California science grade 5 (10)

Chapter 5

Chapter 6

Rocks at the bottom of the Grand Canyon
are 2 billion years old.

Chapter 7


CHAPTER 5

Plate Tectonics
and Earth’s
Structure

What geologic forces have
shaped Earth’s landscape?

264

Mt. Whitney, Sequoia National Park, California


Lesson

1

Earth’s Moving Plates

PAGE

268

Lesson

2

Plate Tectonics:
A Unifying Theory
PAGE

280

Lesson

3

Earthquakes

PAGE

296

Lesson

4

Volcanoes

PAGE

308

Lesson

5

How Plate Tectonics
Affects California
PAGE

324

6 ES 1. Plate tectonics accounts for important features of
Earth’s surface and major geologic events.

265


Literature
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


from CURRENT SCIENCE

by Nicola Jones

Take a close look at Hawaii and you’ll notice something
interesting. The Aloha State is a string of volcanic islands.
If you follow the islands to the northwest, you’ll find
that they get progressively older, smaller, and less active.
Look under the ocean and you’ll discover that the line
continues for thousands of kilometers, with very old, dead
volcanoes, called the Emperor Seamounts, lying on the
seafloor at its farthest reaches.
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.

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.

-Journal Write about it online
@

www.macmillanmh.com

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?

268
ENGAGE

6 ES 1.a. Students know evidence of plate tectonics is derived from the fit 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).


Materials

Are the continents moving?
Form a Hypothesis
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

• world map

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.

• tracing paper
• pencil
• safety scissors

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.

• safety goggles
Step

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?

Step

Explore More
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.

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

269
EXPLORE


What forces shape Earth?
▶ Main Idea

6 ES 1.a

Moving plates cause
Earth’s surface to change.

▶ Vocabulary
continental drift, p. 270
Pangaea, p. 271
geologist, p. 271
mid-ocean ridge, p. 274
ocean trench, p. 275
volcano, p. 276
earthquake, p. 276

-Glossary
@

www.macmillanmh.com

▶ Reading Skill
Draw Conclusions
BSfb1ZcSa

1]\QZcaW]\a

Explore Earth’s
moving plates with
a seismologist.

older weathered mountains

270
EXPLAIN

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.

newer rugged mountains


Motion of Continents

Continental Drift

225
million years
ago

135
million
years ago

65
million
years ago

According to Wegener’s hypothesis,
Earth once had one single landmass,
or “supercontinent.” Wegener called
this landmass Pangaea (pan•JEE•uh),

from the Greek words meaning “all
land.” About 200 million years ago,
Pangaea split into two parts, which are
called Laurasia and Gondwanaland.
Later these two landmasses broke
apart to form North America, Eurasia,
South America, Africa, Australia, and
Antarctica. Over millions of years,
these continents slowly drifted 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.

Quick Check
Draw Conclusions What does

the term Pangaea refer to?
Critical Thinking Compare

present
day

the map of the continents
135 million years ago to the

map of the continents today.
How have the positions of the
continents changed?

Reading Maps
What evidence suggests that Africa and
South America were once connected?
Clue: How have the positions of the
continents changed over time?

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.

272
EXPLAIN

Fossil Evidence

Fossils of

Glossopteris, a fern,
have been found
in South America,
Africa, India,
Antarctica, and
Australia.

Fossils of Cynognathus, a Triassic land
reptile about 3 m (10 ft) long, have been
found in South America and Africa.

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 Mesosaurus,

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

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 the Triassic land reptile
Lystrosaurus have been found in
Africa, India, and Antarctica.

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?
273
EXPLAIN


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

[WR]QSO\`WRUS

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]QSO\WQQ`cab

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


Ocean Trench

]QSO\WQb`S\QV

Model Plate Movement
Stack sheets of paper into two
piles.
Slowly push the short ends of the
two paper piles together.

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,

Observe What happens? How is
this model similar to the formation
of a mountain range such as the
Himalayas?

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?
275
EXPLAIN


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.

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.

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

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.

Earthquake and Volcano Activity
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Reading Maps

BSQb]\WQ^ZObS

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

276
EXPLAIN

Clue: Where are most of the earthquake icons located?


Summarize the Main Idea
Continental drift is
the theory that a past
supercontinent split
apart to form separate
continents.
(pp. 270–271)
Evidence from rocks,
fossils, and the ocean
floor supports the
theory of continental

drift.
(pp. 272–275)

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/@1B71=13/<
B71=13/<

Volcanoes and
earthquakes take place
along plate boundaries
and change Earth’s
surface.
(p. 276)

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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.

Writing Link

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?
BSfb1ZcSa

1]\QZcaW]\a

Critical Thinking Describe the theory
of continental drift.

Test Practice Which of the following
is a long, narrow, deep valley on the
ocean floor?
A ocean trench
B mid-ocean ridge
C earthquake
D volcano

Test Practice Evidence from
supports the theory of continental drift.
A trees
B fish
C fossils
D weather

Math Link

Write a Story

Seafloor Spreading

Suppose you could observe continental
drift in action. Write a story that
explains what happens as Pangaea
breaks apart and moves. Use scientific
terms and descriptive language.

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?

-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.

278
EXTEND

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


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


Lesson 2

Plate Tectonics:
A Unifying
Theory

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?

280
ENGAGE

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.


Materials

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.

• chalk

Procedure

• measuring tape
or meter stick

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

• string
Step

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?

Explore More
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


▶ Main Idea

6 ES 1.b, c

Earth is made up of several
different layers. The
outermost part of Earth

consists of a number of
separate, rigid plates
that move.

▶ Vocabulary
seismic wave, p. 283
crust, p. 284
mantle, p. 285
core, p. 285
lithosphere, p. 286
semimolten, p. 287
asthenosphere, p. 287
magma, p. 287
lava, p. 287
convective flow, p. 288
plate tectonics, p. 290
subduction, p. 291

-Glossary
@

www.macmillanmh.com

▶ Reading Skill
Main Idea
;OW\7RSO

282
EXPLAIN

2SbOWZa

How do scientists study
Earth’s structure?
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?

San Andreas Fault


Waves
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

▶ Seismographs detect, measure,
and record the energy of
earthquake vibrations. As the
ground vibrates, the pen traces
a record of these seismic waves.

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.

Quick Check
Main Idea How are S waves

different from P waves?
Critical Thinking How might

scientists use P waves and S
waves to study Earth’s interior?

283
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


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.
▲ Diamonds form under great pressure.

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).

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

Pressure and Temperature

Main Idea Describe the three main

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.

layers of Earth.
Critical Thinking From which layer

of Earth do people get most of their
resources?

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.

▼ Each plate is constantly in
motion in a set direction. This
causes pressure to build in
locations between plates.

Lithospheric Plates

EURASIAN
PLATE

PHILIPPINE
PLATE

JUAN
DE FUCA
PLATE

EURASIAN
PLATE

NORTH
AMERICAN

PLATE
CARIBBEAN
PLATE

COCOS
PLATE

AUSTRALIAN
PLATE

PACIFIC
PLATE

NAZCA
PLATE

AFRICAN
PLATE

ARABIAN
PLATE

INDIAN
PLATE

SOUTH
AMERICAN
PLATE

SCOTIA PLATE

Convergent boundary
Divergent boundary
Transform boundary

ANTARCTIC PLATE

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

286
EXPLAIN


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),
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the layer of semimolten mantle rock
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that lies directly below the lithosphere.
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The name of this layer comes from
the Greek word asthenos, meaning
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“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.

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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