NAT_IT_SCF_C04_NTEC_S01

Interactive Textbook 61 Plate Tectonics

SECTION

Inside the Earth

Plate Tectonics

Name Class Date

CHAPTER 4

After you read this section, you should be able to answer

these questions:

•

What are the layers inside Earth?

•

How do scientists study Earth’s interior?

What Is Earth Made Of?

Scientists divide the Earth into three layers based on

composition: the crust, the mantle, and the core. These

divisions are based on the compounds that make up each

layer. A compound is a substance composed of two or

more elements. The densest elements make up the core.

Less-dense compounds make up the crust and mantle.

THE CRUST

The thinnest, outermost layer of the Earth is the crust.

There are two main kinds of crust: continental crust and

oceanic crust. Continental crust forms the continents. It

is thicker and less dense than oceanic crust. Continental

crust can be up to 100 km thick. Oceanic crust is found

beneath the oceans. It contains more iron than continen-

tal crust. Most oceanic crust is 5 km to 7 km thick.

Mantle

Continental

crust

5 km

30 km

100 km

Oceanic

crust

Oceanic crust is thinner and denser than continental crust.

BEFORE YOU READ

STUDY TIP

Summarize As you read,

make a chart showing the

features of Earth’s layers. In-

clude both the compositional

layers and the physical layers.

READING CHECK

Compare

How is

oceanic crust different from

continental crust?

Math Focus

Identify What fraction

of the thickness of the

thickest continental crust is

the thickness of the oceanic

crust? Give your answer as a

reduced fraction.

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Interactive Textbook 62 Plate Tectonics

SECTION 1

Name Class Date

Inside the Earth continued

THE MANTLE

The layer of the Earth between the crust and the core

is the mantle. The mantle is much thicker than the crust.

It contains most of the Earth’s mass. The mantle contains

more magnesium and less aluminum than the crust. This

makes the mantle denser than the crust.

No one has ever visited the mantle. The crust is too

thick to drill through to reach the mantle. Therefore, sci-

entists must use observations of Earth’s surface to draw

conclusions about the mantle. In some places, mantle

rock pushes to the surface. This allows scientists to study

the rock directly.

Another place scientists look for clues about the

mantle is the ocean floor. Melted rock from the mantle

flows out from active volcanoes on the ocean floor. These

underwater volcanoes have given scientists many clues

about the composition of the mantle.

THE CORE

The layer beneath the mantle that extends to the cen-

ter of the Earth is the core. Scientists think the core

is made mostly of iron and smaller amounts of nickel.

Scientists do not think that the core contains large

amounts of oxygen, silicon, aluminum, or magnesium.

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READING CHECK

3. Explain Why is the mantle

denser than the crust?

READING CHECK

4. Identify How can

scientists learn about the

mantle if they cannot study

it directly?

TAKE A LOOK

5. List What are the three

compositional layers of the

Earth?

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Interactive Textbook 63 Plate Tectonics

SECTION 1

Name Class Date

Inside the Earth continued

EARTH’S PHYSICAL STRUCTURE

Scientists also divide Earth into five layers based on

physical properties. The outer layer is the lithosphere. It

is a cool, stiff layer that includes all of the crust and a small

part of the upper mantle. The lithosphere is divided into

pieces. These pieces move slowly over Earth’s surface.

The asthenosphere is the layer beneath the litho-

sphere. It is a layer of hot, solid rock that flows very

slowly. Beneath the asthenosphere is the mesosphere,

which is the lower part of the mantle. The mesosphere

flows more slowly than the asthensosphere.

There are two physical layers in Earth’s core. The

outer layer is the outer core. It is made of liquid iron and

nickel. At the center of Earth is the inner core, which

is a ball of solid iron and nickel. The inner core is solid

because it is under very high pressure.

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What Are Tectonic Plates?

Pieces of the lithosphere that move around on top of

the asthenosphere are called tectonic plates. Tectonic

plates can contain different kinds of lithosphere. Some

plates contain mostly oceanic lithosphere. Others contain

mostly continental lithosphere. Some contain both conti-

nental and oceanic lithosphere. The figure on the top of

the next page shows Earth’s tectonic plates.

READING CHECK

6. Deﬁne What is the

lithosphere?

Critical Thinking

7. Infer What do you think

is the reason that scientists

divide the Earth into two

different sets of layers?

TAKE A LOOK

8. Describe What are the

ﬁve layers of Earth, based on

physical properties?

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Interactive Textbook 64 Plate Tectonics

SECTION 1

Name Class Date

Inside the Earth continued

TAKE A LOOK

9. Identify Give the name

of one plate that contains

mostly oceanic lithosphere

and of one plate that

contains mostly continental

lithosphere.

Oceanic:

Continental:

TAKE A LOOK

10. Compare Which type of

crust is thicker, oceanic crust

or continental crust?

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STRUCTURE OF A TECTONIC PLATE

The tectonic plates that make up the lithosphere are

like pieces of a giant jigsaw puzzle. The figure below

shows what a single plate might look like it if were sepa-

rated from the other plates. Notice that the plate contains

both continental and oceanic crust. It also contains some

mantle material.

Oceanic

crust

Mantle

Andes mountain range

Continental

crust

Mid-ocean

ridge

This ﬁ gure shows what the South American plate might look like if it were lifted off

the asthenosphere. Notice that the plate is thickest where it contains continental

crust and thinnest where it contains oceanic crust.

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Interactive Textbook 65 Plate Tectonics

SECTION 1

Name Class Date

Inside the Earth continued

How Do Scientists Study Earth’s Interior?

How do scientists know things about the deepest

parts of the Earth? No one has ever been to these places.

Scientists have never even drilled through the crust, which

is only a thin layer on the surface of the Earth. So how do

we know so much about the mantle and the core?

Much of what scientists know about Earth’s layers

comes from studying earthquakes. Earthquakes create

vibrations called seismic waves. Seismic waves travel

at different speeds through the different layers of Earth.

Their speed depends on the density and composition of

the material that they pass through. Therefore, scientists

can learn about the layers inside the Earth by studying

seismic waves.

Scientists detect seismic waves using instruments

called seismometers. Seismometers measure the times

at which seismic waves arrive at different distances from

an earthquake. Seismologists can use these distances

and travel times to calculate the density and thickness of

each physical layer of the Earth. The figure below shows

how seismic waves travel through the Earth.

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READING CHECK

11. Deﬁne What are seismic

waves?

TAKE A LOOK

12. Explain What is one way

that scientists know the outer

core is liquid?

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Interactive Textbook 66 Plate Tectonics

Name Class Date

SECTION VOCABULARY

asthenosphere the soft layer of the mantle on

which the tectonic plates move

core the central part of the Earth below the

mantle

crust the thin and solid outermost layer of the

Earth above the mantle

lithosphere the solid, outer layer of Earth that

consists of the crust and the rigid upper part

of the mantle

mantle the layer of rock between the Earth’s

crust and core

mesosphere the strong, lower part of the mantle

between the asthenosphere and the outer core

tectonic plates a block of lithosphere that

consists of the crust and the rigid, outermost

part of the mantle

1. Describe Complete the table below.

Crust Mantle Core

Thickness or radius

3,430 km

Location outer layer of the

Earth

Percent of Earth’s

mass

2. Compare

How is the inner core similar to the outer core? How are they different?

3. Compare How is the crust different from the lithosphere? How are they the same?

4. Identify Give three ways scientists can learn about the Earth’s mantle.

Section 1 Review

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SECTION 5 TIME MARCHES ON

1. dinosaurs

2. by studying rocks and fossils

3. about 50 million years

4. Paleocene, Eocene, Oligocene, Miocene,

Pliocene

5. by changes in life on Earth

6. Paleozoic

7. Paleozoic

8. Birds and mammals did not evolve until

after the Paleozoic era.

9. Reptiles were the dominant land animals

during the era.

10. Being warm-blooded and having young

develop inside the mothers’ bodies could

allow mammals to survive in a wider

temperature range than dinosaurs. If the

extinction was caused by climate change,

mammals would have been more likely to

survive.

Review

1. eon, era, period, and epoch

2. about 4.6 billion years

3. Different kinds of rocks form in different

environments. Different kinds of organisms

live in different environments. By studying

the rocks and fossils that formed long ago,

geologists can infer which environments

existed then.

4. Cambrian, Ordovician, Silurian, Devonian,

Mississippian, Pennsylvanian, Permian

5. by mass extinctions

6. Possible answers: sudden events and gradual

events; examples of sudden/gradual events

Chapter 4 Plate Tectonics

SECTION 1 INSIDE THE EARTH

1. The continental crust is thicker and contains

less iron than oceanic crust.

2. about 1/20

3. The rock in the mantle contains more mag-

nesium and less aluminum than the crust.

4. by studying rock that erupts from the mantle

5. crust, mantle, core

6. the upper, rigid layer of Earth made of the

crust and some of the mantle

7. Possible answer: Different scientists are

interested in different properties of the Earth.

8. lithosphere, asthenosphere, mesosphere,

inner core, outer core

9. Oceanic: Pacific

Continental: African

10. continental crust

11. vibrations created by earthquakes

12. Some kinds of seismic waves cannot travel

through it.

Review

Crust Mantle Core

Thickness

or radius

5 km to 100 km 2,900 km 3,430 km

Location

outer layer of

the Earth

middle

layer of the

Earth

inner layer

of the

Earth

Percent

of Earth’s

mass

less than 1% 67% 33%

2. The inner core is solid, but the outer core is

liquid. Both are made of iron and nickel.

3. The lithosphere contains the crust and some

of the mantle.

Both the crust and the lithosphere are the

outermost layers of Earth.

4. by studying mantle rocks that push to the

surface, by studying rocks on the sea floor,

using seismic waves

SECTION 2 RESTLESS CONTINENTS

1. They were once part of a single continent.

2. Europe; they share similar-aged mountain

rocks.

3. The same kinds of fossils are found on

widely separated continents.

4. about 200 million years ago

5. North America and Europe were connected;

India and Asia were separated.

6. an underwater mountain chain

7. when Earth’s magnetic poles change places

8. They are mirror images of each other.

Review

1. The shapes of continental coastlines seem

to match. Similar fossils are found on widely

separated continents. Mountain chains of

similar ages and compositions are found on

widely separated continents.

Interactive Textbook Answer Key 39 Inside the Restless Earth

Inside the Restless Earth Answer Key continued

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