California science grade 5 (10)

Most volcanoes are found at the boundaries between two tectonic plates—giant, slowly moving slabs of Earth’s crust.. ▶ Main Idea 6 ES 1.aMoving plates cause Earth’s surface to change..

Chapter 6

Rocks at the bottom of the Grand Canyon

are 2 billion years old.

Mt Whitney, Sequoia National Park, California

Plate Tectonics and Earth’s

Structure

What geologic forces have

shaped Earth’s landscape?

264

6 ES 1 Plate tectonics accounts for important features of

Earth’s surface and major geologic events.

265

MAGAZINE ARTICLE

ELA R 6.2.7 Make reasonable assertions about a text through accurate, supporting citations

• ELA W 6.2.1 Write narratives.

266

Write About It

Response to Literature In this article the author describes a string of volcanoes in the Pacific Ocean Some of these volcanoes make up the Hawaiian

Islands Others are located on the seafloor Which volcanoes are older? Which ones are most active? Write a story about a scientific expedition to study the volcanoes of the Pacific Describe how the researchers would travel and what they might find.

It’s also interesting that the volcanoes of Hawaii pop up right in the middle of the Pacific Ocean Most volcanoes are found at the boundaries between two tectonic plates—giant, slowly moving slabs of Earth’s crust Molten rock forms in the cracks at plate boundaries and trickles upward

to create volcanoes

But Hawaii is nowhere near a plate boundary It is

located smack-dab in the middle of a plate—the Pacific Plate.What on Earth is going on? For decades, researchers thought they knew how Hawaii formed, but now they’re not sure New theories are shaking up their understanding

of how Earth works

- Journal Write about it online

@www.macmillanmh.com

from CURRENT SCIENCE

267

Lesson 1

Earth’s Moving

Plates

Look at the coastlines of Africa and South

America They look as if they could fit together

like gigantic puzzle pieces Have Earth’s continents

always been in the same locations? Are they moving

now? How will they be arranged in the future?

6 ES 1.a Students know evidence of plate tectonics is derived from the fi t of the continents; the location of earthquakes, volcanoes, and midocean ridges; and the distribution of fossils, rock types, and ancient climatic zones • 6 IE 7.g Interpret events by sequence and time from natural phenomena (e.g., the relative age of rocks and intrusions).

268

ENGAGE

Were the separate continents we know today one

huge supercontinent in the past? Do the outlines

of continents fit together? Write your answer as a

hypothesis in the form “If the continents were once

a supercontinent, then ”

Test Your Hypothesis

Place tracing paper over a map of the world

Trace the coastlines of North America, South

America, Europe and Asia (including India),

Africa, Australia, and Antarctica.

Be Careful Cut the traced continents along

their coastlines, and label them.

Using the continent cutouts like pieces of a jigsaw

puzzle, find ways the continents fit together

Draw a sketch showing ways you can fit them

together.

Draw Conclusions

Analyze Which continents have coastlines that

fit together most closely?

Did your results support your hypothesis?

Infer Which of your sketches shows the

greatest number of continents fitting together?

Do all of the coastlines in the sketch fit together

equally well?

What if the continents in your finished puzzle moved

apart to the positions they are in today? If they kept

moving, how might they be arranged in the distant

future? Make a prediction and test it Then analyze

and present your results.

Explore More

Step

Step

6 IE 7.a Develop a hypothesis • 6 IE 7.e Recognize whether

evidence is consistent with a proposed explanation.

269 EXPLORE

▶ Main Idea 6 ES 1.a

Moving plates cause

Earth’s surface to change.

What forces shape Earth?

Many things change over time Many changes happen quickly, but many other changes happen very slowly Even Earth’s surface has changed over time Mountains rise, only to be worn down by water, wind, and particles of rock The ground is so firm it can support the tallest buildings with ease However, the ground can suddenly shift, bringing those buildings down Even something as large as the continent you live on has slowly moved to its present position The idea that the position of huge continents can change over time might seem strange

Alfred Wegener was a German scientist who proposed a theory to explain changes in Earth’s surface over long time periods Like many other people, he noticed how closely Africa and South America would fit together if the two continents were pushed against each other Wegener wondered if the other continents would fit in similar ways if they were moved together In 1912 Wegener proposed a hypothesis

of continental drift : the idea that a past supercontinent split apart into pieces, which drifted over time to their present locations

A geologist (jee•AHL•uh•jist) is a scientist who studies Earth’s origin, history, structure, composition, and processes In the 1960s new discoveries led geologists to take another look

at Wegener’s work However, during Wegener’s lifetime few geologists accepted his theory

65 million years ago

present day

225 million years ago

135 million years ago

Quick Check

Draw Conclusions What does

the term Pangaea refer to?

Critical Thinking Compare the map of the continents

135 million years ago to the map of the continents today

How have the positions of the continents changed?

271

EXPLAIN

What evidence supports

continental drift?

Wegener provided several kinds of

evidence to support his explanation

of continental drift He noted similar

fossils and rocks on distant continents

He also pointed out changes in the

continents’ climates over millions of

years

Evidence from Rocks

Rock formations can provide

evidence about past events that took

place in a particular location For

example, parts of Africa and

South America contain rocks

of the same age and type If

these continents were once

joined, similar rock layers

would continue across their

borders Mountain ranges

and mineral deposits across

today’s continents would also

line up in the same way These facts

suggest that the continents drifted

apart

Other evidence indicates that

the continents have also drifted to

different climate zones For example,

North America and Antarctica contain

coal deposits Coal is formed from

decaying tropical plants found near

the equator Today neither North

America nor Antarctica lies near the

equator For coal to be found on these

continents, North America must have

moved north from a tropical region,

and Antarctica must have moved

south

Evidence from Rocks’ Ages

How can scientists tell which rocks are older? Scientists compare the age

of one rock with the age of another to find the rocks’ relative ages When two rock layers are found in the same rock formation, normally the lower rock layer is older Scientists also compare ages of similar rock layers that formed

in different areas Sometimes the types

of fossils found in a rock can help scientists determine the age of the rock

Index fossils—fossils of organisms that

lived only during a particular time—can help narrow down the age of the rocks in which they are found

Fossils of Cynognathus, a Triassic land

reptile about 3 m (10 ft) long, have been found in South America and Africa.

272

EXPLAIN

Evidence from Fossils

Ancient fossils of some extinct

animals and plants have been found in

parts of Africa, South America, India,

Australia, and Antarctica These fossils

include three reptiles—Lystrosaurus,

Cynognathus, and Mesosaurus—and

a plant, Glossopteris These organisms

would not have been able to travel

across an ocean However, their fossils

have been found on continents that are

separated by vast oceans today This

suggests that the continents they lived

on were once connected

Another bit of fossil evidence

supports the hypothesis of continental

drift Fossils of Glossopteris and

Lystrosaurus have been found in

Antarctica They could not survive in Antarctica today because it is too cold This suggests that Antarctica drifted from a warmer region to a colder one Despite all of the evidence, some scientists remained skeptical that the continents were together at one time

Quick Check

Draw Conclusions What do the coal deposits found in North America and Antarctica indicate about the way these continents may have drifted?

Critical Thinking What evidence supports the hypothesis of

continental drift?

The map shows where fossils of

ancient organisms have been found

in the southern continents It also

shows how these continents would

once have fit together in a way that

explains the distribution of the fossils.

Fossils of Mesosaurus,

a freshwater reptile, have been found in South America and Africa.

Fossils of the Triassic land reptile

Lystrosaurus have been found in

Africa, India, and Antarctica.

273

EXPLAIN

]QSO\WQQ`cab ]QSO\WQQ`cab

What clues are found

on the ocean floor?

Technology that was not available

to Wegener in the early 1900s helped

answer this question Scientists

discovered that Earth’s crust seemed

to be made of a number of large

pieces These large pieces of Earth’s

surface are called plates The plates

may include continents, ocean floors,

or both When plates move, they carry

the continents and oceans with them

Plates may move apart, move together,

or slide past one another

When plates move apart, new rock

from below the surface may form

between them In the 1960s scientists

found evidence that new rock from

below was being added to plates

moving apart under the oceans

Mid- Ocean Ridge

The addition of new rock has built

up a vast underwater mountain chain called the mid-ocean ridge As new rock is added, it moves away from the ridge in opposite directions This

process is called seafloor spreading

a thermal spring in Iceland

274

EXPLAIN

The rock located farther from the

ridge is older than the rock located

at the center

The mid-ocean ridge extends

through the Atlantic, Pacific, and

Indian oceans In some places parts

of the ridge have emerged as islands

Iceland is an island of this kind As

part of the ridge, Iceland is attached

to the ocean floor

Continental Drift:

Clues from Ocean Trenches

Another feature of the ocean floor

occurs where plates move together

When plates move toward each

other, one sinks under the other,

and this movement creates an ocean

trench Ocean trenches are long,

narrow, deep valleys on the ocean

floor They are the deepest parts of the

oceans Most ocean trenches are found

around the rim of the Pacific Ocean

Many are thousands of kilometers

(miles) long The Challenger Deep,

part of a trench in the western Pacific,

is the deepest part of the Pacific Ocean

It lies about 11,000 m (36,000 ft) below sea level It is deeper than Mount Everest—the world’s tallest mountain—is high

Quick Check

Draw Conclusions Do ocean trenches occur where plates are moving apart or where they are moving together?

Critical Thinking What is unusual about the mid-ocean ridge?

Ocean Trench

Model Plate Movement

Stack sheets of paper into two piles.

Slowly push the short ends of the two paper piles together.

Observe What happens? How is this model similar to the formation

of a mountain range such as the Himalayas?

275

EXPLAIN

Earthquake and Volcano Activity

What other events occur

at plate boundaries?

Besides ridges and trenches, other

geologic events take place along plate

boundaries and change Earth’s surface

One of the most spectacular events in

nature is the eruption of a volcano

A volcano is a place where molten

rock, hot gases, and solid rock erupt

through an opening in the crust

A mountain that formed from these

materials is also called a volcano

Another dramatic natural event

is an earthquake , the shaking of the

ground that occurs when plates shift

and change positions It may be mild

enough to be hardly felt, or it may be

violent enough to cause great damage

Look at the map on this page to see

where most earthquakes and volcanic eruptions occur

In some places where plates move toward each other, the rocks crumple and fold and are pushed up onto the continents These folded bands of rock form mountain ranges The Himalayas

in Asia and the Appalachian Mountains

in North America are examples of mountain ranges that formed this way

Quick Check

Draw Conclusions Examine the map on this page Where do most volcanic eruptions and earthquakes occur?

Critical Thinking Compare and contrast volcanoes and earthquakes

Along which ocean’s coastline do earthquakes occur closest to the shore?

Clue: Where are most of the earthquake icons located?

Reading Maps

276

EXPLAIN

Writing Link Math Link

Seafloor Spreading

Scientists estimate that the seafloor can spread at a rate of about 3 cm per year How long would it take for

1 km of new seafloor to be added?

(pp 270–271)

Evidence from rocks, fossils, and the ocean floor supports the

theory of continental drift

(pp 272–275)

Volcanoes and earthquakes take place

along plate boundaries and change Earth’s surface

(p 276)

Make a Study Guide

Make a three-tab book (see pp 487–490) Use the titles shown On the inside of each tab, draw conclusions about the terms on each tab.

Think, Talk, and Write

Main Idea Earth’s surface changes due to

Vocabulary The vast underwater mountain chain is called the

Draw Conclusions What evidence did Alfred Wegener have to support his theory?

Critical Thinking Describe the theory

of continental drift.

Test Practice Which of the following

is a long, narrow, deep valley on the ocean floor?

Summarize the Main Idea

- Review Summaries and quizzes online @www.macmillanmh.com 277

EVALUATE

Draw Conclusions

Scientists read a lot of data and collect data

themselves through exploration and experimentation

Then they study the data, analyze them, and draw

conclusions, or decide what is and is not true In the

previous lesson, you learned about the evidence

that helped scientists draw the conclusion that the

continental drift theory was correct.

Learn It

When you draw conclusions, you have to look at all the

data and facts before you can decide what is true You have

to be careful not to jump to conclusions Here is an example:

It’s time to go home from school, but you discover that your new

jacket is missing Outside you see a student you don’t know wearing

a jacket just like yours Can you draw the conclusion that this person

took your jacket?

No, that assumption would be jumping to a conclusion You need

to ask questions and maybe even examine the jacket carefully to find

all the facts Suppose you do, and then you discover that the other

person’s jacket merely looks like yours The only conclusion you can

draw is that the other person has really good taste, just as you do.

Try It

▶ Use a hard-boiled egg as a model of Earth to gather evidence

about moving plates Use the evidence to draw conclusions

You will need a hard-boiled egg, a paper plate, and glue.

▶ Crack the egg Pull off the pieces of eggshell, and pile them

on the paper plate They represent Earth’s plates Record the

number of pieces on a table like the one on this page Set

the egg and the pieces of the shell aside for 20 minutes

Can you draw a conclusion at this time

about whether the pieces of eggshell can

be replaced to completely cover the egg?

▶ Try to glue the shell pieces back on the

egg Use a light dab of glue on each piece

Then pick up the egg, and squeeze gently

What happens? Record your observations

on the chart.

6 IE 7.e Recognize whether evidence is consistent with a proposed explanation.

278

EXTEND

Apply It

▶ Now use all the information you have gathered to

draw conclusions, and answer these questions Record

your conclusions on a chart like the one begun here.

▶ How are Earth’s plates similar to the pieces of the eggshell?

▶ Why did the pieces of eggshell push against each other

when you picked the egg up?

▶ What might happen if Earth’s plates broke into as many

pieces as the eggshell?

279

EXTEND

Plate Tectonics:

A Unifying

Theory

Lesson 2

Have you ever wondered what Earth is made

of? Scientists have learned a great deal about

what lies beneath the continents and the ocean

floor What would a model of Earth’s interior

look like?

6 ES 1.b Students know Earth is composed of several layers: a cold, brittle lithosphere; a hot, convecting mantle; and a dense, metallic core • 6 ES 1.c

Students know lithospheric plates the size of continents and oceans move

at rates of centimeters per year in response to movements in the mantle.

280

ENGAGE

How can you make a model

of Earth’s interior?

Purpose

In this activity you will make a model to compare

the thickness of Earth’s layers.

Procedure

Make a Model Draw a small X on the ground

This will be your center point for making three

circles.

Measure Tie one end of a string to a piece of

chalk Then measure the string to a length of 185

cm Hold the string at your center point in the

center of the X, and have a partner draw a circle

around the X, keeping the string straight and

taut all the way around.

Repeat the process two times, first cutting your

string to 182 cm and then cutting it to 100 cm

Draw Conclusions

Analyze The scale for your model is 1 cm = 35 km

How many real kilometers are represented by

each layer in your model?

Are the layers in your model the same thickness?

According to your model, what is the distance

from the surface of Earth to its center?

Research different ways to travel to the center of Earth, using

different modes of transportation Determine how long it

would take to travel there Analyze and present your results.

6 IE 7.b Select and use appropriate tools and technology (including calculators,

computers, balances, spring scales, microscopes, and binoculars) to perform tests,

collect data, and display data • 6 MA NS 1.2 Interpret and use ratios in different contexts

(e.g., batting averages, miles per hour) to show the relative sizes of two quantities, using

appropriate notations (a/b, a to b, a:b).

281 EXPLORE

San Andreas Fault

▶ Main Idea 6 ES 1.b, c

Earth is made up of several

different layers The

outermost part of Earth

Scientists study features on Earth’s surface

to determine how and when these surfaces were formed They are not able to dig holes deep enough to see what goes on in the center of the planet with their own eyes How do you suppose they are able to understand the forces that create these surface features?

;OW\7RSO 2SbOWZa

282

EXPLAIN

One way scientists learn about

Earth’s interior and its structure

is by studying seismic (SIZE•mik)

waves A seismic wave is a vibration

that travels through Earth Seismic

waves are produced by earthquakes

and volcanic eruptions Sometimes

explosions can also cause seismic

waves There are two main kinds of

seismic waves: surface waves and body

waves Each kind vibrates and travels

in a different way and at a different

speed Waves that are trapped near

the surface of Earth are called surface

waves Surface waves move more

slowly than body waves They travel

along the surface of the planet like

ripples on the surface of a pond

Waves that travel through the

interior of Earth are called body

waves There are two kinds of body

waves P waves, also called primary

waves, are the fastest seismic waves

They travel through gases, liquids,

and solids P waves travel by pushing

and pulling against the material they

pass through When the waves push,

they compress, or bunch, the material

together When they pull, they stretch

or expand the material This pushing and pulling causes the material the wave is moving through to vibrate forward and backward in the same direction in which the waves are moving

S waves, or secondary waves, are

much slower than P waves They travel only through solids They vibrate at a right angle to their direction of travel This means that if an S wave is moving ahead, the vibrations will move either

up and down or from side to side This causes the material that the wave is passing through to shake up and down

or from side to side Instruments on Earth’s surface record these movements

or vibrations By studying these waves, scientists learn about the different layers of Earth

▶ Seismographs detect, measure,

and record the energy of

earthquake vibrations As the

ground vibrates, the pen traces

EXPLAIN

What are the main layers of Earth?

By studying seismic waves, scientists have learned that Earth has three main layers Each layer has a different

composition, thickness, temperature, and density Density is

a measure of how much material there is in a given amount

of space Materials with lower densities often float in water, and materials with higher densities often sink in water To observe this, try the Quick Lab on the next page

Layers

The crust is the thin layer of solid rock that makes up the outermost part of Earth The thickness of the crust varies from place to place Earth’s crust is very thin To picture how thin it is, think about the skin of an apple compared

to the rest of the apple Almost all of the natural resources people use are found within this thin crust It is the layer

on which people walk, build buildings, and grow crops

Earth’s Layers

The thin rigid crust

(6–70 km thick)

surrounds Earth.

The mantle (about 2,900

km thick) is less dense

near the crust, denser

near the core.

Lower pressure allows

the outer core (about

2,300 km thick) to

remain liquid.

Intense pressure makes

the inner core a solid

ball about 2,400 km

in diameter.

284

EXPLAIN

The mantle is the thick layer of solid

and molten rock that lies beneath the

crust While the entire mantle is made

of rock, some of the rock in this layer

can move or flow slowly because of

great pressure and high temperatures

The core is the central part of

Earth It lies beneath the mantle and

is made up of an outer, liquid part

and an inner, solid part Earth’s core

is made of iron and nickel, metals

that are denser than rock The core is

almost twice as dense as the mantle

The core is a sphere, and the distance

across it through Earth’s center is

about 6,900 km (4,300 mi)

Pressure and Temperature

Suppose you could move through

Earth’s layers to the core As you

moved deeper, pressure would increase

The weight of the material above you

would cause this increase in pressure

The temperature would also increase

as you traveled deeper into Earth

Measuring Density

Measure 1 cup of vegetable oil,

1 cup of water, and 1 cup of corn syrup.

Add four drops of a different shade of food coloring to each cup Stir each cup.

Observe Pour the three cups together into a large glass bowl Record your observations.

What happened? Why do you think you saw these results?

Which layer of Earth corresponds

to the vegetable oil? The water? The corn syrup?

Quick Check

Main Idea Describe the three main layers of Earth

Critical Thinking From which layer

of Earth do people get most of their resources?

▲ Diamonds form under great pressure.

285

EXPLAIN

How are the main layers

of Earth subdivided?

There are two types of crust:

continental crust and oceanic crust

Continental crust makes up Earth’s land,

while oceanic crust is the floor of the

ocean Continental crust, made mostly

of a relatively lightweight kind of rock

called granite, is thicker and less dense

than oceanic crust Continental crust

has an average thickness of about 32 km

(20 mi) Oceanic crust is made mostly of

basalt, a denser rock than granite The

thickness of the oceanic crust averages

about 6 or 7 km (4 or 5 mi)

The mantle is divided into two parts: the upper mantle and the lower mantle Both continental crust and oceanic crust form the lithosphere The lithosphere (LITH•uh•sfeer) is the rigid outer part of Earth made up

of rocks in the crust attached to the upper part of the mantle The name

comes from the Greek word lithos,

meaning “stone.” The lithosphere

is broken up into plates that move slowly These plates are also called

lithospheric plates.

Convergent boundary Divergent boundary Transform boundary

NORTH AMERICAN PLATE JUAN

DE FUCA PLATE

SCOTIA PLATE

ARABIAN PLATE AFRICAN

PLATE

PHILIPPINE PLATE

ANTARCTIC PLATE

SOUTH AMERICAN PLATE NAZCA

PLATE

CARIBBEAN PLATE COCOS

PLATE

INDIAN PLATE

In which direction is the African Plate moving? Clue: Examine the arrows.

Reading Maps

286

EXPLAIN

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OabVS\]a^VS`S

c^^S`[O\bZS

Mantle

Because of intense heat and pressure, mantle rocks

below the lithosphere are semimolten , or almost

melted These rocks can actually flow, bend,

stretch, and compress They make up

the asthenosphere (as•THEE•nuh•sfeer),

the layer of semimolten mantle rock

that lies directly below the lithosphere

The name of this layer comes from

the Greek word asthenos, meaning

“weak.” These rocks are not as strong

and solid as the rocks closer to Earth’s

surface The lithospheric plates “float”

on the asthenosphere They are supported and

moved around by the movements of the rocks of the

asthenosphere, in much the same way that logs are

carried and moved around by currents in a river

Magma

Magma is molten, or melted, rock deep

below the surface of Earth Its temperature

is between 650°C and 1,200°C (1,202°F and

2,192°F) Magma forms only under specific

conditions in Earth’s asthenosphere Magma

forms when heat melts parts of the mantle and

lower crust Because it is much hotter and less

dense than surrounding rock, magma rises

toward the surface Magma is often found in

magma chambers under Earth’s surface, below

volcanoes When volcanoes erupt, magma can

surface as rock or as lava , the surface form of

magma When magma reaches the surface, it

cools and solidifies over time and crystallizes

into igneous rock

Quick Check

Main Idea What causes magma to rise

out of fissures and cracks in the ground?

Critical Thinking Distinguish between

the lithosphere and the asthenosphere

287

EXPLAIN

Concept sketch

water is

heated

cool water sinks

warm water rises

What moves the plates?

Scientists do not fully understand what causes Earth’s plates to move Most agree that it is caused by movements in the rock in the part of the mantle that lies below the plates—

a process called convective flow Convective flow is the continuous circular pattern of materials as they are heated and cooled

Warm matter rises, and cool matter sinks This principle

is true of solids, of liquids, and of gases Think about a pot of boiling water When the pot of water is heated, the warmer water rises to the surface In this example warmer water is less dense than cooler water When the water

molecules reach the surface, they begin to cool slightly and move sideways across the surface As they cool more, the molecules grow closer together, so their density increases Once they become just a little cooler and denser, they sink back to the bottom of the pot to be heated again This is only one example of convective flow

Convection currents

distribute heat through

the pot of boiling water.

Convection Currents

288

EXPLAIN

Convective Flow in the Mantle

Convective flow in Earth’s mantle

occurs in almost the same way as

it does in water Hotter, less dense

rock at the bottom of the mantle rises

toward the bottom of the plates As

it rises, it becomes cooler and denser,

and it moves sideways In time it

becomes cool enough and dense

enough to move downward As the

rock descends, it becomes hotter and

less dense, and it rises once more

This circular pattern of movement

is repeated over and over

Convective flow in the mantle takes

place much more slowly than it does in

a pot of boiling water As the moving

Plate Movement

rock in the mantle comes up below the rigid plates and moves sideways, the plates above it move as well

Because convective flow is slow, the movement of plates along their boundaries is also slow

in a pot of boiling water?

What happens to warm matter?

Clue: Examine the arrows.

Reading Diagrams

Watch plate movement @ www.macmillanmh.com

289

EXPLAIN

What is plate tectonics?

Plate tectonics (playt tek•TAHN•iks)

is the theory that Earth’s surface is

made up of separate, rigid plates that

move slowly across the mantle These

plates are called tectonic plates The

word tectonic comes from the Greek

word tekton, meaning “builder.”

Earth’s plates are curved like

Earth’s surface There are seven large

plates and about a dozen smaller

ones The plates move along their

boundaries in different ways and at

different speeds Although some may

move much faster, most plates move

only a few centimeters or inches each

year, about as much as your fingernails

grow each year

Types of Plate Boundaries

Most plate boundaries cannot be seen the way continents and coastlines can Most continental plates include both continents and parts of the ocean floor, so many of the boundaries lie below the oceans

Movements along plate boundaries produce earthquakes, volcanic activity, the building of great mountain ranges, and other dramatic surface events These events help show where the plate boundaries lie

How Plates Move

Tectonic plates can move in different directions Boundaries between plates that are moving away from each other,

or pulling apart, are called divergent

boundaries The word divergent means

“moving away from each other.”

rift trench

continental

plate

lithosphere

convection current

mid-ocean ridge

convection current

290

EXPLAIN

At a divergent plate boundary, new

crust is formed as magma wells up

and moves away from the ridge This

is the process of seafloor spreading As

spreading continues the Atlantic Ocean

is slowly becoming wider

Boundaries between plates that are

moving toward each other, or colliding,

are convergent boundaries The word

convergent means “coming together.”

When plates converge, usually one plate

sinks or slides under the other one This

is called subduction Whether a plate

moves up or down depends on whether

the leading edges are continental or

oceanic The leading edge of a plate

is the front part that leads it in the

direction it is moving

When a dense oceanic plate and a

less dense, more buoyant continental

plate converge, the leading edge of the

oceanic plate pulls the rest of the oceanic plate down The oceanic plate moves under the continental plate The convergence of these plates forms deep ocean trenches and, at the same time, can push up mountains

When two oceanic plates collide, one plate sinks below the other plate This kind of convergence is common where the mainly oceanic Pacific Plate converges with other oceanic plates Movements here have formed many volcanic islands and some of the world’s deepest trenches

Something entirely different happens when two continental plates collide In this case the crust buckles and folds It is pushed upward or sideways, creating vast, complex mountain ranges The Himalayas

in Asia and the folded Appalachian Mountains in North America are examples

Boundaries between plates that slide past each other are called

transform boundaries Most transform

boundaries occur on the ocean floor, but some appear on Earth’s surface For example, the San Andreas Fault lies along the boundary between the Pacific and North American plates

in California

Quick Check

Main Idea In what three ways

do tectonic plates move?

Critical Thinking Define

subduction in your own words.

continental plate

291

EXPLAIN

What is a unifying theory?

then began to develop a new theory that came to be called plate tectonics The theory states that Earth’s surface

is broken into continental and oceanic plates that move Plate tectonics

combines the continental drift theory with other evidence that supports it, such as the formation of new crust and the distribution of volcanoes and earthquakes The evidence fits together with Wegener’s theory to create a unifying theory

Most scientists now accept the theory of plate tectonics However, there is still much to learn about the forces that shape Earth’s surface

However, the coelacanth, above, and the sea turtle, right, have changed little since that time.

A unifying theory ties other

theories together to give a complete

picture of natural occurences Plate

tectonics is a unifying theory that

explains much about the history of

Earth Plate tectonics explains what

Earth was like in the past, how

and why it has changed, and how it

continues to change

Alfred Wegener, the scientist who

developed the hypothesis of continental

drift, thought the continents were

floating away from one another The

discovery of seafloor spreading in the

1960s showed scientists that more than

just continents were moving Scientists

292

EXPLAIN

Writing Link Math Link

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Earth’s main layers are

the crust, the mantle, and the core

(pp 284–285)

The lithosphere is

solid rock that makes

up the plates and lies above the semimolten asthenosphere.

(pp 286–289)

Plate tectonics is the

theory that the surface

of Earth is made up of rigid plates that move.

Think, Talk, and Write

Main Idea The outermost part of Earth consists of a number of separate, rigid

.

Vocabulary The central part of the Earth is the

Main Idea Why is Earth’s crust rigid?

Critical Thinking Compare and contrast the theories of continental drift and plate tectonics.

Test Practice What is the theory that Earth’s surface is made up of plates that move slowly across the mantle?

Write a descriptive paragraph about

each of Earth’s three layers Be sure to

include scientific terms.

Estimate Time

Los Angeles is about 8,800 km from Tokyo The cities are on plates that are moving away from each other at the rate

of about 2.5 cm per year In how many years will the cities be 10,000 km apart?

Summarize the Main Idea

- Review Summaries and quizzes online @www.macmillanmh.com 293

EVALUATE

Make a Model

A map is a useful tool that provides information, or

data, about Earth When scientists create a map, they

make a model that shows information about a place

or a region To use a map, you need to learn how to

interpret the information on it.

Learn It

Road maps show streets, roads, and

highways Other maps include different

data, such as borders of countries or

bodies of water When you make a drawing

showing streets in your neighborhood, you

make a model of the area.

All maps have legends, which include

additional information to help you read the

maps For example, the legend on a road map shows the

map’s scale Knowing the scale lets you determine distances.

Try It

▶ Now you will examine two different kinds of maps The first map

is a geologic map It shows the geological features of California,

such as the kinds of rocks found in the state A geologic map can also show features such as faults and volcanoes Scientists include these kinds of information on maps in order to make a model that shows an area’s geological features.

▶ Look at the map, and select one place to study Use the legend

to find out the meaning of the color used in the place you chose What information does the legend contribute to the map?

▶ The second map is a topographic map This map shows only a

small part of California, near San Francisco Topographic maps

show the shape of Earth’s surface They use special contour

lines, which show points that are the same height Contour

lines allow scientists to make a model that tells the height of a hill above sea level or the depth of the ocean below its surface When contour lines are far apart, they tell you that changes

in altitude are gradual When contour lines are close together, they tell you that changes in altitude are steeper.

▶ Select a point on one of the contour lines on the map What is the height of Earth’s surface at the point you chose?

294

EXTEND

Geologic

Apply It

▶ How do geologic maps and topographic maps differ?

▶ Of the two types of maps shown here, which would be a good type to use when planning a hike in the mountains?

Why do you think so?

▶ Make a model by drawing a map of the area around your school Find ways to indicate differences in height In your legend include the scale you used and keys to any features

on your map.

6 IE 7.f Read a topographic map and a geologic map for evidence

provided on the maps and construct and interpret a simple scale map. 295

EXTEND

Lesson 3

Earthquakes are sudden motions in the

crust along plate boundaries These sudden

motions can cause great damage and loss of

life How can something that happens at plate

boundaries cause so much destruction?

earthquake damage, San Francisco, California

6 ES 1.d Students know that earthquakes are sudden motions along breaks in the crust called faults and that volcanoes and fi ssures are locations where magma reaches the surface • 6 ES 1.g Students know how to determine the epicenter

of an earthquake and know that the effects of an earthquake on any region vary, depending on the size of the earthquake, the distance of the region from the epicenter, the local geology, and the type of construction in the region.

296

ENGAGE

• chocolate candy bar with caramel and nougat

• plastic knife

What happens when Earth’s

surface breaks?

Make a Prediction

What would a scientist observe if Earth’s surfaces

were pushed together, pulled apart, or slid against

each other? Write your answer as a prediction in

the form “If Earth’s surfaces were pushed, pulled,

or slid against each other, then ”

Test Your Prediction

Be Careful Place the candy bar on a flat

surface, and cut it in half through the center.

Identify which layers of the bar represent the

layers of Earth.

Then push the pieces together Pull the pieces

apart Take one of those pieces and pull it into

two pieces Slide the pieces past each other

Move the pieces up and down in relation to

each other.

Draw Conclusions

Analyze How would you explain what you

observed?

Did your observations support your prediction?

Infer What layer of the candy bar (“layer of

Earth”) was deformed the most? Why do you

think this happened?

Try to relate each movement of the candy-bar pieces

to an actual Earth event Explain Then analyze each

and present your results.

Explore More

Step

Step

6 IE 7.d Communicate the steps and results from an investigation

in written reports and oral presentations • 6 IE 7.e Recognize

whether evidence is consistent with a proposed explanation.

297 EXPLORE

▶ Main Idea 6 ES 1.d

Faults are breaks in the

lithosphere along plate

An earthquake is a trembling or shaking

of the ground caused by the release of energy following movement along a fault Some of the most damaging earthquakes in U.S history have happened in the state of California In fact, in the last 300 years, California has been shaken by 76 strong earthquakes that have caused loss of life and extensive property damage

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Faults are breaks, or cracks,

in the rocks of the lithosphere

along which movements take place

Fault zones are areas where there

are many interconnected faults Most

faults and fault zones are located along

the boundaries between tectonic plates

When stresses within Earth

become great enough to overcome

the strength of the rocks along faults,

the rocks move, and energy is released,

producing seismic waves, also called

earthquake waves These vibrations

radiate away from the earthquake

California Earthquakes

Where are most earthquakes in California located?

Clue: Look at where the fault line

is located on the map.

Reading Maps

An earthquake can be one of the most destructive of Earth’s natural forces.Only a few earthquakes cause serious damage In California earthquakes are common in the fault zone that includes the San Andreas Fault

Quick Check

Summarize What are fault zones?

Critical Thinking Before an earthquake occurs, what events have to take place?

299

EXPLAIN