny, and dry during the time of your planned vacation, you may actually experience cool, overcast, and rainy weather. There is a well-known saying that summarizes this idea: “Climate is what you expect, but weather is what you get.” The nature of weather and climate is expressed in terms of the same basic elements—quantities or properties that are measured regularly. The most important elements are (1) air temperature, (2) humidity, (3) type and amount of cloudiness, (4) type and amount of precipitation, (5) air pressure, and (6) the speed and direction of the wind. These elements are the major variables from which weather patterns and climate types are deciphered. Although you will study these elements separately at first, keep in mind that they are very much interrelated. A change in any one of the elements will often bring about changes in the others.

10

–16

0

–20 –24

–10

–28

–20

–32 J

F

|

M

A

M

J J Month

A

S

O

N

D

16.1 CONCEPT CHECKS 1 Distinguish between weather and climate. 2 Write two brief statements about your current location: one that relates to weather and one that relates to climate.

3 What is an element? 4 List the basic elements of weather and climate

16.2 COMPOSITION OF THE ATMOSPHERE List the major gases composing Earth’s atmosphere and identify the components that are most important to understanding weather and climate.

Sometimes the term air is used as if it were a specific gas, but it is not. Rather, air is a mixture of many discrete gases, each with its own physical properties, in which varying quantities of tiny solid and liquid particles are suspended. FIGURE 16.4 Composition of the Atmosphere This graph shows the proportional volume of gases composing dry air. Nitrogen and oxygen clearly dominate.

Concentration in parts per million (ppm)

Argon (Ar) 0.934% Carbon dioxide (CO2) 0.0397% or 397 ppm

All others

Neon (Ne) 18.2 Helium (He) 5.24 Methane (CH4) 1.5 Krypton (Kr) 1.14 Hydrogen (H2) 0.5

Oxygen (O2) 20.946%

Nitrogen (N2) 78.084%

Major Components The composition of air is not constant; it varies from time to time and from place to place. If the water vapor, dust, and other variable components were removed from the atmosphere, we would find that its makeup is very stable worldwide up to an altitude of about 80 kilometers (50 miles). As you can see in FIGURE 16.4 , two gases—nitrogen and oxygen—make up 99 percent of the volume of clean, dry air. Although these gases are the most plentiful components of air and are of great significance to life on Earth, they are of minor importance in affecting weather phenomena. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus tiny quantities of a number of other gases.

Carbon Dioxide (CO2) Carbon dioxide, although present in only minute amounts (0.0397 percent, or 397 parts per million [ppm]), is nevertheless an important constituent of air. Carbon dioxide is of great interest to meteorologists because it is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Although the proportion of carbon dioxide in

16.2 Composition of the Atmosphere

Parts per million

the atmosphere is relatively uni400 form, its percentage has been The up-and-down of the red line shows a rising steadily for 200 years. seasonal pattern. Concentrations go down FIGURE 16.5 is a graph that during Northern Hemisphere spring and shows the growth in atmossummer when plants are absorbing CO2. 380 In fall and winter, photosynthesis is pheric CO2 since 1958. Much of greatly reduced and decomposing this rise is attributed to the burnvegetation continues to add CO2 to the ing of ever-increasing quantities atmosphere. of fossil fuels, such as coal and 360 oil. Some of this additional carbon dioxide is absorbed by the Seasonally corrected data ocean or is used by plants, but 340 more than 40 percent remains Monthly CO2 in the air. Estimates project that by sometime in the second half of the twenty-first century, CO2 320 levels will be twice as high as the pre-industrial level. 1960 1970 1980 1969 2000 2010 2020 Most atmospheric scienYear tists agree that increased carbon dioxide concentrations have contributed to a warming of Earth’s atmosphere over the past suspended within it. Although visible dust sometimes clouds several decades and will continue to do so in the decades to the sky, these relatively large particles are too heavy to stay come. The magnitude of such temperature changes is uncer- in the air very long. Many other particles are microscopic tain and depends partly on the quantities of CO2 contributed by and remain suspended for considerable periods of time. They human activities in the years ahead. The role of carbon dioxide may originate from many sources, both natural and human in the atmosphere and its possible effects on climate are exam- made, and include sea salts from breaking waves, fine soil ined in Chapter 20. blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, ash and dust from volcanic eruptions, and more (FIGURE 16.6A ). Collectively, Variable Components these tiny solid and liquid particles are called aerosols. Air includes many gases and particles whose quantities vary From a meteorological standpoint, these tiny, often invissignificantly from time to time and place to place. Impor- ible particles can be significant. First, many act as surfaces tant examples include water vapor, dust particles, and ozone. on which water vapor can condense, an important function in Although usually present in small percentages, they can have the formation of clouds and fog. Second, aerosols can absorb, significant effects on weather and climate. reflect, and scatter incoming solar radiation. Thus, when an airWater Vapor You are probably familiar with the term pollution episode is occurring or when ash fills the sky followhumidity from watching weather reports on TV. Humidity is a ing a volcanic eruption, the amount of sunlight reaching Earth’s reference to water vapor in the air. As you will learn in Chap- surface can be measurably reduced. Finally, aerosols contribute ter 17, there are several ways to express humidity. The amount to an optical phenomenon we have all observed—the varied of water vapor in the air varies considerably, from practically hues of red and orange at sunrise and sunset (FIGURE 16.6B ). none at all up to about 4 percent by volume. Why is such a small fraction of the atmosphere so significant? The fact that water vapor is the source of all clouds and precipitation would be enough to explain its importance. However, water vapor has other roles. Like carbon dioxide, water vapor absorbs heat given off by Earth as well as some solar energy. It is therefore important when we examine the heating of the atmosphere. When water changes from one state to another (see Figure 17.2, page 519), it absorbs or releases heat. This energy is termed latent heat, which means “hidden heat.” As we shall see in later chapters, water vapor in the atmosphere transports this latent heat from one region to another, and it is the energy source that helps drive many storms.

Aerosols The movements of the atmosphere are sufficient to keep a large quantity of solid and liquid particles

Ozone Another important component of the atmosphere is ozone. It is a form of oxygen that combines three oxygen atoms into each molecule (O3). Ozone is not the same as oxygen we breathe, which has two atoms per molecule (O2). There is very little ozone in the atmosphere, and its distribution is not uniform. It is concentrated between 10 and 50 kilometers (6 and 31 miles) above the surface, in a layer called the stratosphere. In this altitude range, oxygen molecules (O2) are split into single atoms of oxygen (O) when they absorb ultraviolet radiation emitted by the Sun. Ozone is then created when a single atom of oxygen (O) and a molecule of oxygen (O2) collide. This must happen in the presence of a third, neutral molecule that acts as a catalyst by allowing the reaction to take place without itself being consumed in the process. Ozone is concentrated in the 10- to 50-kilometer height

509

SmartFigure 16.5 Monthly M t CO2 Concentrations Atmospheric CO2 has been measured at Mauna Loa Observatory, Hawaii, since 1958. There has been a consistent increase since monitoring began. This graphic portrayal is known as the Keeling Curve, in honor of the scientist who originated the measurements. (NOAA)

510 FIGURE 16.6 Aerosols A. This satellite image from November 11, 2002, shows two examples of aerosols. First, a large dust storm is blowing across northeastern China, toward the Korean Peninsula. Second, a dense haze toward the south (bottom center) is humangenerated air pollution. (NASA) B. Dust in the air can cause sunsets to be especially colorful. (Photo by elwynn/

Dust storm

Shutterstock)

Air pollution

A.

B.

range because a crucial balance exists there: The ultraviolet radiation from the Sun is sufficient to produce single atoms of oxygen, and there are enough gas molecules to bring about the required collisions. The presence of the ozone layer in our atmosphere is crucial to those of us who dwell on Earth. The reason is that ozone absorbs much of the potentially harmful ultraviolet (UV) radiation from the Sun. If ozone did not filter a great deal of the ultraviolet radiation, and if the Sun’s UV rays reached the surface of Earth undiminished, our planet would be uninhabitable for most life as we know it. Thus, anything that reduces the amount of ozone in the atmosphere could affect the well-being of life on Earth. Just such a problem exists and is described in the next section.

Ozone Depletion: A Global Issue Although stratospheric ozone is 10 to 50 kilometers (6 to 31 miles) above Earth’s surface, it is vulnerable to human activities. Chemicals produced by people are breaking up ozone molecules in the stratosphere, weakening our shield against UV rays. This loss of ozone is a serious global-scale environmental problem. Measurements over the past three decades confirm that ozone depletion is occurring worldwide and is especially pronounced above Earth’s poles. You can see this effect over the South Pole in FIGURE 16.7 . Over the past 60 years, people have unintentionally placed the ozone layer in jeopardy by polluting the

30

Million square kilometers

25

Area of North America

20 15

Area of Antarctica

10 5

Extent of ozone hole

0

1980 1985 1990 1995 2000 2005 2010 2015

1979

Ozone (Dobson Units) 110

220

330

440

2012

Year

550

SmartFigure 16.7 Antarctic Ozone Hole The two satellite images show ozone distribution in the Southern Hemisphere on the days in September 1979 and 2012 when the ozone hole was largest. The dark blue shades over Antarctica correspond to the region with the sparsest ozone. The ozone hole is not technically a “hole” where no ozone is present but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that occurs in the spring. The small graph traces changes in the maximum size of the ozone hole, 1980 to 2012. (NASA)

GEOGRAPHICS

Acid Precipitation

A Human Impact on the Earth System As a consequence of burning large quantities of coal and petroleum, tens of millions of tons of sulfur dioxide and nitrogen oxides enter the atmosphere each year. Through a series of complex chemical reactions, these pollutants are converted into acids that eventually fall to Earth’s surface. The map shows precipitation pH values for 2008. 14

4.9

5.0 5.9 4.7

4.8

The pH scale measures the degree of acidity or alkalinity of a solution. Each whole number indicates a tenfold difference. Unpolluted rain has a pH of about 5. In the United States, acid rain is most serious in the Northeast.

4.8

5.7 6.2

4.6 5.2 4.7 5.1

LESS ACIDIC

4.8

4.8

4.4

pH VALUE

5.5

13

LYE

12

AMMONIA

11 10 9 8

BAKING SODA

4.5 4.8

5.6

MORE STRONGLY ALKALINE

4.7

4.8 4.8 5.3 5.6

4.6

4.9

5.1 4.8

MORE ACIDIC

>5.3 5.2 - 5.3 5.1 - 5.2 5.0 - 5.1 4.9 - 5.0 4.8 - 4.9 4.7 - 4.8 4.6 - 4.7 4.5 - 4.6 4.4 - 4.5 4.3 - 4.4 5.4

Catastrophic

Hurricane Decay Hurricanes diminish in intensity whenever they (1) move over ocean waters that cannot supply warm, moist tropical air; (2) move onto land; or (3) reach a location where the large-scale flow aloft is unfavorable. When a hurricane moves onto land, it loses its punch rapidly. The most important reason for this rapid demise is the fact that the storm’s source of warm, moist air is cut off. When an adequate supply of water vapor does not exist, condensation and the release of latent heat must diminish. In addition, friction from the increased roughness of the land surface rapidly slows surface wind speeds. This factor causes the winds to move more directly into the center of the low, thus helping to eliminate the large pressure differences.

Hurricane Destruction A location only a few hundred kilometers from a hurricane—just 1 day’s striking distance away—may experience clear skies and virtually no wind. Prior to the age of weather satellites, this situation made the task of warning people of impending storms very difficult. The amount of damage caused by a hurricane depends on several factors, including the size and population density of the area affected and the shape of the ocean bottom near the shore. The most significant factor, of course, is the strength of the storm itself. By studying past storms, a scale has been established to rank the relative intensities of hurricanes. As TABLE 19.2 indicates, a category 5 storm is the worst possible, whereas a category 1 hurricane is least severe.

19.6 Hurricanes

619

FIGURE 19.32 Storm Surge Destruction This is Crystal Beach, Texas, on September 16, 2008, 3 days after Hurricane Ike came ashore. At landfall the storm had sustained winds of 165 kilometers (105 miles) per hour. The extraordinary storm surge caused most of the damage shown here. (Photo by Texas Parks & Wildlife Dept., Earl Nottingham/AP Photo)

During hurricane season, it is common to hear scientists and reporters use the numbers from the Saffir–Simpson hurricane scale. When Hurricane Katrina made landfall, sustained winds were 225 kilometers (140 miles) per hour, making it a strong category 4 storm. Storms that fall into category 5 are rare. Damage caused by hurricanes can be divided into three categories: (1) storm surge, (2) wind damage, and (3) heavy rains and inland flooding.

Storm Surge The most devastating damage in the coastal zone is usually caused by storm surge (FIGURE 19.32 ). It not only accounts for a large share of coastal property losses but also is responsible for a high percentage of all hurricanecaused deaths. A storm surge is a dome of water 65 to 80 kilometers (40 to 50 miles) wide that sweeps across the coast near the point where the eye makes landfall. If all wave activity were smoothed out, the storm surge would be the height of the water above normal tide level. In addition, tremendous wave activity is superimposed on the surge. The worst surges occur in places like the Gulf of Mexico, where the continental shelf is very shallow and gently sloping. In addition, local features such as bays and rivers can cause the surge height to double and increase in speed. As a hurricane advances toward the coast in the Northern Hemisphere, storm surge is always most intense on the right side of the eye (viewed from the ocean), where winds are blowing toward the shore. In addition, on this side of the storm, the forward movement of the hurricane contributes to the storm surge. In FIGURE 19.33 , assume that a hurricane

North Carolina South Carolina

and f eed n o Sp ectio ane dir urric ent h vem mo kph 50

Georgia

ane rric ent Hu vem ph mo 0 k 5 n d do e in t w kph = Win t sid h e N 5 he h p g 22 m t est ri 75 k 1 fro thw sou

ind tw h ne t Ne 5 kp he a c 12 t t rri en Hu vem ph = from hwes t mo 0 k nor 5 n o nd e Wi ft sid h le 5 kp 17 Florida

FIGURE 19.33 An Approaching Hurricane Winds associated with a Northern Hemisphere hurricane that is advancing toward the coast. This hypothetical storm, with peak winds of 175 kilometers (109 miles) per hour, is moving toward the coast at 50 kilometers (31 miles) per hour. On the right side of the advancing storm (as viewed from the ocean), the 175-kilometerper-hour winds are in the same direction as the movement of the storm (50 kilometers per hour). Therefore, the net wind speed on the right side of the storm is 225 kilometers (140 miles) per hour. On the left side, the hurricane’s winds are blowing opposite the direction of storm movement, so the net winds of 125 kilometers (78 miles) per hour are away from the coast. Storm surge will be greatest along the part of the coast hit by the right side of the advancing hurricane.

620

CHAPTER 19

Weather Patterns and Severe Storms

with peak winds of 175 kilometers (109 miles) per hour is moving toward the shore at 50 kilometers (31 miles) per hour. In this case, the net wind speed on the right side of the advancing storm is 225 kilometers (140 miles) per hour. On the left side, the hurricane’s winds are blowing opposite the direction of storm movement, so the net winds are away from the coast at 125 kilometers (78 miles) per hour. Along the shore facing the left side of the oncoming hurricane, the water level may actually decrease as the storm makes landfall.

Heavy Rains and Inland Flooding The torrential rains that accompany most hurricanes contribute to a third significant threat: flooding. Whereas the effects of storm surge and strong winds are concentrated in coastal areas, heavy rains may affect places hundreds of kilometers from the coast for up to several days after the storm has lost its hurricane-force winds. In September 1999, Hurricane Floyd brought flooding rains, high winds, and rough seas to a large portion of the Atlantic seaboard. More than 2.5 million people evacuated their homes from Florida north to the Carolinas and beyond. It was the largest peacetime evacuation in U.S. history up to that time. Torrential rains falling on already saturated ground created devastating inland flooding. Altogether Floyd dumped more than 48 centimeters (19 inches) of rain on Wilmington, North Carolina, 33.98 centimeters (13.38 inches) in a single 24-hour span.

Wind Damage Destruction caused by wind is perhaps the most obvious of the classes of hurricane damage. Debris such as signs, roofing materials, and small items left outside become dangerous flying missiles in hurricanes. For some structures, the force of the wind is sufficient to cause total ruin. Mobile homes are particularly vulnerable. Highrise buildings are also susceptible to hurricane-force winds. Upper floors are most vulnerable because wind speeds usually increase with height. Recent research suggests that people should stay below the 10th floor but remain above any floors at risk for flooding. In regions with good building codes, wind damage is usually not as catastrophic as storm-surge damage. However, hurricane-force winds affect a much larger area than storm surge and can cause huge economic losses. For example, in 1992 it was largely the winds associated with Hurricane Andrew that produced more than $25 billion of damage in southern Florida and Louisiana. A hurricane may produce tornadoes that contribute to the storm’s destructive power. Studies have shown that more than half of the hurricanes that make landfall produce at least one tornado. In 2004 the number of tornadoes associated with tropical storms and hurricanes was extraordinary. Tropical Storm Bonnie and five landfalling hurricanes—Charley, Frances, Gaston, Ivan, and Jeanne—produced nearly 300 tornadoes that affected the southeastern and mid-Atlantic states. FIGURE 19.34 Five-Day Track Forecast for Tropical Storm Dean, Issued at 5 P.M. EDT, Tuesday, August 14, 2007 When a hurricane track forecast is issued by the National Hurricane Center, it is termed a forecast cone. The cone represents the probable track of the center of the storm and is formed by enclosing the area swept out by a set of circles along the forecast track (at 12 hours, 24 hours, 36 hours, etc.). The size of each circle gets larger with time. Based on statistics from 2003 to 2007, the entire track of an Atlantic tropical cyclone can be expected to remain entirely within the cone roughly 60 to 70 percent of the time. (National Weather Service/National Hurricane Center)

100°

95°

90°

Tracking Hurricanes Today we have the benefit of numerous observational tools for tracking tropical storms and hurricanes. Using input from satellites, aircraft reconnaissance, coastal radar, and remote data buoys in conjunction with sophisticated computer models, meteorologists monitor and forecast storm movements and intensity. The goal is to issue timely watches and warnings. An important part of this process is the track forecast— the predicted path of the storm. The track forecast is probably the most basic information because accurate prediction of other storm characteristics (winds and rainfall) is of little value if there is significant uncertainty about where the storm is going. Accurate track forecasts are important because they can lead to timely evacuations from the surge zone, where the greatest number of deaths usually occur. Fortunately, track forecasts have been steadily improving. During the span

85°

80°

35° Approx. distance scale (statute miles) 0 250 500 True at 30.00N 30°

TX

MS LA

AL

75°

70°

65°

60°

50°

45°

40°

35°

Tropical Storm Dean

VA NC

SC

GA

55°

Bermuda 25° FL

August 14, 2007 5 PM EDT Tuesday Current center location 11.6 N, 41.0 W Max sustained wind 40 mph Current movement W at 21 mph Current center location Forecast center positions H Sustained wind > 73 mph S Sustained wind 39–73 mph Potential day 1–3 track area Potential day 4–5 track area

35°

30°

25°

20° Bahamas Cuba

Location of Tropical Storm Dean at time of forecast

Forecast cone

15°

H

Honduras

2 PM Sun

10° Costa Rica 5°

20°

15°

H

2 PM Sat Venezuela

H

S

S

2 PM Fri

2 PM Thu

2 PM Wed

60°

55°

10°

5 PM Tue 5°

90°

85°

80°

75°

70°

65°

50°

45°

621

Concepts in Review

2001 to 2005, forecast errors were roughly half of what they were in 1990. During the very active 2004 and 2005 Atlantic hurricane seasons, 12- to 72-hour track forecast accuracy was at or near record levels. Consequently, the length of official track forecasts issued by the National Hurricane Center was extended from 3 days to 5 days (FIGURE 19.34 ). Current 5-day track forecasts are now as accurate as 3-day forecasts were 15 years ago. Despite improvements in accuracy, forecast uncertainty still requires that hurricane warnings be issued for relatively large coastal areas. During the span 2000 to 2005, the average length of coastline under a hurricane warning in the United States was 510 kilometers (316 miles). This represents a significant improvement over the preceding decade, when the average was 730 kilometers (452 miles). Nevertheless, only about one-quarter of an average warning area experiences hurricane conditions.

19

CONCEPTS IN REVIEW

19.6 CONCEPT CHECKS 1 Define hurricane. What other names are used for this storm?

2 In what latitude zone do hurricanes develop? 3 Distinguish between the eye and the eye wall of a hurricane. How do conditions differ in these zones?

4 What is the source of energy that drives a hurricane? 5 Why do hurricanes not form near the equator? Explain the lack of hurricanes in the South Atlantic and eastern South Pacific.

6 When do most hurricanes in the North Atlantic and Caribbean occur? Why are these months favored?

7 Why does the intensity of a hurricane diminish rapidly when it moves over land?

8 What are the three broad categories of hurricane damage?

|

Weather Patterns and Severe Storms

19.1 AIR MASSES Discuss air masses, their classification, and associated weather. KEY TERMS: air mass, air-mass weather, source region, polar (P) air mass, arctic (A) air mass, tropical (T) air mass, continental (c) air mass, maritime (m) air

mass, lake-effect snow, nor’easter ■

■

■

An air mass is a large body of air, usually 1600 kilometers (1000 miles) or more across, that is characterized by a sameness of temperature and moisture at any given altitude. When this air moves out of its region of origin, called the source region, it carries these temperatures and moisture conditions elsewhere, perhaps eventually affecting a large portion of a continent. Air masses are classified according to the nature of the surface in the source region and the latitude of the source region. Continental (c) designates an air mass of land origin, with the air likely to be dry; a maritime (m) air mass originates over water and, therefore, will be relatively humid. Polar (P) and arctic (A) air masses originate in high latitudes and are cold. Tropical (T) air masses form in low latitudes and are warm. According to this classification scheme, the four basic types of air masses are continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT). Continental polar (cP) and maritime tropical (mT) air masses influence the weather of North America most, especially east of the Rocky Mountains. Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

Q Identify the source region associated with each letter on this map. One letter is not associated with a source region. Which one is it?

A E B D C

622

Concepts in Review

19.2 FRONTS Compare and contrast typical weather associated with a warm front and a cold front. Describe an occluded front and a stationary front.

A.

C.

K EY TERMS: front, overrunning, warm front, cold front, stationary front, occluded front ■

■ ■

Fronts are boundary surfaces that separate air masses of different densities, one usually warmer and more B. D. humid than the other. As one air mass moves into another, the warmer, less dense air mass is forced aloft in a process called overrunning. Along a warm front, a warm air mass overrides a retreating mass of cooler air. As the warm air ascends, it cools adiabatically to produce clouds and, frequently, light to moderate precipitation over a large area. A cold front forms where cold air is actively advancing into a region occupied by warmer air. Cold fronts are about twice as steep as and move more rapidly than warm fronts. Because of these two differences, precipitation along a cold front is generally more intense and of shorter duration than precipitation associated with a warm front.

Q Identify each of these symbols used to designate fronts. On which side of each symbol are the warm air and the cool air?

19.3 MIDLATITUDE CYCLONES Summarize the weather associated with the passage of a mature midlatitude cyclone. Describe how airflow aloft is related to cyclones and anticyclones at the surface.

■

K EY TERMS: midlatitude (middle-latitude) cyclone ■

The primary weather producers in the middle latitudes are large centers of low pressure that generally travel from west to east, called midlatitude cyclones. These bearers of stormy weather, which last from a few days to a week, have a counterclockwise circulation

■

pattern in the Northern Hemisphere, with an inward flow of air toward their centers. Most midlatitude cyclones have a cold front and frequently a warm front extending from the central area of low pressure. Convergence and forceful lifting along the fronts initiate cloud development and frequently cause precipitation. The particular weather experienced by an area depends on the path of the cyclone. Guided by west-to-east-moving jet streams, cyclones generally move eastward across the United States. Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In cyclones, divergence aloft supports the inward flow at the surface.

19.5 TORNADOES

19.4 THUNDERSTORMS

Summarize the atmospheric conditions and locations that are favorable to the formation of tornadoes. Discuss tornado destruction and tornado forecasting.

List the basic requirements for thunderstorm formation and locate places on a map that exhibit frequent thunderstorm activity. Describe the stages in the development of a thunderstorm.

K E Y T E R M S : tornado, mesocyclone, Enhanced Fujita intensity scale (EF-

scale), tornado watch, tornado warning, Doppler radar

K EY TERM: thunderstorm ■

■

Thunderstorms are caused by the upward movement of warm, moist, unstable air. They are associated with cumulonimbus clouds that generate heavy rainfall, lightning, thunder, and occasionally hail and tornadoes. Air mass thunderstorms frequently occur in maritime tropical (mT) air during spring and summer in the middle latitudes. Generally, three stages are involved in the development of these storms: the cumulus stage, mature stage, 0°C and dissipating stage.

■

■

32°F

■

Q Which stage in the development of a thunderstorm is shown in this sketch? Describe what is occurring. Is there a stage that follows this one? If so, describe what occurs during that stage.

■

A tornado is a violent windstorm that takes the form of a rotating column of air called a vortex that extends downward from a cumulonimbus cloud. Many strong tornadoes are multiple-vortex storms. Because of the tremendous pressure gradient associated with a strong tornado, maximum winds can approach 480 kilometers (300 miles) per hour. Tornadoes are most often spawned along the cold front of a midlatitude cyclone or in association with a supercell thunderstorm. Tornadoes also form in association with tropical cyclones (hurricanes). In the United States, April through June is the period of greatest tornado activity, but tornadoes can occur during any month of the year. Most tornado damage is caused by the tremendously strong winds. One commonly used guide to tornado intensity is the Enhanced Fujita intensity scale (EF-scale). A rating on the EF-scale is determined by assessing damage produced by the storm. Because severe thunderstorms and tornadoes are small and short-lived phenomena, they are among the most difficult weather features to forecast precisely. When weather conditions favor the formation of tornadoes, a tornado watch is issued. The National Weather Service issues a tornado warning when a tornado has been sighted in an area or is indicated by Doppler radar.

Give It Some Thought

623

categories: (1) storm surge, (2) wind damage, and (3) heavy rains and inland flooding.

19.6 HURRICANES Identify areas of hurricane formation on a world map and discuss the conditions that promote hurricane formation. List the three broad categories of hurricane destruction.

Q This image taken from the International Space Station shows the inner portion of Hurricane Igor in September 2010. Identify the eye and the eye wall. In which of these zones are winds and rainfall most intense?

KEY TERMS: hurricane, eye wall, eye, tropical depression, tropical storm,

Saffir–Simpson hurricane scale, storm surge ■

■

■

Hurricanes, the greatest storms on Earth, are tropical cyclones with wind speeds in excess of 119 kilometers (74 miles) per hour. These complex tropical disturbances develop over tropical ocean waters and are fueled by the latent heat that is liberated when huge quantities of water vapor condense. Hurricanes form most often in late summer, when ocean-surface temperatures reach 27°C (80°F) or higher and thus are able to provide the necessary heat and moisture to the air. Hurricanes diminish in intensity when they move over cool ocean water that cannot supply adequate heat and moisture, move onto land, or reach a location where large-scale flow aloft is unfavorable. The Saffir–Simpson scale ranks the relative intensities of hurricanes. A 5 on the scale represents the strongest storm possible, and a 1 indicates the lowest severity. Damage caused by hurricanes is divided into three

NASA

GIVE IT SOME THOUGHT 1. Refer to Figure 19.4 to answer these questions: a. Thunder Bay and Marquette are both on the shore of Lake Superior, yet Marquette gets much more snow than Thunder Bay. Why is this the case? b. Notice the narrow, north–south zone of relatively heavy snow east of Pittsburgh and Charleston. This region is too far from the Great Lakes to receive lake-effect snowfall. Speculate on a likely reason for the higher snowfalls here. Does your answer explain the shape of this snowy zone?

2. Refer to the accompanying weather map to answer the following questions: a. What is a likely wind direction at each city? b. Identify the likely air mass that is influencing each city. c. Identify the cold front, warm front, and occluded front. d. What is the barometric tendency at city A and city C? e. Which one of the three cities is probably coldest? Which one is probably warmest?

L

3. What condition is needed to produce precipitation in a front separating two air masses? 4. If you hear that a cyclone is approaching, should you immediately seek shelter? Why or why not? 5. The accompanying diagrams show surface temperatures with isotherms labeled in degrees Fahrenheit for noon and 6 p.m. on January 29, 2008. On this day, a powerful front moved through Missouri and Illinois. a. What type of front passed through the Midwest? b. Describe how the temperature changed in St. Louis, Missouri, over the 6-hour period. c. Describe the likely shift in wind direction in St. Louis during this time span.

624

Concepts in Review

6. If you were located 400 kilometers ahead of the surface position of a typical warm front that had a slope of 1:200, how high would the frontal surface be above you? 7. Assume that after seeing a lightning bolt you heard thunder 10 seconds later. About how far away did the lightning occur? 8. The accompanying table lists the number of tornadoes reported in the United States by decade. Propose a reason to explain why the totals for the 1990s and 2000s are so much higher than for the 1950s and 1960s.

11. Refer to the graph in Figure 19.30. Explain why wind speeds are greatest when the slope of the pressure curve is steepest. 12. Assume that it is late September 2016, and that the eye of Hurricane Gaston, a category 5 storm, is projected to follow the path shown on the accompanying map of Texas. Answer the following questions: a. Name the stages of development that Gaston must have gone through to become a hurricane. At what stage did the storm receive its name? b. If the storm follows the projected path, will the city of Houston experience Gaston’s fastest winds and greatest storm surge? Explain why or why not. c. What is the greatest threat to life and property if this storm approaches the Dallas–Fort Worth area?

9. We know that the greater the contrast when two air masses meet, the more intense the potential storm or tornado. Keeping this in mind, provide an explanation for why tornadoes are very likely to occur in the United States and are rarely observed in Europe. 10. A television meteorologist is able to inform viewers about the intensity of an approaching hurricane. However, the meteorologist can report the intensity of a tornado only after it has occurred. Why is this the case?

EXAMINING THE EARTH SYSTEM 1. This image shows the effects of a major snowstorm that dropped nearly 2 meters (7 feet) of snow on Buffalo, New York, in December 2001. This weather event was unrelated to a midlatitude cyclone. Places not far from Buffalo received only modest amounts of snow or no snow at all. Which spheres of the Earth system interacted in the Great Lakes region to produce this snowstorm? What term is applied to heavy snows such as this? 2. The situations described below involve interactions between the atmosphere and Earth’s surface. In each case indicate whether the air mass is being made more stable or more unstable. Briefly explain each choice. a. An mT air mass moving northward from the Gulf of Mexico over the southeastern United States in winter. b. An mT air mass from the Gulf of Mexico moving northward over the southeastern United States in summer. c. A wintertime cP air mass from Siberia moving eastward from Asia across the North Pacific.

AP Photo/David Duprey

Examining the Earth System

3. This world map shows the tracks and intensities of thousands of hurricanes and other tropical cyclones. It was put together by the National Hurricane Center and the Joint Typhoon Warning Center. a. What area has experienced the greatest number of category 4 and 5 storms? b. Why do hurricanes not form in the very heart of the tropics, astride the equator? c. Explain the absence of storms in the South Atlantic and the eastern South Pacific.

625

4. This satellite image shows Tropical Cyclone Favia as it came ashore along the coast of Mozambique, Africa, on February 22, 2007. This powerful storm was moving from east to west. Portions of the storm had sustained winds of 203 kilometers (126 miles) per hour as it made landfall. Letters A–D relate to Question c. a. Identify the eye and the eye wall of the cyclone. b. Based on wind speed, classify the storm using the Saffir–Simpson hurricane scale. c. Which one of the lettered sites should experience the strongest storm surge? Explain. d. Describe the possible effects of the storm on coastal lands (geosphere), drainage networks (hydrosphere), and plant and animal life (biosphere).

Mozambique Channel

Zimbabwe

A B C D South Africa

N

20

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

20.1

List the five parts of the climate system and provide examples of each.

20.2

Explain why classification is a necessary process when studying world climates. Discuss the criteria used in the Köppen system of climate classification.

20.3

Compare the two broad categories of tropical climates.

20.4

Contrast low-latitude dry climates and middlelatitude dry climates.

20.5

Distinguish among five different humid middlelatitude climates.

20.6 20.7

Contrast ice cap and tundra climates.

20.8

Summarize the nature and cause of the atmosphere’s changing composition since about 1750. Describe the climate’s response.

20.9

Contrast positive- and negative-feedback mechanisms and provide examples of each.

Summarize the characteristics associated with highland climates.

20.10

Discuss the possible impacts of aerosols on climate change.

20.11

Describe some possible consequences of global warming.

Mauna Loa Observatory is an important atmospheric research facility on Hawaii’s Big Island. It has been collecting data and monitoring atmospheric change since the 1950s. (Photo by Forrest M. Mimms III)

627

628

CHAPTER 20

World Climates and Global Climate Change

he focus of this chapter is climate, the longterm aggregate of weather. Climate is more than just an expression of average atmospheric conditions. In order to accurately portray the character of a place or an area, variations and extremes must also be included. Climate strongly influences the nature of plant and animal life, the soil, and many external geologic processes. Climate influences people as well. Although climate has a significant impact on people, we are learning that people also have a strong influence

T

|

on climate. In fact, today global climate change caused by humans is an important global environmental issue. Unlike changes in the geologic past, which represented natural variations, modern climate change is dominated by human influences that are sufficiently large that they exceed the bounds of natural variability. Moreover, these changes are likely to continue for many centuries. The effects of this venture into the unknown with climate could be very disruptive not only to humans but to many other life-forms as well.

20.1 THE CLIMATE SYSTEM List the five parts of the climate system and provide examples of each.

Throughout this book, you have been reminded that Earth is a multidimensional system that consists of many interacting parts. A change in any one part can produce changes in any or all of the other parts—often in ways that are neither obvious nor immediately apparent. This fact is certainly true when it comes to the study of climate and climate change. To understand and appreciate climate, it is important to realize that climate involves more than just the atmosphere. Indeed, we must recognize that there is a climate system that FIGURE 20.1 Earth’s Climate System Schematic view showing several components of Earth’s climate system. Many interactions occur among the various components on a wide range of space and time scales, making the system extremely complex.

includes the atmosphere, hydrosphere, geosphere, biosphere, and cryosphere. (The cryosphere refers to the ice and snow that exist at Earth’s surface.) The climate system involves the exchanges of energy and moisture that occur among the five spheres. These exchanges link the atmosphere to the other spheres so that the whole functions as an extremely complex, interactive unit. Changes to the climate system do not occur in isolation. Rather, when one part of it changes, the other components also react. The major components of the climate system are shown in FIGURE 20.1 .

Changes in amount and type of cloud cover Changes in amount of icecovered land

Changes in amount of outgoing radiation Changes in atmospheric composition

Changes in atmospheric circulation

Changes in amount of evaporationprecipitation

Biosphereatmosphere interactions

Atmosphere-ice interactions

Human influences (burning, land use)

Human influences (cities) Changes in ocean circulation

Ocean

Changes in solar inputs

Oceanatmosphere interaction

Biosphereatmosphere interactions

Changes in amount of sea ice

20.2 World Climates

Climate has a profound impact on many of Earth’s external processes. When climate changes, these processes respond. A glance back at the rock cycle in Chapter 3 (page 61) reminds us about many of the connections. Of course, rock weathering has an obvious climate connection, as do processes that operate in arid, tropical, and glacial landscapes. Phenomena such as debris flows and river flooding are often triggered by atmospheric events such as periods of extraordinary rainfall. Clearly, the atmosphere is a basic link in the hydrologic cycle. Other connections involve the impact of internal processes on the atmosphere. For example, the particles and gases emitted by volcanoes can change the composition of the atmosphere, and mountain building can have a significant impact on regional temperature, precipitation, and wind patterns.

|

The study of sediments, sedimentary rocks, and fossils clearly demonstrates that, through the ages, practically every place on our planet has experienced wide swings in climate, from ice ages to conditions associated with subtropical coal swamps or desert dunes. Chapter 12 reinforces this fact. Time scales for climate change vary from decades to millions of years.

20.1 CONCEPT CHECKS 1 What are the five major parts of the climate system? 2 List at least five connections between climate and Earth’s external and internal processes.

20.2 WORLD CLIMATES Explain why classification is a necessary process when studying world climates. Discuss the criteria used in the Köppen system of climate classification.

Previous chapters have already presented the spatial and seasonal variations of the major elements of weather and climate. Chapter 16 examined the controls of temperature and the world distribution of temperature. In Chapter 18, you studied the general circulation of the atmosphere and the global distribution of precipitation. You are now ready to investigate the combined effects of these variations in different parts of the world. The varied nature of Earth’s surface and the many interactions that occur among atmospheric

EYE ON EY

EARTH E

This image shows a portion of T A Alaska’s Delta Range, a mountainous area southeast of Fairbanks. (Photo by ar Michael Collier) Mic QU ESTIO N 1 What are the five major parts of the climate system?. QU ESTIO N 2 Which of the five parts are represented in this photo? QU ESTIO N 3 Speculate on how climate change in the coming decades might cause change to the area shown here.

processes give every location on our planet a distinctive, even unique, climate. However, we are not going to describe the unique climatic character of countless different locales. Instead, we introduce the major climate regions of the world. The discussion in this chapter examines large areas and uses particular places only to illustrate the characteristics of these major climate regions. Temperature and precipitation are the most important elements in a climate description because they have the

629

630

CHAPTER 20

World Climates and Global Climate Change

greatest influence on people and their activities, and they also have an important impact on the distribution of such phenomena as vegetation and soils. Nevertheless, other factors are also important for a complete climatic description. When possible, some of these factors are introduced into our discussion of world climates.

Climate Classification The worldwide distribution of temperature, precipitation, pressure, and wind is, to say the least, complex. Because of the many differences from place to place and time to time, it is unlikely that any two places that are more than a very short distance apart can experience identical weather. The virtually infinite variety of places on Earth makes it apparent that the number of different climates must be extremely large. Having such a diversity of information to investigate is not unique to the study of the atmosphere. It is a problem basic to all science. (Consider astronomy, which deals with billions of stars, and biology, which studies millions of complex organisms.) To cope with such variety, we must devise some means of classifying the vast array of data to be studied. By establishing groups of items that have common characteristics, order and manageability are introduced. Bringing order to large quantities of information not only aids comprehension and understanding but also facilitates analysis and explanation. One of the first attempts at climate classification was made by the ancient Greeks, who divided each hemisphere into three zones: torrid, temperate, and frigid (FIGURE 20.2 ). Arctic Circle

Frigid zone

Temperate zone

The basis of this simple scheme was Earth–Sun relationships. The boundaries were the four astronomically important parallels of latitude: the Tropic of Cancer (23.5° north), the Tropic of Capricorn (23.5° south), the Arctic Circle (66.5° north), and the Antarctic Circle (66.5° south). Thus, the globe was divided into winterless climates and summerless climates and an intermediate type that had features of the other two. Few other attempts were made until the beginning of the twentieth century. Since then, many climate-classification schemes have been devised. Remember that the classification of climates (or of anything else) is not a natural phenomenon but the product of human ingenuity. The value of any particular classification system is determined largely by its intended use. A system designed for one purpose may not work well for another.

The Köppen Classification In this chapter, we use a classification devised by Russianborn German climatologist Wladimir Köppen (1846–1940). As a tool for presenting the general world pattern of climates, the Köppen classification has been the best-known and most used system for decades. It is widely accepted for many reasons. For example, it uses only easily obtained data: mean monthly and annual values of temperature and precipitation. Furthermore, the criteria are unambiguous, are relatively simple to apply, and divide the world into climate regions in a realistic way. Köppen believed that the distribution of natural vegetation is an excellent expression of the totality of climate. Consequently, the boundaries he chose were largely based on the limits of certain plant associations. Five principal groups were recognized, and each group was designated by a capital letter, as follows:

Tropic of Cancer

Torrid zone Equator

Torrid zone Tropic of Capricorn

Temperate zone

Antarctic Circle

Frigid zone

FIGURE 20.2 An Early Climate Classification Among the first attempts at climate classification was one made by the ancient Greeks. They divided each hemisphere into three zones. The winterless torrid zone was separated from the summerless frigid zone by the temperate zone, which had features of the other two.

A. Humid tropical. Winterless climates; all months have a mean temperature above 18°C (64°F). B. Dry. Climates where evaporation exceeds precipitation; there is a constant water deficiency. C. Humid middle-latitude, mild winters. The average temperature of the coldest month is below 18°C (64°F) but above –3°C (27°F). D. Humid middle-latitude, severe winters. The average temperature of the coldest month is below –3°C (27°F), and the warmest monthly mean exceeds 10°C (50°F). E. Polar. Summerless climates; the average temperature of the warmest month is below 10°C (50°F). Notice that four of the major groups (A, C, D, and E) are defined on the basis of temperature characteristics, and the fifth, the B group, has precipitation as its primary criterion. Each of the five groups is further subdivided by using the criteria and symbols presented in FIGURE 20.3 .

Letter Symbol 1st 2nd 3rd A

Average temperature of the coldest month is 18°C or higher. f

Every month has 6 cm of precipitation or more.

m

Short dry season; precipitation in driest month less than 6 cm but equal to or greater than 10 – R/25 (R is annual rainfall in cm).

w

Well-defined winter dry season; precipitation in driest month less than 10 – R/25.

s

Well-defined summer dry season (rare).

B

Potential evaporation exceeds precipitation. The dry–humid boundary is defined by the following formulas: (Note: R is the average annual precipitation in cm, and T is the average annual temperature in °C.) R < 2T + 28 when 70% or more of rain falls in warmer 6 months. R < 2T when 70% or more of rain falls in cooler 6 months. R < 2T + 14 when neither half year has 70% or more of rain. S

Steppe

W

Desert

The BS–BW boundary is 1/2 the dry–humid boundary.

h

Average annual temperature is 18°C or greater.

k

Average annual temperature is less than 18°C.

C

Average temperature of the coldest month is under 18°C and above –3°C. w

At least 10 times as much precipitation in a summer month as in the driest winter month.

s

At least three times as much precipitation in a winter month as in the driest summer month; precipitation in driest summer month less than 4 cm.

f

Criteria for w and s cannot be met. a

Warmest month is over 22°C; at least 4 months over 10°C.

b

No month above 22°C; at least 4 months over 10°C.

c

One to 3 months above 10°C.

D

Average temperature of coldest month is –3°C or below; average temperature of warmest month is greater than 10°C. w

Same as under C.

s

Same as under C.

f

E

Same as under C. a

Same as under C.

b

Same as under C.

c

Same as under C.

d

Average temperature of the coldest month is –3°C or below. Average temperature of the warmest month is below 10°C.

T

Average temperature of the warmest month is greater than 0°C and less than 10°C.

F

Average temperature of the warmest month is 0°C or below.

FIGURE 20.3 The Köppen System of Climate Classification This system uses easily obtained data: mean monthly and annual values of temperature and precipitation. When using this figure to classify climate data, first determine whether the data meet the criteria for the E climates. If the station is not a polar climate, proceed to the criteria for B climates. If the data do not fit into either the E or B groups, check the data against the criteria for A, C, and D climates, in that order. (Photos, in order from A–E, are by Michael Collier, Marek Zak/Alamy Images, Ed Reschke/Getty Images, Michael Collier, J.G. Paren/Science Source)

631

632

CHAPTER 20

World Climates and Global Climate Change ARCTIC OCEAN

EF

ET 60°

Dfc

Cfc

ET Cfc

Dfc Dfc

H Cfb

Dfb BSk

PACIFIC

Dry

40°

Humid

ATLANTIC

Dfa

40°

H

Csb Csa

OCEAN

OCEAN

BWk BSh

Af

Cfa

BWh H

20°

60°

BSh

BSh Aw

Aw

Tropic of Cancer

Am

20°

Af H Aw

Humid Tropical Tropical wet Af

Aw

Tropical monsoon Am

BSh

BWh BSh Aw

BSh

0°

Tropical savanna Aw 105˚

100˚

95˚

90˚

Af BSh

Dry 110˚

Am

BWh

Aw

Subtropical desert BWh

FIGURE 20.4 Conditions Change from Year to Year Yearly fluctuations in the dry–humid boundary during a 5-year period. The small inset shows the average position of the dry–humid boundary.

Midlatitude desert BWk

H

Midlatitude steppe BSk

BSk

Humid Middle-Latitude (mild winter)

Humid continental Dfa Dwa Dfb Dwb Subarctic Dfc Dwc Dfd Dwd

Cfa

BS

Csb

Humid subtropical Cfa Cwa Cwb Marine west coast Cfb Cfc Dry-summer subtropics Csa Csb

Humid Middle-Latitude (severe winter)

Af 20°

BWk

Subtropical steppe BSh

40°

BWk

Cfb

BSk Cfb

ET 60°

60° 80°

60°

40°

A strength of the Köppen system is the Polar relative ease with which boundaries are Tundra ET Ice cap EF determined. However, these boundaries cannot be viewed as fixed. On the contrary, FIGURE 20.5 Climates of the Highland all climate boundaries shift from year to World This map is based on the Cold climates due to elevation H Köppen classification. year (FIGURE 20.4 ). The boundaries shown on climate maps are simply average locations based on data collected over many years. Thus, a cli20.2 CONCEPT CHECKS mate boundary should be regarded as a broad transition zone 1 Why is classification often a necessary task in science? and not a sharp line. 2 What climate data are needed to classify a climate using the The world distribution of climates according to the KöpKöppen system? pen classification is shown in FIGURE 20.5 . You will refer to 3 Should climate boundaries, such as those shown on the this map several times as Earth’s climates are discussed in world map in Figure 20.5, be regarded as fixed? Explain. the following pages.

|

20.3 HUMID TROPICAL (A) CLIMATES Compare the two broad categories of tropical climates.

Within the A group of climates, two main types are recognized: wet tropical climates (Af and Am) and tropical wet and dry (Aw).

The Wet Tropics The constantly high temperatures and year-round rainfall in the wet tropics combine to produce the most luxuriant

vegetation found in any climatic realm: the tropical rain forest (FIGURE 20.6 ). The environment of the wet tropics characterizes almost 10 percent of Earth’s land area. An examination of Figure 20.5 shows that Af and Am climates form a discontinuous belt astride the equator that typically extends 5° to 10° into each hemisphere. The poleward margins are most often marked by diminishing rainfall, but occasionally decreasing

20.3 Humid Tropical (A) Climates

EF

Dwd ET

Arctic Circle

ET 60°

Dfc

Cfc Cfb

BSk 40°

Csb

Dfd

Dfb H

Cfb

Csa

BWh

BSh

BSh

BSh

Aw

Am BSh BAY OF BENGAL

BWh Equator

H

PACIFIC

INDIAN

Cwa

Tropic of Cancer 20°

Aw

Am

Am

Af

Af

Af

Cfa

Aw

H

BSh 0°

40°

OCEAN

Cwa

Am

BSh

Aw

Dfa

Cwa Cfa

ARABIAN SEA

Am

Dwa

BSk

H BWh

BWh BWh

20°

Dfb

BWk

BSk

BSh

Dfc

Dwb

BSk

BWk

Csa

ET

Dwc

Dfa

Cfa

FIGURE 20.5 Climates of the World (continued)

ET

ARCTIC OCEAN

Af

OCEAN

H Af

Aw Aw

Aw Aw

20°

Af BWh

Tropic of Capricorn

BSh

Cwb

Am Cwa

20°

Cfa

BSh

Cfa Csb

CORAL SEA

BSh

BSh BWh

0°

H

Af

ATLANTIC OCEAN

Csb

Cfb

Csa

Cfb

Cfb

40° 100°

120°

0 0°

20°

40°

633

60°

0

140°

1,000 1,000

2,000

2,000

160°

40° 180°

3,000 Miles

3,000 Kilometers

MODIFIED GOODE'S HOMOLOSINE EQUAL-AREA PROJECTION

SmartFigure 20.6 Rain Forest TTropical i Unexcelled in luxuriance and characterized by hundreds of different species per square kilometer, the tropical rain forest is a broadleaf evergreen forest that dominates the wet tropics. This image shows Borneo’s Segama River passing through virgin tropical rain forest. (Photo by Peter Lilja/AGE Fotostock)

634

CHAPTER 20

World Climates and Global Climate Change

FIGURE 20.7 Humid Tropical Climates By comparing these three climatic diagrams, the primary differences among the A climates can be seen. A. Iquitos, the Af station, is wet throughout the year. B. Monrovia, the Am station, has a short dry season. C. As is true for all Aw stations, Normanton has an extended dry season and a higher annual temperature range than the others.

Temp. ˚C (F)

Iquitos, Peru (Af )

Monrovia, Liberia (Am)

Normanton, Australia (Aw)

4˚ S 73˚ W

6˚ N 10˚ W

17˚ S 141˚ E

Precip: 261 cm Temp. range: 2˚C

Cm (In.) 60 (23)

40 (104) 30 (86)

50 (20) Temp.

20 (68)

40 (16)

10 (50) 0 (32)

Precip.

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

30 (12)

J FMAM J J A S O ND

A.

0

Temp. ˚C (F)

Precip: 300 cm Temp. range: 2˚C

40 (104) 30 (86)

50 (20)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

J FMAM J J A S O ND

tropical savanna in Tanzania’s Serengeti National Park, with its stunted, drought-resistant trees, was probably strongly influenced by seasonal burnings carried out by native human populations. (Photo by Kondrachov Vladimir/Shutterstock)

0

Temp. ˚C (F)

Precip: 94 cm Temp. range: 8˚C

40 (104) 30 (86)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

■

Temperatures usually average 25°C (77°F) or more each month. Consequently, not only is the annual mean temperature high, but the annual temperature range is very small.

■

The total precipitation for the year is high, often exceeding 200 centimeters (80 inches).

■

Although rainfall is not evenly distributed throughout the

Cm (In.) 60 (23) 50 (20)

J FMAM J J A S O ND

0

C.

B.

temperatures mark the boundary. Because of the general decrease in temperature with height in the troposphere, this climate region is restricted to elevations below 1000 meters (nearly 3300 feet). Consequently, the major interruptions near the equator are principally cooler highland areas. Data for some representative stations in the wet tropics are shown in FIGURE 20.7A,B . A brief examination reveals the most obvious features that characterize the climate in these areas:

FIGURE 20.8 Tropical Savanna Grassland This

Cm (In.) 60 (23)

year, tropical rain forest stations are generally wet in all months. If a dry season exists, it is very short. Because places with an Af or Am designation lie near the equator, the reason for the uniform temperature rhythm experienced in such locales is clear: The intensity of solar radiation is consistently high. The vertical rays of the Sun are always relatively close, and changes in the length of daylight throughout the year are slight; therefore, seasonal temperature variations are minimal. The region is strongly influenced by the equatorial low. Its converging trade winds and the accompanying ascent of warm, humid, unstable air produce conditions that are ideal for the formation of precipitation.

Tropical Wet and Dry In the latitude zone poleward of the wet tropics and equatorward of the subtropical deserts lies the transitional tropical wet and dry climate. Here the rain forest gives way to the savanna, a tropical grassland with scattered drought-tolerant trees (FIGURE 20.8). Because temperature characteristics among all A climates are quite similar, the primary factor that distinguishes the Aw climate from Af and Am is

20.4 Dry (B) Climates

precipitation. Although the overall amount of precipitation in the tropical wet and dry realm is often considerably less than in the wet tropics, the most distinctive feature of this climate is not the annual rainfall total but the markedly seasonal character of the rainfall. The climate diagram for Normanton, Australia (FIGURE 20.7C), clearly illustrates this trait. As the equatorial low advances poleward in summer, the rainy season commences and features weather patterns typical of the wet tropics. Later, with the retreat of the equatorial low, the subtropical high advances into the region and brings with it pronounced dryness. In some Aw regions, such as India, Southeast Asia, and portions of

|

635

Australia, the alternating periods of rainfall and dryness are associated with a well-established monsoon circulation (see Chapter 18).

20.3 CONCEPT CHECKS 1 What is the main factor that distinguishes Aw climates from Af and Am? How is this difference reflected in the vegetation of these climate regions?

2 How do the equatorial low and the subtropical high influence the seasonal distribution of rainfall in the Aw climate?

20.4 DRY (B) CLIMATES Contrast low-latitude dry climates and middle-latitude dry climates.

It is important to realize that the concept of dryness is a relative one and refers to any situation in which a water deficiency exists. Climatologists define a dry climate as one in which the yearly precipitation is not as great as the potential loss of water by evaporation. Thus, dryness is not only related to annual rainfall totals but is also a function of evaporation, which in turn is closely dependent on temperature. To establish the boundary between dry and humid climates, the Köppen classification uses formulas that involve three variables: average annual precipitation, average annual temperature, and seasonal distribution of precipitation. The use of average annual temperature reflects its importance as an index of evaporation. The amount of rainfall defining the humid–dry boundary increases as the annual mean temperature increases. The use of seasonal precipitation as a variable is also related to this idea. If rain is concentrated in the warmest months, loss to evaporation is greater than if the precipitation is concentrated in the cooler months. Within the regions defined by a general water deficiency are two climatic types: arid, or desert (BW), and semiarid, or steppe (BS). These two groups have many features in common; their differences are primarily a matter of degree. The semiarid is a marginal and more humid variant of the arid climate type and represents a transition zone that surrounds the desert and separates it from the bordering humid climates (see Figure 6.30, page 193).

continent is desert, and much of the remainder is steppe. In addition, arid and semiarid areas are found in southern Africa and make a limited appearance in coastal Chile and Peru. The existence of this dry subtropical realm is primarily the result of the prevailing global distribution of air pressure and winds. Earth’s low-latitude deserts and steppes coincide with the subtropical high-pressure belts where air is subsiding (see Figure 18.17, page 561). When air sinks, it is compressed and warmed. Such conditions are opposite of what is needed for cloud formation and precipitation. Therefore, clear skies, a maximum of sunshine, and dryness are to be expected. The classic view of Africa and the Arabian Peninsula from space in FIGURE 20.9 reinforces this idea. The climate diagrams for Cairo, Egypt, and Monterrey, Mexico (FIGURE 20.10A,B ), illustrate the characteristics of lowlatitude dry climates.

Arabian

Sahara

Low-Latitude Deserts and Steppes The heart of low-latitude dry climates lies in the vicinities of the Tropics of Cancer and Capricorn. A glance at Figure 20.5 shows a virtually unbroken desert environment stretching for more than 9300 kilometers (nearly 6000 miles) from the Atlantic coast of North Africa to the dry lands of northwestern India. In addition to this single great expanse, the Northern Hemisphere contains another, much smaller area of subtropical desert and steppe in northern Mexico and the southwestern United States. In the Southern Hemisphere, dry climates dominate Australia. Almost 40 percent of the

Namib

Kalahari

FIGURE 20.9 Deserts of Africa and the Arabian Peninsula In this view of Earth from space, North Africa’s Sahara Desert, the adjacent Arabian Desert, and the Kalahari and Namib Deserts in southern Africa are clearly visible as tancolored, cloud-free zones. These low-latitude deserts are dominated by the dry, subsiding air associated with pressure belts known as the subtropical highs. By contrast, the band of clouds that extends across central Africa and the adjacent oceans coincides with the equatorial low-pressure belt, the rainiest region on Earth. (NASA)

636

CHAPTER 20

FIGURE 20.10 Climate Diagrams for Arid and Semiarid Stations Stations A. and B. are in the subtropics, whereas C. is in the middle latitudes. Cairo and Lovelock are classified as deserts; Monterrey is a steppe. Lovelock, Nevada, may also be called a rain shadow desert.

World Climates and Global Climate Change

Temp. ˚C (F)

Cairo, Egypt (BWh)

Monterey, Mexico (BSh)

31˚ N 31˚ E

26˚ N 100˚ W

Precip: 2.5 cm Temp. range: 16˚C

40 (104) 30 (86)

50 (20)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

Cm (In.) 60 (23)

J FMAM J J A S O ND

0

Temp. ˚C (F)

Precip: 51 cm Temp. range: 14˚C

40 (104) 30 (86)

40˚ N 119˚ W Cm (In.) 60 (23) 50 (20)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

Lovelock, Nevada, USA (BWk)

J FMAM J J A S O ND

A.

0

Temp. ˚C (F)

Precip: 10 cm Temp. range: 25˚C

40 (104) 30 (86)

50 (20)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

J FMAM J J A S O ND

B.

Middle-Latitude Deserts and Steppes Unlike their low-latitude counterparts, middle-latitude deserts and steppes are not controlled by the subsiding air masses associated with high pressure. Instead, these dry lands exist principally because of their positions in the deep interiors of large landmasses far removed from the oceans, which are the ultimate source of moisture for cloud formation and precipitation. In addition, the presence of high

mountains across the paths of prevailing winds further acts to separate these areas from water-bearing maritime air masses. Windward sides of mountains are often wet. As prevailing winds meet mountain barriers, the air is forced to ascend, producing clouds and precipitation. By contrast, the leeward sides of mountains are usually much dryer and are often arid enough to be referred to as rain shadow deserts (see Figure 17.11, page 526). Because

EARTH E

North America

QUESTIO N 1 What is the name of the desert that occupies the cloudless area along the western margin of South America? QUESTIO N 2 An ocean current flows just offshore of this desert area. Is it warm or cold? Toward what direction must the current flow—toward or away from the equator?

0

C.

EYE ON EY

This classic view of Earth from space was taken in December 1968 by T a Apollo 8 astronaut. The image shows the Western Hemisphere. Dense an clo clouds cover much of North America. A large portion of South America is also cloud covered. However, the western margin of South America is cloud free free. As you answer the following questions, you will find it helpful to consult Figure 6.30 (page 193) and Figures 15.2 and 15.3 (pages 455–456) and the discussions that relate to these figures.

Cm (In.) 60 (23)

r ato Equ

QUESTIO N 3 How does this desert’s close proximity to the Pacific Ocean influence its aridity? South America

NASA

20.5 Humid Middle-Latitude Climates (C and D Climates)

many middle-latitude deserts occupy sites on the leeward sides of the mountains, they can also be classified as rain shadow deserts (FIGURE 20.10C). In North America, the Coast Ranges, Sierra Nevada, and Cascades are the foremost mountain barriers. In Asia, the great Himalayan chain prevents the summertime monsoon flow of moist Indian Ocean air from reaching the interior. Because the Southern Hemisphere lacks extensive land areas in the middle latitudes, only a small area of desert and steppe is found in this latitude range, existing primarily in the rain shadow of the towering Andes. In the case of middle-latitude deserts, we have an example of the impact of tectonic processes on climate.

637

Rain shadow deserts exist because of the mountains produced when plates collide. Without such mountain-building episodes, wetter climates would prevail where many dry regions exist today.

20.4 CONCEPT CHECKS 1 Why is the amount of precipitation that defines the boundary between humid and dry climates variable?

2 What is the primary reason (control) for the existence of the dry subtropical realm (BWh and BSh)?

3 What factors contribute to the existence of middle-latitude deserts and steppes?

20.5 | HUMID MIDDLE-LATITUDE CLIMATES (C AND D CLIMATES) Distinguish among five different humid middle-latitude climates.

The humid middle-latitude climates dominate large portions of North America, Europe, and Asia. Humid middlelatitude climates are divided into two categories: C climates, which have mild winters, and D climates, which have severe winters.

Humid Middle-Latitude Climates with Mild Winters (C Climates) Although the term subtropical is often used for the C climates, it can be misleading. Although many areas with C climates do indeed possess some near-tropical characteristics, other regions do not. For example, we would be stretching

Guangzhou, China (Cfa) Precip: 165 cm Temp. range: 16˚C

40 (104) 30 (86)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

Cm (In.) 60 (23)

J FMAM J J A S O ND

A.

0

Temp. ˚C (F)

Precip: 215 cm Temp. range: 13˚C

40 (104) 30 (86)

Cm (In.) 60 (23)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

34˚ S 19˚ E

50 (20)

20 (68)

J FMAM J J A S O ND

B.

FIGURE 20.11 Examples of C Climates A. humid

Capetown, South Africa (Cs)

57˚ N 135˚ W

50 (20)

20 (68)

Humid Subtropics Located on the eastern sides of the continents, in the 25° to 40° latitude range, the humid subtropical climate dominates the southeastern United States, as well as other similarly situated areas around the world (see Figure 20.5). The climate diagram for Guangzhou, China (FIGURE 20.11A ), shows a typical example. In summer, the humid subtropics experience hot, sultry weather of the type one expects to find in the rainy tropics. Daytime temperatures

Sitka, Alaska, USA (Cfb)

23˚ N 113˚ E Temp. ˚C (F)

the use of the term subtropical to describe the climates of coastal Alaska and Norway, which belong to the C group. Within the C group of climates, several subgroups are recognized.

0

Temp. ˚C (F)

Precip: 51 cm Temp. range: 8˚C

40 (104) 30 (86)

50 (20)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

Cm (In.) 60 (23)

J FMAM J J A S O ND

C.

0

subtropical, B. marine west coast, and C. dry-summer subtropical.

638

CHAPTER 20

World Climates and Global Climate Change

are generally high, and because both mixing ratio and relative humidity are high, the night brings little relief. An afternoon or evening thunderstorm is also possible, for these areas experience such storms on an average of 40–100 days each year, the majority during the summer months. As summer turns to autumn, the humid subtropics lose their similarity to the rainy tropics. Although winters are mild, frosts are common in the higher-latitude Cfa areas and occasionally plague the tropical margins as well. The winter precipitation is also different in character from the summer. Some is in the form of snow, and most is generated along fronts of the frequent middle-latitude cyclones that sweep over these regions.

Marine West Coast Situated on the western (windward) side of continents, from about 40° to 65° north and south latitude, is a climate region dominated by the onshore flow of oceanic air (FIGURE 20.12 ). In North America, the marine west coast climate extends from near the U.S.–Canadian border northward as a narrow belt into southern Alaska (FIGURE 20.11B ). The largest area of Cfb climate is found in Europe because there are no mountain barriers blocking the movement of cool maritime air from the North Atlantic. The prevalence of maritime air masses means that mild winters and cool summers are the rule, as is an ample amount of rainfall throughout the year. Although there is no pronounced dry period, there is a drop in monthly precipitation totals during the summer. The reason for the reduced summer rainfall is the poleward migration of the oceanic subtropical highs. Although the areas of marine west FIGURE 20.12 Marine West Coast Climate Fog is common along the rocky Pacific coastline at Olympic National Park, Washington. As the name of this climate implies, the ocean exerts a strong influence. (Photo by Richard J. Green/Science Source)

coast climate are situated too far poleward to be dominated by these dry anticyclones, their influence is sufficient to cause a decrease in warm season rainfall.

Dry-Summer Subtropics The dry-summer subtropical climate is typically located along the west sides of continents between latitudes 30° and 45°. Situated between the marine west coast climate on the poleward side and the subtropical steppes on the equatorward side, this climate is best described as transitional in character. It is unique because it is the only humid climate that has a strong winter rainfall maximum, a feature that reflects its intermediate position (FIGURE 20.11C). In summer, the region is dominated by stable conditions associated with the oceanic subtropical highs. In winter, as the wind and pressure systems follow the Sun equatorward, the region is within range of the cyclonic storms of the polar front. Thus, during the course of a year, these areas alternate between becoming a part of the dry subtropics and an extension of the humid middle latitudes. Whereas middle-latitude changeability characterizes the winter, subtropical constancy describes the summer. As is the case for the marine west coast climate, mountain ranges limit the dry-summer subtropics to a relatively narrow coastal zone in both North and South America. Because Australia and southern Africa barely reach to the latitudes where dry-summer climates exist, the development of this climatic type is limited on those continents as well. Consequently, because of the arrangement of the continents and their mountain ranges, inland development occurs only in the Mediterranean basin. Here the zone of subsidence extends far to the east in summer; in winter, the sea is a major route of cyclonic disturbances. Because the dry-summer climate is particularly extensive in this region, the name Mediterranean climate is often used as a synonym.

Humid Middle-Latitude Climates with Severe Winters (D Climates) The C climates that were just described characteristically have mild winters. By contrast, D climates experience severe winters. Two types of D climates are recognized: the humid continental and the subarctic climates. Climatic diagrams of representative locations are shown in FIGURE 20.13. The D climates are land-controlled climates, the result of broad continents in the middle latitudes. Because continentality is a basic feature, D climates are absent in the Southern

20.5 Humid Middle-Latitude Climates (C and D Climates)

Hemisphere, where the middlelatitude zone is dominated by the oceans.

Chicago, Illinois, USA (Dfa) Precip: 84 cm Temp. range: 26˚C

FIGURE 20.13 Examples of D Climates Climates in

Moose Factory, Ontario (Dfc)

42˚ N 88˚ W Temp. ˚C (F)

51˚ N 80˚ W Cm (In.) 60 (23)

Temp. ˚C (F)

Precip: 58 cm Temp. range: 38˚C

Cm (In.) 60 (23)

Humid Continental The 40 40 humid continental climate is (104) (104) confined to the central and east30 30 ern portions of North America 50 50 (86) (86) and Eurasia in the latitude range (20) (20) between approximately 40° and 20 20 40 40 50° north latitude. It may at first (68) (68) (16) (16) seem unusual that a continental 10 10 climate should extend eastward (50) (50) 30 30 to the margins of the ocean. (12) (12) However, because the prevailing 0 0 (32) (32) atmospheric circulation is from 20 20 the west, deep and persistent -10 -10 (8) (8) incursions of maritime air from (14) (14) the east are not likely to occur. 10 10 -20 -20 (4) (4) Both winter and summer tem(-4) (-4) peratures in the humid continental -30 -30 climate can be characterized as 0 0 (-22) J F M A M J J A S O N D (-22) J F M A M J J A S O N D relatively severe. Consequently, annual temperature ranges are A. B. high throughout the climate. Precipitation is generally greater in summer than in winter. expanses from western Alaska to Newfoundland in North Precipitation totals generally decrease toward the interior of the America and from Norway to the Pacific coast of Ruscontinents, as well as from south to north, primarily because of sia in Eurasia (see Figure 20.5). It is often referred to as increasing distance from the sources of maritime tropical (mT) the taiga climate, for its extent closely corresponds to the air. Furthermore, the more northerly stations are also influenced northern coniferous forest region of the same name (FIGURE 20.14). Although scrawny, the spruce, fir, larch, and birch for a greater part of the year by drier polar air masses. Wintertime precipitation in humid continental climates trees in the taiga represent the largest stretch of continuous is chiefly associated with the passage of fronts connected forest on the surface of Earth. with traveling midlatitude cyclones. Part of this precipitation is in the form of snow, and the proportion of snow increases with latitude. Although precipitation is often considerably less during the cold season, it is usually more conspicuous than the greater amounts that fall during summer. An obvious reason is that snow remains on the ground, often for extended periods. Subarctic Situated north of the humid continental climate and south of the polar tundra is an extensive subarctic climate region covering broad, uninterrupted

639

this category are associated with the interiors of large landmasses in the mid- to high latitudes of the Northern Hemisphere. Winters can be harsh in Chicago’s humid continental (Dfa) climate, and the subarctic environment (Dfc) of Moose Factory is more extreme.

FIGURE 20.14 Subarctic Climate The northern coniferous forest, also called the taiga, is associated with subarctic climates. This climate typically experiences the highest annual temperature ranges on Earth. This scene is in Quebec’s Gaspésie Peninsula. (Photo by Michael P. Gadomski/Science Source)

640

CHAPTER 20

World Climates and Global Climate Change

Here in the source regions of continental polar air masses, the outstanding feature is certainly the dominance of winter. Winter is long, and temperatures are bitterly cold. Winter minimum temperatures are among the lowest ever recorded outside the ice sheets of Greenland and Antarctica. In fact, for many years, the world’s coldest temperature was attributed to Verkhoyansk in eastcentral Siberia, where the temperature dropped to –68°C (–90°F) on February 5 and 7, 1892. Over a 23-year period, this same station had an average monthly minimum of –62°C (–80°F) during January. Although exceptional temperatures, they illustrate the extreme cold that envelops the taiga in winter. By contrast, summers in the subarctic are remarkably warm, despite their short duration. However, when compared with regions farther south, this short season must be characterized as cool. The extremely cold winters and relatively warm summers combine to produce the highest annual temperature ranges on Earth. Because these far northerly continental interiors are the source regions

|

for cP air masses, there is very limited moisture available throughout the year. Precipitation totals are therefore small, with a maximum occurring during the warmer summer months.

20.5 CONCEPT CHECKS 1 Describe and explain the differences between summertime and wintertime precipitation in the humid subtropical climate (Cfa).

2 Why is the marine west coast climate (Cfb) represented by only slender strips of land in North and South America, and why is it very extensive in Western Europe?

3 The dry-summer subtropics are described as transitional. Explain why this is true.

4 Why is the humid continental climate confined to the Northern Hemisphere?

5 Describe and explain the annual temperature range you should expect in the realm of the taiga.

20.6 POLAR (E) CLIMATES Contrast ice cap and tundra climates.

Polar climates are those in which the mean temperature of the warmest month is below 10°C (50°F). Thus, just as the tropics are defined by their year-round warmth, the polar realm is known for its enduring cold. As winters are periods of perpetual night, or nearly so, temperatures at most polar locations are understandably bitter. During the summer months temperatures remain cool despite the long days, because the Sun is so low in the sky that its oblique rays are not effective in bringing about a genuine warming. Although polar climates are classified as humid, precipitation is generally meager. Evaporation, of course, is also limited. The FIGURE 20.15 Ice Cap Climate Greenland and Antarctica are the major examples of this extreme climate. In this image, scientists are conducting research on the Greenland ice sheet. (Photo by Dr. Joel Harper)

scanty precipitation totals are easily understood in view of the temperature characteristics of the region. The amount of water vapor in the air is always small because low mixing ratios must accompany low temperatures. Usually precipitation is most abundant during the warmer summer months, when the moisture content of the air is highest. Two types of polar climates are recognized. The tundra climate (ET) is a treeless climate found almost exclusively in the Northern Hemisphere (FIGURE 20.15). Because of the combination of high latitude and continentality, winters are severe, summers are cool, and annual temperature ranges are

20.7 Highland Climates

Point Barrow, Alaska, USA (ET)

Eismitte, Greenland (EF)

71˚ N 156˚ W

70˚ N 40˚ W

Temp. ˚C (F)

Precip: 13 cm Temp. range: 34˚C

40 (104) 30 (86)

50 (20)

20 (68)

40 (16)

10 (50)

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) -30 (-22)

Cm (In.) 60 (23)

J FM AM J J A S O ND

0

Temp. ˚C (F)

Temp. range: 36˚C

40 (104) 30 (86)

40 (16)

10 (50)

climatic diagrams represent the two basic types of polar climates. A. Barrow, Alaska, exhibits a tundra (ET) climate. B. Eismitte, Greenland, a station located on a massive ice sheet, is classified as an ice cap (EF) climate.

30 (12)

0 (32)

20 (8)

-10 (14)

10 (4)

-20 (-4) J FM AM J J A S O ND

A.

high (FIGURE 20.16A). Furthermore, yearly precipitation is small, with a modest summer maximum. The ice cap climate (EF) does not have a single monthly mean above 0°C (32°F) (FIGURE 20.16B). Consequently, because the average temperature for all months is below freezing, the growth of vegetation is prohibited, and the landscape is one of permanent ice and snow. This climate of perpetual frost covers a surprisingly large area of more than 15.5 million square kilometers (6 million square miles), or about 9 percent of Earth’s land area. Aside from scattered occurrences in high mountain areas, it is confined

|

Cm (In.) 60 (23) 50 (20)

20 (68)

-30 (-22)

SmartFigure 20.16 EExamples of E Climates These

0

B.

to the ice sheets of Greenland and Antarctica (see Figure 6.2, page 173).

20.6 CONCEPT CHECKS 1 Although polar regions experience extended periods of sunlight in the summer, temperatures remain cool. Explain.

2 Why are precipitation totals low in polar climates? Which season has the most precipitation? Why?

3 Where are EF climates most extensively developed?

20.7 HIGHLAND CLIMATES Summarize the characteristics associated with highland climates.

It is a well-known fact that mountains have climate conditions that are distinctly different from those found in adjacent lowlands. Compared to nearby places at lower elevations, sites with highland climates are cooler and usually wetter. Unlike the world climate types already discussed, which consist of large, relatively homogeneous regions, the outstanding characteristic of highland climates is the great diversity of climatic conditions that occur. The best-known climatic effect of increased altitude is lower temperatures. In addition, an increase in precipitation due to orographic lifting usually occurs at higher elevations. Despite the fact that mountain stations are colder and often wetter than locations at lower elevations,

highland climates are often very similar to those in adjacent lowlands in terms of seasonal temperature cycles and precipitation distribution. FIGURE 20.17 illustrates this relationship. Phoenix, at an elevation of 338 meters (1109 feet), lies in the desert lowlands of southern Arizona. By contrast, Flagstaff is located at an altitude of 2100 meters (7000 feet) on the Colorado Plateau in northern Arizona. When summer averages climb to 34°C (93°F) in Phoenix, Flagstaff is experiencing a pleasant 19°C (66°F), which is a full 15°C (27°F) cooler. Although the temperatures at each city are quite different, the pattern of monthly temperature changes for each place is similar. Both experience their minimum and maximum monthly means in the same months. When

641

642

CHAPTER 20

World Climates and Global Climate Change

SmartFigure 20.17 Highland Cli Climate Diagrams for two stations in Arizona illustrate the general influence of elevation on climate. Flagstaff is cooler and wetter because of its position on the Colorado Plateau, nearly 1800 meters (6000 feet) higher than Phoenix. Only scanty drought-tolerant natural vegetation can survive in the hot, dry climate of southern Arizona, near Phoenix. (Photo by Design Pics/ SuperStock) The natural vegetation associated with the cooler, wetter highlands near Flagstaff, Arizona, is much different from the desert lowlands. (Photo by Jim Cole/Alamy)

Temperature ˚C (F)

40 (104)

Precipitation cm (In.)

Temperature– Phoenix, AZ 33° 30' N. lat. elevation = 338 m

30 (86)

50 (20)

20 (68) 10 (50) 0 (32) -10 (14) -20 (-4) -30 (-22)

60 (23)

40 (16) 30 (12) Temperature– Flagstaff, AZ 35° 15' N. lat. 20 elevation = 2134 m (8) Precipitation– Flagstaff 10 Precipitation– (4) Phoenix J FM AM J J A S O ND

0

precipitation data are examined, both places have a similar seasonal pattern, but the amounts at Flagstaff are higher in every month. In addition, because of its higher altitude, much of Flagstaff’s winter precipitation is in the form of snow. By contrast, all of the precipitation at Phoenix is rain. Because topographic variations are pronounced in mountains, every change in slope with respect to the Sun’s rays produces a different microclimate. In the Northern Hemisphere, south-facing slopes are warmer and dryer because they receive more direct sunlight than northfacing slopes and deep valleys. Wind direction and speed in mountains can be highly variable. Mountains create various obstacles to winds. Locally, winds may be funneled through valleys or forced over ridges and around mountain peaks. When weather conditions are fair, mountain and valley breezes are created by the topography itself.1 We know that climate strongly influences vegetation, which is the basis for the Köppen system. Thus, where there are vertical differences in climate, we should expect a vertical zonation of vegetation as well. By ascending a mountain, we can view dramatic vegetation changes that 1

Phoenix, AZ

Mountain and valley breezes are described in the section “Local Winds” in Chapter 18. Also see Figure 18.20, page 564.

Flagstaff, AZ

otherwise might require a poleward journey of thousands of kilometers (see Figure 20.17). This occurs because altitude duplicates, in some respects, the influence of latitude on the distribution of vegetation. Perhaps the terms variety and changeability best describe mountain climates. Because atmospheric conditions fluctuate rapidly with changes in altitude and exposure, a nearly limitless variety of local climates occur in mountainous regions. The climate in a protected valley is very different from that of an exposed peak. Conditions on windward slopes contrast sharply with those on the leeward sides, whereas slopes facing the Sun are unlike those that lie mainly in the shadows.

20.7 CONCEPT CHECKS 1 The Arizona cities of Flagstaff and Phoenix are relatively close to one another yet have contrasting climates. Briefly describe the differences and why they occur.

20.8 Human Impact on Global Climate

|

20.8 HUMAN IMPACT ON GLOBAL CLIMATE Summarize the nature and cause of the atmosphere’s changing composition since about 1750. Describe the climate’s response.

Proposals to explain global climate change are many and varied. In Chapter 6, we examined some possible causes for ice-age climates. These hypotheses, which involved plate tectonics and variations in Earth’s orbit, were natural causes. Another natural cause, discussed in Chapter 9, is the possible role of explosive volcanic eruptions in modifying the atmosphere. It is important to remember that these mechanisms, as well as others, were not only responsible for climate change in the geologic past but will also contribute to future shifts in climate. When relatively recent and future changes in our climate are considered, we must also examine the impact of human beings. In this section, we examine how humans contribute to global climate change. One impact largely results from the addition of carbon dioxide and other greenhouse gases to the atmosphere. A second impact is related to the addition of human-generated aerosols to the atmosphere. Human influence on regional and global climate did not just begin with the onset of the modern industrial period. There is good evidence that people have been modifying the environment over extensive areas for thousands of years. The use of fire and the overgrazing of marginal lands by domesticated animals have both reduced the abundance and distribution of vegetation. By altering ground cover, humans have modified such important climate factors as surface albedo, evaporation rates, and surface winds.

Rising CO2 Levels In Chapter 16 you learned that carbon dioxide (CO2) represents only about 0.0397 percent (397 parts per million) of the gases that make up clean, dry air. Nevertheless, it is a very significant component meteorologically. Carbon dioxide is influential because it is transparent to incoming shortwavelength solar radiation, but it is not transparent to some of the longer-wavelength outgoing Earth radiation. A portion of the energy leaving the ground is absorbed by atmospheric CO2. This energy is subsequently re-emitted, part of it back toward the surface, thereby keeping the air near the ground warmer than it would be without CO2. Thus, along with water vapor, carbon dioxide is largely responsible for the greenhouse effect of the atmosphere. Carbon dioxide is an important heat absorber, and it follows logically that any change in the air’s CO2 content could alter temperatures in the lower atmosphere. Earth’s tremendous industrialization of the past two centuries has been fueled—and still is fueled—by burning fossil fuels: coal, natural gas, and petroleum (FIGURE 20.18 ). Combustion of these fuels has added great quantities of carbon dioxide to the atmosphere. Figure 16.5 (page 489) shows changes in CO2 concentrations at Hawaii’s Mauna Loa Observatory, where measurements have been made since 1958. The graph shows an annual seasonal cycle and

EYE ON EY

EARTH E

Amundsen–Scott South Pole Station is an AmeriA c research facility. Among the phenomena that can are monitored there are variations in atmospheric ar composition. The accompanying graph shows com changes in the air’s CO2 content at South Pole Station change (90° south latitude) and at a similar facility at Barrow, Alaska (71° north latitude). (Photo by Vicky Beaver/Alamy) QU ESTIO N 1 Describe how the two lines on the graph differ. 400

QU ESTIO N 2 Which line on the graph represents the South Pole, and which represents Barrow, Alaska?

390

QU ESTIO N 3 Explain how you were able to determine which line is which. CO2 (ppm)

380 370 360 350 340 330 320

1980

1988

1996 Year

2004

2012

643

644

CHAPTER 20

World Climates and Global Climate Change

FIGURE 20.18 U.S. Energy Consumption The Quadrillion Btu

graph shows energy consumption in 2011. The total was 97.5 quadrillion Btu. A quadrillion is 10 raised to the 12th power, or a million million. The burning of fossil fuels represents slightly more than 82 percent of the total.

40 30 20 10 0

Petroleum

Natural Gas

Coal

Nuclear Renewable Electric Energy Power

(Data from U.S. Energy Information Administration)

The Atmosphere’s Response Given the increase in the atmosphere’s carbon dioxide content, have global temperatures actually increased? The answer is “yes.” According to the Intergovernmental Panel on Climate Change (IPCC), warming of the climate system

CO2 parts per million

a steady upward trend over the years. The up-and-down of the seasonal cycle is due to the vast land area of the Northern Hemisphere, which contains the majority of land-based vegetation. During spring and summer in the Northern Hem- FIGURE 20.19 Tropical Deforestation Clearing the tropical isphere, when plants are absorbing CO2 as part of photosyn- rain forest is a serious environmental issue. In addition to causing thesis, concentrations decrease. The annual increase in CO2 a loss of biodiversity, it is a significant source of carbon dioxide. Fires are frequently used to clear the land. This scene is in Brazil’s during the cold months occurs as vegetation dies and leaves Amazon basin. (Photo by Pete Oxford/Nature Picture Library) fall and decompose, which releases CO2 back into the air. The use of coal and other fuels is the most prominent is unequivocal, as is now evident from observations of means by which humans add CO2 to the atmosphere, but it increases in global average air and ocean temperatures, is not the only way. The clearing of forests also contributes widespread melting of snow and ice, and rising global sea substantially because CO2 is released as vegetation is burned level.2 Most of the observed increase in global average temor decays. Deforestation is particularly pronounced in the peratures since the mid-twentieth century is very likely due tropics, where vast tracts are cleared for ranching and agriculto the observed increase in human-generated greenhouse gas ture or are subjected to inefficient commercial logging operaconcentrations. (As used by the IPCC, very likely indicates tions (FIGURE 20.19 ). According to United Nations estimates, a probability of 90–99 percent.) Global warming since the nearly 10.2 million hectares (25.1 million acres) of tropical mid-1970s is now about 0.6°C (1°F), and total warming in forest were permanently destroyed each year during the 1990s. Between the years 2000 and 2005, the average figure increased 2IPCC, “Summary for Policy Makers.” In Climate Change 2013: The to 10.4 million hectares (25.7 million acres) per year. Physical Science Basis. The Intergovernmental Panel on Climate Change is Some of the excess CO2 is taken up by plants or is dis- an authoritative group that provides advice to the world community through solved in the ocean. It is estimated that about 45 percent periodic reports that assess the state of knowledge of causes of climate change. More than 259 authors and 1089 scientific reviewers from more remains in the atmosphere. FIGURE 20.20 is a graphic record of than 55 countries contributed to the 2013 report. changes in atmospheric CO2 extending back more than 400,000 years. Over this long span, natural fluctuations have varied from about 180 to 300 ppm. As a result of human activities, the present CO2 level is about 30 percent higher than its highest level 460 over at least the past 650,000 440 years. The rapid increase in CO2 420 2013 400 concentrations since the onset 380 of industrialization is obvious. 360 The annual rate at which atmos340 pheric CO2 concentrations are 320 growing has been increasing For 650,000 years, atmospheric CO2 was never higher than this level 300 over the past several decades. 1950 280 260 240 220 200 180 160

400,000 350,000 300,000 250,000 200,000 150,000 100,000 Years before today

50,000

0

FIGURE 20.20 CO2 Concentrations over the Past 400,000 Years Most of these data come from the analysis of air bubbles trapped in ice cores. The record since 1958 comes from direct measurements at Mauna Loa Observatory, Hawaii. The rapid increase in CO2 concentrations since the onset of the Industrial Revolution is obvious. (NOAA)

645 6.0

FIGURE 20.22 Temperature Projections to 2100 The right half of

.4

0 –.2 –.4 1880

1900

1920

1940

1960

1980

2000

2020

A.

s

5.0 4.0

g d loba iff l a e r ve en ra t s ge ce na rio

.2

Global surface warming (°C)

Temperature difference (°C) compared to 1951-80 mean

.6

3.0 2.0

Global temperature changes during the 20th century

1.0

g ted sin c oje u Pr ures rat pe m te

0.0 –1.0 1900

1950

2000 Year

2050

2100

the graph shows projected global warming based on different emissions scenarios. The shaded zone adjacent to each colored line shows the uncertainty range for each scenario. The basis for comparison (0.0 on the vertical axis) is the global average for the period 1980 to 1999. The orange line represents the scenario in which CO2 concentrations were held constant at values for the year 2000. (NOAA)

unlikely” (1 to 10 percent probability) to be less than 1.5°C (2.7°F), and values higher than 4.5°C (8.1°F) are possible.

The Role of Trace Gases B.

–2 –1 0 1 2 Temperature difference (°C) compared to 1951-80 mean

FIGURE 20.21 Global Temperatures The year 2012 was among the 10 warmest years on record. A. The graph depicts global temperature change in degrees Celsius since the year 1880. B. The world map shows global temperature differences averaged from 2008 through 2012 compared to the mean for the 1951–1980 base period. The high latitudes in the Northern Hemisphere clearly stand out. (NASA/Goddard Institute for Space Studies)

the past century is about 0.8°C (1.4°F). The upward trend in surface temperatures is shown in FIGURE 20.21A. The world map in FIGURE 20.21B compares surface temperatures for 2012 to the base period (1951–1980). You can see that the greatest warming has been in the Arctic and neighboring high-latitude regions. Here are some related facts: ■

When we consider the 132-year span for which there are instrumental records (since 1880), the 10 warmest years have all occurred since 1998.

■

Global mean temperature is now higher than at any time in at least the past 500 to 1000 years.

■

The average temperature of the global ocean has increased to depths of at least 3000 meters (10,000 feet).

Are these temperature trends caused by human activities, or would they have occurred anyway? The scientific consensus of the IPCC is that human activities were very likely responsible for most of the temperature increase since 1950. What about the future? Projections for the years ahead depend in part on the quantities of greenhouse gases that are emitted. FIGURE 20.22 shows the best estimates of global warming for several different scenarios. The IPCC estimates that if there is a doubling of the preindustrial level of carbon dioxide (280 ppm) to 560 ppm, the “likely” temperature increase will be in the range of 2° to 4.5°C (3.5° to 8.1°F). The increase is “very

Carbon dioxide is not the only gas that contributes to the global increase in temperature. In recent years, atmospheric scientists have come to realize that the industrial and agricultural activities of people are causing a buildup of several trace gases that also play significant roles. The substances are called trace gases because their concentrations are much lower than the concentration of carbon dioxide. The trace gases that are most important are methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). These gases absorb wavelengths of outgoing radiation from Earth that would otherwise escape into space (FIGURE 20.23 ). Although individually their impact is FIGURE 20.23 Methane Methane is produced by anaerobic bacteria in wet places, where oxygen is scarce. (Anaerobic means “without air,” specifically oxygen.) Such places include swamps, bogs, wetlands, and the guts of termites and grazing animals such as cattle and sheep. Methane is also generated in flooded paddy fields (“artificial swamps”) used for growing rice. Coal mining and drilling for oil and natural gas are other sources of methane. (Photo by Robert Harding/ SuperStock)

648

CHAPTER 20

World Climates and Global Climate Change

20.8 CONCEPT CHECKS

modest, taken together, these trace gases play a significant role in warming the troposphere. Sophisticated computer models show that the warming of the lower atmosphere caused by CO2 and trace gases will not be the same everywhere. Rather, the temperature response in polar regions could be two to three times greater than the global average. One reason is that the polar troposphere is very stable, which suppresses vertical mixing and thus limits the amount of surface heat that is transferred upward. In addition, the expected reduction in sea ice will contribute to the greater temperature increase. This topic will be explored more fully in the next section.

|

1 Why has the CO2 level of the atmosphere been increasing over the past 200 years?

2 How has the atmosphere responded to the growing CO2 levels? 3 How are temperatures in the lower atmosphere likely to change as CO2 levels continue to increase?

4 Aside from CO2, what trace gases are contributing to global temperature change?

20.9 CLIMATE-FEEDBACK MECHANISMS Contrast positive- and negative-feedback mechanisms and provide examples of each.

Climate is a very complex interactive physical system. Thus, when any component of the climate system is altered, scientists must consider many possible outcomes, some of which amplify the initial effect and some of which balance it out. These possible outcomes are called climate-feedback mechanisms. They complicate climate-modeling efforts and add greater uncertainty to climate predictions.

Types of Feedback Mechanisms What climate-feedback mechanisms are related to carbon dioxide and other greenhouse gases? One important mechanism is that warmer surface temperatures increase evaporation rates. This in turn increases the atmosphere’s water vapor content. Remember that water vapor is an even more powerful absorber of radiation emitted by Earth than is carbon dioxide. Therefore, with more water vapor in the air, the temperature increase caused by carbon dioxide and the trace gases is reinforced. Recall that the temperature increase at high latitudes may be two to three times greater than the global average. FIGURE 20.24 Sea Ice As a Feedback Mechanism The image shows the springtime breakup of sea ice near Antarctica. The diagram shows a likely feedback loop. A reduction in sea ice acts as a positive-feedback mechanism because surface albedo decreases, and the amount of energy absorbed at the surface increases. (Photo by Radius Images/Alamy)

Decline in the perennial ice cover Longer melt period

Warmer ocean

Reduced reflectivity Increased absorption of solar radiation

This projection is based in part on the likelihood that the area covered by sea ice will decrease as surface temperatures rise. Because ice reflects a much larger percentage of incoming solar radiation than does open water, the melting of the sea ice causes a surface having a high albedo to be replaced by a surface with a much lower albedo (FIGURE 20.24). The result is a substantial increase in the solar energy absorbed at the surface. This in turn feeds back to the atmosphere and magnifies the initial temperature increase created by higher levels of greenhouse gases. So far, the climate-feedback mechanisms discussed have magnified the temperature rise caused by the buildup of carbon dioxide. Because these effects reinforce the initial change, they are called positive-feedback mechanisms. However, other effects must be classified as negativefeedback mechanisms because they produce results that are just the opposite of the initial change and tend to offset it. One probable result of a global temperature rise would be an accompanying increase in cloud cover due to the higher moisture content of the atmosphere. Most clouds are good reflectors of solar radiation. At the same time, however, they are also good absorbers and emitters of radiation emitted by Earth. Consequently, clouds produce two opposite effects. They are a negativefeedback mechanism because they increase the reflection of solar radiation and thus diminish the amount of solar energy available to heat the atmosphere. On the other hand, clouds act as a positive-feedback mechanism by absorbing and emitting radiation that would otherwise be lost from the troposphere. Which effect, if either, is stronger? Scientists still are not sure whether clouds will produce net positive or negative feedback. Although recent studies have not settled the question, they seem to lean toward the idea that clouds do not dampen global warming but rather produce a small positive feedback overall.3 Global warming caused by human-induced changes in atmospheric composition continues to be one of the most-studied aspects of climate change. Although no models yet incorporate 3

A. E. Dessler, “A Determination of the Cloud Feedback from Climate Variations over the Past Decade,” Science 330: 1523–1526, December 10, 2010.

the full range of potential factors and feedbacks, there is strong scientific consensus that the increasing levels of atmospheric carbon dioxide and trace gases have already warmed the planet and will continue to do so into the foreseeable future.

Computer Models of Climate: Important yet Imperfect Tools Earth’s climate system is amazingly complex. Comprehensive state-of-the-science climate simulation models are among the basic tools used to develop possible climate-change scenarios. Called general circulation models (GCMs), they are based on fundamental laws of physics and chemistry and incorporate human and biological interactions. The models simulate many variables, including temperature, rainfall, snow cover, soil moisture, winds, clouds, sea ice, and ocean circulation over the entire globe through the seasons and over spans of decades. In many other fields of study, hypotheses can be tested by direct experimentation in the laboratory or by observations and measurements in the field. However, this is often not possible in the study of climate. Rather, scientists must construct computer models of how our planet’s climate system works. If we understand the climate system correctly and construct a model appropriately, then the behavior of the model climate system should mimic the behavior of Earth’s climate system (FIGURE 20.25 ). What factors influence the accuracy of climate models? Clearly, mathematical models are simplified versions of the real Earth and cannot capture its full complexity, especially at smaller geographic scales. Moreover, when computer models are used to simulate future climate change, many assumptions have to be made that significantly influence

|

Temperature (°F)

20.10 How Aerosols Influence Climate

FIGURE 20.25 Computer Models The blue band

With human effects

58 Observed 57

56

Natural forces only

1900

1950 Year

2000

Observations Models using only natural forces Models using both natural and human forces

the outcome. They must consider a wide range of possibilities for future changes in population, economic growth, consumption of fossil fuels, technological development, improvements in energy efficiency, and more. Despite many obstacles, our ability to use supercomputers to simulate climate continues to improve. Although today’s models are far from infallible, they are powerful tools for understanding what Earth’s future climate might be like.

20.9 CONCEPT CHECKS 1 Distinguish between positive and negative climate-feedback mechanisms.

2 Provide at least one example of each type of feedback mechanism.

3 List some factors that influence the accuracy of computer models of climate.

20.10 HOW AEROSOLS INFLUENCE CLIMATE Discuss the possible impacts of aerosols on climate change.

Increasing the levels of carbon dioxide and other greenhouse gases in the atmosphere is the most direct human influence on global climate. But it is not the only impact. Global climate is also affected by human activities that contribute to the atmosphere’s aerosol content. Aerosols are the tiny, often microscopic, liquid and solid particles that are suspended in the air. Unlike cloud droplets, aerosols are present even in relatively dry air. Atmospheric aerosols are composed of many different materials, including soil, smoke, sea salt, and sulfuric acid. Natural sources are numerous and include such phenomena as dust storms and volcanoes. Most human-generated aerosols come from the sulfur dioxide emitted during the combustion of fossil fuels and as a consequence of burning vegetation to clear agricultural land. Chemical reactions in the atmosphere convert the sulfur dioxide into sulfate aerosols, the same material that produces acid precipitation. The satellite images in FIGURE 20.26 provide an example. How do aerosols affect climate? Aerosols act directly by reflecting sunlight back to space and indirectly by making

649

clouds “brighter” reflectors. The second effect relates to the fact that many aerosols (such as those composed of salt or sulfuric acid) attract water and thus are especially effective as cloud condensation nuclei. The large quantity of aerosols produced by human activities (especially industrial emissions) trigger an increase in the number of cloud droplets that form within a cloud. A greater number of small droplets increases the cloud’s brightness, causing more sunlight to be reflected back to space. One category of aerosols, called black carbon, is soot generated by combustion processes and fires. Unlike most other aerosols, black carbon warms the atmosphere because it is an effective absorber of incoming solar radiation. In addition, when deposited on snow and ice, black carbon reduces surface albedo, thus increasing the amount of light absorbed. Nevertheless, despite the warming effect of black carbon, the overall effect of atmospheric aerosols is to cool Earth. Studies indicate that the cooling effect of humangenerated aerosols offsets a portion of the global warming caused by the growing quantities of greenhouse gases in the

shows how global average temperatures would have changed due to natural forces only, as simulated by climate models. The red band shows model projections of the effects of human and natural forces combined. The black line shows actual observed global average temperatures. As the blue band indicates, without human influences, temperatures over the past century would actually have first warmed and then cooled slightly over recent decades. Bands of color are used to express the range of uncertainty. (U.S. Global Change Research Program)

650 Beijing

FIGURE 20.26 HumanGenerated Aerosols These satellite images show a serious air pollution episode that plagued China on October 8, 2010. (NASA)

The source of these pollutants was coal-burning power plants, agricultural burning, and industrial processes.

Beijing

Zhengzhou

N 100 km

Aerosol Index 0.0

1.75

This satellite image shows the extremely high levels of aerosols associated with this air pollution episode. At an index value of 4, aerosols are so dense that you would have difficulty seeing the midday sun.

3.5

Zhengzhou

N 200 km

atmosphere. The magnitude and extent of the cooling effect of aerosols is uncertain. This uncertainty is a significant hurdle in advancing our understanding of how humans alter Earth’s climate. It is important to point out some significant differences between global warming by greenhouse gases and aerosol cooling. After being emitted, greenhouse gases, such as carbon dioxide, remain in the atmosphere for many decades. By contrast, aerosols released into the troposphere remain there for only a few days or, at most, a few weeks before they are “washed out” by precipitation. Because of their short lifetime in the troposphere, aerosols are distributed unevenly over the globe. As expected, human-generated aerosols are concentrated near the areas that produce them—namely industrialized regions that burn fossil fuels and land areas where vegetation is burned. The lifetime of aerosols in the atmosphere is short. Therefore, the effect of aerosols on today’s climate is determined

|

by the amount emitted during the preceding couple weeks. By contrast, the carbon dioxide and trace gases released into the atmosphere remain for much longer spans and thus influence climate for many decades.

20.10 CONCEPT CHECKS 1 What are the main sources of human-generated aerosols? 2 What effect does black carbon have on atmospheric temperatures?

3 What is the net effect of aerosols on temperatures in the troposphere?

4 How long do aerosols remain in the atmosphere before they are removed?

5 How does the residence time of aerosols compare to that of CO2?

20.11 SOME POSSIBLE CONSEQUENCES OF GLOBAL WARMING Describe some possible consequences of global warming.

What consequences can be expected if the carbon dioxide content of the atmosphere reaches a level that is twice what it was early in the twentieth century? Because the climate system is complex, predicting the distribution of particular regional changes can be speculative. It is not yet possible to pinpoint specifics, such as where or when it will become drier or wetter. Nevertheless, plausible scenarios can be given for larger scales of space and time. As noted, the magnitude of the temperature increase will not be the same everywhere. The temperature rise will probably be smallest in the tropics and increase toward the

poles. As for precipitation, the models indicate that some regions will experience significantly more precipitation and runoff, whereas others will experience a decrease in runoff due to reduced precipitation or greater evaporation caused by higher temperatures. TABLE 20.1 summarizes some of the most likely effects and their possible consequences. The table also provides the IPCC’s estimate of the probability of each effect. Levels of confidence for these projections vary from “likely” (67 to 90 percent probability) to “very likely” (90 to 99 percent probability) to “virtually certain” (greater than 99 percent probability).

20.11 Some Possible Consequences of Global Warming

TABLE 20.1

651

Projected Changes and Effects of Global Warming in the Twenty-First Century

Projected Changes and Estimated Probability*

Examples of Projected Impacts

Higher maximum temperatures; more hot days and heat waves over nearly all land areas (virtually certain)

Increased incidence of death and serious illness in older age groups and urban poor. Increased heat stress in livestock and wildlife. Shift in tourist destinations. Increased risk of damage to a number of crops. Increased electric cooling demand and reduced energy supply reliability. Decreased cold-related human morbidity and mortality.

Higher minimum temperatures; fewer cold days, frost days, and cold waves over nearly all land areas (virtually certain)

Decreased risk of damage to a number of crops and increased risk to others. Extended range and activity of some pest and disease vectors. Reduced heating energy demand. Increased flood, landslide, avalanche, and debris flow damage.

Increases in frequency of heavy precipitation events over most areas (very likely)

Increased soil erosion. Increased flood runoff could increase recharge of some floodplain aquifers. Increased pressure on government and private flood insurance systems and disaster relief.

Increases in area affected by drought (likely)

Decreased crop yields. Increased damage to building foundations caused by ground shrinkage. Decreased water-resource quantity and quality. Increased risk of wildfires.

Increases in intense tropical cyclone activity (likely)

Increased risks to human life, risk of infectious-disease epidemics, and many other risks. Increased coastal erosion and damage to coastal buildings and infrastructure. Increased damage to coastal ecosystems, such as coral reefs and mangroves.

* Virtually certain indicates a probability greater than 99 percent, very likely indicates a probability of 90–99 percent, and likely indicates a probability of 67–90 percent. Source: Adapted from Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Figure SPM.3. IPCC, Geneva, Switzerland.

level? One significant factor is thermal expansion. Higher air temperatures warm the adjacent upper layers of the ocean, which in turn causes the water to expand and sea level to rise. Perhaps a more easily visualized contributor to global sea-level rise is melting glaciers. With few exceptions, glaciers around the world have been retreating at unprecedented rates over the past century. Some mountain glaciers have disappeared altogether. A recent 18-year satellite study showed that the mass of the Greenland and Antarctic Ice Sheets dropped an average of 475 gigatons per year. (A gigaton is 1 billion metric tons.) That is enough water to raise sea level 1.5 millimeters (0.05 inch) per year. The loss of ice was not steady but was occurring at an accelerating rate during the study period. Each year over the course of the study period, the two ice sheets lost a combined average of 36.3 gigatons more than they did the year before. During the same span, mountain FIGURE 20.27 Rising Sea Level These graphs show

60

Sea Level Change (mm)

A significant impact of human-induced global warming is a rise in sea level. As this occurs, coastal cities, wetlands, and low-lying islands could be threatened with more frequent flooding, increased shoreline erosion, and saltwater encroachment into coastal rivers and aquifers. Research indicates that sea level has risen about 25 centimeters (9.75 inches) since 1870. As FIGURE 20.27 indicates, the rate of sea-level rise has been greater in recent years. Some models indicate that additional rise may approach or even exceed 50 centimeters (20 inches) by the end of the twenty-first century. Such a change may seem modest, but scientists realize that any rise in sea level along a gently sloping shoreline, such as the Atlantic and Gulf coasts of the United States, will lead to significant erosion and severe and permanent inland flooding (FIGURE 20.28 ). If this happens, many beaches and wetlands will be eliminated, and coastal civilization will be severely disrupted. Low-lying and 250 densely populated places such Annual Rate of Change 200 as Bangladesh and the small 1.70 mm/yr island nation of the Maldives 150 are especially vulnerable. The 100 average elevation in the Mal50 dives is 1.5 meters (less than 5 feet), and its highest point 0 is just 2.4 meters (less than 8 –50 feet) above sea level. 1880 1900 1920 1940 1960 1980 Year How is a warmer atmosphere related to a rise in sea A. Sea Level Change 1870 to 2000

Sea Level Change (mm)

Sea-Level Rise

2000

50

Annual Rate of Change 3.17 mm/yr

40 30 20 10 0 –10 1993 1996

2000

2004 Year

2008

B. Sea Level Change 1993 to 2012

2012

sea-level change from 1870 until 2012. Note the difference in the rate of change between the two graphs. A. This graph shows historical sea-level data derived from coastal tide gauges. B. This graph shows the average sea level since 1993, derived from global satellite measurements. (NASA)

652

CHAPTER 20

Where the slope is gentle a small rise in sea level causes a substantial shift

SmartFigure 20.28 Slope Sl of the Shoreline The slope of

Original shoreline

Shoreline shift Sea level rise

the shoreline is critical to determining the degree to which sea-level changes will affect it. As sea level gradually rises, the shoreline retreats, and structures that were once thought to be safe from wave attack become vulnerable.

The Changing Arctic A 2005 study of climate change in the Arctic began with the following statement:

Original shoreline

For nearly 30 years, Arctic sea ice extent and thickness have been falling dramatically. Permafrost temperatures are rising and coverage is decreasing. Mountain glaciers Sea level and the Greenland ice sheet are rise shrinking. Evidence suggests we are witnessing the early stage of an anthropogenically induced global warming superimposed on natural cycles, reinforced by reductions in Arctic ice.4

Where the slope is steep the same sea level rise causes a small shift

Shoreline shift

glaciers and ice caps lost an average of slightly more than 400 gigatons per year. Because rising sea level is a gradual phenomenon, coastal residents may overlook it as an important contributor to shoreline erosion problems. Rather, the blame may be assigned to other forces, especially storm activity. Although a given storm may be the immediate cause, the magnitude of its destruction may result from the relatively small sea-level rise that allowed FIGURE 20.29 Siberian Lakes This false-color image pair shows lakes dotting the tundra in 1973 and 2002. The tundra vegetation is colored a faded red, whereas lakes appear blue or blue-green. Many lakes disappeared or shrunk considerably between 1973 and 2002. After studying satellite imagery of about 10,000 large lakes in a 500,000-square-kilometer (195,000-square-mile) area in northern Siberia, scientists documented an 11 percent decline in the number of lakes, at least 125 of which disappeared completely.

the storm’s power to cross a much greater land area.

(a) June 27, 1973

Arctic Sea Ice Climate models are in general agreement that one of the strongest signals of global warming should be a loss of sea ice in the Arctic. This is indeed occurring. The area covered by sea ice naturally grows during the frigid Arctic winters and shrinks when temperatures climb in the spring and summer. Since 1979, satellites have observed a 13 percent decline per decade in the minimum summertime extent of sea ice in the Arctic. The thickness of the ice has also been declining. The map in Figure 14.3A (page 435) compares the average sea ice extent for early September 2012 to the long-term average for the period 1979–2000. The extent of sea ice in September 2012 set a record. On that date the extent was less than 4 million square kilometers (1.54 million square miles)— 70,000 square kilometers (27,000 square miles) less than the previous record low set in September 2007. The trend is also clear when you examine the graph in Figure 14.3B. Is it possible that this trend may be part of a natural cycle? Yes, but it is more likely that the sea ice decline represents a combination of natural variability and human-induced global warming, with the latter becoming increasingly evident in coming decades. As was noted in the section “Climate-Feedback Mechanisms,” a reduction in sea ice represents a positive-feedback mechanism that reinforces global warming.

(NASA)

Permafrost During the past decade, mounting evidence has indicated that the extent of permafrost in the Northern Hemisphere has decreased, as would be expected under long-term warming conditions. FIGURE 20.29 presents one example which shows that such a decline is occurring. In the Arctic, short summers thaw only the top layer of frozen ground. The permafrost beneath this active layer is like the cement bottom of a swimming pool. In summer, water cannot percolate downward, so it saturates the soil 4

(b) July 2, 2002

J. T. Overpeck, et al., “Arctic System on Trajectory to New, Seasonally Ice-Free States,” EOS, Transactions, American Geophysical Union, 86(34): 309, August 23, 2005.

Concepts in Review

above the permafrost and collects on the surface in thousands of lakes. However, as Arctic temperatures climb, the bottom of the “pool” seems to be “cracking.” Satellite imagery shows that over a 20-year span, a significant number of lakes have shrunk or disappeared altogether. As the permafrost thaws, lake water drains deeper into the ground. Thawing permafrost represents a potentially significant positive-feedback mechanism that may reinforce global warming. When vegetation dies in the Arctic, cold temperatures inhibit its total decomposition. As a consequence, over thousands of years, a great deal of organic matter has become stored in the permafrost. When the permafrost thaws, organic matter that may have been frozen for millennia comes out of “cold storage” and decomposes. The result is the release of carbon dioxide and methane—greenhouse gases that contribute to global warming.

The Potential for “Surprises” You have seen that climate in the twenty-first century, unlike in the preceding 1000 years, is not expected to be stable. Rather, a constant state of change is very likely. Many of the changes will probably be gradual environmental shifts, imperceptible from year to year. Nevertheless, the effects, accumulated over decades, will have powerful economic, social, and political consequences. Despite our best efforts to understand future climate shifts, there is also the potential for “surprises.” This simply means that, due to the complexity of Earth’s climate system, we might experience relatively sudden, unexpected changes or see some aspects of climate shift in an unexpected manner. The report Climate Change Impacts on the United States describes the situation like this:

treme precipitation events, and damaging winds could become common. What if large quantities of methane, a potent greenhouse gas currently frozen in icy Arctic tundra and sediments, began to be released to the atmosphere by warming, potentially creating an amplifying “feedback loop” that would cause even more warming? We simply do not know how far the climate system or other systems it affects can be pushed before they respond in unexpected ways. There are many examples of potential surprises, each of which would have large consequences. Most of these potential outcomes are rarely reported, in this study or elsewhere. Even if the chance of any particular surprise happening is small, the chance that at least one such surprise will occur is much greater. In other words, while we can’t know which of these events will occur, it is likely that one or more will eventually occur.5 The impact on climate of an increase in atmospheric carbon dioxide and trace gases is obscured by some uncertainties. Yet climate scientists continue to improve our understanding of the climate system and the potential impacts and effects of global climate change. Policymakers are confronted with responding to the risks posed by emissions of greenhouse gases, knowing that our understanding is imperfect. However, they are also faced with the fact that climate-induced environmental changes cannot be reversed quickly, if at all, due to the lengthy time scales associated with the climate system. 5

National Assessment Synthesis Team, Climate Change Impacts on the United States: The Potential Consequences of Climate Variability and Change (Washington, DC: U.S. Global Research Program, 2000), p. 19.

20.11 CONCEPT CHECKS

Surprises challenge humans’ ability to adapt, because of how quickly and unexpectedly they occur. For example, what if the Pacific Ocean warms in such a way that El Niño events become much more extreme? This could reduce the frequency, but perhaps not the strength, of hurricanes along the East Coast, while on the West Coast, more severe winter storms, ex-

20

CONCEPTS IN REVIEW

1 List and describe the factors that are causing sea level to rise.

2 Is global warming greater near the equator or near the poles? Explain.

3 Based on Table 20.1, what projected changes relate to something other than temperature?

|

World Climates and Global Climate Change

20.1 THE CLIMATE SYSTEM KEY TERM: climate system ■ ■

Climate is the aggregate of weather conditions for a place or region over a long period of time. Earth’s climate system involves the exchanges of energy and moisture that occur among the atmosphere, hydrosphere, solid Earth, biosphere, and cryosphere (the ice and snow that exist at Earth’s surface).

Q Which sphere of the climate system dominates this image? What other sphere or spheres are present?

James Balog/Getty Images

List the five parts of the climate system and provide examples of each.

653

Concepts in Review

■

20.2 WORLD CLIMATES Explain why classification is a necessary process when studying world climates. Discuss the criteria used in the Köppen system of climate classification.

■

K EY TERM: Köppen classification ■

■

Climate classification brings order to large quantities of information, which aids comprehension and understanding and facilitates analysis and explanation. The most important elements in climate descriptions are temperature and precipitation because they have the greatest influence on people and their activities, and they also have an important impact on the distribution of vegetation and the development of soils.

20.3 HUMID TROPICAL (A) CLIMATES Compare the two broad categories of tropical climates.

■

■

Q When does the rainy season occur in a tropical wet and dry climate: winter or summer? Explain.

is considered dry?

Contrast low-latitude dry climates and middle-latitude dry climates. K E Y T E R M S : arid (desert), semiarid (steppe)

K EY TERMS: tropical rain forest, tropical wet and dry

Humid tropical (A) climates are winterless, with all months having a mean temperature above 18°C (64°F). Wet tropical climates (Af and Am), which lie near the equator, have constantly high temperatures and enough rainfall to support the most luxuriant vegetation (tropical rain forest) found in any climatic realm. Tropical wet and dry climates (Aw) are found poleward of the wet tropics and equatorward of the subtropical deserts, where the rain forest gives way to the tropical grasslands and scattered droughttolerant trees of the savanna. The most distinctive feature of this climate is the seasonal character of the rainfall.

Q Why are three different formulas used to determine whether a climate

20.4 DRY (B) CLIMATES

■

■

■

An early attempt at climate classification by the Greeks divided each hemisphere into three zones: torrid, temperate, and frigid. Many climate classifications have been devised, with the value of each determined by its intended use. The Köppen classification, which uses mean monthly and annual values of temperature and precipitation, is a widely used system. The boundaries Köppen chose were largely based on the limits of certain plant associations. Five principal climate groups, each with subdivisions, were recognized. Each group is designated by a capital letter. Four of the climate groups— A, C, D, and E—are defined on the basis of temperature characteristics, and the fifth, the B group, has precipitation as its primary criterion.

■ ■

■

Dry (B) climates, in which the yearly precipitation is less than the potential loss of water by evaporation, are subdivided into two types: arid or desert (BW) and semiarid or steppe (BS). Differences between desert and steppe are primarily a matter of degree, with semiarid being a marginal and more humid variant of arid. Low-latitude deserts and steppes coincide with the clear skies caused by subsiding air beneath the subtropical high-pressure belts. Middle-latitude deserts and steppes exist principally because of their position in the deep interiors of large landmasses far removed from the ocean. Because many middle-latitude deserts occupy sites on the leeward sides of mountains, they can also be classified as rain shadow deserts.

Q This photo was taken in Nevada’s Great Basin Desert, looking west toward the Sierra Nevada. What is the basic cause of the arid conditions in this region?

Dennis Tasa

654

20.5 HUMID MIDDLE-LATITUDE CLIMATES (C AND D CLIMATES) Distinguish among five different humid middle-latitude climates. K EY TERMS: humid subtropical climate, marine west coast climate, dry-summer subtropical climate, humid continental climate, subarctic climate ■

■

■

■

Middle-latitude climates with mild winters (C climates) occur where the average temperature of the coldest month is below 18°C (64°F) but above –3°C (27°F). Three C climate subgroups exist. Humid subtropical climates (Cfa) are located on the eastern sides of the continents, in the 25°–40° latitude range. Summer weather is hot and sultry, and winters are mild. In North America, the marine west coast climate (Cfb, Cfc) extends from near the U.S.–Canada border northward as a narrow belt into southern Alaska. The prevalence of maritime air masses means that mild winters and cool summers are the rule. Dry-summer subtropical climates (Csa, Csb) are typically located along the west sides of continents between latitudes 30° and 45°. In summer, the regions are dominated by stable, dry conditions associated with the oceanic subtropical highs. In winter, they are within range of the cyclonic storms of the polar front. Humid middle-latitude climates with severe winters (D climates) are land-controlled climates that are absent in the Southern Hemisphere. The D climates have severe winters. The average temperature of the coldest month is –3°C (27°F) or below, and the warmest monthly mean exceeds 10°C (50°F). Humid continental climates (Dfa, Dfb, Dwa, Dwb) are confined to the eastern portions of North America and Eurasia in the latitude range between approximately 40° and 50° north latitude. Both winter and summer temperatures can be characterized as relatively severe. Precipitation is generally greater in summer than in winter. Subarctic climates (Dfc, Dfd, Dwc, Dwd) are situated north of the humid continental climates and south of the polar tundras. The outstanding feature of subarctic climates is the dominance of winter. By contrast, summers in the subarctic are remarkably warm, despite their short duration. The highest annual temperature ranges on Earth occur here.

Concepts in Review

20.6 POLAR (E) CLIMATES

655

20.7 HIGHLAND CLIMATES

Contrast ice cap and tundra climates.

Summarize the characteristics associated with highland climates.

KEY TERMS: polar climate, tundra climate, ice cap climate ■

■

■

Polar climates (ET, EF) are those in which the mean temperature of the warmest month is below 10°C (50°F). Annual temperature ranges are extreme, with the lowest annual means on the planet. Although polar climates are classified as humid, precipitation is generally meager, with many nonmarine stations receiving less than 25 centimeters (10 inches) annually. Two types of polar climates are recognized. The tundra climate (ET) is found almost exclusively in the Northern Hemisphere. The 10°C (50°F) summer isotherm represents its equatorward limit. It is a treeless region of grasses, sedges, mosses, and lichens with permanently frozen subsoil, called permafrost. The ice cap climate (EF) does not have a single monthly mean above 0°C (32°F). Consequently, the growth of vegetation is prohibited, and the landscape is one of permanent ice and snow. The ice sheets of Greenland and Antarctica are important examples.

K E Y T E R M : highland climates ■

Highland climates are characterized by a great diversity of climatic conditions over a small area. Although the bestknown climatic effect of increased altitude is lower temperatures, greater precipitation due to orographic lifting is also common. Variety and changeability best describe highland climates. Because atmospheric conditions fluctuate with altitude and exposure to the Sun’s rays, a nearly limitless variety of local climates occur in mountainous regions.

Q Which of the local winds discussed in Chapter 18 are likely associated with highland climates?

20.8 HUMAN IMPACT ON GLOBAL CLIMATE Summarize the nature and cause of the atmosphere’s changing composition since about 1750. Describe the climate’s response. ■

■

■

■

■

Humans have been modifying the environment for thousands of years. By altering ground cover with the use of fire and the overgrazing of land, people have modified such important climatic factors as surface albedo, evaporation rates, and surface winds. Human activities produce climate change through the release of carbon dioxide (CO2) and trace gases. Humans release CO2 when they cut down forests and when they burn fossil fuels such as coal, oil, and natural gas. A steady rise in atmospheric CO2 levels has been documented at Mauna Loa, Hawaii, and other locations around the world. More than half of the carbon released by humans is absorbed by new plant matter or dissolved in the oceans. About 45 percent remains in the atmosphere, where it can influence climate for decades. Air bubbles trapped in glacial ice reveal that there is currently about 30 percent more CO2 than the atmosphere has contained in the past 650,000 years. As a result of the extra heat retention due to added CO2, Earth’s atmosphere has warmed by about 0.8°C (1.4°F) in the past 100 years, most of it since the 1970s. Temperatures are projected to increase by another 2° to 4.5°C (3.6° to 8.1°F) in the future. Trace gases such as methane, nitrous oxide, and CFCs also play a significant role in increasing global temperature.

20.9 CLIMATE-FEEDBACK MECHANISMS Contrast positive- and negative-feedback mechanisms and provide examples of each.

■

■

■

A change in one part of the climate system may trigger changes in other parts of the climate system that amplify or diminish the initial effect. These climate-feedback mechanisms are called positive-feedback mechanisms if they reinforce the initial change and negative-feedback mechanisms if they counteract the initial effect. The melting of sea ice due to global warming (decreasing albedo and increasing the initial effect of warming) is one example of a positive-feedback mechanism. The production of more clouds (blotting out incoming solar radiation, leading to cooling) is an example of a negative-feedback mechanism. Computer models of climate give scientists a tool for testing hypotheses about climate change. Although these models are far simpler than the real climate system, they are useful tools for predicting the future climate.

Michael Collier

KEY TERMS: climate-feedback mechanism, positive-feedback mechanism, negative-feedback mechanism

Q Changes in precipitation and temperature due to climate change can increase the risk of forest fires. Describe two ways that the event shown in this photo could contribute to global warming.

656

Concepts In Review

20.10 HOW AEROSOLS INFLUENCE CLIMATE

20.11 SOME POSSIBLE CONSEQUENCES OF GLOBAL WARMING

Discuss the possible impacts of aerosols on climate change.

Describe some possible consequences of global warming.

K EY TERMS: aerosols, black carbon ■

■

■

Aerosols are tiny liquid and solid particles that are suspended in the air. Global climate is affected by human activities that contribute to the atmosphere’s aerosol content. Most aerosols reflect a portion of incoming solar radiation back to space and therefore have a cooling effect. Overall, aerosols have a cooling effect, yet some aerosols called black carbon (soot from combustion processes and fires) absorb incoming solar radiation and warm the atmosphere. When black carbon is deposited on snow and ice, it reduces surface albedo and increases the amount of light absorbed at the surface.

■

■

■

■

Q Do aerosols spend more or less time in the atmosphere than green-

■

house gases such as carbon dioxide? What is the significance of this difference in residence time? Explain.

In the future, Earth’s surface temperature is likely to continue to rise. The temperature increase will likely be greatest in the polar regions and least in the tropics. Some areas will get drier, and other areas will get wetter. Sea level is predicted to rise for several reasons, including the melting of glacial ice and thermal expansion (a given mass of seawater takes up more volume when it is warm than when it is cool). Low-lying, gently sloped, highly populated coastal areas are most at risk. Sea ice cover and thickness in the Arctic have been declining since satellite observations began in 1979. Because of the warming of the Arctic, permafrost is melting, releasing CO2 and methane to the atmosphere in a positive-feedback mechanism. Because the climate system is complicated, dynamic, and imperfectly understood, it could produce sudden, unexpected changes with little warning.

GIVE IT SOME THOUGHT 1. Refer to Figure 20.1, which illustrates various components of Earth’s climate system. Boxes represent interactions or changes that occur in the climate system. Select three boxes and provide an example of an interaction or change associated with each. Explain how these interactions may influence temperature. 2. Using figure 20.1, describe what may be the main cause of the following climate-related changes: (a) a local change in precipitation; (b) a global change in precipitation; (c) a fall in sea level; (d) global warming. 3. Refer to the monthly rainfall data (in millimeters) for three cities in Africa. Their locations are shown on the accompanying map. Match the data for each city to the correct location (1, 2, or 3) on the map. How were you able to figure this out? Bonus: Which figure in Chapter 18 would be especially useful in explaining or illustrating why these places have rainfall maximums and minimums when they do? 4. Refer to Figure 20.5, which shows climates of the world. Humid continental (Dfb and Dwb) and subarctic (Dfc) climates are usually described as being “land controlled”—that is, they lack marine influence. J

F

M

A

M

J

J

A

S

O

N

D

81

102

155

140

133

119

99

109

206

213

196

122

0

2

0

0

1

15

88

249

163

49

5

6

CITY C

236

168

86

46

13

8

0

3

8

38

94

201

Nevertheless, these climates are found along the margins of the North Atlantic and the North Pacific oceans. Explain why this occurs. 5. It has been suggested that global warming over the past several decades likely would have been greater were it not for the effect of certain types of air pollution. Explain how this could be true. 6. Motor vehicles are a significant source of CO2. Using electric cars, such as the one pictured here, is one way to reduce emissions from this source. Although these vehicles emit little or no CO2 or other air pollutants directly into the air, can they still be connected to such emissions? If so, explain.

David Pearson/Alamy

  CITY A CITY B

Examining the Earth System

7. If a fellow student who, unlike you, had not studied climate were to ask, “Isn’t the greenhouse effect a bad thing because it’s responsible for global warming?” how would you respond? 8. During a conversation, an acquaintance indicates that he is skeptical about global warming. When you ask why he feels that way, he says, “The past couple of years in this area have been among the coolest I

657

can remember.” While you assure this person that it is useful to question scientific findings, you suggest to him that his reasoning in this case may be flawed. Use your understanding of the definition of climate along with one or more graphs in the chapter to persuade this person to reevaluate his reasoning.

EXAMINING THE EARTH SYSTEM 1. The Köppen climate classification is based on the fact that there is an excellent association between natural vegetation (biosphere) and climate (atmosphere). Briefly describe the climate conditions (temperature and precipitation) and natural vegetation associated with each of the following Köppen climates: Af, BWh, Dfc, and ET. 2. Examine the precipitation map for the state of Nevada. Notice that the areas receiving the most precipitation resemble long, slender “islands” scattered across the state. Provide an explanation for this pattern. Are average temperatures in these wetter areas likely different from those in nearby less rainy places? Why or why not? A look back at Section 6.9 and Figure 6.32 (page 195) might be helpful.

4. This satellite image from August 2007 shows the effects of tropical deforestation in a portion of the Amazon basin in western Brazil. Intact forest is dark green, whereas cleared areas are tan (bare ground) or light green (crops and pasture). Notice the relatively dense smoke in the left center of the image. How does deforestation of tropical forests change the composition of the atmosphere? Describe the effect that tropical deforestation has on global warming.

< 10 cm (4 in.) 10–20 cm (4–8 in.) 20–40 cm (8–16 in.) > 40 cm (16 in.)

3. How might the burning of fossil fuels, such as the gasoline to run your car, influence global temperature? If such a temperature change occurs, how might sea level be affected? How might the intensity of hurricanes change? How might these changes impact people who live on a beach or barrier island along the Atlantic or Gulf coasts?

NASA

UNIT SEVEN | EARTH’S PLACE IN THE UNIVERSE 

21

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

21.1

Explain the geocentric view of the solar system and describe how it differs from the heliocentric view.

21.2

List and describe the contributions to modern astronomy of Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Isaac Newton.

21.3

Compare the equatorial system of coordinates used to establish the position of the stars with longitude and latitude. Explain how the positions of stars are described using declination and right ascension.

21.4

Describe the two primary motions of Earth and explain the difference between a solar day and a sidereal day.

21.5

Sketch the changing positions of the Earth–Moon system that produce the regular cycle we call the phases of the Moon.

21.6

Sketch the positions of the Earth–Moon system that produce a lunar eclipse, as well as a solar eclipse.

* This chapter was revised with the assistance of Professors Mark Watry and Teresa Tarbuck.

An observer at a very dark site with the Milky Way galaxy in the background. (Photo by Babak Tafreshi/Science Source)

659

660

CHAPTER 21

Origins of Modern Astronomy

he science of astronomy provides a rational way of knowing and understanding the origins of Earth, the solar system, and the universe. Earth was once thought to be unique, different in every way from everything else in the universe. However, through the science of astronomy, we have discovered that Earth and the Sun are similar to other objects in the universe and that the physical laws that apply on

T

|

Earth seem to apply everywhere else in the universe. How did our understanding of the universe change so drastically? In this chapter we examine the transformation from the ancient view of the universe, which focused on the positions and movements of celestial objects, to the modern perspective, which focuses on understanding how these objects came to be and why they move the way they do.

21.1 ANCIENT ASTRONOMY Explain the geocentric view of the solar system and describe how it differs from the heliocentric view.

Long before recorded history, people were aware of the close relationship between events on Earth and the positions of heavenly bodies. They realized that changes in the seasons and floods of great rivers such as the Nile in Egypt occurred when certain celestial bodies, including the Sun, Moon, planets, and stars, reached particular places in the heavens. Early agrarian cultures, whose survival depended on seasonal change, believed that if these heavenly objects could control the seasons, they could also strongly influence all Earthly events. These beliefs undoubtedly encouraged early civilizations to begin keeping records of the positions of celestial objects. The origin of astronomy began more than 5000 years ago, when humans began to track the motion of celestial objects so they knew when to plant their crops or prepare to hunt migrating herds. The ancient Chinese, Egyptians, and Babylonians are well known for their record keeping. These cultures recorded the FIGURE 21.1 The Bayeux Tapestry that Hangs in Bayeux, France This tapestry shows the apprehension caused by Halley’s Comet in A.D. 1066. This event preceded the defeat of King Harold by William the Conqueror. (“Sighting of a comet.” Detail from Bayeux Tapestry. Musee de la Tapisserie, Bayeux. “With special authorization of the City of Bayeux.” Bridgeman-Giraudon/Art Resource, NY)

locations of the Sun, the Moon, and the five visible planets as these objects moved slowly against the background of “fixed” stars. Eventually, it was not enough to track the motions of celestial objects; predicting their future positions (to avoid getting married at an unfavorable time, for example) became important. A study of Chinese archives shows that the Chinese recorded every appearance of the famous Halley’s Comet for at least 10 centuries. However, because this comet appears only once every 76 years, they were unable to link these appearances to establish that what they saw was the same object multiple times. Like most other ancients, the Chinese considered comets to be mystical. Generally, comets were seen as bad omens and were blamed for a variety of disasters, from wars to plagues (FIGURE 21.1 ). In addition, the Chinese kept quite accurate records of “guest stars.” Today we know that a “guest star” is a normal star, usually too faint to be visible, which increases its brightness as it explosively ejects gases from its surface, a phenomenon we call a nova (novus 5 new) or supernova (FIGURE 21.2 ).

The Golden Age of Astronomy The “Golden Age” of early astronomy (600 b.c.–a.d. 150) was centered in Greece. Although the early Greeks have been criticized for using purely philosophical arguments to explain natural phenomena, they employed observational data as well. The basics of geometry and trigonometry, which they developed, were used to measure the sizes of and distances to the largestappearing bodies in the heavens—the Sun and the Moon. The early Greeks held the incorrect geocentric (geo 5 Earth, centric 5 centered) view of the universe— which professed that Earth was a sphere that remained motionless at the

661 center of the universe. Orbiting Earth were the Moon, Sun, and known planets—Mercury, Venus, Mars, Jupiter, and Saturn. The Sun and Moon were thought to be perfect crystal spheres. Beyond the planets was a transparent, hollow celestial sphere to which the stars were attached and which traveled daily around Earth. (Although it appears that the stars and planets move across the sky, this effect is actually caused by Earth’s rotation on its axis.) Some early Greeks realized that the motion of the stars could be explained just as easily by a rotating Earth, but they rejected that idea because people can’t sense Earth’s motion, and the planet seemed too large to be movable. In fact, proof of Earth’s rotation was not demonstrated until 1851. To the Greeks, all except seven of the heavenly bodies appeared to remain in the same position relative to one another. These seven wanderers (planetai in Greek) were the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Each was thought to have a circular orbit around Earth. Although this system was incorrect, the Greeks refined it to the point that it explained the apparent movements of all celestial bodies. The famous Greek philosopher Aristotle (384–322 b.c.) concluded that Earth is spherical because it always casts a curved shadow when it eclipses the moon. Although many considered most of Aristotle’s teachings infallible, his belief in a spherical Earth was lost during the Middle Ages.

A Sun-Centered Universe? The first Greek to profess a Sun-centered, or heliocentric (helios 5 Sun, centric 5 centered), universe was Aristarchus (312–230 b.c.).

the sudden appearance of a “guest star” in 1054 A.D. The scattered remains of that supernova is the Crab Nebula in the constellation Taurus. This image comes from the Hubble Space Telescope. (NASA/Jet Propulsion Laboratory)

Aristarchus also used simple geometric relations to calculate the relative distances from Earth to the Sun and the Moon. He later used these data to calculate their sizes. As a result of an observational error beyond his control, he came up with measurements that were much too small. However, he did discover that the Sun was many times more distant than the Moon and many times larger than Earth. The latter fact may have prompted him to suggest a Sun-centered universe. Nevertheless, because of the strong influence of Aristotle’s writings, the Earth-centered view dominated Western thought for nearly 2000 years. Alexandria

31˚N

N i le Riv

Measuring the Earth’s Circumference The first successful attempt to establish the size of Earth is credited to Eratosthenes (276–194 b.c.). Eratosthenes observed the angles of the noonday Sun in two Egyptian cities that were roughly north and south of each other—Syene (presently Aswan) and Alexandria (FIGURE 21.3 ). Finding that the angles of the noonday sun differed by 7 degrees, or 1/50 of a complete circle, he concluded that the circumference of Earth must be 50 times the distance between these two cities. The cities were 5000 stadia apart, giving him a measurement of 250,000 stadia. Many historians believe the stadia was 157.6 meters (517 feet), which would make Eratosthenes’s calculation of Earth’s circumference—39,400 kilometers (24,428 miles)—very close to the modern value of 40,075 kilometers (24,902 miles).

FIGURE 21.2 The Sudden Appearance of a “Guest Star” The Chinese recorded

24˚N

7˚ angle

EGYPT

er

Syene (Aswan)

Post at Alexandria

Center of Earth 7˚ angle ( 1/50 of a circle)

Well at Syene Sun’s rays

SmartFigure 21.3 O i Orientation of the Sun’s Rays at Syene (Aswan) and Alexandria, Egypt on June 21 From these data, Eratosthenes calculated Earth’s circumference.

662

CHAPTER 21

Origins of Modern Astronomy

FIGURE 21.4 The Universe According to Ptolemy, Second Century A.D. A. Ptolemy believed that

Jupiter

the star-studded celestial sphere made a daily trip around a motionless Earth. In addition, he proposed that the Sun, Moon, and planets made trips of various lengths along individual orbits. B. A three-dimensional model of an Earthcentered system. Ptolemy likely utilized something similar to this to calculate the motions of the heavens. (Photo by Science

Deferent

Moon Venus

Earth

Museum, London/ The Bridgeman Art Library)

Mercury

Sun Saturn Mars Epicycle

A.

B.

Mapping the Stars Probably the greatest of the early Greek astronomers was Hipparchus (second century b.c.), best known for his star catalogue. Hipparchus determined the location of almost 850 stars, which he divided into six groups according to their brightness. (This system is still used today.) He measured the length of the year to within minutes of the modern value and developed a method for predicting the times of lunar eclipses to within a few hours. Although many of the Greek discoveries were lost during the Middle Ages, the Earth-centered view that the Greeks proposed became entrenched in Europe. Presented in its finest form by Claudius Ptolemy, this geocentric outlook became known as the Ptolemaic System. FIGURE 21.5 Retrograde Motion of Mars, as Seen Against the Background of Distant Stars When viewed from Earth, Mars moves eastward among the stars each day and then periodically appears to stop and reverse direction. This apparent westward drift is a result of the fact that Earth has a faster orbital speed than Mars and overtakes it. As this occurs, Mars appears to be moving backward— that is, it exhibits retrograde motion.

Sun

Earth Mars

Ptolemy’s Model Much of our knowledge of Greek astronomy comes from a 13-volume treatise, Almagest (meaning “the great work”), which was compiled by Ptolemy in a.d. 141. In addition to presenting a summary of Greek astronomical knowledge, Ptolemy is credited with developing a model of the universe that accounted for the observable motions of the celestial bodies (FIGURE 21.4). In the Greek tradition, the Ptolemaic model had the planets moving in perfect circular orbits around a motionless Earth. (The Greeks considered the circle to be the pure and perfect shape.) However, the motion of the planets, as seen against the background of stars, is not so simple. Each planet, if watched night after night, moves slightly eastward among the stars. Periodically, each planet appears to stop, reverse direction for a period of time, and then resume an eastward motion. The apparent westward drift is called retrograde (retro 5 to go back, gradus 5 walking) motion. This rather odd apparent motion results from the combination of the motion of Earth and the planet’s own motion around the Sun. The retrograde motion of Apparent path of Mars Mars is shown in FIGURE 21.5 . seen against Because Earth has a faster background stars orbital speed than Mars, it overtakes its neighbor. While doing so, Mars appears to be moving backward, in

21.2 The Birth of Modern Astronomy

retrograde motion. This is analoPlanet gous to what a driver sees out the side window when passing a slower car. The slower planet, like the slower car, appears to be going backward, although its actual motion is in the same direction as the faster-moving body. Although it is difficult to accuEarth rately represent retrograde motion using the incorrect Earth-centered model, Ptolemy did so successfully (FIGURE 21.6 ). Rather than using a Orbit (deferent) single circle for each planet’s orbit, he proposed that the planets orbited on small circles (epicycles), revolving along large circles (deferents). By trial and error, he found the right combination of circles to produce the amount of retrograde motion observed for each planet. (An interesting note is that almost any closed curve can be produced by the combination of two circular motions, a fact that can be verified by anyone who has used the Spirograph™ design-drawing toy.) It is a tribute to Ptolemy’s genius that he was able to account for the planets’ motions as well as he did, considering that he used an incorrect model. The precision with which his model was able to predict planetary motion is attested to by the fact that it went virtually unchallenged, in principle if not in detail, until the seventeenth century. When Ptolemy’s predicted positions for the planets became out of step with the observed positions (which took 100 years or more), his model was simply recalibrated, using the new observed positions as a starting point. With the decline of the Roman Empire around the fourth century, much of the accumulated knowledge disappeared as libraries were destroyed. After the decline of Greek and Roman civilizations, the center of astronomical study moved east to Baghdad where, fortunately, Ptolemy’s work was translated into

|

Normal motion Epicycle Retrograde motion

Earth

Planet makes small circles during its orbit

Arabic. Later, Arabic astronomers expanded Hipparchus’s star catalog and divided the sky into 48 constellations—the foundation of our present-day constellation system. It wasn’t until sometime after the tenth century that the ancient Greeks’ contributions to astronomy were reintroduced to Europe through the Arabic community. The Ptolemaic model soon dominated European thought as the correct representation of the heavens, which created problems for anyone who found errors in it.

21.1 CONCEPT CHECKS 1 Why did the ancients believe that celestial objects had some influence over their lives?

2 What is the modern explanation of “guest stars” that suddenly appear in the night sky?

3 Explain the geocentric view of the universe. 4 In the Greek model of the universe, what were the seven wanderers, or planetai? How were they different from stars?

5 Describe what produces the retrograde motion of Mars. What geometric arrangements did Ptolemy use to explain this motion?

21.2 THE BIRTH OF MODERN ASTRONOMY List and describe the contributions to modern astronomy of Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Isaac Newton

Ptolemy’s Earth-centered universe was not discarded overnight. Modern astronomy’s development was more than a scientific endeavor; it required a break from deeply entrenched philosophical and religious views that had been a basic part of Western society for thousands of years. Its development was brought about by the discovery of a new and much larger universe, governed by discernible laws. We examine the work of five noted scientists involved in this transition from an astronomy that merely describes what is observed to an astronomy that tries to explain what is observed and, more importantly, why the universe behaves the way it does. They are Nicolaus Copernicus,

Tycho Brahe, Johannes Kepler, Galileo Galilei, and Sir Isaac Newton.

Nicolaus Copernicus For almost 13 centuries after the time of Ptolemy, very few astronomical advances were made in Europe; some were even lost, including the notion of a spherical Earth. The first great astronomer to emerge after the Middle Ages was Nicolaus Copernicus (1473–1543) from Poland (FIGURE 21.7 ). After discovering Aristarchus’s writings, Copernicus became convinced that Earth is a planet, just

663

SmartFigure 21.6 Pt l Ptolemy’s Explanation of Retrograde Motion Retrograde motion is the apparent backward motion of planets against the background of fixed stars. In Ptolemy’s model, the planets move on small circles (epicycles) while they orbit Earth on larger circles (deferents). Through trial and error, Ptolemy discovered the right combination of circles to produce the retrograde motion observed for each planet.

664

CHAPTER 21

Origins of Modern Astronomy

FIGURE 21.7 Painting of Polish Astronomer Nicolaus Copernicus (1473–1543) Copernicus’s proclamation that Earth was just another planet was very controversial for more than 100 years after his death. (Detlev van Ravenswaay/Science Source)

like the other five then-known planets. The daily motions of the heavens, he reasoned, could be more simply explained by a rotating Earth. Having concluded that Earth is a planet, Copernicus constructed a heliocentric model for the solar system, with FIGURE 21.8 Tycho Brahe (1546–1601) in His Observatory, in Uraniborg, on the Danish Island of Hveen Tycho (central figure) and the background are painted on the wall of the observatory within the arc of the sighting instrument called a quadrant. In the far right, Tycho can be seen “sighting” a celestial object through the “hole” in the wall. Tycho’s accurate measurements of Mars enabled Johannes Kepler to formulate his three laws of planetary motion. (Courtesy of Royal Geographic Society, London/ The Bridgeman Library International)

the Sun at the center and the planets Mercury, Venus, Earth, Mars, Jupiter, and Saturn orbiting it. This was a major break from the ancient and prevailing idea that a motionless Earth lies at the center of all movement in the universe. However, Copernicus retained a link to the past and used circles to represent the orbits of the planets. Because of this, Copernicus was unable to accurately predict the future locations of the planets. Copernicus found it necessary to add smaller circles (epicycles) like those that Ptolemy had used. The discovery that the planets actually have elliptical orbits occurred a century later and is credited to Johannes Kepler. Like his predecessors, Copernicus also used philosophical justifications to support his point of view: “In the midst of all stands the Sun. For who could in this most beautiful temple place this lamp in another or better place than that from which it can at the same time illuminate the whole?” Copernicus’s monumental work De Revolutionibus, Orbium Coelestium (On the Revolution of the Heavenly Spheres), which set forth his controversial Sun-centered solar system, was published as he lay on his deathbed. Hence, he never suffered the criticisms that fell on many of his followers. Although Copernicus’s model was a vast improvement over Ptolemy’s, it did not attempt to explain how planetary motions occurred or why. The greatest contribution of the Copernican system to modern science is its challenge of the primacy of Earth in the universe. At the time, many Europeans considered this heretical. Professing the Sun-centered model cost at least one person his life. Giordano Bruno was seized by the Inquisition, a Church tribunal, in 1600, and, refusing to denounce the Copernican theory, was burned at the stake.

Tycho Brahe Tycho Brahe (1546–1601) was born of Danish nobility 3 years after the death of Copernicus. Reportedly, Tycho became interested in astronomy while viewing a solar eclipse that astronomers had predicted. He persuaded King Frederick II to establish an observatory near Copenhagen, which Tycho headed. There he designed and built pointers (the telescope would not be invented for a few more decades), which he used for 20 years to systematically measure the locations of the heavenly bodies in an effort to disprove the Copernican theory (FIGURE 21.8 ). His observations, particularly of Mars, were far more precise than any made previously and are his legacy to astronomy. Tycho did not believe in the Copernican model because he was unable to observe an apparent shift in the position of stars that should result if Earth traveled around the Sun. His argument went like this: If Earth orbits the Sun, the position of a nearby star, when observed from two locations in Earth’s orbit 6 months apart, should shift with respect to the more distant stars. Tycho was correct, but his measurements did not have great enough precision to show any displacement. The apparent shift of the stars is called stellar parallax, and today it is used to measure distances to the nearest stars. (Stellar parallax is discussed in Appendix C, page 745.)

21.2 The Birth of Modern Astronomy

665

FIGURE 21.10 Drawing Ellipses with Various Eccentricities Using two

Focus

Focus

Focus

the derivation of his three laws of planetary motion. (Photo by Imagno/ Getty Images)

The principle of parallax is easy to visualize: Close one eye, and with your index finger vertical, use your eye to line up your finger with some distant object. Now, without moving your finger, view the object with your other eye and notice that the object’s position appears to change. The farther away you hold your finger, the less the object’s position seems to shift. Herein lay the flaw in Tycho’s argument. He was right about parallax, but the distance to even the nearest stars is enormous compared to the width of Earth’s orbit. Consequently, the shift that Tycho was looking for is too small to be detected without the aid of a telescope—an instrument that had not yet been invented. With the death of his patron, the king of Denmark, Tycho was forced to leave his observatory. Known for his arrogance and extravagant nature, Tycho was unable to continue his work under Denmark’s new ruler. As a result, Tycho moved to Prague in the present-day Czech Republic, where, in the last year of his life, he acquired an able assistant, Johannes Kepler. Kepler retained most of the observations made by Tycho and put them to exceptional use. Ironically, the data Tycho collected to refute the Copernican view of the solar system would later be used by Kepler to support it.

If Copernicus ushered out the old astronomy, Johannes Kepler (1571–1630) ushered in the new (FIGURE 21.9 ). Armed with Tycho’s data, a good mathematical mind, and, of greater importance, a strong belief in the accuracy of

1. The path of each planet around the Sun, while almost circular, is actually an ellipse, with the Sun at one focus (see Figure 21.10). 2. Each planet revolves so that an imaginary line connecting it to the Sun sweeps over equal areas in equal intervals of time (FIGURE 21.11 ). This law of equal areas geometrically expresses the variations in orbital speeds of the planets. Figure 21.11 illustrates the second law. Note that in order for a planet to sweep equal areas in the same amount of time, it must travel more rapidly when it is nearer the Sun and more slowly when it is farther from the Sun. FIGURE 21.11 Kepler’s Law of Equal Areas A line

January

Sun

July

Slower

Johannes Kepler

Focus

Tycho’s work, Kepler derived three basic laws of planetary motion. The first two laws resulted from his inability to fit Tycho’s observations of Mars to a circular orbit. Unwilling to concede that the discrepancies were a result of observational error, he searched for another solution. This endeavor led him to discover that the orbit of Mars is not a perfect circle but is slightly elliptical (FIGURE 21.10 ). About the same time, he realized that the orbital speed of Mars varies in a predictable way. As it approaches the Sun, it speeds up, and as it moves away, it slows down. In 1609, after nearly a decade of work, Kepler proposed his first two laws of planetary motion:

Faster

FIGURE 21.9 German Astronomer Johannes Kepler (1571–1630) Kepler’s contribution to modern astronomy was

straight pins for foci and a loop of string, trace out a curve while keeping the string taut, and you will have drawn an ellipse. The farther the pins (the foci) are moved apart, the more flattened (more eccentric) is the resulting ellipse.

connecting a planet (Earth) to the Sun sweeps out an area in such a manner that equal areas are swept out in equal times. Thus, Earth revolves slower when it is farther from the Sun (aphelion) and faster when it is closest (perihelion). The eccentricity of Earth’s orbit is greatly exaggerated in this diagram.

666

CHAPTER 21

Origins of Modern Astronomy

TABLE 21.1 Planet

Period of Revolution and Solar Distances of Planets Solar Distance (AU) *

Period (years)

Ellipticity (0 5 circle)

Mercury

0.39

0.24

0.205

Venus

0.72

0.62

0.007

Earth

1.00

1.00

0.017

Mars

1.52

1.88

0.094

Jupiter

5.20

11.86

0.049

Saturn

9.54

29.46

0.057

Uranus

19.18

84.01

0.046

Neptune

30.06

164.80

0.011

* AU 5 astronomical unit

Kepler was devout and believed that the Creator made an orderly universe and that this order would be reflected in the positions and motions of the planets. The uniformity he tried to find eluded him for nearly a decade. Then in 1619, Kepler published his third law in The Harmony of the Worlds: 3. The orbital periods of the planets and their distances to the Sun are proportional. In its simplest form, the orbital period is measured in Earth years, and the planet’s distance to the Sun is expressed in terms of Earth’s mean distance to the Sun. The latter “yardstick” is called the astronomical unit (AU) and is equal to about 150 million kilometers (93 million miles). Using these units, Kepler’s third law states that the planet’s orbital period squared is equal to its mean solar distance cubed. Consequently, the solar distances of the planets can be calculated when their periods of revolution are known. For example, Mars has an orbital period of 1.88 years, and

1.88 squared equals 3.54. The cube root of 3.54 is 1.52, and that is the average distance from Mars to the Sun, in astronomical units (TABLE 21.1 ). Kepler’s laws assert that the planets revolve around the Sun and therefore support the Copernican theory. Kepler, however, did not determine the forces that act to produce the planetary motion he had so ably described. That task would remain for Galileo Galilei and Sir Isaac Newton.

Galileo Galilei Galileo Galilei (1564–1642) was the greatest Italian scientist of the Renaissance (FIGURE 21.12 ). He was a contemporary of Kepler and, like Kepler, strongly supported the Copernican theory of a Sun-centered solar system. Galileo’s greatest contributions to science were his descriptions of the behavior of moving objects, which he derived from experimentation. The method of using experiments to determine natural laws had essentially been lost since the time of the early Greeks. All astronomical discoveries before Galileo’s time were made without the aid of a telescope. In 1609, Galileo heard that a Dutch lens maker had devised a system of lenses that magnified objects. Apparently without ever having seen a telescope, Galileo constructed his own, which magnified distant objects three times the size seen by the unaided eye. He

FIGURE 21.12 Italian Scientist Galileo Galilei (1564–1642) Galileo was the first scientist to use a new invention, the telescope, to observe the Sun, Moon, and planets in more detail than ever before. (Nimatallah/Art Resource, NY)

FIGURE 21.13 One of Galileo’s Many Telescopes Although Galileo did not invent the telescope, he built several—the largest of which had a magnification of 30. (Photo by Gianni Tortoli/Science Source)

21.2 The Birth of Modern Astronomy

667

SmartFigure 21.15 Sun

Venus

Earth

A. In the Ptolemaic (Earth-centered) system, the orbit of Venus lies between the Sun and Earth, as shown here. Thus, in an Earth-centered solar system, only the crescent phase of Venus would be visible from Earth. Venus

Sun

U i a Telescope, Using Galileo Discovered That Venus Has Phases Like Earth’s Moon A. In the Ptolemaic (Earthcentered) system, the orbit of Venus lies between the Sun and Earth, as shown in Figure 21.4A. Thus, in an Earth-centered solar system, only the crescent phase of Venus would be visible from Earth. B. In the Copernican (Sun-centered) system, Venus orbits the Sun, and hence all the phases of Venus should be visible from Earth. C. As Galileo observed, Venus goes through a series of Moonlike phases. Venus appears smallest during the full phase, when it is farthest from Earth, and largest in the crescent phase, when it is closest to Earth. This verified Galileo’s belief that the Sun is the center of the solar system. (Photo courtesy

FIGURE 21.14 Sketch by Galileo of Jupiter and Its Four Largest Satellites Using a telescope, Galileo discovered Jupiter’s four largest Moons (drawn as stars) and noted that their positions change nightly. You can observe these same changes with binoculars. (Yerkes Observatory Photograph/University of Chicago)

immediately made others, the best having a magnification of about 30 (FIGURE 21.13 ). With the telescope, Galileo was able to view the universe in a new way. He made many important discoveries that supported the Copernican view of the universe, including the following: 1. The discovery of Jupiter’s four largest satellites, or moons (FIGURE 21.14 ). This finding dispelled the old idea that Earth was the sole center of motion in the universe; for here, plainly visible, was another center of motion—Jupiter. It also countered the frequently used argument that the Moon would be left behind if Earth revolved around the Sun. 2. The discovery that the planets are circular disks rather than just points of light, as was previously thought. This indicated that the planets must be Earth-like as opposed to star-like. 3. The discovery that Venus exhibits phases just as the Moon does and that Venus appears smallest when it is in full phase and thus is farthest from Earth (FIGURE 21.15B,C ). This observation demonstrates that Venus orbits its source of light—the Sun. In the Ptolemaic system, shown in FIGURE 21.15A , the orbit of Venus lies between Earth

of Lowell Observatory)

Earth

B. In the Copernican (Sun-centered) system, Venus orbits the Sun and hence all of the phases of Venus should be visible from Earth.

C. As Galileo observed, Venus goes through a series of Moonlike phases. Venus appears smallest during the full phase when it is farthest from Earth and largest in the crescent phase when it is closest to Earth. This led Galileo to conclude that the Sun was the center of the solar system.

668

CHAPTER 21

Origins of Modern Astronomy

and the Sun, which means that only the crescent phases of Venus should ever be seen from Earth. 4. The discovery that the Moon’s surface is not a smooth glass sphere, as the ancients had proclaimed. Rather, Galileo saw mountains, craters, and plains, indicating that the Moon is Earth-like. He thought the plains might be bodies of water, and this idea was strongly promoted by others, as we can tell from the names given to these features (Sea of Tranquility, Sea of Storms, etc.). 5. The discovery that the Sun (the viewing of which may have caused the eye damage that later blinded him) had sunspots—dark regions caused by slightly lower temperatures. He tracked the movement of these spots and estimated the rotational period of the Sun as just under a month. Hence, another heavenly body was found to have both “blemishes” and rotational motion. Each of these observations eroded a bedrock principle held by the prevailing view on the nature of the universe. In 1616, the Church condemned the Copernican theory as contrary to Scripture because it did not put humans at their rightful place in the center of Creation, and Galileo was told to abandon this theory. Undeterred, Galileo began writing his most famous work, Dialogue of the Great World Systems. Despite poor health, he completed the project and in 1630 went to Rome, seeking permission from Pope Urban VIII to publish. Since the book was a dialogue that expounded both the Ptolemaic and Copernican systems, publication was allowed. However, Galileo’s detractors were quick to realize that he was promoting the Copernican view at the expense of FIGURE 21.16 Prominent English Scientist Sir Isaac Newton (1642–1727) Newton discovered that gravity is the force that holds planets in orbit around the Sun. (Photo by De Agostini/Getty Images)

the Ptolemaic system. Sale of the book was quickly halted, and Galileo was called before the Inquisition. Tried and convicted of proclaiming doctrines contrary to religious teachings, he was sentenced to permanent house arrest, under which he remained for the last 10 years of his life. Despite this restriction, and his grief following the death of his eldest daughter, Galileo continued to work. In 1637 he became totally blind, yet during the next few years, he completed his finest scientific work, a book on the study of motion in which he stated that the natural tendency of an object in motion is to remain in motion. Later, as more scientific evidence in support of the Copernican system was discovered, the Church allowed Galileo’s works to be published.

Sir Isaac Newton Sir Isaac Newton (1642–1727) was born in the year of Galileo’s death (FIGURE 21.16 ). His many accomplishments in mathematics and physics led a successor to say, “Newton was the greatest genius that ever existed.” Although Kepler and those who followed attempted to explain the forces involved in planetary motion, their explanations were less than satisfactory. Kepler believed that some force pushed the planets along in their orbits. Galileo, however, correctly reasoned that no force is required to keep an object in motion. Instead, Galileo proposed that the natural tendency for a moving object that is unaffected by an outside force is to continue moving at a uniform speed and in a straight line. Newton later formalized this concept, inertia, as his first law of motion. The problem, then, was not to explain the force that keeps the planets moving but rather to determine the force that keeps them from going in a straight line out into space. It was to this end that Newton conceptualized the force of gravity. At the early age of 23, he envisioned a force that extends from Earth into space and holds the Moon in orbit around Earth. Although others had theorized the existence of such a force, he was the first to formulate and test the law of universal gravitation. It states: Every body in the universe attracts every other body with a force that is directly proportional to their masses and inversely proportional to the square of the distance between them.

Thus, gravitational force decreases with distance, so that two objects 3 kilometers apart have 32, or 9, times less gravitational attraction than if the same objects were 1 kilometer apart. The law of gravitation also states that the greater the mass of an object, the greater its gravitational force. For example, the large mass of the Moon has a gravitational force strong enough to cause ocean tides on Earth, whereas the tiny mass of a communications satellite has very little effect on Earth. With his laws of motion, Newton proved that the force of gravity—combined with the tendency of a planet to remain in straight-line motion—would result in a planet having an elliptical orbit as established by Kepler. Earth, for example, moves forward in its orbit about 30 kilometers (18.5 miles)

21.3 Positions in the Sky

on oti ) tm B Ne (A +

B. Gr av ita tio na lf

or ce

Planet

Orbit

Sun

SmartFigure 21.17 Orbital O bit Motion of Earth and Other Planets

Newton used the law of universal gravitation to express Kepler’s third law, which defines the relationship between the orbital periods of the planets and their solar distances. In its new form, Kepler’s third law takes into account the masses of the bodies involved and thereby provides a method for determining the mass of a body when the orbit of one of its satellites is known. For example, the mass of the Sun is known from Earth’s orbit, and Earth’s mass has been determined from the orbit of the Moon. In fact, the mass of any body with a satellite can be determined. The masses of bodies that do not have satellites can be determined only if the bodies noticeably affect the orbit of a neighboring body or of a nearby artificial satellite.

21.2 CONCEPT CHECKS 1 What major change did Copernicus make in the Ptolemaic system? Why was this change philosophically different?

2 What data did Tycho Brahe collect that was useful to Johannes Kepler in his quest to describe planetary motion?

each second, and during the same second, the force of gravity pulls it toward the Sun about 0.5 centimeter (1/8 inch). Therefore, as Newton concluded, it is the combination of Earth’s forward motion and its “falling” motion that defines its orbit (FIGURE 21.17 ). If gravity were somehow eliminated, Earth would move in a straight line out into space. Conversely, if Earth’s forward motion suddenly stopped, gravity would pull it until it crashed into the Sun.

|

3 Who discovered that planetary orbits are ellipses rather than circles?

4 Does Earth move faster in its orbit near perihelion (January) or near aphelion (July)?

5 Explain why Galileo’s discovery of a rotating Sun supports the Copernican view of a Sun-centered universe.

6 Newton discovered that the orbits of the planets result from opposing forces. Briefly explain these forces.

21.3 POSITIONS IN THE SKY

Compare the equatorial system of coordinates used to establish the position of the stars with longitude and latitude. Explain how the positions of stars are described using declination and right ascension.

If you gaze at the stars away from city lights, you will get the distinct impression that the stars produce a spherical shell surrounding Earth. This impression seems so real that it is easy to understand why the early Greeks regarded the stars as being fixed to a crystalline celestial sphere. Although we realize that no such sphere exists, it is convenient to use this concept to map the stars and other celestial objects. We describe two mapping systems that use the concept of celestial sphere: (1) the division of the sky into areas called constellations and (2) the extension of Earth’s lines of longitude and latitude into space (the equatorial system).

Constellations The natural fascination people have with the star-studded skies led them to name the patterns they saw. These configurations, called constellations (con 5 with, stella 5 star), were named in honor of mythological characters or great heroes, such as Orion the hunter (see GEOgraphics on pages 000–000). Sometimes it takes a bit of imagination to identify the intended subjects, as

most constellations were probably not originally thought of as likenesses. Although many of the constellations originated from Greek mythology, the Greeks adopted most of them from the Babylonians, Egyptians, and Mesopotamians. Although the stars that make up constellations all appear to be the same distance from Earth, this is not the case. Some are many times farther away than others. Thus, the stars in a particular constellation are not associated with each other in any important physical way. In addition, various cultural groups, including Native Americans and the Chinese, attached their own names, pictures, and stories to the constellations. For example, the stars that compose the constellation Leo the lion are said to represent a horse in the ancient Chinese zodiac. Because the solar system is “flat,” like a whirling Frisbee, the planets orbit the Sun along nearly the same plane. Therefore, the planets, Sun, and Moon all appear to move along a band around the sky known as the zodiac. Because Earth’s Moon cycles through its phases about 12 times each year, the Babylonians divided the zodiac into

669

HST/NASA

GEOGRAPHICS

Orion the Hunter Orion, a prominent constellation located on the celestial equator, is visible around the world. It is one of the most conspicuous constellations to decorate the winter skies in the Northern Hemisphere. Named after a hunter in Greek mythology, its brightest stars are Rigel, a luminous blue-white star and Betelgeuse, a red supergiant. Many of the other brightest stars in the constellation are hot, massive blue stars.

Size of Earth’s Orbit Size of Jupiter’s Orbit

US Navy

Artist’s depiction of Orion based on Greek mythology. Ancient Chinese observers visualized a different image and named this constellation the White Tiger.

Size of Betelgeuse

John Chumack/Photo Researchers, Inc.

Rigel, also known as Beta Orionis (β Orionis), is the brightest star in the constellation and the sixth brightest star in the night sky. As seen from Earth, Rigel is actually a triple star system. The primary star (Rigel A) of the three-star system is a blue-white massive star that is about 130,000 times more luminous than the Sun. Although Rigel has the designation “beta,” it is almost always brighter than Alpha Orionis (Betelgeuse). Rigel is one of the “model stars” by which other stars are compared and classified.

Betelgeuse (Alpha Orionis) is a red supergiant that makes up the shoulder of the winter constellation Orion the hunter. Betelgeuse is so huge that if placed at the center of our solar system, its outer atmosphere would extend beyond the orbit of Jupiter.

Occurring together at the heart of the Orion Nebula is a cluster of four dazzling, young stars, known as the Trapezium. This quartet of stars is much hotter and brighter than the Sun, and collectively provides enough radiation to make the entire nebula glow.

HST/NASA

HST/NASA

The Orion Nebula is visible with the naked eye as the middle “star” in the sword of Orion, which consists of the three stars located south of Orion’s Belt. The star appears fuzzy to sharp-eyed observers, and its cloud-like nature is obvious through binoculars or a small telescope. It is one of the brightest nebulae in the night sky and the closest region to Earth where massive stars are born. Within this nebula, astronomers have observed disk-shaped structures composed of dust and gases that orbit protostars. These structures provide insight into the processes of how stars and planetary systems develop from collapsing clouds of gas and dust.

672

CHAPTER 21

Origins of Modern Astronomy

FIGURE 21.18 Astronomical Coordinate System on the Celestial Sphere

North celestial pole Celestial sphere

Star North Pole

Equator

Declination

North latitude

East longitude Celestial equator

Vernal equinox Position of sun on March 21

Right ascension

South celestial pole

12 constellations. The dozen constellations of the zodiac (“Zone of Animals,” so named because some constellations represent animals) are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. These names may be familiar to you as the astrological signs of the zodiac. Today, 88 constellations are recognized, and they are used to divide the sky into units, just as state boundaries divide the United States. Every star in the sky is within the boundaries of one of these constellations. Astronomers use FIGURE 21.19 Star Trails in the Region of Polaris (North Celestial Pole) on a Time Exposure (Photo by Douglas Kirkland/CORBIS)

constellations when they want to roughly identify the area of the heavens they are observing. For a student, constellations provide a good way to become familiar with the night sky. Some of the brightest stars in the heavens were given proper names, such as Sirius, Arcturus, and Betelgeuse. In addition, the brightest stars in a constellation are generally named in order of their brightness by the letters of the Greek alphabet—alpha (a), beta (b), and so on—followed by the name of the parent constellation. For example, Sirius, the brightest star in the constellation Canis Major (Larger Dog), is also called Alpha (a) Canis Majoris.

The Equatorial System The equatorial system divides the celestial sphere into coordinates that are similar to the latitude and longitude system we use for establishing locations on Earth’s surface (FIGURE 21.18 ). Because the celestial sphere (night sky) appears to rotate around an imaginary line extending from Earth’s axis, the north and south celestial poles are aligned with the terrestrial North Pole and South Pole. The north celestial pole happens to be very near the bright star whose various names reflect its location: “pole star,” Polaris, and North Star. To an observer in the Northern Hemisphere, the stars appear to circle Polaris, because it, like the North Pole, is in the center of motion (FIGURE 21.19 ). (FIGURE 21.20 shows how to locate the North Star using two stars, called pointer stars, located on the outside of the “dipper” in the easily located constellation the Big Dipper.) Now, imagine a plane through Earth’s equator, a plane that extends outward from Earth and intersects the celestial sphere. The intersection of this plane with the celestial sphere is called the celestial equator (see Figure 21.18). In the equatorial system, the term declination is analogous to latitude, and the term right ascension is analogous to longitude (see Figure 21.18). Declination (declinare 5 to turn away), like latitude, is the angular distance north or south of the celestial equator. Right ascension (ascendere 5 to climb up) is the angular distance measured eastward along the celestial equator from the position of the vernal equinox. (The vernal equinox is at the point in the sky where the Sun crosses the celestial equator, at the onset of spring.) While declination (Dec) is expressed in

21.4 The Motions of Earth

degrees, right ascension (RA) is expressed in hours, minutes, and seconds, where each hour is equivalent to 15 degrees. (Earth rotates 15 degrees each hour.) For example, the position of Betelgeuse, a bright star in the constellation Orion, is RA: 5h55m10.3s, Dec: +7°24’25.4”. To visualize distances in the night sky, it helps to remember that the Sun and full Moon have an apparent width of about 0.5 degree.

April

673

SmartFigure 21.20 L t Locating the North Star (Polaris) from the Pointer Stars in the Big Dipper The Big

August

21.3 CONCEPT CHECKS

North Star

Dipper, which is part of the constellation Ursa Major, is shown soon after sunset in December (lower figure), April (upper figure), and August (left).

Pointer stars

December

1 How do modern astronomers use constellations? 2 How many constellations are currently recognized? 3 How are the brightest stars in a constellation denoted?

4 Briefly describe the equatorial system.

|

What an observer in the Northern Hemisphere sees looking north on a clear night three times a year.

21.4 THE MOTIONS OF EARTH Describe the two primary motions of Earth and explain the difference between a solar day and a sidereal day.

The two primary motions of Earth are rotation and revolution. A lesser motion is axial precession. Rotation is the turning, or spinning, of a body on its axis. Revolution is the motion of a body, such as a planet or moon, along a path around some point in space. For example, Earth revolves around the Sun, and the Moon revolves around Earth. Earth also has another very slow motion, known as axial precession, which is the gradual change in the orientation of Earth’s axis over a period of 26,000 years.

Rotation The main consequences of Earth’s rotation are day and night. Earth’s rotation has become a standard method of measuring time because it is so dependable and easy to use. You may be surprised to learn that Earth’s rotation is measured in two ways, making two kinds of days. Most familiar is the

Distant stars

Distant stars

mean solar day, the time interval from one noon to the next, which averages about 24 hours. Noon is when the Sun has reached its highest point in the sky. The sidereal (sider 5 star, at 5 pertaining to) day, on the other hand, is the time it takes for Earth to make one complete rotation (360 degrees) with respect to a star other than our Sun. The sidereal day is measured by the time required for a star to reappear at the identical position in the sky. The sidereal day has a period of 23 hours, 56 minutes, and 4 seconds (measured in solar time), which is almost 4 minutes shorter than the mean solar day. This difference results because the direction to distant stars changes only infinitesimally, whereas the direction to the Sun changes by almost 1 degree each day. This difference is shown in FIGURE 21.21 . Why do we use the mean solar day rather than the sidereal day to measure time? Consider the fact that in sidereal time, “noon” occurs 4 minutes earlier each day. Therefore,

Day 1 Sun’s noon rays X

Sidereal day X 23 hr 56 min

Y

FIGURE 21.21 Illustration of the Difference Between a Solar Day and a Sidereal Day Sun

Day 2 rays noon ’s Sun

Solar day 24 hr 4 minutes Y

(Not to scale)

Locations X and Y are directly opposite each other. It takes Earth 23 hours and 56 minutes to make one rotation with respect to the stars (sidereal day). However, notice that after Earth has rotated once with respect to the stars, point Y is not yet returned to the “noon position” with respect to the Sun. Earth has to rotate another 4 minutes to complete the solar day.

North celestial pole

CHAPTER 21

FIGURE 21.22 Depiction of the Ecliptic and the Plane of the Ecliptic Earth’s orbital motion causes the apparent position of the Sun to shift about 1 degree each day on the celestial sphere. The ecliptic is the path the Sun appears to trace through the stars. The plane of the ecliptic is an imaginary plane that connects the points on the ecliptic.

Celestial sphere

Revolution Sun against the backdrop of stars in October

Earth revolves around the Sun in an elliptical orbit at an average speed of 107,000 kilometers (66,000 miles) per hour. Its average distance from the Sun is 150 million kilometers (93 million Libra miles), but because its orbit is an ellipse, Scorpio Virgo Earth’s distance from the Sun varies. At perihelion (peri 5 near, helios 5 sun) it is 147 million kilometers (91.5 million miles) distant, which occurs about January 3 each year. At aphelion (apo 5 away, helios 5 Sun sun) Earth is 152 million kilometers (94.5 million miles) distant, which occurs about Sept Nov Oct July 4. Because of Earth’s orbital moveEcliptic ment, the Sun appears to move relative the constellations. Each day, this apparPlane of ent movement amounts to a distance equal Earth the equator Plane of to about 1 degree, or about twice the Sun’s the ecliptic width. The apparent annual path of the Sun against the backdrop of the celestial sphere is called the ecliptic (FIGURE 21.22 ). The planets and the Moon travel in nearly the same plane as Earth. Plane of 23½ Hence, their paths on the celestial sphere also lie near the the ecliptic degrees ecliptic. The imaginary plane that connects points along the after a span of 6 months, “noon” would occur at “midnight.” ecliptic is called the plane of the ecliptic. As measured However, observatories use clocks that keep sidereal time from this imaginary plane, Earth’s axis is tilted about because the stars appear to move through the sky in sidereal 23 12 degrees (see Figure 21.22). This angle is very time. Simply, if a star is sighted directly south of an observa- important to Earth’s inhabitants because the inclinatory at 9:00 p.m. (sidereal time), it will appear in the same tion of Earth’s axis causes the yearly cycle of seasons, direction at that time every (sidereal) day. a topic discussed in detail in Chapter 16.

EYE ON THE EY

UNIVERSE U

Stonehenge, a prehistoric monument whose construction S b began about 5000 years ago, is located north of the modern city of Salisbury, England. It is the remains of a ring er of m massive stones; the largest stands 9 meters (30 feet) tall and we weighs 25 tons. Some of the smaller stones appear to have been transported 250 kilometers (150 miles) from southwestern Wales. The arrangement of the stones indicates that Stonehenge must have an astronomical connection. Each year on June 21 or 22, an observer standing within the stone circle, looking though the entrance, would see the Sun rising above the heel stone, as shown in the inset image. (To answer the following questions, you may want to refer to the section “Earth–Sun Relationships” in Chapter 16) QUESTIO N 1 What is the significance of June 21–22? Does it mark the occurrence of the spring equinox, the summer equinox, the spring solstice, or the summer solstice? QUESTION 2 On June 21–22, at what latitude do the vertical rays of the Sun strike Earth’s surface? QUESTION 3 Most world maps and globes label the latitude that is the answer to Question 2. What is this line of latitude called?

Robin Scagell/Science Source

Adam Woolfitt/Robert Harding

674

Vega Precession

Precession

SmartFigure 21.23

A third and very slow movement of Earth is called axial precession. Although Earth’s axis maintains approximately the same angle of tilt, the direction in which the axis points continually changes (FIGURE 21.23A ). As a result, the axis traces a circle on the sky. This movement is similar to the “wobble” of a spinning top (FIGURE 21.23B ). At the present time, the axis points toward the bright star Polaris. In a.d. 14000, it will point toward the bright star Vega, which will then be the North Star for about 1000 years or so (FIGURE 21.23C ). The period of precession is 26,000 years. By the year 28000, Polaris will once again be the North Star. Precession has only a minor effect on the seasons because Earth’s angle of tilt changes only slightly. In addition to its own movements, Earth accompanies the Sun as it speeds in the direction of the bright star Vega at 20 kilometers (12 miles) per second. Also, the Sun, like other nearby stars, revolves around the galaxy, a trip that requires 230 million years to complete at speeds approaching 250 kilometers (150 miles) per second. In addition, the galaxies themselves are in motion. We are presently approaching one of our nearest galactic neighbors, the Great Galaxy in Andromeda.

Precession

23½°

A.

Vega

CYGNUS

14,000 AD Dzneb

DRACO

Thuban CEPHEUS 2000 AD

1 Describe the three primary motions of Earth. 2 Explain the difference between the mean solar day and the

URSA MAJOR

sidereal day.

|

P Precession of Earth’s Axis A. The precession

B.

21.4 CONCEPT CHECKS

3 Define the ecliptic. 4 Why does axial precession have little effect on the seasons?

675

Polaris

Polaris (North Star)

C.

21.5 MOTIONS OF THE EARTH–MOON SYSTEM Sketch the changing positions of the Earth–Moon system that produce the regular cycle we call the phases of the Moon.

Earth has one natural satellite, the Moon. In addition to accompanying Earth in its annual trek around the Sun, our Moon orbits Earth about once each month. When viewed from a Northern Hemisphere perspective, the Moon moves counterclockwise (eastward) around Earth. The Moon’s orbit is elliptical, causing the Earth–Moon distance to vary by about 6 percent, averaging 384,401 kilometers (238,329 miles). The motions of the Earth–Moon system constantly change the relative positions of the Sun, Earth, and Moon. The results are some of the most noticeable astronomical phenomena: the phases of the Moon and the occasional eclipses of the Sun and Moon.

Lunar Motions The cycle of the Moon through its phases requires 2912 days—a time span called the synodic month. This cycle was the basis for the first Roman calendar. However, this

is the apparent period of the Moon’s revolution around Earth and not the true period, which takes only 2713 days and is known as the sidereal month. The reason for the difference of nearly 2 days each cycle is shown in FIGURE 21.24 . Notice that as the Moon orbits Earth, the Earth– Moon system also moves in an orbit around the Sun. Consequently, even after the Moon has made a complete revolution around Earth, it has not yet reached its starting position with respect to the Sun, which is directly between the Sun and Earth (new-Moon phase). This motion takes an additional 2 days. An interesting fact concerning the motions of the Moon is that its period of rotation around its axis and its revolution around Earth are the same—2713 days. Because of this, the same lunar hemisphere always faces Earth. All of the landings of the manned Apollo missions were confined to the Earth-facing side. Only orbiting satellites and astronauts have seen the “back” side of the Moon.

of Earth’s axis causes the North Pole to “trace” a circle through the sky during a 26,000-year cycle. Currently, the North Pole points toward Polaris (North Star). In about 12,000 years, Vega will be the North Star. Around 3000 B.C., the North Star was Thuban, a bright star in the constellation Draco. B. Precession illustrated by a spinning toy top. C. The circle shows the path of the North Pole among some prominent stars and constellations in the northern sky.

676

CHAPTER 21

Earth’s orbit

FIGURE 21.24 The Difference Between a Sidereal Month (2713 days) and a Synodic Month (2912 days) Distances and

Day 1 New Moon

Sun

angles are not shown to scale.

To distant star

Day 29½, New Moon (synodic month) Day 14 ¾ Full Moon

2 days

Day 27½, 3 Earth, Moon, and distant star are once again aligned (Sidereal month)

SmartFigure 21.25 Phases of the Moon Ph A. The outer figures show the phases as seen from Earth. B. Compare these photographs with the diagram. (Photos © UC

Sunset

3. First quarter West

Regents/Lick Observatory)

2. Crescent (waxing)

4. Gibbous (waxing) Sunset

Midnight

Rays from

Noon

sun 5. Full

1. New Sunrise

6. Gibbous (waning)

8. Crescent (waning) 7. Third quarter

Sunrise

A.

East

B.

Crescent

Third quarter

Gibbous

Full Moon

21.6 Eclipses of the Sun and Moon

Because the Moon rotates on its axis only once every 2713 days, any location on its surface experiences periods of daylight and darkness lasting about 2 weeks. This, along with the absence of an atmosphere, accounts for the high surface temperature of 127°C (261°F) on the day side of the Moon and the low surface temperature of –173°C (–280°F) on its night side.

Phases of the Moon The first astronomical phenomenon to be understood was the regular cycle of the phases of the Moon. On a monthly basis, we observe the phases as a systematic change in the amount of the Moon that appears illuminated (FIGURE 21.25 ). We will choose the “new-Moon” position in the cycle as a starting point. About 2 days after the new Moon, a thin sliver (crescent phase) can be seen with the naked eye low in the western sky just after sunset. During the following week, the illuminated portion of the Moon that is visible from Earth increases (waxing) to a half-circle (first-quarter phase) that can be seen from about noon to midnight. In another week, the complete disk (full-Moon phase) can be seen rising in the east as the Sun sinks in the west. During the next 2 weeks, the percentage of the Moon that can be seen steadily declines (waning), until the Moon disappears altogether (new-Moon phase). The cycle soon begins anew, with the reappearance of the crescent Moon.

|

677

The lunar phases are a result of the motion of the Moon and the sunlight that is reflected from its surface (see Figure 21.25B). Half of the Moon is illuminated at all times (note the inner group of Moon sketches in Figure 21.25A). But to an earthbound observer, the percentage of the bright side that is visible depends on the location of the Moon with respect to the Sun and Earth. When the Moon lies between the Sun and Earth, none of its bright side faces Earth, so we see the new-Moon (“noMoon”) phase. Conversely, when the Moon lies on the side of Earth opposite the Sun, all of its lighted side faces Earth, so we see the full Moon. At all positions between these extremes, an intermediate amount of the Moon’s illuminated side is visible from Earth.

21.5 CONCEPT CHECKS 1 Compare the synodic month with the sidereal month. 2 What is the approximate length of the cycle of the phases of the Moon?

3 What phenomenon results from the fact that the Moon’s periods of rotation and revolution are the same? 1

4 The Moon rotates very slowly (once in 273 days) on its axis. How does this affect the lunar surface temperature?

5 What is different about the crescent phase that precedes the new-Moon phase and that which follows the new-Moon phase?

6 What phase of the Moon occurs approximately 1 week after the new Moon? 2 weeks?

21.6 ECLIPSES OF THE SUN AND MOON Sketch the positions of the Earth–Moon system that produce a lunar eclipse, as well as a solar eclipse.

Along with understanding the Moon’s phases, the early Greeks also realized that eclipses are simply shadow effects. When Moon the Moon moves in a line Sunlight directly between Earth and the Sun, which can occur only during the newMoon phase, it casts a dark shadow on Earth, producing a solar eclipse (eclipsis 5 failure to appear) (FIG- A. URE 21.26 ). Conversely, the Moon is eclipsed (lunar eclipse) when it moves within Earth’s shadow, a situation that is possible only during the full-Moon phase (FIGURE 21.27 ). Why does a solar eclipse not occur with every new-Moon phase and a lunar eclipse with every full Moon? They would, if the orbit of the Moon lay exactly along the plane of Earth’s orbit. However, the Moon’s orbit is inclined about 5 degrees to the plane of the ecliptic. Thus, during most new-Moon

FIGURE 21.26 Solar Eclipse A. Observers in the zone of the umbral shadow see a total solar eclipse. Those located in the penumbra see only a partial eclipse. The path of the solar eclipse moves eastward across the Earth. B. During a total solar eclipse, the blotted-out solar disk is surrounded by an irregularly shaped halo called the corona. (Photo by Maksim Nikalayenka/

Earth Umbra

Penumbra

B.

Path of total solar eclipse

Shutterstock)

678

CHAPTER 21

SmartFigure 21.27 L Lunar Eclipse A. During a total lunar eclipse, the Moon’s orbit carries it into the dark shadow of Earth (umbra). During a partial eclipse, only a portion of the Moon enters the umbra. B. During a total lunar eclipse, a dark, copper-colored Moon is observed. The color is a result of a small amount of sunlight that is reddened by Earth’s atmosphere—for the same reason sunsets appear red. This light is refracted (bent) toward the Moon’s surface. (Photo by Eckhard Slawik/Science Source)

Origins of Modern Astronomy

beginning, middle, and end. These occur as a solar eclipse flanked by two lunar eclipses or Penumbra vice versa. Furthermore, it occasionally happens Umbra that the first set of eclipses for the year occurs at the very beginning of a year, the second set in the middle, and a third set before the calendar Sunlight year ends, resulting in six eclipses in that year. Moon More rarely, if one of these sets consists of three eclipses, the total number of eclipses in a year can reach seven, which is the maximum. Earth During a total lunar eclipse, Earth’s circular shadow moves slowly across the disk of the full Moon. When totally eclipsed, the Moon is completely within Earth’s shadow but is still visible A. as a coppery disk because Earth’s atmosphere bends some long-wavelength light (red) into its shadow. Some of this light reflects off the Moon and back to us. A total eclipse of the Moon can last up to 4 hours and is visible to anyone on the side of Earth facing the Moon. B. During a total solar eclipse, the Moon casts a circular shadow that is never wider phases, the shadow of the Moon passes either above or below Earth; during most full-Moon phases, the shadow than 275 kilometers (170 miles), about the size of South of Earth misses the Moon. An eclipse can take place only Carolina. This shadow traces a stripe on Earth’s surwhen a new- or full-Moon phase occurs while the Moon’s face. Anyone observing in this region will see the Moon slowly block the Sun from view and the sky darken orbit crosses the plane of the ecliptic. Because these conditions are normally met only twice (FIGURE 21.28 ). Near totality, a sharp drop in temperaa year, the usual number of eclipses is four. These occur as ture of a few degrees is experienced. The solar disk is a set of one solar and one lunar eclipse, followed 6 months completely blocked for a maximum of only 7 minutes later with another set. Occasionally the alignment is such because the Moon’s shadow is so small. At totality, the that three eclipses can occur in a 1-month period—at the dark Moon is seen covering the complete solar disk, and

FIGURE 21.28 Montage of Images Showing a Complete Solar Eclipse This sequence of photos moving from the upper left to the lower right shows the stages of a total solar eclipse. (Photo by Dr. Fred Espenak/ Science Source)

Concepts in Review

21.6 CONCEPT CHECKS

only the Sun’s brilliant white outer atmosphere is visible (see Figure 21.26). Total solar eclipses are visible only to people in the dark part of the Moon’s shadow (umbra), while partial eclipses are seen by those in the light portion (penumbra) (see Figure 21.26). Partial solar eclipses are most common in the polar regions because it is these areas that the penumbra blankets when the dark umbra of the Moon’s shadow just misses Earth. A total solar eclipse is a rare event at any given location. The next one that will be visible from the contiguous United States will occur on August 21, 2017.

1 Sketch the locations of the Sun, Moon, and Earth during a solar eclipse and during a lunar eclipse.

2 How many eclipses normally occur each year? 3 Solar eclipses are slightly more common than lunar eclipses. Why, then, is it more likely that your region of the country will experience a lunar eclipse than a solar eclipse?

4 How long can a total eclipse of the Moon last? How about a total eclipse of the Sun?

21

| Origins of Modern Astronomy

21.1 ANCIENT ASTRONOMY

21.2 THE BIRTH OF MODERN ASTRONOMY

CONCEPTS IN REVIEW

679

Explain the geocentric view of the solar system and describe how it differs from the heliocentric view.

List and describe the contributions to modern astronomy of Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Isaac Newton. K E Y T E R M S : astronomical unit (AU), inertia, law of universal gravitation,

KEY TERMS: geocentric, celestial sphere, heliocentric,

Ptolemaic system, retrograde motion ■

■

■

Early agrarian cultures, whose survival depended on seasonal change, believed that if the heavenly objects could control the seasons, they must also strongly influence all other Earth events. The early Greeks held a geocentric (“Earth-centered”) view of the universe, believing that Earth is a sphere that stays motionless at the center of the universe. They believed that the Moon, the Sun, and the known planets—Mercury, Venus, Mars, Jupiter, and Saturn—orbited Earth. The early Greeks believed that the stars traveled daily around Earth on a transparent, hollow celestial sphere. In a.d. 141, Claudius Ptolemy documented this geocentric view, called the Ptolemaic system, which became the dominant view of the solar system for over 15 centuries.

■

■

■ ■ ■

■

Modern astronomy evolved during the 1500s and 1600s. The scientists involved in the transition from an astronomy that merely describes what is observed to an astronomy that tries to explain why the universe behaves the way it does included Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Isaac Newton. Nicolaus Copernicus (1473–1543) reconstructed the solar system with the Sun at the center and the planets orbiting around it, but he erroneously continued to use circles to represent the orbits of planets. The establishment of his day rejected his Sun-centered view. Tycho Brahe’s (1546–1601) observations of the planets were far more precise than any made previously and are his legacy to astronomy. Johannes Kepler (1571–1630) used Tycho Brahe’s observations to usher in a new astronomy with the formulation of his three laws of planetary motion. After constructing his own telescope, Galileo Galilei (1564–1642) made many important discoveries that supported the Copernican view of a Sun-centered solar system. This included charting the movement of Jupiter’s four largest moons, proving that Earth was not the center of all planetary motion. Sir Isaac Newton (1642–1727) demonstrated that the orbit of a planet is a result of a planet’s inertia (the tendency of a planet to move in a straight line) and the Sun’s gravitational attraction, which bends the planet’s path into an elliptical orbit.

21.3 POSITIONS IN THE SKY Compare the equatorial system of coordinates used to establish the position of the stars with longitude and latitude. Explain how the positions of stars are described using declination and right ascension. KEY TERMS: constellations, equatorial system, declination, right ascension ■ ■

As early as 5000 years ago, people began naming the configurations of stars, called constellations, in honor of mythological characters or great heroes. Today, 88 constellations are recognized that divide the sky into units, just as state boundaries divide the United States. One method for locating stars, called the equatorial system, divides the celestial sphere into a coordinate system similar to the grid system of latitude–longitude used for locating places on Earth’s surface. Declination, like latitude, is the angular distance north or south of the celestial equator. Right ascension is the angular distance measured eastward from the position of the vernal equinox (the point in the sky where the Sun crosses the celestial equator at the onset of spring).

680

Concepts in Review

21.4 THE MOTIONS OF EARTH

21.5 MOTIONS OF THE EARTH–MOON SYSTEM

Describe the two primary motions of Earth and explain the difference between a solar day and a sidereal day.

Sketch the changing positions of the Earth–Moon system that produce the regular cycle we call the phases of the Moon.

K E Y TERMS: rotation, revolution, axial precession, mean solar day, sidereal day,

perihelion, aphelion, ecliptic, plane of the ecliptic ■

■

■

■

The two primary motions of Earth are rotation (the turning, or spinning, of a body on its axis) and revolution (the motion of a body, such as a planet or moon, along a path around some point in space). Earth’s rotation can be measured in two ways, making two kinds of days. The mean solar day is the time interval from one noon (the time of day when the Sun is highest in the sky) to the next, which averages about 24 hours. In contrast, the sidereal day is the time it takes for Earth to make one complete rotation with respect to a star other than the Sun, a period of 23 hours, 56 minutes, and 4 seconds. Earth revolves around the Sun in an elliptical orbit at an average distance from the Sun of 150 million kilometers (93 million miles). At perihelion (closest to the Sun), which occurs in January, Earth is 147 million kilometers (91.5 million miles) from the Sun. At aphelion (farthest from the Sun), which occurs in July, Earth is 152 million kilometers (94.5 million miles) distant. The imaginary plane that connects Earth’s orbit with the celestial sphere is called the plane of the ecliptic. Another very slow motion of Earth is precession—the slow motion of Earth’s axis that traces out a cone over a period of 26,000 years.

K E Y T E R M S : synodic month, sidereal month, phases of the Moon ■

■

One of the first astronomical phenomena to be understood was the regular cycle of the phases of the Moon. The phases of the Moon are a result of the motion of the Moon around Earth and the portion of the bright side of the Moon that is visible to an observer on Earth. The cycle of the Moon through its phases requires 2912 days, a time span called the synodic month. However, the true period of the Moon’s revolution around Earth takes 2713 days and is known as the sidereal month. The difference of nearly 2 days is due to the fact that as the Moon orbits Earth, the Earth–Moon system also moves in an orbit around the Sun.

21.6 ECLIPSES OF THE SUN AND MOON Sketch the positions of the Earth–Moon system that produce a lunar eclipse, as well as a solar eclipse K E Y TERMS: solar eclipse, lunar eclipse ■ ■ ■

In addition to understanding the Moon’s phases, the early Greeks also realized that eclipses are simply shadow effects. When the Moon moves in a line directly between Earth and the Sun, which can occur only during the new-Moon phase, it casts a dark shadow on Earth, producing a solar eclipse. A lunar eclipse takes place when the Moon moves within the shadow of Earth during the full-Moon phase. Because the Moon’s orbit is inclined about 5 degrees to the plane that contains the Earth and Sun (the plane of the ecliptic), during most new- and fullMoon phases, no eclipse occurs. Only if a new- or full-Moon phase occurs as the Moon crosses the plane of the ecliptic can an eclipse take place. The usual number of eclipses is four per year.

GIVE IT SOME THOUGHT 1. Refer to Figure 21.3 and imagine that Eratosthenes had measured the difference in the angles of the noonday Sun between Syene and Alexandria to be 10 degrees instead of 7 degrees. Consider how this measurement would have affected his calculation of Earth’s circumference as you answer the following questions. a. Would this new measurement lead to a more accurate calculation? b. Would this new measurement lead to an estimate for the circumference of Earth that is larger or smaller than Eratosthenes’s original estimate?

2. Use Kepler’s third law to answer the following questions: a. Determine the period of a planet with a solar distance of 10 AU. b. Determine the distance between the Sun and a planet with a rotational period of 5 years. c. Imagine two bodies, one twice as large as the other, orbiting the Sun at the same distance. Which of the bodies, if either, would move faster than the other?

3. Galileo used his telescope to observe the planets and moons in our solar system. These observations allowed him to determine the positions and relative motions of the Sun, Earth, and other objects in the solar system. Refer to Figure 21.15A, which shows an Earth-centered solar system, and Figure 21.15B, which shows a Sun-centered solar system, to complete the following: a. Describe the phases of Venus that an observer on Earth would see for the Earth-centered model of the solar system. b. Describe the phases of Venus that an observer on Earth would see for the Sun-centered model of the solar system. c. Explain how Galileo used observations of the phases of Venus to determine the correct positions of the Sun, Earth, and Venus.

4. Refer to the accompanying diagram, which shows three asteroids (A, B, and C) that are being pulled by the gravitational force exerted on them by their partner asteroid shown on the left. How will the strength of the

Examining the Earth System

gravitational force felt by each asteroid (A, B, and C) compare? (Assume that all these asteroids are composed of the same material.)

681

7. Imagine you are driving westward along the equator before sunset. At what velocity should you drive if you never want to see the sun setting? 8. Refer to the accompanying photo of the Moon to complete the following: a. When you observe the phase of the Moon shown, is the Moon waxing or waning? b. What time of day can this phase of the Moon be observed?

5. Refer to the accompanying diagram, which shows two pairs of asteroids, Pair D and Pair E. Is it possible for the asteroids in Pair D to be experiencing the same degree of gravitational force as the asteroids in Pair E? Explain your answer.

6. Imagine that Earth rotates on its axis at half its current rate. How much time would be required to capture the time-lapse photo shown in Figure 21.19?

9. Imagine that you are looking up at a full Moon. At the same time, an astronaut on the Moon is viewing Earth. In what phase will Earth appear to be from the astronaut’s vantage point? Sketch a diagram to illustrate your answer. 10. If the Moon’s orbit were precisely aligned with the plane of Earth’s orbit, how many eclipses (solar and lunar) would occur in a 6-month period of © UC Regents/Lick Oservatory time? If the Moon’s orbit were tilted 90 degrees with respect to the plane of Earth’s orbit, how many eclipses (solar and lunar) would occur in a 6-month period?

EXAMINING THE EARTH SYSTEM 1. Currently, Earth is closest to the Sun (perihelion) in January (147 million kilometers [91.5 million miles]) and farthest from the Sun in July (152 million kilometers [94.5 million miles]). As a result of the precession of Earth’s axis, 12,000 years from now perihelion (closest) will occur in July, and aphelion (farthest) will take place in January. Assuming no other changes, how might this change average summer temperatures for your location? What about average winter temperatures? What might the

impact be on the biosphere and hydrosphere? (To aid your understanding of the effect of Earth’s orbital parameters on the seasons, you may want to review the section “Variations in Earth’s Orbit” in Chapter 6, page 191.) 2. Speculate about what would happen on Earth if the following increased: (a) the period of Earth’s rotation, and (b) the period of Earth’s revolution. What would be the effects of these increases on the major spheres of the Earth system?

22

1

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

22.1

Describe the formation of the solar system according to the nebular theory. Compare and contrast the terrestrial and Jovian planets.

22.2

List and describe the major features of Earth’s Moon and explain how maria basins were produced.

22.3

Outline the principal characteristics of Mercury, Venus, and Mars. Describe their similarities to and differences from Earth.

22.4 22.5

Compare and contrast the four Jovian planets. List and describe the principal characteristics of the small bodies that inhabit the solar system.

View of the windblown Martian surface obtained by the NASA rover Curiosity. The black-colored rocks are volcanic and have a composition similar to that of the rock basalt found on the Hawaiian islands. (Photo courtesy of NASA/JPL-Caltech/MSSS)

1 This chapter was revised with the assistance of Professors Teresa Tarbuck and Mark Watry.

683

684

CHAPTER 22

Touring Our Solar System

lanetary geology is the study of the formation and evolution of the bodies in our solar system—including the eight planets and myriad smaller objects: moons, dwarf planets, asteroids, comets, and meteoroids. Studying these objects provides valuable insights into the dynamic processes that operate on Earth. Understanding how other atmospheres evolve helps scientists build better models

P

|

for predicting climate change. Studying tectonic processes on other planets helps us appreciate how these complex interactions alter Earth. In addition, seeing how erosional forces work on other bodies allows us to observe the many ways landscapes are created. Finally, the uniqueness of Earth, a body that harbors life, is revealed through the exploration of other planetary bodies.

22.1 OUR SOLAR SYSTEM: AN OVERVIEW Describe the formation of the solar system according to the nebular theory. Compare and contrast the terrestrial and Jovian planets.

The Sun is at the center of a revolving system, trillions of miles wide, consisting of eight planets, their satellites, and numerous smaller asteroids, comets, and meteoroids. An estimated 99.85 percent of the mass of our solar system is

SmartFigure 22.1

A.

contained within the Sun. Collectively, the planets account for most of the remaining 0.15 percent. Starting from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (FIGURE 22.1 ). Pluto was

Kuiper belt

Orbits O bit of the Planets A. Artistic view of the solar system, in which planets are not drawn to scale. B. Positions of the planets shown to scale using astronomical units (AU), where 1 AU is equal to the average distance from Earth to the Sun—150 million kilometers (93 million miles).

Neptune

Uranus Asteroid belt Earth

Sun

Mercury

Venus

Mars Jupiter Saturn

B.

Mercury Venus Earth Mars SUN Neptune

30

Uranus

25

20

Saturn

15 10 Distance in astronomical units (AU)

Jupiter

5

0

22.1 Our Solar System: An Overview

TABLE 22.1  

Planetary Data

 

 

Mean Distance from Sun

Planet

Symbol

AU*

Millions of Miles

Mercury



0.39

36

 

Orbital Velocity 

Millions of Kilometers

Period of Revolution

Inclination of Orbit

mi/s

km/s

58

88d

7°00´

29.5

47.5 35.0

Venus

씸

0.72

67

108

225

3°24´

21.8

Earth

{

1.00

93

150

365.25d

0°00´

18.5

29.8

Mars

쯑

1.52

142

228

687d

1°51´

14.9

24.1

Jupiter

첎

5.20

483

778

12yr

1°18´

8.1

13.1

Saturn



9.54

886

1427

30yr

2°29´

6.0

9.6

Uranus

췜

19.18

1783

2870

84

yr

0°46´

4.2

6.8

Neptune

°

30.06

2794

4497

165yr

1°46´

3.3

5.3

Relative Mass (Earth = 1)

Average Density (g/cm3)

Polar Flattening (%)

Eccentricity†

Number of Known Satellites††

 Planet

Period of Rotation d

Diameter Miles

Kilometers

d

Mercury

59

3015

4878

0.06

5.4

0.0

0.206

0

Venus

243d

7526

12,104

0.82

5.2

0.0

0.007

0

Earth

23 56 04

s

7920

12,756

1.00

5.5

0.3

0.017

1

Mars

24h37m23s

4216

6794

0.11

3.9

0.5

0.093

2

Jupiter

9h56m

88,700

143,884

317.87

1.3

6.7

0.048

67

Saturn

10h30m

75,000

120,536

95.14

0.7

10.4

0.056

62

Uranus

17h14m

29,000

51,118

14.56

1.2

2.3

0.047

27

Neptune

16h07m

28,900

50,530

17.21

1.7

1.8

0.009

13

h

m

*AU = astronomical unit, Earth’s mean distance from the Sun. †

Eccentricity is a measure of the amount an orbit deviates from a circular shape. The larger the number, the less circular the orbit.

††

Includes all satellites discovered as of December 2012.

recently reclassified as a member of a new class of solar system bodies called dwarf planets. Tethered to the Sun by gravity, all the planets travel in the same direction, on slightly elliptical orbits (TABLE 22.1 ). Gravity causes objects nearest the Sun to travel fastest. Therefore, Mercury has the highest orbital velocity, 48 kilometers (30 miles) per second, and the shortest period of revolution around the Sun, 88 Earth-days. By contrast, the distant dwarf planet Pluto has an orbital speed of just 5 kilometers (3 miles) per second and requires 248 Earth-years to complete one revolution. Most large bodies orbit the Sun approximately in the same plane. The planets’ inclination with respect to the Earth–Sun orbital plane, known as the ecliptic, is shown in Table 22.1.

Nebular Theory: Formation of the Solar System The nebular theory, which explains the formation of the solar system, proposes that the Sun and planets formed from a rotating cloud of interstellar gases (mainly hydrogen and helium) and dust called the solar nebula. As the solar nebula contracted due to gravity, most of the material collected in the center to form the hot protosun. The remaining materials formed a thick, flattened, rotating disk, within which matter gradually cooled and condensed into grains and clumps of icy, rocky material. Repeated collisions resulted in most of the material clumping together into larger and larger chunks that eventually became asteroid-sized objects called planetesimals.

The composition of planetesimals was largely determined by their proximity to the protosun. As you might expect, temperatures were highest in the inner solar system and decreased toward the outer edge of the disk. Therefore, between the present orbits of Mercury and Mars, the planetesimals were composed of materials with high melting temperatures—metals and rocky substances. Then, through repeated collisions and accretion, these asteroid-sized rocky bodies combined to form the four protoplanets that eventually became Mercury, Venus, Earth, and Mars. The planetesimals that formed beyond the orbit of Mars, where temperatures were low, contained high percentages of ices—water, carbon dioxide, ammonia, and methane—as well as small amounts of rocky and metallic debris. It was mainly from these planetesimals that the four outer planets eventually formed. The accumulation of ices accounts, in part, for the large sizes and low densities of the outer planets. The two most massive planets, Jupiter and Saturn, had surface gravities sufficient to attract and retain large quantities of hydrogen and helium, the lightest elements. It took roughly 1 billion years after the protoplanets formed for the planets to gravitationally accumulate most of the interplanetary debris. This was a period of intense bombardment as the planets cleared their orbits by collecting much of the leftover material. The “scars” of this period are still evident on the Moon’s surface. Because of the gravitational effect of the planets, particularly Jupiter, small bodies were flung into planet-crossing orbits or into interstellar space. The small fraction of interplanetary matter that escaped this violent

685

686

CHAPTER 22

Touring Our Solar System

period became either asteroids or comets. By comparison, the present-day solar system is a much quieter place, although many of these processes continue today at a reduced pace.

The Planets: Internal Structures and Atmospheres The planets fall into two groups, based on location, size, and density: the terrestrial (Earth-like) planets (Mercury, Venus, Earth, and Mars) and the Jovian (Jupiter-like) planets (Jupiter, Saturn, Uranus, and Neptune). Because of their relative locations, the four terrestrial planets are also known as inner planets, and the four Jovian planets are known as outer planets. A correlation exists between planetary locations and sizes: The inner planets are substantially smaller than the outer planets, also known as gas giants. For example, the diameter of Neptune (the smallest Jovian planet) is nearly 4 times larger than the diameter of Earth. Furthermore, Neptune’s mass is 17 times greater than that of Earth or Venus. Other properties that differ among the planets include densities, chemical compositions, orbital periods, and numbers of satellites. Variations in the chemical composition of planets are largely responsible for their density differences. Specifically, the average density of the terrestrial planets is about 5 times the density of water, whereas the average density of the Jovian planets is only 1.5 times that of water. In fact, Saturn has a density only 0.7 times that of water, which means that it would float in a sufficiently large tank of water. The outer planets are also characterized by long orbital periods and numerous satellites. TERRESTRIAL PLANETS

FIGURE 22.2 Comparing Internal Structures of the Planets

Mercury Key Venus Rocky crust Rocky mantle Metallic core Inner metallic core

Earth

JOVIAN PLANETS

Jupiter Key for Jupiter/Saturn Visible clouds Gaseous hydrogen/helium Liquid molecular hydrogen Liquid metallic hydrogen Rocky/iron core

Saturn

Internal Structures Shortly after Earth formed, segregation of material resulted in the formation of three major layers, defined by their chemical composition—the crust, mantle, and core. This type of chemical separation occurred in the other planets as well. However, because the terrestrial planets are compositionally different from the Jovian planets, the nature of these layers is different as well (FIGURE 22.2 ). The terrestrial planets are dense, having relatively large cores of iron and nickel. The outer cores of Earth and Mercury are liquid, whereas the cores of Venus and Mars are thought to be only partially molten. This difference is attributable to Venus and Mars having lower internal temperatures than those of Earth and Mercury. Silicate minerals and other lighter compounds make up the mantles of the terrestrial planets. Finally, the silicate crusts of terrestrial planets are relatively thin compared to their mantles. The two largest Jovian planets, Jupiter and Saturn, likely have small, solid cores consisting of iron compounds, like the cores of the terrestrial planets, and rocky material similar to Earth’s mantle. Progressing outward, the layer above the core consists of liquid hydrogen that is under extremely high temperatures and pressures. There is substantial evidence that under these conditions, hydrogen behaves like a metal in that its electrons move freely about and are efficient conductors of both heat and electricity. Jupiter’s intense magnetic field is thought to be the result of electric currents flowing within a spinning layer of liquid metallic hydrogen. Saturn’s magnetic field is much weaker than Jupiter’s, due to its smaller shell of liquid metallic hydrogen. Above this metallic layer, both Jupiter and Saturn are thought to be composed of molecular liquid hydrogen that is intermixed with helium. The outermost layers are gases of hydrogen and helium, as well as ices of water, ammonia, Moon and methane—which mainly Mars account for the low densities of these giants. Uranus and Neptune also have small iron-rich, rocky cores, but their mantles are likely hot, dense water and ammonia. Above their mantles, the amount of hydrogen and helium increases, but these gases exist in much lower concentrations than in Jupiter and Saturn. All planets, except Venus Uranus Neptune and Mars, have significant magnetic fields generated by Key for Uranus/Neptune flow of metallic materials in Visible clouds Gaseous hydrogen/helium their liquid cores, or mantles. Ices (water/methane) Venus has a weak field due to Rocky/iron core the interaction between the solar wind and its uppermost atmosphere (ionosphere), while the weak Martian magnetic field is thought

22.1 Our Solar System: An Overview

to be a remnant from when its interior was hotter. Magnetic fields play an important role in protecting a planet’s surface from bombardment by charged particles of the solar wind—a necessary condition for the survival of life-forms.

Airless worlds have relatively warm surface temperatures and/or weak gravities.

SmartFigure 22.3 B di with Bodies Atmospheres Versus Airless Bodies Two

Airless bodies Mercury

The Atmospheres of the Planets The

Venus

Jovian planets have very thick atmospheres comMoon Earth posed mainly of hydrogen and helium, with lesser Mars amounts of water, methane, ammonia, and other Jupiter Galilean hydrocarbons. By contrast, the terrestrial planets, Asteroids moons including Earth, have relatively meager atmosEuropa pheres composed of carbon dioxide, nitrogen, and Saturn Titan oxygen. Two factors explain these significant differences—solar heating (temperature) and gravity Uranus (FIGURE 22.3 ). These variables determine what Triton Charon planetary gases, if any, were captured by planNeptune Pluto ets during the formation of the solar system and which were ultimately retained. During planetary formation, the inner regions of the developing solar system were too hot for ices Bodies with an atmosphere and gases to condense. By contrast, the Jovian planets formed where temperatures were low and solar heating of planetesimals was minimal. This allowed water vapor, Bodies with significant atmospheres have low ammonia, and methane to condense into ices. Hence, the surface temperatures and strong gravities. gas giants contain large amounts of these volatiles. As the planets grew, the largest Jovian planets, Jupiter and Saturn, Gravity also attracted large quantities of the lightest gases, hydrogen and helium. How did Earth acquire water and other volatile gases? It space. The speed required to escape a planet’s gravity is called seems that early in the history of the solar system, gravita- escape velocity. Because hydrogen is the lightest gas, it most tional tugs by the developing protoplanets sent planetesimals easily reaches the speed needed to overcome Earth’s gravity. Billions of years in the future, the loss of hydrogen (one into very eccentric orbits. As a result, Earth was bombarded with icy objects that originated beyond the orbit of Mars. of the components of water) will eventually “dry out” Earth’s This was a fortuitous event for organisms that currently oceans, ending its hydrologic cycle. Life, however, may inhabit our planet. Mercury, our Moon, and numerous other remain sustainable in Earth’s polar regions. The massive Jovian planets have strong gravitational small bodies lack significant atmospheres, even though they certainly would have been bombarded by icy bodies early in fields and thick atmospheres. Furthermore, because of their great distance from the Sun, solar heating is minimal. This their development. Airless bodies develop where solar heating is strong and/or explains why Saturn’s moon Titan, which is small compared gravities are weak. Simply stated, small warm bodies have a to Earth but much further from the Sun, retains an atmosbetter chance of losing their atmospheres because gas molecules phere. Because the molecular motion of a gas is temperature are more energetic and need less speed to escape their weak dependent, even hydrogen and helium move too slowly to gravities. For example, warm bodies with small surface gravity, escape the gravitational pull of the Jovian planets. such as our Moon, are unable to hold even heavy gases such as carbon dioxide and nitrogen. Mercury is massive enough to Planetary Impacts hold trace amounts of hydrogen, helium, and oxygen gas. The slightly larger terrestrial planets, Earth, Venus, and Planetary impacts between solar system bodies have Mars, retain some heavy gases, including water vapor, nitrogen, occurred throughout the history of the solar system. On bodand carbon dioxide. However, their atmospheres are miniscule ies that have little or no atmosphere (like the Moon) and, compared to their total mass. Early in their development, the therefore, no air resistance, even the smallest pieces of interterrestrial planets probably had much thicker atmospheres. planetary debris (meteorites) can reach the surface. At high Over time, however, these primitive atmospheres gradually enough velocities, this debris can produce microscopic cavichanged as light gases trickled away into space. For example, ties on individual mineral grains. By contrast, large impact Earth’s atmosphere continues to leak hydrogen and helium (the craters result from collisions with massive bodies, such as two lightest gases) into space. This phenomenon occurs near asteroids and comets. Planetary impacts were considerably more common the top of Earth’s atmosphere, where air is so tenuous that nothing stops the fastest-moving particles from flying off into in the early history of the solar system than they are today.

Solar heating (temperature)

687

factors largely explain why some solar system bodies have thick atmospheres, whereas others are airless. Airless worlds have relatively warm surface temperatures and/or weak gravities. Bodies with significant atmospheres have low surface temperatures and strong gravities.

688

CHAPTER 22

FIGURE 22.4 Formation of an Impact Crater

Touring Our Solar System

Crater ray

Compressional wave

Meteoroid Secondary crater chain Central peak

A. The energy of a rapidly moving body is transformed into heat and shock waves. Continuous ejecta

High-speed ejecta Discontinuous ejecta

FIGURE 22.5 Lunar Crater Euler This 20-kilometer-wide Rebound B. The rebound of over-compressed rock causes debris to be explosively ejected from the crater.

(12-mile-wide) crater is located in the southwestern part of Mare Imbrium. Clearly visible are the bright rays, central peak, secondary craters, and large accumulation of ejecta near the crater rim. (Courtesy of NASA)

The formation of a large impact crater is illustrated in FIGURE 22.4 . The meteoroid’s high-speed impact com-

Melt

Central peak

Fractured rock C. Heating melts some material that may be ejected from the crater as glass beads. Uplifted crater rim

Ejecta blanket

D. Small secondary craters often form when the material “splashed” from the impact crater strikes the surrounding landscape.

Following that early period of intense bombardment, the rate of cratering diminished dramatically and now remains essentially constant. Because weathering and erosion are almost nonexistent on the Moon and Mercury, evidence of their cratered past is clearly evident. On larger bodies, thick atmospheres may cause the impacting objects to break up and/or decelerate. For example, Earth’s atmosphere causes meteoroids with masses of less than 10 kilograms (22 pounds) to lose up to 90 percent of their speed as they penetrate the atmosphere. Therefore, impacts of low-mass bodies produce only small craters on Earth. Our atmosphere is much less effective in slowing large bodies; fortunately, they make very rare appearances.

presses the material it strikes, causing an almost instantaneous rebound, which ejects material from the surface. On Earth, impacts can occur at speeds that exceed 50 kilometers (30 miles) per second. Impacts at such high speeds produce shock waves that compress both the impactor and the material being impacted. Almost instantaneously, the over-compressed material rebounds and explosively ejects material out of the newly formed crater. This process is analogous to the detonation of an explosive device that has been buried underground. Craters excavated by objects that are several kilometers across often exhibit a central peak, such as the one in the large crater in FIGURE 22.5 . Much of the material expelled, called ejecta, lands in or near the crater, where it accumulates to form a rim. Large meteoroids may generate sufficient heat to melt and eject some of the impacted rock as glass beads. Specimens of glass beads produced in this manner, as well as melt breccia consisting of broken fragments welded by the heat of impact, have been collected on Earth, as well as the Moon, allowing planetary geologists to more fully understand such events.

22.1 CONCEPT CHECKS 1 Briefly outline the steps in the formation of our solar system, according to the nebular theory.

2 By what criteria are planets considered either terrestrial or Jovian?

3 What accounts for the large density differences between the terrestrial and Jovian planets?

4 Explain why the terrestrial planets have meager atmospheres, as compared to the Jovian planets.

5 Why are impact craters more common on the Moon than on Earth, even though the Moon is a much smaller target and has a weaker gravitational field?

6 When did the solar system experience the period of heaviest planetary impacts?

22.2 Earth’s Moon: A Chip Off the Old Block

|

22.2 EARTH’S MOON: A CHIP OFF THE OLD BLOCK List and describe the major features of Earth’s Moon and explain how maria basins were formed.

The Earth–Moon system is unique because the Moon is the largest satellite relative to its planet. Mars is the only other terrestrial planet that has moons, but its tiny satellites are likely captured asteroids. Most of the 150 or so satellites of the Jovian planets are composed of low-density rock–ice mixtures, none of which resemble the Moon. As we will see later, our unique planet–satellite system is closely related to the mechanism that created it. The diameter of the Moon is 3475 kilometers (2160 miles), about one-fourth of Earth’s 12,756 kilometers (7926 miles). The Moon’s surface temperature averages about 107°C (225°F) during daylight hours and −153°C (−243°F) at night. Because its period of rotation on its axis equals its period of revolution around Earth, the same lunar hemisphere always faces Earth. All of the landings of staffed Apollo missions were confined to the side of the Moon that faces Earth. The Moon’s density is 3.3 times that of water, comparable to that of mantle rocks on Earth but considerably less than Earth’s average density (5.5 times that of water). The Moon’s relatively small iron core is thought to account for much of this difference. The Moon’s low mass relative to Earth results in a lunar gravitational attraction that is one-sixth that of Earth. A person who weighs 150 pounds on Earth weighs only 25 pounds on the Moon, although the person’s mass remains the same. This difference allows an astronaut to carry a heavy life-support system with relative ease. If not burdened with such a load, an astronaut could jump six times higher on the Moon than on Earth. The Moon’s small mass (and low gravity) is the primary reason it was not able to retain an atmosphere.

EYE ON THE EY

UNIVERSE U

Mariner 9 obtained this image of Phobos, one of M tw tiny satellites of Mars. Phobos has a diameter two of only 24 kilometers (15 miles). The two moons of M Mars were not discovered until the late 1800s because ttheir size made them nearly impossible to view telescopically. (Photo by NASA) Q UESTION 1 In what way is Phobos similar to Earth’s Moon? Q UESTION 2 List characteristics of Phobos that make it different from Earth’s Moon. Q UESTION 3 Do an Internet search to learn how Phobos and its companion moon, Deimos, got their names.

How Did the Moon Form? Until recently, the origin of the Moon—our nearest planetary neighbor—was a topic of considerable debate among scientists. Current models show that Earth is too small to have formed with a moon, particularly one so large. Furthermore, a captured moon would likely have an eccentric orbit similar to the captured moons that orbit the Jovian planets. The current consensus is that the Moon formed as a result of a collision between a Mars-sized body and a youthful, semimolten Earth about 4.5 billion years ago. During this collision, some of the ejected debris was thrown into orbit around Earth and gradually coalesced to form the Moon. Computer simulations show that most of the ejected material would have come from the rocky mantle of the impactor, while its core was assimilated into the growing Earth. This impact model is consistent with the Moon having a proportionately smaller core than Earth’s and, hence, a lower density.

The Lunar Surface When Galileo first pointed his telescope toward the Moon, he observed two different types of terrain: dark lowlands and brighter, highly cratered highlands (FIGURE 22.6 ). Because the dark regions appeared to be smooth, resembling seas on Earth, they were called maria (mar = sea, singular mare). The Apollo 11 mission showed conclusively that the maria are exceedingly smooth plains composed of basaltic lavas. These vast plains are strongly concentrated on the side of the Moon facing Earth and cover about 16 percent of the lunar surface. The lack of large volcanic cones on these surfaces is

689

690

CHAPTER 22

Touring Our Solar System

FIGURE 22.6 Telescopic View of the Lunar Surface The major features are the dark maria and the light, highly cratered highlands. (NASA)

Mare Imbrium (Sea of Rains)

Kepler crater

SmartFigure 22.7 Formation and Filling F of Large Impact Basins

Copernicus crater

evidence of high eruption rates of very fluid basaltic lavas similar to the Columbia Plateau flood basalts on Earth. By contrast, the Moon’s light-colored areas resemble Earth’s continents, so the first observers dubbed them terrae (Latin for “lands”). These areas are now generally referred to as the lunar highlands because they are elevated several kilometers above the maria. Rocks retrieved from the highlands are mainly breccias, pulverized by massive bombardment early in the Moon’s history. The arrangement of terrae and maria has resulted in the legendary Mare Tranquillitatus “face” of the “man in the Moon.” (Sea of Tranquility Some of the most obvious lunar features are impact craters. A meteoroid 3 meters (10 feet) in diameter can blast out a crater 50 times larger, or about 150 meters (500 feet) in Lunar diameter. The larger craters shown in Highlands Figure 22.6, such as Kepler and Copernicus (32 and 93 kilometers [20 and 57 miles] in diameter, respectively), were created from bombardment by bodies 1 kilometer (0.62 mile) or more in diameter. These two craters are thought to be relatively young because of the bright rays (light-colored ejected material) that radiate from them for hundreds of kilometers.

Impact of an asteroid-size body produced a huge crater hundreds of kilometers in diameter and disturbed the lunar crust far beyond the crater.

(Photo © UC Regents/Lick Observatory)

Filling of the impact crater with fluid basalts, perhaps derived from partial melting deep within the lunar mantle.

History of the Lunar Surface The evidence used to unravel the history of the lunar surface comes primarily from radiometric dating of rocks returned from Apollo missions and studies of crater densities—counting the number of craters per unit area. The greater the crater density, the older the feature is inferred to be. Such evidence suggests that, after the Moon coalesced, it passed through the following four phases: (1) formation of the original crust, (2) excavation of the large impact basins, (3) filling of maria basins, and (4) formation of rayed craters. During the late stages of its accretion, the Moon’s outer shell was most likely completely melted—literally a magma ocean. Then, about 4.4 billion years ago, the magma ocean began to cool and underwent magmatic differentiation (see Chapter 4). Most of the dense minerals, olivine and pyroxene, sank, while less-dense silicate minerals floated to form the Moon’s crust. The highlands are made of these igneous Today these basins make up the lunar maria and a few similar large structures on Mercury.

22.2 Earth’s Moon: A Chip Off the Old Block

691

FIGURE 22.8 Astronaut Harrison Schmitt, Sampling the Lunar Surface Notice the footprint (inset) in the lunar “soil,” called regolith, which lacks organic material and is therefore not a true soil. (Courtesy of NASA)

rocks, which rose buoyantly like “scum” from the crystallizing magma. The most common highland rock type is anorthosite, which is composed mainly of calcium-rich plagioclase feldspar. Once formed, the lunar crust was continually impacted as the Moon swept up debris from the solar nebula. During this time, several large impact basins were created. Then, about 3.8 billion years ago, the Moon, as well as the rest of the solar system, experienced a sudden drop in the rate of meteoritic bombardment. The Moon’s next major event was the filling of the large impact basins, which were created at least 300 million years earlier (FIGURE 22.7 ). Radiometric dating of the maria basalts puts their age between 3.0 billion and 3.5 billion years, considerably younger than the initial lunar crust. The maria basalts are thought to have originated at depths between 200 and 400 kilometers (125 and 250 miles). They were likely generated by a slow rise in temperature attributed to the decay of radioactive elements. Partial melting probably occurred in several isolated pockets, as indicated by the diverse chemical makeup of the rocks retrieved during the Apollo missions. Recent evidence suggests that some mare-forming eruptions may have occurred as recently as 1 billion years ago. Other lunar surface features related to this period of volcanism include small shield volcanoes (8–12 kilometers [5–7.5 miles] in diameter), evidence of pyroclastic eruptions, rilles (narrow winding valleys thought to be lava channels), and grabens (down-faulted valleys). The last prominent features to form were rayed craters, as exemplified by the 90-kilometer-wide (56-mile-wide) Copernicus crater shown in Figure 22.7. Material ejected from these craters blankets the maria surfaces and many older, rayless craters. The relatively young Copernicus crater is thought to be about 1 billion years old. Had it formed on Earth, weathering and erosion would have long since obliterated it.

Today’s Lunar Surface The Moon’s small mass and low gravity account for its lack of atmosphere and flowing water. The processes of weathering and erosion that continually modify Earth’s surface are absent on the Moon. In addition, tectonic forces are no longer active on the Moon, so quakes and volcanic eruptions have ceased. Because the Moon is unprotected by an atmosphere, erosion is dominated by the impact of tiny particles from space (micrometeorites) that continually bombard its surface and gradually smooth the landscape. This activity has crushed and repeatedly mixed the upper portions of the lunar crust. Both the maria and terrae are mantled with a layer of gray, unconsolidated debris derived from a few billion years of meteoric bombardment (FIGURE 22.8 ). This soillike layer, properly called lunar regolith (rhegos = blanket, lithos = stone), is composed of igneous rocks, breccia, glass beads, and fine lunar dust. The lunar regolith is anywhere from 2 to 20 meters (6.5 to 65 feet) thick, depending on the age of the surface.

22.2 CONCEPT CHECKS 1 Briefly describe the origin of the Moon. 2 Compare and contrast the Moon’s maria and highlands. 3 How are maria on the Moon similar to the Columbia Plateau in the Pacific Northwest?

4 How is crater density used in the relative dating of surface features on the Moon?

5 List the major stages in the development of the modern lunar surface.

6 Compare and contrast the processes of weathering and erosion on Earth with the same processes on the Moon.

692

CHAPTER 22

Touring Our Solar System

|

22.3 TERRESTRIAL PLANETS Outline the principal characteristics of Mercury, Venus, and Mars. Describe their similarities to and differences from Earth.

The terrestrial planets, in order from the Sun, are Mercury, Venus, Earth, and Mars. Because most of the book focuses on Earth, we consider the three other Earth-like planets next.

Mercury: The Innermost Planet Mercury, the innermost and smallest planet, revolves around the Sun quickly (88 days) but rotates slowly on its axis. Mercury’s day–night cycle, which lasts 176 Earth-days, is very long compared to Earth’s 24-hour cycle. One “night” on Mercury is roughly equivalent to 3 months on Earth and is followed by the same duration of daylight. Mercury has the greatest temperature extremes, from as low as −173°C (−280°F) at night to noontime temperatures exceeding 427°C (800°F), hot enough to melt tin and lead. These extreme temperatures make life as we know it impossible on Mercury. FIGURE 22.9 Two Views of Mercury On the left is a monochromatic image, while the image on the right is color enhanced. These are high-resolution mosaics constructed from thousands of images obtained by the Messenger orbiter. (Courtesy of NASA)

EYE ON THE EY

UNIVERSE U

IIn 2012, the Hubble Space Telescope captured th these images of auroras above the giant planet Uranus. These light shows on Uranus appear to Ur last for only a few minutes and consist of faint glowing dots dots. These are unlike auroras on Earth, which can color the sky shades of green, red, or purple for hours. (Photo by NASA) QUE S T I ON 1 What is unusual about the location of the auroras on Uranus? QUE S T I ON 2 What does this indicate about the locations of Uranus’s “north” and “south” magnetic poles?

Mercury absorbs most of the sunlight that strikes it, reflecting only 6 percent into space, a characteristic of terrestrial bodies that have little or no atmosphere. The minuscule amount of gas that is present on Mercury may have originated from several sources, including ionized gas from the Sun, ices that vaporized during a recent comet impact, and/or outgassing of the planet’s interior. Although Mercury is small and scientists expected the planet’s interior to have already cooled, Mariner 10 measured Mercury’s magnetic field in 2012. It found Mercury’s magnetic field to be about 100 times less than Earth’s, which suggests that Mercury has a large core that remains hot and fluid—a requirement for generating a magnetic field. Mercury resembles Earth’s Moon in that it has very low reflectivity, no sustained atmosphere, numerous volcanic features, and a heavily cratered terrain (FIGURE 22.9 ). The largest-known impact crater (1300 kilometers [800 miles] in diameter) on Mercury is Caloris Basin. Images and other data gathered by Mariner 10 show evidence of volcanism in and around Caloris Basin and a few other smaller basins. Also like our Moon, Mercury has smooth plains that cover nearly 40 percent of the area imaged by Mariner 10. Most of these smooth areas are associated with large impact basins, including Caloris Basin, where lava partially filled the basins and the surrounding lowlands. Consequently, these smooth plains appear to be similar in origin to lunar maria. Recently, Messenger found evidence

22.3 Terrestrial Planets

of volcanism by revealing thick volcanic deposits similar to those on Earth in the Columbia Basin. In addition, researchers were surprised by the recent detection of probable ice caps on Mercury.

Venus: The Veiled Planet Venus, second only to the Moon in brilliance in the night sky, is named for the Roman goddess of love and beauty. It orbits the Sun in a nearly perfect circle once every 225 Earth-days. However, Venus rotates in the opposite direction of the other planets (retrograde motion) at an agonizingly slow pace: 1 Venus s and l day is equivalent to about 244 h g hi Earth days. Venus has the densest dite o r atmosphere of the terrestrial planAph ets, consisting mostly of carbon dioxide (97 percent)—the prototype for an extreme greenhouse effect. As a consequence, the surface temperature of Venus averages about 450°C (900°F) day and night. Temperature variations at the surface are generally minimal because of the intense mixing within the planet’s dense atmosphere. Investigations of the extreme and uniform surface temperature led scientists to more fully understand how the Elevation of surface greenhouse effect operates on Earth. The composition of the Venusian interior is Low High probably similar to Earth’s. However, Venus’s weak magnetic field means its internal dynamics must be very dif- traveled along lava channels that extend hundreds of kilferent from Earth’s. Mantle convection is thought to operate ometers (FIGURE 22.11 ). Venus’s Baltis Vallis, the longeston Venus, but the processes of plate tectonics, which recycle known lava channel in the solar system, meanders 6800 rigid lithosphere, do not appear to have contributed to the pre- kilometers (4255 miles) across the planet. More than 1000 volcanoes with diameters greater than 20 kilometers (12 sent Venusian topography. The surface of Venus is completely hidden from view by a miles) have been identified on Venus. However, high surthick cloud layer composed mainly of tiny sulfuric acid drop- face pressures keep the gaseous components in lava from lets. In the 1970s, despite extreme temperatures and pressures, escaping. This retards the production of pyroclastic material four Russian spacecraft landed successfully and obtained sur- and lava fountaining, phenomena that tend to steepen volface images. As expected, however, all the probes were crushed canic cones. In addition, because of Venus’s high temperaby the planet’s immense atmospheric pressure, approximately ture, lava remains mobile longer and thus flows far from 90 times that on Earth, within an hour of landing. Using radar the vent. Both of these factors result in volcanoes that tend imaging, the unstaffed spacecraft Magellan mapped Venus’s to be shorter and wider than those on Earth or Mars (FIGURE 22.12 ). Maat Mons, the largest volcano on Venus, surface in stunning detail (FIGURE 22.10 ). A few thousand impact craters have been identified on is about 8.5 kilometers high (5 miles) and 400 kilometers Venus—far fewer than on Mercury and Mars but more than on (250 miles) wide. By comparison, Mauna Loa, the largest Earth. Researchers expected that Venus would show evidence volcano on Earth, is about 9 kilometers high (5.5 miles) and of extensive cratering from the heavy bombardment period but only 120 kilometers (75 miles) wide. Venus also has major highlands that consist of plateaus, found instead that a period of extensive volcanism was responsible for resurfacing Venus. The planet’s thick atmosphere also ridges, and topographic rises that stand above the plains. The limits the number of impacts by breaking up large incoming rises are thought to have formed where hot mantle plumes encountered the base of the planet’s crust, causing uplift. meteoroids and incinerating most of the small debris. About 80 percent of the Venusian surface consists of Much as with mantle plumes on Earth, abundant volcanism low-lying plains covered by lava flows, some of which is associated with mantle upwelling on Venus. Recent data

693

FIGURE 22.10 Global View of the Surface of Venus This computergenerated image of Venus was constructed from years of investigations, culminating with the Magellan mission. The twisting bright features that cross the globe are highly fractured ridges and canyons of the eastern Aphrodite highland. (Courtesy of NASA)

694

CHAPTER 22

Touring Our Solar System

thin atmosphere of Mars (only 1 percent as dense as Earth’s). The tenuous Martian atmosphere consists primarily of carbon dioxide (95 percent), with small amounts of nitrogen, oxygen, and water vapor.

FIGURE 22.11 Extensive Lava Flows on Venus This Magellan radar image shows a system of lava flows that originated from a volcano named Ammavaru, which lies approximately 300 kilometers (186 miles) west of the scene. The lava, which appears bright in this radar image, has rough surfaces, whereas the darker flows are smooth. Upon breaking through the ridge belt (left of center), the lava collected in a 100,000-square-kilometer pool. (Courtesy of NASA)

Topography Mars,

collected by the European Space Agency’s Venus Express suggest that Venus’s highlands contain silica-rich granitic rock. As such, these elevated landmasses resemble Earth’s continents, albeit on a much smaller scale.

Mars: The Red Planet Mars, approximately one-half the diameter of Earth, revolves around the Sun in 687 Earth-days. Mean surface temperatures range from lows of −140°C (−220°F) at the poles in the winter to highs of 20°C (68°F) at the equator in the summer. Although seasonal temperature variations are similar to Earth’s, daily temperature variations are greater due to the very FIGURE 22.12 Volcanoes on Venus Sapas Mons is a broad volcano, 400 kilometers (250 miles) wide. The bright areas in the foreground are lava flows. Another large volcano, Maat Mons, is in the background.

Maat Mons

Sapas Mons

(Courtesy of NASA)

Lava flows

Lava flows

like the Moon, is pitted with impact craters. The smaller craters are usually filled with wind-blown dust—confirming that Mars is a dry, desert world. The reddish color of the Martian landscape is due to iron oxide (rust). Large impact craters provide information about the nature of the Martian surface. For example, if the surface is composed of dry dust and rocky debris, ejecta similar to that surrounding lunar craters is to be expected. But the ejecta surrounding some Martian craters has a different appearance: It looks like a muddy slurry that was splashed from the crater. Planetary geologists infer that a layer of permafrost (frozen, icy soil) lies below portions of the Martian surface and that impacts heated and melted the ice to produce the fluid-like appearance of these ejecta. About two-thirds of the surface of Mars consists of heavily cratered highlands, concentrated mostly in its southern hemisphere (FIGURE 22.13 ). The period of extreme cratering occurred early in the planet’s history and ended about 3.8 billion years ago, as it did in the rest of the solar system. Thus, Martian highlands are similar in age to the lunar highlands. Based on relatively low crater counts, the northern plains, which account for the remaining one-third of the planet, are younger than the highlands. If Mars once had abundant water, it would have flowed to the north, which is lower in elevation, forming an expansive ocean (shown in blue in Figure 22.13, indicating lower elevation). The relatively flat topography of the northern plains, possibly the smoothest surface in the solar system, is consistent with vast outpourings of fluid basaltic lavas. Visible on these plains are volcanic cones, some with summit pits (craters) and lava flows with wrinkled edges. Located along the Martian equator is an enormous elevated region, about the size of North America, called the Tharsis bulge. This feature, about 10 kilometers (6 miles) high, appears to have been uplifted and capped with a massive accumulation of volcanic rock that includes the solar system’s largest volcanoes. The tectonic forces that created the Tharsis region also produced fractures that radiate from its center, like spokes on a bicycle wheel. Along the

22.3 Terrestrial Planets

Two hemispheres of Mars

695

FIGURE 22.13 Two Hemispheres of Mars Color represents height above (or below) the mean planetary radius: White is about 12 kilometers above average, and dark blue is 8 kilometers below average.

Northern lava plains

Elysium Mons

(Courtesy of NASA)

Olympus Mons Tharsis bulge Equator

Equator Cratered highlands Valles Marineris Hellas Argyre

–8 Low

–4

0

4 Altitude (km)

eastern flanks of the bulge, a series of vast canyons called Valles Marineris (Mariner Valleys) developed. Valles Marineris is so vast that it can be seen in the image of Mars in Figure 22.13. This canyon network was largely created by down-faulting, not by stream erosion, as is the case for Arizona’s Grand Canyon. Thus, it consists of grabenlike valleys similar to the East African Rift valleys. Once formed, Valles Marineris grew by water erosion and collapse of the rift walls. The main canyon is more than 5000 kilometers (3000 miles) long, 7 kilometers (4 miles) deep, and 100 kilometers (60 miles) wide. Other prominent features on the Martian landscape are large impact basins. Hellas, the largest identifiable impact structure on the planet, is about 2300 kilometers (1400 miles) in diameter and has the planet’s lowest elevation. Debris ejected from this basin contributed to the elevation of the adjacent highlands. Other buried crater basins that are even larger than Hellas probably exist.

8

12 High

keep the crust in constant motion. Consequently, mantle plumes tend to produce a chain of volcanic structures, like the Hawaiian islands. By contrast, plate tectonics on Mars is absent, so successive eruptions accumulate in the same location. As a result, enormous volcanoes such as Olympus Mons form rather than a string of smaller ones.

Wind Erosion on Mars Currently, the dominant force shaping the Martian surface is wind erosion. Extensive dust storms, with winds up to 270 kilometers (170 miles) per hour, can persist for weeks. Dust devils have also been photographed.

SmartFigure 22.14 Olympus Mons This Ol massive inactive shield volcano on Mars covers an area about the size of the state of Arizona.

Volcanoes on Mars Volcanism has been prevalent

(Courtesy of

on Mars during most of its history. The scarcity of impact craters on some volcanic surfaces suggests that the planet is still active. Mars has several of the largest-known volcanoes in the solar system, including the largest, Olympus Mons, which is about the size of Arizona and stands nearly three times higher than Mount Everest. This gigantic volcano was last active about 100 million years ago and resembles Earth’s Hawaiian shield volcanoes (FIGURE 22.14 ). How did the volcanoes on Mars grow so much larger than similar structures on Earth? The largest volcanoes on the terrestrial planets tend to form where plumes of hot rock rise from deep within their interiors. On Earth, moving plates

NASA)

Caldera of Olympus Mons

Outline of the state of Arizona

GGEO EOGRAPHICS

696

CHAPTER 22

Touring Our Solar System

Mars Exploration Since the first close-up picture of Mars was obtained in 1965, spacecraft voyages to the fourth planet from the Sun have revealed a world that is strangely familiar. Mars has a thin atmosphere, polar ice caps, volcanoes, lava plains, sand dunes, and seasons. Unlike Earth, Mars appears to lack liquid water on its surface; however, many Martian landscapes suggest that, in the past, running water was an effective erosion agent. The defining question for Mars exploration is “has Mars ever harbored life?”

NASA's Phoenix lander dugg into the Martian surfacee to uncover water ice in a northern region of thee planet. Whether icee becomes available as liquid water to supportt microbial life remains unanswered.

PHOENIX VIKING 2

VIKING 1

PATHFINDER A CURIOSITY

OPPORTUNITY

SPIRIT

The U.S. has successfully landed seven rovers on the surface of Mars. The most recent was NASA's Curiosity, which landed in Gale Crater in August, 2012.

MARS LANDING SITES NASA

NASA's Curiosity rover, the size of a car, gracefully landed on Mars after decelerating from 13,000 miles per hour to a complete stop. The landing, ,” in and around Gale Crater to discover signs of past or present microbial life.

NASA

NASA

22.3 Terrestrial Planets

NASA

The layers in the background, located at the base of Mount Sharp, are thought to be surviving remnants of extensive deposits laid down in a lake long ago, or possibly wind-delivered sediments subsequently cemented together by ground water. Curiosity will use 10 instruments to investigate whether Mars had ever provided a water-rich environment.

NASA

This image captured by Curiosity shows Mount Sharp, a central peak located in Craterr the 96-mile wide Gale Crater. CAPE ST. VINCENT

NASA

VICTORIA CRATER A

NASA

This false-color image obtained by Rover Opportunity shows Cape St. Vincent, one of many promontories that jut out from the walls of Victoria Crater. Below the loose, jumbled rocks, layering in the crater walls shows evidence of ancient wind-blown dunes.

1 2 3 4 5 6 7 8

Spacecraft Curiosity Phoenix Mars Reconnaissance Spirit Opportunity Odyssey Viking I Viking II

Type Rover Lander Orbiter Rover Rover Orbiter Lander* Lander*

* Also had an orbiter; ** As of late 2012

Landed on Mars August 2012 May 2008 March 2006 January 2004 January 2004 October 2001 July 1976 September 1976

Years Active** Remains in operation Ran out of power during its first Martian winter Planned 2-year mission, remains in operation Planned 4-year mission, operated for more than 6 years Planned 90-day mission, remains in operation Remains active, longest active spacecraft in orbit around another planet Operational for more than 6 years Operational for more than 3 years NASA

697

698

CHAPTER 22

Touring Our Solar System

FIGURE 22.15 Similar Rock Outcrops on Mars and Earth This set of images compares a rock outcrop on Mars (left) with similar rocks on Earth. The rock outcrop on Earth formed in a streambed, which suggests that the Martian rocks formed in a similar environment. Based on this finding, the scientist John Grotzinger concluded that there was once “a vigorous flow on the surface of Mars.” (Courtesy of NASA)

This NASA image obtained by Curiosity rover shows A typical sample of the sedimentary rock conglomerate rounded gravel fragments within a rock outcrop consis- that contains rounded gravel fragments deposited in a tent with the sedimentary rock conglomerate. Weathered stream bed on Earth. rock fragments can be seen below.

1 cm

FIGURE 22.16 Earthlike Stream Channels Are Strong Evidence That Mars Once Had Flowing Water Inset shows a close-up of a streamlined island where running water encountered resistant material along its channel. (Courtesy of NASA)

Most of the Martian landscape resembles Earth’s rocky deserts, with abundant dunes and low areas partially filled with dust.

Water Ice on Mars Liquid water does not appear to exist anywhere on the Martian surface. However, poleward of about 30° latitude, ice can be found within 1 meter (3 feet) of the surface. In the polar regions, it forms small permanent ice caps. Current estimates place the maximum amount of water ice held by the Martian polar ice caps at about 1.5 times the amount covering Greenland. Considerable evidence indicates that in the first 1 billion years of the planet’s history, liquid water flowed on the surface, creating stream valleys and related features (FIGURE 22.15 ). One location where running water was

1 cm

involved in carving valleys can be seen in the Mars Reconnaissance Orbiter image in FIGURE 22.16 . Notice the streamlike banks that contain numerous teardrop-shaped islands. These valleys appear to have been cut by catastrophic floods with discharge rates that were more than 1000 times greater than those of the Mississippi River. Most of these large flood channels emerge from areas of chaotic topography that appear to have formed when the surface collapsed. The most likely source of water for these flood-created valleys was the melting of subsurface ice. If the meltwater was trapped beneath a thick layer of permafrost, pressure could mount until a catastrophic release occurred. As the water escaped, the overlying surface would collapse, creating the chaotic terrain. Not all Martian valleys appear to have resulted from water released in this manner. Some exhibit branching, treelike patterns that resemble dendritic drainage networks on Earth. In addition, the Opportunity rover investigated structures similar to features created by water on Earth—including layered sedimentary rocks, playas (salt flats), and lake beds. Minerals that form only in the presence of water, such as hydrated sulfates, were also detected. Small spherical concretions of hematite, dubbed “blueberries,” were found that probably precipitated from water to form lake sediments. Nevertheless, except in the polar regions, water does not appear to have significantly altered the topography of Mars for more than 1 billion years. On August 6, 2012, the Mars rover Curiosity landed in Gale Crater, near what NASA calls Mount Sharp (officially known as Aeolis Mons). We can only imagine what Curiosity will uncover in its quest to study the habitability, climate, and geology of Mars.

22.3 CONCEPT CHECKS 1 What body in our solar system is most like Mercury? 2 Why are the surface temperatures so much higher on Venus than on Earth?

3 Venus was once referred to as “Earth’s twin.” How are these two planets similar? How do they differ from one another?

4 What surface features do Mars and Earth have in common? 5 Why are the largest volcanoes on Earth so much smaller than the largest ones on Mars?

6 What evidence suggests that Mars had an active hydrologic cycle in the past?

22.4 Jovian Planets

|

699

22.4 JOVIAN PLANETS Compare and contrast the four Jovian planets.

The four Jovian (Jupiter-like) planets, in order from the Sun, are Jupiter, Saturn, Uranus, and Neptune. Because of their location within the solar system and their size and composition, they are also commonly called the outer planets and the gas giants.

Jupiter: Lord of the Heavens The giant among planets, Jupiter has a mass 2.5 times greater than the combined mass of all other planets, satellites, and asteroids in the solar system. However, it pales in comparison to the Sun, with only 1/800 of the Sun’s mass. Jupiter orbits the Sun once every 12 Earth-years, and it rotates more rapidly than any other planet, completing one rotation in slightly less than 10 hours. When viewed telescopically, the effect of this fast spin is noticeable. The bulge of the equatorial region and the slight flattening at the poles are evident (see the Polar Flattening column in Table 22.1). Jupiter’s appearance is mainly attributable to the colors of light reflected from its three main cloud layers (FIGURE 22.17 ). The warmest, and lowest, layer is composed mainly of water ice and appears blue-gray; it is generally not seen in visible-light images. The middle layer, where temperatures are lower, consists of brown to orange-brown clouds of ammonium hydrosulfide droplets. These colors are thought to be by-products of chemical reactions occurring in Jupiter’s atmosphere. Near the top of its atmosphere lie white wispy clouds of ammonia ice. Because of its immense gravity, Jupiter is shrinking a few centimeters each year. This contraction generates most of the heat that drives Jupiter’s atmospheric circulation. Thus, unlike winds on Earth, which are driven by solar energy, the heat emanating from Jupiter’s interior produces the huge convection currents observed in its atmosphere. Jupiter’s convective flow produces alternating darkcolored belts and light-colored zones, as shown in Figure 22.17. The light clouds (zones) are regions where warm material is ascending and cooling, whereas the dark belts represent cool material that is sinking and warming. This convective circulation, along with Jupiter’s rapid rotation, generates the high-speed, east–west flow observed between the belts and zones. The largest storm on the planet is the Great Red Spot. This enormous anticyclonic storm that is twice the size of Earth has been known for 300 years. In addition to the Great Red Spot, there are various white and brown ovalshaped storms. The white ovals are the cold cloud tops of huge storms many times larger than hurricanes on Earth. The brown storm clouds reside at lower levels in the atmosphere. Lightning in various white oval storms has been photographed by the Cassini spacecraft, but the strikes appear to be less frequent than on Earth.

Jupiter’s magnetic field, the strongest in the solar system, is probably generated by a rapidly rotating, liquid metallic hydrogen layer surrounding its core. Bright auroras, associated with the magnetic field, have been photographed over Jupiter’s poles (FIGURE 22.18 ). Unlike Earth’s auroras, which occur only in conjunction with heightened solar activity, Jupiter’s auroras are continuous.

Jupiter’s Moons Jupiter’s satellite system, consisting of 67 moons discovered thus far, resembles a miniature solar system. Galileo discovered the four largest satellites, referred to as Galilean satellites, in 1610 (FIGURE 22.19 ). The two largest, Ganymede and Callisto, are roughly the size of Mercury, whereas the two smaller ones, Europa and Io, are about the size of Earth’s Moon. The eight largest moons appear to have formed around Jupiter as the solar system condensed. Jupiter also has many very small satellites (about 20 kilometers [12 miles] in diameter) that revolve in the opposite direction (retrograde motion) of the largest moons and have

FIGURE 22.17 The Structure of Jupiter’s Atmosphere The areas of light clouds (zones) are regions where gases are ascending and cooling. Sinking dominates the flow in the darker cloud layers (belts). This convective circulation, along with the rapid rotation of the planet, generates the high-speed winds observed between the belts and zones.

Belts s) loud r a (d k c ds g win Stron Zones uds) ht clo ig r b (

ds g win Stron Belts uds) clo r (da k

700

CHAPTER 22

A.

Touring Our Solar System

FIGURE 22.18 View of Jupiter’s Aurora, Taken by the Hubble Space Telescope This phenomenon is produced by high-energy electrons racing along Jupiter’s magnetic field. The electrons excite atmospheric gases and make them glow. (Courtesy of NASA/

This plume of volcanic gases and debris is rising more than 100 kilometers (60 miles) above Io’s surface.

John Clark)

B.

eccentric (elongated) orbits steeply inclined to the Jovian equator. These satellites appear to be asteroids or comets that passed near enough to be gravitationally captured by Jupiter or are remnants of the collisions of larger bodies. The Galilean moons can be observed with binoculars or a small telescope and are interesting in their own right. Images from Voyagers 1 and 2 revealed, to the surprise of most geoscientists, that each of the four Galilean satellites is a unique world (Figure 22.19). The Galileo mission also unexpectedly revealed that the composition of each satellite is strikingly different, implying a different evolution for each. For example, Ganymede has a dynamic core that generates a strong magnetic field not observed in other satellites.

The bright red area on the left side of the image (see arrow) is newly erupted lava.

FIGURE 22.20 A Volcanic Eruption on Jupiter’s Moon Io (Courtesy of NASA; Jet Propulsion Laboratory/University of Arizona/NASA)

FIGURE 22.19 Jupiter’s Four Largest Moons These moons are often referred to as the Galilean moons because Galileo discovered them. (Courtesy of NASA)

A. Io is one of only three volcanically active bodies other than Earth known to exist in the solar system.

D. Callisto, the outermost of the Galilean satellites, is densely cratered, much like Earth’s Moon

B. Europa, the smallest of the Galilean moons, has an icy surface that is crisscrossed by many linear features.

C. Ganymede, the largest Jovian satellite, exhibits cratered areas, smooth regions, and areas covered by numerous parallel grooves.

The innermost of the Galilean moons, Io, is perhaps the most volcanically active body in our solar system. In all, more than 80 active, sulfurous volcanic centers have been discovered. Umbrella-shaped plumes have been observed rising from Io’s surface to heights approaching 200 kilometers (125 miles) (FIGURE 22.20A ). The heat source for volcanic activity is tidal energy generated by a relentless “tug of war” between Jupiter and the other Galilean satellites— with Io as the rope. The gravitational field of Jupiter and the other nearby satellites pull and push on Io’s tidal bulge as its slightly eccentric orbit takes it alternately closer to and farther from Jupiter. This gravitational flexing of Io is transformed into heat (similar to the back-and-forth bending of a piece of sheet metal) and results in Io’s spectacular sulfurous volcanic eruptions. Moreover, lava, thought to be mainly composed of silicate minerals, regularly erupts on its surface (FIGURE 22.20B ). The planets closer to the Sun than Earth are considered too warm to contain liquid water, and those farther from the Sun are generally too cold (although some features on Mars indicate that it probably had abundant liquid water at some point in its history). The best prospects of finding liquid water within our solar system lie beneath the icy surfaces of some of Jupiter’s moons. For instance, an ocean of liquid water is possibly hidden under Europa’s outer covering of ice. Detailed images from Galileo have revealed that Europa’s icy surface is quite young and exhibits cracks apparently filled with dark fluid from below. This suggests that under its icy shell, Europa must have a warm, mobile interior—perhaps an ocean. Because

22.4 Jovian Planets

FIGURE 22.21 Saturn’s Dynamic Ring System The

Cassini division

Encke gap

Saturn

liquid water is a necessity for life as we know it, there is considerable interest in sending an orbiter to Europa—and, eventually, a lander capable of launching a robotic submarine—to determine whether it harbors life.

Jupiter’s Rings One of the surprising aspects of the Voyager 1 mission was the discovery of Jupiter’s ring system. More recently, the ring system was thoroughly investigated by the Galileo mission. By analyzing how these rings scatter light, researchers determined that the rings are composed of fine, dark particles that are similar in size to smoke particles. Furthermore, the faint nature of the rings indicates that these minute particles are widely dispersed. The main ring is composed of particles believed to be fragments blasted from the surfaces of Metis and Adrastea, two small moons of Jupiter. Impacts on Jupiter’s moons Amalthea and Thebe are believed to be the source of the debris from which the outer gossamer ring formed.

D C

B

A

two bright rings, called A ring (outer) and B ring (inner), are separated by the Cassini division. A second small gap (Encke gap) is also visible as a thin line in the outer portion of the A ring. (Courtesy of NASA)

Saturn’s Moons The Saturnian satellite system consists of 62 known moons, of which 53 have been named. The moons vary significantly in size, shape, surface age, and origin. Twenty-three of the moons are “original” satellites that formed in tandem with their parent planet. At least three (Rhea, Dione, and Tethys) show evidence of tectonic activity, where internal forces have ripped apart their icy surfaces. Others, like Hyperion, are so porous that impacts punch into their surfaces (FIGURE 22.22 ). Many of Saturn’s smallest moons have irregular shapes and are only a few tens of kilometers in diameter. FIGURE 22.22 Hyperion, Saturn’s Impact-Pummeled Satellite Planetary geologists think Hyperion’s surface is so weak and porous that impacts punch into its surface. (Courtesy of NASA)

Saturn: The Elegant Planet Requiring more than 29 Earth-years to make one revolution, Saturn is almost twice as far from the Sun as Jupiter, yet their atmospheres, compositions, and internal structures are remarkably similar. The most striking feature of Saturn is its system of rings, first observed by Galileo in 1610 (FIGURE 22.21 ). Through his primitive telescope, the rings appeared as two small bodies adjacent to the planet. Their ring nature was determined 50 years later by Dutch astronomer Christian Huygens. Saturn’s atmosphere, like Jupiter’s, is dynamic. Although the bands of clouds are fainter and wider near the equator, rotating “storms” similar to Jupiter’s Great Red Spot occur in Saturn’s atmosphere, as does intense lightning. Although the atmosphere is nearly 75 percent hydrogen and 25 percent helium, the clouds (or condensed gases) are composed of ammonia, ammonia hydrosulfide, and water, each segregated by temperature. Like Jupiter, the atmosphere’s dynamics are driven by the heat released by gravitational compression.

701

702

CHAPTER 22

Touring Our Solar System

FIGURE 22.23 Enceladus, Saturn’s Tectonically Active, Icy Satellite The Northern Hemisphere contains a 1-kilometer-deep (0.6 mile) chasm, and linear features, called tiger stripes, are visible in the lower right. Inset image shows jets spurting ice particles, water vapor, and organic compounds from the area of the tiger stripes.

Labtayt Sulci

(Courtesy of NASA)

Tiger stripes

Jets spurting ice and water

Saturn’s largest moon, Titan, is larger than Mercury and is the second-largest satellite in the solar system. Titan and Neptune’s Triton are the only satellites in the solar system known to have substantial atmospheres. Titan was visited and photographed by the Cassini-Huygens probe in 2005. The atmospheric pressure at Titan’s surface is about 1.5 times that at Earth’s surface, and the atmospheric composition is about 98 percent nitrogen and 2 percent methane, with trace organic compounds. Titan has Earth-like geologic landforms and geologic processes, such as dune formation and stream-like erosion caused by methane “rain.” In addition, the northern latitudes appear to have lakes of liquid methane. Enceladus is another unique satellite of Saturn—one of the few where active eruptions have been observed (FIGURE 22.23 ). The outgassing, comprised mostly of water, is thought to be the source that replenishes the material in Saturn’s E ring. The volcanic-like activity occurs in areas called “tiger stripes” that consists of four large fractures with ridges on either side.

Saturn’s Ring System In the early 1980s, the nuclearpowered Voyagers 1 and 2 explored Saturn within 160,000 kilometers (100,000 miles) of its surface. More information

was collected about Saturn in that short time than had been acquired since Galileo first viewed this “elegant planet” in the early 1600s. More recently, observations from ground-based telescopes, the Hubble Space Telescope, and the Cassini-Huygens spacecraft, have added to our knowledge of Saturn’s ring system. In 1995 and 1996, when the positions of Earth and Saturn allowed the rings to be viewed edge-on, Saturn’s faintest rings and satellites became visible. (The rings were visible edge-on again in 2009.) Saturn’s ring system is more like a large rotating disk of varying density and brightness than a series of independent ringlets. Each ring is composed of individual particles— mainly water ice, with lesser amounts of rocky debris—that circle the planet while regularly impacting one another. There are only a few gaps; most of the areas that look like empty space either contain fine dust particles or coated ice particles that are inefficient reflectors of light. Most of Saturn’s rings fall into one of two categories, based on density. Saturn’s main (bright) rings, designated A and B, are tightly packed and contain particles that range in size from a few centimeters (pebble-size) to tens of meters (house-size), with most of the particles being roughly the size of a large snowball (see Figure 22.21). In the dense rings, particles collide frequently as they orbit the planet. Although Saturn’s main rings (A and B) are 40,000 kilometers (25,000 miles) wide, they are very thin, only 10–30 meters (30–100 feet) from top to bottom. At the other extreme are Saturn’s faint rings. Saturn’s outermost ring (E ring), not visible in Figure 22.21, is composed of widely dispersed, tiny particles. Recall that volcanic-like activity on Saturn’s satellite Enceladus is thought to be the source of material for the E ring. Studies have shown that the gravitational tugs of nearby moons tend to shepherd the ring particles by gravitationally altering their orbits (FIGURE 22.24 ). For example, the F ring, which is very narrow, appears to be the work of satellites located on either side that confine the ring by pulling back particles that try to escape. On the other hand, the Cassini division, a clearly visible gap in Figure 22.21, arises from the gravitational pull of Mimas, one of Saturn’s moons. Some of the ring particles are believed to be debris ejected from the moons embedded in them. It is also possible that material is continually recycled between the rings and the ring moons. The ring moons gradually sweep up particles, which are subsequently ejected by collisions with large chunks of ring material, or perhaps by energetic collisions with other moons. It seems, then, that planetary rings are not the timeless features that we once thought; rather, they are continually recycled. The origin of planetary ring systems is still being debated. Perhaps the rings formed simultaneously and from the same material as the planets and moons—condensing from a flattened cloud of dust and gases that encircled the parent planet. Or perhaps the rings formed later, when a moon or large asteroid was gravitationally pulled apart after straying too close to a planet. Yet another hypothesis suggests that a foreign body collided catastrophically with one

22.4 Jovian Planets

of the planet’s moons, the fragments of which would tend to jostle one another and form a flat, thin ring. Researchers expect more light to be shed on the origin of planetary rings as the Cassini spacecraft continues its tour of Saturn.

703

FIGURE 22.24 Two of Saturn’s Ring Moons (Courtesy of NASA)

Encke gap

Moon

Moon

Uranus and Neptune: Twins B. Prometheus, a potato-shaped moon, acts Although Earth and A. Pan is a small moon about 30 kilometers in diameter that orbits in the Encke gap, located as a ring shepherd. Its gravity helps confine Venus have many similar in the A ring. It is responsible for keeping the the moonlets in Saturn’s thin Fring. traits, Uranus and NepEncke gap open by sweeping up any stray material that may enter. tune are perhaps more deserving of being called “twins.” They are nearly equal in diameter (both about four the star “wink” briefly five times (meaning five rings) before times the size of Earth), and they are both bluish in appear- the primary occultation and again five times afterward. More ance, as a result of methane in their atmospheres. Their days recent ground- and space-based observations indicate that are nearly the same length, and their cores are made of rocky Uranus has at least 10 sharp-edged, distinct rings orbiting its silicates and iron—similar to the other gas giants. Their equatorial region. Interspersed among these distinct strucmantles, made mainly of water, ammonia, and methane, are tures are broad sheets of dust. thought to be very different from Jupiter and Saturn. One of the most pronounced differences between Uranus and Nep- Neptune: The Windy Planet Because of Neptune’s tune is the time they take to complete one revolution around great distance from Earth, astronomers knew very little about the Sun—84 and 165 Earth-years, respectively.

Uranus: The Sideways Planet Unique to Uranus is the orientation of its axis of rotation. Whereas the other planets resemble spinning toy tops as they circle the Sun, Uranus is like a top that has been knocked on its side but remains spinning (FIGURE 22.25 ). This unusual characteristic of Uranus is likely due to one or more impacts essentially knocking the planet sideways from its original orientation early in its evolution. Uranus shows evidence of huge storm systems equivalent in size to those in the United States. Recent photographs from the Hubble Space Telescope also reveal banded clouds composed mainly of ammonia and methane ice—similar to the cloud systems of the other gas giants.

Uranus’s Moons Spectacular views from Voyager 2 showed that Uranus’s five largest moons have varied terrains. Some have long, deep canyons and linear scars, whereas others possess large, smooth areas on otherwise crater-riddled surfaces. Studies conducted at California’s Jet Propulsion Laboratory suggest that Miranda, the innermost of the five largest moons, was recently geologically active—most likely driven by gravitational heating, as occurs on Io.

Uranus’s Rings A surprise discovery in 1977 showed that Uranus has a ring system. The discovery was made as Uranus passed in front of a distant star and blocked its view, a process called occultation (occult = hidden). Observers saw

FIGURE 22.25 Uranus, Surrounded by Its Major Rings and a Few of Its Known Moons Also visible in this image are cloud patterns and several oval storm systems. This falsecolor image was generated from data obtained by Hubble’s Near Infrared Camera. (Image by Hubble Space Telescope, courtesy of NASA)

704

CHAPTER 22

Touring Our Solar System

appear to have comparatively short life spans—usually only a few years. Another feature that Neptune has in common with the other Jovian planets is layers of white, cirruslike clouds (probably frozen methane) about 50 kilometers (30 miles) above the main cloud deck.

FIGURE 22.26 Neptune’s Dynamic Atmosphere (Courtesy of NASA)

Great dark spot

Cirrus-like clouds Dark spot with bright core

this planet until 1989. Twelve years and nearly 3 billion miles of Voyager 2 travel provided investigators an amazing opportunity to view the outermost planet in the solar system. Neptune has a dynamic atmosphere, much like that of the other Jovian planets (FIGURE 22.26 ). Record wind speeds exceeding 2400 kilometers (1500 miles) per hour encircle the planet, making Neptune one of the windiest places in the solar system. In addition, Neptune exhibits large dark spots thought to be rotating storms similar to Jupiter’s Great Red Spot. However, Neptune’s storms FIGURE 22.27 Triton, Neptune’s Largest Moon The bottom of the image shows Triton’s wind and sublimation-eroded south polar cap. Sublimation is the process whereby a solid (ice) changes directly to a gas.

Neptune’s Moons Neptune has 13 known satellites, the largest of which is the moon Triton; the remaining 12 are small, irregularly shaped bodies. Triton is the only large moon in the solar system that exhibits retrograde motion, indicating that it most likely formed independently and was later gravitationally captured by Neptune (FIGURE 22.27 ). Triton and a few other icy moons erupt “fluid” ices—an amazing manifestation of volcanism. Cryovolcanism (from the Greek kryos, meaning “frost”) describes the eruption of magmas derived from the partial melting of ice instead of silicate rocks. Triton’s icy magma is a mixture of water ice, methane, and probably ammonia. When partially melted, this mixture behaves as molten rock does on Earth. In fact, upon reaching the surface, these magmas can generate quiet outpourings of ice lavas or occasionally produce explosive eruptions. An explosive eruptive column can generate the ice equivalent of volcanic ash. In 1989, Voyager 2 detected active plumes on Triton that rose 8 kilometers (5 miles) above the surface and were blown downwind for more than 100 kilometers (60 miles). In other environments, ice lavas develop that can flow great distances from their source— similar to the fluid basaltic flows on Hawaii. Neptune’s Rings Neptune has five named rings; two of them are broad, and three are narrow, perhaps no more than 100 kilometers (60 miles) wide. The outermost ring appears to be partially confined by the satellite Galatea. Neptune’s rings are most similar to Jupiter’s in that they appear faint, which suggests that they are composed mostly of dust-size particles. Neptune’s rings also display red colors, indicating that the dust is composed of organic compounds.

(Courtesy of NASA)

22.4 CONCEPT CHECKS 1 2 3 4

What is the nature of Jupiter’s Great Red Spot? Why are the Galilean satellites of Jupiter so named? What is distinctive about Jupiter’s satellite Io? Why are many of Jupiter’s small satellites thought to have been captured?

5 How are Jupiter and Saturn similar to one another? 6 What two roles do ring moons play in the nature of planetary ring systems?

7 How are Saturn’s satellite Titan and Neptune’s satellite Triton similar to one another?

8 Name three bodies in the solar system that exhibit active volcanism.

22.5 Small Solar System Bodies

|

705

22.5 SMALL SOLAR SYSTEM BODIES List and describe the principal characteristics of the small bodies that inhabit the solar system.

There are countless chunks of Asteroid belt debris in the vast spaces separating the eight planets and in the outer reaches of the solar system. In 2006, the International Astronomical Union organized Mars solar system objects not clasEarth sified as planets or moons into two broad categories: (1) small solar system bodies, including Jupiter asteroids, comets, and meteoroids, and (2) dwarf planets. The newest grouping, dwarf planets, includes Ceres, the largest known object in the asteroid and collected information that has planetary geologists both intrigued and perplexed. Images obtained as the spacecraft belt, and Pluto, a former planet. Asteroids and meteoroids are composed of rocky and/or drifted toward the surface of Eros revealed a barren, rocky metallic material with compositions somewhat like the terres- surface composed of particles ranging in size from fine dust trial planets. They are distinguished according to size: Aster- to boulders up to 10 meters (30 feet) across. Researchers oids are larger than 100 meters (60 miles) in diameter, whereas unexpectedly discovered that fine debris tends to concenmeteoroids have diameters less than 100 meters. Comets, on the trate in the low areas, where it forms flat deposits resembling other hand, are loose collections of ices, dust, and small rocky ponds. Surrounding the low areas, the landscape is marked by an abundance of large boulders. particles that originate in the outer reaches of the solar system. One of several hypotheses to explain the boulder-strewn topography is seismic shaking, which would cause the boulAsteroids: Leftover Planetesimals ders to move upward as the finer materials sink. This is Asteroids are small bodies (planetesimals) that remain analogous to what happens when a jar of sand and variousfrom the formation of the solar system, which means they sized pebbles is shaken: The larger pebbles rise to the top, are about 4.6 billion years old. Most asteroids orbit the while the smaller sand grains settle to the bottom (sometimes Sun between Mars and Jupiter, in the region known as the referred to as the Brazil nut effect). asteroid belt (FIGURE 22.28 ). Only five asteroids are more Indirect evidence from meteorites suggests that some than 400 kilometers (250 miles) in diameter, but the solar asteroids might have been heated by a large impact event. system hosts an estimated 1 to 2 million asteroids larger than A few large asteroids may have completely melted, causing 1 kilometer (0.6 mile) and many millions that are smaller. Some travel along eccentric orbits that take them very near the Sun, and others regularly pass close to Earth and the Moon (Earth-crossing asteroids). Many of the recent large-impact craters on the Moon and Earth probably resulted from collisions with asteroids. There are an estimated 1000 to 2000 Earth-crossing asteroids that are more than 0.6 kilometer in diameter. Inevitably, Earth–asteroid collisions will occur again. Because most asteroids have irregular shapes, planetary geologists initially speculated that they might be fragments of a broken planet that once orbited between Mars and Jupiter. However, the combined mass of all asteroids is now estimated to be only 1/1000 of the modest-sized Earth. Today, most researchers agree that asteroids are leftover debris from the solar nebula. Asteroids have lower densities than scientists originally thought, suggesting that they are porous bodies, like “piles of rubble,” loosely bound together (FIGURE 22.29 ). In February 2001, an American spacecraft became the first visitor to an asteroid. Although it was not designed for landing, NEAR Shoemaker landed successfully on Eros

FIGURE 22.28 The Asteroid Belt The orbits of most asteroids lie between Mars and Jupiter. Also shown in red are the orbits of a few known near-Earth asteroids.

FIGURE 22.29 Giant Asteroid Vesta (Photo courtesy of NASA)

706

CHAPTER 22

FIGURE 22.30 Changing Orientation of a Comet’s Tail as It Orbits the Sun (Photo by Dan Schechter/Science Source)

FIGURE 22.31 Coma of Comet Holmes The nucleus of the comet is within the bright spot in the center. Comet Holmes, which orbits the Sun every six years, was uncharacteristically active during its most recent entry into the inner solar system. (Courtesy of NASA)

Touring Our Solar System

and carbon dioxide), thus the nickname “dirty snowballs.” Tail of ionized Recent space missions to comgases Tail of ionized ets have shown their surfaces gases to be dry and dusty, which indicates that their ices are hidden beneath a rocky layer. Most comets reside in the outer reaches of the solar Fully formed, system and take hundreds of curved thousands of years to comdust tail Sun plete a single orbit around Tail composed of dust the Sun. However, a smaller number of short-period comets (those having orbital periOrbit ods of less than 200 years), Dust tail Ion tail beginning such as the famous Halley’s to form Comet, make regular encounters with the inner solar sysFIGURE 22.30 ). The shortest-period comet (Encke’s tem ( them to differentiate into a dense iron core and a rocky Comet) orbits around the Sun once every 3 years. mantle. In November 2005, the Japanese probe Hayabusa landed on a small near-Earth asteroid named 25143 Itokawa; it returned to Earth in June 2010. Analyzed sam- Structure and Composition of Comets All the ples suggest that the surface of the asteroid was identical phenomena associated with comets come from a small cenin composition to meteorites and was once part of a larger tral body called the nucleus. These structures are typically 1 asteroid. Hayabusa 2 is scheduled to launch in 2014, to to 10 kilometers in diameter, but nuclei 40 kilometers across eventually expose subsurface samples by blasting a crater have been observed. When comets reach the inner solar system, solar energy begins to vaporize their ices. The escapin asteroid 1999 JU3. ing gases carry dust from the comet’s surface, producing a highly reflective halo called a coma (FIGURE 22.31 ). Within Comets: Dirty Snowballs the coma, the small glowing nucleus with a diameter of only Comets, like asteroids, are leftover material from the forma- a few kilometers can sometimes be detected. tion of the solar system. They are loose collections of rocky As comets approach the Sun, most develop tails that material, dust, water ice, and frozen gases (ammonia, methane, can extend for millions of kilometers. The tail of a comet points away from the Sun in a slightly curved manner (see Figure 22.30), which led early astronomers to believe that the Sun has a repulsive force that pushes away particles of the coma to form the tail. Scientists have identified two solar forces known to contribute to tail formation. One is radiation pressure caused by radiant energy (light) emitted by the Sun, and the second is the solar wind, a stream of charged particles ejected from the Sun. Sometimes a single tail composed of both dust and ionized gases is produced, but two tails are often observed (see Figure 22.30). The heavier dust particles produce a slightly curved tail that follows the comet’s orbit, whereas the extremely light ionized gases are “pushed” directly away from the Sun, forming the second tail. As a comet’s orbit carries it away from the Sun, the gases forming the coma recondense, the tail disappears, and the comet returns to cold storage. Material that was blown from the coma to form the tail is lost forever. When all the gases are expelled, the inactive comet, which closely resembles an asteroid, continues its orbit without a coma or tail. It is believed that few comets remain active for more than a few hundred close orbits of the Sun. The very first samples from a comet’s coma (Comet Wild 2) were returned to Earth in January 2006 by NASA’s

22.5 Small Solar System Bodies

effect of a distant passing star may send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the Sun. However, only a tiny fraction of Oort cloud comets have orbits that bring them into the inner solar system.

Meteoroids: Visitors to Earth

FIGURE 22.32 Comet Wild 2 This image shows Comet Wild 2, as seen by NASA’s Stardust spacecraft. The inset shows an artist’s depiction of jets of gas and dust erupting from Comet Wild 2. (Courtesy of NASA)

Stardust spacecraft (FIGURE 22.32 ). Images from Stardust show that the comet’s surface was riddled with flat-bottomed depressions and appeared dry, although at least 10 gas jets were active. Laboratory studies revealed that the coma contained a wide range of organic compounds and substantial amounts of silicate crystals.

The Realm of Comets: The Kuiper Belt and Oort Cloud Most comets originate in one of two regions: the Kuiper belt or the Oort cloud. Named in honor of astronomer Gerald Kuiper, who predicted its existence, the Kuiper belt hosts comets that orbit in the outer solar system, beyond Neptune (see Figure 22.1). This disc-shaped structure is thought to contain about a billion objects over 1 kilometer in size. However, most comets are too small and too distant to be observed from Earth, even using the Hubble Space Telescope. Like the asteroids in the inner solar system, most Kuiper belt comets move in slightly elliptical orbits that lie roughly in the same plane as the planets. A chance collision between two Kuiper belt comets or the gravitational influence of one of the Jovian planets occasionally alters their orbits sufficiently to send them into our view. Halley’s Comet originated in the Kuiper belt. Its orbital period averages 76 years, and every one of its 29 appearances since 240 b.c. has been recorded, thanks to ancient Chinese astronomers—testimony to their dedication as astronomical observers and the endurance of Chinese culture. In 1910, Halley’s Comet made a very close approach to Earth, making for a spectacular display. Named for Dutch astronomer Jan Oort, the Oort cloud consists of comets that are distributed in all directions from the Sun, forming a spherical shell around the solar system. Most Oort cloud comets orbit the Sun at distances greater than 10,000 times the Earth–Sun distance. The gravitational

Nearly everyone has seen meteors, commonly (but inaccurately) called “shooting stars.” These streaks of light can be observed in as little as the blink of an eye or can last as “long” as a few seconds. They occur when a small solid particle, a meteoroid, enters Earth’s atmosphere from interplanetary space. Heat, created by friction between the meteoroid and the air, produces the light we see trailing across the sky. Most meteoroids originate from one of the following three sources: (1) interplanetary debris missed by the gravitational sweep of the planets during formation of the solar system, (2) material that is continually being ejected from the asteroid belt, or (3) the rocky and/or metallic remains of comets that once passed through Earth’s orbit. A few meteoroids are probably fragments of the Moon, Mars, or possibly Mercury, ejected by a violent asteroid impact. Before Apollo astronauts brought Moon rocks back to Earth, meteorites were the only extraterrestrial materials that could be studied in the laboratory. Meteoroids less than about 1 meter (3 feet) in diameter generally vaporize before reaching Earth’s surface. Some, called micrometeorites, are so tiny and their rate of fall so slow that they drift to Earth continually as space dust. Researchers estimate that thousands of meteoroids enter Earth’s atmosphere every day. After sunset on a clear, dark night, many are bright enough to be seen with the naked eye from Earth.

Meteor Showers Occasionally, meteor sightings increase dramatically to 60 or more per hour. These displays, called meteor showers, result when Earth encounters a swarm of meteoroids traveling in the same direction at nearly the same speed as Earth. The close association of these swarms to the orbits of some short-term comets strongly suggests that they represent material lost by these comets (TABLE 22.2 ). Some swarms, not associated with the orbits of known comets, are probably the scattered remains TABLE 22.2

Major Meteor Showers

Shower

Approximate Dates

Associated Comet

Quadrantids

January 4–6

 

Lyrids

April 20–23

Comet 1861 I

Eta Aquarids

May 3–5

Halley’s Comet

Delta Aquarids

July 30

 

Perseids

August 12

Comet 1862 III

Draconids

October 7–10

Comet Giacobini-Zinner

Orionids

October 20

Halley’s Comet

Taurids

November 3–13

Comet Encke

Andromedids

November 14

Comet Biela

Leonids

November 18

Comet 1866 I

Geminids

December 4–16

 

707

708

CHAPTER 22

Touring Our Solar System

SmartFigure 22.33 Meteor Crater, Near Winslow, Arizona This cavity is about 1.2 kilometers (0.75 mile) across and 170 meters (560 feet) deep. The solar system is cluttered with asteroids and comets that can strike Earth with explosive force. (Photo by Michael Collier)

of the nucleus of a long-defunct comet. The notable Perseid meteor shower that occurs each year around August 12 is likely material ejected from the comet Swift–Tuttle on previous approaches to the Sun. Most meteoroids large enough to survive passage through the atmosphere to impact Earth probably originate among the asteroids, where chance collisions or gravitational interactions with Jupiter modify their orbits and send them toward Earth. Earth’s gravity does the rest. A few very large meteoroids have blasted craters on Earth’s surface that strongly resemble those on our Moon. At least 40 terrestrial craters exhibit features that could be produced only by an explosive impact of a large asteroid, or perhaps even a comet nucleus. More than 250 others may be of impact origin. Notable among them is Arizona’s Meteor Crater, a huge cavity more than 1 kilometer (0.6 mile) wide and 170 meters (560 feet) deep, with an upturned rim that rises above the surrounding countryside (FIGURE 22.33 ). More than 30 tons of iron fragments have been found in the immediate area, but attempts to locate the main body have been unsuccessful. Based on the

amount of erosion observed on the crater rim, the impact likely occurred within the past 50,000 years.

Types of Meteorites The remains of meteoroids, when found on Earth, are referred to as meteorites (FIGURE 22.34 ). Classified by their composition, meteorites are either (1) irons, mostly aggregates of iron with 5–20 percent nickel; (2) stony (also called chondrites), silicate minerals with inclusions of other minerals; or (3) stony–irons, mixtures of the two. Although stony meteorites are the most common, irons are found in large numbers because metallic meteorites withstand impacts better, weather more slowly, and are easily distinguished from terrestrial rocks. Iron meteorites are probably fragments of once-molten cores of large asteroids or small planets. One type of stony meteorite, called a carbonaceous chondrite, contains organic compounds and occasionally simple amino acids, which are some of the basic building blocks of life. This discovery confirms similar findings in

22.5 Small Solar System Bodies

observational astronomy, which indicate that numerous organic compounds exist in interstellar space. Data from meteorites have been used to ascertain the internal structure of Earth and the age of the solar system. If meteorites represent the 10 1 0 0 cm cm composition of the terrestrial planets, as some planetary geologists suggest, our planet must contain a much larger percentage of iron than is indicated by surface rocks. This is one reason that geologists think Earth’s core is mostly iron and nickel. In addition, radiometric dating of meteorites indicates that the age of solar system. The Kuiper belt objects are rich in ices and have our solar system is about 4.6 billion years. This “old age” physical properties similar to those of comets. Many other has been confirmed by data obtained from lunar samples. planetary objects, some perhaps larger than Pluto, are thought to exist in this belt of icy worlds beyond Neptune’s orbit. Researchers soon recognized that Pluto was unique among the Dwarf Planets planets—completely different from the four rocky, innermost Since its discovery in 1930, Pluto has been a mystery to planets, as well as the four gaseous giants. In 2006, the International Astronomical Union, the group astronomers who were searching for another planet in order to explain irregularities in Neptune’s orbit. At the time of its dis- responsible for naming and classifying celestial objects, covery, Pluto was thought to be the size of Earth—too small to voted to designate a new class of solar system objects called significantly alter Neptune’s orbit. Later, estimates of Pluto’s dwarf planets. These are celestial bodies that orbit the Sun diameter, adjusted because of improved satellite images, indi- and are essentially spherical due to their own gravity but are cated that it was less than half Earth’s diameter. Then, in 1978, not large enough to sweep their orbits clear of other debris. astronomers realized that Pluto appeared much larger than it By this definition, Pluto is recognized as a dwarf planet and really is because of the brightness of its newly discovered sat- the prototype of this new category of planetary objects. Other ellite, Charon (FIGURE 22.35 ). Most recently, calculations based on images obtained by the Hubble Space Telescope show that Pluto’s diameter is Nix 2300 kilometers (1430 miles), about one-fifth the diameter of Earth and less than half that of Mercury (long considered the solar system’s “runt”). In P5 P4 fact, seven moons in the solar Pluto system, including Earth’s, are Hydra larger than Pluto. Even more attention was given to Pluto’s status as a Charon planet when astronomers discovered another large icy body in orbit beyond Neptune. Soon, more than 1000 of these Kuiper belt objects were discovered 40,250 km forming a band of objects— a second “asteroid belt,” but 25,000 miles located at the outskirts of the

709

FIGURE 22.34 Iron Meteorite Found Near Meteor Crater, Arizona (Courtesy of M2 Photography/Alamy)

FIGURE 22.35 Pluto, with Its Five Known Moons (Courtesy of NASA)

710

Moon

Eris

Ceres

dwarf planets include Eris, a Kuiper belt object, and Ceres, the largest-known asteroid (FIGURE 22.36 ). Pluto’s reclassification was not the first such “demotion.” In the mid-1800s, astronomy textbooks listed as many as 11 planets in our solar system, including the asteroids Vesta, Juno, Ceres, and Pallas. Astronomers continued to discover dozens of other “planets,” a clear signal that these small bodies represent a class of objects separate from the planets. The new classification will give a home to the hundreds of additional dwarf planets astronomers assume exist in the solar system. New Horizons, the first spacecraft designed to explore the outer solar system, was launched in January 2006. As of September 2012, New Horizons was halfway between the orbits of Uranus and Neptune. Scheduled to fly by Pluto in July 2015 and later explore the Kuiper belt, New Horizons carries tremendous potential for aiding researchers in further understanding the solar system.

22.5 CONCEPT CHECKS Pluto and its moon Charon

1 Where are most asteroids found? 2 Compare and contrast asteroids and comets. 3 What do you think would happen if Earth passed through the tail of a comet?

FIGURE 22.36 An Artist’s Drawing Showing the Relative Sizes of the Best-Known Dwarf Planets Compared to Earth and Its Moon Eris, the largest known dwarf planet, has a very eccentric orbit that takes it as far as 100 AU from the Sun. Both Eris and Pluto are composed mainly of ices of water, methane, and ammonia. Ceres is the only identified dwarf planet in the asteroid belt. (Courtesy of NASA)

22

CONCEPTS IN REVIEW

4 Where are most comets thought to reside? What eventually becomes of comets that orbit close to the Sun?

5 Differentiate among the following solar system bodies: meteoroid, meteor, and meteorite.

6 What are the three main sources of meteoroids? 7 Why was Pluto demoted from the ranks of the officially recognized planets?

| Touring Our Solar System

22.1 OUR SOLAR SYSTEM: AN OVERVIEW Describe the formation of the solar system according to the nebular theory. Compare and contrast the terrestrial and Jovian planets. K EY TERMS: nebular theory, solar nebula, planetesimal, protoplanet, terrestrial (Earth-like) planet, Jovian (Jupiter-like) planet, escape velocity, impact crater ■ ■

■ ■

Our Sun is the most massive body in a solar system, which includes planets, dwarf planets, moons, and other small bodies. The planets all orbit in the same direction and at speeds proportional to their distance from the Sun, with inner planets moving faster and outer planets moving more slowly. The solar system’s formation is described by the nebular theory, which proposes that the system began as a solar nebula before condensing due to gravity. While most of the matter ended up in the Sun, some material formed a thick disc around the early Sun and later clumped together into larger and larger bodies. Planetesimals collided to form protoplanets, and protoplanets grew into planets. The four terrestrial planets are enriched in rocky materials, whereas the Jovian planets have a higher proportion of ice and gas. The terrestrial planets are relatively dense, with thin atmospheres, while the Jovian planets are less dense and have thick atmospheres. Smaller planets have less gravity to retain gases in their atmosphere. It’s easier for lightweight gases such as hydrogen and helium to reach escape velocity in this situation, so the atmospheres of the terrestrial planets tend to be enriched in heavier gases, such as water vapor, carbon dioxide, and nitrogen.

Concepts in Review

711

22.2 EARTH’S MOON: A CHIP OFF THE OLD BLOCK List and describe the major features of Earth’s Moon and explain how maria basins were formed. KEY TERMS: maria, lunar highlands (terrae), lunar regolith ■

■

Earth’s Moon is the largest moon relative to its planet, and its composition is unique in the solar system, approximately the same as the composition of Earth’s mantle (density = 3.3 g/cm3). The Moon likely formed due to a collision between a Mars-sized protoplanet and the early Earth. The bulk of the protoplanet’s iron core material was incorporated into Earth, and its rocky mantle material spun off to make the Moon. Two types of topography dominate the lunar surface: (1) light-colored lunar highlands (or terrae) dominated by relatively old anorthosite breccia and (2) darker lowlands called maria, which are dominated by younger flood basalts. Both terrae and maria are partially covered by a layer called lunar regolith, which is produced by micrometeorite bombardment.

Q Briefly describe the formation of our Moon and how its formation accounts for its low density compared to that of Earth.

22.3 TERRESTRIAL PLANETS Outline the principal characteristics of Mercury, Venus, and Mars. Describe their similarities to and differences from Earth.

■

■

Mercury is the planet closest to the Sun. It has a very thin atmosphere and a weak magnetic field. Because it has a very thin atmosphere and its rate of rotation is extremely slow, the temperature on the surface varies from less than Idealized graph comparing the daily temperature −173°C (−280°F) at night to 427°C (800°F) during daylight hours. The lobate scarps variations on Venus and Mercury. on Mercury’s surface are likely the traces of thrust faults, which formed due to the planet’s cooling and contraction. 500 Venus, the second planet from the Sun, has a very dense atmosphere that is dominated Venus by carbon dioxide. The resulting extreme greenhouse effect produces surface tempera400 tures around 450°C (900°F). The topography of Venus has been resurfaced by active volcanism. 300 Mars is the fourth planet from the Sun. It has about 1 percent as much atmosphere as 200 Earth, so it is relatively cold (−140°C to 20°C [2220°F to 68°F]). Mars appears to be the closest planetary analogue to Earth, showing surface evidence of rifting, volcanism, and 100 modification by flowing water. Volcanoes on Mars, such as Olympus Mons, are much bigger than volcanoes on Earth because of the lack of plate motion on Mars: The lava ac0 cumulates in a single cone rather than forming a long chain of cones, as exemplified by the Hawaiian islands. Temperature (°C)

■

–100

Q As you can see from this graph, Mercury’s temperature varies a lot from “day” to “night,” but Venus’s temperature is relatively constant “around the clock.” Suggest a reason for this difference.

–200

Mercury

Time (1 day)

22.4 JOVIAN PLANETS Compare and contrast the four Jovian planets. KEY TERM: cryovolcanism ■

■ ■ ■

Jupiter is the fifth planet from the Sun. It is very big—several times greater than the combined mass of everything else in the solar system except for the Sun. Convective flow among its three layers of clouds produces its characteristic banded appearance. Persistent, giant rotating storms exist between these bands. Many moons orbit Jupiter, including Io, which shows active volcanism, and Europa, which has an icy shell. Saturn is the sixth planet from the Sun. Like Jupiter, it is big, gaseous, and endowed with dozens of moons. Some of these moons show evidence of tectonics, while Titan has its own atmosphere. Saturn’s well-developed rings are made of many particles of water ice and rocky debris. Uranus is the seventh planet from the Sun. Like its “twin” Neptune, it has a blue atmosphere dominated by methane, and its diameter is about four times greater than Earth’s. Uranus rotates sideways relative to the plane of the solar system. It has a relatively thin ring system and at least five moons. Neptune, the eighth planet from the Sun, has an active atmosphere, with fierce wind speeds and giant storms. It has one large moon, Triton, which shows evidence of cryovolcanism, as well as a dozen smaller moons and a ring system.

Q Prepare and label a sketch comparing the typical characteristics of terrestrial planets and those of Jovian planets.

712

Concepts in Review

22.5 SMALL SOLAR SYSTEM BODIES List and describe the principal characteristics of the small bodies that inhabit the solar system. K EY TERMS: small solar system body, dwarf planet, asteroid, asteroid belt, comet, nucleus, coma, Kuiper belt, Oort cloud, meteor, meteoroid, meteor shower, meteorite ■

■

■

■

■

Small solar system bodies include rocky asteroids and icy comets. Both are basically scraps left over from the formation of the solar system or fragments from later impacts. Most asteroids are concentrated in a wide belt between the orbits of Mars and Jupiter. Some are rocky, some are metallic, and some are basically “piles of rubble” loosely held together by their own weak gravity. Comets are dominated by ices, “dirtied” by rocky material and dust. They originate in either the Kuiper belt (a second “asteroid belt” beyond Neptune) or the Oort cloud (a spherical “shell” around the otherwise planar “disc” of the solar system). When the orbit of a comet brings it through the inner solar system, solar radiation (sunlight) causes its ices to begin to vaporize, generating the coma (gaseous envelope around the comet’s nucleus) and its characteristic “tail.” A meteoroid is debris that enters Earth’s atmosphere, flaring briefly as a meteor before either burning up or striking Earth’s surface to become a meteorite. Asteroids and material lost from comets as they travel through the inner solar system are the most common source of meteoroids. Dwarf planets include Ceres (located in the asteroid belt), the dwarf planet Pluto, and Eris, a Kuiper belt object. They are spherical bodies that orbit the Sun but are not massive enough to have cleared their orbits of debris.

Muellek Josef/Shutterstock

National Science Foundation

Jerry Schad/Science Source

NASA/JPL-Caltech/UCAL/MPS/DLR/IDA

Q Shown here are four small solar system bodies. Identify each and explain the differences among them.

GIVE IT SOME THOUGHT 1. Assume that a solar system has been discovered in a nearby region of the Milky Way Galaxy. The accompanying table shows data that have been gathered about three of the planets orbiting the central star of this newly discovered solar system. Using Table 22.1 as a guide, classify each planet as either Jovian, terrestrial, or neither. Explain your reasoning.

2. In order to conceptualize the size and scale of Earth and Moon as they relate to the solar system, complete the following: a. Approximately how many Moons (diameter 3475 kilometers [2160 miles]) would fit side-by-side across the diameter of Earth (diameter 12,756 kilometers [7926 miles])? b. Given that the Moon’s orbital radius is 384,798 kilometers, approximately how many Earths would fit side-by-side between Earth and the Moon? c. Approximately how many Earths would fit side-by-side across the Sun, whose diameter is about 1,390,000 kilometers? d. Approximately how many Suns would fit side-by-side between Earth and the Sun, a distance of about 150,000,000 kilometers?

3. The accompanying graph shows the temperatures at various distances from the Sun during the formation of our solar system. Use it to complete the following: a. Which planets formed at locations where the temperature in the solar system was hotter than the boiling point of water? b. Which planets formed at locations where the temperature in the solar system was cooler than the freezing point of water?

Examining the Earth System

4. The accompanying sketch shows four primary craters (A, B, C, and D). The impact that produced crater A produced two secondary craters (labeled “a”) and three rays. Crater D has one secondary crater (labeled “d”). Rank the four primary craters from oldest to youngest and explain your ranking. 5. The accompanying diagram shows two of Uranus’s moons, Ophelia and Cordelia, which act as shepherd moons for the Epsilon ring. Explain what would happen to the Epsilon ring if a large asteroid struck Ophelia, knocking it out of the Uranian system. 6. The rim of a meteor crater is being eroded at a rate of 0.2 millimeters per year. The vertical offset between the current and the original rim profile is 4 meters. Estimate when the meteoroid impacted Earth.

713

7. The accompanying diagram shows a comet traveling toward the Sun at the first position where it has both an ion tail and a dust tail. Refer to this diagram to complete the following: a. For each of the three numbered sites, indicate whether the comet will have no tails, one tail, or two tails. If one tail or two tails are present, in what direction will they point? b. Would your answers to the preceding question change if the Sun’s energy output were to increase significantly? If so, how would they change? c. If the solar wind suddenly ceased, how would this affect the comet and its tails?

8. Imagine that two comets, Comet A and Comet B, are orbiting the Sun. The orbital period for Comet A is 50 years, and for Comet B it is 60 years. Comet A initially passes at a distance of 600,000 kilometers from Earth, while Comet B initially passes 450,000 kilometers away. At each orbital period, the minimum distances between Earth and the comets decrease by 40,000 kilometers for Comet A and 25,000 kilometers for Comet B. Which comet will impact Earth first?

EXAMINING THE EARTH SYSTEM 1. On Earth the four major spheres (atmosphere, hydrosphere, geosphere, and biosphere) interact as a system with occasional influences from our near-space neighbors. Which of these spheres are absent, or nearly absent, on the Moon? Because the Moon lacks these spheres, list at least five processes that operate on Earth but are absent on the Moon. 2. Among the planets in our solar system, Earth is unique because water exists in all three states (solid, liquid, and gas) on and near its surface. In what state(s) of matter is water found on Mercury, Venus, and Mars?

a. How would Earth’s hydrologic cycle be different if its orbit were inside the orbit of Venus? b. How would Earth’s hydrologic cycle be different if its orbit were outside the orbit of Mars?

3. Of the four major Earth spheres, which displays the most striking analogy with Mars and which is the most different? Describe the processes that occur in the most similar major sphere which allow you to say it is a good analogy. Speculate on the potential development of life on Mars.

23

1

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

23.1

List and describe the various types of electromagnetic radiation.

23.2

Explain how the three types of spectra are generated and what they tell astronomers about the radiating body that produced them.

23.3

Compare and contrast refracting and reflecting telescopes. Explain why modern telescopes are built on mountaintops.

23.4

Explain the advantages of radio telescopes and orbiting observatories over optical telescopes.

23.5

Write a statement explaining why the Sun is important to the study of astronomy. Sketch the Sun’s structure and describe each of its four major layers.

23.6

List and describe the three types of explosive activity that occur at the Sun’s surface.

23.7

Summarize the process called the proton–proton chain reaction.

Comet Hale-Bopp above the observatories on the summit of Mauna Kea, Hawaii. (Photo by David Nunuk/Science Source)

1

This chapter was revised with the assistance of Mark Watry and Teresa Tarbuck.

715

716

CHAPTER 23

Light, Astronomical Observations, and the Sun

ince astronomers cannot study the universe by bringing it into the laboartory, and because the vast majority of celestial objects are too far away to visit, astronomers collect and study those things that come to Earth from space. Overwhelmingly, this means collecting and studying light emitted or reflected by objects found in the

S

|

universe. In fact, everything that is known about the universe beyond the solar system comes from the analysis of the light from distant sources. This chapter examines the properties and utility of light, some of the tools astronomers use to collect and study light, and what is known about the nearest source of light, the Sun.

23.1 SIGNALS FROM SPACE List and describe the various types of electromagnetic radiation.

Although visible light is most familiar to us, it constitutes only a tiny sliver of an array of energy referred to as electromagnetic radiation (FIGURE 23.1 ). Included in this array are gamma rays, X-rays, ultraviolet light, visible light, infrared radiation (heat), microwaves, and radio waves (FIGURE 23.2 ). All forms of radiant energy travel through the vacuum of space in a straight line at the rate of 300,000 kilometers (186,000 miles) per second.2 Over 24 hours, this is a staggering 26 billion kilometers. The light that we collect tells us about the processes that created it and about the matter lying between us and the source of the light. 2 Light rays are “bent” slightly when they pass near a very massive object such as the Sun.

FIGURE 23.1 A Face-on View of Galaxy NGC 1232 Despite being 100 million light-years away, modern telescopes allow astronomers to study its intricate details. Older, reddish stars are located mainly in the galaxy’s central region, while young, hot blue stars make up the spiral arms. (Photo by NASA)

Nature of Light Experiments have demonstrated that light can be described in two ways. In some instances, light behaves like waves, and in others, it behaves like discrete particles. In the wave sense, light is analogous to swells in the ocean. This motion is characterized by wavelength—the distance from one wave crest to the next. Wavelengths vary from several kilometers for some radio waves to less than one-billionth of a centimeter for gamma rays (see Figure 23.2). Most of these waves are either too long or too short for our eyes to detect; however, the primary characteristics of all electromagnetic radiation can be described using visible light as an example. The extremely narrow band of electromagnetic radiation we can see (which is labeled visible light in Figure 23.2) is

23.1 Signals from Space

FM VLF

AM

VHF

Microwave (Long) 700

Radio 1 GHz

100 GHz

FIGURE 23.2 Electromagnetic Radiation

Visible light

UHF

Nanometers 500 600

(Short) 400

Infrared Far

Near

100 1 Microns

717

Visible

The electromagnetic spectrum ranges from longwavelength radio waves to short-wavelength gamma radiation.

Ultraviolet Far

Near

X rays “Soft”

“Hard”

Gamma rays

Wavelength (meters)

104

102

0

10–2

10–4

10–6

10–8

10–10

10–12

10–14

Window for visible and infrared waves

Window for radio waves

100 Opacity (percent) 50

1

Atmosphere is opaque

Atmosphere is opaque 100 m

1m

1 cm

100 m

1 m

sometimes referred to as white light. White light consists of TABLE 23.1 Colors and Corresponding Wavelengths an array of waves having various wavelengths, a fact easily Color Wavelength (Nanometers*) demonstrated with a prism (FIGURE 23.3A ). As white light Violet 380–440 passes through a prism, the color with the shortest waveBlue 440–500 length, violet, is bent more than blue, which is bent more than Green 500–560 green, and so forth (TABLE 23.1 ). Thus, white light can be Yellow 560–590 separated into its component colors, producing the familiar Orange 590–640 “rainbow of colors” (see Figure 23.3A). Red 640–750 Wave theory, however, cannot explain some of the –9 *1 nanometer is 10 meter. observed characteristics of light. In these cases, light acts like a stream of particles, analogous to infinitesimally small Incandescent solid bullets fired from a machine Visable (filament) gun. These particles, called light photons, can exert a presPrism sure (push) on matter, which is called radiation pressure. Continuous A. Recall that photons from the spectrum Sun are responsible for pushIncandescent ing material away from a comet solid (filament) to produce its dust tail. Each photon has a specific amount of energy, which is related Cool to its wavelength in a simple Dark-line gas spectrum B. way: Shorter wavelengths correspond to more energetic photons. Thus, blue light has more energetic photons than red light. Incandescent (hot) gas Which theory of light— the wave theory or the partiC. Bright-line cle theory—is correct? The spectrum answer is that both are correct

SmartFigure 23.3 FFormation of the Three Types of Spectra A. Continuous spectrum. B. Dark-line spectrum. C. Bright-line spectrum.

718

CHAPTER 23

Light, Astronomical Observations, and the Sun

because each will predict the behavior of light for certain phenomena. As George Abell, a prominent astronomer, stated about all scientific laws, “The mistake is only to apply them to situations that are outside their range of validity.”

Light As Evidence of Events and Processes Light energy from all parts of the electromagnetic spectrum reaches Earth from stars, interstellar matter, and galaxies throughout the universe. This light provides evidence of events and processes that we could not otherwise observe. For example, when matter is engulfed by a black hole, the result is the emission of high-energy x-rays. By contrast, when less violent processes occur, small amounts of lowenergy radiation are released. This occurs when a shock wave moves through a gas cloud, and it raises the cloud’s temperature, causing infrared (heat) energy to be emitted.

|

The intensity of the light emitted and its wavelength distribution tell us a lot about the type of process that is occurring. This information can often be used to support or refute scientific hypotheses. For example, theoretical studies predicted the existence of black holes long before observational evidence existed. The concept of black holes gained considerable support when x-rays matching the wavelengths predicted by this hypothesis were detected around objects suspected of being black holes.

23.1 CONCEPT CHECKS 1 What term is used to describe the collection that includes gamma rays, x-rays, ultraviolet light, visible light, infrared radiation, microwaves, and radio waves?

2 Which color has the longest wavelength? The shortest? 3 How does the amount of energy contained in a photon relate to its wavelength?

23.2 SPECTROSCOPY Explain how the three types of spectra are generated and what they tell astronomers about the radiating body that produced them.

When Sir Isaac Newton used a prism to disperse white light into its component colors, he unknowingly initiated the field of spectroscopy—the study of those properties of light that are wavelength dependent. The rainbow of colors Newton produced is called a continuous spectrum because all wavelengths of visible light are included. Later it was learned that two other types of spectra (dark line and bright line) exist and that each one is generated under somewhat different conditions (see Figure 23.3). Just as visible light can produce a spectrum, other regions of the electromagnetic spectrum can be dispersed to produce spectra.

Continuous Spectrum A continuous spectrum is produced by an incandescent (glowing) solid, liquid, or gas under high pressure. (Incandescent means “to emit light when hot.”) It consists of a continuous band of wavelengths like that generated by a common 100-watt incandescent light bulb (see Figure 23.3A). A continuous spectrum contains two important pieces of information about radiating bodies. First, a continuous spectrum provides information about the total energy output of the radiating body. If the temperature of a radiating surface increases, the total amount of energy emitted increases. The rate of increase is stated in the Stefan–Boltzmann law: The energy radiated by a body is directly proportional to the fourth power of its absolute temperature. For example, if the temperature of a star is twice that of another star, the total radiation emitted by the hotter star is 24 5 2 3 2 3 2 3 2, or 16 times greater than that of the cooler star. Second, a continuous spectrum contains information about the surface temperature of the radiating body. As

the surface temperature of an object increases, a larger proportion of its energy is radiated at shorter wavelengths (higher energy). To illustrate, imagine a metal rod that is heated slowly. Initially, the rod appears dull red (longer wavelengths), then yellow, and later bluish-white (shorter wavelengths). All incandescent bodies show this behavior, so it follows that blue stars are hotter than yellow stars (like the Sun), which are hotter than red stars (see Table 23.1).

Dark-Line Spectrum When a telescope collects light radiating from a star and the light is passed through an instrument called a spectroscope (which spreads out the wavelengths in a manner similar to a prism), a “continuous spectrum” that contains a series of dark is produced. This type of spectrum, called a darkline (or absorption) spectrum, is generated whenever visible light is passed through a comparatively cool gas at low pressure (FIGURE 23.3B ). For example, when visible light is passed through a glass jar containing hydrogen gas, the hydrogen atoms absorb specific wavelengths of light, resulting in a unique set of dark lines. Each set of dark lines, like a set of fingerprints, identifies the matter that is present. Elements such as iron that exist in the gaseous state on the Sun have been identified in numerous other stars through study of their spectra. Even organic molecules have been discovered in distant interstellar clouds of dust and gases using this technique. Thus, when carefully analyzed, a dark-line spectrum revels the composition of the radiating body. The spectra of most stars are of the dark-line type. Imagine the light produced in the Sun’s interior passing outward through its atmosphere. The gas in the solar atmosphere is

23.2 Spectroscopy

cooler than that inside, and when it absorbs some of the sunlight (and re-emits it in a random direction), we do not see it, and a dark area (line) appears in the spectrum. Although the lines appear black, they just look that way next to the bright parts of the spectrum. The relative intensities of the light in the dark lines contain information about the relative amounts of each kind of matter present.

Bright-Line Spectrum A bright-line (or emission) spectrum is a series of bright lines that appear in the same locations as the dark lines for the same gas. They are produced by hot (incandescent), gaseous materials, at low pressure (FIGURE 23.3C ). These spectra contain information about the temperature of the gas and the matter in it. FIGURE 23.4 shows the emission spectra of hydrogen and helium, the two most abundant elements in the universe. Bright-line, or emission, spectra are produced by large interstellar clouds (nebula) consisting largely of hydrogen gas excited by extremely hot stars. Because the brightest emission line produced by hydrogen is red, these clouds tend to have a red glow that is characteristic of excited hydrogen gas. The Orion Nebula is a well-known emission nebula that is bright enough to be seen by the naked eye (FIGURE 23.5 ). It is located in the constellation Orion, in the sword of the hunter.

The Doppler Effect The positions of the bright and dark lines in the spectra described earlier shift when the source of energy moves relative to the observer. This effect is observed for all types of

719

FIGURE 23.4 Bright-Line Spectra of Hydrogen and Helium These gases are the

Hydrogen

two most abundant elements in the universe.

Helium 650

600

550 500 Wavelength (nanometers)

450

400

waves. You may have heard the change in pitch of a car horn or an ambulance siren as it passes by. When it is approaching, the sound seems to have a higher-than-normal pitch, and when it is moving away, the pitch sounds lower than normal. This effect, first explained by Christian Doppler in 1842, is called the Doppler effect. The reason for the difference in pitch is that it takes time for the wave to be emitted. If the source is moving away, the beginning of the wave is emitted nearer to you than the end, which stretches the wave—that is, gives it a longer wavelength (FIGURE 23.6 ). The opposite is true for an approaching source. In the case of light, when a source is moving away, its light appears redder than it actually is because its waves are lengthened. Objects approaching have their light waves shifted toward the blue (shorter-wavelength) end of the spectrum. The same effect is produced if you are moving and the light remains stationary. Doppler shifts are generally established by comparing the dark lines in the spectra of a star (or other celestial body) with the spectrum of a motionless body produced in FIGURE 23.5 The Orion Nebula, an Emission Nebula The red color of Orion Nebula is produced by hydrogen gas that is being heated by nearby hot stars. Bright enough to be seen by the naked eye, the Orion Nebula is located in the sword of the hunter in the constellation of the same name. (Courtesy of National Optical Astronomy Observatory/ Association of Universities for Research in Astronomy/National Science Foundation [NOAO/AURA/ NSF])

720

CHAPTER 23

SmartFigure 23.6

Light, Astronomical Observations, and the Sun

Approaching ambulance

Apparent wavelength

Th The Doppler D Effect Illustration showing the apparent lengthening and shortening of wavelengths caused by the relative motion between a source and an observer.

Receding ambulance

Apparent wavelength

the laboratory (FIGURE 23.7 ). When the spectral lines are shifted toward the long end of the electromagnetic spectrum, called a red shift, the star is moving away from the observer. By contrast, when the spectral lines are moving toward the short end (blue/violet end) of the spectrum, the object is approaching Earth. In addition, the amount of shift allows us to calculate the rate at which the relative movement is occurring. Large A. Standard sodium lines

FIGURE 23.7 Determining the Relative Motion of Two Bodies We can determine whether Earth is approaching or receding from a celestial body by comparing the spectrum of a motionless light source to the spectrum of a moving body. A. Standard dark-line spectrum for sodium produced in the laboratory. B. and C. Sodium lines as they would appear when a light source is receding (red shift). D. Sodium lines produced by an approaching star (blue shift).

B. Red-shifted sodium lines

Doppler shifts indicate high velocities; small Doppler shifts indicate low velocities. For example, notice that the amount of the red shift in Figure 23.7B is less than that shown in Figure 23.7C. Two types of Doppler shifts are important in astronomy: those caused by local motions and those caused by the expansion of the universe. Doppler shifts due to local motions are used to measure how fast one star orbits another in a binary (two-star) system or how fast a pulsing star expands and contracts. Those shifts caused by the expansion of the universe (where space is continually being created between the galaxies) can tell us how far away distant objects are. These measurements coupled with the speed of light tell us how long ago the light left these distant objects, and as we look farther out, we can get a sense of the age of the universe. (The expansion of the universe will be discussed in depth in Chapter 24.)

23.2 CONCEPT CHECKS 1 What is spectroscopy? 2 Describe a continuous spectrum. Give an example of a natural phenomenon that exhibits a continuous spectrum.

3 What can a continuous spectrum tell astronomers about a C. Large red-shifted sodium lines

star?

4 What can be learned about a star (or other celestial objects) from a dark-line (absorption) spectrum?

D. Blue-shifted sodium lines

|

5 What produces emission lines (bright lines) in a spectrum? 6 Briefly describe the Doppler effect and describe how astronomers determine whether a star is moving toward or away from Earth.

23.3 COLLECTING LIGHT USING OPTICAL TELESCOPES Compare and contrast refracting and reflecting telescopes. Explain why modern telescopes are built on mountaintops.

The earliest tools used to observe the heavens were human eyes—the only mechanisms early astronomers like Tycho Brahe had available to them. However, the human eye is a poor instrument for astronomical observation because it cannot collect much light, is not very sensitive to faint colors, and collects only visible light. Early optical telescopes vastly improved over the naked eye, allowing for the collection of large amounts of light. Optical telescopes collect light with visible (or nearly visible) wavelengths and come in two basic types—refracting and reflecting telescopes.

Refracting Telescopes Much like the one used by Galileo, refracting telescopes employ lenses to collect and focus light (FIGURE 23.8 ). The light coming from a distant object can be thought of as a ray or beam by the time it reaches Earth. Our eye or a telescope lens intercepts some portion of the incoming light. To collect more light, one simply uses a larger lens. Two major problems prevent the manufacture of large refracting telescopes. First, the lens acts like a prism,

23.3 Collecting Light Using Optical Telescopes

Light

721

used for spectroscopic work (and other observations), but it suffers from the problems described above.

Reflecting Telescopes Small refracting telescopes work very well for observing objects in the solar system and for observing any other bright source. However, through experimentation, Sir Isaac Newton discovered that a large lens would cause white light to separate into its constituent parts (chromatic aberration), causing a halo of colored light to form around the object being viewed. By designing a telescope that used a mirror rather than a lens, Newton avoided this problem because the light does not travel through glass but is reflected from a coated surface instead (FIGURE 23.10 ). As a result, refracting telescopes have mainly been replaced by reflecting telescopes that use a curved mirror to collect and focus the light (FIGURE 23.11A ). All large telescopes built today are of the reflecting type, having a mirror that is made of a special glass that is finely ground to a nearly perfect parabolic shape. A parabola is the geometric shape that takes parallel lines—or parallel light rays—and focuses them to a point. The Hubble Space Telescope has a 2.4-meter (94.5-inch) mirror that is ground to within about one-millionth of an inch of being a perfect paraboloid. (If you have the time and patience, you could grind your own 8-, 10-, or even 12-inch mirror.) Once ground, the surface of the mirror is coated with a highly reflective material. Reflecting telescopes collect more light as the diameter of the mirror increases, just like refracting telescopes with larger lenses. However, there are difficulties in increasing the size of the mirror beyond several meters. These include Eyepiece

Focus

Objective lens

FIGURE 23.8 Refracting Telescope A refracting telescope uses a lens to collect and focus light.

spreading out the colors in the light, an effect called chromatic aberration. This is a problem that grows quickly as lenses get larger. Second, large lenses weigh so much that they sag under their own weight, changing their shape and, hence, changing their focusing properties. The world’s largest refracting telescope is the 1-meter (40-inch) telescope at Yerkes Observatory in Williams Bay, Wisconsin (FIGURE 23.9 ). This telescope was successfully

FIGURE 23.10 Newton’s Reflecting Telescope (Photo by Dave King © Dorling Kindersley, Courtesy of The Science Museum, London)

FIGURE 23.9 World’s Largest Refracting Telescope This 1-meter (40-inch) refractor is located at Yerkes Observatory, Williams Bay, Wisconsin. (Photo by AP Photo/The Janesville Gazette, Lukas Keapproth)

722

CHAPTER 23

Light, Astronomical Observations, and the Sun

SmartFigure 23.11 Reflecting R fl Telescope A. Diagram illustrating how paraboloid-shaped mirrors, like those used in reflecting telescopes, gather light. B. Preparation of the 2.4meter (94.5-inch) mirror for the Hubble Space Telescope. (Courtesy of NASA)

Light Eyepiece Focus

A. Secondary flat mirror

with inputs from an active optics system. Active optics are a recent development that corrects for distortions caused by turbulence in the atmosphere and became practical only recently, due to the availability of fast, relatively inexpensive computers. Larger telescopes not only allow us to collect more light from faint nearby objects, Paraboloid-shaped they also allow us to collect more light from primary mirror very distant objects. Since the speed of light is finite (about 300,000 kilometers [186,000 miles] per second), it takes time for the light to get to us. Even light from the Sun takes about 81/2 minutes to reach Earth, and light from the nearest large galaxy takes 2 million years to reach us. Larger telescopes allow us to literally look back in time. Our desire to understand the nature and evolution of the universe has motivated us to develop telescopes that look farther and farther back in time. Larger telescopes also generally provide better resolution, or clarity (FIGURE 23.12 ).

Light Collection

supporting such a large mass, moving that mass to realign the telescope, warping of the mirror surface under its own weight, and the time required to grind a nearly perfect surface over such a large area. For example, it took 14 years to construct the mirror for the Hale Telescope. These difficulties have recently been overcome in two ways. First, we can use an array of several smaller deformable mirrors under computer control to give the effect of one large mirror. Second, we can use a single, very thin mirror mounted on actuators that control the mirror shape, FIGURE 23.12 Comparing the Resolutions of Two Telescopes Appearance of the galaxy in the constellation Andromeda using telescopes of different resolutions. (Images Copyright © The Association of Universities for Research in Astronomy, Inc. All Rights Reserved)

Telescopes simply collect light. The earliest light collectors were the astronomer’s eyes. Astronomers would look through telescopes and draw what they saw (FIGURE 23.13 ). Each person’s eyes perceive light intensity and faint color differB. ently (and each person has a different amount of drawing talent), so under the same conditions, different images of the same object were produced. In addition, personal biases can influence what a person observes. For example, in the early twentieth century, noted astronomer Percival Lowell (1855– 1916) was convinced that there were canals on the surface of Mars. Thus, he “saw” them in his telescope and drew them in his images. Subsequent studies did not support Lowell’s observations. Photographic film was a revolutionary improvement. It is not impeded by personal biases, it records reasonably accurate relative light intensities, and it records faint colors more

23.3 Collecting Light Using Optical Telescopes

accurately than does the human eye. However, only about 2 percent of the light that strikes film is recorded. This means that long exposure times are required when recording faint images. Furthermore, photographic film, like the human eye, is not equally sensitive to all wavelengths. There are also differences between individual batches or even between pieces of film that need to be considered when making quantitative comparisons. Advances in semiconductor technology have produced the charge-coupled device (CCD), which takes an electronic photograph and effectively uses the same piece of “film” over and over again. (Charge-coupled devices are used in digital cameras, like cell phone cameras, as the light-sensing component.) CCD cameras offer a tremendous improvement over photographic film for detection of visible and near-visible light. They typically detect 70 percent, or more, of all incoming light and are easily calibrated for variations in wavelength sensitivity. Using CCD cameras, astronomers can collect light from distant objects for hours, as long as the telescope is accurately steered. Light can also be collected over several nights and synthesized to make a single image. Once collected, light emitted from distant sources is analyzed to determine the temperature, composition, relative motion, and distance to celestial objects. For faint or distant sources, as much light as possible must be collected, for the longest amount of time that is reasonable. This requires very large instruments with very sensitive detectors and

723

FIGURE 23.13 Percival Lowell’s Drawing of Mars Percival Lowell believed that life existed on Mars and drew these canals, influenced perhaps by his personal biases. (Photo SPL/ Science Source)

virtually no interference from other sources of electromagnetic energy. Further, Earth’s atmosphere is very turbulent, which turns faint points of light into very faint smudges. The largest telescopes were built on mountaintops away from urban areas to get above as much of the turbulent atmosphere as possible and to reduce the effects of light pollution (FIGURE 23.14 ). In addition, astronomers have

FIGURE 23.14 Kitt Peak National Observatory on a Starlit Night (Photo by Bryan Allen/CORBIS)

724

CHAPTER 23

Light, Astronomical Observations, and the Sun

designed adaptive optics to overcome much of the blurring introduced by the constant motion of Earth’s atmosphere. However, these instruments are limited to collecting light in the visible (or radio) regions of the electromagnetic spectrum because other wavelengths do not penetrate Earth’s atmosphere (see Figure 23.2). Finally, with the dawn of the space age, even these wavelength limitations have been overcome. It has become practical to put astronomical observatories in space, avoiding the turbulent atmosphere and allowing for the collection of electromagnetic radiation at all wavelengths.

23.3 CONCEPT CHECKS 1 What is the main difference between reflecting and refracting telescopes?

2 Why do astronomers seek to design telescopes with larger and larger mirrors?

3 Why do all large optical telescopes use mirrors rather than lenses to collect light?

4 What are the advantages of charge-coupled devices (CCDs) over photographic film?

5 Why is the human eye an ineffective tool for astronomical observation?

6 Provide two reasons why the largest telescopes are built on mountaintops away from large cities.

|

23.4 RADIO- AND SPACE-BASED ASTRONOMY Explain the advantages of radio telescopes and orbiting observatories over optical telescopes.

Sunlight consists of more than the visible portion of the electromagnetic spectrum. Gamma rays, x-rays, ultraviolet radiation, infrared radiation, and radio waves are also produced by stars and other celestial objects. CCD cameras that are sensitive to ultraviolet and infrared radiation have been developed to extend the limits of our vision. However, much of the radiation produced by celestial objects cannot penetrate our atmosphere or is not detectable by optical telescopes. As a result, astronomers have developed other observational techniques covering the remaining portions of the electromagnetic spectrum.

Radio Telescopes Of great importance is a narrow band of radio waves that penetrates the atmosphere (see Figure 23.2). One particular wavelength is the 21-centimeter line produced by neutral hydrogen (hydrogen atoms that still hold their electron). Measurement of this radiation has permitted us to map the galactic distribution of hydrogen, the material from which stars are made. The detection of radio waves is accomplished by “big dishes” called radio telescopes (FIGURE 23.15A ). In principle,

Secondary reflector Receiver Primary reflector

A.

B.

FIGURE 23.15 Radio Telescopes A. The 100-meter (330-foot) steerable Robert C. Byrd radio telescope at Green Bank, West Virginia. The dish acts like the mirror of a reflecting optical telescope to focus radio waves onto the receiver. (Photo by National Radio Astronomy Observatory) B. Twenty-seven identical radio telescopes operate together to form the Very Large Array near Socorro, New Mexico. (Photo by Prisma/SuperStock)

23.4 Radio- and Space-Based Astronomy

the dish of a radio telescope operates in the same manner as the mirror of an optical telescope. It is parabolic in shape and focuses the incoming radio waves on an antenna, which collects and transmits these waves to an amplifier. Because radio waves are about 100,000 times longer than visible radiation, the surface of a dish need not be as smooth as a mirror. In fact, except for the shortest radio waves, wire mesh is an adequate reflector. On the other hand, because radio signals from celestial sources are very weak, large dishes are necessary in order to intercept a signal that is strong enough to be detected. The largest radio telescope is a bowl-shaped antenna hung in a natural depression in Puerto Rico (FIGURE 23.16 ). It is 300 meters (1000 feet) in diameter and has some directional flexibility because of its movable antenna. The largest steerable types have about 100-meter (330foot) dishes. The National Radio Astronomy Observatory in Green Bank, West Virginia, provides an example (see Figure 23.15A). Radio telescopes have relatively poor resolution, making it difficult to pinpoint the radio source. Pairs or groups of telescopes are used to reduce this problem. When several radio telescopes are wired together, the resulting network is called a radio interferometer (FIGURE 23.15B ).

Orbiting Observatories Orbiting observatories circumvent all the problems caused by Earth’s atmosphere and have led to many significant discoveries in astronomy. NASA’s series of “four great observatories” provides a good illustration.

The Hubble Space Telescope Launched in 1990, the Hubble Space Telescope (HST) is an optical reflecting telescope in orbit around Earth (see the GEOgraphics on page 712). Its images are not distorted by the atmosphere, and there is no atmospherically scattered light to drown out faint sources of light. In addition, it can collect ultraviolet light that is absorbed by Earth’s ozone layer and is thus unavailable to ground-based telescopes. Hubble must be considered to be one of the most important instruments in the history of astronomy because of the large number of discoveries that have been made from its images. The 2.4-meter (94.5-inch) mirror has produced images with a sensitivity and resolution that are only now being matched by much larger (10-meter [33-foot]) ground-based telescopes. Here are just a few of the many discoveries made with Hubble. HST provided visual proof that pancake-shaped disks of dust are common around young stars, providing support for the nebular hypothesis of solar system formation. Hubble provided decisive evidence that super-massive black holes reside in the center of many galaxies by imaging the

725

FIGURE 23.16 The 300Meter (1000-Foot) Radio Telescope at Arecibo, Puerto Rico (Courtesy of David Parker/Science Source)

Receiver Reflecting surface

movements of dust and gas in the interiors of galaxies. The HST has also allowed us to look farther out into the universe (and farther back in time) than ever before, in the process producing the most “elusive” astronomical image ever taken, the Ultra-Deep Field (see the GEOgraphics on page 713). This image was acquired by looking at a patch of “empty” sky for a total of 1 million seconds; the faintest objects put only one photon per minute into the exposure. The scientific successor to the Hubble Telescope, called the James Webb Space Telescope, is scheduled for launch in 2018. The Webb telescope will have a large 6.6–meter-wide (21-foot-wide) mirror and a sunshield the size of a tennis court.

The Compton Gamma Ray Observatory Designed to collect data on some of the most violent physical processes in the universe, the Compton Gamma Ray Observatory (CGRO) was launched in 1991. It had a sensitivity 10 times greater than any previous gamma ray instrument and collected an incredible range of high-energy radiation. One of the main scientific discoveries made by CGRO was the uniform distribution of gamma ray bursts, which suggest that they are common events associated with ordinary objects. Gamma ray bursts are flashes of gamma rays that come from seemingly random places deep in the universe at random times. They are probably the most luminous and, therefore, the most energetic events occurring in the universe since the Big Bang. It is quite likely that many of them are caused by rapidly rotating massive stars as they collapse to form black holes (see Chapter 24). The Chandra X-Ray Observatory The Chandra X-Ray Observatory (CXO), launched in 1999, was designed to observe objects such as black holes, quasars, and high-temperature gases at x-ray wavelengths to better understand the

726

CHAPTER 23

Light, Astronomical Observations, and the Sun

FIGURE 23.17 Supernova Remnant Captured by Chandra X-Ray Observatory This scattered

The

Spitzer

Space

Telescope

Launched in 2003, the Spitzer Space Telescope (SST ) was designed to collect infrared (heat) energy that is mostly blocked by Earth’s atmosphere. Its instruments must be cooled to near zero Kelvin so that heat from nearby objects (and the satellite itself) does not interfere with the measurements. The telescope is actually in an orbit around the Sun to keep it away from the thermal energy radiated by Earth, and it is outfitted with a shield to deflect solar radiation. Spitzer’s highly sensitive instruments give us unique views of the universe and allow us to peer into regions of space that are hidden from optical telescopes by vast, dense clouds of gas and dust (nebula). Fortunately, infrared light can pass through these clouds, allowing us to peer into regions of star formation, the centers of galaxies, and newly forming planetary systems. Infrared light also brings us information about cool celestial objects, such as small stars that are too dim to be detected at visible wavelengths, planets that lay outside our solar system, and molecular clouds.

glowing debris ejected from a massive star that is barely visible in the optical part of the spectrum is awash in brilliantly glowing gases emitting x-rays. (NASA/Marshall Space Flight Center)

structure and evolution of the universe. With a resolution 25 times greater than any other x-ray observatory, it uses only as much power as an ordinary hair dryer (FIGURE 23.17 ). The CXO has observed a black hole pulling in matter and two black holes merging into one. In addition, it has provided an independent measurement of the age of the universe, reinforcing the estimated age of 12–14 billion years. The CXO has shown what galaxies were like when the universe was only a few billion years old.

|

23.4 CONCEPT CHECKS 1 Why are radio telescopes much larger than optical telescopes?

2 What are some of the advantages of radio telescopes over optical telescopes?

3 Explain why space makes a good site for an optical observatory.

4 What can astronomers learn about the universe by studying it at multiple wavelengths?

23.5 THE SUN Write a statement explaining why the Sun is important to the study of astronomy. Sketch the Sun’s structure and describe each of its four major layers.

The Sun is one of the 200 billion stars that make up the Milky Way Galaxy. Although the Sun is of little significance to the universe as a whole, to those of us who inhabit Earth, it is the primary source of energy. Everything from the food we eat to the fossil fuels we burn in our automobiles and power plants is ultimately derived from solar energy (FIGURE 23.18 ). The Sun is also important in astronomy, since it is the only star close enough to permit easy study of its surface. Even with the largest telescopes, most other stars appear only as points of light. Because the Sun is so bright and emits eye-damaging radiation, observing it directly is unsafe. However, we can study it safely by using a telescope to project the Sun’s image on a piece of cardboard placed behind

the telescope’s eyepiece. Several telescopes around the world use this method to keep a constant vigil of the Sun. One of the finest is at the Kitt Peak National Observatory in southern Arizona (FIGURE 23.19 ). It consists of a 150-meter (500-foot) sloped enclosure that directs sunlight to a mirror situated below ground. From the mirror, an 85-centimeter (33-inch) image of the Sun is projected to an observing room, where it is studied. Compared to other stars of the universe, many of which are larger, smaller, hotter, cooler, more red, or more blue, the Sun is an “average star.” However, on the scale of our solar system, it is truly gigantic, having a diameter equal to 109 Earth diameters (1.35 million kilometers) and a volume 1.25 million times as great as that of Earth. Yet, because of its

23.5 The Sun

727

FIGURE 23.18 The Sun Is the Source of 99.9 Percent of All Energy on Earth (Photo by Jerry and Marcy Monkman/Danita Delimont/Alamy)

gaseous nature, the density is only one-quarter that of Earth, a little greater than the density of water. For convenience of discussion, we divide the Sun into four parts: the solar interior; the visible surface, or photosphere; and the two layers of its atmosphere, the chromosphere and the corona (FIGURE 23.20 ). Because the Sun is gaseous throughout, no sharp boundaries exist between these layers. The Sun’s interior makes up all but a tiny fraction of the solar mass, and unlike the outer three layers, it is not accessible to direct observation. We discuss the visible layers first.

Photosphere The photosphere (photos = light, sphere = ball) is aptly named because it radiates most of the sunlight we see and therefore appears as the bright disk of the Sun. Although it is FIGURE 23.19 The Robert J. McMath Solar Telescope Located at Kitt Peak, near Tucson, Arizona, this telescope has movable mirrors at the top to follow the Sun and reflect the Sun’s light down the sloping tunnel. (Photo by Kent Wood/ Science Source) Inset photo shows a view of the solar disk obtained by a solar telescope. (Photo by SOHO/ LASCO [ESA & NASA])

728

CHAPTER 23

Light, Astronomical Observations, and the Sun

amounts of the other detectable elements. Other stars also show similar disproportionate percentages of these two lightest elements, a fact we consider later.

SmartFigure 23.20 Di Diagram of the Sun’s Structure Earth is shown for scale.

Corona

Photosphere

FIGURE 23.21 Granules of the Solar Photosphere Granules appear as yellowish-orange patches. Each granule is about the size of Texas and lasts for only 10–20 minutes before being replaced by a new granule. (Courtesy of National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

Chromosphere

Just above the photosphere lies the chromosphere (color Radiation Spicules sphere), a relatively thin layer zone of hot, incandescent gases a few Convection zone thousand kilometers thick. The Core chromosphere is observable for a few moments during a total Chromosphere solar eclipse or by using a special instrument that blocks out the Granulation light from the photosphere. Under Sunspots such conditions, it appears as a Prominence thin red rim around the Sun. Because the chromosphere conCorona Spicules Corona sists of hot, incandescent gases under low Chromosphere pressure, it produces a bright-line spectrum that is nearly the reverse of the darkconsidered to be the Sun’s “surface,” it is unlike most line spectrum of the surfaces to which we are accustomed. The photosphere photosphere. One of Photosphere consists of a layer of incandescent gas less than 500 the bright lines of kilometers (300 miles) thick, having a pressure less hydrogen contributes than 1/100 of our atmosphere. Furthermore, it is neither a good portion of its total output and accounts for this smooth nor uniformly bright, as the ancients had imagined. It sphere’s red color. has numerous blemishes. In 1868, a study of the chromospheric spectrum revealed When viewed telescopically, the photosphere’s grainy the existence of an element unknown on Earth. It was named texture is apparent. This is the result of numerous comparahelium, from helios, the Greek word for Sun. Originally, tively small, bright markings called granules (granum = helium was thought to be an element unique to the stars, small grain) that are surrounded by narrow, dark regions but 27 years later, it was discovered in a natural-gas well on (FIGURE 23.21 ). Granules are typically the size of Texas Earth. and owe their brightness to hotter gases that are rising from The top of the chromosphere contains numerous spicules below. As this gas spreads laterally, cooling causes it to (spica 5 point), flamelike structures that extend upward about darken and sink back into the interior. Each granule survives 10,000 kilometers (6000 miles) into the lower corona, almost for only 10–20 minutes, while the combined motion of old like trees that reach into our atmosphere (FIGURE 23.22 ). granules being replaced by new ones gives the photosphere Spicules are produced by the turbulent motion of the granules the appearance of boiling. This up-and-down movement of below. gas, called convection, produces the grainy appearance of the photosphere and is responsible for the transfer of energy in the uppermost part of the Sun’s interior (see Figure 23.20). The composition of the photosphere has been determined Corona from the dark lines of its absorption spectrum (see Figure 23.3). The outermost portion of the solar atmosphere, the corona When these fingerprints are compared to the spectra of (corona 5 crown) is very tenuous and, like the chromoknown elements, they indicate that most of the elements sphere, is visible only when the brilliant photosphere is found on Earth also exist on the Sun. When the strengths blocked (FIGURE 23.23 ). This envelope of ionized gases norof the absorption lines are analyzed, the relative abundance mally extends 1 million kilometers or so from the Sun and of the elements can be determined. These studies show that produces a glow about half as bright as the full Moon. At the outer fringe of the corona, the ionized gases have 90 percent of the Sun’s surface atoms are hydrogen and almost 10 percent are helium. That leaves only minor speeds great enough to escape the gravitational pull of the

23.6 The Active Sun

729

FIGURE 23.22 Spicules of the Chromosphere This view shows the outer edge of the solar disk. (National Solar Observatory/ Sacramento Peak)

Sun. The streams of charged particles (protons and electrons) that boil from the corona constitute the solar wind. The solar wind travels outward through the solar system at high speeds, about 250–800 kilometers per second. The solar wind and the Sun’s magnetic field form a sort of bubble that reaches far beyond the orbit of Pluto. This immense region, called the heliosphere, extends to the outermost boundary of the Sun’s influence, where interstellar space begins. During its journey, the solar wind interacts with the bodies of the solar system, continually bombarding lunar rocks and altering their appearance. Although Earth’s magnetic field prevents the solar winds from reaching the surface, these streams of charged particles interact with gases in our atmosphere—a topic we will discuss later. Studies of the energy emitted from the Sun indicate that its temperature averages about 6000K (10,000°F) in the photosphere. Upward from the photosphere, the temperature unexpectedly increases, exceeding 1 million K at the top of the corona. It should be noted that although the coronal temperature exceeds that of the photosphere by many times, it radiates much less energy overall because of its very low density.

|

NASA

FIGURE 23.23 Solar Corona This photograph was obtained during a total eclipse.

Surprisingly, the high temperature of the corona is probably caused by sound waves generated by the convective motion of the photosphere. Just as boiling water makes noise, energetic sound waves generated in the photosphere are absorbed by the gases that compose the corona, thereby increasing its temperature.

23.5 CONCEPT CHECKS 1 2 3 4

Why is the Sun significant to the study of astronomy? Describe the Sun in relationship to other stars in the universe. Describe the photosphere, chromosphere, and corona. Why are there no distinct boundaries between the layers of the Sun?

5 Why is the photosphere considered the Sun’s “surface”? 6 Briefly describe the solar wind.

23.6 THE ACTIVE SUN List and describe the three types of explosive activity that occurs at the Sun’s surface.

Most of the Sun’s energy that reaches Earth is a result of a rather steady, continuous emission from the photosphere. In addition to this predictable aspect of our Sun’s energy output, there is a much more irregular component, characterized by explosive surface activity that includes sunspots, prominences, and solar flares.

Sunspots The most conspicuous features on the surface of the Sun are the dark blemishes called sunspots (FIGURE 23.24A ). Although large sunspots were occasionally observed before the advent of the telescope, they were generally regarded as

730

CHAPTER 23

Light, Astronomical Observations, and the Sun

FIGURE 23.24 Sunspots A. Large sunspot group on the solar disk. (Solar & Heliospheric Observatory consortium (ESA & NASA)/Science Source) B. Sunspots having visible umbra (dark central area) and penumbra (lighter area surrounding umbra). (Courtesy of National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation [NOAO/AURA/NSF])

A.

opaque objects located somewhere between the Sun and Earth. In 1610 Galileo concluded that they were residents of the solar surface, and from their motion, he deduced that the Sun rotates on its axis about once a month. Later observations indicated that the time required for one rotation varied by latitude. The Sun’s equator rotates once in 25 days, whereas a place located 70° from the solar equa- B. tor, either north or south, requires 33 days for one rotation. If Earth rotated in a similar disjointed manner, imagine the consequences! The Sun’s nonuniform rotation is a testament to its gaseous nature. Sunspots begin as small, dark pores about 1600 kilometers (1000 miles) in diameter. Although most sunspots last for only a few hours, some grow into blemishes many times larger than Earth and last for a month or more. The largest sunspots often occur in pairs surrounded by several smaller sunspots. An individual spot contains a black

150 100 50 0

1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020

of sunspots reaches a maximum on roughly an 11year cycle.

200 Annual number of sunspots

FIGURE 23.25 Mean Annual Sunspot Numbers The number

center, the umbra (umbra = shadow), which is rimmed by a lighter region, the penumbra (paene 5 almost, umbra 5 shadow) (FIGURE 23.24B ). Sunspots appear dark only by contrast with the brilliant photosphere, a fact accounted for by their temperature, which is about 1500K less than that of the solar surface. If these dark spots could be observed away from the Sun, they would appear many times brighter than the full moon. During the early nineteenth century, it was believed that a tiny planet named Vulcan orbited between Mercury and the Sun. In the search for Vulcan, an accurate record of sunspot occurrences was kept. Although the planet was never found, the sunspot data showed that the number of sunspots on the solar disk varies in an 11-year cycle (FIGURE 23.25 ). First, the number of sunspots increases to a maximum, with perhaps 100 or more visible at a given time. Then, over a period of five to seven years, their numbers decline to a minimum, when only a few or even none are visible. At the beginning of each cycle, the first sunspots form about 30° from the solar equator, but as the cycle progresses and their numbers increase, they form nearer the equator. During the period when sunspots are most abundant, the majority form about 15° from the equator. They rarely occur more than 40° away from the Sun’s equator or within 5° of it. Another interesting characteristic of sunspots was discovered by astronomer George Hale, for whom the Hale Telescope is named. Hale deduced that the large spots are strongly magnetized, and when they occur in pairs, they have opposite magnetic poles. For instance, if one member of the pair is a north magnetic pole, then the other member is a south magnetic pole. Also, every pair located in the same hemisphere is magnetized in the same manner. However, all pairs in the other hemisphere are magnetized in the opposite manner. At the beginning of each sunspot cycle, the situation reverses, and the polarity of these sunspot pairs is opposite those of the previous cycle. The cause of this change in polarity—in fact, the cause of sunspots themselves—is not fully understood. However, other modes of solar activity vary in the same cyclic manner as sunspots, indicating the likelihood of a common origin.

23.6 The Active Sun

731

Prominences Among the most spectacular features of the active Sun are prominences (prominere 5 to jut out). These huge cloudlike structures, consisting of concentrations of chromospheric gases, are best observed when they are on the edge, or limb, of the Sun, where they often appear as bright arches that extend well into the corona (FIGURE 23.26 ). Quiescent prominences have the appearance of a fine tapestry and seem to hang motionless for days at a time, but motion pictures reveal that the material within them is continually falling like luminescent rain. By contrast, eruptive prominences rise almost explosively away from the Sun. These active prominences reach velocities up to 1000 kilometers (620 miles) per second and may leave the Sun entirely. Whether eruptive or quiescent, prominences are ionized chromospheric gases trapped by magnetic fields that extend from regions of intense solar activity.

Solar Prominence

Solar Flares Solar flares are brief outbursts that normally last an hour or so and appear as a sudden brightening of the region above a sunspot cluster. During their existence, enormous quantities of energy are released across the entire electromagnetic spectrum, much of it in the form of ultraviolet, radio, and x-ray radiation. Simultaneously, fast-moving atomic particles are ejected, causing the solar wind to intensify noticeably. Although a major flare could conceivably endanger a staffed space flight, these are relatively rare events. Within hours after a large outburst, the ejected particles reach Earth

NASA

FIGURE 23.26 Solar Prominence

EYE ON THE EY

UNIVERSE U

The accompanying image shows one of two telescopes T lo located at Keck Observatory atop Hawaii’s Mauna Kea volcano, at an elevation of 4200 meters (13,800 feet). The vo 10-meter (33-foot) mirrors of these telescopes consist of 36 10-m distinct pieces. These telescopes can work independently or in tandem to double their light-gathering capacity. (Photo by Enrico Sacchetti/Science Source) QU ESTIO N 1 Examine the primary mirror located near the center of the image. Based on what you see, describe the shape of the 36 segments that make up this mirror. Q UESTION 2 Assume that these mirrors have a perfect circular shape and a diameter of 10 meters (33-foot) and that they are working in tandem (gathering light from exactly the same source). How much more light-gathering capacity do they have compared to the Hale Telescope, which has a mirror with a 5-meter (16-foot) diameter? (Hint: Compare the surface areas of these instruments.) Q UESTION 3 The Keck Observatory telescopes are located far out in the Pacific, on the Big Island of Hawaii, and the Hale Telescope is located 80 kilometers (50 miles) northeast of San Diego. Does the location of the Keck telescopes represent an advantage or a disadvantage as compared to the Hale Telescope? Explain.

GEOGRAPHICS

Hubble Space Telescope In 1990, the Space Shuttle Atlantis carried the Hubble Space Telescope (HST) into an orbit 557 kilometers (347 miles) above Earth’s surface. At this altitude, the HST avoids many of the problems experienced by Earth-bound telescopes, which are affected by the atmosphere that distorts and blocks much of the light reaching our planet. The thousands of images beamed back to Earth have helped scientists solve many great mysteries. For example, data from the HST allowed scientists to determine that the age of the universe is between 13 and 14 billion years, significantly narrowing the previous range of 10 to 20 billion years.

These pillar-likes structures are columns of cool hydrogen gas and dust: the incubators of new stars. Captured in stunning detail by the HST, they are part of the Eagle Nebula, a nearby star-forming area about 6500 light-years away. HST/NASA

HST/NASA

The Hubble Space Telescope produced images of remote galaxies with unprecedented clarity and sharpness, including small galaxies dubbed “tadpoles” that existed when the universe was young. This image, called the Hubble Deep Field, was acquired in what appears to be an empty part of the sky located near the Big Dipper. Thousands of galaxies in various stages of evolution are visible in the image, captured by the HST focusing entirely on the same tiny patch of sky for ten consecutive days.

HST/NASA

This striking view of The Ring Nebula was obtained by the Hubble Space Telescope. Barrel-shaped clouds, called planetary nebula because they appeared spherical to early observers, are actually composed of ejected material from a dying sun-like star. The HST is also credited with imaging several supernova remnants, including the Crab Nebula, which formed just 900 years ago.

HST/NASA

From Hubble images, astronomers deduced that galaxies grow by the accretion of smaller galaxies. Despite many successes, they cannot yet explain the variety of galaxies that exist, from large spiral galaxies such as the one shown in the accompanying image, to round, featureless elliptical galaxies.

HST/NASA

atmosphere above the magnetic poles is set aglow for several nights. The auroras appear in a wide variety of forms. Sometimes the display consists of vertical streamers with considerable movement. At other times, the auroras appear as a series of luminous, expanding arcs or as a quiet, almost foglike, glow. Auroral displays, like other solar activities, vary in intensity with the 11-year sunspot cycle.

FIGURE 23.27 Aurora Borealis (Northern Lights) This phenomenon, caused by particles ejected during a solar flare, also occurs near the South Pole, where it is called the Aurora Australis (Southern Lights). (Photo by Andy Farrer/Getty Images)

and disturb the ionosphere,3 affecting long-distance radio communications. The most spectacular effects of solar flares are the auroras, also called the Northern and Southern Lights (FIGURE 23.27 ). Following a strong solar flare, Earth’s upper 3

The ionosphere is a complex atmospheric zone of ionized gases extending between about 80 and 400 kilometers (50 and 250 miles) above Earth’s surface.

|

23.6 CONCEPT CHECKS 1 What did Galileo learn about the Sun from his observations of sunspots?

2 Briefly describe the 11-year sunspot cycle. 3 What are prominences? 4 How do solar flares affect the solar wind?

23.7 THE SOURCE OF SOLAR ENERGY Summarize the process called the proton–proton chain reaction.

The interior of the Sun cannot be observed directly. For that reason, what we know about it is based on information acquired from the energy radiated from the photosphere and solar atmosphere and from theoretical studies. The source of the Sun’s energy, nuclear fusion (fusus = to melt), was not discovered until the late 1930s. Deep in its interior, a nuclear reaction called the proton–proton chain reaction converts four hydrogen nuclei (protons) into the nucleus of a helium atom. The energy released from the proton–proton reaction results because some of the matter involved is actually converted to radiant energy. This can be illustrated by noting that four hydrogen atoms have a combined atomic mass of 4.032 (4 3 1.008), whereas the atomic mass of helium is 4.003, which is 0.029 less than the combined mass of the hydrogen. The tiny missing mass is emitted as energy according to Einstein’s formula E 5 mc2, where E equals energy, m equals mass, and c equals the speed of light. Because the speed of light is very great, the amount of energy released from even a small amount of mass is enormous. The conversion of just one pinhead’s worth of hydrogen to helium generates more energy than burning thousands of tons of coal. (This process is often referred to as “hydrogen burning,” but it is nothing like the type of burning to which we are accustomed.) Most of this energy is in the form of very highenergy photons that work their way toward the solar surface,

734

being absorbed and re-emitted many times until they reach an opaque layer just below the photosphere. Here, convection currents transport this energy to the solar surface, where it radiates through the nearly transparent chromosphere and corona as mostly visible light (see Figure 23.20). Only a small percentage (0.7%) of the hydrogen in the proton–proton reaction is actually converted to energy. Nevertheless, the Sun consumes an estimated 600 million tons of hydrogen each second, with about 4 million tons of it being converted to energy. The by-product of hydrogen burning is helium, which forms the solar core. Consequently, the core continually grows in size. How long can the Sun produce energy at its present rate before all of its fuel (hydrogen) is consumed? Even at the enormous rate of consumption, the Sun has enough fuel to easily last another 100 billion years. However, evidence from other stars indicates that the Sun will grow dramatically and engulf Earth long before all of its hydrogen is gone. It is likely that a star the size of the Sun can remain in a stable state for about 10 billion years. Since the Sun is already 5 billion years old, it is “middle-aged.” To initiate the proton–proton reaction, the Sun’s internal temperature must have reached several million degrees. What was the source of this heat? As previously noted, the solar system formed from an enormous cloud of dust and gases

Concepts in Review

(mostly hydrogen) that gravitationally collapsed. When a gas is squeezed (compressed), its temperature increases. Although all the bodies in the solar system were heated in this manner, the Sun was the only one that, because of its mass, became hot enough to trigger the proton–proton reaction. Astronomers currently estimate its internal temperature at 15 million K. The planet Jupiter is basically a hydrogen-rich gas ball. Why didn’t it become a star? Although Jupiter is a huge

23

CONCEPTS IN REVIEW

23.1 SIGNALS FROM SPACE

planet, the stars with the lowest mass are between 75 and 80 times the size of Jupiter.

23.7 CONCEPT CHECKS 1 What “fuel” does the Sun consume? 2 What happens to the matter that is consumed in the proton– proton chain reaction?

|

Light, Astronomical Observations, and the Sun

23.2 SPECTROSCOPY

List and describe the various types of electromagnetic radiation.

Explain how the three types of spectra are generated and what they tell astronomers about the radiating body that produced them.

KEY TERMS: electromagnetic radiation, photons,

K E Y T E R M S : spectroscopy, continuous spectrum, spectroscope, dark-line (absorption) spectrum,

radiation pressure

bright-line (emission) spectrum, Doppler effect

■

■

■

Electromagnetic radiation consists of an array of energy that includes gamma rays, x-rays, ultraviolet light, visible light, infrared radiation (heat), microwaves, and radio waves. Light, a type of electromagnetic radiation, can be described in two ways: (1) as waves and (2) as a stream of particles, called photons. The wavelengths of electromagnetic radiation vary from several kilometers for radio waves to less than one-billionth of a centimeter for gamma rays. Shorter wavelengths correspond to more energetic photons, and the longer wavelengths are less energetic.

Q List the following types of electromagnetic radiation in order from long-wavelength radiation to short-wavelength radiation: visible light, gamma rays, ultraviolet radiation, x-rays, infrared radiation, and radio waves.

735

■

■

■

Spectroscopy is the study of the properties of light that depend on wavelength. When a prism is used to disperse visible light into its component parts (wavelengths), one of three possible types of spectra is produced. (A spectrum, the singular form of spectra, is the light pattern produced by passing light through a prism.) The three types of spectra are (1) continuous spectrum, (2) dark-line (absorption) spectrum, and (3) bright-line (emission) spectrum. The spectra of most stars are of the dark-line type, which indicate the composition of the radiating body as well as the relative amount of each type of matter. Continuous spectra provide information about the total output of energy and the surface temperature of the radiating body. Finally, bright-line spectra are produced by gaseous bodies (nebula) at low pressure and contain information about the temperature of the gas and its composition. Spectroscopy can be used to determine the motion of an object, Spectrum of motionless light source a phenomena called the Doppler effect.

Q Use the accompanying diagram, which shows two dark-line spectra, to determine whether the moving body is going toward the observer or receding from the observer. Explain. Spectrum of moving body

23.3 COLLECTING LIGHT USING OPTICAL TELESCOPES Compare and contrast refracting and reflecting telescopes. Explain why modern telescopes are built on mountaintops. KEY TERMS: refracting telescope, chromatic aberration, reflecting telescope ■ ■ ■

There are two types of optical telescopes: (1) the refracting telescope, which uses a lens to bend or refract light, and (2) the reflecting telescope, which uses a concave mirror to focus (gather) the light. Most large modern telescopes use mirrors to collect light. Telescopes simply collect light. When correctly analyzed, the collected light can be used to determine the temperature, composition, relative motion, and distance to a celestial object. Historically, astronomers relied on their eyes to collect light. Then photographic film was developed, which was a revolutionary advancement. Presently, light is collected using a charge-coupled device (CCD). A CCD camera produces a digital image akin to that of a digital camera. Despite these advances, Earth-bound optical telescopes can only “view” a tiny portion of the electromagnetic spectrum and are hindered by “poor seeing” as a result of atmospheric turbulence.

736

Concepts in Review

23.4 RADIO- AND SPACE-BASED ASTRONOMY Explain the advantages of radio telescopes and orbiting observatories over optical telescopes. K EY TERMS: radio telescope, radio interferometer ■ ■ ■

The detection of radio waves is accomplished by “big dishes” known as radio telescopes. A parabolic-shaped dish, often made of wire mesh, operates in a manner similar to the mirror of a reflecting telescope. Of great importance is a narrow band of radio waves that is able to penetrate Earth’s atmosphere. Because this radiation is produced by neutral hydrogen, it has permitted us to map the galactic distribution of this gaseous material from which stars are made. Orbiting observatories, like the Hubble Space Telescope, circumvent all the problems caused by the Earth’s atmosphere and have led to many significant discoveries in astronomy.

23.5 THE SUN Write a statement explaining why the Sun is important to the study of astronomy. Sketch the Sun’s structure and describe each of its four major layers.

23.6 THE ACTIVE SUN List and describe the three types of explosive activity that occur at the Sun’s surface. KE Y T E R M S : sunspot, prominence, solar flare, aurora ■

K EY TERMS: photosphere, granule, chromosphere,

spicule, corona, solar wind, heliosphere ■

■

■

■

The Sun is an average star, one of 200 billion stars that make up the Milky Way Galaxy. The Sun can be divided into four parts: (1) the solar interior, (2) the photosphere (visible surface) and the two layers of its atmosphere, (3) the chromosphere, and (4) the corona. The photosphere radiates most of the light we see. Unlike most surfaces to which we are accustomed, it consists of a layer of incandescent gas less than 500 kilometers (300 miles) thick and has a grainy texture consisting of numerous relatively small, bright markings called granules. Just above the photosphere lies the chromosphere, a relatively thin layer of hot incandescent gases a few thousand kilometers thick. At the edge of the uppermost portion of the solar atmosphere, called the corona, ionized gases escape the gravitational pull of the Sun and stream toward Earth at high speeds, producing the solar wind. The solar wind and the Sun’s magnetic field form a sort of bubble that extends far beyond the orbit of Pluto. This immense region, called the heliosphere, extends to the outermost boundary of the Sun’s influence, where interstellar space begins.

■

■

Numerous features have been identified on the active Sun. Sunspots are dark blemishes with a black center, the umbra, which is rimmed by a lighter region, the penumbra. The number of sunspots observable on the solar disk varies in an 11-year cycle. Prominences, huge cloudlike structures best observed when they are on the edge, or limb, of the Sun, are produced by ionized chromospheric gases trapped by magnetic fields that extend from regions of intense solar activity. The most explosive events associated with sunspots are solar flares. Flares are brief outbursts that release enormous quantities of energy that appear as a sudden brightening of the region above sunspot clusters. During the event, radiation and fast-moving atomic particles are ejected, causing the solar wind to intensify. When the ejected particles reach Earth 2 and disturb the ionosphere, radio communication is 1 disrupted, and the auroras, also called the Northern and Southern Lights, occur.

Q The accompanying image is a close-up view of the Sun’s surface. What name is given to the Sun’s surface? Name the labeled features.

NASA

23.7 THE SOURCE OF SOLAR ENERGY Summarize the process called the proton–proton chain reaction. K EY TERMS: nuclear fusion, proton–proton chain reaction ■

■

The source of the Sun’s energy is nuclear fusion. Deep in the solar interior, at a temperature of 15 million K, a type of nuclear fusion called the proton– proton chain reaction converts four hydrogen nuclei (protons) into the nucleus of a helium atom. During the reaction, some of the matter is converted to the energy that the Sun radiates to space. The Sun can continue to operate its nuclear furnace and exist in its present stable state for 10 billion years. As the Sun is already 5 billion years old, it is a “middle-aged” star.

Examining the Earth System

737

GIVE IT SOME THOUGHT 1. Refer to Figure 23.2 to answer the following questions: a. Is the atmosphere mostly transparent or opaque to visible light? b. Is the atmosphere mostly transparent or opaque to radio waves with a wavelength of 1 meter (3 feet)? c. Is the atmosphere mostly transparent or opaque to gamma rays?

2. Imagine that the composition of Earth’s atmosphere were altered so that its ability to absorb visible and far infrared light were reversed. a. If you were outdoors when the Sun was at its highest point in the sky, how would the sky appear? b. Would there be an increase or a decrease in Earth’s average surface temperature?

3. Explain the process by which we can estimate the velocity of a moving star. When we study the movement of a star due to the expansion of the universe, toward which color of the spectrum should we notice the Doppler shift to be occurring? a. What type of object in the galaxy could you study to investigate whether stars consist primarily of helium or hydrogen? b. How could spectroscopy help you verify or disprove the scientist’s claim? Explain your reasoning.

4. Imagine that you are responsible for funding the construction of observatories. After considering the four proposals listed below, state whether you would or would not recommend funding for each proposal and explain your reasoning. Proposal A: A ground-based x-ray telescope on top of a mountain in Arizona, designed to observe supernovae in distant galaxies. Proposal B: A space-based 3-meter reflecting infrared telescope designed to observe very distant galaxies. Proposal C: A ground-based 8-meter refracting optical telescope located on the top of Mauna Kea in Hawaii, designed to measure the spectra of binary stars in our galaxy. Proposal D: A ground-based 250-meter radio telescope array in New Mexico, designed to measure the distribution of hydrogen gas clouds in the disk of our galaxy. 5. An important absorption line in the spectrum of stars occurs at a wavelength of 656 nm for stars not moving toward or away from Earth.

Imagine that you observe four stars in our galaxy and discover that this absorption line is at the wavelength shown in the accompanying diagram. Using this data, complete the following questions. Explain the reasoning behind your answers. If you are unable to determine the answer to any of these questions from the given information, explain. a. Which of these stars is moving toward Earth the fastest? b. Which of these stars is closest to Earth? c. Which of these stars is moving away from Earth?

6. Consider the following discussion among three of your classmates regarding why telescopes are put in space. Support or refute each statement. Student 1: “I think it is because the atmosphere distorts and magnifies light, which causes objects to look larger than they actually are.” Student 2: “I thought it was because some of the wavelengths of light being sent out from the telescopes can be blocked by Earth’s atmosphere, so the telescopes need to be above the atmosphere.” Student 3: “Wait, I thought it was because by moving the telescope above the atmosphere, the telescope is closer to the objects, which makes them appear brighter.” 7. Refer to the accompanying spectra, which represent four identical stars in our galaxy. One star is not moving, another is moving away from you, and two stars are moving toward you. Determine which star is which and explain how you reached your conclusion.

EXAMINING THE EARTH SYSTEM 1. Of the two sources of energy that power the Earth system, the Sun is the main driver of Earth’s external processes. If the Sun increased its energy output by 10 percent, what would happen to global temperatures? What effect would this temperature change have on the percentage of water that exists as ice? What would be the impact on the position of the ocean

shoreline? Speculate about whether the change in temperature might produce an increase or a decrease in the amount of surface vegetation. In turn, what impact might this change in vegetation have on the level of atmospheric carbon dioxide? How would such a change in the amount of carbon dioxide in the atmosphere affect global temperatures?

24

1

FOCUS ON CONCEPTS

Each statement represents the primary LEARNING OBJECTIVE for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

24.1

Define cosmology and describe Edwin Hubble’s most significant discovery about the universe.

24.2

Explain why interstellar matter is often referred to as a stellar nursery. Compare and contrast bright and dark nebulae.

24.3

Define main-sequence star. Explain the criteria used to classify stars as giants.

24.4

List and describe the stages in the evolution of a typical Sun-like star.

24.5

Compare and contrast the final state of Sun-like stars to the remnants of the most massive stars.

24.6

List the three major types of galaxies. Explain the formation of large elliptical galaxies.

24.7

Describe the Big Bang theory. Explain what it tells us about the universe.

The Whirlpool Galaxy (M51) has excited astronomers for nearly three centuries. Its majestic spiral arms are star-forming factories. (NASA) 1

Revised with the assistance of Professors Teresa Tarbuck and Mark Watry.

739

740

CHAPTER 24

Beyond Our Solar System

stronomers and cosmologists study the nature of our vast universe, attempting to answer questions such as these: Is our Sun a typical star? Do other stars have solar systems with planets similar to Earth? Are galaxies distributed randomly, or are they organized into groups? How do stars

A

|

form? What happens when stars expend their fuel? If the early universe consisted mostly of hydrogen and helium, how did other elements come into existence? How large is the universe? Did it have a beginning? Will it have an end? This chapter explores these as well as other questions.

24.1 THE UNIVERSE Define cosmology and describe Edwin Hubble’s most significant

FIGURE 24.1 The Trifid Nebula, in the Constellation Sagittarius This colorful nebula is a cloud consisting mostly of hydrogen and helium gases. These gases are excited by light emitted by hot, young stars within and produce a reddish glow. (National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation)

discovery about the universe.

The universe is more than a collection of dust clouds, stars, stellar remnants, and galaxies (FIGURE 24.1 ). It is an entity with its own properties. Cosmology is the study of the universe, including its properties, structure, and evolution. Over the years, cosmologists have developed a comprehensive theory that describes the structure and evolution of the universe. Some of the questions cosmologists seek to answer with this theory include these: How did the universe evolve to its present state? How long has it existed, and how will it end? Modern cosmology addresses these important issues and helps us understand the universe we inhabit.

How Large Is It? For most of human existence, our universe was thought to be Earth centered, containing only the Sun, Moon, 5 wandering stars, and about 6000 fixed stars. Even after

the Copernican view of a Sun-centered universe became widely accepted, the entire universe was believed to consist of a single galaxy, the Milky Way, composed of innumerable stars, along with many faint “fuzzy patches,” thought to be clouds of dust and gases. In the mid-1700s, German philosopher Immanuel Kant proposed that many of the telescopically visible fuzzy patches of light scattered among the stars were actually distant galaxies similar to the Milky Way. Kant described them as “island universes.” Each galaxy, he believed, contained billions of stars and was a universe in itself. In Kant’s time, however, the weight of opinion favored the hypothesis that the faint patches of light occurred within our galaxy. Admitting otherwise would have implied a vastly larger universe, thereby diminishing the status of Earth and, likewise, humankind. In 1919 Edwin Hubble arrived at the observatory at Mount Wilson, California, to conduct research using a 2.5-meter

24.1 The Universe

(100-inch) telescope, then the world’s largest and most advanced astronomical instrument. Armed with this modern tool, Hubble set out to solve the mystery of the “fuzzy patches.” At that time, the debate was still raging as to whether the fuzzy patches were “island universes,” as Kant had proposed more than 150 years earlier, or clouds of dust and gases (nebulae). To accomplish this task, Hubble studied a group of pulsating stars known as Cepheid variables—extremely bright variable stars that increase and decrease in brightness in a repetitive cycle. This group of stars is significant because their “true” brightness, called absolute magnitude, can be determined by know- A. ing the rate at which they pulsate (see Appendix C). When the absolute magnitude of a star is compared to its observed brightness, a reliable approximation of distance can be established. (This is similar to how we judge the distance of an oncoming vehicle when driving at night.) Thus, Cepheid variables are important because they can be used to measure large astronomical distances. Using the telescope at Mount Wilson, Hubble found several Cepheid variables embedded in one of the fuzzy patches. However, because these intrinsically bright stars appeared faint, Hubble concluded that they must lie outside the Milky Way. Indeed, one of the objects Hubble observed lies more than 2 million light-years away and is now known as the Andromeda Galaxy (FIGURE 24.2 ). Based on his observations, Hubble determined that the universe extended far beyond the limits of our imagination. Today, we know that there are hundreds of billions of galaxies, each containing hundreds of billions of stars. For example, researchers estimate that a million galaxies exist in the portion of the sky bounded by the cup of the Big Dipper. There are literally more stars in the heavens than grains of sand in all the beaches on Earth.

A Brief History of the Universe Large telescopes can actually “look back in time,” which accounts for much of the knowledge astronomers have acquired regarding the history of the universe. Light from

741

FIGURE 24.2 Andromeda: A Nearby Galaxy That Is Larger Than Our Milky Way A. Photo of Andromeda

B.

Galaxy, taken by an optical telescope aboard the GALEX orbiter. (NASA) B. Andromeda Galaxy as it appears under low magnification. When viewed with the naked eye, Andromeda appears as a fuzzy patch surrounded by stars. (European Southern Observatory/ESO)

celestial objects that are great distances from Earth require millions or even billions of years to reach Earth. The distance light travels in 1 year is called a light-year (slightly less than 10 trillion kilometers [6 trillion miles]). Therefore, the farther out telescopes can “see,” the farther back in time astronomers are able to study. Even the closest large galaxy, the Andromeda Galaxy, is a staggering 2.5 million light-years away. Light that left Andromeda Galaxy 2.5 million years ago is just now reaching Earth, allowing scientists to observe Andromeda as it was 2.5 million years ago. Light from the farthest-known objects, about 13 billion light-years away, came from stars that have long since burned out. The time line for the history of the universe, shown in FIGURE 24.3 , highlights some of the major events in the evolution of matter and energy. The model that most accurately describes the birth and current state of the universe is the Big Bang theory. According to this theory, all of the energy and matter of the universe existed in an incomprehensibly hot and dense state. About 13.7 billion years ago, our universe began as a cataclysmic explosion, which continued to expand, cool, and evolve to its current state. In the earliest moments of this expansion, only energy and quarks (subatomic particles that are the building blocks of protons and neutrons) existed. Not until 380,000 years after the initial expansion did the universe cool sufficiently for electrons and protons to combine to form hydrogen and helium atoms—the lightest elements in the universe. For the

742

CHAPTER 24

Beyond Our Solar System

FIGURE 24.3 Time Line for the Evolution of the Universe According to the

Present day 13.7 billion years

ive he Un t f o Age

Big Bang theory, the universe began 13.7 billion years ago and has been expanding ever since.

rse 9.1 billion years

400–500 million years 1 million years

BIG BANG Emission of cosmic background radiation

First galaxies and stars

Galaxie

s evolv

e

Formation of our Solar System 4.6 billion years ago

first time, light traveled through space. Eventually, temperatures decreased sufficiently to allow clumps of matter to collect. This material formed massive clouds of dust and gases (nebulae), which quickly evolved into the first stars and galaxies. Our Sun and planetary system, having formed about 5 billion years ago (nearly 9 billion years after the Big Bang), is a latecomer to the universe.

|

Modern galaxies

24.1 CONCEPT CHECKS 1 What is cosmology? 2 Explain how Edwin Hubble used Cepheid variables to change our view of the structure of the universe.

3 When do cosmologists think the universe began? 4 What two elements were the first to form?

24.2 INTERSTELLAR MATTER: NURSERY OF THE STARS Explain why interstellar matter is often referred to as a stellar nursery. Compare and contrast bright and dark nebulae.

As the universe expanded, gravity caused matter to accumulate into large “clumps” and “strands” of interstellar matter known as nebulae (meaning “clouds”; the singular of nebulae is nebula). In addition, considerable amounts of interstellar matter once resided in the interiors of stars and were subsequently returned to space. Some stars ejected matter as part of their normal life cycle, some exploded when they died, and some formed black holes that ejected streams of matter through structures called jets. Interstellar matter resides between stars within the galaxies and consists of roughly 90 percent hydrogen and 9 percent helium. The remainder, interstellar dust, is composed of atoms, molecules, and larger dust grains of the heavier elements. These huge concentrations of interstellar dust and gases are extremely diffuse, similar to fog, with no distinct edges or boundaries. However, because nebulae are enormous, their total mass is many times that of the Sun. If a

nebula is dense enough, it will contract due to gravity, leading to processes that form stars and planets. When nebulae are in close proximity to very hot (blue) stars, they glow and are called bright nebulae. By contrast, when clouds of interstellar material are too far from bright stars to be illuminated, they are referred to as dark nebulae.

Bright Nebulae There are three main types of bright nebulae: emission, reflection, and planetary nebulae. Emission and reflection nebulae consist of the material from which new stars are born—stellar nurseries. By contrast, planetary nebulae form when stars die and expel material into space. Thus, all stars form from nebulae, and many ultimately return to the material of nebulae when their life cycle ends.

24.2 Interstellar Matter: Nursery of the Stars

743

Emission Nebulae Glowing clouds of hydrogen gas, called emission nebulae, are produced in active star-forming regions of galaxies. Energetic ultraviolet light emitted from hot, young stars ionizes the hydrogen atoms in the nebulae. Because these gases exist under extremely low pressure, they radiate, or emit, their energy as less energetic visible light. The conversion of ultraviolet light to visible light is known as fluorescence—the same phenomenon that causes neon lights to glow. Hydrogen emits much of its energy in the red portion of the spectrum, which accounts for the red glow from emission nebulae (FIGURE 24.4 ). When elements other than hydrogen are ionized, the glowing nebula may exhibit a broader range of colors. An emission nebula that is easily seen with binoculars is located in the constellation Orion, in the sword of the hunter. Reflection Nebulae As the name implies, reflection nebulae merely reflect the light of nearby stars (FIGURE 24.5 ). Reflection nebulae are likely composed of significant amounts of comparatively large debris, including grains of carbon compounds. This view is supported by the fact that atomic gases with low densities could not reflect light sufficiently to produce the glow observed. Reflection nebulae are usually blue because blue light (shorter wavelength) is scattered more efficiently than red light (longer wavelength)—a process that also produces the blue color of the sky. The blue wisps in the Pleiades, shown in Figure 24.5, are reflection nebulae.

FIGURE 24.5 Reflection Nebulae This blue reflection nebula, located in the Pleiades star cluster, is a result of the scattering of starlight by relatively large molecules and interstellar dust. The Pleiades star cluster, barely visible to the naked eye in the constellation Taurus, is spectacular when viewed through binoculars or a small telescope.(David Malin /AAO/Royal Observatory, Edinburgh/Science Source)

Planetary Nebulae Planetary nebulae are generally not as diffuse as other nebulae, and they originate from the remnants of dying Sun-like stars (FIGURE 24.6 ). Planetary

FIGURE 24.4 Emission Nebulae Lagoon Nebula is a large emission nebula, composed mainly of hydrogen. Its red color is attributed to ionized gases, which are excited by the energetic light emitted from young, hot stars embedded in the nebula. (National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation [NOAO/AURA/NSF])

FIGURE 24.6 Planetary Nebulae The Helix Nebula is the nearest planetary nebula to our solar system. A planetary nebula is the ejected outer envelope of a Sun-like star that formed during the star’s collapse from a red giant to a white dwarf. (© Anglo-Australian Observatory. Photography by David Malin)

744

CHAPTER 24

Beyond Our Solar System

name. A good example of a planetary nebula is the Helix Nebula in the constellation Aquarius (see Figure 24.6).

FIGURE 24.7 Dark Nebulae The Horsehead Nebula is a dark nebula in the constellation Orion.

Dark Nebulae

(Courtesy of the European Southern Observatory)

Recall that when clouds of interstellar material are too distant from bright stars to be illuminated, they are referred to as dark nebulae. Exemplified by the Horsehead Nebula in Orion, dark nebulae appear as opaque objects silhouetted against bright backgrounds (FIGURE 24.7 ). In addition, dark nebulae can also easily be seen as starless regions— “holes in the heavens”—when viewing the Milky Way (see Figure 24.14). Although dark nebulae often appear dense, they are made of the same matter as bright nebulae and consist of thinly scattered matter.

24.2 CONCEPT CHECKS 1 Why is the phrase “nursery of stars” an appropriate way of describing interstellar matter (nebulae)?

nebulae consist of glowing clouds of dust and hot gases that have been expelled near the end of a star’s life. When first viewed through primitive optical telescopes, planetary nebulae resembled giant planets such as Jupiter—hence their

|

2 Compare and contrast bright and dark nebulae. 3 Why are reflection nebulae generally blue? 4 How are planetary nebulae different from other types of bright nebulae?

24.3 CLASSIFYING STARS: HERTZSPRUNG–RUSSELL DIAGRAMS (H-R DIAGRAMS) Define main-sequence star. Explain the criteria used to classify stars as giants.

Early in the twentieth century, Einar Hertzsprung and Henry Russell independently studied the relationship between the true brightness (absolute magnitude) of stars and their respective temperatures. Their work resulted in the development of a graph, called a Hertzsprung–Russell diagram (H-R diagram). By studying H-R diagrams, we can learn a great deal of information about the relationships among the sizes, colors, and temperatures of stars (see Appendix D). For example, we learned that the hottest stars are blue in color and the coolest are red. To produce an H-R diagram, astronomers survey a portion of the sky and plot each star according to its absolute magnitude (stellar brightness) and temperature (FIGURE 24.8 ). Notice that the stars in Figure 24.8 are not uniformly distributed. Rather, about 90 percent of all stars fall along a band that runs from the upper-left corner to the lower-right corner of the H-R diagram. These “ordinary” stars are called main-sequence stars. As you can see in Figure 24.8, the hottest main-sequence stars are intrinsically the brightest, and, conversely, the coolest are the dimmest.

The absolute magnitude of main-sequence stars is also related to their mass. The hottest (blue) stars are about 50 times more massive than the Sun, whereas the coolest (red) stars are only 1/10 as massive. Therefore, on the H-R diagram, the main-sequence stars appear in decreasing order, from hotter, more massive blue stars to cooler, less massive red stars. Note the location of the Sun in Figure 24.8. The Sun is a yellow main-sequence star with an absolute magnitude, or “true” brightness, of about 5 (see Appendix D). Because the vast majority of main-sequence stars have magnitudes between –10 and 20, the Sun’s midpoint position in this range results in its classification as an “average star.” (Note that stellar magnitudes are measured so that the lower the number, the brighter the star and vice versa.) Just as all humans do not fall into the normal size range, some stars differ significantly from main-sequence stars. Above and to the right of the main sequence stars lies a group of very luminous stars called giants, or, on the basis

24.3 Classifying Stars: Hertzsprung–Russell Diagrams (H-R Diagrams)

of their color, red giants (see Figure 24.8). The size of these giants can be estimated by comparing them with stars of known size that have the same surface temperature. Scientists have discovered that objects having equal surface temperatures radiate the same amount of energy per unit area. Any difference in the brightness of two stars having the same surface temperature can be attributed to their relative sizes. Therefore, if one red star is 100 times more luminous than another red star, it must have a surface area that is 100 times larger. Stars with large radiating surfaces appear in the upper-right position of an H-R diagram and are appropriately called giants. Some stars are so immense that they are called supergiants. Betelgeuse, a bright red supergiant in the constellation Orion, has a radius about 800 times that of the Sun. If this star were at the center of our solar system, it would extend beyond the orbit of Mars, and Earth would find itself buried inside this supergiant! In the lower portion of the H-R diagram, opposite conditions are observed. These stars are much fainter than

main-sequence stars of the same temperature and likewise are much smaller. Some probably approximate Earth in size. These stars are called white dwarfs. H-R diagrams have proved to be an important tool for interpreting stellar evolution: The stages in which stars, similar to living things, are born, age, and die. Considering that almost 90 percent of stars lie on the main sequence, we can be relatively certain that they spend most of their active years as main-sequence stars. Only a few percent are giants, and perhaps 10 percent are white dwarfs.

24.3 CONCEPT CHECKS 1 On an H-R diagram, where do stars spend most of their life span?

2 How does the Sun compare in size and brightness to other main-sequence stars?

3 Describe how the H-R diagram is used to determine which stars are “giants.”

Spectral class 1,000,000

O

B

A

F

G

K

M

-10

SUPERGIANTS -5

10,000

GIANTS

Luminosity (sun = 1)

Sun

Main

1

0.01

sequ

enc

+5

e st ars

Absolute magnitude

0

100

+10

WH

ITE

DW AR

FS

0.0001

0.000001 30,000

+15

20,000

14,000

10,000 7000 Surface temperature (K)

5000

3500

SmartFigure 24.8 Hertzsprung–Russell Diagram Astronomers use these diagrams to study stellar evolution by plotted stars according to their temperatures and luminosities (absolute magnitudes).

+20 2500

745

746

CHAPTER 24

Beyond Our Solar System

|

24.4 STELLAR EVOLUTION List and describe the stages in the evolution of a typical Sun-like star.

The idea of describing how a star is born, ages, and dies may seem a bit presumptuous, for most stars have life spans that exceed billions of years. However, by studying stars of different ages, at different points in their life cycles, astronomers have been able to assemble a model for stellar evolution. The method used to create this model is analogous to how an alien, upon reaching Earth, might determine the developmental stages of human life. By observing large numbers of humans, this stranger would witness the onset of life, the progression of life in children and adults, and the death of the elderly. From this information, the alien could put the stages of human development into their natural sequence. Based on the relative abundance of humans in each stage of development, it would even be possible to conclude that humans spend more of their lives as adults than as toddlers. Similarly, astronomers have pieced together the life story of stars. The first stars probably formed about 300 million years after the Big Bang. The most massive nebulae were the birthplaces of the first stars because their immense gravity caused them to collapse quickly. As a result, the stars that formed early in the history of the universe, often referred to as first-generation stars, were very massive. First-generation stars consisted mostly of hydrogen, with lesser amounts of helium, the primary elements formed during the Big Bang. Massive stars have relatively short lifetimes, followed by violent, explosive deaths. These explosions create the heavier elements and expel them into space. Some of this

ejected matter is incorporated into subsequent generations of stars such as our Sun. Every stage of a star’s life is ruled by gravity. The mutual gravitational attraction of particles in a thin, gaseous nebula causes the cloud to collapse on itself. As the cloud is squeezed to unimaginable pressures, its temperature increases, eventually igniting its nuclear furnace, and a star is born. A star is a ball of very hot gases, caught between the opposing forces of gravity trying to contract it and thermal nuclear energy trying to expand it. Eventually, all of a star’s nuclear fuel will be exhausted, and gravity will prevail, collapsing the stellar remnant into a small, dense body.

Stellar Birth The birthplaces of stars are interstellar clouds, rich in dust and gases (see Figure 24.4). In the Milky Way, these gaseous clouds are about 92 percent hydrogen, 7 percent helium, and less than 1 percent heavier elements. If these thin gaseous clouds become sufficiently concentrated, they begin to gravitationally contract (FIGURE 24.9 ). A mechanism that may trigger star formation is a shock wave from a catastrophic explosion (supernova) of a nearby star. Slow dissipation of thermal energy is also thought to cause nebulae to collapse. Regardless of how the process is initiated, once it begins, mutual gravitational attraction of the particles causes the cloud to contract, pulling every particle toward the center.

FIGURE 24.9 H-R Diagram Illustrating the Evolution of a Sun-Like Star GIANT STAGE

–5 VARIABLE STAGE Absolute magnitude

0 Protostar

Dust and gases

+5 Main-sequence star

+10

PLANETARY NEBULA STAGE

WHITE DWARF STAGE To black dwarf stage

+15

25,000

10,000

7000 Surface temperature (K)

5000

3000

24.4 Stellar Evolution

As the cloud collapses, gravitational energy is converted into energy of motion, or thermal energy, causing the contracting gases to gradually increase in temperature. When the temperature of these gaseous bodies increases sufficiently, they begin to radiate energy in the form of long-wavelength red light. Because these large red objects are not hot enough to engage in nuclear fusion, they are not yet stars. The name protostar is applied to these bodies (see Figure 24.9).

Protostar Stage During the protostar stage, gravitational contraction continues, slowly at first and then much more rapidly. This collapse causes the core of the developing star to heat much faster than its outer envelope. (Stellar temperatures are expressed in kelvin [K]; see Appendix A.) When the core reaches a temperature of 10 million K, the pressure within is so intense that groups of four hydrogen nuclei (through a several-step process) fuse into a single helium nucleus. Astronomers refer to this nuclear reaction, in which hydrogen nuclei are fused into helium, as hydrogen fusion. The immense amount of heat released by hydrogen fusion causes the gases inside stars to move with increased vigor, raising the internal gas pressure. At some point, the increased atomic motion produces an outward force (gas pressure) that balances the inward-directed force of gravity. Upon reaching this balance, the stars become stable mainsequence stars (see Figure 24.9). In other words, a mainsequence star is one in which the force of gravity, in an effort to squeeze the star into the smallest possible ball, is precisely balanced by gas pressure created by hydrogen fusion in the star’s interior.

Main-Sequence Stage During the main-sequence stage, stars experience minimal changes in size or energy output. Hydrogen is continually being converted into helium, and the energy released

maintains the gas pressure sufficiently high to prevent gravitational collapse. How long can stars maintain this balance? Hot, massive blue stars radiate energy at such an enormous rate that they substantially deplete their hydrogen fuel in only a few million years, approaching the end of their main-sequence stage rapidly. By contrast, the smallest (red) main-sequence stars may take hundreds of billions of years to burn their hydrogen; they live practically forever. A yellow star, such as the Sun, typically remains a main-sequence star for about 10 billion years. Because the solar system is about 5 billion years old, the Sun is expected to remain a stable main-sequence star for another 5 billion years. An average star spends 90 percent of its life as a hydrogen-burning main-sequence star. Once the hydrogen fuel in a star’s core is depleted, the star evolves rapidly and dies. However, with the exception of the least-massive stars, death is delayed when another type of nuclear reaction is triggered and the star becomes a red giant (see Figure 24.9).

Red Giant Stage Evolution to the red giant stage begins when the usable hydrogen in a star’s interior is consumed, leaving a heliumrich core. Although hydrogen fusion is still progressing in the star’s outer shell, no fusion is taking place in the core. Without a source of energy, the core no longer has the gas pressure necessary to support itself against the inward force of gravity. As a result, the core begins to contract. The collapse of a star’s interior causes its temperature to rise rapidly as gravitational energy is converted into thermal energy. Some of this energy is radiated outward, initiating a more vigorous level of hydrogen fusion in the shell surrounding the star’s core. The additional heat from the accelerated rate of hydrogen fusion expands the star’s outer gaseous shell enormously. Sun-like stars become bloated red giants, while the most massive stars become

EYE ON THE EY

UNIVERSE U

The accompanying image shows the remnant T o a supernova that Johannes Kepler observed of in 1604. The red, green, and blue show low-, intermediate-, and high-energy x-rays observed by inter NASA’s Chandra X-ray Observatory. QU ESTIO N 1 If a supernova explosion were to occur within the immediate vicinity of our solar system, what might be some possible consequences of the intense x-ray and gamma radiation that would reach Earth? Q UESTION 2 Explain the difference between a nova and a supernova.

NASA

747

748

CHAPTER 24

Beyond Our Solar System

supergiants, which are thousands of times larger than their main-sequence size. As a star expands, its surface cools, which explains the star’s color: Relatively cool objects radiate more energy as long-wavelength radiation (nearer the red end of the visible spectrum). Eventually the star’s gravitational force stops this outward expansion, and the two opposing forces—gravity and gas pressure—once again achieve balance. The star enters a stable state but is much larger in size. Some red giants overshoot the equilibrium point and instead rebound like an overextended spring. These stars, which alternately expand and contract, and never reach equilibrium, are known as variable stars. While the outer envelope of a red giant expands, the core continues to collapse, and the internal temperature eventually reaches 100 million K. This astonishing temperature triggers another nuclear reaction, in which helium is converted to carbon. At this point, a red giant consumes both hydrogen and helium to produce energy. In stars more massive than the Sun, other thermonuclear reactions occur that generate the elements on the periodic table up to and including number 26, iron.

Birth

SmartFigure 24.10

Burnout and Death What happens to stars after the red giant phase? We know that stars, regardless of their size, must eventually exhaust their usable nuclear fuel and collapse in response to their immense gravity. Because the gravitational field of a star is dependent on its mass, low-mass stars and high-mass stars have different fates.

Death of Low-Mass Stars Stars less than one-half the mass of the Sun (0.5 solar mass) consume their fuel at relatively low rates (FIGURE 24.10A ). Consequently, there are many small, cool red stars that may remain stable for as long as 100 billion years. Because the interiors of low-mass stars never attain sufficiently high temperatures and pressures to fuse helium, their only energy source is hydrogen fusion. Thus, a low-mass star never becomes a bloated red giant. Rather, it remains a stable main-sequence star until it consumes its usable hydrogen fuel and collapses into a hot, dense white dwarf.

Stellar Stage

Death

Stages of Stars EEvolutionary l Having Various Masses

Nebula

Protostar

A. Low-mass stars

Nebula

Protostar

Main-sequence star

Main-sequence star

White dwarf

Red giant

B. Medium-mass (Sun-like) stars

Planetary nebula

White dwarf

Neutron star

or

Nebula

C. High-mass stars

Protostar

Main-sequence star

Red supergiant

Supernova explosion

Black hole

749 FIGURE 24.11 Crab Nebula in the Constellation Taurus This

Death of Medium-Mass (Sun-Like) Stars Stars with masses ranging between one-half and eight times that of the Sun have a similar evolutionary history (FIGURE 24.10B ). During their red giant phase, Sun-like stars fuse hydrogen and helium fuel at accelerated rates. Once this fuel is exhausted, these stars (like low-mass stars) collapse into Earth-size bodies of great density—white dwarfs. Without a source of nuclear energy, white dwarfs become cooler and dimmer as they continually radiate thermal energy into space. During their collapse from red giants to white dwarfs, medium-mass stars cast off their bloated outer atmosphere, creating an expanding spherical cloud of gas. The remaining hot, central white dwarf heats the gas cloud, causing it to glow. Recall that these spectacular spherical clouds are called planetary nebulae (see Figure 24.6).

Death of Massive Stars In contrast to Sun-like stars, which expire nonviolently, stars that are more than eight times the mass of the Sun have relatively short life spans and terminate in brilliant explosions called supernovae (FIGURE 24.10C ). During supernova events, these stars become millions of times brighter than they were in prenova stages. If a star located near Earth produced such an outburst, its brilliance would surpass that of the Sun. Fortunately for us, supernovae are relatively rare events; none have been observed in our galaxy since the advent of the telescope, although Tycho Brahe and Johannes Kepler each recorded one, about 30 years apart, late in the sixteenth century. Chinese astronomers recorded an even brighter supernova in a.d. 1054. Today, the remnant of that great outburst is the Crab Nebula, shown in FIGURE 24.11 . A supernova event is triggered when a massive star has consumed most of its nuclear fuel. Without a heat engine to generate the gas pressure required to balance its immense gravitational field, it collapses. This implosion is enormous, resulting in a shock wave that rebounds out from the star’s

|

spectacular nebula is thought to be the remains of the supernova of A.D. 1054. (Courtesy of NASA)

interior. This energetic shock wave blasts the star’s outer shell into space, generating the supernova event. Theoretical work predicts that during a supernova, the star’s interior condenses into an incredibly hot object, possibly no larger than 20 kilometers (12 miles) in diameter. These incomprehensibly dense bodies have been named neutron stars. Some supernova events are thought to produce even smaller and more intriguing objects called black holes. We consider the nature of neutron stars and black holes in the following section.

24.4 CONCEPT CHECKS 1 What element is the fuel for main-sequence stars? 2 Describe how main-sequence stars become giants. 3 Why are less massive stars thought to age more slowly than more massive stars, despite the fact they have much less “fuel”?

4 List the steps thought to be involved in the evolution of Sunlike stars.

24.5 STELLAR REMNANTS Compare and contrast the final state of Sun-like stars to the remnants of the most massive stars.

Eventually, all stars consume their nuclear fuel and collapse into one of three celestial objects—white dwarfs, neutron stars, or black holes. How a star’s life ends, and what final form it takes, depends largely on the star’s mass during its main-sequence stage (TABLE 24.1 ).

White Dwarfs After low- and medium-mass stars consume their remaining fuel, gravity causes them to collapse into white dwarfs. The density of these Earth-sized objects, having masses

TABLE 24.1

Summary of Evolution for Stars of Various Masses

Initial Mass of Main-Sequence Star (Sun = 1)* 0.001

Main-Sequence Stage

Giant Phase

Evolution After Giant Phase

Terminal State (Final Mass)

None (planet)

No

Not applicable

Planet (0.001)

0.1

Red

No

Not applicable

White dwarf (0.1)

1–3

Yellow

Yes

Planetary nebula

White dwarf (,1.4)

8

White

Yes

Supernova

Neutron star (1.4–3)

25

Blue

Yes (supergiant)

Supernova

Black hole (.3)

*These mass numbers are estimates.

750

CHAPTER 24

Beyond Our Solar System

FIGURE 24.12 Crab Pulsar: A Young Neutron Star Centered in the Crab Nebula This is the first pulsar to have been associated with a supernova. The energy emitted from this star illuminates the Crab Nebula. (Courtesy of NASA)

roughly equal to the Sun, may be 1 million times greater than water. A spoonful of such matter would weigh several tons on Earth. Densities of this magnitude are possible only when electrons are displaced inward from their regular orbits around an atom’s nucleus. Material in this state is called degenerate matter. The atoms in degenerate matter have been squeezed together so tightly that the electrons are pushed very close to the nucleus. However, the electrical repulsion that occurs between the negatively charged electrons supports the star against complete gravitational collapse. As main-sequence stars contract into white dwarfs, their surfaces become extremely hot, sometimes exceeding 25,000K. Without sources of energy, main-sequence stars slowly cool and eventually become small, cold, burned-out embers called black dwarfs. However, our galaxy is not yet old enough for any black dwarfs to have formed.

Neutron Stars A study of white dwarfs produced a surprising conclusion: The smallest white dwarfs are the most massive, and the largest are the least massive. Researchers have discovered that more massive stars, because of their greater gravitational fields, are squeezed into smaller, more densely packed objects than less massive stars. Thus, the smallest white dwarfs were produced from the collapse of larger, more massive main-sequence stars than are the largest white dwarfs. This conclusion led to the prediction that stellar remnants even smaller and more massive than white dwarfs must exist. Named neutron stars, these objects are the remnants of explosive supernova events. In white dwarfs, the electrons are pushed close to the nucleus, whereas in neutron stars, the electrons are forced to combine with

protons in the nucleus to produce neutrons (hence the name neutron star). A pea-size sample of this matter would weigh 100 million tons. This is approximately the density of an atomic nucleus; thus, a neutron star can be thought of as a large atomic nucleus, composed entirely of neutrons. During a supernova implosion, the outer envelope of the star is ejected, while the core collapses into a very hot star that is only about 20 to 30 kilometers (12 to 18 miles) in diameter. Although neutron stars have high surface temperatures, their small size greatly limits their luminosity, making them nearly impossible to locate visually. Theoretical models predict that neutron stars have a very strong magnetic field and a high rate of rotation. As stars collapse, they rotate faster, for the same reason ice skaters rotate faster as they pull their arms in as they spin. Radio waves generated by the rotating magnetic fields of neutron stars are concentrated into two narrow zones that align with the star’s magnetic poles. Consequently, these stars resemble rapidly rotating beacons emitting strong radio waves. If Earth happened to be in the path of these beacons, the star would appear to blink on and off, or pulsate, as the waves swept past. In the early 1970s, a source that radiates short pulses of radio energy named a pulsar (pulsating radio source) was discovered in the Crab Nebula (FIGURE 24.12 ). Visual inspection of this radio source indicated that it was coming from a small star centered in the nebula. The pulsar found in the Crab Nebula is most likely the remains of the supernova of a.d. 1054 (see Figure 24.11).

Black Holes Although neutron stars are extremely dense, they are not the densest objects in the universe. Stellar evolutionary theory predicts that neutron stars cannot exceed three times the mass of the Sun. Above this mass, not even tightly packed neutrons can withstand the star’s gravitational pull. Following supernova explosions, if the core of a remaining star exceeds three solar masses, gravity prevails, and the stellar remnant collapses into an object that is denser than a neutron star. (The pre-supernova mass of such a star was likely 25 times that of our Sun.) The incredible objects, or celestial phenomena, created by such a collapse are called black holes. Einstein’s theory of general relativity predicts that even though black holes are extremely hot, their surface gravity is so immense that even light cannot escape. Consequently, they literally disappear from sight. Anything that moves too close to a black hole can be swept in and devoured by its immense gravitational field. How do astronomers find objects whose gravitational field prevents the escape of all matter and energy? Theory predicts that as matter is pulled into a black hole, it should become extremely hot and emit a flood of x-rays before it is engulfed. Because isolated black holes do not have a source of matter to engulf, astronomers decided to look at binary-star

24.6 Galaxies and Galactic Clusters

751

solar masses. Since the discovery of Cygnus X-1, many other x-ray sources have been discovered that are assumed to Giant be black holes. Jet companion Astronomers have estabstar lished that black holes are common objects in the universe and vary considerably in size. Small black holes have masses approximately 10 times that of our Sun but are only about 32 kilometers (20 miles) across, Accretion disc around black hole less than the distance of a marathon course. Intermediate black holes have masses 1000 times our Sun, and the largest black holes (supermassive black holes), found in the centers of galaxies, are estimated FIGURE 24.13 Artist’s Depiction of a Black Hole and a Giant Companion Star Note the to be millions of solar masses. accretion disk surrounding the black hole. (Courtesy of European Southern Observatory/L. Calcada/M. Kornmesser) Because the earliest stars were thought to be massive, their systems for evidence of matter emitting x-rays while being deaths could have provided the seeds that eventually formed rapidly swept into a region of apparent nothingness. the supermassive black holes at the centers of galaxies. X-rays cannot penetrate our atmosphere; therefore, the existence of black holes was not confirmed until the advent 24.5 CONCEPT CHECKS of orbiting observatories. The first black hole to be identified, Cygnus X-1, orbits a massive supergiant companion 1 Describe degenerate matter. once every 5.6 days. The gases that are pulled from the com2 What is the final state of a medium-mass (Sun-like) star? panion form an accretion disk that spirals around a “void” 3 How do the “lives” of the most massive stars end? What are while emitting a steady stream of x-rays (FIGURE 24.13 ). the two possible products of this event? Recent observations have determined that pairs of jets extend 4 Explain how it is possible for the smallest white dwarfs to be outward from these accretion disks and are thought to return the most massive. some of this material back to space (see Figure 24.13). 5 Black holes are thought to be abundant, yet they are hard to Cygnus X-1, which is about 8 or 9 times as massive as find. Explain why. our Sun, probably formed from a star of approximately 40

|

24.6 GALAXIES AND GALACTIC CLUSTERS List the three major types of galaxies. Explain the formation of large elliptical galaxies.

FIGURE 24.14 View of Our Milky Way Galaxy at Sunset The dark patches in the “milky” band of light are caused by the presence of dark nebulae. (Courtesy of ESO/ European Southern Observatory)

On a clear and moonless night away from city lights, you can see a truly marvelous sight—a band of light stretching from horizon to horizon. With his telescope, Galileo discovered that this band of light was composed of countless individual stars. Today, we realize that the Sun is actually a part of this vast system of stars, the Milky Way Galaxy (FIGURE 24.14 ). Galaxies (galaxias = milky) are collections of interstellar matter, stars, and stellar remnants that are gravitationally bound (see Figure 24.14). Recent observational data indicate that supermassive black holes may exist at the centers of most galaxies. In addition, spherical halos of very tenuous gas and numerous star clusters (globular clusters) appear to surround many of the largest galaxies.

The Milky Way derives it name from its appearance as a dim, “milky” glowing band that arches across the night sky. In this magnificent 360-degree panoramic image we see the plane of our galaxy, edge-on from our perspective on Earth. From this vantage point, the components of the Milky Way come into view. We can see the galaxy’s bright central bulge, its disc that contains both dark and glowing nebulae that harbor bright, young stars. Also visible adjacent to the Milky Way are a few of its satellite galaxies. ESO

f old stars and hot g lo o as a H

Galactic disc

This profile view shows that, in addition to its dense central bulge and galactic disc containing spiral arms, the Milky Way is surrounded by a spherical halo that extends far beyond the galactic disc. Recent evidence indicates that the halo contains a large amount of hot gas, but lacks star formation. The halo also contains old stars and numerous large stellar groups called globular clusters.

Central bulge

Solar system Globular clusters

The age of the stars in globular cluster NGC 6397 are more than 13 billion years old, which confirms that the Milky Way is among the oldest of galaxies.

NGC 6397

ESO

GEOGRAPHICS

The Milky Way

Scutum-Centaurus Arm

An artist’s conception of the Milky Way Galaxy showing its dense central bulge and flat disc consisting of curved spiral arms. Our galaxy is a barred-spiral type that contains more than 200 billion stars. Its diameter exceeds 100,000 light-years and it rotates such that our solar system makes one complete trip around the galactic center every 250 million years. The galactic center of our galaxy houses a supermassive black hole with a mass of at least 40,000 Suns. Like the black holes found in the center of most large galaxies this behemoth tries to consume anything that happens to be nearby.

Supermassive Black Hole

Central Bulge

Outer Arm

Sagittarius Arm

Orion Spur

Solar System

Perseus Arm

NASA

Our galaxy is just one of an assemblage of more than 50 galaxies that collectively make up the Local Group. Members include Andromeda Galaxy, which is an even larger spiral galaxy than the Milky Way. For many years astronomers thought that the Magellanic Clouds, that can be seen as fuzzy patches in the southern sky, were our closest galactic neighbor. But, in fact, the closest galaxy so far discovered, named Canis Major Dwarf Galaxy, lies within our galaxy. Astronomers have recently concluded that the Milky Way grew to its current size by “eating up” dwarf galaxies like Canis Major.

Large Magellanic Cloud

ESO

Small Magellanic Cloud

754

CHAPTER 24

FIGURE 24.15 Dramatic Image of the Spiral Galaxy Messier 83 Although

Types of Galaxies Among the hundreds of billions of galaxies, three basic types have been identified: spiral, elliptical, and irregular. Within each of these categories are many variations, the causes of which are still a mystery.

smaller, Messier 83 is thought to be very similar to the Milky Way Galaxy. (ESO/ European Space Observatory)

Spiral Galaxies Our Milky

The first galaxies were small and composed mainly of massive stars and abundant interstellar matter. These galaxies grew quickly by accreting nearby interstellar matter and by colliding and merging with other galaxies. In fact, our galaxy is currently absorbing at least two tiny satellite galaxies.

A. Face-on view

Way Galaxy is an example of a large spiral galaxy (FIGURE 24.15 ). Spiral galaxies are flat, disk-shaped objects that range from 20,000 to about 125,000 light-years in diameter. Typically, spiral galaxies have a greater concentration of stars near their centers, but there are numerous variations. As shown in FIGURE 24.16 , spiral galaxies have arms (usually two) extending from the central nucleus. Spiral galaxies rotate rapidly in the center, while the outermost stars rotate more slowly, which gives these galaxies the appearance of a fireworks pinwheel. Generally, the central bulge contains older stars that give it a yellowish color, while younger hot stars are located in the arms. The young, hot stars in the arms are found in large groups that appear as bright patches of blue and violet light shown in Figure 24.15. Many spiral galaxies have a band of stars extending outward from the central bulge that merges with the spiral arms. These are known as barred spiral galaxies (FIGURE 24.17 ). Recent investigations have found evidence that our galaxy probably has a bar structure. What produces these bar-shaped structures is a matter of ongoing research.

B. Edge-on view

SmartFigure 24.16 Spiral Galaxies A. Spiral galaxies typically have a greater concentration of older stars near their center, which gives the central bulge its yellowish color. By contrast, the arms of spiral galaxies contain numerous hot, young stars that give these structures a bluish or violet tint. B. Edge-on view showing the central bulge. C. Surrounding most large galaxies are spherical halos of very tenuous gases and groups of stars called globular clusters. This large globular cluster contains an estimated 10 million stars. (Image A courtesy of NASA; images B and C courtesy of ESO/European Southern Observatory)

C. Globular cluster

24.6 Galaxies and Galactic Clusters

Large elliptical galaxies tend to be composed of older, low-mass stars (red) and have minimal amounts of interstellar matter. Thus, unlike the arms of spiral galaxies, they have low rates of star formation. As a result, elliptical galaxies appear yellow to red in color, as compared to the bluish tint emanating from the young, hot stars in the arms of spiral galaxies.

Irregular Galaxies Approximately 25 percent of

FIGURE 24.17 Barred Spiral Galaxy (Courtesy of NASA, ESA, and the Hubble Heritage Team [STSci/AURA])

Elliptical Galaxies As the name implies, elliptical galaxies have an ellipsoidal shape that can be nearly spherical, and they lack arms (FIGURE 24.18 ). Some of the largest and the smallest galaxies are elliptical. The smallest of these are known as dwarf galaxies. The two small companions of Andromeda shown in Figure 24.2 are dwarf galaxies. The very largest known galaxies (1 million light-years in diameter) are also elliptical. For comparison, the Milky Way, a large spiral galaxy, is about one-half the diameter of a large elliptical galaxy. Most large elliptical galaxies are believed to result from the merger of two or more smaller galaxies.

known galaxies show no symmetry and are classified as irregular galaxies. Some were once spiral or elliptical galaxies that were subsequently distorted by the gravity of a larger neighbor. Two well-known irregular galaxies, the Large and Small Magellanic Clouds, are named for explorer Ferdinand Magellan, who observed them when he circumnavigated the globe in 1520. They are among our nearest galactic neighbors. Recent images of the Large Magellanic Cloud revealed a central bar-shaped structure. Thus, the Large Magellanic Cloud was once a barred spiral galaxy that was subsequently distorted by gravity exerted by another galaxy as it passed by.

Galactic Clusters Once astronomers discovered that stars occur in groups (galaxies), they set out to determine whether galaxies also occur in groups or whether they are just randomly distributed. They found that galaxies are grouped into gravitationally bound clusters (FIGURE 24.19 ). Some large galactic clusters contain thousands of galaxies. Our own galactic

FIGURE 24.18 Large Elliptical Galaxy This large elliptical galaxy belongs to the Fornax Cluster. Dark clouds of interstellar matter are visible within the central nucleus of this galaxy. Some of the starlike objects in this image are large groups of stars (globular clusters) that belong to the galaxy. (Courtesy of ESO/European Southern Observatory)

FIGURE 24.19 The Fornax Galaxy Cluster This is one of the nearest groupings of galaxies to our Local Group. Although many of the galaxies shown are elliptical, an elegant barred spiral galaxy is visible in the lower right. (Courtesy of ESO/European Southern Observatory/ J. Emerson/VISTA)

755

756

CHAPTER 24

Galactic Collisions

FIGURE 24.20 The Collision of the Antennae Galaxies When two galaxies collide, the stars generally do not. However, the clouds of dust and gas that are common to both collide. During these galactic encounters, there is a rapid birth of millions of stars, shown as the bright regions in this image. (Courtesy of NASA)

cluster, called the Local Group, consists of more than 40 galaxies and may contain many undiscovered dwarf galaxies. Of the Local Group galaxies, three are large spiral galaxies, including the Milky Way and Andromeda Galaxies. Galactic clusters also reside in huge groups called superclusters. There are possibly 10 million superclusters; our Local Group is found in the Virgo Supercluster. From visual observations, it appears that superclusters are the largest entities in the universe.

|

Within galactic clusters, interactions between galaxies, often driven by one galaxy’s gravity disturbing another, are common. For example, a large galaxy may engulf a dwarf satellite galaxy. In this case, the larger galaxy will retain its form, while the smaller galaxy will be torn apart and assimilated into the larger galaxy. Recall that two dwarf satellite galaxies are currently merging with the Milky Way. Galactic interactions may also involve two galaxies of similar size that pass through one another without merging. It is unlikely that the individual stars within these galaxies will collide because they are so widely dispersed. However, the interstellar matter will likely interact, triggering an intense period of star formation. In an extreme case, two large galaxies may collide and merge into a single large galaxy (FIGURE 24.20 ). Many of the largest elliptical galaxies are thought to have been produced by the merger of two large spiral galaxies. Some studies have predicted that in 2 to 4 billion years, there is a 50 percent probability that the Milky Way and Andromeda Galaxies will collide and merge.

24.6 CONCEPT CHECKS 1 Compare the three main types of galaxies. 2 What type of galaxy is our Milky Way? 3 Describe a possible scenario for the formation of a large elliptical galaxy.

24.7 THE BIG BANG THEORY Describe the Big Bang theory. Explain what it tells us about the universe.

The Big Bang theory describes the birth, evolution, and fate of the universe. According to the Big Bang theory, the universe was originally in an extremely hot, supermassive state that expanded rapidly in all directions. Based on astronomers’ best calculations, this expansion began about 13.7 billion years ago. What scientific evidence exists to support this theory?

Evidence for an Expanding Universe In 1912 Vesto Slipher, while working at the Lowell Observatory in Flagstaff, Arizona, was the first to discover that galaxies exhibit motion. The motions he detected were twofold: Galaxies rotate, and galaxies move relative to each other. Slipher’s efforts focused on the shifts in the spectra of the light emanating from galaxies. (See the section titled “The Doppler Effect” in Chapter 23.) When a source of light is moving away

from an observer, the spectral lines shift toward the red end of the spectrum (longer wavelengths). Conversely, when celestial objects approach the observer, the spectral lines shift to the blue end of the spectrum (shorter wavelengths). In 1929, a study of galaxies conducted by Edwin Hubble expanded the groundwork established by Slipher. Hubble noticed that most galaxies have spectral shifts toward the red end of the spectrum—which occurs when an object emitting light is receding from an observer (FIGURE 24.21 ). Therefore, all galaxies (except those in the Local Group) appear to be moving away from the Milky Way. These patterns were later named cosmological red shifts because the movement they reveal is a result of the expansion of the universe. Recall that Hubble had also found a way to measure galactic distances. By comparing the distance to a galaxy with Vesto Slipher’s measurements of its red shift, Hubble

24.7 The Big Bang Theory

Standard spectral lines (unshifted)

created between two objects located farther apart than between two objects that are closer together. Another feature of the expanding universe can be demonstrated using the same bread analogy. Regardless of which raisin you look at, it will move away from all the other raisins. Likewise, at any point in the universe, all other galaxies (except those in the same cluster) are receding from an observer at that location. Hubble’s law implies a centerless universe that is expanding uniformly. The Hubble Space Telescope is named in honor of Edwin Hubble’s invaluable contributions to the scientific understanding of the universe.

Red shift moves spectral lines to longer wavelengths

Predictions of the Big Bang Theory

FIGURE 24.21 Red Shift Illustration of the shift in spectral lines

Recall from the Introduction that in order for a hypothesis to become an accepted component of scientific knowledge (a theory), it must incorporate predictions that can be tested. One prediction of the Big Bang model is that if the universe was initially unimaginably hot, then researchers should be able to detect the remnant of that heat. The electromagnetic radiation (light) emitted by a white-hot universe would have extremely high energy and short wavelengths. However, according to the Big Bang theory, the continued expansion of the universe would have stretched the waves so that by now they should be detectable as long-wavelength radio waves called microwave radiation. Scientists began to search for this “missing” radiation, which they named cosmic microwave background radiation. As predicted, in 1965, this microwave radiation was detected and found to fill the entire visible universe. Detailed observations of the cosmic microwave background radiation since its original discovery have confirmed many theoretical details of the Big Bang theory, including the order and timing of important events in the early history of the universe.

toward the red end of the spectrum, which occurs when an object emitting light is receding from an observer.

made an unexpected discovery: He discovered that the red shifts of galaxies increase with distance and that the most distant galaxies are receding from the Milky Way at the fastest rate. This concept, now called Hubble’s law, states that galaxies recede at speeds proportional to their distances from the observer. This discovery surprised Hubble because conventional wisdom was that the universe was unchanging and would likely remain unchanged. What cosmological theory could explain Hubble’s findings? Researchers concluded that an expanding universe accounts for the observed red shifts. To help visualize why Hubble’s law implies an expanding universe, imagine a batch of raisin bread dough that has been set out to rise for a few hours (FIGURE 24.22 ). In this analogy, the raisins represent galaxies, and the dough represents space. As the dough doubles in size, so does the distance between all the raisins. The distance between raisins that were originally 2 centimeters apart will become 4 centimeters, while the distance between raisins originally 6 centimeters apart will increase to 12 centimeters. The raisins that were originally farthest apart travel greater distances than those located closer together. Therefore, in an expanding universe, as in our analogy, more space is

What Is the Fate of the Universe?

Cosmologists have developed different scenarios for the ultimate fate of the universe (FIGURE 24.23 ). In one scenario, the stars will slowly burn out and be replaced by invisible degenerate matter and black holes that travel outward through an endless, dark, cold universe. This scenario is sometimes called the Big Chill because the universe will slowly cool 4 cm 12 cm as it expands, to the point that it is unable to sustain life. 2 cm Another possibility is that the 6 cm outward flight of the galaxies will slow and eventually stop. Gravitational contraction would follow, causing all matter to A. Raisin bread dough before B. Raisin bread dough a few hours later. it rises. eventually collide and coalesce into the high-energy, highSmartFigure 24.22 Raisin Bread Analogy for an Expanding density state, from which the Universe U i As the dough rises, raisins originally farthest apart travel greater universe began. This fiery death distances than those located closer together. Thus, in an expanding universe of the universe, the Big Bang (as with the raisins), more space is created between two objects that are operating in reverse, has been farther apart than between two objects that are closer together. called the Big Crunch.

757

758

CHAPTER 24

Beyond Our Solar System

Possible Fates of the Universe Big Bang

same speed. This implies that something surrounding our galaxy is tugging on the stars. This yet undetected material was named dark matter. Approximately one-quarter of the universe consists of dark matter, which produces no detectable light energy but exerts a gravitational force that pulls on all “visible” matter in the universe. Thus, dark matter exerts a force that helps hold our galaxy together and at the same time works to slow the expansion of the universe as a whole. Although the concept of dark matter may sound foreboding, it simply allows for the possibility that matter exists that does not interact with electromagnetic radiation. Recall that most of our knowledge about the universe comes to us via light. If there is a form of matter that does not interact with light, we will not be able to “see” it—hence the term dark matter.

Dark Energy In the early 1990s, most cosmologists held the view that gravity was certain to slow the Current Universe expansion of the universe over time, resulting eventually in the Big Crunch. However, in 1998 observations of very distant galaxies by the Hubble Space Telescope showed that the universe is actually expanding faster today than it was early in its history. Therefore, the expansion of the universe was not slowing due to gravity as scientists thought; rather, it was accelerating. To explain this unexpected result, researchers concluded that some unusual material, generally referred to as dark energy, must exist. Unlike dark Big Crunch Big Chill Indefinite expansion the result of Gravitational collapse of the matter, which works to slow the expansion of the unithe dominance of dark energy universe caused by the dominance verse, dark energy exerts a force that pushes matter pushing the universe apart of dark matter pulling it together outward, causing the expansion to speed up. It has not been determined whether dark matter FIGURE 24.23 Cosmic and dark energy are related—or exactly what they are. Most Whether the universe will expand forever or eventually colTug of War Illustration researchers think that dark matter consists of a type of subalapse on itself is contingent on its density. If the average density showing two possible fates tomic particle that has not yet been detected. Dark energy may of the universe is greater than an amount known as its critical of the universe. The gravity have its own particle, but there is no evidence of its existence. density (about one atom for every cubic meter), gravitational of dark matter tries to pull There is growing consensus among cosmologists that the universe together, while attraction would be sufficient to stop the outward expansion and dark energy tries to push dark energy, which is propelling the universe outward, is the cause the universe to collapse. On the other hand, if the density it apart. There is growing dominant force. If dark energy is, in fact, the driving force of the universe is less than the critical value, the universe will consensus that dark energy behind the fate of the universe, the universe will expand forcontinue to expand forever. Complicating the possibilities in the will prevail and produce an ever (see Figure 24.23). Consider the following as astronofate of the universe are two other types of matter that may exist. ever-expanding universe. mers search for dark energy: “Absence of evidence is not These are called dark matter and dark energy. evidence of absence.” Dark Matter The universe contains perhaps 100 billion galaxies, each with billions of stars, massive clouds of gas and dust, and large numbers of planets, moons, and other debris. 24.7 CONCEPT CHECKS Yet everything we see is like the tip of the cosmic iceberg; it 1 In your own words, explain how astronomers determined accounts for less than 5 percent of the total mass of the unithat the universe is expanding. verse. Astronomers came to this conclusion when studying the 2 What did the Big Bang theory predict that was finally rotational periods of stars as they orbit the center of the Milky confirmed years after it was formulated? Way Galaxy. The law of gravity states that the stars closest 3 Which view of the fate of the universe is currently favored: to galactic center should travel faster than those near the galthe Big Crunch or the Big Chill? axy’s outer edge. (This is the reason Mercury travels around 4 What property does the universe possess that will the Sun at a much faster speed than Pluto.) Yet these researchdetermine its final state? ers found that all stars orbit the galactic center at roughly the

24

CONCEPTS IN REVIEW

| Beyond Our Solar System

24.1 THE UNIVERSE Define cosmology and describe Edwin Hubble’s most significant discovery about the universe. KEY TERMS: cosmology, absolute magnitude, light-year, Big Bang theory ■ ■ ■

Cosmology is the study of the universe, including its properties, structure, and evolution. The universe consists of hundreds of billions of galaxies, each containing billions of stars. The model that most accurately describes the birth and current state of the universe is the Big Bang theory. According to this model, the universe began about 13.7 billion years ago, in a cataclysmic explosion, and then it continued to expand, cool, and evolve to its current state.

24.2 INTERSTELLAR MATTER: NURSERY OF THE STARS Explain why interstellar matter is often referred to as a stellar nursery. Compare and contrast bright and dark nebulae. KEY TERMS: interstellar matter, nebula, bright nebula, dark nebula, emission nebula, reflection nebula,

planetary nebula ■ ■

New stars are born out of enormous accumulations of dust and gases, called nebulae, which are scattered between existing stars. Emission nebulae derive their visible light from nearby hot stars or stars that are embedded in them. Reflection nebulae contain comparatively large debris, including grains of carbon compounds that reflect the light of nearby stars. Planetary nebulae consist of glowing clouds of dust and hot gases that have been expelled near the end of a star’s life. Nebulae that are too distant from bright stars to be illuminated are referred to as dark nebulae.

Q Based on color, what type of nebulae is likely shown in the accompanying image? National Optical Astronomy Observatory (NOAO)

24.3 CLASSIFYING STARS: HERTZSPRUNG–RUSSELL DIAGRAMS (H-R DIAGRAMS) Define main-sequence star. Explain the criteria used to classify stars as giants.

A

supergiant

■

Hertzsprung–Russell diagrams are constructed by plotting the absolute magnitudes and temperatures of stars on graphs. Considerable information about stars and stellar evolution has been discovered through the use of H-R diagrams. Stars are positioned within H-R diagrams as follows: (1) Main-sequence stars—90 percent of all stars—are in the band that runs from the upper-left corner (massive, hot blue stars) to the lower-right corner (low-mass, red stars); (2) red giants and supergiants—very luminous stars with large radii—are located in the upper-right position; and (3) white dwarfs—small, dense stars—are located in the lower portion.

Q Identify the type of stars located in the positions labeled A, B, and C on the

B Luminosity

■

C

Absolute magnitude

KEY TERMS: Hertzsprung–Russell diagram (H-R diagram), main-sequence star, red giant,

Surface temperature

accompanying H-R diagram.

759

760

Concepts in Review

24.4 STELLAR EVOLUTION List and describe the stages in the evolution of a typical Sun-like star. K EY TERMS: protostar, hydrogen fusion, variable star, supernova ■ ■

■ ■

Stars are born when their nuclear furnaces are ignited by the unimaginable pressures and temperatures generated during the collapse of nebulae. Red star-like objects not yet hot enough for nuclear fusion are called protostars. When the core of a protostar reaches a temperature of about 10 million K, a process called hydrogen fusion begins, marking the birth of the star. Hydrogen fusion involves the conversion of four hydrogen nuclei into a single helium nucleus and the release of thermal nuclear energy. Two opposing forces act on a star: gravity, which tries to contract it into the smallest possible ball, and gas pressure (created by thermal nuclear energy), which tries to blow it apart. When the forces are balanced, the star becomes a stable main-sequence star. Medium- and high-mass stars experience another type of nuclear fusion that causes their outer envelopes to expand enormously (hundreds to thousands of times larger), making them red giants or supergiants. When a star exhausts all its usable nuclear fuel, gravity takes over, and the stellar remnant collapses into a small, dense body.

24.5 STELLAR REMNANTS Compare and contrast the final state of Sun-like stars to the remnants of the most massive stars. K EY TERMS: white dwarf, degenerate matter, neutron star, pulsar, black hole ■ ■ ■ ■

The final fate of a star is determined by its mass. Stars with less than one-half the mass of the Sun collapse into hot, dense white dwarfs. Medium-mass stars, like the Sun, become red giants, collapse, and end up as white dwarfs, often surrounded by expanding spherical clouds of glowing gas called planetary nebulae. Massive stars terminate in a brilliant explosion called a supernova. Supernova events can produce small, extremely dense neutron stars, composed entirely of neutrons, or smaller, even denser black holes—objects that have such immense gravity that light cannot escape their surface.

24.6 GALAXIES AND GALACTIC CLUSTERS List the three major types of galaxies. Explain the formation of large elliptical galaxies. K EY TERMS: spiral galaxy, barred spiral galaxy, elliptical galaxy, dwarf galaxy, irregular ■

■

The various types of galaxies include (1) irregular galaxies, which lack symmetry and account for about 25 percent of the known galaxies; (2) spiral galaxies, which are disk shaped and have a greater concentration of stars near their centers and arms extending from their central nucleus; and (3) elliptical galaxies, which have an ellipsoidal shape that may be nearly spherical. Galaxies are grouped in galactic clusters, some containing thousands of galaxies. Our own, called the Local Group, contains at least 40 galaxies.

Q What type of galaxy is shown in the accompanying image?

(NASA)

galaxy, galactic cluster, Local Group

24.7 THE BIG BANG THEORY Describe the Big Bang theory. Explain what it tells us about the universe. K EY TERMS: cosmological red shift, Hubble’s law, dark matter, dark energy ■ ■

Evidence for an expanding universe came from the study of red shifts in the spectra of galaxies. Edwin Hubble concluded that the observed red shifts, called cosmological red shifts, result from the expansion of space. This evidence strongly supports the Big Bang model of an expanded universe. One question that remains is whether the universe will expand forever in a Big Chill or gravitationally contract in a Big Crunch. Dark matter slows the expansion of the universe, while dark energy exerts a force that pushes matter outward, causing the expansion to speed up. Most cosmologists favor an endless, ever-expanding universe.

Give It Some Thought

761

GIVE IT SOME THOUGHT 1. Assume that NASA is sending a space probe to each of the following locations:

5. Refer to the accompanying images (A, B, C, and D) to complete the following:

a. Polaris (the North Star)

a. Which of these nebulae, if any, is an emission nebula?

b. A comet near the outer edge of our solar system

b. Which of these nebulae, if any, formed near the end of a star’s lifetime?

c. Jupiter

c. Which of these nebulae, if any, is a reflection nebula?

d. The far edge of the Milky Way Galaxy e. The near side of the Andromeda Galaxy f. The Sun

List the locations in order, from nearest to farthest. 2. Use the information provided below about three main-sequence stars (A, B, and C) to complete the following and explain your reasoning: ■

Star A has a main-sequence life span of 5 billion years.

■

Star B has the same luminosity (absolute magnitude) as the Sun.

■

Star C has a surface temperature of 5000K.

a. Rank the mass of these stars from greatest to least. b. Rank the energy output of these stars from greatest to least. c. Rank the main-sequence life span of these stars from longest to shortest.

3. The masses of three clouds of gas and dust (nebulae) are provided below. Imagine that each cloud will collapse to form a single star. Use this information to complete the following and explain your reasoning. ■

Cloud A is 60 times the mass of the Sun.

■

Cloud B is 7 times the mass of the Sun.

■

Cloud C is 2 times the mass of the Sun.

a. Which cloud or clouds, if any, will evolve into a red main-sequence star? b. Which of the stars that will form from these clouds, if any, will reach the giant stage? c. Which of the stars that will form from these clouds, if any, will go through the supernova stage?

4. The accompanying photo shows the Trifid Nebula, which can be easily observed with a small telescope. What unique properties does this nebula exhibit?

6. How a star evolves is closely related to its mass as a main-sequence star. Complete the accompanying diagram by labeling the evolutionary stages for the three groups of main-sequence stars shown.

762

Concepts in Review

7. Refer to the accompanying photos of an elliptical galaxy and a spiral galaxy to complete the following: a. Which image (A or B) is an elliptical galaxy? b. Which of these galaxies appears to contain more young, hot massive stars? How did you determine your answer?

c. When stars are born from a cloud of dust and gases, large and small stars form at about the same time. Which group of stars, the large or the small stars, will die out first? Over time, how will this affect the color of the light we observe coming from this group of stars? Based on your response, which of these galaxies appears to be older? Explain.

8. Can a hypothesis or a theory describe (a) the past, (b) the present, and (c) the fate of the universe? Describe in your own words what this hypothesis or theory is. In case a theory is used, what evidences allow us to describe the evolution of the universe?

EXAMINING THE EARTH SYSTEM 1. Briefly describe how the atmosphere, hydrosphere, geosphere, and biosphere are each related to the death of stars that occurred billions of years ago. 2. What are the most significant interactions occurring between each of Earth’s major spheres and processes related to the universe. You can consider a source of energy as a source of interaction.

3. Based on your knowledge of the Earth system, the planets in our solar system, and the universe in general, speculate about the likelihood that extra-solar planets exist with atmospheres, hydrospheres, geospheres, and biospheres similar to Earth’s. Explain your speculation.

APPENDIX A |

Metric and English Units Compared  Masses and Weights

Units 1 kilometer (km) 1 meter (m) 1 centimeter (cm) 1 mile (mi) 1 foot (ft) 1 inch (in.) 1 square mile (mi2) 1 kilogram (kg) 1 pound (lb) 1 fathom

= 1000 meters (m) = 100 centimeters = 0.39 inch (in.) = 5280 feet (ft) = 12 inches (in.) = 2.54 centimeters (cm) = 640 acres (a) = 1000 grams (g) = 16 ounces (oz) = 6 feet (ft)

Conversions Length When you want to convert: inches centimeters feet meters yards meters miles kilometers

multiply by: 2.54 0.39 0.30 3.28 0.91 1.09 1.61 0.62

to find: centimeters inches meters feet meters yards kilometers miles

multiply by: 6.45 0.15 0.09 10.76 2.59 0.39

to find: square centimeters square inches square meters square feet square kilometers square miles

multiply by: 16.38 0.06 0.028 35.3 4.17 0.24 1.06 0.26 3.78

to find: cubic centimeters cubic inches cubic meters cubic feet cubic kilometers cubic miles quarts gallons liters

When you want to convert: ounces grams pounds kilograms

multiply by: 28.35 0.035 0.45 2.205

to find: grams ounces kilograms pounds

Temperature When you want to convert degrees Fahrenheit (°F) to degrees Celsius (°C), subtract 32 degrees and divide by 1.8. When you want to convert degrees Celsius (°C) to degrees Fahrenheit (°F), multiply by 1.8 and add 32 degrees. When you want to convert degrees Celsius (°C) to kelvin (K), delete the degree symbol and add 273. When you want to convert kelvin (K) to degrees Celsius (°C), add the degree symbol and subtract 273.

Area When you want to convert: square inches square centimeters square feet square meters square miles square kilometers

Volume When you want to convert: cubic inches cubic centimeters cubic feet cubic meters cubic miles cubic kilometers liters liters gallons

FIGURE A.1 Temperature Scales

763

APPENDIX B |

Relative Humidity and Dew-Point Tables

TABLE B.1 Relative Humidity (Percent)

Dry-Bulb (Air) Temperature

Dry bulb (°C)   −20 −18 −16 −14 −12 −10 –8 –6 –4 –2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Dry-Bulb Temperature Minus Wet-Bulb Temperature = Depression of the Wet Bulb 1 28 40 48 55 61 66 71 73 77 79 81 83 85 86 87 88 88 89 90 91 91 92 92 92 93 93 93 93 94 94 94

2     0 11 23 33 41 48 54 58 63 67 70 72 74 76 78 79 80 81 82 83 84 85 86 86 86 86 87 87 89

3           0 13 20 32 37 45 51 56 59 62 65 67 69 77 72 74 75 76 77 78 79 80 81 81 82 82

4               0 11 20 28 36 42 46 51 54 57 60 62 64 66 68 69 70 71 72 73 74 75 76 76

5                   1 11 20 27 35 39 43 48 50 54 56 58 60 62 64 65 66 68 69 69 70 71

6                       6 14 22 28 38 38 41 45 48 51 53 55 57 59 61 62 63 64 66 67

7                           10 17 24 28 33 37 40 44 46 49 51 53 55 56 58 59 60 61

8 9 10 11                                                                                                 R ela        tive 0     Hu   m 6     id   i t y 13 4   V al 19 10 2   ues 25 16 8 1 29 21 14 7 33 26 19 12 36 30 23 17 40 33 27 21 42 36 30 25 45 39 34 28 45 42 36 31 49 44 39 34 51 46 41 36 52 48 43 38 54 50 44 40 55 51 46 42 57 52 48 44

12                                     1 6 11 15 20 23 26 29 32 34 36 38 40

13                                       0 5 10 14 18 21 25 27 30 32 34 36

14                                           4 9 13 17 20 22 26 28 30 33

15                                           0 4 9 12 16 19 22 24 26 29

16                                             0 5 8 12 14 18 21 23 25

17                                                 4 8 11 14 17 20 22

18                                                   4 8 11 13 16 19

19                                                     4 8 10 13 16

20                                                       5 7 10 13

21                                                         4 7 10

22                                                           5 7

To determine the relative humidity, find the air (dry-bulb) temperature on the vertical axis (far left) and the depression of the wet bulb on the horizontal axis (top). Where the two meet, the relative humidity is found. For example, when the dry-bulb temperature is 20°C and a wet-bulb temperature is 14°C, then the depression of the wet bulb is 6°C (20°C –14°C). From TABLE B.1, the relative humidity is 51 percent and from TABLE B.2, the dew point is 10°C.

TABLE B.2 Dew-Point Temperature (°C)

Dry-Bulb (Air) Temperature

Dry bulb (°C)

764

  −20 −18 −16 −14 −12 −10 –8 –6 –4 –2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Dry-Bulb Temperature Minus Wet-Bulb Temperature = Depression of the Wet Bulb 1 −33 −28 −24 −21 −18 −14 −12 −10 –7 –5 –3 –1 1 4 6 8 10 12 14 16 19 21 23 25 27 29 31 33 35 37 39

2       −36 −28 −22 −18 −14 −12 –8 –6 –3 –1 1 3 6 8 11 13 15 17 19 21 23 25 27 29 31 33 35 37

3             −29 −22 −17 −13 –9 –6 –4 –1 1 4 6 9 11 13 15 17 20 22 24 26 28 30 32 34 36

4                 −29 −20 −15 −11 –7 –4 2 1 4 6 9 11 14 16 18 20 22 24 27 29 31 33 35

5                     −24 −17 −11 –7 –5 –2 1 4 7 9 12 14 16 18 27 23 25 27 29 32 34

6                         −19 −13 –9 –5 –2 1 4 7 10 12 14 17 19 21 24 26 28 30 32

7 8 9 10                                                                                                 D e w  -P       −21   oint V  a   −14     lues   –9 −14 −18   –5 –9 −16   –2 –5 −10 −17 1 –1 –6 −10 4 2 –2 5 7 4 2 –2 10 8 5 3 12 10 8 6 15 13 11 9 17 16 14 11 19 18 16 14 22 21 19 17 24 23 21 20 27 25 24 22 29 28 26 25 31 30 28 27

11                                     −17 10 –5 –1 2 6 9 12 15 18 20 23 25

12                                       −19 −10 –5 –1 3 7 10 13 16 19 21 24

13                                         −19 −10 –5 0 4 8 11 14 17 19 22

14                                           −19 −10 –4 1 5 8 12 15 17 20

15                                             −18 –9 –3 1 5 9 13 15 18

16                                               −18 –9 –2 2 6 10 13 16

17                                                 16 −8 −2 3 7 11 14

18                                                   −15 –7 –1 4 8 12

19                                                     −14 –5 0 5 9

20                                                       −12 –4 1 6

21                                                       −29 −10 –3 2

22                                                           9 −2

APPENDIX C |

Stellar Properties 

Measuring Distances to the Closest Stars

with certainty—nearly all others have such small parallax shifts that accurate measurements are not possible. Fortunately, other methods have been developed to estimate distances to more distant stars. In addition, the Hubble Space Telescope, which is not hindered by Earth’s light-distorting atmosphere, has obtained accurate parallax distances for many more stars.

Measuring the distance to stars is difficult. Nevertheless, astronomers have developed some direct as well as indirect methods to measure stellar distances. One simple measurement, called stellar parallax, is effective in determining the distances to only the closest stars. Stellar parallax is the slight back-and-forth shift of the apparent position of a nearby star due to the orbital motion of Earth around the Sun. The principle of parallax is easy to visualize. Close one eye, and with your index finger in a vertical position, use your open eye to align your finger with some distant object. Without moving your finger, view the object with your other eye and notice that its position appears to have changed. Now repeat the exercise, holding your finger farther away, and notice that the farther away you hold your finger, the less its position seems to shift. In principle, this method of measuring stellar distances is elementary and was practiced by ancient Greek astronomers. Modern cosmologists determine parallax by photographing a nearby star against the background of distant stars. Then, when Earth has moved halfway around its orbit six months later, the same star is photographed again. When these two photographs are compared, the position of the nearby star appears to have shifted with respect to the background stars. FIGURE C.1 illustrates this shift, and the parallax angle that is determined from it. The nearest stars have the largest parallax angles, whereas those of distant stars are much too small to measure. In practice, conducting parallax measurements is quite complex because of the miniscule angles being measured. The process is further complicated because both the Sun and the star being measured are moving relative to each other. The first accurate stellar parallax was not determined until 1838. Even today, parallax angles for only a few thousand of the nearest stars are known

Stellar Brightness The oldest means of classifying stars is based on their brightness, also called luminosity or magnitude. Three factors control the brightness of a star as seen from Earth: how large it is, how hot it is, and its distance from Earth. The stars in the night sky come in a grand assortment of sizes, temperatures, and distances, so their apparent brightness varies widely.

Apparent Magnitude Stars have been classified according to their apparent brightness since at least the second century b.c., when Hipparchus placed about 850 stars into six categories, based on his ability to see differences in brightness. Because he could only reliably see six different brightness levels, he created six categories. These categories were later called magnitudes, with first magnitude being the brightest and sixth magnitude the dimmest. Because some stars may appear dimmer than others only because they are farther away, a star’s brightness, as it appears when viewed from Earth, is called its apparent magnitude. With the invention of the telescope, many stars fainter than the sixth magnitude were discovered. In the mid-1800s, a method was developed to standardize the magnitude scale. An absolute comparison was made between the light coming from stars of the first magnitude and those of the sixth magnitude. It was determined that

FIGURE C.1 Geometry of Stellar Parallax The parallax angle shown here is enormously exaggerated to illustrate the principle. Because distances to even the nearest stars are thousands of times greater than the Earth–Sun distance, the triangles that astronomers work with are extremely long and narrow, making the angles that are measured very small. Original photo

Line

Earth’s orbit

of sig

Parallax angle

ht

Sun

Line

of sig

ht s

nths ix mo

later

Nearby star

Apparent shift Distant stars

Photo taken 6 months later

765

766

APPENDIX C Stellar Properties

TABLE C.1 Ratios of Star Brightness Difference in Magnitude

Brightness Ratio

0.5

1.6:1

1

2.5:1

2

6.3:1

3

16:1

4

40:1

5

100:1

10

10,000:1

20

100,000,000:1

*Calculations: 2.512 × 2.512 × 2.512 × 2.512 × 2.512, or 2.512 raised to the fifth power, equals 100.

a first-magnitude star was about 100 times brighter than a sixth-magnitude star. On the scale that was devised, any two stars that differ by five magnitudes have a ratio in brightness of 100 to 1. Hence, a third-magnitude star is 100 times brighter than an eighth-magnitude star. It follows, then, that the brightness ratio of two stars differing by only one magnitude is about 2.5.1 A star of the first magnitude is about 2.5 times brighter than a star of the second magnitude. TABLE C.1 shows how differences in magnitude correspond to brightness ratios. Because some celestial bodies are brighter than first-magnitude stars, zero and negative magnitudes were introduced. On this scale, the Sun has an apparent magnitude of –26.7. At its brightest, Venus has a magnitude of –4.3. At the other end of the scale, the Hubble Space Telescope can view stars with an apparent magnitude of 30, more than 1 billion times dimmer than stars that are visible to the unaided eye.

Absolute Magnitude Apparent magnitudes were good approximations of the true brightness of stars when astronomers thought that the universe was very small—containing no more than a few thousand stars that were all at very similar distances from Earth. However, we now know that the universe is unimaginably large and contains innumerable stars at wildly varying distances. Since astronomers are interested in the “true” brightness of stars, they devised a measure called absolute magnitude. Stars of the same apparent magnitude usually do not have the same brightness because their distances from us are not equal. Astronomers correct for distance by determining what brightness (magnitude) the stars would have if they were at a standard distance—about 32.6 light-years. For example, if the Sun, which has an apparent magnitude of –26.7, were located 32.6 light-years 1

The more negative, the brighter; the more positive, the dimmer.

from Earth, it would have an absolute magnitude of about +5. Thus, stars with absolute magnitudes greater than 5 (smaller numerical value) are intrinsically brighter than the Sun but appear much dimmer because of their distance from Earth. TABLE C.2 lists the absolute and apparent magnitudes of some stars as well as their distances from Earth. Most stars have an absolute magnitude between 25 (very bright) and 15 (very dim). The Sun is near the midpoint of this range.

Stellar Color and Temperature The next time you are outside on a clear night, look carefully at the stars and note their colors (FIGURE C.2 ). Because human eyes do not respond well to color in low-intensity light (when it is very dark, we see in only black and white), we tend to look at the brightest stars. Some that are quite colorful can be found in the constellation Orion. Of the two brightest stars in Orion, Rigel (b Orionis) appears blue, whereas Betelgeuse (a Orionis) is definitely red. Very hot stars with surface temperatures above 30,000K emit most of their energy in the form of short-wavelength light and therefore appear blue. On the other hand, cooler red stars, with surface temperatures generally less than 3000K, emit most of their energy as longer-wavelength red light. Stars such as the Sun with surface temperatures between 5000 and 6000K appear yellow. Because color is primarily a manifestation of a star’s surface temperature, this characteristic provides astronomers with useful information. Combining temperature data with stellar magnitude tells us a great deal about the size and mass of stars.

Binary Stars and Stellar Mass One of the night sky’s best-known constellations, the Big Dipper, appears to consist of seven stars. But those with good eyesight can recognize that the second star in the handle is actually two stars. In the early nineteenth century, careful examination of numerous star pairs by William Herschel showed that many stars found in pairs actually orbit one another. In such cases, the two stars are in fact united by their mutual gravitation. These pairs, in which the members are far enough apart to be telescopically identified as two stars, are called visual binaries (binaries 5 double). The idea of one star orbiting another may seem unusual, but many stars in the universe exist in pairs or multiples. Binary stars can be used to determine the star property most difficult to calculate—its mass. The mass of a body can be established if it is gravitationally attached to a partner. Binary stars orbit each other around a common point called the center of mass (FIGURE C.3 ). For stars of equal mass, the center of mass lies exactly halfway between them. When one star is more massive than its partner, their common center will be located closer to the more massive one. Thus, if the sizes of their orbits can be observed, their individual masses

TABLE C.2 Distance, Apparent Magnitude, and Absolute Magnitude of Some Stars Name

Distance (light-years)

Apparent Magnitude

NA

Absolute Magnitude

226.7

5.0

Alpha Centauri

4.27

0.0

4.4

Sirius

8.70

21.4

1.5 20.3

Sun

Arcturus Betelgeuse Deneb

36

20.1

520

0.8

25.5

1600

1.3

26.9

APPENDIX C Stellar Properties

767

Center of mass

1 unit

1 unit

A. Two stars of equal mass

Center of mass

FIGURE C.2 Time-Lapse Photograph of Stars in the Constellation Orion These star trails show some of the various star colors. It is important to note that the human eye sees color somewhat differently than photographic film. (Courtesy of

s

2 unit 1 unit

National Optical Astronomy Observatories)

can be determined. You can experience this relationship on a seesaw by trying to balance a person who has a much greater (or smaller) mass. For illustration, when one star has an orbit half the size (radius) of its companion, it is twice as massive as its companion. If their combined masses are equal to three solar masses, then the larger will be twice as massive as the Sun, and the smaller will have a mass equal to that of the Sun. Most stars have a mass that falls in a range between 1/10 and 50 times the mass of the Sun.

B. One star twice as massive as its companion

FIGURE C.3 Binary Stars Orbit Each Other Around Their Common Center of Mass A. For stars of equal mass, the center of mass lies exactly halfway between them. B. If one star is twice as massive as its companion, it is twice as close to their common center of mass. Therefore, more massive stars have proportionately smaller orbits than their less massive companions.

GLOSSARY Aa flow A type of lava flow that has a jagged, blocky surface. Abrasion The grinding and scraping of a rock surface by the friction and impact of rock particles carried by water, wind, or ice.

Absolute humidity The weight of water vapor in a given volume of air (usually expressed in grams/m3).

Absolute instability Air that has a lapse rate greater than the dry adiabatic rate.

Absolute magnitude The apparent brightness of a star if it were viewed from a distance of 10 parsecs (32.6 light-years). Used to compare the true brightness of stars.

Absolute stability Air with a lapse rate less than the wet adiabatic rate.

Absorption spectrum A continuous spectrum with dark lines superimposed. Also known as dark-line spectrum.

Abyssal plain A very level area of the deep-ocean floor, usually lying at the foot of the continental rise.

Abyssal zone A subdivision of the benthic zone characterized by extremely high pressures, low temperatures, low oxygen, few nutrients, and no sunlight.

Accretionary wedge A large wedge-shaped mass of sediment that accumulates in subduction zones. Here, sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block.

Acid precipitation Rain or snow with a pH value that is less than the pH of unpolluted precipitation.

Active continental margin A portion of the seafloor adjacent to the continents that is usually narrow and consisting of highly deformed sediments. These margins occur where oceanic lithosphere is being subducted beneath the margin of a continent.

Adiabatic temperature change Cooling or warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted.

Advection Horizontal convective motion, such as wind. Advection fog A fog formed when warm, moist air is blown over a cool surface.

Aerosols Tiny solid and liquid particles suspended in the atmosphere.

Aftershocks Smaller earthquakes that follow the main earthquake.

Air A mixture of many discrete gases, of which nitrogen and oxygen are most abundant, in which varying quantities of tiny solid and liquid particles are suspended.

Air mass A large body of air that is characterized by a sameness of temperature and humidity.

Air pollutants Airborne particles and gases that occur in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the environment.

Air pressure The force exerted by the weight of a column of air above a given point.

Air-mass weather The conditions experienced in an area as an air mass passes over it. Because air masses are large and fairly homogenous, air-mass weather will be fairly constant and may last for several days.

Albedo The reflectivity of a substance, usually expressed as a percentage of the incident radiation reflected.

768

Alluvial fan A fan-shaped deposit of sediment formed when a stream’s slope is abruptly reduced. Alluvium Unconsolidated sediment deposited by a stream. Alpine glacier A glacier confined to a mountain valley, which in most instances had previously been a stream valley. Also known as a valley glacier.

Altitude (of the Sun) The angle of the Sun above the horizon. Andean-type plate margin A plate boundary that generates continental volcanic arcs.

Andesitic composition See Intermediate composition. Anemometer An instrument used to determine wind speed. Also known as a cup anemometer.

Aneroid barometer An instrument for measuring air pressure that consists of evacuated metal chambers that are very sensitive to variations in air pressure.

Angiosperm A flowering plant in which fruits contain the seeds.

Angle of repose The steepest angle at which loose material remains stationary, without sliding downslope.

Angular unconformity An unconformity in which the strata below dip at an angle different from that of the beds above.

Annual mean temperature An average of the 12 monthly temperature means.

Annual temperature range The difference between the highest and lowest monthly temperature means.

Anthracite A hard, metamorphic form of coal that burns clean and hot.

Anticline A fold in sedimentary strata that resembles an arch; the opposite of syncline.

Anticyclone A high-pressure center characterized by a clockwise flow of air in the Northern Hemisphere.

Aphelion The place in the orbit of a planet where the planet is farthest from the Sun.

Aphotic zone The portion of the ocean where there is no sunlight.

Apparent magnitude The brightness of a star when viewed from Earth.

Aquifer Rock or soil through which groundwater moves easily.

Aquitard Impermeable beds that hinder or prevent groundwater movement.

Archean eon The second eon of Precambrian time,

Asteroids Thousands of small planetlike bodies, ranging in size from a few hundred kilometers to less than a kilometer, whose orbits lie mainly between those of Mars and Jupiter.

Asthenosphere A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers (60 miles) and in some regions extends as deep as 700 kilometers (430 miles). The rock within this zone is easily deformed. Also known as the low-velocity zone.

Astronomical theory A theory of climatic change first developed by Yugoslavian astronomer Milutin Milankovitch. It is based on changes in the shape of Earth’s orbit, variations in the obliquity of Earth’s axis, and the wobbling of Earth’s axis.

Astronomical unit (AU) Average distance from Earth to the Sun; 1.5 3 108 kilometers (93 3 106 miles).

Astronomy The scientific study of the universe; it includes the observation and interpretation of celestial bodies and phenomena.

Atmosphere The gaseous portion of a planet; the planet’s envelope of air. One of the traditional subdivisions of Earth’s physical environment.

Atoll A continuous or broken ring of coral reef surrounding a central lagoon.

Atom The smallest particle that exists as an element. Atomic number The number of protons in the nucleus of an atom.

Atomic weight The average of the atomic masses of isotopes for a given element.

Aurora A bright display of ever-changing light caused by solar radiation interacting with the upper atmosphere in the region of the poles.

Autumnal equinox The equinox that occurs on September 21–23 in the Northern Hemisphere and on March 21–22 in the Southern Hemisphere.

Axial precession A slow motion of Earth’s axis that traces out a cone over a period of 26,000 years. Also known simply as precession.

Backshore The inner portion of the shore, lying landward of the high-tide shoreline. It is usually dry, being affected by waves only during storms.

Backswamp A poorly drained area on a floodplain that results when natural levees are present.

Bajada An apron of sediment along a mountain front created

following the Hadean and preceding the Proterozoic. It extends between 3.8 billion and 2.5 billion years before the present.

Banded iron formations A finely layered iron and silica-

Arctic (A) air mass A bitterly cold air mass that forms over

Bar Common term for sand and gravel deposits in a stream

the frozen Arctic Ocean.

Arête A narrow knifelike ridge separating two adjacent glaciated valleys.

Arid See Desert. Arid climate See Dry climate. Arkose A feldspar-rich sandstone. Artesian well A well in which the water rises above the level where it was initially encountered.

Asteroid belt The region in which most asteroids orbit the Sun between Mars and Jupiter.

by the coalescence of alluvial fans. rich (chert) layer deposited mainly during the Precambrian. channel.

Barchan dune A solitary sand dune shaped like a crescent with its tips pointing downward.

Barchanoid dune Dunes forming scalloped rows of sand oriented at right angles to the wind. This form is intermediate between isolated barchans and extensive waves of transverse dunes.

Barograph A recording barometer. Barometer An instrument that measures atmospheric pressure.

GLOSSARY

Barometric tendency See Pressure tendency. Barred spiral galaxy A galaxy having straight arms extending from its nucleus.

Barrier island A low, elongate ridge of sand that parallels the coast.

Basalt A fine-grained igneous rock of mafic composition. Basalt plateau The broad and extensive accumulation of lava from a succession of flows emanating from fissure eruptions.

Basaltic composition A compositional group of igneous rocks indicating that the rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar.

Base level The level below which a stream cannot erode. Basin A circular downfolded structure. Batholith A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion.

Bathymetry The measurement of ocean depths and the charting of the shape or topography of the ocean floor.

Baymouth bar A sandbar that completely crosses a bay, sealing it off from the open ocean.

Beach An accumulation of sediment found along the landward margin of the ocean or a lake.

Beach drift The transport of sediment in a zigzag pattern along a beach caused by the uprush of water from obliquely breaking waves.

Beach face The wet, sloping surface that extends from the berm to the shoreline.

Beach nourishment The process by which large quantities of sand are added to the beach system to offset losses caused by wave erosion.

Bed load Sediment that is carried by a stream along the bottom of its channel.

Benioff zone The zone of inclined seismic activity that extends from a trench downward into the asthenosphere.

Benthic zone The marine life zone that includes any seafloor surface, regardless of its distance from shore.

Benthos The forms of marine life that live on or in the ocean bottom.

Bergeron process A theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.

Berm The dry, gently sloping zone on the backshore of a beach at the foot of the coastal cliffs or dunes.

Big Bang theory The theory which proposes that the universe originated as a single mass, which subsequently exploded.

Binary stars Two stars revolving around a common center of mass under their mutual gravitational attraction.

Biochemical sedimentary rock Sediment that forms when material dissolved in water is precipitated by waterdwelling organisms. Shells are common examples.

Biogenous sediment Seafloor sediments consisting of material of marine-organic origin.

Biomass The total mass of a defined organism or group of organisms in a particular area or ecosystem.

Biosphere The totality of life on Earth; the parts of the solid Earth, hydrosphere, and atmosphere in which living organisms can be found.

Bituminous coal The most common form of coal, often called soft, black coal.

Black carbon Soot generated by combustion processes and fires.

Black dwarf A final state of evolution for a star, in which all of its energy sources are exhausted and it no longer emits radiation.

769

Black hole A massive star that has collapsed to such a small

Chemical bond A strong attractive force that exists between

volume that its gravity prevents the escape of all radiation.

atoms in a substance. It involves the transfer or sharing of electrons that allows each atom to attain a full valence shell.

Blowout (deflation hollow) A depression excavated by the wind in easily eroded deposits.

Bode’s law A sequence of numbers that approximates the mean distances of the planets from the Sun.

Body waves Seismic waves that travel through Earth’s interior.

Bowen’s reaction series A concept proposed by N. L. Bowen that illustrates the relationships between magma and the minerals crystallizing from it during the formation of igneous rocks.

Braided stream A stream consisting of numerous intertwining channels.

Breakwater A structure protecting a nearshore area from breaking waves.

Breccia A sedimentary rock composed of angular fragments that were lithified.

Bright nebula A cloud of glowing gas excited by ultraviolet radiation from hot stars.

Bright-line spectrum The bright lines produced by an incandescent gas under low pressure. Also known as emission spectrum.

Brittle deformation Deformation that involves the fracturing of rock. Associated with rocks near the surface.

Cactolith A quasi-horizontal chonolith composed of anastomosing ductoliths, whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an akmolith or ethmolith.

Caldera A large depression typically caused by collapse or ejection of the summit area of a volcano.

Calorie The amount of heat required to raise the temperature of 1 gram of water 1°C.

Calving Wastage of a glacier that occurs when large pieces of ice break off into water.

Cambrian explosion The huge expansion in biodiversity that occurred at the beginning of the Paleozoic era.

Cap rock A necessary part of an oil trap. The cap rock is impermeable and hence keeps upwardly mobile oil and gas from escaping at the surface.

Capacity The total amount of sediment a stream is able to transport.

Carbonate group Mineral group whose members contain the carbonate ion (CO222) and one or more kinds of positive ions. Calcite is a common example.

Chemical compound A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.

Chemical sedimentary rock Sedimentary rock consisting of material that was precipitated from water by either inorganic or organic means.

Chemical weathering The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements.

Chinook A wind blowing down the leeward side of a mountain and warming by compression.

Chromatic aberration The property of a lens whereby light of different colors is focused at different places.

Chromosphere The first layer of the solar atmosphere found directly above the photosphere.

Cinder cone A rather small volcano built primarily of pyroclastics ejected from a single vent. Also known as a scoria cone.

Circle of illumination The great circle that separates daylight from darkness.

Circular orbital motion A reference to the movement of water in a wave. As a wave travels, energy is passed along by moving in a circle. The waveform advances but the water does not advance appreciably.

Circum-Pacific belt An area approximately 40,000 kilometers (24,000 miles) in length surrounding the basin of the Pacific Ocean where oceanic lithosphere is continually subducted beneath the surrounding continental plates causing most of Earth’s largest earthquakes.

Cirque An amphitheater-shaped basin at the head of a glaciated valley produced by frost wedging and plucking.

Cirrus One of three basic cloud forms; also one of the three high cloud types. They are thin, delicate ice-crystal clouds often appearing as veil-like patches or thin, wispy fibers.

Clastic rock A sedimentary rock made of broken fragments of preexisting rock.

Cleavage The tendency of a mineral to break along planes of weak bonding.

Climate A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Carbonic acid A weak acid formed when carbon dioxide is

Climate system The exchanges of energy and moisture that

dissolved in water. It plays an important role in chemical weathering.

occur among the atmosphere, hydrosphere, solid Earth, biosphere, and cryosphere.

Cassini division A wide gap in the ring system of Saturn between the A ring and the B ring.

Catastrophism The concept that Earth was shaped by catastrophic events of a short-term nature.

Cavern A naturally formed underground chamber or series of chambers most commonly produced by solution activity in limestone.

Celestial sphere An imaginary hollow sphere upon which the ancients believed the stars were hung and carried around Earth.

Cementation One way in which sedimentary rocks are lithified. As material precipitates from water that percolates through the sediment, open spaces are filled, and particles are joined into a solid mass.

Cenozoic era A span on the geologic time scale beginning about 65 million years ago following the Mesozoic era.

Cepheid variable A star whose brightness varies periodically because it expands and contracts. A type of pulsating star.

Climate-feedback mechanism Several different possible outcomes that may result when one of the atmosphere’s elements is altered.

Climatology The scientific study of climate. Closed system A system that is self-contained with regard to matter—that is, no matter enters or leaves.

Cloud A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.

Clouds of vertical development Clouds that have their bases in the low-height range but extend upward into the middle or high altitudes.

Cluster (star) A large group of stars. Coarse-grained texture An igneous rock texture in which the crystals are roughly equal in size and large enough so that individual minerals can be identified with the unaided eye.

Coast A strip of land that extends inland from the coastline as far as ocean-related features can be found.

770

GLOSSARY

Coastline The coast’s seaward edge. The landward limit of the effect of the highest storm waves on the shore.

Col A pass between mountain valleys where the headwalls of two cirques intersect.

Cold front A front along which a cold air mass thrusts beneath a warmer air mass.

Collision–coalescence process A theory of raindrop formation in warm clouds (above 0°C) in which large cloud droplets (giants) collide and join together with smaller droplets to form a raindrop. Opposite electrical charges may bind the cloud droplets together.

Color A phenomenon of light by which otherwise identical objects may be differentiated.

Column A feature found in caves that is formed when a stalactite and stalagmite join.

Columnar joints A pattern of cracks that form during cooling of molten rock to generate columns that are generally six sided.

Coma The fuzzy, gaseous component of a comet’s head. Comet A small body that generally revolves about the Sun in an elongated orbit.

Compaction A type of lithification in which the weight of overlying material compresses more deeply buried sediment. It is most important in the fine-grained sedimentary rocks such as shale.

Competence A measure of the largest particle a stream can transport; a factor that is dependent on velocity.

Composite cone A volcano composed of both lava flows and pyroclastic material. Also known as a stratovolcano.

Compound A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.

Compressional mountains Mountains in which great horizontal forces have shortened and thickened the crust. Most major mountain belts are of this type.

Compressional stress Differential stress that shortens a rock body.

Concordant A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock.

Condensation nuclei Tiny bits of particulate matter that serve as surfaces on which water vapor condenses.

Condensation The change of state from a gas to a liquid. Conditional instability Moist air with a lapse rate between the dry and wet adiabatic rates.

Conduction The transfer of heat through matter by molecular activity. Energy is transferred through collisions from one molecule to another.

Conduit A pipelike opening through which magma moves toward Earth’s surface. It terminates at a surface opening called a vent.

Cone of depression A cone-shaped depression in the water table immediately surrounding a well.

Confined aquifer An aquifer that has impermeable layers (aquitards) both above and below.

Confining pressure Stress that is applied uniformly in all directions.

Conformable Layers of rock that were deposited without interruption.

Conglomerate A sedimentary rock composed of rounded, gravel-size particles.

Constellation An apparent group of stars originally named for mythical characters. The sky is presently divided into 88 constellations.

Contact metamorphism Changes in rock caused by the heat from a nearby magma body. Also known as thermal metamorphism.

Continent Large, continuous areas of land that include the adjacent continental shelf and islands that are structurally connected to the mainland.

Continental (c) air mass An air mass that forms over land; it is normally relatively dry.

Continental drift A theory that originally proposed that the continents are rafted about. It has essentially been replaced by the plate tectonics theory.

Continental margin The portion of the seafloor adjacent to the continents. It may include the continental shelf, continental slope, and continental rise.

Continental rift A linear zone along which continental lithosphere stretches and pulls apart. Its creation may mark the beginning of a new ocean basin.

Continental rise The gently sloping surface at the base of the continental slope.

Continental shelf The gently sloping submerged portion of the continental margin, extending from the shoreline to the continental slope.

Continental slope The steep gradient that leads to the deepocean floor and marks the seaward edge of the continental shelf.

Continental volcanic arc Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.

Continuous spectrum An uninterrupted band of light emitted by an incandescent solid, liquid, or gas under pressure.

Convection The transfer of heat by the movement of a mass or substance. It can take place only in fluids.

Convergence The condition that exists when the distribution of winds in a given area results in a net horizontal inflow of air into the area. Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favorable to cloud formation and precipitation.

Convergent plate boundary A boundary in which two plates move together, causing one of the slabs of lithosphere to be consumed into the mantle as it descends beneath on an overriding plate.

Coral reef A structure formed in a warm, shallow, sunlit

Creep The slow downhill movement of soil and regolith. Crevasse A deep crack in the brittle surface of a glacier. Cross-bedding A structure in which relatively thin layers are inclined at an angle to the main bedding. Formed by currents of wind or water.

Cross-cutting A principle of relative dating which says that a rock or fault is younger than any rock (or fault) through which it cuts.

Crust The very thin outermost layer of Earth. Cryovolcanism A type of volcanism that results from the eruption of magmas derived from the partial melting of ice.

Crystal An orderly arrangement of atoms. Crystal form See Habit. Crystal settling During the crystallization of magma, the settling of the earlier-formed minerals that are denser than the liquid portion to the bottom of the magma chamber.

Crystal shape See Habit. Crystallization The formation and growth of a crystalline solid from a liquid or gas.

Cumulus One of three basic cloud forms; also the name given one of the clouds of vertical development. Cumulus are billowy individual cloud masses that often have flat bases.

Cup anemometer See Anemometer. Curie point The temperature above which a material loses its magnetization.

Cut bank The area of active erosion on the outside of a meander.

Cutoff A short channel segment created when a river erodes through the narrow neck of land between meanders.

Cyclone A low-pressure center characterized by a counterclockwise flow of air in the Northern Hemisphere.

Daily mean temperature The mean temperature for a day, which is determined by averaging the hourly readings or, more commonly, by averaging the maximum and minimum temperatures for a day.

Daily temperature range The difference between the maximum and minimum temperatures for a day.

Dark matter Undetected matter that is thought to exist in great quantities in the universe.

ocean environment that consists primarily of the calciterich remains of corals as well as the limy secretions of algae and the hard parts of many other small organisms.

Dark nebula A cloud of interstellar dust that obscures the

Core The innermost layer of Earth, located beneath the mantle.

Dark silicate mineral A silicate mineral that contains ions

The core is divided into an outer core and an inner core.

Coriolis force (effect) The deflective force of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Corona The outer, tenuous layer of the solar atmosphere. Correlation Establishing the equivalence of rocks of similar age in different areas.

Cosmological red shift Changes in the spectra of galaxies which indicate that they are moving away from the Milky Way as a result of the expansion of space.

Cosmology The study of the universe. Country breeze A circulation pattern characterized by a light wind blowing into a city from the surrounding countryside. It is best developed on clear and otherwise calm nights when the urban heat island is most pronounced.

Covalent bond A chemical bond produced by the sharing of electrons.

Crater The depression at the summit of a volcano, or that which is produced by a meteorite impact.

Craton The part of the continental crust that has attained stability; that is, it has not been affected by significant tectonic activity during the Phanerozoic eon. It consists of the shield and stable platform.

light of more distant stars and appears as an opaque curtain. of iron and/or magnesium in its structure. Dark silicates are dark in color and have a higher specific gravity than nonferromagnesian silicates.

Dark-line spectrum See Absorption spectrum. Daughter product An isotope that results from radioactive decay.

Debris flow A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water. Also called mudflow.

Declination (stellar) The angular distance north or south of the celestial equator denoting the position of a celestial body.

Decompression melting Melting that occurs as rock ascends due to a drop in confining pressure.

Deep-ocean basin The portion of seafloor that lies between the continental margin and the oceanic ridge system. This region comprises almost 30 percent of Earth’s surface.

Deep-ocean trench See Trench. Deep-sea fan A cone-shaped deposit at the base of the continental slope. The sediment is transported to the fan by turbidity currents that follow submarine canyons.

Deflation The lifting and removal of loose material by wind. Deformation General term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces.

GLOSSARY

Degenerate matter Extremely dense solar material created by electrons being displaced inward toward an atom’s nucleus.

Delta An accumulation of sediment formed where a stream enters a lake or an ocean.

Dendritic pattern A stream system that resembles the pattern of a branching tree.

Density Mass per unit volume of a substance, usually expressed as grams per cubic centimeter (g/cm3).

Deposition The process by which water vapor is changed directly to a solid without passing through the liquid state.

Desalination The removal of salts and other chemicals from seawater.

Desert One of the two types of dry climate; the driest of the dry climates. Also known as arid.

Desert pavement A layer of coarse pebbles and gravel created when wind removed the finer material.

Detachment fault A nearly horizontal fault that may extend hundreds of kilometers below the surface. Such a fault represents a boundary between rocks that exhibit ductile deformation and rocks that exhibit brittle deformation.

Detrital sedimentary rock Rock formed from the accumulation of material that originated and was transported in the form of solid particles derived from both mechanical and chemical weathering.

Dew-point temperature The temperature to which air has to be cooled in order to reach saturation.

Differential stress Forces that are unequal in different directions.

Differential weathering The variation in the rate and degree of weathering caused by such factors as mineral makeup, degree of jointing, and climate.

Diffused light Solar energy scattered and reflected in the atmosphere that reaches Earth’s surface in the form of diffuse blue light from the sky.

Dike A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.

Dip-slip fault A fault in which the movement is parallel to the dip of the fault.

Discharge The quantity of water in a stream that passes a given point in a period of time.

Disconformity A type of unconformity in which the beds above and below are parallel.

Discordant A term used to describe plutons that cut across existing rock structures, such as bedding planes.

Disseminated deposit Any economic mineral deposit in which the desired mineral occurs as scattered particles in the rock but in sufficient quantity to make the deposit an ore.

Dissolved load That portion of a stream’s load that is carried in solution.

Distributary A section of a stream that leaves the main flow. Diurnal tidal pattern A tidal pattern exhibiting one high tide and one low tide during a tidal day; a daily tide.

Divergence The condition that exists when the distribution of winds in a given area results in a net horizontal outflow of air from the region. In divergence at lower levels, the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavorable to cloud formation and precipitation.

Divergent plate boundary A region where the rigid plates

Doppler radar In addition to performing the tasks of conventional radar, a new generation of weather radar that can detect motion directly and hence greatly improve tornado and severe storm warnings.

Drainage basin The land area that contributes water to a stream.

Drawdown The difference in height between the bottom of a cone of depression and the original height of the water table.

Drift See Glacial drift. Drumlin A streamlined asymmetrical hill composed of glacial till. The steep side of the hill faces the direction from which the ice advanced.

Dry adiabatic rate The rate of adiabatic cooling or warming in unsaturated air. The rate of temperature change is 1°C per 100 meters.

Dry climate A climate in which yearly precipitation is not as great as the potential loss of water by evaporation. Also known as arid climate.

Dry-summer subtropical climate A climate located on the west sides of continents between latitudes 30° and 45°. It is the only humid climate with a strong winter precipitation maximum.

Ductile deformation A type of solid state flow that produces a change in the size and shape of a rock body without fracturing. Occurs at depths where temperatures and confining pressures are high.

Dune A hill or ridge of wind-deposited sand. Dwarf galaxy Very small galaxies, usually elliptical and lacking spiral arms.

Dwarf planets Celestial bodies that orbit stars and are massive enough to be spherical but have not cleared their neighboring regions of planetesimals.

Earth science The name for all the sciences that collectively seek to understand Earth. It includes geology, oceanography, meteorology, and astronomy.

Earth system science An interdisciplinary study that seeks to examine Earth as a system composed of numerous interacting parts or subsystems.

Earthflow The downslope movement of water-saturated, clayrich sediment. Most characteristic of humid regions.

Earthquake The vibration of Earth produced by the rapid release of energy.

Ebb current The movement of a tidal current away from the shore.

Eccentricity The variation of an ellipse from a circle. Echo sounder An instrument used to determine the depth of water by measuring the time interval between emission of a sound signal and the return of its echo from the bottom.

Eclipse The cutting off of the light of one celestial body by another passing in front of it.

Ecliptic The yearly path of the Sun plotted against the background of stars.

Economic mineral A concentration of a mineral resource or reserve that can be profitably extracted from Earth.

El Niño The name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. A major El Niño episode can cause extreme weather in many parts of the world.

Elastic deformation Rock deformation in which the rock

are moving apart, typified by the mid-ocean ridges. Also known as a spreading center.

will return to nearly its original size and shape when the stress is removed.

Divide An imaginary line that separates the drainage of two

Elastic rebound The sudden release of stored strain in rocks

streams; often found along a ridge.

Dome A roughly circular upfolded structure similar to an anticline.

Doppler effect The apparent change in wavelength of radiation caused by the relative motions of the source and the observer.

that results in movement along a fault.

Electromagnetic radiation See Radiation. Electromagnetic spectrum The distribution of electromagnetic radiation by wavelength.

Electron A negatively charged subatomic particle that has a negligible mass and is found outside an atom’s nucleus.

771

Element A substance that cannot be decomposed into simpler substances by ordinary chemical or physical means.

Elements of weather and climate Quantities or properties of the atmosphere that are measured regularly and that are used to express the nature of weather and climate.

Elliptical galaxy A galaxy that is round or elliptical in outline. It contains little gas and dust, no disk or spiral arms, and few hot, bright stars.

Eluviation The washing out of fine soil components from the horizon by downward-percolating water.

Emergent coast A coast where land that was formerly below sea level has been exposed either because of crustal uplift or a drop in sea level or both.

Emission nebula A gaseous nebula that derives its visible light from the fluorescence of ultraviolet light from a star in or near the nebula.

Emission spectrum See Bright-line spectrum. End moraine A ridge of till marking a former position of the front of a glacier.

Energy The capacity to do work. Energy levels Spherically shaped, negatively charged zones that surround the nucleus of an atom. Also known as principal shells.

Enhanced Fujita intensity scale (EF-scale) A scale originally developed by T. Theodore Fujita for classifying the severity of a tornado, based on the correlation of wind speed with the degree of destruction.

Environment Everything that surrounds and influences an organism.

Environmental lapse rate The rate of temperature decrease with increasing height in the troposphere.

Eon The largest time unit on the geologic time scale, next in order of magnitude above era.

Ephemeral stream A stream that is usually dry because it carries water only in response to specific episodes of rainfall. Most desert streams are of this type.

Epicenter The location on Earth’s surface that lies directly above the focus of an earthquake.

Epoch A unit of the geologic calendar that is a subdivision of a period.

Equatorial low A belt of low pressure lying near the equator and between the subtropical highs.

Equatorial system A method of locating stellar objects much like the coordinate system used on Earth’s surface.

Equinox The time when the vertical rays of the Sun are striking the equator. The length of daylight and darkness is equal at all latitudes at equinox.

Era A major division on the geologic calendar; eras are divided into shorter units called periods.

Erosion The incorporation and transportation of material by a mobile agent, such as water, wind, or ice.

Eruption column Buoyant plumes of hot, ash-laden gases that can extend thousands of meters into the atmosphere.

Eruptive variable A star that varies in brightness. Escape velocity The initial velocity an object needs to escape from the surface of a celestial body.

Esker A sinuous ridge composed largely of sand and gravel deposited by a stream flowing in a tunnel beneath a glacier near its terminus.

Estuary A partially enclosed coastal water body that is connected to the ocean. Salinity here is measurably reduced by the freshwater flow of rivers.

Eukaryotes An organism whose genetic material is enclosed in a nucleus; plants, animals, and fungi are eukaryotes.

Euphotic zone The portion of the photic zone near the surface where light is bright enough for photosynthesis to occur.

Evaporation The process of converting a liquid to a gas.

772

GLOSSARY

Evaporite deposits A sedimentary rock formed of material deposited from solution by evaporation of water.

Evapotranspiration The combined effect of evaporation and transpiration.

Evolution (theory of) A fundamental theory in biology and paleontology that sets forth the process by which members of a population of organisms come to differ from their ancestors. Organisms evolve by means of mutations, natural selection, and genetic factors. Modern species are descended from related but different species that lived in earlier times.

Exfoliation dome A large, dome-shaped structure, usually composed of granite, formed by sheeting.

Exotic stream A permanent stream that traverses a desert and has its source in well-watered areas outside the desert.

External process Process such as weathering, mass wasting, or erosion that is powered by the Sun and transforms solid rock into sediment.

Extrusive Igneous activity that occurs outside the crust. Eye A zone of scattered clouds and calm averaging about 20 kilometers (12 miles) in diameter at the center of a hurricane.

Eye wall The doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane.

Eyepiece A short-focal-length lens used to enlarge the image in a telescope. The lens nearest the eye.

Fall A type of movement common to mass-wasting processes that refers to the free falling of detached individual pieces of any size.

Fault A break in a rock mass along which movement has occurred.

Fault creep Displacement along a fault that is so slow and gradual that little seismic activity occurs.

Fault scarp A cliff created by movement along a fault. It represents the exposed surface of the fault prior to modification by weathering and erosion.

Fault-block mountain A mountain formed by the displacement of rock along a fault.

Felsic The group of igneous rocks composed primarily of feldspar and quartz.

Fetch The distance that wind has traveled across open water. It is one of three factors that influence the height, length, and period of a wave.

Filaments Dark, thin streaks that appear across the bright solar disk.

Fine-grained texture A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished with the unaided eye.

Fiord A steep-sided inlet of the sea formed when a glacial trough was partially submerged.

Fissure A crack in rock along which there is a distinct separation.

Fissure eruption An eruption in which lava is extruded from narrow fractures or cracks in the crust.

Flare A sudden brightening of an area on the Sun. Flood basalts Flows of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters.

Flood current The tidal current associated with the increase in the height of the tide.

Floodplain The flat, low-lying portion of a stream valley subject to periodic inundation.

Flow A type of movement common to mass-wasting processes in which water-saturated material moves downslope as a viscous fluid.

Fluorescence The absorption of ultraviolet light, which is reemitted as visible light.

Focal length The distance from a lens to the point where it focuses parallel rays of light.

Focus (earthquake) The zone within Earth where rock displacement produces an earthquake. Also known as the hypocenter.

Focus (light) The point where a lens or mirror causes light rays to converge.

Fog A cloud with its base at or very near Earth’s surface. Fold A bent rock layer or series of layers that were originally horizontal and subsequently deformed.

Foliation A texture of metamorphic rocks that gives the rock a layered appearance.

Food chain A succession of organisms in an ecological community through which food energy is transferred from producers through herbivores and on to one or more carnivores.

Food web A group of interrelated food chains. Footwall block The rock surface below a fault. Forearc basin The region located between a volcanic arc and an accretionary wedge where shallow-water marine sediments typically accumulate.

Foreshocks Small earthquakes that often precede a major earthquake.

Foreshore That portion of the shore lying between the normal high and low water marks; the intertidal zone.

Fossil The remains or traces of organisms preserved from the geologic past.

Fossil assemblage The overlapping ranges of a group of fossils (assemblage) collected from a layer. By examining such an assemblage, the age of the sedimentary layer can be established.

Fossil fuel General term for any hydrocarbon that may be used as a fuel, including coal, oil, natural gas, bitumen from tar sands, and shale oil.

Fossil magnetism See Paleomagnetism. Fossil succession A principle in which fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content.

Fracture zone Any break or rupture in rock along which no appreciable movement has taken place.

Fragmental texture See Pyroclastic texture. Freezing The change of state from a liquid to a solid. Freezing nuclei Solid particles that serve as cores for the formation of ice crystals.

Freezing rain See Glaze. Front The boundary between two adjoining air masses having contrasting characteristics.

Frontal fog Fog formed when rain evaporates as it falls through a layer of cool air.

Frontal wedging Lifting of air resulting when cool air acts as a barrier over which warmer, lighter air will rise.

Frost wedging The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.

Fumarole A vent in a volcanic area from which fumes or gases escape.

Galactic cluster Groups of gravitationally bound galaxies that sometimes contain thousands of galaxies.

Geocentric The concept of an Earth-centered universe. Geologic structure See Rock structure. Geologic time The span of time since the formation of Earth, about 4.6 billion years.

Geologic time scale The division of Earth history into blocks of time—eons, eras, periods, and epochs. The time scale was created using relative dating principles.

Geology The science that examines Earth, its form and composition, and the changes it has undergone and is undergoing.

Geosphere The solid Earth, the largest of Earth’s four major spheres.

Geostrophic wind A wind, usually above a height of 600 meters (2000 feet), that blows parallel to the isobars.

Geothermal energy Natural steam used for power generation.

Geothermal gradient The gradual increase in temperature with depth in the crust. The average is 30°C per kilometer in the upper crust.

Geyser A fountain of hot water ejected periodically. Giant (star) A luminous star of large radius. Glacial budget The balance, or lack of balance, between ice formation at the upper end of a glacier and ice loss in the zone of wastage.

Glacial drift An all-embracing term for sediments of glacial origin, no matter how, where, or in what shape they were deposited. Also known simply as drift.

Glacial erratic An ice-transported boulder that was not derived from bedrock near its present site.

Glacial striations Scratches and grooves on bedrock caused by glacial abrasion.

Glacial trough A mountain valley that has been widened, deepened, and straightened by a glacier.

Glacier A thick mass of ice originating on land from the compaction and recrystallization of snow that shows evidence of past or present flow.

Glassy texture A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.

Glaze A coating of ice on objects formed when supercooled rain freezes on contact.

Globular cluster A nearly spherically shaped group of densely packed stars.

Globule A dense, dark nebula thought to be the birthplace of stars. Also known as glaze.

Gondwanaland The southern portion of Pangaea, consisting of South America, Africa, Australia, India, and Antarctica.

Graben A valley formed by the downward displacement of a fault-bounded block.

Graded bed A sediment layer that is characterized by a decrease in sediment size from bottom to top.

Gradient The slope of a stream; generally measured in feet per mile.

Granitic composition A compositional group of igneous rocks that indicates a rock is composed almost entirely of light-colored silicates.

Granule A fine structure visible on the solar surface caused by convective cells below.

Gravitational collapse The gradual subsidence of mountains caused by lateral spreading of weak material located deep within these structures.

Great Oxygenation Event A time about 2.5 billion years ago, when a significant amount of oxygen appeared in the atmosphere.

Greenhouse effect The transmission of short-wave solar radiation by the atmosphere, coupled with the selective absorption of longer-wavelength terrestrial radiation, especially by water vapor and carbon dioxide.

Groin A short wall built at a right angle to the shore to trap moving sand.

Ground moraine An undulating layer of till deposited as the ice front retreats.

Groundmass The matrix of smaller crystals within an igneous rock that has porphyritic texture.

Groundwater Water in the zone of saturation. Guyot A submerged flat-topped seamount. Gymnosperm A group of seed-bearing plants that includes conifers and Ginkgo. The term means “naked seed,” a reference to the unenclosed condition of the seeds.

GLOSSARY

Gyre The large circular surface current pattern found in each ocean.

Habit Refers to the common or characteristic shape of a crystal, or aggregate of crystals. Also known as crystal form and crystal shape.

Hadean eon A term found on some versions of the geologic time scale. It refers to the earliest interval (eon) of Earth history and ended 4 billion years ago.

Hail Nearly spherical ice pellets having concentric layers and formed by the successive freezing of layers of water.

Half graben A tilted fault block in which the higher side is associated with mountainous topography and the lower side is a basin that fills with sediment.

Half-life The time required for one-half of the atoms of a radioactive substance to decay.

Halocline A layer of water in which there is a high rate of change in salinity in the vertical dimension.

Hanging valley A tributary valley that enters a glacial trough at a considerable height above its floor.

Hanging wall block The rock surface immediately above a fault.

Hard stabilization Any form of artificial structure built to protect a coast or to prevent the movement of sand along a beach. Examples include groins, jetties, breakwaters, and seawalls.

Hardness The resistance a mineral offers to scratching. Heat The kinetic energy of random molecular motion. Heliocentric The view that the Sun is at the center of the solar system.

Heliosphere A large region of space that extends far beyond Pluto’s orbit, marked by solar winds and the Sun’s magnetic field.

Hertzsprung-Russell diagram See H-R diagram. High A center of high pressure characterized by anticyclonic winds.

High cloud A cloud that normally has its base above 6000 meters (3,728 miles); the base may be lower in winter and at high-latitude locations.

Highland climate A complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Hogback A narrow, sharp-crested ridge formed by the upturned edge of a steeply dipping bed of resistant rock.

Horizon A layer in a soil profile. Also known as the soil horizon.

Horn A pyramid-like peak formed by glacial action in three or more cirques surrounding a mountain summit.

Horst An elongated, uplifted block of crust bounded by faults.

Hot spot A concentration of heat in the mantle capable of producing magma, which in turn extrudes onto Earth’s surface. The intraplate volcanism that produced the Hawaiian islands is one example.

Hot spot track A chain of volcanic structures produced as a lithospheric plate moves over a mantle plume.

Hot spring A spring in which the water is 6–9°C (10–15°F) warmer than the mean annual air temperature of its locality.

H-R diagram A plot of stars according to their absolute magnitudes and spectral types. Stands for HertzsprungRussell diagram.

Hubble’s law A law that relates the distance to a galaxy and its velocity.

Humid continental climate A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40° and 50° north latitude. This climate is not found in the Southern Hemisphere, where the middle latitudes are dominated by the oceans.

Humid subtropical climate A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

Humidity A general term referring to water vapor in the air but not to liquid droplets of fog, cloud, or rain.

Humus Organic matter in soil produced by the decomposition of plants and animals.

Hurricane A tropical cyclonic storm having winds in excess of 119 kilometers (74 miles) per hour.

Hydraulic fracturing A method of opening up pore space in otherwise impermeable rocks, permitting natural gas to flow out into wells.

Hydrogen burning The conversion of hydrogen through fusion to form helium.

Hydrogen fusion The nuclear reaction in which hydrogen nuclei are fused into helium nuclei.

Hydrogenous sediment Seafloor sediments consisting of minerals that crystallize from seawater. An important example is manganese nodules.

Hydrosphere The water portion of our planet; one of the traditional subdivisions of Earth’s physical environment.

Hydrothermal solution The hot, watery solution that escapes from a mass of magma during the later stages of crystallization. Such solutions may alter the surrounding country rock and are frequently the source of significant ore deposits.

Hygrometer An instrument designed to measure relative humidity.

Hygroscopic nuclei Condensation nuclei having a high affinity for water, such as salt particles.

Hypocenter See Focus (earthquake). Hypothesis A tentative explanation that is tested to determine whether it is valid.

Ice cap A mass of glacial ice covering a high upland or plateau and spreading out radially.

Ice cap climate A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice sheet A very large, thick mass of glacial ice flowing outward in all directions from one or more accumulation centers.

Ice shelf A large, relatively flat mass of floating ice that forms where glacial ice flows into bays and extends seaward from the coast but remains attached to the land along one or more sides.

Iceberg A mass of floating ice produced by a calving glacier. Usually 20 percent or less of the iceberg protrudes above the waterline.

Igneous rock A rock formed by the crystallization of molten magma.

Immature soil A soil lacking horizons. Impact craters Depressions result from collisions with bodies such as asteroids and comets.

Incised meander A meandering channel that flows in a steep, narrow valley. Incised meanders form either when an area is uplifted or when base level drops.

Inclination of the axis The tilt of Earth’s axis from the perpendicular to the plane of Earth’s orbit.

Inclusion A piece of one rock unit contained within another. Inclusions are used in relative dating. The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.

Index fossil A fossil that is associated with a particular span of geologic time.

Inertia A property of matter that resists a change in its motion.

Infiltration The movement of surface water into rock or soil through cracks and pore spaces.

773

Infrared Radiation with a wavelength from 0.7 to 200 micrometers.

Inner core The solid innermost layer of Earth, about 1300 kilometers (800 miles) in radius.

Inner planets See Terrestrial planets. Inselberg An isolated mountain remnant characteristic of the late stage of erosion in an arid region.

Intensity (earthquake) A measure of the degree of earthquake shaking at a given locale, based on the amount of damage.

Interface A common boundary where different parts of a system interact.

Interior drainage A discontinuous pattern of intermittent streams that do not flow to the ocean.

Intermediate composition The composition of igneous rocks lying between felsic and mafic. Also known as andesitic composition.

Interstellar matter Dust and gases found between stars. Intertidal zone The area where land and sea meet and overlap; the zone between high and low tides.

Intertropical convergence zone (ITCZ) The zone of general convergence between the Northern and Southern Hemisphere trade winds.

Intraplate volcanism Igneous activity that occurs within a tectonic plate away from plate boundaries.

Intrusion See Pluton. Intrusive Igneous rock that formed below Earth’s surface. Ion An atom or a molecule that possesses an electrical charge.

Ionic bond A chemical bond between two oppositely charged ions formed by the transfer of valence electrons from one atom to the other.

Ionosphere A complex zone of ionized gases that coincides with the lower portion of the thermosphere.

Iron meteorite One of the three main categories of meteorites. This group is composed largely of iron with varying amounts of nickel (5–20 percent). Most meteorite finds are irons.

Irregular galaxy A galaxy that lacks symmetry. Island arc See Volcanic island arc. Isobar A line drawn on a map connecting points of equal atmospheric pressure, usually corrected to sea level.

Isostasy The concept that Earth’s crust is floating in gravitational balance on the material of the mantle.

Isostatic adjustment Compensation of the lithosphere when weight is added or removed. When weight is added, the lithosphere responds by subsiding, and when weight is removed, there is uplift.

Isotherms Lines connecting points of equal temperature. Isotopes Varieties of the same element that have different mass numbers; their nuclei contain the same number of protons but different numbers of neutrons.

Jet stream Swift (120–240 kilometers per hour), highaltitude winds.

Jetties A pair of structures extending into the ocean at the entrance to a harbor or river that are built for the purpose of protecting against storm waves and sediment deposition.

Joint A fracture in rock along which there has been no movement.

Jovian planets The Jupiter-like planets: Jupiter, Saturn, Uranus, and Neptune. These planets have relatively low densities. Also known as the outer planets.

Kame A steep-sided hill composed of sand and gravel that originates when sediment is collected in openings in stagnant glacial ice.

Karst A topography consisting of numerous depressions called sinkholes.

774

GLOSSARY

Kettle holes Depressions created when blocks of ice became lodged in glacial deposits and subsequently melted.

Köppen classification A system for classifying climates devised by Wladimir Köppen that is based on mean monthly and annual values of temperature and precipitation.

Kuiper belt A region outside the orbit of Neptune where most short-period comets are thought to originate.

La Niña An episode of strong trade winds and unusually low sea-surface temperatures in the central and eastern Pacific. The opposite of El Niño.

Laccolith A massive igneous body intruded between preexisting strata.

Lahar Mudflows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

Lake-effect snow Snow showers associated with a cP air mass to which moisture and heat are added from below as the air mass traverses a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.

Laminar flow The movement of water particles in straight-

Local Group The cluster of 20 or so galaxies to which our galaxy belongs.

Local wind A small-scale wind produced by a locally generated pressure gradient. Examples include land and sea breezes and mountain and valley breezes.

Localized convective lifting Unequal surface heating that causes localized pockets of air (thermals) to rise because of their buoyancy.

Loess Deposits of windblown silt, lacking visible layers, generally buff-colored, and capable of maintaining a nearly vertical cliff.

Longitudinal (seif) dunes Long ridges of sand oriented parallel to the prevailing wind; these dunes form where sand supplies are limited.

Longshore current A nearshore current that flows parallel to the shore.

Low A center of low pressure characterized by cyclonic winds.

Low cloud A cloud that forms below a height of 2000 meters (1,200 miles).

Lower mantle The part of the mantle that extends from the

line paths that are parallel to the channel. The water particles move downstream, without mixing.

core–mantle boundary to a depth of 660 kilometers (400 miles).

Land breeze A local wind blowing from land toward the

Low-velocity zone See Asthenosphere. Luminosity The brightness of a star. The amount of energy

water during the night in coastal areas.

Lapse rate (normal) The average drop in temperature (6.5°C per kilometer [3.5°F per 1000 feet]) with increased altitude in the troposphere.

Latent heat The energy absorbed or released during a change in state.

Lateral continuity (principle of) A principle which states that sedimentary beds originate as continuous layers that extend in all directions until they grade into a different type of sediment or thin out at the edge of a sedimentary basin.

Lateral moraine A ridge of till along the sides of an alpine glacier composed primarily of debris that fell to the glacier from the valley walls.

Laurasia The northern portion of Pangaea, consisting of North America and Eurasia.

radiated by a star.

Lunar breccia A lunar rock formed when angular fragments and dust are welded together by the heat generated by the impact of a meteoroid.

Lunar eclipse An eclipse of the Moon. Lunar highlands See Terrae. Lunar regolith A thin, gray layer on the surface of the Moon, consisting of loosely compacted, fragmented material believed to have been formed by repeated meteoritic impacts.

Luster The appearance or quality of light reflected from the surface of a mineral.

Mafic Igneous rocks with a low silica content and a high iron–magnesium content.

Lava Magma that reaches Earth’s surface. Lava tube A tunnel in hardened lava that acts as a horizontal

Magma A body of molten rock found at depth, including any

conduit for lava flowing from a volcanic vent. Lava tubes allow fluid lavas to advance great distances.

Magmatic differentiation The process of generating more

Law of conservation of angular momentum The product

Magnetic reversal A change in Earth’s magnetic field from

of the velocity of an object around a center of rotation (axis), and the distance squared of the object from the axis is constant.

Leaching The depletion of soluble materials from the upper soil by downward-percolating water.

Light silicate mineral A silicate mineral that lacks iron and/or magnesium. Light silicates are generally lighter in color and have lower specific gravities than dark silicates.

Lightning A sudden flash of light generated by the flow of electrons between oppositely charged parts of a cumulonimbus cloud or between the cloud and the ground.

Light-year The distance light travels in a year; about 6 trillion miles.

Liquefaction A phenomenon, sometimes associated with earthquakes, in which soils and other unconsolidated materials containing abundant water are turned into a fluidlike mass that is not capable of supporting buildings.

Lithification The process, generally cementation and/or compaction, of converting sediments to solid rock.

Lithosphere The rigid outer layer of Earth, including the crust and upper mantle.

Lithospheric plate A coherent unit of Earth’s rigid outer layer that includes the crust and upper unit. Also known simply as a plate.

dissolved gases and crystals. than one rock type from a single magma. normal to reverse or vice versa.

Magnetic time scale A scale that shows the ages of magnetic reversals and is based on the polarity of lava flows of various ages.

Magnetometer A sensitive instrument used to measure the intensity of Earth’s magnetic field at various points.

Magnitude (earthquake) The total amount of energy released during an earthquake.

Magnitude (stellar) A number given to a celestial object to express its relative brightness.

Main-sequence stars A sequence of stars on the Hertzsprung-Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right.

Manganese nodules Rounded lumps of hydrogenous sediment scattered on the ocean floor, consisting mainly of manganese and iron and usually containing small amounts of copper, nickel, and cobalt.

Mantle The 2900-kilometer- (1800-mile-) thick layer of Earth located below the crust.

Mantle plume A mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core–mantle boundary.

Maria The Latin name for the smooth areas of the Moon formerly thought to be seas.

Marine terrace A wave-cut platform that has been exposed above sea level.

Marine west coast climate A climate found on windward coasts from latitudes 40° to 65° and dominated by maritime air masses. Winters are mild and summers are cool.

Maritime (m) air mass An air mass that originates over the ocean. These air masses are relatively humid.

Mass extinction An event in which a large percentage of species become extinct.

Mass number The number of neutrons and protons in the nucleus of an atom.

Mass wasting The downslope movement of rock, regolith, and soil under the direct influence of gravity.

Massive An igneous pluton that is not tabular in shape. Mean solar day The average time between two passages of the Sun across the local celestial meridian.

Meander A looplike bend in the course of a stream. Mechanical weathering The physical disintegration of rock, resulting in smaller fragments.

Medial moraine A ridge of till formed when lateral moraines from two coalescing alpine glaciers join.

Megathrust fault The plate boundary separating a subducting slab of oceanic lithosphere and the overlying plate.

Melt The liquid portion of magma, excluding the solid crystals.

Melting The change of state from a solid to a liquid. Mercalli intensity scale See Modified Mercalli intensity scale.

Mercury barometer A mercury-filled glass tube in which the height of the mercury column is a measure of air pressure.

Mesocyclone A vertical cylinder of cyclonically rotating air (3 to 10 kilometers in diameter) that develops in the updraft of a severe thunderstorm and that often precedes the development of damaging hail or tornadoes.

Mesopause The boundary between the mesosphere and the thermosphere.

Mesosphere The layer of the atmosphere immediately above the stratosphere and characterized by decreasing temperatures with height.

Mesozoic era A span on the geologic time scale between the Paleozoic and Cenozoic eras from about 248 million to 65 million years ago.

Metallic bond A chemical bond present in all metals that may be characterized as an extreme type of electron sharing in which the electrons move freely from atom to atom.

Metamorphic rock Rocks formed by the alteration of preexisting rock deep within Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids.

Metamorphism The changes in mineral composition and texture of a rock subjected to high temperature and pressure within Earth.

Meteor The luminous phenomenon observed when a meteoroid enters Earth’s atmosphere and burns up; popularly called a “shooting star.”

Meteor shower Many meteors appearing in the sky, caused by Earth intercepting a swarm of meteoritic particles.

Meteorite Any portion of a meteoroid that survives its traverse through Earth’s atmosphere and strikes Earth’s surface.

Meteoroid Small solid particles that have orbits in the solar system.

Meteorology The scientific study of the atmosphere and atmospheric phenomena; the study of weather and climate.

GLOSSARY

Microcontinents Relatively small fragments of continental crust that may lie above sea level, such as the island of Madagascar, or may be submerged, as exemplified by the Campbell Plateau located near New Zealand.

Middle cloud A cloud occupying the height range from 2000 to 6000 meters.

Midlatitude (middle-latitude) cyclone A large center of low pressure with an associated cold front and often a warm front. Frequently accompanied by abundant precipitation.

Mid-ocean ridge A continuous mountainous ridge on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers (300 to 3000 miles). The rifts at the crests of these ridges represent divergent plate boundaries.

Mineral A naturally occurring, inorganic crystalline material with a unique chemical composition.

Nebular theory The basic idea that the Sun and planets formed from the same cloud of gas and dust in interstellar space.

Negative-feedback mechanism A feedback mechanism that tends to maintain a system as it is—that is, maintain the status quo.

Nekton Pelagic organisms that can move independently of ocean currents by swimming or other means of propulsion.

Neritic zone The marine-life zone that extends from the low tideline out to the shelf break.

Neutron A subatomic particle found in the nucleus of an atom. A neutron is electrically neutral and has a mass approximately that of a proton.

Neutron star A star of extremely high density, composed entirely of neutrons.

Nonconformity An unconformity in which older

Mineral resource All discovered and undiscovered deposits

metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata.

of a useful mineral that can be extracted now or at some time in the future.

Nonfoliated texture Metamorphic rocks that do not exhibit

Mineralogy The study of minerals. Mixed tidal pattern A tidal pattern exhibiting two high tides and two low tides per tidal day, with a large inequality in high water heights, low water heights, or both. Coastal locations that experience such a tidal pattern may also show alternating periods of diurnal and semidiurnal tidal patterns. Also called mixed semidiurnal.

Mixing depth The height to which convectional movements extend above Earth’s surface. The greater the mixing depth, the better the air quality.

Mixing ratio The mass of water vapor in a unit mass of dry air; commonly expressed as grams of water vapor per kilogram of dry air.

Model A term often used synonymously with hypothesis but that is less precise because it is sometimes used to describe a theory as well.

Modified Mercalli intensity scale A 12-point scale developed to evaluate earthquake intensity based on the amount of damage to various structures.

Mohorovicˆ ic´ discontinuity (Moho) The boundary separating the crust from the mantle, discernible by an increase in seismic velocity.

Mohs scale A series of 10 minerals used as a standard in determining hardness.

Moment magnitude A more precise measure of earthquake magnitude than the Richter scale that is derived from the amount of displacement that occurs along a fault zone.

Monocline A one-limbed flexure in strata. The strata are unusually flat-lying or very gently dipping on both sides of the monocline.

Monsoon Seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea; in summer, from sea to land.

Monthly mean temperature The mean temperature for a month that is calculated by averaging the daily means.

Mountain belt A geographic area of roughly parallel and geologically connected mountain ranges developed as a result of plate tectonics.

Mountain breeze The nightly downslope winds commonly encountered in mountain valleys.

Natural levees The elevated landforms that parallel some streams and act to confine their waters, except during floodstage.

Neap tide The lowest tidal range, which occurs near the times of the first- and third-quarter phases of the Moon.

Nearshore zone The zone of beach that extends from the low-tide shoreline seaward to where waves break at low tide.

Nebula A cloud of interstellar gas and/or dust.

foliation.

Nonmetallic mineral resource A mineral resource that is not a fuel or processed for the metals it contains.

Nonrenewable resource A resource that forms or accumulates over such long time spans that it must be considered as fixed in total quantity.

Nonsilicates Mineral groups that lack silicas in their structures and account for less than 10 percent of Earth’s crust.

Nor’easter The term used to describe the weather associated with an incursion of mP air from the North Atlantic into the Northeast and Mid-Atlantic regions; strong northeast winds, freezing or near-freezing temperatures, and the possibility of precipitation make this an unwelcome weather event.

Normal fault A fault in which the rock above the fault plane has moved down relative to the rock below.

Normal polarity A magnetic field that is the same as that which exists at present.

Nova A star that explosively increases in brightness. Nuclear fusion The source of the Sun’s energy. Nucleus The small heavy core of an atom that contains all of its positive charge and most of its mass.

Nuée ardente Incandescent volcanic debris buoyed up by hot gases that moves downslope in an avalanche fashion.

Numerical date A date that specifies the actual number of years that have passed since an event occurred.

Obliquity The angle between the planes of Earth’s equator and orbit.

Obsidian A volcanic glass of felsic composition. Occluded front A front formed when a cold front overtakes a warm front. It marks the beginning of the end of a middle-latitude cyclone.

Occlusion The overtaking of one front by another. Occultation An eclipse of a star or planet by the Moon or a planet.

Ocean basin A deep submarine region that lies beyond the continental margins.

Oceanic plateau An extensive region on the ocean floor composed of thick accumulations of pillow basalts and other mafic rocks that in some cases exceed 30 kilometers (20 miles) in thickness.

Oceanic ridge system A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers (300–3000 miles). The rifts at the crests of ridges represent divergent plate boundaries.

Oceanic rise See Mid-ocean ridge. Oceanic zone The marine-life zone beyond the continental shelf.

775

Oceanography The scientific study of the oceans and oceanic phenomena.

Octet rule A rule which says that atoms combine in order that each may have the electron arrangement of a noble gas; that is, the outer energy level contains eight neutrons.

Offshore zone The relatively flat submerged zone that extends from the breaker line to the edge of the continental shelf.

Oil trap A geologic structure that allows for significant amounts of oil and gas to accumulate.

Oort cloud A spherical shell composed of comets that orbit the Sun at distances generally greater than 10,000 times the Earth–Sun distance.

Open cluster A loosely formed group of stars of similar origin.

Open system A system in which both matter and energy flow into and out of the system. Most natural systems are of this type.

Orbit The path of a body in revolution around a center of mass.

Ore Usually a useful metallic mineral that can be mined at a profit. The term is also applied to certain nonmetallic minerals such as fluorite and sulfur.

Ore deposit A naturally occurring concentration of one or more metallic minerals that can be extracted economically.

Organic matter Material composed of organic compounds consisting of the remains of once-living plants and animals and their waste products in the environment.

Original horizontality Layers of sediments are generally deposited in a horizontal or nearly horizontal position.

Orogenesis The processes that collectively result in the formation of mountains.

Orographic lifting Mountains acting as barriers to the flow of air, forcing the air to ascend. The air cools adiabatically, and clouds and precipitation may result.

Outer core A layer beneath the mantle about 2200 kilometers (1364 miles) thick that has the properties of a liquid.

Outer planets See Jovian planets. Outgassing The escape of gases that had been dissolved in magma.

Outlet glacier A tongue of ice that normally flows rapidly outward from an ice cap or ice sheet, usually through mountainous terrain to the sea.

Outwash plain A relatively flat, gently sloping plain consisting of materials deposited by meltwater streams in front of the margin of an ice sheet.

Overrunning Warm air gliding up a retreating cold air mass. Oxbow lake A curved lake produced when a stream cuts off a meander.

Ozone A molecule of oxygen that contains three oxygen atoms.

Pahoehoe flow A lava flow with a smooth-to-ropey surface. Paleomagnetism The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized. Also known as fossil magnetism.

Paleontology The systematic study of fossils and the history of life on Earth.

Paleozoic era A span on the geologic time scale between the eons of the Precambrian and Mesozoic era from about 540 million to 248 million years ago.

Pangaea The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.

Parabolic dunes Dunes that resemble barchans, except that their tips point into the wind; they often form along coasts that have strong onshore winds, abundant sand, and vegetation that partly covers the sand.

776

GLOSSARY

Paradigm A theory that is held with a very high degree of confidence and is comprehensive in scope.

Parallax The apparent shift of an object when viewed from two different locations.

Parasitic cone A volcanic cone that forms on the flank of a larger volcano.

Parcel An imaginary volume of air enclosed in a thin elastic cover. Typically it is considered to be a few hundred cubic meters in volume and is assumed to act independently of the surrounding air.

Parent material The material on which a soil develops. Parsec The distance at which an object would have a parallax angle of 1 second of arc (3.26 light-years).

Partial melting The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

Passive continental margin A margin that consists of a continental shelf, continental slope, and continental rise. These margins are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.

Pegmatite A very coarse-grained igneous rock (typically granite) commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a water-rich environment is believed to be responsible for the very large crystals.

Pelagic zone Open ocean of any depth. Animals in this zone swim or float freely.

Penumbra The portion of a shadow from which only part of the light source is blocked by an opaque body.

Perched water table A localized zone of saturation above the main water table created by an impermeable layer (aquiclude).

Peridotite An igneous rock of ultramafic composition thought to be abundant in the upper mantle.

Perihelion The point in the orbit of a planet where it is closest to the Sun.

Period A basic unit of the geologic calendar that is a subdivision of an era. Periods may be divided into smaller units called epochs.

Periodic table The tabular arrangement of the elements according to atomic number.

Permeability A measure of a material’s ability to transmit water.

Perturbation The gravitational disturbance of the orbit of one celestial body by another.

pH scale A common measure of the degree of acidity or alkalinity of a solution, it is a logarithmic scale ranging from 0 to 14. A value of 7 denotes a neutral solution, values below 7 indicate greater acidity, and numbers above 7 indicate greater alkalinity.

Phanerozoic eon That part of geologic time represented by rocks containing abundant fossil evidence. The eon extending from the end of the Proterozoic eon (about 540 million years ago) to the present.

Phases of the Moon The progression of changes in the Moon’s appearance during the month.

Phenocryst In an igneous rock with a porphyritic texture, a conspicuously large crystal embedded in a matrix of finergrained crystals called the groundmass.

Photic zone The upper part of the ocean into which any sunlight penetrates.

Photochemical reaction A chemical reaction in the atmosphere that is triggered by sunlight, often yielding a secondary pollutant.

Photon A discrete amount (quantum) of electromagnetic energy. Photosphere The region of the Sun that radiates energy to space. The visible surface of the Sun.

Photosynthesis The process by which plants and algae produce carbohydrates from carbon dioxide and water in the presence of chlorophyll, using light energy and releasing oxygen.

Physical environment The part of the environment that encompasses water, air, soil, and rock, as well as conditions such as temperature, humidity, and sunlight.

Phytoplankton Algal plankton, which are the most important community of primary producers in the ocean.

Piedmont glacier A glacier that forms when one or more valley glaciers emerge from the confining walls of mountain valleys and spread out to create a broad sheet in the lowlands at the base of the mountains.

Pillow lava Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.

Pipe A vertical conduit through which magmatic materials have passed.

Placer A deposit formed when heavy minerals are mechanically concentrated by currents, most commonly streams and waves. Placers are sources of gold, tin, platinum, diamonds, and other valuable minerals.

Plane of the ecliptic The imaginary plane that connects Earth’s orbit with the celestial sphere.

Planetary nebula A shell of incandescent gas expanding from a star.

Planetesimal A solid celestial body that accumulated during the first stages of planetary formation. Planetesimals aggregated into increasingly larger bodies, ultimately forming the planets.

Plankton Passively drifting or weakly swimming organisms that cannot move independently of ocean currents. Includes microscopic algae, protozoa, jellyfish, and larval forms of many animals.

Plate See Lithospheric plate. Plate tectonics The theory which proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

Playa A flat area on the floor of an undrained desert basin. Following heavy rain, the playa becomes a lake.

Playa lake A temporary lake in a playa. Pleistocene epoch An epoch of the Quaternary period beginning about 1.8 million years ago and ending about 10,000 years ago. Best known as a time of extensive continental glaciation.

Plucking (quarrying) The process by which pieces of bedrock are lifted out of place by a glacier.

Plug See Volcanic neck. Pluton A structure that results from the emplacement and crystallization of magma beneath the surface of Earth. Also known as an intrusion.

Pluvial lake A lake formed during a period of increased rainfall. During the Pleistocene epoch, this occurred in some nonglaciated regions during periods of ice advance elsewhere.

Point bar A crescent-shaped accumulation of sand and gravel deposited on the inside of a meander.

Polar (P) air mass A cold air mass that forms in a highlatitude source region.

Polar easterlies In the global pattern of prevailing winds,

Polar wandering As a result of paleomagnetic studies in the 1950s, researchers proposed that either the magnetic poles migrated greatly through time or the continents had gradually shifted their positions.

Population I Stars rich in atoms heavier than helium. Nearly always relatively young stars found in the disk of the galaxy.

Population II Stars poor in atoms heavier than helium. Nearly always relatively old stars found in the halo, globular clusters, or nuclear bulge.

Porosity The volume of open spaces in rock or soil. Porphyritic texture An igneous texture consisting of large crystals embedded in a matrix of much smaller crystals.

Positive-feedback mechanism A feedback mechanism that enhances or drives change.

Pothole A circular depression in a bedrock stream channel created by the abrasive action of particles swirling in fastmoving eddies.

Precambrian All geologic time prior to the Paleozoic era. Precession See Axial precession. Precipitation fog Fog formed when rain evaporates as it falls through a layer of cool air.

Pressure gradient The amount of pressure change occurring over a given distance.

Pressure tendency The nature of the change in atmospheric pressure over the past several hours. It can be a useful aid in short-range weather prediction. Also known as barometric tendency.

Prevailing wind A wind that consistently blows from one direction more than from another.

Primary (P) wave A type of seismic wave that involves alternating compression and expansion of the material through which it passes.

Primary pollutants Pollutants emitted directly from identifiable sources.

Primary productivity The amount of organic matter synthesized by organisms from inorganic substances through photosynthesis or chemosynthesis within a given volume of water or habitat in a unit of time.

Principal shells See Energy levels. Proglacial lake A lake created when a glacier acts as a dam, blocking the flow of a river or trapping glacial meltwater. The term refers to the position of such lakes just beyond the outer limits of a glacier.

Prokaryotes Cells or organisms such as bacteria whose genetic material is not enclosed in a nucleus.

Prominence A concentration of material above the solar surface that appears as a bright archlike structure.

Proterozoic eon The eon following the Archean and preceding the Phanerozoic. It extends between about 2500 million (2.5 billion) and 540 million years ago.

Proton A positively charged subatomic particle found in the nucleus of an atom.

Proton–proton chain A chain of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium.

Protoplanet A developing planetary body that grows by the accumulation of planetesimals.

Protostar A collapsing cloud of gas and dust destined to become a star.

winds that blow from the polar high toward the subpolar low. These winds, however, should not be thought of as persistent winds, such as the trade winds.

Psychrometer A device consisting of two thermometers (wet

Polar front The stormy frontal zone separating air masses of

Ptolemaic system An Earth-centered system of the universe. Pulsar A variable radio source of small size that emits radio

polar origin from air masses of tropical origin.

Polar high Anticyclones that are assumed to occupy the inner polar regions and are believed to be thermally induced, at least in part.

bulb and dry bulb) that is rapidly whirled and, with the use of tables, yields the relative humidity and dew point.

pulses in very regular periods.

Pulsating variable A variable star that pulsates in size and luminosity.

GLOSSARY

Pycnocline A layer of water in which there is a rapid change of density with depth. Pyroclastic flow A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground.

Pyroclastic material The volcanic rock ejected during an eruption, including ash, bombs, and blocks.

Pyroclastic texture An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. Also known as fragmental texture.

Quaternary period The most recent period on the geologic time scale. It began about 2.6 million years ago and extends to the present.

Radial pattern A system of streams running in all directions away from a central elevated structure, such as a volcano.

Radiation The transfer of energy (heat) through space by electromagnetic waves. Also known as electromagnetic radiation. Radiation fog Fog resulting from radiation heat loss by Earth. Radiation pressure The force exerted by electromagnetic radiation from an object such as the Sun.

Radio interferometer Two or more radio telescopes that combine their signals to achieve the resolving power of a larger telescope. Radio telescope A telescope designed to make observations in radio wavelengths.

Radioactive decay The spontaneous decay of certain unstable atomic nuclei.

Radioactivity The spontaneous emission of certain unstable atomic nuclei.

Radiocarbon (carbon-14) The radioactive isotope of carbon, which is produced continuously in the atmosphere and is used in dating events from the very recent geologic past (the last few tens of thousands of years).

Radiometric dating The procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.

Radiosonde A lightweight package of weather instruments fitted with a radio transmitter and carried aloft by a balloon.

Rain Drops of water that fall from clouds that have a diameter of at least 0.5 millimeter (0.02 inch).

Rain shadow desert A dry area on the lee side of a mountain range. Many middle-latitude deserts are of this type.

Rapids A part of a stream channel in which the water suddenly begins flowing more swiftly and turbulently because of an abrupt steepening of the gradient.

Ray (lunar) Any of a system of bright elongated streaks, sometimes associated with a crater on the Moon.

Recessional moraine An end moraine formed as the ice front stagnated during glacial retreat.

Rectangular pattern A drainage pattern characterized by numerous right-angle bends that develops on jointed or fractured bedrock. Red giant A large, cool star of high luminosity; a star occupying the upper-right portion of the HertzsprungRussell diagram.

Reflecting telescope A telescope that concentrates light from distant objects by using a concave mirror. Reflection The process whereby light bounces back from an object at the same angle at which it encounters a surface and with the same intensity. Reflection nebula A relatively dense dust cloud in interstellar space that is illuminated by starlight. Refracting telescope A telescope that uses a lens to bend and concentrate the light from distant objects.

Refraction The process by which the portion of a wave in shallow water slows, causing the wave to bend and tend to align itself with the underwater contours. Also known as wave refraction.

Regional metamorphism Metamorphism associated with large-scale mountain-building processes.

Regolith The layer of rock and mineral fragments that nearly everywhere covers Earth’s surface.

Relative dating Placing rocks in their proper sequence or order to determine the chronological order of events.

Relative humidity The ratio of the air’s water-vapor content to its water-vapor capacity.

Renewable resource A resource that is virtually inexhaustible or that can be replenished over relatively short time spans.

Reserve An already identified deposit from which minerals can be extracted profitably.

Reservoir rock The porous, permeable portion of an oil trap that yields oil and gas.

Residual soil Soil developed directly from the weathering of the bedrock below.

Resolving power The ability of a telescope to separate objects that would otherwise appear as one.

Retrograde motion The apparent westward motion of the planets with respect to the stars.

Reverse fault A fault in which the material above the fault plane moves up in relation to the material below.

Reverse polarity A magnetic field opposite to that which exists at present.

Revolution The motion of one body about another, as Earth about the Sun.

Richter scale A scale of earthquake magnitude based on the motion of a seismograph.

Ridge push A mechanism that may contribute to plate motion. It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.

Rift valley A long, narrow trough bounded by normal faults. It represents a region where divergence is taking place.

Rift zone A region of Earth’s crust along which divergence is taking place.

Right ascension An angular distance measured eastward along the celestial equator from the vernal equinox. Used with declination in a coordinate system to describe the position of celestial bodies.

Rime A thin coating of ice on objects produced when supercooled fog droplets freeze on contact.

Ring of Fire The zone of active volcanoes surrounding the Pacific Ocean.

Rip current A strong narrow surface or near-surface current of short duration and high speed flowing seaward through the breaker zone at nearly right angles to the shore. It represents the return to the ocean of water that has been piled up on the shore by incoming waves.

Rock A consolidated mixture of minerals. Rock avalanche Very rapid downslope movement of rock and debris. These rapid movements may be aided by a layer of air trapped beneath the debris, and they have been known to reach speeds of over 200 kilometers (125 miles) per hour.

Rock cycle A model that illustrates the origin of the three basic rock types and the interrelatedness of Earth materials and processes.

Rock flour Ground-up rock produced by the grinding effect of a glacier.

Rock structure All features created by the processes of deformation from minor fractures in bedrock to a major mountain chain. Also known as geologic structure.

Rock-forming minerals The minerals that make up most of the rocks of Earth’s crust.

Rockslide The rapid slide of a mass of rock downslope along planes of weakness.

777

Rotation The spinning of a body, such as Earth, about its axis.

Runoff Water that flows over the land rather than infiltrating into the ground.

Saffir–Simpson hurricane scale A scale, from 1 to 5, used to rank the relative intensities of hurricanes.

Salinity The proportion of dissolved salts to pure water, usually expressed in parts per thousand (‰).

Saltation Transportation of sediment through a series of leaps or bounces.

Santa Ana The local name given a chinook wind in southern California.

Saturation The maximum quantity of water vapor that the air can hold at any given temperature and pressure.

Scattering The redirecting (in all directions) of light by small particles and gas molecules in the atmosphere. The result is diffused light.

Scoria Hardened lava that has retained the vesicles produced by escaping gases.

Scoria cone See Cinder cone. Sea arch An arch formed by wave erosion when caves on opposite sides of a headland unite.

Sea breeze A local wind blowing from the sea during the afternoon in coastal areas.

Sea ice Frozen seawater that is associated with polar regions. The area covered by sea ice expands in winter and shrinks in summer.

Sea stack An isolated mass of rock standing just offshore, produced by wave erosion of a headland.

Seafloor spreading The process of producing new seafloor between two diverging plates.

Seamount An isolated volcanic peak that rises at least 1000 meters (3000 feet) above the deep-ocean floor.

Seawall A barrier constructed to prevent waves from reaching the area behind the wall. Its purpose is to defend property from the force of breaking waves.

Secondary enrichment The concentration of minor amounts of metals that are scattered through unweathered rock into economically valuable concentrations by weathering processes.

Secondary pollutants Pollutants that are produced in the atmosphere by chemical reactions that occur among primary pollutants.

Secondary (S) wave A seismic wave that involves oscillation perpendicular to the direction of propagation.

Sediment Unconsolidated particles created by the weathering and erosion of rock, by chemical precipitation from solution in water, or from the secretions of organisms and transported by water, wind, or glaciers.

Sedimentary rock Rock formed from the weathered products of preexisting rocks that have been transported, deposited, and lithified.

Seismic gap A segment of an active fault zone that has not experienced a major earthquake over a span when most other segments have. Such segments are probable sites for future major earthquakes.

Seismic waves A rapidly moving ocean wave generated by earthquake activity capable of inflicting heavy damage in coastal regions.

Seismogram The record made by a seismograph. Seismograph An instrument that records earthquake waves. Also known as a seismometer.

Seismology The study of earthquakes and seismic waves. Seismometer See Seismograph. Selective absorbers Gases that absorb and emit radiation only in certain wavelengths.

Semiarid See Steppe.

778

GLOSSARY

Semidiurnal tidal pattern A tidal pattern exhibiting two high tides and two low tides per tidal day with small inequalities between successive highs and successive lows; a semi-daily tide.

Settling velocity The speed at which a particle falls through a still fluid. The size, shape, and specific gravity of particles influence settling velocity.

Shadow zone The zone between 104 and 143 degrees distance from an earthquake epicenter in which direct waves do not arrive because of refraction by Earth’s core.

Shear Stress that causes two adjacent parts of a body to slide past one another.

Sheeting A mechanical weathering process characterized by the splitting-off of slablike sheets of rock.

Shelf break The point where a rapid steepening of the gradient occurs, marking the outer edge of the continental shelf and the beginning of the continental slope.

Shield A large, relatively flat expanse of ancient metamorphic rock within the stable continental interior.

Shield volcano A broad, gently sloping volcano built from fluid basaltic lavas.

Shore Seaward of the coast, a zone that extends from the highest level of wave action during storms to the lowest tide level.

Shoreline The line that marks the contact between land and sea. It migrates up and down as the tide rises and falls.

Sidereal day The period of Earth’s rotation with respect to the stars.

Sidereal month A time period based on the revolution of the Moon around Earth with respect to the stars.

Silicate Any one of numerous minerals that have the oxygen and silicon tetrahedron as their basic structure.

Silicon-oxygen tetrahedron A structure composed of four oxygen atoms surrounding a silicon atom that constitutes the basic building block of silicate minerals.

Sill A tabular igneous body that was intruded parallel to the layering of preexisting rock.

Sinkhole A depression produced in a region where soluble rock has been removed by groundwater.

Slab pull A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along.

Sleet Frozen or semifrozen rain formed when raindrops freeze as they pass through a layer of cold air.

Slide A movement common to mass-wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.

Slip face The steep, leeward slope of a sand dune; it maintains an angle of about 34 degrees.

Slump The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface.

Small solar system bodies Solar system objects not classified as planets or moons that include dwarf planets, asteroids, comets, and meteoroids.

Snow A solid form of precipitation produced by sublimination of water vapor.

Snowfield An area where snow persists year-round. Snowline The lower limit of perennial snow. Soil A combination of mineral and organic matter, water, and air; the portion of the regolith that supports plant growth.

Soil horizon A layer of soil that has identifiable characteristics produced by chemical weathering and other soil-forming processes.

Soil profile A vertical section through a soil, showing its succession of horizons and the underlying parent material.

Soil taxonomy A soil classification system consisting of six hierarchical categories based on observable soil characteristics. The system recognizes 12 soil orders.

Soil texture The relative proportions of clay, silt, and sand

Stationary front A situation in which the surface position of a front does not move; the flow on either side of such a boundary is nearly parallel to the position of the front.

Steam fog Fog having the appearance of steam, produced

in a soil. A soil’s texture strongly influences its ability to retain and transmit water and air.

by evaporation from a warm water surface into the cool air above.

Solar constant The rate at which solar radiation is received

Stellar parallax A measure of stellar distance. Steppe One of the two types of dry climate. A marginal and

outside Earth’s atmosphere on a surface perpendicular to the Sun’s rays when Earth is at an average distance from the Sun.

Solar eclipse An eclipse of the Sun. Solar flare A sudden and tremendous eruption in the solar chromosphere.

Solar nebula The cloud of interstellar gas and/or dust from which the bodies of our solar system formed.

Solar winds Subatomic particles ejected at high speed from the solar corona.

Solifluction A slow, downslope flow of water-saturated materials common to permafrost areas.

Solstice The time when the vertical rays of the Sun are striking either the Tropic of Cancer or the Tropic of Capricorn. Solstice represents the longest or shortest day (length of daylight) of the year.

Solum The O, A, and B horizons in a soil profile. Living roots and other plant and animal life are largely confined to this zone.

Sorting The process by which solid particles of various sizes are separated by moving water or wind. Also, the degree of similarity in particle size in sediment or sedimentary rock.

Source region The area where an air mass acquires its characteristic properties of temperature and moisture.

Specific gravity The ratio of a substance’s weight to the weight of an equal volume of water.

Spectral class A classification of a star according to the characteristics of its spectrum.

Spectroscope An instrument for directly viewing the spectrum of a light source.

Spectroscopy The study of spectra. Spheroidal weathering Any weathering process that tends to produce a spherical shape from an initially blocky shape.

Spicule A narrow jet of rising material in the solar chromosphere.

Spiral galaxy A flattened, rotating galaxy with pinwheellike arms of interstellar material and young stars winding out from its nucleus.

Spit An elongated ridge of sand that projects from the land into the mouth of an adjacent bay.

Spreading center See Divergent plate boundary. Spring A flow of groundwater that emerges naturally at the ground surface.

Spring equinox The equinox that occurs on March 21–22 in the Northern Hemisphere and on September 21–23 in the Southern Hemisphere.

Spring tide The highest tidal range, which occurs near the times of the new and full moons.

Stable air Air that resists vertical displacement. If it is lifted, adiabatic cooling will cause its temperature to be lower than the surrounding environment; if it is allowed, it will sink to its original position.

Stable platform The part of a craton that is mantled by relatively undeformed sedimentary rocks and underlain by a basement complex of igneous and metamorphic rocks.

Stalactite An icicle-like structure that hangs from the ceiling of a cavern.

Stalagmite A columnlike form that grows upward from the floor of a cavern.

Star dune An isolated hill of sand that exhibits a complex form and develops where wind directions are variable.

more humid variant of the desert that separates it from bordering humid climates. Also known as semiarid.

Stock A pluton similar to but smaller than a batholith. Stony meteorite One of the three main categories of meteorites. Such meteorites are composed largely of silicate minerals with inclusions of other minerals.

Stony-iron meteorite One of the three main categories of meteorites. This group, as the name implies, is a mixture of iron and silicate minerals.

Storm surge The abnormal rise of the sea along a shore as a result of strong winds.

Strain An irreversible change in the shape and size of a rock body that is caused by stress.

Strata Parallel layers of sedimentary rock. Stratified drift Sediments deposited by glacial meltwater. Stratopause The boundary between the stratosphere and the mesosphere.

Stratosphere The layer of the atmosphere immediately above the troposphere, characterized by increasing temperatures with height, due to the concentration of ozone.

Stratovolcano See Composite cone. Stratus One of three basic cloud forms; also, the name given one of the flow clouds. They are sheets or layers that cover much or all of the sky.

Streak The color of a mineral in powdered form. Stream valley The channel, valley floor, and sloping valley walls of a stream.

Stress The force per unit area acting on any surface within a solid.

Striations (glacial) Scratches or grooves in a bedrock surface caused by the grinding action of a glacier and its load of sediment.

Strike-slip fault A fault along which the movement is horizontal.

Stromatolite Structures that are deposited by algae and consist of layered mounds of calcium carbonate.

Subarctic climate A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short, cool summers. Places within this climatic realm experience the highest annual temperature ranges on Earth.

Subduction The process of thrusting oceanic lithosphere into the mantle along a convergent boundary.

Subduction erosion A process in subduction zones in which sediment and rock are scraped off the bottom of the overriding plate and transported into the mantle.

Subduction zone A long, narrow zone where one lithospheric plate descends beneath another.

Sublimation The conversion of a solid directly to a gas without passing through the liquid state.

Submarine canyon A seaward extension of a valley that was cut on the continental shelf during a time when sea level was lower, or a canyon carved into the outer continental shelf, slope, and rise by turbidity currents.

Submergent coast A coast with a form that is largely the result of the partial drowning of a former land surface either because of a rise of sea level or subsidence of the crust or both.

GLOSSARY

Subpolar low Low pressure located at about the latitudes of the Arctic and Antarctic Circles. In the Northern Hemisphere the low takes the form of individual oceanic cells; in the Southern Hemisphere there is a deep and continuous trough of low pressure.

Subsoil A term applied to the B horizon of a soil profile. Subtropical high Not a continuous belt of high pressure but rather several semipermanent, anticyclonic centers characterized by subsidence and divergence located roughly between latitudes 25° and 35°.

Summer solstice The solstice that occurs on June 21–22 in the Northern Hemisphere and on December 21–22 in the Southern Hemisphere.

Sunspot A dark spot on the Sun, which is cool in contrast to the surrounding photosphere.

Supercontinent A large landmass that contains all, or nearly all, of the existing continents.

Supercontinent cycle The idea that the rifting and dispersal of one supercontinent is followed by a long period during which the fragments gradually reassemble into a new supercontinent.

Supercooled The condition of water droplets that remain in the liquid state at temperatures well below 0°C (32°F).

Supergiant A very large star of high luminosity. Supernova An exploding star that increases in brightness many thousands of times.

Superposition The principle that in any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below.

Supersaturation The condition of being more highly concentrated than is normally possible under given temperature and pressure conditions. When describing humidity, it refers to a relative humidity that is greater than 100 percent.

Surf A collective term for breakers; also, the wave activity in the area between the shoreline and the outer limit of breakers.

Surface soil The uppermost layer in a soil profile: the A horizon.

Surface waves Seismic waves that travel along the outer layer of Earth.

Suspended load The fine sediment carried within the body of flowing water.

Suture A zone along which two crustal fragments are jointed together. For example, following a continental collision, the two continental blocks are sutured together.

Swells Wind-generated waves that have moved into an area of weaker winds or calm.

Syncline A linear downfold in sedimentary strata; the opposite of anticline.

Synodic month The period of revolution of the Moon with respect to the Sun, or its cycle of phases.

System Any size group of interacting parts that form a complex whole.

Talus An accumulation of rock debris at the base of a cliff. Tarn A small lake in a cirque. Tectonic plate A coherent unit of Earth’s rigid outer layer that includes the crust and upper unit.

Tectonics The study of the large-scale processes that collectively deform Earth’s crust.

Temperature A measure of the degree of hotness or coldness of a substance; a measure of the average kinetic energy of individual atoms or molecules in a substance.

Temperature gradient The amount of temperature change per unit of distance.

Temperature inversion A layer in the atmosphere of limited depth where the temperature increases rather than decreases with height.

Temporary (local) base level The level of a lake, resistant rock layer, or any other base level that stands above sea level.

Tenacity A mineral’s toughness or resistance to breaking or deforming.

Tensional stress The type of stress that tends to pull apart a body.

Terminal moraine The end moraine marking the farthest advance of a glacier.

Terrace A flat, benchlike structure produced by a stream, which was left elevated as the stream cut downward.

Terrae The extensively cratered highland areas of the Moon. Also known as lunar highlands.

Terrane A crustal block bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks.

Terrestrial planets Any of the Earth-like planets, including Mercury, Venus, Mars, and Earth. Also known as the inner planets.

Terrigenous sediment Seafloor sediments derived from terrestrial weathering and erosion.

Texture The size, shape, and distribution of the particles that collectively constitute a rock.

Theory A well-tested and widely accepted view that explains certain observable facts.

Thermal gradient The increase in temperature with depth. It averages 1°C per 30 meters (1–2°F per 100 feet) in the crust.

Thermal metamorphism See Contact metamorphism. Thermocline A layer of water in which there is a rapid change in temperature in the vertical dimension.

Thermohaline circulation Movements of ocean water caused by density differences brought about by variations in temperature and salinity.

Thermosphere The region of the atmosphere immediately above the mesosphere and characterized by increasing temperatures due to absorption of very shortwave solar energy by oxygen.

Thrust fault A low-angle reverse fault. Thunder The sound emitted by rapidly expanding gases along the channel of lightning discharge.

Thunderstorm A storm produced by a cumulonimbus cloud and always accompanied by lightning and thunder. It is of relatively short duration and usually accompanied by strong wind gusts, heavy rain, and sometimes hail.

Tidal current The alternating horizontal movement of water associated with the rise and fall of the tide.

Tidal delta A deltalike feature created when a rapidly moving tidal current emerges from a narrow inlet and slows, depositing its load of sediment.

Tidal flat A marshy or muddy area that is covered and uncovered by the rise and fall of the tide.

Tide Periodic change in the elevation of the ocean surface. Till Unsorted sediment deposited directly by a glacier. Tombolo A ridge of sand that connects an island to the mainland or to another island.

Tornado A small, very intense cyclonic storm with exceedingly high winds, most often produced along cold fronts in conjunction with severe thunderstorms.

Tornado warning A warning issued when a tornado has actually been sighted in an area or is indicated by radar.

Tornado watch A warning issued for areas of about 65,000 square kilometers (25,000 square miles), indicating that conditions are such that tornadoes may develop; it is intended to alert people to the possibility of tornadoes.

779

Trade winds Two belts of winds that blow almost constantly from easterly directions and are located on the equatorward sides of the subtropical highs.

Transform fault A major strike-slip fault that cuts through the lithosphere and accommodates motion between two plates.

Transform plate boundary A boundary in which two plates slide past one another without creating or destroying lithosphere.

Transpiration The release of water vapor to the atmosphere by plants.

Transported soil Soils that form on unconsolidated deposits. Transverse dunes A series of long ridges oriented at right angles to the prevailing wind; these dunes form where vegetation is sparse and sand is very plentiful.

Travertine A form of limestone (CaCO3) that is deposited by hot springs or as a cave deposit.

Trellis pattern A system of streams in which nearly parallel tributaries occupy valleys cut in folded strata.

Trench An elongated depression in the seafloor produced by bending of oceanic crust during subduction. Also known as deep-ocean trench.

Trophic level A nourishment level in a food chain. Plant and algae producers constitute the lowest level, followed by herbivores and a series of carnivores at progressively higher levels.

Tropic of Cancer The parallel of latitude, 23½° north latitude, marking the northern limit of the Sun’s vertical rays.

Tropic of Capricorn The parallel of latitude, 23½° south latitude, marking the southern limit of the Sun’s vertical rays.

Tropical depression By international agreement, a tropical cyclone with maximum winds that do not exceed 61 kilometers (38 miles) per hour.

Tropical rain forest A luxuriant broadleaf evergreen forest; also, the name given the climate associated with this vegetation.

Tropical storm By international agreement, a tropical cyclone with maximum winds between 61 and 119 kilometers (38 and 74 miles) per hour.

Tropical wet and dry A climate that is transitional between the wet tropics and the subtropical steppes.

Tropopause The boundary between the troposphere and the stratosphere.

Troposphere The lowermost layer of the atmosphere. It is generally characterized by a decrease in temperature with height.

Tsunami The Japanese word for a seismic sea wave. Tundra climate A climate found almost exclusively in the Northern Hemisphere or at high altitudes in many mountainous regions. A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by a long, bitterly cold winter.

Turbidite A turbidity current deposit characterized by graded bedding.

Turbidity current A downslope movement of dense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension.

Turbulent flow The movement of water in an erratic fashion, often characterized by swirling, whirlpool-like eddies. Most streamflow is of this type.

Ultimate base level Sea level; the lowest level to which stream erosion could lower the land.

Ultramafic composition Igneous rocks composed mainly of iron and magnesium-rich minerals.

Ultraviolet Radiation with a wavelength from 0.2 to 0.4 micrometer.

780

GLOSSARY

Umbra The central, completely dark part of a shadow produced during an eclipse.

Unconformity A surface that represents a break in the rock record, caused by erosion or nondeposition.

Uniformitarianism The concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today.

Unsaturated zone The area above the water table where openings in soil, sediment, and rock are not saturated but filled mainly with air.

Unstable air Air that does not resist vertical displacement. If it is lifted, its temperature will not cool as rapidly as the surrounding environment, so it will continue to rise on its own.

Upslope fog Fog created when air moves up a slope and cools adiabatically.

Upwelling The rising of cold water from deeper layers to replace warmer surface water that has been moved away.

Urban heat island The phenomenon of temperatures within a city being generally higher than in surrounding rural areas.

Valence electron The electrons involved in the bonding process; the electrons occupying the highest principal energy level of an atom.

Valley breeze The daily upslope winds commonly encountered in a mountain valley.

Valley glacier See Alpine glacier. Valley train A relatively narrow body of stratified drift deposited on a valley floor by meltwater streams that issue from a valley glacier.

Vapor pressure The part of the total atmospheric pressure attributable to water-vapor content.

Variable stars Red giants that overshoot equilibrium and then alternately expand and contract.

Vein deposit A mineral filling a fracture or fault in a host rock. Such deposits have a sheetlike, or tabular, form.

Vent The surface opening of a conduit or pipe. Ventifact A cobble or pebble polished and shaped by the sandblasting effect of wind.

Vesicular texture A term applied to igneous rocks that contain small cavities called vesicles, which are formed when gases escape from lava.

Viscosity A measure of a fluid’s resistance to flow. Visible light Radiation with a wavelength from 0.4 to 0.7 micrometer.

Volatiles Gaseous components of magma dissolved in melt. Volatiles readily vaporize (form a gas) at surface pressures.

Volcanic bomb A streamlined pyroclastic fragment ejected from a volcano while molten.

Volcanic cone A cone-shaped structure built by successive eruptions of lava and/or pyroclastic materials.

Volcanic island arc A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occurring. Also known simply as island arc.

Volcanic neck An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano. Also known as a plug.

Warm front A front along which a warm air mass overrides a retreating mass of cooler air.

Wash A common term for a desert stream course that is typically dry except for brief periods immediately following a rain.

Water table The upper level of the saturated zone of groundwater.

Wave base A depth equal to one-half the wavelength measured from the still water level. Below this depth, water movement associated with a wave is negligible.

Wave height The vertical distance between the trough and crest of a wave.

Wave of oscillation A water wave in which the waveform advances as the water particles move in circular orbits.

Wave of translation The turbulent advance of water created by breaking waves.

Wave period The time interval between the passage of successive crests at a stationary point.

Wave refraction See Refraction. Wave-cut cliff A seaward-facing cliff along a steep shoreline formed by wave erosion at its base and mass wasting.

Wave-cut platform A bench or shelf in the bedrock at sea level, cut by wave erosion.

Wavelength The horizontal distance separating successive crests or troughs.

Weather The state of the atmosphere at any given time. Weathering The disintegration and decomposition of rock at or near Earth’s surface.

Welded tuff A pyroclastic rock composed of particles that have been fused together by the combination of heat still contained in the deposit after it has come to rest and by the weight of overlying material.

Well An opening bored into the zone of saturation. Westerlies The dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.

Wet adiabatic rate The rate of adiabatic temperature change in saturated air. The rate of temperature change is variable, but it is always less than the dry adiabatic rate.

White dwarf A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; believed to be near its final stage of evolution.

White frost Ice crystals instead of dew that form on surfaces when the dew point is below freezing.

Wind Air flowing horizontally with respect to Earth’s surface.

Wind vane An instrument used to determine wind direction. Winter solstice The solstice that occurs on December 21–22 in the Northern Hemisphere and on June 21–22 in the Southern Hemisphere.

Yazoo tributary A tributary that flows parallel to the main stream because a natural levee is present.

Zodiac A band along the ecliptic containing the 12 constellations of the zodiac.

Zone of accumulation The part of a glacier characterized by snow accumulation and ice formation. Its outer limit is the snowline.

Zone of fracture The upper portion of a glacier, consisting of brittle ice.

Zone of saturation The zone where all open spaces in sediment and rock are completely filled with water.

Zone of wastage The part of a glacier beyond the zone of accumulation where all of the snow from the previous winter melts, as does some of the glacial ice.

Zooplankton Animal plankton.

INDEX A Aa flow, 302 Abrasion, 200, 483 wave impact and, 483 wind, 218 Absolute instability, 549–550 Absolute magnitude, 741 Absolute stability, 549 Absorption, 523–524 Absorption spectrum, 718–719 Abyssal plains, 45, 442 Abyssal zone, 465 Acadian Orogeny, 356, 357 Accretion, 352–353 Accretionary wedge, 351, 439 Acid rain, 511 burning of coal, 108 Active continental margin, 350, 439 Adiabatic temperature changes, 544–545 Advection fog, 556 Aerosols, 509, 510 from volcanic ash and gases, 316 Aftershocks, 268 Agate, 94, 95 Age of Earth, determining, 369 Aggregates, 55, 107 Air, 546–548 Airflow aloft, 606 Air-mass, 598–601 described, 598–599 source regions of, 599 weather associated with, 599–601 Air-mass weather, 599 Air pollution Beijing, China, 27 burning of coal, 107–108 Air pollution, types of, 510, 512 Air pressure, 572–574 Air temperature data, 525–526 Albedo, 523, 528–529 Alexandrite, 74 Alleghanian Orogeny, 356, 357 Alluvial channels, 161, 162 Alluvial fan, 167, 169, 215 Alluvium, 161 Alpine glaciers, 192 Altitude, temperature changes based on, 528 Aluminum (Al), 67, 103 Amethyst, 74 Amphibole, 68, 70, 87, 89 Andean-type plate margins, 350, 350–351 Andesitic (intermediate) composition, 83–84, 88 Aneroid barometer, 573 Angiosperms, 420 Angle of repose, 137 Angular unconformity, 373

Antarctica, 196–197 fact file, 196–197 ice sheets, 193–194 Anthracite coal, 95 Anticlines, 108–109, 341–342 Anticyclone, 578 Apatite, 107 Aphelion, 674 Aphotic zone, 461 Appalachians, 44, 355–357 Aquifer, 175 confined, 178 Aquitards, 174–175 Archaeology vs. paleontology, 377 Archean eon, 385 Arctic (A) air mass, 599 Arctic, global warming and, 652–653 Arêtes, 202 Argentite, 103 Arid climate, 635 Arrowheads, 88, 95 Artesian systems, 177–178 Artificial levees, 171 Asbestos, 107 Asteroid belt, 705 Asteroids, 33, 705–706 Asthenosphere, 40, 237, 291 Astrology, Orion the Hunter, 670–671 Astronomical unit (AU), 666 Astronomy, 25, 659–681 eclipses of the Sun and Moon, 677–679 motions of Earth, 673–675 motions of Earth-Moon system, 675–677 positions in sky, 669, 672–673 Astronomy, ancient, 660–663 Golden Age of early astronomy, 660–662 mapping the stars, 662 measuring the Earth’s circumference, 661 Ptolemy’s model, 662–663 Astronomy, modern, 663–669 Brahe and, 664–665 Copernicus and, 663–664 Galileo and, 666–668 Kepler and, 665–666 Newton and, 668–669 Atlantic coast, 492 Atmosphere, 36–37, 506–514 air pollution, types of, 512 air temperature data, 525–526 altering, 512 carbon dioxide (CO2), 508–509 carbon dioxide in, response to, 644–645 composition of, 508–510, 512 Earth-Sun relationship in, 514–519

evolution of, 399 heating of, 524–525 heat transfer mechanisms in, 520–522 major components of, 508 ozone depletion in, 510, 512 of planets, 687 solar radiation in, incoming, 522–525 temperature controls for, 526–529 temperatures in, world distribution of, 530–531 variable components of, 509–510 vertical structure of, 512–514 weather and, 506–508 Atmospheric stability, 548–551 absolute instability, 549–550 absolute stability, 549 conditional instability, 550 daily weather and, 550–551 types of, 548–550 Atomic number, 56 Atoms, 56–57, 60–62 atomic number and, 56 bonding of, 60–62 chemical compounds and, 57 defined, 56–57 models of, 56 orderly packing of, 55 periodic table and, 57 proton, neutron, and electron properties, 56 structure, review of, 381 Aurora, 734 Automated Surface Observing System (ASOS), 29 Autumnal equinox, 518 Axial precession, 673, 675

B Backshore, 479–480 Back swamp, 168 Bajada, 215 Banded iron formations, 400 Bar, 167 Barchan dunes, 220 Barchanoid dunes, 220–221 Barograph, 574 Barometric pressure, 573–574 Barometric tendency, 579 Barred spiral galaxy, 754 Barrier islands, 487 Basalt, 39 Basaltic composition, 83, 84, 88, 88–89 Basalt plateau, 319 Base level erosion, 164 Base level meanders, 165–166 Basin and Range landscape, 215–216 Basins, 343 Batholiths, 323–324 emplacement, 350–351

Bathymetry, 432 Bauxite, 103 Baymouth bar, 486–487 Beach, 480 drift, 485 face, 480 nourishment, 490–491 sand movement on, 483–485 shoreline of, 480 Beach drift, 485 Bedding planes, 97 Bed load, 160, 161 Bedrock channels, 161–162 Benthic zone, 465 Benthos, 460–461 Bergeron process, 558 Berms, 480 Big Bang theory, 32, 397, 741, 756–758 Bingham Canyon copper mine, 74, 106 Biochemical sedimentary rock, 93–95 Biogenous sediment, 444–445 Biological activity, in weathering, 120–121 Biomass, 460 Biosphere, 37–38 Biotite, 68, 70, 89 Bituminous coal, 95 Black carbon aerosols, 649 Black hole, 750–751 Blowouts, 217, 218, 222 “The Blue Marble” view from Apollo 37, 34 Blue skies, 523 Blue whale, 467 Body wave, 271, 272 Bonding of atoms, 60–62 Bornite, 103 Bottom dwellers, 460–461 Bowen’s reaction series, 89–90 Brahe, Tycho, 664–665 Braided channels, 163 Braided streams, 163 Breakwater, 489 Bright-line (emission) spectrum, 719 Bright nebulae, 742–743 Brittle deformation, 345–348 faults formed by, 345–347 joints formed by, 347–348 Building materials, 107

C Calcite, 67, 70, 71, 107 Calcium (Ca), 67 Calcium carbonate, 71 Calderas, 317–319 Calorie, 538 Calving, 199 Cambrian period, 411 Canadian Shield, 44–45 Capacity, 161 Cap rock, 108

Carbon-34, 383 Carbonates, 67, 70, 71 Carbon dioxide (CO2), 508–509 Carbon dioxide, global climate change and, 643–644 Carbonic acid, 121–122 Cassiterite, 103 Catastrophism, 368 Cat’s-eye, 74 Caverns, 173, 182–183 Celestial sphere, 661 Cementation, 91–93, 96 Cenozoic era, 385, 408–409, 420–422. See also Mammals Eastern North America, 409 flowering plants and, 420 Western North America, 409 Chalcedony, 74 Chalcocite, 103 Chalcopyrite, 103 Chalk, 93 Chandra X-Ray Observatory (CXO), 725–726 Channelization, 171 Charleston earthquake, 282, 283 Chemical bond, 60 Chemical compounds, 60 Chemical sedimentary rock, 93, 93–95 Chemical weathering, 119, 121–123 carbonic acid, 121–122 in deserts, 214 of granite, 122–123 water in, 121, 122–123 Chert, 94, 95 Chinook, 584 Chlorofluorocarbons (CFCs), 512 Chromatic aberration, 721 Chromite, 103 Chromium, 103 Chromosphere, 728 Chrysoberyl, 74 Cinder cones, 310–311 Cinnabar, 103 Circle of illumination, 515 Circular orbital motion, 482 Circulation idealized global, 580–581 on nonrotating Earth, 580 Circumference of Earth, 661 Circum-Pacific Belt, 44, 281 Cirques, 202 Cirrus, 552 Citrine, 74 Clay, 107 minerals, 69 Cleavage, 64–65 Climate-feedback mechanisms, 648–649 Climates, 507, 626–657. See also Global climate change ancient, continental drift and, 234 classification, 629–632 computer models of, 649

781

782

INDEX

dry (B), 635–637 highland, 641–642 human impact on global, 643–644 humid middle-latitude climates with mild winters (C), 637–638 humid middle-latitude climates with severe winters (D), 638–640 humid tropical (A), 632–635 influence on ocean current patterns, 476–477 polar (E), 640–641 volcanoes and climate changes, 316 weather and, 506–508 world, 629–632 Climate system, 628–629 Climatology of tornado, 611–612 Clouds, 551–555 adiabatic temperature changes and, 544–545 cold, 558 condensation and, 551–555 cover, temperature changes based on, 528–529 fog and dew vs., 544 formations, 544–545 types of, 552–555 vertical development of, 553, 555 warm, 559 Coal, 95 as fossil fuel, 107–108 Coarse-grained texture, 84, 85, 86, 87, 88 Coast, 479 brief tour of, 494–495 classification of, 493, 496 emergent, 493, 496 submergent, 496 Coastline, 479 Coast Ranges in California, 352 Cold clouds, 558 Cold front, 602–603 Collisional mountain belts, 352–357 Appalachians, 355–357 Himalayas, 354–355 terranes, 352 Collision-coalescence process, 559 Color (mineral), 63 Columnar joint, 323 Coma, 706 Comets, 706–707 Compaction, 91–93, 95–96 Competence, 161 Composite cones, 311–313 lahars, 315 nuée ardentes, 314–315 Compositional layers of Earth. See Internal structure of Earth Compressional forces, 360–361 Compressional mountain, 349 Compton Gamma Ray Observatory (CGRO), 725 Conchoidal fractures, 65, 66 Concordant, 321 Condensation, 539, 545 cloud formation and, 551–555 Condensation nuclei, 551 Conditional instability, 550 Conduction, 520–521 Conduit, 304

Cone of depression, 177 Confining pressure, 99–100, 339 Conformable, 372 Constellations, 669, 672 Contact (thermal) metamorphism, 98, 99 Continental (c) air mass, 599 Continental-continental convergence, 244 Continental drift, 40–41 debate over, 235–236 development of, 230–234 evidence of, 231–234 Continental margin, 45, 436–439 Continental mountain belt collisions Appalachians, 355–357 Himalayas, 354–355 Continental rift, 240–241 Continental rise, 437 Continental shelf, 45, 437 Continental slope, 45, 437 Continental volcanic arc, 243, 327, 439 Continents features of, major, 42–43, 43–45 first on Earth, 401–404 influence on winds, 581–582 mountain belts, 44 stable interior, 44–45 Continuous spectrum, 718 Convection, 521 Convective lifting, 546 Convergence, 547, 578 Convergent boundaries, 41, 237, 238, 241–244 continental-continental convergence and, 244 faults associated, 269–270 oceanic-continental convergence and, 242–243 oceanic-oceanic convergence and, 243 volcanism at, 326–327 Copernicus, Nicolaus, 663–664 Copper, 57, 103 Bingham Canyon copper mine, 74, 106 as resource, 26 Coquina, 93, 94 Coral atolls, 440–441 Coral reefs, 452–453 Core, 40, 291 Coriolis effect, 474, 575–576, 577 Corona, 728–729 Correlation of rock layers, 378–380 Corundum, 74, 107 Cosmological red shift, 756 Cosmology, 740 Country breezes, 583–584 Covalent bonds, 60–61 Crater, 306 Crater Lake-type calderas, 317 Cratons, 403 Creep, 140, 142–143 Crevasse, 195 Cross beds, 220 Cross-cutting relationships, principle of, 371 Crust, 39, 289, 290 Crustal deformation, 338–340

Cryovolcanism, 704 Crystallization, 80, 82–83, 91 Crystal settling, 90 Crystal shape, 63–64 Cumulonimbus clouds, 555 Cumulus clouds, 552 Cup anemometer, 585 Curie point, 251 Cut bank, 163 Cutoff, 163 Cyclones, 578, 607 Cyclonic anticyclonic, 578 Cyclonic winds, 578

D Dams, flood-control, 171 Dark energy, 758 Dark-line (absorption) spectrum, 718–719 Dark matter, 758 Dark nebulae, 744 Dark silicates, 83 Darwin, Charles, 440–441 Death Valley landscape, 216 Debris flow, 139, 141 Declination, 672 Decompression melting, 325 Deep-ocean basins, 45, 439, 442 abyssal plains, 442 deep-ocean trenches, 439, 442 guyots, 442 oceanic plateaus, 442 seamonts, 442 Deep-ocean circulation, 477–478 Deep-ocean drilling, 289 Deep-ocean trenches, 45, 241, 439, 442 Deep-sea fan, 437 Deep-sea hydrothermal vent biocommunities, 463 Deflation, 217, 218 Deforestation, 644 Deformation, 338–340 brittle, 339, 345–348 ductile, 340, 341–344 elastic, 339 rock strength and, 340 Degenerate matter, 750 Deltas, 167, 167–168 Dendritic pattern, 155 Density, 66, 457 Deposition, 540 Depositional features of shoreline, 486–487 Depositional landforms, 167–169 alluvial fans, 167, 169 deltas, 167, 167–168 natural levees, 168–169 Desert pavement, 217–218, 218 Deserts, 212–216 climate of, 635 deserts and steppes, 213 dry climates, 213 geologic processes in, 214 mountainous landscape in, 215–216 sand dunes in, 219–222, 220–222 subtropical highs, 635 water’s role in, 214 weathering in, 214 wind deposits in, 218–222

wind erosion in, 217–218 Detachment fault, 346 Detrital sedimentary rock, 92–93 Devonian period, 413–414, 414 Dew, 544 Dew-point temperature, 542–543, 543 Diamonds, 64, 74, 107 Diatoms, 445 Differential stress, 100, 339 Differential weathering, 124–125 Diffused light, 523 Dikes, 322–323 Dinosaurs, 388, 395, 418–419 demise of, 416–417 Diorite, 87, 88 Dip-slip faults, 345–346 normal faults, 345–346 reverse faults, 346 thrust faults, 346 Discharge, 157 Disconformity, 373 Discordant, 321 Disseminated deposit, 106 Dissolved load, 160 Distributary, 167 Diurnal tidal pattern, 498 Divergence, 578 Divergent boundaries, 41, 237, 238–241 continental rift and, 240–241 oceanic ridge system and, 239 seafloor spreading and, 239 volcanism at, 327 Divide, 154 Dolomite, 71, 103 Domes, 342–343 Doppler effect, 719–720 Doppler radar, 614 Double refraction, 66 Drainage basins, 154 Drainage patterns, 155–156 Drawdown, 177 Drumlins, 206–207 Dry adiabatic rate, 545 Dry (B) climates, 635–637 low-latitude deserts and steppes, 635–636 middle-latitude deserts and steppes, 636–637 Dry-summer subtropical climate, 638 Ductile deformation, 340, 341–344 Dust Bowl, 589 Dwarf galaxy, 755 Dwarf planets, 705, 709–710, 716–718

E Earth age of, determining, 369 albedo of, 523 circulation on nonrotating, 580 external processes of, 116, 116–117 face of, 42–46 interior of, 289–291 internal structure of, 38–40 motions of, 515 movement of features on, 247–249 orientation of, 516 origin of, 32–34 spheres of, 34–41 sun, relationship with, 514–519

as a system, 46–47 temperature of crust, 99 winds friction with surface of, 576–577 Earthflow, 139, 142 Earth-Moon system, motions of, 675–677 Earthquake destruction, 276–280 fire, 278 ground subsidence, 278 landslides, 278 liquefaction, 277–278 from seismic waves, 276–278 from tsunami, 279–280 Earthquake prediction, 284–288 long-range, 284–288 short-range, 284 Earthquakes, 264–295 aftershocks, 268 belts, 281 causes of, 267–268 in central and eastern U.S., 282–283 circum-Pacific belt, 281 defined, 266–267 faults and, 266, 269–270 finding the epicenter, 275 foreshocks, 269 frequency of, 274 in Haiti, 2010, 266, 268 in Indonesia, 2004, 279–280 interior of Earth and, 289–291 notable, 285 plates, 281 seismology and, 270–272 size of, measuring, 272–274, 276 as triggers, 138 waves triggered by, 279 “Earthrise” view from Apollo 28, 34 Earth science defined, 24 people and earth processes, 26–27 scales of space and time in, 27–28 scientific inquiry and, 29–32 Earth’s evolution through geologic time, 392–427 atmosphere, 399–400 Big Bang theory and, 397 Cenozoic era, 420–422 early evolution, 397, 399 of life, 395, 410–411 life forms, 412 major events leading to formation of early Earth, 397–399, 398 Mesozoic era, 418–419 oceans, 400–401 oxygen, 399–400 Paleozoic era, 411, 413–414 Phanerozoic eon, 406–409 planetismals and, 397 Precambrian period, 401–406 protoplanets and, 397 timely, Earth altering events, 395 unique characteristics of, 394–395 Earth’s orbit, glaciers and, 211–212 Earth system, 46–47 cycles in, 47 energy sources for, 47 hydrologic cycle in, 152–153 parts are linked, 47 people and, 47 time and space scales, 47

INDEX

Earth system science, 46 East African Rift Valley, 240–241 Eastern North America, Cenozoic era, 409 Echo sounder, 432 Eclipses of the Sun and Moon, 677–679 Ecliptic, 674 Economic minerals, 67 Effervescency, 66, 96 Elastic deformation, 339 Elastic rebound, 268 Electrical charge, 56 Electromagnetic radiation, 521, 716–718 Electrons, 56 Elements (atoms), 56–57 Elements (weather and climate), 508 Elliptical galaxy, 755 El Niño, 586–587, 588 Eluviation, 129 Emerald, 74 Emergent coast, 493, 496 Emission nebulae, 743 Emission spectrum, 719 Enchanced Fujita intensity scale (EFscale), 613 End moraine, 204, 205–206 Energy resources, from seafloor, 446–448 Energy sources for Earth, 47 Environment astronomy used in probing origins of, 25 Dust Bowl disaster, 589 natural hazards to, 25–26 problems associated with groundwater, 179–181 Environmental lapse rate, 513 Eon, 385 Ephemeral stream, 214 Epicenter, 266, 271 finding, 275 Epoch, 385 Equatorial low, 580 Equatorial system, 672–673 Equinoxes, 516–519 Era, 385 Erosion, 117 base level, 164 features of shoreline, 486 problems along U.S. coasts, 492–493, 496 stream, 164 Eruption column, 300 Escape velocity, 687 Eskers, 207 Estuaries, 496 Eukaryotes, 411 Euphotic zone, 461 Evaporation, 152, 539 Evaporation fog, 557 Evaporative salts, 447 Evaporite deposit, 95 Evapotranspiration, 152 Exfoliation dome, 120 Explosive eruptions, 300–301 External processes of Earth, 116, 116–117 erosion, 132–134 mass wasting, 134–142 weathering and soil, 116–124, 116–127

Extrusive (volcanic) rock, 82 Eye, 616 Eye wall, 616

F Fall, 138, 466–467 Fault-block mountains, 345 Fault creep, 269 Faults, 266, 269–270, 345–347 dip-slip, 345–346 relative motion across, 346 strike-slip, 346–347 Fault scarp, 345 Fault trap, 109 Feldspars, 55, 67, 69, 83, 89 Felsic, 83, 86 Fetch, 481 Fibrous fractures, 65 Fine-grained texture, 84, 85, 87, 88 Fiords, 202 Fire, from earthquakes, 278 Fissure, 319 Fissure eruptions, 319 Flash floods, 169, 170 Flint, 94, 95 Floaters, 459–460 Flood basalts, 319–320 Floodplain, 165 Floods and flood control, 169–172 artificial levees, 171 causes of floods, 169 channelization, 171 flash floods, 169, 170 flood-control dams, 171 nonstructural approach, 171–172 Flourite, 107 Flow, 139 Focus, 266 Fog, 555–557 advection, 556 caused by cooling, 556–557 dew and, 544 evaporation, 557 frontal, 557 radiation, 556–557 steam, 557 upslope, 557 Folds, 341–342 anticlines, 341–342 basins, 343 domes, 342–343 monoclines, 343–344 synclines, 341–342 Foliation, 101, 101–102 Food chain, 467 Food web, 467–468 Forearc basins, 352 Foreshocks, 269 Foreshore, 479 Fossil assemblage, 380 Fossil fuels, 107–109 carbon dioxide and, 643–644 coal, 107–108 hydraulic fracturing (fracking), 109 oil and natural gas, 108–109 Fossil magnetism, 252 Fossils, 97, 232–233, 375–378 conditions favoring preservation, 378 correlation and, 378–379 of Glossopteris, 27, 232

of Mesosaurus, 232 types of, 376, 378 Fossil succession, principle of, 379–380 Fracking, 109 Fracture, 65, 66 Fracture zone, 245 Fragmental texture, 86 Freezing nuclei, 558 Front, 547, 601–603 cold, 602–603 occluded, 603 stationary, 603 warm, 602 Frontal fog, 557 Frontal wedging, 546–547 Frost wedging, 119–120 Fujiyama, 313 Fumarole, 306

G Gabbro, 87, 89 Galactic clusters, 755–756 Galactic collisions, 756 Galaxies, types of, 751, 754–755 elliptical, 755 irregular, 755 spiral, 754 Galena, 103 Galileo, 666–668 Garnet, 70, 74, 107 Gases extruded during volcanic eruptions, 303 Gas hydrate, 446–447 Gemstones, 74 General atmospheric conditions for tornado, 611 Geocentric, 660 Geographic position, temperature changes based on, 528 Geologic structures, 338 Geologic time, 366–391 correlation of rock layers and, 378–380 dating with radioactivity and, 380–383 Earth’s evolution through, 395–396 fossils and, 375–378 Geologic time, magnitude of, 28 Geologic time scale, 28, 384–386, 396 dating, difficulties in, 386–387 dinosaurs vs. humans, 388 need for, 368 Precambrian time and, 385–386 structure of, 385 terminology and, 386 Geology, defined, 24 Geology, history of, 368–375 geology today, 369 inclusions, 372 law of superposition, 370 modern geology, birth of, 368–369 principle of cross-cutting relationships, 371 principle of original horizontality, 370 relative dating principles, 374–375 unconformities, 372–374 Geosphere, 38–41

Geostrophic wind, 576 Geothermal gradient, 324 Geysers, 176 Glacial budget, 198–199 Glacial deposits, 203–207 drumlins, 206–207 eskars, 207 glacial drift, 203–204 kames, 207 moraines, 204–206 Glacial drift, 203–204 Glacial erosion, 199–202 landforms created by, 200–202 process of, 200 Glacial erratic, 204 Glacial striations, 200 Glacial troughs, 201 fiords, 202 Glaciers, 192–212 Antarctica, 196–197 chemical composition of atmosphere and, 212 depositional work carried out by, 198–207 erosional work carried out by, 199–202 Ice Age, 207–209 ice caps and, 194, 196 ice sheets and, 192–194, 207 movement of, 195, 198–199 outlet, 194 outlet glaciers, 194 piedmont, 194 retreating, 199 valley (alpine), 192 water storage in, 153 Glass rocks, 82–83 Glassy texture, 86, 87 Glaze, 560 Global climate change, 643–653 aerosols and, 649–650 carbon dioxide and, 643–644 climate-feedback mechanisms, 648–649 global warming and, 650–653 human impact on, 643–644 temperature variations, 644–645, 648 trace gases and, 648 Global warming, 650–653 changing Artic, 652–653 potential for surprises, 653 projected changes/effects of, in 21st century, 651 sea-level rise, 491, 651–652 Glossopteris, 232 Gneiss, 102 Gold, 57, 58–59, 103 Super Pit gold mine, 62 Graben, 346 Gradient, 156–157 Grand Canyon, 24 Granite, 55, 86, 87, 88, 90, 122–123 Granitic composition, 83, 86 Granodiorite, 39 Granules, 728 Graphite, 107 Gravitation, law of universal, 668

783

Gravitational collapse, 361 Great Lakes, creation of, 207 Great Oxygenation Event, 400 Greenhouse effect, 524–525, 643 Greenhouse gases (GHG) emissions, 646–647 Greenland ice sheet, size of, 193 Groin, 489 Groundmass, 86 Ground moraine, 205 Ground subsidence, 278 Groundwater, 172–175 aquifers, 175 aquitards, 174–175 artesian systems, 177–178 caverns formed by, 182–183, 183–184 contamination, 180–181 distribution of, 172–173 environmental problems associated with, 179–181 geological roles of, 172 geologic work of, 182–184 importance of, 172 karst topography formed by, 183–184 land subsidence caused by withdrawal, 179–180 movement, 175 permeability, 174 porosity, 174 speed of, 175 springs, 175–176 storage, 174–175 treating as nonrenewable resource, 179 underground zones, 173 wells, 176–177 Gulf Coast, 492 Gulf stream, 474 Guyot, 442 Gypsum, 70, 71, 107 Gyres, 474

H Habit, 63–64 Hadean eon, 386, 397, 399 Hail, 561 largest hailstone in U.S., 561 Haiti earthquake, 2010, 266, 268 Half-graben, 346 Half-life, 382 Halides, 67, 70 Halite, 55, 70, 71, 94, 107 Hanging valleys, 201–202 Hardness, 64 Hard stabilization, 489–490 alternatives to, 490–491 breakwater, 489 groins, 489 seawalls, 490 Hawaii, intraplate volcanism in, 328 Hawaiian-type calderas, 317–318 Hawaiian-type volcanic eruptions, 300 Heat, 520 Heat conduction, 520–521 Heating of atmosphere, 524–525 Heat transfer mechanisms, 520–522 conduction, 520–521 convection, 521

784

INDEX

radiation, 521–522 Heliocentric, 661 Heliosphere, 729 Hematite, 71–72, 103 Hertzsprung–Russell (H-R) diagram, 744–745 High, 578 High cloud, 553 Highland climate, 641–642 Himalayas, 44, 354–355 Historical geology, 25 Hoar frost, 540 Homo sapiens, 421 Horizon, 129 Hornblende, 70 Horns, 202 Horst, 346 Hot spot, 250, 330 Hot spot track, 250 Hot spring, 176 Hubble’s law, 757 Hubble Space Telescope (HST), 725, 732–733 Humid continental climate, 639 Humidity, 540–544 dewpoint temperature, 542–543 measuring, 543–544 mixing ratio, 541 relative, 541–542 saturation, 540–541 Humid middle-latitude climates with mild winters (C) climates, 637–638 with severe winters (D) climates, 638–640 Humid subtropical climate, 637–638 Humid subtropics, 637–638 Humid tropical (A) climates, 632–635 Hurricane Katrina, 617 Hurricanes, 615–616 destruction caused by, 618–620 formation and decay of, 618 names of, explained, 615–616 names given to, 618 profile of, 615–616 tracking, 620–621 Hurricane Sandy, 25, 596–597 Hydraulic fracturing (fracking), 109 Hydrogen fusion, 747 Hydrogenous sediment, 445 Hydrologic cycle, 47, 152–153 Hydrosphere, 36 Hydrothermal solutions, 106 Hydrothermal vents, 461, 462–463 Hygrometer, 543 Hygroscopic nuclei, 551 Hygroscopic particles, 559 Hypocenter, 266 Hypothesis, 29

I Ice, 538 Ice Age glaciers, 207–209 extent of, 209 sea-level changes, 208–209 Ice ages causes of, 210–212 dating, 209 Icebergs, 199 Ice cap climate, 641 Ice caps, 194, 196

Ice sheets, 192–194, 207 Ice shelf, 194 Idealized global circulation, 580–581 Idealized weather, 604–606 Igneous rock, 80, 82–91 classifying, 86–88 compositions of, 83–84 crystallization, 80, 82–83 formation of, 89–91 textures, 84–86, 86, 87 Ilmenite, 103 Impact craters, 687–688 Incised meanders, 165–166 Inclination of the axis, 516 Inclusions, 372 Index fossil, 380 Industrial minerals, 107 Infiltration, 152 Infrared, 522 Inland flooding, 620 Inner core, 40, 291 Inorganic compounds, 54 Integrated Ocean Drilling Program (IODP), 428–429 Intensity, 272 Intergovernmental Panel on Climate Change (IPCC), 644 Interior drainage, 215 Internal processes, 116 Internal structure of Earth, 38–40, 289–291 core, 40, 291 crust, 39, 289, 290 experiments on composition and temperature, 291 formation of layered structure, 289 mantle, 39–40, 291 probing, seismic waves and, 289–290 Interstellar matter, 742–744 bright nebulae, 742–743 dark nebulae, 744 Intertidal zone, 464 Intertropic convergence zone (ITCZ), 580 Intraplate volcanism, 327, 330 Intrusions, 321–324 batholiths, 323–324 dikes, 322–323 laccoliths, 324 nature of, 321–322 sills, 322–323 stocks, 324 Intrusive (plutonic) rock, 82 Ionic bonds, 60 Ionic compounds, 60 Ions, 60 Iron (Fe), 67, 103 Irregular fractures, 65, 66 Irregular galaxy, 755 Island arc, 243, 327 Island arc-type mountain building, 350 Isobar, 574 Isostasy, 360–361 vs. changes in elevation, 360–361 compressional forces and, 360–361 gravitational collapse and, 361 principle of, 360 Isostatic adjustment, 360 Isotherm, 525

J Jade, 74 Jasper, 94, 95 Jet stream, 576–577 Jetties, 489 Joints, 347–348 Joshua Tree National Park, 123 Jovian planet, 686 Jupiter, 699–701 moons of, 699–701 rings of, 701

K Kames, 207 Kaolinite, 69 Karst topography, 183–184 tower karsts, 184 Kepler, Johannes, 665–666 Kettles, 206 Kilauea, 298–299, 307–309 Komatiite, 87, 90 Köppen classification, 630–632 Kuiper belt, 707

Liquid water, 538 Lithification, 80, 95–96 Lithosphere, 40, 236, 291 Lithospheric plates, 41, 237–238 Local Group, 756 Localized convective lifting, 547–548 Local wind, 583–584 Loess, 219 Longitudinal dunes, 221 Longitudinal profile, 157 Longshore currents, 485 Longshore transport, 485 Low, 578 Low clouds, 553 Lower mantle, 40 Low-latitude deserts and steppes, 635–636 Lunar eclipse, 677 Lunar highlands, 690 Lunar motions, 675, 677 Lunar regolith, 691 Lunar surface, 689–691 Luster, 62–63

M L Laccoliths, 324 La Conchita, California mud slide, 46 Lahars, 141, 315 Lake Agassiz, 208 Lake-effect snow, 598, 599 Laminar flow, 156 Land breezes, 583 Landslides, 139, 278 as geologic hazards, 134 as natural disasters, 135 Land, temperature changes based on, 526–527 La Niña, 586, 587–588, 590 Laramide Rockies, 358–359 Laramide Orogeny, 408 Latent heat, 538–540 condensation, 539 deposition, 540 evaporation, 539 sublimation, 540 Lateral continuity, principle of, 371 Lateral moraine, 204–205 Lava, 80, 82 Lava flows, 301–303 Lava plateaus, 45 Lava tube, 302 Law of superposition, 370 Layered plate-mantle convection model, 257–258 Layers of Earth. See Internal structure of Earth Leaching, 129 Lead, 103 Life cycle of middle-latitude cyclone, 605 Light collection, 722–724 nature of, 716–718 processes and, 718 Light silicates, 83 Light-year, 741 Lignite coal, 95 Limestone, 55, 71, 93, 94, 107, 182 Limonite, 103 Liquefaction, 142, 277–278

Mafic, 83, 88–89, 90 Mafic compositions, 83, 84 Magma, 80, 82, 83, 324–326 composition, 299–300 decompression melting and, 325 generating from solid rock, 324–326 geothermal gradient and, 324 partial melting and, 324–326 water and, 325 Magma mixing, 90–91 Magmatic differentiation, 90–91, 103 Magnesite, 103 Magnesium (Mg), 67, 103 Magnetic reversals, 252–254 Magnetic time scale, 253 Magnetite, 71–72, 103 Magnetometer, 253 Magnitude, 272 Magnitude scales, 273–274, 276 moment magnitude, 276 Richter, 273–274 Main-sequence stars, 744, 747 Mammals, 418, 420–422 extinction of large mammals, 421–422 humans, 421 marsupials, 420–421 placental, 421 from reptiles to, 420 Manganese, 103 Manganese nodules, 448 Mantle, 39–40, 291 Mantle plumes, 250, 257, 330 Mapping the seafloor, 431–433 modern bathymetric techniques, 432–433 from space, 433 Marble, 101, 102, 104–105 Maria, 689 Mariana Trench, 439 Marine life zones, 461, 464–465 shore, distance from, 464 sunlight, availability of, 461 water depth, 464–465 Marine organisms, 459–461

Marine terrace, 486 Marine west coast climate, 638 Maritime (m) air mass, 599 Mars, 694–698 exploration, 696–697 topography of, 694–695 water ice on, 698 Marsupials, 420–421 Massive, 321 Mass wasting, 117, 134–142 controls and triggers of, 136–138 damage caused by, 142 deaths caused by, 142 earthquakes and, 138 landslides and, 139 motion types, 138–140 oversteepened slopes and, 137 processes, classifying, 138–140 rate of movement, 139–140 rock avalanches and, 139–140 role of, 134 slope change through time, 134, 136 vegetation, and removal of, 137–138 water’s role in, 136–137 Mauna Loa, 306–307 Observatory, 627 Meander, 162 Meandering streams, 162–163 Mean solar day, 673 Mechanical weathering, 119–121 biological activity, 120–121 frost wedging, 119–120 salt crystal growth, 120 sheeting, 120 Medial moraine, 205 Megathrust fault, 269–270, 281 Mercury, 103, 692–693 Mercury barometer, 573 Mesosaurus, 232 Mesosphere, 514 Mesozoic era, 385, 407–408 reptiles as first terrestrial vertebrates, 414, 418–419 Metallic bonds, 61–62 Metallic mineral resources, 103, 106–107 Metallic substances, 60 Metamorphic rock, 80, 81, 98–102. See also Metamorphism foliated, 101–102 nonfoliated, 102 textures, 101 Metamorphism, 98–101 chemically active fluids as metamorphic agent, 100–101 confining pressure as metamorphic agent, 99–100 contact or thermal, 98 defined, 98 differential stress as metamorphic agent, 100 heat as metamorphic agent, 98–99 metamorphic grade, 99 regional, 98 Meteor, 707 Meteorites, 33, 708–709 Meteoroids, 707–709 Meteorology, 25 Meteor shower, 707–708 Mica, 65

INDEX

Mica schists, 102 Microcontinent, 352 Midaltitude ocean productivity, 466–467 Middle clouds, 553 Middle-latitude deserts and steppes, 636–637 Middle-latitude or mid-latitude cyclone, 604–606 airflow aloft, role of, 606 idealized weather of, 604–606 life cycle of, 605 Mid-ocean ridge, 46, 443 Milky Way Galaxy, 46, 752–753 Mineral groups. See Rock-forming minerals Mineralogy, 54 Mineral resources, 72–73, 73–74 Minerals, 54–55 atoms as building blocks of, 56–57 characteristics of, 54–55 groups (See Rock-forming minerals) natural resources, 72–73 properties of (See Minerals, properties of) rocks, 55 use of term, 54 Minerals, properties of, 62–66 crystal shape or habit, 63–64 density and specific gravity, 66 effervescency, 66 feel, 66 magnetic, 66 odor, 66 optical, 62–63, 66 strength (See Mineral strength) taste, 66 Mineral strength, 64–66 cleavage, 64–65 fracture, 65, 66 hardness, 64 tenacity, 65–66 Mixed tidal pattern, 498 Mixing ratio, 541 Model, 29 Modified Mercalli Intensity Scale, 273 Mohs scale, 64 Moisture, adding or subtracting, 541–542 Molecules, 60 Molybdenite, 103 Molybdenum, 103 Moment magnitude, 276 Monoclines, 343–344 Monsoon, 581 Montreal Protocol, 512 Moon, Earth’s, 689–691 eclipses of, 677–679 phases of, 677 Moons Jupiter, 699–701 Neptune, 704 Saturn, 701–702 Uranus, 703 Moonstone, 74 Moraines, 204–206 kettles, 206 outwash plains, 206 valley trains, 206

Morenci, Arizona copper mine, 26 Motions of Earth, 515, 673–675 precession, 675 revolution, 673, 674 rotation, 673–674 Mountain belts, 44 continental drift and, 233 distribution of, 44 Mountain breezes, 583–584 Mountain building, 349–361. See also Isostasy Cordilleran-type, 352–354 at subduction zones, 350–352 (See also Collisional mountain belts) terranes and, 352 Mountainous desert landscape, 215–216 Mount Etna, Italy, volcano, 296–297 Mount Everest, formation of, 354–355 Mount St. Helens, 47, 298–299 Movement of features on Earth, 247–249 Mudflow, 141 Mud slide, 46 Muscovite, 68, 69, 89, 107

N Native Americans, tools made by, 88 Native elements, 70, 73 Native gold, 59, 103 Native platinum, 103 Native silver, 103 Natural gas, 108–109, 446 Natural gas and oil, 108–109 Natural hazards, 25–26 Natural levee, 168–169 Naturally occurring crystalline, 54 Natural resources, 72–73 mineral resources, 72–73 renewable vs. nonrenewable, 72 Neap tides, 497 Nearshore zone, 480 Nebulae, 32, 742 Nebular theory, 32, 33, 682–713 Negative-feedback mechanism, 648 Nekton, 460 Neptune, 703–704 moons of, 704 rings of, 704 Neritic zone, 464 Neutrons, 56 Neutron star, 750 Newfoundland herring, 467, 468 New Madrid Seismic Zone, 283 Newton, Sir Isaac, 668–669 Nickel, 103 Nimbostratus clouds, 553 Noctilucent clouds, 37 Nonconformity, 373 Nonfoliated texture, 101 Nonmetallic mineral resources, 107 Nonmetallic resources, 103 Nonrenewable resources, 72 treating groundwater as, 179 Nonsilicates, 67, 70–71, 72 Nor’easter, 601 Normal fault, 345–346 Normal polarity, 252 North America, making of, 404 North American Cordillera, 354

formation of, 407–408 North Pole ice, sea vs. glacial, 193 North Sea herring, 467, 468 Nuclear fusion, 734 Nucleus, 56, 706 Nuée ardente, 314–315 Numerical date, 369–370

O Observatories on mountaintops, 723 Obsidian, 55, 86, 88 Occluded front, 603 Occlusion, 605–606 Ocean acidity, 456 Ocean basins average depth, 43 continental margins, 45 deep-ocean basins, 45 features of, major, 43, 45–46 oceanic ridges, 46 Ocean circulation deep-ocean, 477–478 glaciers and, 212 surface, 474–478 Ocean current patterns, 474–477 influence on climate, 476–477 Ocean density variation, 457–459 with depth, 458 factors affecting, 457–458 ocean layering and, 458–459 Ocean drilling, 249–250 Ocean dynamics of coast, 492–493, 496 of erosion problems along U.S. coasts, 492–493, 496 of ocean circulation, 474–478 of sand movement on beach, 483–485 of shore, 488–491 of shoreline, 478–480 of tides, 496–499 Ocean floor, 428–451 continental margins, 436–439 deep-ocean basin, 439, 442 mapping, 431–433 oceanic ridge, 443–444 provinces of, 434–435 resources from, 446–448 sediments (See Seafloor sediments) seismic reflection profiles, 442 Oceanic-continental convergence, 242–243 Oceanic feeding relationships, 467–468 food chains, 467, 468 food webs, 467–468 transfer efficiency, 467 trophic levels, 467 Oceanic-oceanic convergence, 243 Oceanic plateau, 442 Oceanic ridge, 46, 443–444 anatomy of, 443 elevated position of, reasons for, 443–444 movement of, 254 system, 236, 239 Oceanic zone, 464 Ocean layering, 458–459 Ocean life diversity of, 459–465

marine life zones, 461, 464–465 marine organisms, 459–461 oceanic feeding relationships, 467–468 Oceanography, 25, 430 Ocean productivity, 465–467 deep-sea hydrothermal vent biocommunities, 463 midaltitude, 466–467 polar, 465 tropical, 465–466, 467 Oceans, 38 acidity of, 456 comparing to continents, 431 density variations in, 457–459 evolution of, 400–401 geography of, 430–431 hydrothermal vents, 461, 462–463 productivity in, 465–467 seawater, composition of, 454–456 temperature variations in, 456–457 Ocean trenches, deepest explored, 439 Octet rule, 60 Offshore zone, 480 Oil, 446 Oil and natural gas, 108–109 Oil trap, 108 Old Man of the Mountain, 122 Olivine, 68, 70, 87, 89 Oort cloud, 33, 707 Opal, 74 Opaque minerals, 63 Optical telescopes, 720–724 light collection, 722–724 reflecting, 721–722 refracting, 720–721 world’s largest, 723 Orbiting observatories, 725–726, 732–733 Chandra X-Ray Observatory (CXO), 725–726 Compton Gamma Ray Observatory (CGRO), 725 Hubble Space Telescope (HST), 725, 732–733 Spitzer Space Telescope (SST), 726 Ordovician period, 411, 413 Ore, 73 Ore deposits, 73–74 Ore minerals of important metals, 103 Organic compounds, 54 Organic sedimentary rock, 93 Orientation of Earth, 516 Original horizontality, principle of, 370 Orion the Hunter, 670–671 Orion nebula, 719 Orogenesis, 349, 352–353 Orographic lifting, 546 Outer core, 40, 291 Outgassing, 399 Outlet glaciers, 194 Outwash plains, 206 Overrunning, 602 Oxbow lake, 163 Oxides, 70, 72 Oxygen (O), 67, 399–400 Ozone, 509–510 Ozone depletion, 510, 512

785

P Pacific coast, 492–493 Pahoehoe flow, 302 Paleomagnetism, 251, 252 Paleontology, 375, 377 Paleoseismology, 288 Paleozoic era, 210, 385, 413–414 early life-forms, 411, 413 Pangaea formation during, 210, 406–407 vertebrates move to land, 414 Pangaea, 40, 210, 231, 234, 245, 248, 406–407 breakup of, 247–248, 407 Parabolic dunes, 222 Parasitic cones, 306 Parcel, 545 Parent material, 127 Parent rock, 98 Parícutin, 310–311 Partial melting, 243, 324–326 Passive continental margin, 350, 436 Peat, 96 Pegmatite, 103, 106 Pelagic zone, 464 Pennsylvanian period, 414 Pentlandite, 103 Perched water table, 176 Peridot, 74 Peridotite, 40, 84, 90 Perihelion, 674 Period, 385 Periodic table, 57 Permafrost, 143, 652–653 Permeability, 174 Permian extinction, 415 Phanerozoic eon, 385, 406–409 Cenozoic era, 408–409 Mesozoic era, 407–408 Paleozoic era, 406–407 Phases of the Moon, 677 Phenocrysts, 86 Photic zone, 461 Photon, 717 Photosphere, 727–728 Photosynthesis, 459 Phyllite, 101, 102 Physical geology, 24–25 Phytoplankton, 459 Piedmont glaciers, 194 Pillow lavas, 302–303 Pipe, 320–321, 321 Placental mammals, 421 Plagioclase feldspar, 68, 69, 89 Plane of the ecliptic, 674 Planetary nebulae, 743–744, 749 Planetesimals, 33, 34, 397, 685 Planets. See also Small solar system bodies atmospheres of, 687 data, planetary, 685 dwarf, 705, 709–710, 716–718 Earth’s moon, 689–691 formation of (Nebular theory), 685–686 internal structures, 686–687 Jovian, 686, 699–704 overview of, 684–688 planetary impacts, 687–688 terrestrial, 686, 692–698 Plankton, 459

786

INDEX

Plants flowering, in Cenozoic era, 420 soil and, 128 water vapor emitted from, 152 Plateaus, 352–353 Plate boundaries, 237–238 convergent, 237, 238, 241–244 divergent, 237, 238–241 transform plate, 238, 245–246 Plate-mantle convection, 256–258 models, 257–258 Plate motion 36–37, 254–258 drivers of, 256–258 measuring, 254–256 Plates boundaries separating, 40 lithospheric, 41 plate motion, 41 Plate tectonics, 40–41, 236–238. See also Continental drift boundaries of, 238–241 cessation of, 249 future, 248–249 glaciations and, 210–211 major plates and, 237–238 motion of, 254–258 theory of, 236 volcanoes and, 326–330 Plate tectonics model, 249–254 hot spots as evidence, 250–251 magnetic reversals and, 252–254 ocean drilling for evidence, 249–250 paleomagnetism as evidence, 251, 252 polar wandering and, 252 seafloor spreading and, 252–254 Platinum, 103 Playa lake, 215 Pleistocene epoch, 211 Plucking, 200 Plutonic (intrusive) rock, 82 Plutons, 321 Pluvial lake, 207, 209 Point bar, 163 Polar easterlies, 581 Polar (E) climates, 640–641 Polar front, 581 Polar high, 581 Polar ocean productivity, 465 Polar (P) air mass, 599 Polar wandering, 252 Population of world, 30–31 Porosity, 174 Porphyritic texture, 85, 86, 87 Positive-feedback mechanism, 648 Potassium feldspar, 68, 69, 89 Potassium (K), 67 Potholes, 160 Precambrian period, 210, 385–386, 401–406 first continents, 401–404 making of North America, 404 supercontinents of, 404–406 Precious gems, 74 Precipitation from cold clouds, 558 forms of, 559–561, 564 global distribution of, 590–591

measuring, 564–565 processes that generate, 558–559 from warm clouds, 559 Precipitation fog, 557 Predictions, 29 Pressure centers, high and low, 578–581 circulation, 578–581 of continents, influences of, 581–582 weather generalizations about, 578–580 Westerlies, 582–583 winds, cyclonic and anticyclonic, 578 Pressure changes, 512 Pressure gradient, 574 Pressure-gradient force, 574–575 Pressure tendency, 579 Prevailing wind, 585 Primary productivity, 465 Primary (P) waves, 271 Principal shells, 56 Principle of original horizontality, 370 Processes that lift air, 546–548 convergence, 547 frontal wedging, 546–547 localized convective lifting, 547–548 orographic lifting, 546 Proglacial lakes, 208 Prokaryotes, 410–411 Prominence, 731 Proterozoic eon, 385 Proton–proton chain reaction, 734 Protons, 56 Protoplanets, 397, 685 Protostar, 747 Protosun, 32 Provinces of ocean floor, 434–435 Psychrometer, 543 Ptolemaic system, 662–663 Pulsar, 750 Pumice, 85, 86, 88, 304 Pycnocline, 458 Pyrite, 63 Pyroclastic flow, 314 Pyroclastic material, 86, 303–304 Pyrolusite, 103 Pyroxene, 68, 70, 87, 89

Q Quartz, 54, 55, 67, 68, 69, 74, 87, 89, 107 Quartzite, 102 Quaternary period, 209

R Radial pattern, 155–156 Radiation, 521–522 Radiation fog, 556–557 Radiation pressure, 717 Radioactivity, dating with, 380–383 atomic structure and, 381 half-life and, 382 importance of, 383 radiocarbon dating, 383 radiometric dating, 382 Radiocarbon dating, 383 Radio interferometer, 725

Radiometric dating, 382 of sedimentary strata, 386–387 Radio telescopes, 724–726, 724–725 Rain, 560 measuring, 564 standard rain gauge, 564 TRMM data showing rainfall in Malaysia, Dec. 2004, 31 water in clouds, 555 Rain shadow desert, 546, 637 Rectangular pattern, 156 Red giant, 745, 747 Red sunsets, 509 Reflecting telescope, 721–722 Reflection, 522–523 Reflection nebulae, 743 Reflectivity of Earth’s surface, glaciers and, 212 Refracting telescope, 720–721 Regional metamorphism, 98 Regolith, 126 Relative dating, 370, 374–375 Relative humidity, 541–542 moisture, adding or subtracting, 541–542 temperature changes, 542 Relative motion, 247, 248, 254–256, 346, 720 Relocation of buildings, 491 Renewable resources, 72 Reptiles dominating Mesozoic environment, 418–419 as first terrestrial, 414 Reservoir rock, 108 Resources, 26 Resurgent dome, 319 Retrograde motion, 662 Reverse fault, 346 Reverse polarity, 252 Revolution, 515, 673, 674 Rhyolite, 86, 87 Richter scale, 273–274 Ridge push, 256 Rift valley, 239, 443 Right ascension, 672 Rime, 564 Ring of Fire composite volcanoes, 311 deep-ocean trenches, 442 plate tectonics and, 326, 327 Rip currents, 485 Rise, 443 River systems, 154–155 largest, 159 Rock avalanches, 139–140 Rock cycle, 47, 80–82 alternative paths, 80, 82 basic cycle, 80 defined, 80 magma, 80, 82 Rock flour, 200 Rock-forming minerals, 66–73 aluminum (Al), 67 calcium (Ca), 67 iron (Fe), 67 listed, 67 magnesium (Mg), 67 nonsilicates, 67, 70–71, 72 oxygen (O), 67

potassium (K), 67 relative abundance of, 67 silicates, 67, 67–69 silicon (Si), 67 sodium (Na), 67 vs. economic minerals, 67 Rock gypsum, 94 Rocks, 55, 80–113 continental drift and, 233 igneous, 80, 82–91 metaphoric, 81, 98–102 resources from, 103–107 rock cycle and, 80–82 sedimentary, 80, 81, 91–97 Rock salt, 94 Rockslide, 139, 141 Rock strength, factors that affect, 340 confining pressure, 340 rock type, 340 temperature, 340 time, 340 Rock structures, 338 Rocky Mountains, Laramide Rockies, 358–359 Rodinia, 405 Rotation, 515, 673–674 Rubies, 74 Running water, 152–172 base level and stream erosion, 164 depositional landforms, 167–169 drainage basins, 154 drainage patterns, 155–156 floods and flood control, 169–172 river systems, 154–155 sediment transportation, 160–161 shaping stream valleys, 164–166 stream channels, 161–163 stream erosion, 158, 160 streamflow, 156–158 work of, 158–161 Runoff, 152, 153 Rutile, 103

S Saffir-Simpson hurricane scale, 619 Sahara Desert, satellite images of, 217 Salinity, 454–456 Saltation, 161 Salt crystal growth, 120 Salt dome, 109 Salt flats, 95 San Andreas Fault, 286–287, 288 Sand dunes, 219–222 in deserts, 219, 220–222 types of, 220–221 wind deposits and, 219–220 Sand and gravel, 447 Sand movement on beach, 483–485 longshore transport, 485 perpendicular to shoreline, 483 rip currents, 485 wave refraction, 483–484 Sandstone, 92 San Francisco earthquake (1906), 267–268, 278 Santa Ana, 584 Sapphire, 74 Saturation, 540–541 Saturn, 701–703 moons of, 701–702

rings of, 702–703 Scales of space and time, 27–28. See also Geologic time scale Scattering, 523 Scenarios, 650 Scheelite, 103 Schists, 102 Scientific inquiry, 29–32 hypothesis, 29 scientific methods, 30, 32 theory, 30 Scientific methods, 30, 32 Scoria, 304 Scoria cone, 310–311 Sea arch, 486 Sea breezes, 583 Seafloor, major features, 434–435 Seafloor sediments, 444–446 climate data in, 445–446 types of, 444–445 Seafloor spreading in divergent boundaries, 239 in plate tectonics model, 252–254 Sea ice, 455, 652 Sea level changes, supercontinents and, 405–406 global warming’s effect on, 491 rise, global warming and, 651–652 Seamount, 45, 442 Sea salt, sources of, 454–455 Seasons, 515–519 Sea stack, 486 Seawall, 490 Seawater composition of, 454–456 saltiest water in the world, 458 Secondary (S) waves, 271 Sediment, 80, 91 river systems, 154–155 Sedimentary rock, 80, 81, 91–97 biochemical, 93–95 cemented, 91–93, 96 chemical, 93, 93–95 classifying, 92–95 color of, 92 compacted, 91–93, 96 detrital, 92–93 features of, 96–97 formation of, 91 lithification and, 95–96 organic, 93, 95 radiometric dating of, 386–387 Sedimentation, 91 Sediment deposition, 161 Sediment transportation, 160–161 bed load and, 161 capacity and, 161 competence and, 161 dissolved load and, 160 suspended load and, 160 Seismic gaps, 285 Seismic reflection profiles, 442 Seismic waves, 266, 289–290 destruction from, 276–278 sea, 279–280 (See also Tsunami) Seismogram, 271 Seismograph, 270–271 Seismology, 270–272 Semiarid climate, 635 Semidiurnal tidal pattern, 498

INDEX

Semiprecious gems, 74 Settling velocity, 160 Severe storms hurricane, 615–621 middle-latitude or mid-latitude cyclone, 604–606 thunderstorm, 607–609 tornado, 610–614 Shale, 92, 98 Sheeting, 120 Shield, 44–45 crustal materials, 403 Shield volcanoes, 306–307 Kilauea, 307–309 Mauna Loa, 306–307 Shore, 479 evolution of, 487–488 stabilization of, 488–491 Shoreline, 36, 478–480 beaches, 480 circular orbital motion, 482 features of, 486–488 wave, 481–483 Sidereal day, 673 Sidereal month, 675 Sierra Nevada, 352 Silicates, 67, 67–69, 123, 124 in igneous rocks, 84 Silicon-oxygen tetrahedron, 67 Silicon (Si), 67 Sills, 322–323 Silver, 103 Sinkhole (sink), 173, 183–184 Slab pull, 256 Slate, 98, 101, 102 Sleet, 560 Slide, 139 Slip face, 220 Slope orientation, 128 Slow movements, 142–143 creep, 140, 142–143 solifluction, 143 Slump, 139, 140–141 Small solar system bodies, 705–710 asteroids, 705–706 comets, 706–707 meteoroids, 707–709 Snow, 560 measuring of, 564 Sodium (Na), 67 Soil, 125–127 animals and, 128 classifying, 130–131 climate and, 128 components of, 125 formation, 127–129 as interface, 125–126, 126 parent material and, 127 plants and, 128 profile, 129–130 structure, 126–127 texture, 126 time and, 127 topography and, 128 Soil erosion, 132–134 controlling, 133–134 process of, 132 rates of, 132–133 sedimentation and, 132 water erosion, 132 wind erosion, 132, 134

Soil horizons, 129 Soil Taxonomy, 131 Soil texture, 126 Solar eclipse, 677 Solar energy, 72 source of, 734–735 Solar flare, 731, 734 Solar nebula, 32, 397, 685 Solar radiation, incoming, 522–525 absorption, 523–524 reflection, 522–523 scattering, 523 Solar system, 32, 33, 46. See also Planets; Small solar system bodies formation of, according to nebular theory, 685–686 overview, 684–688 size and scale, 35 Solar wind, 729 Solidification, 80, 82–83 Solifluction, 143 Solstices, 516–519 Solum, 129, 131 Sonar, 432, 432–433 Sonoran Desert, 213 Sorting, 161 Source regions for air masses, 599 Southern Oscillation, 590 Space, scales of, 27–28 Specific gravity, 66 Specific heat, 527 Spectroscope, 718 Spectroscopy, 718–720 Sphalerite, 103 Spheres of Earth, 34–38 atmosphere, 36–37 biosphere, 37–38 geospheres, 38–41 hydrosphere, 36 Spheroidal weathering, 123 Spicule, 728 Spinels, 74 Spiral galaxy, 754 Spit, 486 Spitzer Space Telescope (SST), 726 Splintery fractures, 65 Spreading centers, 238, 239 Springs, 175–176 geysers, 176 hot springs, 176 Spring, temperate ocean productivity in, 466 Spring tides, 497 Stabilization of shore, 488–491 Stable air, 548 Stable platform, 45 Stalactite, 182 Stalagmite, 182–183 Star dunes, 222 Stars classifying, 744–745 mapping, 662 States of water, changes, 538–540 ice, 538 latent heat, 538–540 liquid water, 538 water vapor, 538 Stationary front, 603 Steam fog, 557 Stellar evolution, 746–749

burnout and death, 748–749 main-sequence stage, 747 protostar stage, 747 red giant stage, 747–748 stellar birth, 746–747 Stellar remnants, 749–751 black holes, 750–751 neutron stars, 750 white dwarfs, 745, 749–750 Steppe climate, 213, 635–637 Stocks, 324 Storm surge, 619–620 Strata (beds), 96 Stratified drift, 203 Stratigraphic (pinchout) trap, 109 Stratosphere, 513–514 Stratovolcano, 311 Stratus, 552, 553 Streak, 63 Stream channels, 161–163 alluvial, 161, 162 bedrock, 161–162 Stream erosion, 164 Streamflow, 156–158 changes from upstream to downstream, 157–158 channel characteristics, 157 discharge, 157 flow velocity, 156–157 gradient characteristics, 156–157 laminar, 156 turbulent, 156 Stream valleys, 164–166 changing meanders, 165–166 valley deepening, 164–165 valley widening, 165 Stress, 339 Strike-slip faults, 269, 346–347 Stromatolites, 410–411 Subarctic climate, 639–640 Subduction erosion, 439 Subduction zones, 241 Andean-type plate margins, 350–351 mountain building at, 350–352 volcanic island arcs, 350 Sublimation, 540 Submarine canyon, 438 Submergent coast, 493, 496 Subpolar low, 581 Subtropical high, 580 Suction vortices, 610 Sulfates, 67, 70, 71 Sulfides, 70, 72 Sulfur, 57, 107 Summer solstice, 516 Summer, temperate ocean productivity in, 466 Sun, 726–735 chromosphere, 728 corona, 728–729 death of, 747 Earth’s relationship with, 514–519 eclipses of, 677–679 photosphere, 727–728 solar energy and, 734–735 solar flares, 731, 734 structure of, 726–729 sunspots, 729–730 surface of, 728 Sunlight, 461

Sunspots, 729–730 Supercontinent cycle, 405 Supercontinents, 210, 231, 404–406 climate of, 405 sea level changes and, 405–406 Supercooling, 558 Supergiants, 745 Supernova, 32, 397, 749 Superposition, law of, 370 Surf, 483 Surface ocean circulation, 474–478 Gulf Stream, 474 ocean current patterns, 474–477 upwelling, 477 Surface wave, 271–272 Surf zone, waves in, 482–483 Suspended load, 160 Suture, 354 Swimmers, 460 Sylvite, 107 Synclines, 341–342 Synodic month, 675 System, 46

T Tabular, 321 Taconic Orogeny, 356, 357 Talc, 107 Tectonic plates, 236–238 Temperature, 520 in Alaska, 641 of Earth’s crust, 99 greatest contrast between summer and winter, 531 heat compared, 520 rock strength and, 340 variations, global climate change and, 644–645, 648 world distribution of, 530–531 Temperature changes, 513–514 due to relative humidity, 542 mesophere, 514 stratosphere, 513–514 thermosphere, 514 troposphere, 513 Temperature controls, 526–529 albedo, 528–529 altitude, 528 cloud cover, 528–529 geographic position, 528 land, 526–527 water, 526–527 Temperature gradient, 525 air temperature data, 525 Temperature inversion, 549 Tenacity, 65–66 Terrae, 690 Terranes, 352 accretion, 352–353 nature of, 352 North American Cordillera, 354 orogenesis, 352–353 vs. terrains, 352 Terrestrial planet, 686 Terrestrial planets, 686, 692–698 Terrigenous sediment, 444 Texture, 84–86 coarse-grained, 84, 85, 87 fine-grained, 84, 85, 87 foliated, 101

787

glassy, 82, 85, 86, 87 nonfoliated, 101 porphyritic, 85, 86, 87 vesicular, 86 Theory, 30, 32 Thermal (contact) metamorphism, 98, 99 Thermal energy, 32, 98–99, 520, 749 Thermocline, 457 Thermohaline circulation, 478 Thermosphere, 514 Thrust fault, 269, 346 Thunderstorm, 607–609 development, stages of, 608–609 mesocyclone and, 611 occurrence, 608 Tidal current, 498–499 Tidal cycles, monthly, 497 Tidal deltas, 499 Tidal flats, 498–499 Tidal patterns, 498 Tidal waves. See also Tsunami Tides, 496–499 causes of, 496–497 tidal currents, 498–499 tidal cycles, monthly, 497 tidal patterns, 498 tidal ranges, 497 Till, 203 Time. See also Geologic time rock strength and, 340 scales of, 27–28 (See also Geologic time scales) Tin, 103 Titanium, 103 Tombolo, 487 Topaz, 74 Tornadoes, 610–614 climatology of, 611–612 destruction by, 612–613 EF-scale, 613 forecasting, 613–614 general atmospheric conditions for, 611 most destructive on record, 619 occurrence of, 610–611 profile of, 612 tornado occurrence and development of, 610–611 warnings, 614 watches, 614 Tower karst, 184 Trace gases and global climate change, 645, 648 Track forecast, 620 Trade winds, 580 Transfer efficiency, 467 Transform fault, 346–347 Transform fault boundaries, 41, 269 Transform plate boundaries, 238, 245–246 Translucent minerals, 63 Transparent minerals, 63 Transpiration, 152 Transverse dunes, 220 Travertine, 93–94 Trellis pattern, 156 Trophic level, 467 Tropical depression, 618 Tropical ocean productivity, 465–466, 467

788

INDEX

Tropical Rainfall Measuring Mission (TRMM), 31 Tropical rain forest, 38, 632 Tropical storm, 618 Tropical (T) air mass, 599 Tropical wet and dry climate, 634–635 Tropic of Cancer, 516 Tropic of Capricorn, 518 Troposphere, 37, 513 Trunk streams, 155 Tsunami, 279–280 damage from Indonesian earthquake, 2004, 279–280 defined, 279 Japan (2011), 264–266, 280 volcano-related, 315 warning system, 280 Tundra climate, 640–641 Tungsten, 103 Turbidity current, 438–439 Turbulent flow, 156 Turquoise, 74

U Ultramafic composition, 84, 90 Ultraviolet (UV) rays, 522 Unconformities, 372–374 Uniformitarianism, 368–369 Universe, 738–767 Big Bang theory, 756–758 fate of, 757–758 galactic clusters, 755–756 galactic collisions, 756 galaxies, types of, 751, 754–755 history of, 741–742 interstellar matter, 742–744 size of, 740–741 stars, classifying, 744–745 stellar evolution, 746–749 stellar remnants, 749–751 Unsaturated zone, 173 Unstable air, 548 Upslope fog, 557 Upwelling, 477

Uraninite (pitchblende), 103 Uranium, 103 Uranus, 703 moons of, 703 rings of, 703

V Valence electrons, 56, 60 Valley breezes, 583–584 Valley glaciers, 192 Valley trains, 206 Vapor pressure, 540 Variable stars, 748 Vein deposit, 106 Vent, 304 Venus, 693–694 Vertebrates evolution from a fish-like ancestor, 414 mammals, 420–422 Permian extinction of, 415 reptiles as first terrestrial, 414 Vertical development of clouds, 553, 555 Vesicular texture, 86, 87 Vesuvius, 312 Viscosity, 299–300, 300 Visible light, 521–522 Volatiles, 300 Volcanic ash, 83 Volcanic cone, 304–306 Volcanic eruptions, 299–313 explosive, 300, 300–301 gases extruded during, 303 Hawaiian-type, 300 hazards, 313–316 lava extruded during, 301–303 nature of, 299–301 pyroclastic materials extruded during, 303–304 styles of (See Volcanic structures) Vesuvius, 312 viscosity, 299–300, 300 Volcanic (extrusive) rock, 82

Volcanic island arcs, 45, 243, 327, 350, 442 Volcanic landforms, 317–321 basalt plateaus, 319–320 calderas, 317–319 fissures, 319 pipes and necks, 320–321 Volcanic neck, 320–321 Volcanic structures, 304–310 cinder cones, 310–311 composite cones, 311–313 shield volcanoes, 306–307 Volcanoes, 296–335 climate changes and, 316 hazards of living with, 313–316 intrusions and, 321–324 magma and, 324–326 Mt. St. Helens vs. Kilauea, 298–299 plate tectonics and, 326–330 St. Pierre, 314–315

W Warm clouds, precipitation from, 559 Warm front, 602 Water. See also Running water changes to state of, 538–540 in chemical weathering, 121, 122–123, 123 in deserts, 214 distribution of, on Earth, 36, 152–153 earth’s distribution of, 152–153 in solar system (liquid), 700–701 temperature changes based on, 526–527 Water balance, 153 Water in clouds, 551–552 Water depth, 464–465 Water erosion, 132 Water ice on Mars, 698 Water supply, 562–563 Water table, 173, 174 perched, 176

Water vapor, 509, 538 from plants, 152 Wave-cut cliffs, 486 Wave-cut platform, 486 Wave erosion, 483 Wave height, 481 Wavelength, 481 Wave period, 481 Wave refraction, 483–484 Waves characteristics of, 481–482 erosion caused by, 36 in surf zone, 482–483 Weather, 507 air-mass, 599–601 associated with air masses, 599–601 atmospheric stability and daily, 550–551 and climate, 506–508 front, warm and cold, 601–603 patterns in U.S., 506 Weathering, 80, 116, 116–125 chemical, 119, 121–123 climate and, 124 in deserts, 214 differential weathering and, 124–125 everyday examples, 118 mechanical, 119–121 products of, 123 rates of, 124–125 rock characteristics and, 124 of silicate, 123, 124 spheroidal, 123 Weather radar, 564–565 Wegener, Alfred, 231–236, 237, 247, 252 Wegener’s hypothesis. See Continental drift Wells, 176–177 artesian, 177–178 Westerlies, 580, 582–583 Western North America, Cenozoic era, 409

Wet adiabatic rate, 545 Wet tropics climate, 632–634 White dwarfs, 745, 749–750 Whole-mantle convection, 257 Wildfires, mass wasting and, 138 Wind abrasion, 218 Wind damage, 620 Wind deposits, 218–222, 219 loess, 219 sand dunes, 219–222 Wind erosion, 132, 217–218 blowouts and, 217, 218 by deflation, 217, 218 desert pavement and, 217–218, 218 Winds, 217–222, 574 breezes, 583–584 chinook, 584 cyclonic and anticyclonic, 578 factors affecting, 574–577 local, 583–584 measuring, 585–586 Santa Ana, 584 solar, 729 Wind vane, 585 Winter, 466 Winter solstice, 518, 519 Wolframite, 103 World population, 30–31

Y Yazoo tributary, 169 Yellowstone-type calderas, 318–319

Z Zinc, 103 Zircon, 74 Zodiac, 669, 672 Zone of accumulation, 198 Zone of fracture, 195 Zone of saturation, 173 Zone of wastage, 198 Zooplankton, 459