The Earth's Crust

Citation preview

DAY

JOHN

TEC

The “Reason Why”

Series

THE EARTH’S CRUST By IRVING and Mountains and

and

RUTH ADLER

valleys,

ocean

islands are all part of the outside

layer of our earth— the part crust. In this

scientists is

floors

made

book you

know what

of.

You

call its

will learn

how

the earth's crust

how they how it was

will discover

know how

thick

formed, and

why

You

we

will find out

act like icebergs

it it

is,

keeps changing.

about mountains that

and rocks that

act like

magnets. r I

You

will learn

how

scientists dis-

covered that the inside of the earth

is

and how they guess what it is made of. And you will read about the deep hole that they are digging solid

through the crust of the earth to see

if

their guesses are right. 0080-0100 Jacket design by

The Etheredges

THE JOHN DAY COMPANY New York $2 1

JOHN

0

Gua

LIBRARY EDITION AVAILABLE

Library

PT.r-

O'"

VERMONT

FREE PUBLIC LIBRARY SERVI regional LIBRARY ST. JOHNSBURY, VERMONT

Digitized by the Internet Archive in

2016

https://archive.org/details/earthscrustOOadle

The

**

Reason Why'* Series

THE EARTH’S CRUST Irving

and Ruth Adler

The John Day Company

New

York

The

‘‘Reason

Why”

Series

by Irving and Ruth Adler

AIR

AND PLANTS NUMBERS OLD AND NEW

INSECTS

OCEANS RIVERS

SHADOWS THE earth's crust THE STORY OF A NAIL THINGS THAT SPIN

WHY? A BOOK OF REASONS YOUR EARS YOUR EYES

(c)

1963 by Irving and Ruth Adler

must not be reproduced in any form without permission. Published by The John Day Company, 62 West 45th Street, New York 36, N.Y., and simultaneously in Canada by Longmans Canada Limited, Toronto.

All rights reserved. This book, or parts thereof,

Library of Congress Catalogue Card Number: 63-10011

MANUFACTURED

0080-0100

IN

THE UNITED STATES OF AMERICA

Contents Crusts, Crusts, Crusts

4

Our The

6

Planet:

The Earth

Earth’s Gravity

8

Floating Mountains

10

Earthquake Waves

12

The Moho

14

‘"X-Raying” the Earth

16

Above the Moho

18

Volcanoes

20

Weathering: The Making of

Taking Care of

A Rocky Rocks

Soil

24

Soil

Layer Cake: Sedimentary Rocks

Made from Other

22

26

Rocks: Metamorphic Rocks 28

Taking the Earth’s Temperature

30

The Storv of Volcanoes The Puzzle of the Continents Underneath the Ocean

32

34

Treasures in the Earth’s Crust

38

Glaciers

40

The Earth as a Magnet The Age of the Earth The Mohole

42

Word

47

List

6G1 1399

36

44

46

Crusts, Crusts, Crusts

A A of

crust

is

a hard outside covering.

crust of bread

is

baked bread. The

under the

A

crust

the hard,

brown

soft inside of the

bread

on snow

is

the hard top layer that

When snow stands for a while,

the top layer melts a

little bit.

Then when

gets colder, the top layer freezes crust.

The hard crust is good for sledding.

day. If the crust

The

is

thin,

it

will

break

a cold

when you

it.

crust of the earth

layer of the earth.

ocean

it

making the

A crust of ice forms over a puddle on walk on

is

crust.

forms on snow.

hard

outside

floors

and

is

the hard outside

Mountains and

valleys,

islands are all part of the

earth’s crust.

A

long time ago, people thought that the

inside of the earth this

was

liquid.

They thought

because hot liquids come out of volca-

-V

'•••“'as.

V

.

N< V..,v

noes. So they called the hard outside layer the earth’s ‘‘crust.”

Now

earth scientists

of the earth

they

still

is

know

that the inside

very hard, like the crust. But

word

use the

“crust”

when

they talk

about the earth’s outside layer.

how scientists know what the earth’s crust is made of. You will find out how they know how thick the crust is. You will find out how the crust was In this book you will find out

formed and keeps changing will find out

all

the time.

about mountains that act

You

like ice-

bergs and rocks that act like magnets.

You

will find out

how

scientists

that the inside of the earth find out

how

is

is

made

of.

You will what the in-

solid.

scientists guess at

side of the earth

discovered

And you

will find

out about the deep hole that they are digging

through the crust of the earth to see guesses are right.

if

their

North pole

South pole

Our The

earth

(AX-iss).

is

The

Planet:

The Earth

a flattened ball, spinning around axis

is

an imaginary

line

axis.

of the axis.

The south

pole

is

at the other

The equator (ee-KWAY-tur)

is

at

one

end of the

a circle around the

halfway between the north and south poles. The

earth,

earth

is

axis

that passes

through the center of the earth. The north pole

end

its

is

The

flatter at

the south pole than at the north pole.

flattened earth bulges at the equator. So the big-

gest distance around the earth

distance

is

about 25,000 miles.

distance from

The

New York City to

crust of the earth

is

at the equator. This

It is

about ten times the

Los Angeles, California.

not smooth.

places and low places.

V

is

6

It is full

of high

Mt. Everest

ASIA

is

29,000

30.000 feet

feet high

20.000 feet 10.000 feet

PACIFIC

(XEAN

*

V’*

Sea

level

10.000 feet

The deepest part of the ocean

is

38,000 fee

20.000

feet

30.000

feet

40.000 feet

Large high surfaces make up the continents. Mountains

and plateaus (plaa-TOWS) are high places on the

continents. Valleys

and plains are low places on the con-

tinents.

Large low surfaces make the than

/3

of the earth’s crust

is

floors of the oceans.

More

underneath the oceans.

There are high places and low places

in the floors of the

oceans, too.

The

highest part of the earth’s crust

Mt. Everest

is

is

Mt. Everest.

a mountain peak in the Himalaya

Moun-

more than 29,000 feet the ocean. Mt. Everest is more than

tains in Asia. Mt. Everest rises

above the surface of 5 miles high.

The lowest

part of the earth’s crust

floor of the Pacific

a trench in the

Ocean, near the island of Guam. Here

more than 38,000 feet below the the ocean. The Pacific Trench near Guam is

the floor of the ocean surface of

is

is

more than 7 miles deep. Islands are small pieces of the earth’s crust that are

higher than the oceans around them. 7

The easier to

It is

stairs.

The

down

all

Earth's Gravity

walk downstairs than

earth’s gravity

it is

(GRAAV-i-tee)

is

walk up-

to

pulling you

The pull downward helps you when you go downstairs. The pull downward makes it harder to

the time.

go upstairs.

The

The The

the crust of snow.

down

makes

earth’s gravity

to the sea.

blanket of

The The

air

sled speed downhill over

earth’s gravity

makes

earth’s gravity

keeps the earth’s

from getting

earth’s gravity

a*

is

rivers flow

lost in space.

not the same everywhere.

earth pulls harder on things that are closer to

Because the earth bulges

center.

its

at the equator, the

poles are nearer to the center of the earth than the

equator

than

is.

it is

So the earth’s gravity

at the equator.

The

is

greater at the poles

Pacific

Trench

is

closer to

the center of the earth than the top of Mt. Everest

Gravity

than

is

it is

The

greater at the bottom of the Pacific Trench

at the top of

down.

It

ter of the earth.

plumb

makes the

We

a

call

weight attached to a string

string point

toward the cen-

such a weight on a string a

line.

Mountains on the If

Mt. Everest.

earth’s gravity pulls a

straight

is.

plumb

line

is

earth’s surface also pull

on

things.

held near the bottom of a mountain,

the weight will not hang straight down. 8

It will

be pulled

a

little bit in

the direction of the mountain. So the earth's

gravity at a particular place also depends on whether

there are mountains nearby. little less

The

pull

down

will

be a

because of the pull of the mountains.

The earth's gravity can be measured with a gravity pendulum (PEN-dyou-lum). You can use a plumb line to see how a pendulum works. Pull the weight to one side and let go. Gravity pulls the weight down. The weight begins to swing back and forth. Each swing takes the same amount of time. By counting the number of swings in a second a gravity pendulum makes at a place, the earth's gravity can be measured there.

Measuring gravity with a pendulum takes a

lot of

time because millions of swings of the pendulum must

be counted. So

scientists

meet-er) instead. It

A

use a gravimeter (GRAAV-i-

gravimeter

is

a kind of spring scale.

measures the earth's gravity in 4 or 5 minutes.

No mountain

is

nearby. So the earth’s gravity pulls a plumb line straight down.

...JWP

How

nearby mountains

9

pull

on a plumb

line

Floating Mountains Scientists

used to think that mountains were

just

heaps of rock piled up on top of the continents. ever,

when

they

big

How-

made measurements with plumb

lines

and gravimeters, they discovered that there was some-

wrong with

They found that the pull of a mountain on a plumb line was not as great as if the mountain were just a big heap of rocks. They found that the pull of gravity at the top of a mountain was not as

thing

great as

This

if

made

this idea.

the mountain were just a big heap of rocks. earth scientists decide that mountains

and

the rest of the earth's crust were lighter than the inside of the earth.

They decided

that the lighter crust floats

on a heavier material underneath

To

see

what

two blocks be

thin.

scientists

of the

The

it.

mean, do an experiment with

same kind

of

wood. One block should

other block should be thick. Place both

blocks in a basin of water.

The

blocks

of the thick block is above the water. block is below the water, too.

More

10

float,

More

because

of the thick

wood

There

is

more

block above the water than there

is

of the thin block.

is

There

The

is

lighter than water.

more

of the thick block

of the thick

below the water,

blocks are like the earth’s crust.

too.

The water

is

like

the heavier material underneath the crust.

A

mountain

is

like the thick block.

up very high above the heavier It

has a great root that

earth’s crust

is

thickest

were

just a

A plain

mountain

layer on

mountain

which

is

why

the pull of gravity

not as great as

if

a mountain

big heap of rocks.

is

like the thin block.

A

plain does not rise

the crust under a plain does not go

under a mountain. The

earth’s crust

down is

it

as

up

floats.

So

deep

as

thinner under

under mountains.

plains than

The idea is

floats.

under mountains. This thick

very high above the heavier layer on which

material

it

rises

deep underground. So the

is

layer of lighter crust explains at the top of a

A

that the lighter crust floats

on a heavier

called isostasy (eye-50S-ta-sea).

11

S waves are shake waves. They travel only

through

solids.

Earthquake Waves Earthquakes take place where there are great cracks called faults in the earth’s crust.

The

earth heaves

and

trembles along these cracks. Earthquakes are so strong that the smallest earthquake

is

more powerful than 1,000

atom bombs. Earthquakes send waves traveling through the ground.

The waves are not always strong enough to be seen or felt by people. But the waves of even the smallest earthquakes can be felt and measured by an instrument called a seismograph (SIZE-mow-graf). Scientists have set up more than 500 seismographs all over the earth’s surface. The earthquake waves that reach these seismographs help scientists find out what the inside of the earth is like. Earthquakes send out several different kinds of wave.

Two

waves, called S and P waves, have helped scientists

get information about the earth’s crust. 12 X

They have

also

P waves are push-pull waves. They

travel

through liquids and

solids.

helped

scientists get information

about the inside of the

earth.

P and

S waves are alike in

in all directions starts.

They

two ways. They

travel out

from the place where the earthquake

travel faster through

some materials than

through other materials. They travel fastest where the material of the earth

is

packed

closest together, or

is

most dense. So the speed with which P and S waves travel through the earth

a clue to the density of the

is

inside of the earth.

S

get

waves are shake waves. They are

when you shake a rope.

liquids.

They

P waves

S

like the

waves you

waves cannot travel through

travel only through solids.

are push-pull waves.

They

are like the

you get when you push the end of a long

spring.

travel through liquids as well as through solids.

the fastest earthquake waves. 13

waves

P waves They are

The Moho In 1909 there

part of Europe. is

now

was an earthquake

A

Yugoslavia

seientist

made

who

in the southeastern

lived in the country that

a careful study 'of the

waves that were sent out by

this

earthquake.

the records

made on seismographs

earthquake.

He

He

P and

S

studied

were near the

that

also studied the records that

were made

away from the earthquake. All the seismographs recorded two pairs of P waves and two pairs of S waves. The scientist found that one pair of P and S waves arrived at a seismograph near the

on seismographs that were

far

earthquake before the other pair of

graph

He

P and

first,

S

pair.

But he found that the

waves that arrived

at a

arrived at a seismograph far

nearby seismo-

away

last.

explained his strange discovery in this way.

said that the crust of the earth

was made

of material that

was not very dense. So earthquake waves did not very fast through

it.

He

travel

said that the rest of the earth

below the crust was denser than the The

He

crust.

So earthquake

The S wave reached

waves traveled

The P and by

first,

faster

through the earth below the

crust.

S waves that reached the seismograph near-

had traveled along the surface

of the earth. So

they traveled only a short distance through the ground.

The P and

S waves that arrived later,

into the denser part of the earth

had traveled a very great

and bounced back. They

distance.

traveled faster than the surface

had traveled deep

P and

Even though they S waves, they trav-

eled so far that they arrived after the surface waves.

The P and S waves that reached the seismograph far away first had traveled a long distance through the denser part of the earth. But the surface P and S waves also

had a great distance

to travel to the distant seismo-

graph. Since they traveled the distant seismograph

The

scientist

more

slowly, they arrived at

last.

who made this

discovery was

horovicic (Moe-hoe-roe-VEECH-ick).

named Mo-

The place where

the crust ends and the denser part of the earth begins has

been named

after him. It

is

called the

Moho,

for short.

The P wave reached

‘"X-Raying” the Earth

We

cannot see through the earth with ordinary light

or even with X-rays.

But the records of P and S earth-

quake waves on seismographs give

scientists clues that

help them “X-Ray” the inside of the earth underneath the Moho.

The part of

picture on the next page shows the earth with it

cut

away

to

show the

the paths followed by

inside.

P and

The

picture shows

waves

as they travel

waves follow curved paths

as they travel

S

through the earth.

P and

S

through the earth. The shape of the curved path shows that, usually, the

and S waves

deeper

travel.

down

they go, the faster the P

But P and S waves travel

faster

through denser material. So the shape of the curved paths of the

P and

S

inside of the earth.

waves It

is

a clue to the density of the

shows that the material of the

earth gets denser and denser the deeper

down

it is.

There are places where the curved paths of the P and S

waves bend suddenly. These bends are

at places

where

the density of the earth increases suddenly. These places separate the layers of the earth.

The first bend in the curved paths of the P and S waves is at the Moho, which separates the crust from the layer underneath it. The picture doesn’t show the bend at the Moho, because the earth’s crust is so thin. The layer 16

under the

Moho

is

called the mantle.

shell 1,800 miles thick.

The mantle

We know that the mantle

because both P and S waves pass through

is

is

a

solid

it.

S waves do not pass through the inside of the earth

1,800 miles below the Moho. This

is

how we know where

the mantle ends and another layer begins. This

is

the

beginning of the core of the earth.

The

core

is

made up

called the inner core,

The

inner core

inner core

is

is

is

of

two

much

layers.

The

inside layer,

denser than the outer core.

the densest material on the earth.

a ball about 1,600 miles across.

entists think that

it is

solid.

The outer core

is

Some

The sci-

a shell about

1,300 miles thick. Because S waves do not pass through it,

scientists think that the outer core

is

liquid.

An earthquake here sends out S and P waves Crust

Mantle Inner core

Outer core

— Only P waves pass through the core

S waves P waves

Above

The

earth’s crust

is

the

Moho Moho.

a rocky shell above the

Using the measurements of waves made by earthquakes

and explosions,

scientists are finding out the

rocky shell and what

With very

it is

made

shape of

this

of.

careful measurements, scientists have dis-

covered that some earthquakes send out three pairs of

P and

From

S waves.

have decided that the separate layers. layer.

P and

the records of these waves, they earth’s crust

The upper

S

waves

layer

travel

is

made up

is

with a

more slowly through the

layer of the earth’s crust

lot of silicon

and aluminum

rocks of the upper layer sial

(

upper

in

it,

too.

in

So sometimes

is

made

,

So they

muh),

call

the

after silicon

and

and

alu-

lot of silicon

scientists call this sialic

lower layer of the crust

the rock basalt. Basalt has a lot of silicon and it.

of rocks

layer, the granitic layer.

Scientists think that the

in

layer.

them. They

SEYE-al)

a/uminum. The rock granite has a

minum

two

lighter than the lower

upper layer than they do through the lower

The upper

of

call

this simatic

like

magnesium

the rocks of the lower level sima

after silicon

is

(

SEYE-

and magnesium. They sometimes

call

lower layer, the basaltic layer.

Scientists are

making maps

of the crust

under the

oceans and the continents. They do not use earthquake

waves because earthquakes do not happen often or 18

in

They use man-made waves to give them the same facts that earthquake waves do. The man-made waves are made by explosions. The man-made waves are picked up by special receivers. This kind of mapping the right places.

is

called seismic (SIZE-mick) mapping.

On

land, holes are drilled in the rock. Charges are

exploded in the holes. The waves sent out by the explosions are picked

up by

receivers called

geophones and

are recorded.

At

sea,

charges are dropped over the side of a ship.

The charges explode in the ocean. The waves sent out by the explosions are picked up by receivers called hydrophones on a second ship and are recorded.

By crust

seismic

mapping

under the ocean

have found that

is

this crust

They have found is

scientists

have found that the

only 3 or 4 miles thick. is

made mostly

of sima.

that the crust under the continents

15 to 25 miles thick.

It is

40 miles thick under very

high mountains. The continents have a layer of top of a layer of sima.

Waves

They

travel faster

through the lower crust

sial

on

The

inside of a volcano

Volcanoes

A

volcano

is

of the earth.

comes out

like a

doorway

that opens into the inside

Lava (LAH-va), the melted rock

of the opening of a volcano, starts

its

that

trip to

When the lava is underground it is called magma MAAG-ma) Some scientists think that magma comes from the the top of the volcano from deep underground. (

20

They think

mantle.

that an earthquake in the mantle

makes the mantle rock shake the melted rock, or

so hard that

magma, pushes

its

cracks in the ground, to the outside.

magma and

lava give

them

it

Then

melts.

way up through They think

that

clues about the material that

makes up the mantle.

When

lava pours out of the opening of a volcano,

hardens quickly. The rock formed this still

way

is

Sometimes

basalt.

underground.

It

when

magma

it

lava hardens in

hardens while

it is

hardens slowly, forming granite.

Basalt and granite are igneous (IG-nee-us) rocks.

There are deep layers of basalt over large parts of the states of

These layers are

States. left

Washington, Oregon and Idaho

in the

like footprints that

United

have been

behind by volcanoes. There are no active volcanoes

there any more. But there are mountain peaks that used to

be active volcanoes. Mounts Hood, Rainier and Shasta

are

all

dead volcanoes.

The island of Hawaii is mostly basalt. It was formed when the lava that poured out of volcanoes hardened. Lava is still piling up on Hawaii. The lava pours out of the openings of two volcanoes. Each volcano becomes active every five years. piled

up

Most

so

much

One

of the volcanoes,

lava on

its

sides that

of the volcanoes of the

earthquakes happen often. 21

Mauna it is

world are

Loa, has

3 miles high.

in places

where

Weathering: The Making of

Soil

Soil covers the top of the crust of continents

lands. It tains

and places covered by

ice

may be a few inches deep.

Soil

is

It

and snow

all

the time.

may be many feet deep.

the only part of the crust in which plants can grow.

Once

work

perture,

was no

there

a rocky shell.

the

is-

everywhere, except on steep rocky moun-

is

Soil

is

and

It

of

took

soil.

many

wind and

The

outside of the earth

was

be made.

Soil

years for

rain

soil to

and great changes

which make rocks crumble. This work

in

is

tem-

called

weathering.

The heating and

cooling of rocks helps

make

Heating of rocks during the daytime and cooling

may make

soil.

at night

the rocks crack. Rain water that collects in

the cracks freezes

when

the weather gets very cold. But

water takes up more space when

it

freezes.

So the ice

pushes against the sides of the cracks and makes the cracks larger. In this way, the rocks begin to crumble.

Carbon dioxide (CAR-bun dye-OX-ide)

in the air

make soil. Rain carries carbon dioxide out of the air when it falls. Some of the rain water and carbon dioxide come together to make a chemical that wears away rock. The chemical slowly turns the surface of the rock to a powdery dust. Sand and clay are made when rain

helps

water and carbon dioxide change granite 22

in this

way.

Strong winds help

make

soil.

Strong winds pick up

The dust sandpaper on wood and wear away

dust and sand and blow them against rocks.

and sand work

like

the soft parts of the rock.

Crumbled rock

is

broken into smaller pieces by living

things. Roots of trees

and burrowing animals help do

Chemicals made by earthworms as they crawl

this.

through the ground help do

The work

of

making

soil is

this, too.

begun by weathering.

It is

helped along by the animals and plants that grow in the soil

and

live in

You can time.

On

see

it.

what weathering can do even

in a short

tombstones in old cemeteries, you can hardly

read the writing even though they aren’t even 100 years old.

Weathering has worn away the stone and the writing

with

it.

Earthworms help make 23

soil

Heavy rains cannot wash away

Taking Care of Soil

is

plowed

like this

Soil

the most precious part of the earth’s crust.

Plants take root and

grow

and water from the

soil.

soil.

soil

in the soil.

They

get their food

So plants cannot

live

without

Animals get their food by eating plants or other

animals. So animals, too, cannot live without Soil that

is

soil.

baked by the sun becomes dry and pow-

Then it can be carried away by the wind. It can be washed away by heavy rains. The carrying away of soil by wind and flowing water is called erosion ( e-ROWdery.

zhun )

.

A

cover of grass and trees can keep the sun from 24

baking the

soil

and drying

it

out.

Then

erosion does not

take place.

There are some places where there

and where the

air is

such places. The

from the is

at

soil

hot, dry air.

work

all

Few

very hot.

is

very

little

plants can

rain

grow

in

has no natural cover to protect

These places are

the time in deserts.

it

deserts. Erosion

Some

deserts can

be

turned into places where plants can grow by irrigation.

By

bringing water to them, the

be made

soil of

some

deserts can

useful. Imperial Valley in California

used to be

a desert. Imperial Valley has been turned into valuable

farmland by

irrigation.

There are some places where bad farming has caused erosion. This has

happened

not very good. Such grass or trees.

soil

in places

where the

should be used only for growing

When farmers

use this land for row crops,

like peas, potatoes, or cotton, a lot of soil

between the rows. The hot sun

Then soil

is left

uncovered

dries out the bare

soil.

strong winds and sudden heavy rains carry the

away. Plants cannot grow any longer in the

is left.

The land has been changed

Farmers can do many things

plow

soil is

their fields in such a

way

soil

that

into a desert.

to stop erosion.

that sudden,

They can

heavy rains

They can plant rows of trees to slow down the wind. They can learn to know their soil, and use it wisely. Then there will always be enough soil in which to grow food for all the people on the earth. do not wash away the

soil.

25

A Some

Rocky Layer Cake: Sedimentary Rocks of the soil

rock again.

made by weathering

The streams and

is

changed

into

rivers that flow over the \

land help change

it

into rock.

Rain water flows downhill.

on

its

way.

It

Little streams of rain

form brooks. Brooks with

their larger load of soil

pebbles come together to form the sea. Brooks and rivers look carries a lot of

Rivers slow

up soil and pebbles water come together to

picks

and

Rivers flow into

rivers.

muddy when

their

water

soil.

down when they reach the

slow down, they drop their load of

and pebbles

soil

sea.

When they

and pebbles. The

out of the water and

fall to

the

bottom. They pile up on the floor of the sea.

When

bits

soil

settle

of solids settle out of liquids this

way

they are called

sediments (SED-ih-ments). So the layers of

soil

and

pebbles that pile up on the ocean floor are sediments. As

Sediments:

Soil

and pebbles that 26

settle out of

water

they pile up, they sometimes bury the shells of sea ani-

mals under them.

As the sediments

pile up, their

more and more. The weight

gether.

soil

and pebble

too.

in the

At the same time, chemicals

the spaces around the bits of

soil

sediments closer in the sea

it

water

and pebble. In

into a rock called shale. Shale

mentary rock because

sedi-

These weights press

the sediments are cemented together. In this

changed

down

above the

of the water

ments presses down on them the bits of

weight presses

is

to-

fill

this

in

way

way they are

called a sedi-

was formed from sediments.

Sandstone and limestone are also sedimentary rocks.

Much rock. It

of the top layer of the continents is

the layer that

The sedimentary

lies

right

gave

sedimentary

underneath the

soil.

rocks are a clue to the story of the

earth’s crust. Prints of sea shells in first

is

scientists the idea that

sedimentary rocks

sedimentary rocks were

formed under water.

Sedimentary rock: Sediments pressed and cemented together 27

Rocks

The flat

Made from Other

slate

blackboard that you write on has a smooth,

surface. Slate that

for flagstone walks

is

Because

it is flat, it

mentary

rocks. This

thought that Scientists

them study

Rocks: Metamorphic Rocks

slate

is

used for roofing on houses and

smooth and

flat,

reminds us of the is

too. Slate flat

a rock.

layers of sedi-

why, a long time ago,

was a kind

is

scientists

of sedimentary rock.

have used microscopes and X-rays

to help

They have also studied the imprints of They have compared these imprints with

slate.

shells in slate.

shell-imprints in sedimentary rocks. Their studies

them decide

made

that slate belongs to a different family of

They named this family metamorphic (met-aMORE-fick) rocks, because they are changed rocks. They are rocks that are ( Metamorphic means changed. ) formed when igneous rocks and sedimentary rocks

rock.

change

in a certain

way.

Sedimentary and igneous rocks are changed to...

28

Metamorphic rocks are formed when other rocks deep under the ground are squeezed hard. They may be squeezed hard by the weight of big pushes

down on them from above. They may be squeezed

by big pushes from the pressure.

sure

The

side.

Slate

is

These pushes are called

pressure makes the rocks hot.

and heat change the rocks

when

piles of rock that

into

The

metamorphic

one kind of metamorphic rock. Slate

the sedimentary rock, shale,

and pressure. Marble, metamorphic

schist

is

is

presrocks.

formed

changed by heat

and gneiss (NICE) are

also

rocks.

All rocks in the earth’s crust belong to

one of the three

families of rock. All rocks are either igneous, sedimen-

By studying

the rocks of the earth’s

and where they are found,

scientists are learning

tary or metamorphic. crust

the stories of the continents, islands and oceans.

...metamorphic rock by heat and pressure 29

Taking the EartKs Temperature

No one

can ever get deep inside the earth to take the

But

earth’s temperature.

scientists

can get some idea

about the earth’s temperature in other ways. They can

do

this

and

by taking the temperature

tunnels. Scientists can get

of rocks in mines, wells

some idea

of the earth’s

temperature, too, from the temperature of lava that

comes out

of volcanoes.

Using the measurements they have made,

scientists

have discovered that the deeper you go into the the hotter

it is. It is

that the mines

work

in

so hot at the

earth,

bottom of some mines

must be cooled. Otherwise miners cannot

them. Using the measurements they have made,

scientists

have decided that the inside of the earth

hot. It

hotter than the inside of a blast furnace.

is

The heat from crust in the

inside the earth

same way

comes up

is

very

to the earth’s

that heat rises in a room.

The

crust

loses this heat to the earth’s blanket of air.

How

did the inside of the earth get so hot? People

used to think that the inside of the earth the earth

they said, If this

was once part is

hot because

of the sun or of a star.

cool because the earth

is

The

idea were correct, the earth would have lost

is

Now

more than 3

crust,

slowly cooling

heat in 100 million years. But scientists earth

is

know

off.

all its

that the

billion years old.

scientists think that the inside of the earth

30

is

hot

AIR

Heat from inside the earth comes up to the earth’s crust

The pressure of the crust on the mantle makes the mantle hot. The greater pressure of the mantle on the core makes the core even hotter. Pressure helps make the inside of the earth hot. Also certain rocks make heat. Rocks that have uranium in them make heat when the uranium changes

for

two

to lead.

reasons. Pressure

Rocks

makes

heat.

like this are called radioactive.

active rocks help

make

the inside of the earth hot.

scientists think that radioactive rocks are

earth’s crust

Radio-

warmer and warmer. 31

Some

making the

The Story Scientists think that

currents

mantle

moving

is

The top

of

mountains are formed by great

in the earth’s mantle.

very hot because

of the mantle

cool crust.

The

Mountains

is

it is

of the

near the earth’s hot core.

cool because

currents are

The bottom

it is

made by

near the earth’s

the difference in

temperature between the top and the bottom of the mantle.

The

currents

move

slowly in big circles between

32

They

the core and the crust.

carry heat from deep inside

the earth to the surface.

The

pictures

show how the moving

currents in the

mantle make mountains. At some places near the

sur-

two currents come together and then flow down-

face,

The downward flow pulls the crust of making a low place in the ground. (2) Rivers

wards again. the earth,

(1)

flowing to this low place drop their sediments there. ( 3 After millions of years the sediments harden into rock. ( 4

Meanwhile, the currents keep on moving

The

down below the surface of the great folds. (5) Some of the rock

currents pull the rock

crust squeezing is

in big circles.

it

into

changed by heat and pressure

Some

into

metamorphic

rock.

even changed into magma. (6) The metamorphic rock and magma are the beginnings of a mountain.

They

is

float

on the mantle, because they are

lighter than

the mantle. ( 7 ) But the pull of the currents in the mantle

keeps them below the surface of the

crust.

After millions of years, the currents carry

much

away

so

heat from the bottom of the mantle that they stop

moving

altogether.

There

rocks down. Isostasy

is

nothing to hold the lighter

makes them

rise to

the surface of

the crust. Mountains have been born. (8)

New work

of

formed

currents in other parts of the mantle keep

up the

mountain building. So new mountains are being all

the time. 33

The Puzzle

The

of the Continents

and ocean

continents, islands

of the earth’s crust.

they are

made

The lower

But they are

floors are all parts

all

different because

of different materials.

part of the earth’s crust

is

a shell of sima

around the mantle. Large parts of the crust have a layer of sial

on top of the layer of sima. These large

are the continents. Places sial

on top of

Many like

where the simatic layer has no

little like

continents and a

Some of their rock is sialic like Some of their rock is simatic, like

floors.

of continents.

places

are the ocean floors.

it

islands are a

ocean

sialic

little

the rocks the rocks

of the ocean floor.

Isostasy explains

why

above the oceans. Their float is

the continents stand so high lighter sialic rock

higher on the mantle.

It

also explains

deeper under continents than

The big puzzle

Some

all

why

the crust

under oceans.

of the earth’s crust

there a sialic layer over

the continents

it is

makes them

is

of the earth’s

Why isn’t crust? How did this:

come about?

scientists think that the

how

shape of the continents

came about. If they were pushed together, the continents would fit like the pieces of a jig saw puzzle. These scientists say that the earth was once is

a clue to

they

completely covered with

sial.

Great earthquakes inside

the earth are stretching the crust 34

all

the time and slowly

the continents were pushed together, they would pieces of a jig saw puzzle If

making the earth sialic

bigger.

The

They

like

stretching has broken the

layer into large jagged pieces.

are the continents.

fit

The jagged

pieces

are like big floating sialic rafts,

that have drifted apart.

Other

scientists think that islands that are a little like

continents and a find out

how

little like

the continents

ocean

came

can help them

about.

They think

new

continents.

these islands are the beginnings of 35

floors

that

Continental shelf

Underneath the Ocean

Most

of the earth's crust lies

But not

underneath the ocean.

all

the crust under the ocean belongs to the

ocean

floor.

There are wide shelves of crust under the

ocean

all

around the continents. These shelves are

like the continents.

continents.

They

They

sialic

away from the continental shelves. The

slope gently

are called the

continental shelves are covered with a thick layer of sedi-

ment. The sediment of the continental shelves settled out of the water of streams and rivers.

The

earth's crust drops sharply

away from

nental shelves to the deep part of the ocean.

under the deep part of the ocean the ocean

floor.

The ocean 36

floor

is

the conti-

The

crust

simatic. This crust

is

is

covered with sedi-

merits too. These sediments took millions of years to

form.

They formed from

dropped

to the

ocean

and

tiny sea animals that died

floor.

Some

of the sediments

have been carried down from the edges of the

may

conti-

nental shelves.

The

floor of the

ocean

is

made up

of great undersea

mountain ranges. The mountains of the like

dead volcanoes. This

are

still

many

is

Pacific floor look

not surprising because there

active volcanoes near the Pacific Ocean.

A lot of earthquakes happen near the Pacific Ocean,

too.

The earthquakes and volcanoes make a ring of fire around the Pacific Ocean. The mountains under the Atlantic Ocean look like mountain ranges on the continents. But. they are higher and longer than any mountains on land.

The

floor of the

ocean has

many deep

valleys.

There

are also great cracks in the crust under the ocean.

The

deepest valleys are the trenches of the Pacific Ocean.

These trenches nents and a

lie

near islands that are a

little like

ocean

along the Pacific ring of

fire.

floors.

The

little like

conti-

These trenches

lie

great crack under the

Ocean is hundreds of miles long. In many places it is more than a mile deep and 30 miles wide. This great crack lies in the belt under the Atlantic Ocean where Atlantic

many

earthquakes happen. Scientists think that the

trenches and cracks have something to do with continents

came

about. 37

how

the

Treasures in the EartKs Crust

Rocks are made up of minerals.

Some

common

a very ite

and

phires

common. Quartz

mineral. There

is

(quarts)

is

a lot of quartz in gran-

in sand.

Some They

minerals are very

minerals are very rare. Diamonds, rubies, sap-

and emeralds are precious (PRESH-us)

stones.

are very valuable because they are so rare.

Some

minerals are very useful.

The minerals from

which we get metals and the minerals from which we get fuels are the

most important minerals of

all.

The

precious

stones are useful, too, because they are the hardest stones there are.

Many

minerals are like buried treasures.

They

are

locked up in the rocks of the earth’s crust. Metals like iron ores.

and copper are locked up

in rocks called mineral

They must be freed from the

rocks and ores in

which they have been hidden. It is

the job of the prospector (PROS-peck-tur) to find

the places where minerals are hidden. uses

many

The prospector

different tools to help him.

The most important knows about different

tool of the prospector

rocks.

mentary

rocks.

what he

rocks and the minerals in them.

For example, he does not look for

morphic

is

He knows

oil in

igneous or meta-

that oil forms only in sedi-

38

The prospector uses erals. Oil is

a gravimeter to help

often found

of salt underground. Salt

him

find min-

where there are large amounts is

lighter than

most other

rocks.

So a gravimeter will show a smaller reading near a deposit. This

may mean

that there

does seismic mapping to find

oil,

is oil

salt

He

there too.

too.

The prospector uses a cover to help him find minerals. The corer bites out pieces of rock from deep holes that have been drilled in the earth’s crust. The prospector can examine the pieces to find out what minerals are in them. The prospector uses chemicals to test rocks and soils to find out

if

they have mineral ores in them.

The prospector radioactive rocks.

found

uses a Geiger counter to help

him

find

The minerals radium and uranium

are

in radioactive rocks.

The prospector too, to help him

uses magnets and electric currents, in his search for the earth’s buried

treasures.

Mining iron ore

A

boulder

left

behind by a glacier

Glaciers

Snow

that falls

on the land

in the wintertime usually

melts before the next winter comes. There are some places, however, ter

where

so

much snow

snows never melt. The snow

Each

layer presses

down on

The

land

covered with a sheet of

form

pressure changes the

at the tops of

The neath

is

ice.

ice.

down on

Under the pressure

A

a river of

snow under-

to ice.

Then the

The whole continent

of

its

moving

ice,

of

ice.

the ground under-

weight, the sheet be-

gins to slide slowly over the ground.

become

in great layers.

Sheets of ice like this

covered with a sheet of

sheet of ice presses it.

snow

that the win-

very high mountains. Parts of Alaska

are covered with sheets of

Antarctica

up

piles

the layers of

neath. is

falls

The

or a glacier

ice sheet has

(GLAY-sher)

glacier acts like a great bulldozer. It pushes dirt.

40

rocks and even large boulders in front of

rocks and dirt on

A

its

it.

It

also carries

back.

glacier acts like a powerful rock crusher. It crushes

some rocks

A

into a fine

powder.

glacier acts like a great polishing machine. It pol-

ishes

some rocks

smooth and round.

so that they are

At one time large parts of North America and Europe

were covered by

and the

glaciers

glaciers.

began

Then

to melt.

warmer the work of

the weather got

We

can see

the glaciers where they have melted.

Boulders that are different from the rocks around them are the

work

of glaciers.

Sometimes

have carried

glaciers

large boulders hundreds of miles.

Some

are the

hills

and

piles of rocks

work

of glaciers.

These

dirt that the glaciers

hills

are the

pushed before

them. The piles were left behind when the glaciers melted.

Long

Island, east of

length,

which was

New

left

York City, has a

hill

along

its

behind by the North American

when it stopped. Some valleys are the work of like the letter U were made by glacier

through the

V

glaciers. Valleys

shaped

glaciers as they

pushed

shaped valleys of streams.

There are long scratches and deep holes are the

work

of glaciers, too.

These are some of the changes are

made by

in rocks that

glaciers.

41

in the earth's crust that

The Earth

The two

earth

poles.

like a

is

earth

is

big magnet. Like a magnet

One magnetic pole

other magnetic pole

Magnet

as a

is

in northern

is

in Antarctica.

surrounded by a magnetic

it

has

Canada. The

Like a magnet the

field.

At any point

in

the earth’s magnetic field a compass needle can feel the strength of the field and the direction of

its

pull.

There are some magnets that are made of special metals.

They

lasts a

long time. There are other magnets

magnetism

are permanent magnets. Their

made

of a

wound around a core of soft iron. These become magnets when an electric current flows through the coil of wire. They lose their magnetism when the curcoil of

rent

is

wire

turned

off.

This kind of magnet

is

called an

electromagnet. Scientists think that the earth gets

ism from

its

core.

They think

most of

that the core

its

magnet-

is

made

of

Magnetic

Magnetic

pole

pole

The grey magnet

lines

show the magnetic 42

field

around a permanent

iron

and

They think

nickel.

there are electric currents

in the core flowing in big circles.

currents

make

These flowing

electric

the core act like a big electromagnet.

Scientists think that the earth gets

some

of

its

mag-

netism from a magnetic rock in the earth’s crust. The rock, magnetite,

is

an iron

Deposits of magnetite in

ore.

the earth’s crust act like magnets.

They add

magnetic

fields.

magnetic

field of

The

earth’s

around the

westward

movement

magnetic

their

own

fields to the

the core.

magnetic

earth.

their

They make

field is

slowly moving westward

The magnetic

poles are slowly

moving

in big circles, too. Scientists think that this is

caused by the spinning of the earth. 43

The Age

No one knows

exactly

of the Earth

how

old the earth

is.

Scientists

They use certain rocks that they find in the earth’s crust to help them guess. Radioactive rocks inside the earth make some of the earth’s heat. Some of these rocks have uranium in them. Scientists use uranium rocks to help them guess at the

try to guess at

its

age.

earth’s age.

Uranium keeps changing.

When

uranium has finished

It

changes several times.

all its

changes,

it

ends up as

lead.

The uranium in a rock does not all change to lead at the same time. As time passes, a piece of uranium rock will have less and less uranium in it. It will have more and more lead in it. Uranium changes to lead very slowly. Scientists have figured out the time it takes for this change. They measure the time by finding the half-life of uranium. By this they mean the time it takes half of the uranium in a piece of uranium rock to change to lead. The half-life of uranium

is 7/2

billion years.

Scientists think that all

when

the earth

of the earth

uranium rocks were made

was formed. They can

by finding out the age

of

find out the age

uranium

rocks.

They do this by measuring the amount of lead in uranium rocks. In this way they have found out the age of a 44

rock taken from a mine in South Africa. billion years old.

So

know

scientists

It is

about 3

that the earth

is

at

least 3 billion years old.

There are reasons is

for scientists to think that the earth

even older. They think that the earth, the sun and the

planets were

all

formed

same

at the

time.

They

think

and

fall to

that the meteorites that flash through the sky

the ground were formed at the same time, too.

By meas-

uring the amount of lead in meteorites scientists have

decided that they are

them think

4/2 billion

that the earth

years old. This

makes

billion years old, too.

is 4/2

any piece of uranium In

.

.

about 1/10 of it has changed to lead 1 billion years .

.

.

.

.

.

about 1/4 of

changed to lead billion

.

.

.

it

has

In

3

it

has

years

about 1/3 of

changed to lead in 4V2 billion years 45

In

The Mohole

The

He

earth’s mantle.

gets

make up

has never seen the rocks that

scientist

has clues about these rocks that he

by making measurements. He measures the

gravity

the

and the speed

of earthquake waves.

earth’s

He measures

the earth’s magnetism and the temperature in deep mines

and

Using these clues he makes guesses about

drill holes.

what the

inside of the earth

is like.

These guesses are

theories.

The

scientist tries to test these theories to see

are correct.

from the

as his

measurements are

earth’s crust, the scientist finds

So American and Russian

theories.

cided to

As long

all

scientists

They are going to

drill

through the

test his

have de-

holes through the earth’s crust to the

drill

they

gathered

hard to

it

if

Moho.

Moho so that they can

get samples of the earth’s mantle.

The Americans call their hole through the Moho a Mohole. They are going to drill the Mohole under the ocean were the will

have

crust to

go

Moho. They

warm and

thin.

Because the ocean

down about

deep, they

through the

Moho from

They will drill the Mohole where the ocean

the tides are weak.

The Russians little like

Moho

will drill their hole to the

the crust of an island that

and a

is

7 miles before they reach the

will drill their hole

a great barge. is

is

a

is

a continent.

little like

The Moho

the surface of this island. 46

through

the ocean floor

is

7 miles below

Word

List

— The line around which the earth spins. earth — A ball of iron and nickel, about 1600

Axis (AX-iss)

Core

of the

miles

across, at the center of the earth.

Dense

— Packed

close together.

Equator (e-KWAY-tur)

way between

—A

big circle around the earth half-

the north pole and the south pole.

Erosion (e-ROW-zhun)

— The

carrying

away

of soil

by wind

and flowing water. Gravity (GRAAV-i-tee)

— The pull of the earth towards its cen-

ter.

Igneous rock

(

— Rock made when magma

IG-nee-us )

or lava

hardens. Isostasij floats

Lava

(

(

eye-SOS-ta-sea )

on

its

— The

idea that the earth's crust

mantle.

LAH-va — The melted

rock that comes out of the open-

)

ing of a volcano.

Magma

(

M AAG-ma — Lava )

Mantle of the earth and the crust.

— The

that

is

underground.

layer of the earth

between the core

Metamorphic rock (Met-a-MORE-fick) — Rock formed when igneous or sedimentary rock is changed by heat and pressure. Sediment (SED-ih-ment) Sedimentary rock

— Rock

— Solids

formed when sediments harden.

Seismograph ( SIZE-mow-graf ) records earthquake waves. Sial

(

SEYE-al)

that settle out of a liquid.

— An

instrument that feels and

— The rocks that make up

the upper layer of the

earth's crust.

Sima (SEYE-muh)

— The

rocks that

of the earth's crust.

47

make up

the lower layer

About the Authors Irving

and Ruth Adler have written more than three

dozen books about science and mathematics. Dr. Adler has been an instructor in mathematics at Columbia University

and

at

Bennington College, and was formerly

head of the mathematics department high school. Mrs. Adler, matics, science,

and

who

of a

New York City

formerly taught mathe-

art in schools in the

New York

area,

recently also taught at Bennington. In addition to work-

ing with her husband writing this book, she

drew the

illustrations.

The Adlers now

live in the

country in Shaftsbury

Township, near Bennington, Vermont.

Books by Irving Adler alone and books by him

in col-

laboration with Ruth Adler have been printed in 58 different editions, in 8 languages

and

in 9 reprint editions.

PICTURE CREDITS Page 24

Page 39

—

— United

States

Department

of Agriculture

Hibbing Chamber of Commerce, Hibbing, Minnesota

48

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•

.

M

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