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Holt McDougal Earth Science 13 Earth as a System

SECTION

1 Earth: A Unique PlanetEarth as a System

Name Class Date

CHAPTER 2

As you read this section, keep these questions in mind:

• What are the size and shape of Earth?

• What are Earth’s compositional and structural layers?

• What is a possible source for Earth’s magnetic field?

• What does Newton’s law of gravitation state?

What Shape Is Earth?Earth formed about 4.6 billion years ago. It is the only

planet in our solar system that scientists think has liquid water on its surface. Earth is the only planet we know of that supports life.

If you looked at Earth from space, it would look like a perfect sphere, or ball. However, Earth is not perfectly round. It is an oblate spheroid, or flattened sphere. The diagram below shows what an oblate spheroid looks like.

What Is Inside Earth? Scientists can study Earth’s interior directly by drilling.

However, scientists can drill only a few kilometers into Earth’s surface. To learn about the rest of Earth’s interior, scientists must use indirect methods.

One way scientists learn about Earth’s interior is by studying seismic waves. Seismic waves are vibrations that travel through Earth. Earthquakes cause most seis-mic waves. Seismic waves move differently in different substances. Therefore, by studying these waves, scien-tists can learn what Earth’s interior is made of. Scientists divide Earth’s interior into three compositional zones and five structural zones.

KEY IDEAS

Re-Read After you read the section, write out answers to the Key Ideas questions. If you cannot answer the ques-tions, re-read the text under the appropriate heading.

READING TOOLBOX

2. Explain Why must scientists use indirect meth-ods to study Earth’s interior?

READING CHECK

Critical Thinking1. Apply Concepts Which would take longer, a trip around Earth’s equator or a trip around Earth from pole to pole? Why? (Assume you travel at the same speed for both trips.)

Earth’s shape is exaggerated in this picture. In real life, you would not be able to see the difference between Earth and a perfect sphere.

Oblate spheroidPerfect sphere

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SECTION 1

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Earth: A Unique Planet continued

COMPOSITIONAL ZONESCompositional zones are made up of different mate-

rials. The thin, solid, outermost compositional zone is the crust. Crust beneath oceans is called oceanic crust. Crust that makes up continents is called continental crust. In general, continental crust is much thicker than oceanic crust. The crust is between 5 km and 35 km thick.

The mantle is the compositional zone that lies beneath the crust. The mantle is made of denser rock than the crust and is almost 2,900 km thick. The inner-most compositional zone is the core. The core is a sphere with a radius of about 3,500 km. Scientists think the core is made up mainly of iron and nickel.

STRUCTURAL ZONESStructural zones have different properties. The

lithosphere is made up of the crust and the top part of the mantle. The lithosphere is relatively cool and brittle. The asthenosphere is made of hot, solid mantle rock. The rock of the asthenosphere is also under a great deal of pressure. The heat and pressure allow the solid rock to flow. The mesosphere is a layer of solid mantle rock beneath the asthenosphere.

The core is divided into the outer core and inner core. The outer core is made of liquid iron and nickel. Scientists think the inner core is a made of solid iron and nickel.

Talk About ItDiscuss In a small group, talk about ways you have learned about something when you could not observe it directly. For example, many people try to learn what is in a gift box by shaking it. Talk about how these methods are related to how scientists learn about Earth’s interior.

LOOKING CLOSER3. Identify Relationships Which three structural zones overlap with the mantle?

4. Calculate Earth’s layers have the following thick-nesses: crust, 35 km; mantle, 2,900 km; outer core, 2,250 km; inner core, 1,228 km. A seismic wave moves at the following speeds through each layer: crust, 8 km/s; mantle, 12 km/s; outer core, 9.5 km/s; inner core, 10.5 km/s. How long would a seismic wave take to travel from Earth’s surface to its center?

SkillsMath

5. Compare How are the inner core and outer core different?

READING CHECK

Inner core (solid)

Outer core (liquid)

Core

Mesosphere

Lithosphere

Asthenosphere

Structural Zones

CompositionalZones

Mantle

Crust

Ocean

Moho

Oceanic crust

Continentalcrust

Lithosphere

Asthenosphere

Mantle

Crust

The black labels show the compositional zones. The white labels show the structural zones. The compositional and structural zones of Earth’s interior overlap.

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SECTION 1

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Earth: A Unique Planet continued

What Is the Source of Earth’s Magnetic Field?

Earth acts as a giant magnet. Like all magnets, it has two magnetic poles. Earth’s magnetic field extends beyond Earth’s atmosphere, and it affects a region of space called the magnetosphere.

Most scientists think that the liquid iron in Earth’s outer core is the source of Earth’s magnetic field. They think that motions within the core produce electric currents that produce the magnetic field.

What Is Newton’s Law of Gravitation? Gravity is a force that pulls matter together. In the

1600s, Isaac Newton explained how gravity affects objects in his law of universal gravitation. This law states that the force of gravity between two objects depends on the masses of the objects and the distance between them.

Earth’s gravity pulls objects toward Earth’s center. Weight is a measure of the strength of this pull. The newton (N) is the SI unit of weight. The mass of an object does not change with location, but the weight of an object can change.

The magnetic fi eld lines around Earth show the shape of Earth’s magne-tosphere. The solar wind affects the shape of the magnetosphere.

Weight = 50 NMass = 50 kg



The force of gravity decreases as an object moves farther from Earth’s center.

6. Identify What do most scientists think is the source of Earth’s magnetic fi eld?

READING CHECK

LOOKING CLOSER7. Describe How does the distance between two ob-jects affect the gravitational force between them?

Magneticfield linesSolar wind

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Name Class Date

Section 1 Review SECTION VOCABULARY

asthenosphere the solid, plastic layer of the mantle beneath the lithosphere; made of mantle rock that fl ows very slowly, which allows tectonic plates to move on top of it

crust the thin and solid outermost layer of the Earth above the mantle

core the central part of the Earth below the mantle

lithosphere the solid, outer layer of Earth that consists of the crust and the rigid upper part of the mantle

mantle in Earth science, the layer between Earth’s crust and core

mesosphere literally the “middle sphere”; the strong, lower part of the mantle between the asthenosphere and the outer core

1. Organize Complete the concept map below to show the relationship between Earth’s compositional zones and structural zones.

2. Apply Concepts A compass needle is a very small magnet that can move. Why can you use a compass to determine direction on Earth?

3. Analyze Ideas Why would you weigh less on a high mountain peak than you would at sea level?

Earth’s interior has fi ve

zones

core

Earth’s interior has three

zones

lithosphere

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2 earth as a system section 1 earth: a unique planet · holt mcdougal earth science 13 earth as a...

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