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8.4 Earth’s Layered Structure

Reading StrategySequencing Copy the flowchart. After youread, complete the sequence of layers inEarth’s interior.

Earth’s Internal Structure

Key ConceptsWhat is Earth’s internalstructure?

What is the compositionof Earth’s interior?

Vocabulary◆ crust◆ mantle◆ lithosphere◆ asthenosphere◆ outer core◆ inner core◆ Moho

Lithosphere a. ? b. ? Outercore c. ?

Earth’s interior lies not very far beneath our feet, but we can’t reachit. The deepest well has drilled only 12 kilometers into Earth’s crust.With such limited access, how do we know what Earth’s interior is like?Most knowledge of the interior comes from the study of earthquakewaves that travel through Earth.

Layers Defined by CompositionIf Earth were made of the same materials throughout, seismic waveswould spread through it in straight lines at constant speed. However, thisis not the case. Seismic waves reaching seismographs located fartherfrom an earthquake travel at faster average speeds than thoserecorded at locations closer to the event. This general increase inspeed with depth is due to increased pressure, which changes theelastic properties of deeply buried rock. As a result, the paths ofseismic waves through Earth are refracted, or bent, as they travel.Figure 14 shows this bending. Earth’s interior consists ofthree major zones defined by its chemical composition—thecrust, mantle, and core.

Crust The crust, the thin, rocky outer layer of Earth, is dividedinto oceanic and continental crust. The oceanic crust is roughly7 kilometers thick and composed of the igneous rocks basalt andgabbro. The continental crust is 8–75 kilometers thick, but averages athickness of 40 kilometers. It consists of many rock types. The averagecomposition of the continental crust is granitic rock called granodiorite.Continental rocks have an average density of about 2.7 g/cm3and someare over 4 billion years old. The rocks of the oceanic crust are younger(180 million years or less) and have an average density of about 3.0 g/cm3.

Earthquakes and Earth’s Interior 233

Figure 14 The arrows show onlya few of the many possible pathsthat seismic waves take throughEarth.Inferring What causes the wavepaths to change?

FOCUS

Section Objectives8.10 List the layers of Earth based

on composition and physicalproperties.

8.11 Describe the composition ofeach layer of Earth.

Build VocabularyLINCS Have students: List the parts ofthe vocabulary that they know, such ascore, sphere, and litho-. Imagine whatthe interior of Earth might look like andhow the terms might fit together. Notea reminding, sound-alike term, such asapple core or atmosphere. Connect theterms, perhaps in a long sentence or aslabels on a diagram. Self-test.

Reading Strategya. asthenosphereb. lower mantlec. inner core

INSTRUCT

Layers Defined byCompositionUse VisualsFigure 14 Have students look at themodel of Earth and seismic waves in thediagram. Ask: One seismic wave travelsstraight through the center of Earth.Would this be a P wave or an S wave?(P wave)Visual

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Section 8.4

Answer to . . .

Figure 14 Seismic rays changedirection because as pressure increaseswith depth, elastic properties of rockschange.

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Uppermantle

Continentalcrust

Oceanic crust

Dep

th (k

m)

100

200

300

660

Lithosphere(sphere of rock)

Asthenosphere(weak sphere)

400

Figure 15 Earth’s Layered Structure The left side of the globeshows that Earth’s interior is divided into three different layersbased on compositional differences—the crust, mantle, and core.The right side of the globe shows the five main layers of Earth’sinterior based on physical properties and mechanical strength—the lithosphere, asthenosphere, mesosphere, outer core, and innercore. The block diagram shows an enlarged view of the upperportion of Earth’s interior.

234 Chapter 8

Mantle Over 82 percent of Earth’s volume is contained in themantle—a solid, rocky shell that extends to a depth of 2890 kilometers.The boundary between the crust and mantle represents a change inchemical composition. The dominant rock type in the uppermostmantle is peridotite, which has a density of 3.4 g/cm3.

Core The core is a sphere composed of an iron-nickel alloy. At theextreme pressures found in the center of the core, the iron-rich materialhasan average density of almost 13 g/cm3 (13 times heavier than water).

Layers Defined by Physical PropertiesEarth’s interior has a gradual increase in temperature, pressure, and den-sity with depth. When a substance is heated, the transfer of energyincreases the vibrations of particles. If the temperature exceeds the melt-ing point, the forces between particles are overcome and melting begins.

If temperature were the only factor that determined whether a sub-stance melted, our planet would be a molten ball covered with a thin,solid outer shell. Fortunately, pressure also increases with depth andincreases rock strength. Depending on the physical environment (tem-perature and pressure), a material may behave like a brittle solid, aputty, or a liquid. Earth can be divided into layers based on phys-ical properties—the lithosphere, asthenosphere, outer core, andinner core.

Lithosphere and Asthenosphere Earth’s outermost layerconsists of the crust and uppermost mantle and forms a relativelycool, rigid shell called the lithosphere. This layer averages about100 kilometers in thickness.

What is the composition of the core?

234 Chapter 8

Layers Defined byPhysical PropertiesBuild Reading LiteracyRefer to p. 502D in Chapter 18, whichprovides the guidelines for usingvisualization.

Visualize Have students keep theirbooks closed. Tell them to listencarefully while you read the paragraphabout defining the layers of Earth basedon physical properties. Ask students todescribe how they visualize the interiorof Earth. Then, ask students to work inpairs and discuss how they visualized theprocess.Visual

Floating CrackersPurpose To model for students thecharacteristics and behavior of thelithosphere and asthenosphere.

Materials shallow baking pan, packageof chocolate pudding, 2 cups of milk,several animal crackers

Procedure Review with students thegeneral characteristics and thicknessesof the lithosphere and asthenosphere.You may want to introduce the idea ofthe lithosphere being broken into smallerpieces called plates. These plates moveabout on top of the asthenosphere.Then make the pudding and pour it intothe shallow baking pan. This will modelthe asthenosphere. Once the puddinghas set, place the animal crackers on topof the asthenosphere to represent thelithosphere.

Expected Outcomes Students shouldsee that the lithospheric plates arerelatively thin compared to theasthenosphere. They also can see howthe lithosphere “floats” on top of theasthenosphere, without sinking into it.The asthenosphere has a solid consistencyyet has some ability to move.Logical, Visual

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Section 8.4 (continued)

Customize for English Language Learners

Imagine that Earth was a ball. If you could cutit in half, you would see that Earth is made upof layers. The deepest layer is a solid core ofmetal, which is surrounded by a core of liquidmetal. The liquid metal spins as Earth rotates.These two parts are thick and unbelievablyhot. The next layer is called the mantle. Themantle is much cooler than the core, but it is

still so hot that some of the rock is completelyliquid.

A brittle crust of solid rock covers themantle. All life on Earth exists on the top layerof this crust. Now imagine that you are takinga trip through Earth. Write and illustrate ajournal entry for your trip. Share your journalentry with the class.

234_PH_ES_TE_2p 9/23/04 6:50 PM 34

Crust 5–70 km

Outercore

Innercore

2230km

2260km1220

km

Mantle Lowermantle

2890km

3480km

Lithosphere5–250 km

660km

Upper mantle

Core

Beneath the lithosphere lies a soft, comparatively weak layer knownas the asthenosphere. The asthenosphere has temperature/pressure con-ditions that may result in a small amount of melting. Within theasthenosphere, the rocks are close enough to their melting temperaturesthat they are easily deformed. Thus, the asthenosphere is weak because itis near its melting point, just as hot wax is weaker than cold wax. Thelower lithosphere and asthenosphere are both part of the upper mantle.

Lower Mantle From a depth of about 660 kilometers down tonear the base of the mantle lies a more rigid layer called the lowermantle. Despite their strength, the rocks of the lower mantle are stillvery hot and capable of gradual flow. The bottom few hundred kilo-meters of the mantle, laying on top of the hot core, contains softer,more flowing rock like that of the asthenosphere.

Inner and Outer Core The core, which is composed mostly ofan iron-nickel alloy, is divided into two regions with different physicalproperties. The outer core is a liquid layer 2260 kilometers thick. Theflow of metallic iron within this zone generates Earth’s magnetic field.The inner core is a sphere having a radius of 1220 kilometers. Despiteits higher temperature, the material in the inner core is compressedinto a solid state by the immense pressure.

Why is the inner core solid?


Build Science SkillsCalculating The mantle makes uproughly 82 percent of Earth. Themantle is composed of two differentlayers, the upper mantle and the lowermantle. The mantle reaches toa depth of approximately 2900 km.Using the numbers given on Figure15, what percent of the mantle isupper mantle? (660 km/2900 km =23 percent) What percent is lowermantle? (2230 km/2900 km =77 percent)Logical

IntegrateLanguage ArtsWord Parts Students can remembervocabulary by recognizing word parts incertain words. For example, the Greeksuffix litho- means “rock, or stone.” Ask:What other word part is a clue tomeaning of vocabulary terms suchas lithosphere and asthenosphere?(–sphere) What do you think it means?(Sample answer: rounded)

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Answer to . . .

The core is composed ofan iron-nickel alloy.

because it is underextreme pressure and

is compressed into a solid as a result

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Earthquake epicenter

100°

140°

100°

140°

KeyP-waveS-wave

Area whereP waves are

deflectedaround theliquid outer

core

Discovering Earth’s LayersIn 1909, a Croatian seismologist, Andrija Mohorovicic, presented evi-dence for layering within Earth. By studying seismic records, he foundthat the velocity of seismic waves increases abruptly below about 50 kilometers of depth. This boundary separates the crust from theunderlying mantle and is known as the Mohorovicic discontinuity.The name is usually shortened to Moho.

Another boundary was discovered between the mantle and outercore. Seismic waves from even small earthquakes can travel around theworld. This is why a seismograph in Antarctica can record earthquakesin California or Italy. However, it was observed that P waves were bentaround the liquid outer core beyond about 100 degrees away from anearthquake. The outer core also causes P waves that travel through thecore to arrive several minutes later than expected. This region, wherebent P waves arrive, is sometimes called the shadow zone.

The bent wave paths can be explained if the core is composed ofmaterial that is different from the overlying mantle. The P waves bendaround the core in a way similar to sound waves being bent around thecorner of a building. For example, you can hear people talking fromaround the side of a building even if you cannot see them. In this way,rather than actually stopping the P waves in the shadow zone, the outercore bends them, as you can see modeled in Figure 16. It was furthershown that S waves could not travel through the outer core. Therefore,geologists concluded that this region is liquid.

What is the Moho?

236 Chapter 8

Figure 16 Earth’s Interior Showing P and SWave Paths The change in physical propertiesat the mantle-core boundary causes the wavepaths to bend sharply. Any location more than100 degrees from an earthquake epicenter willnot receive direct S waves because the liquidouter core will not transmit them.

236 Chapter 8

DiscoveringEarth’s LayersIntegrate PhysicsPhysical and Chemical PropertiesHave students read the caption forFigure 16. Then ask students forexamples of physical and chemicalproperties. Make a two-column chart onthe board and compile a list of physicaland chemical properties. (Examples ofphysical properties: conductivity, hardness,melting point, density, pressure. Examplesof chemical properties: flammability,reactivity.) Ask: What physical prop-erties change between the mantleand outer core? (hardness, density,pressure, state; outer core is liquid,mantle is solid)

Build Reading LiteracyRefer to p. 186D in Chapter 7, whichprovides guidelines for relating text andvisuals.

Relate Text and Visuals Instructstudents to look at Figure 16. Referthem to the key and point out that Pwaves and S waves are different colors inthe picture. Ask: What happens when Pwaves hit the mantle-core boundary?(They bend around the core, or gothrough the core.) What sentences inthe text support this observation?(“It was observed that P waves were bentaround the liquid outer core. . . P wavesthat travel through the core. . . ”) Whathappens when S waves meet theboundary? (They stop travelling.) Whatsentence in the text supports this?(“It was further shown that S waves couldnot travel through the outer core.”)

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Section 8.4 (continued)

Andrija Mohorovicic was a Croatian scientistwho lived from 1857 to 1936. Best known forhis work as a seismologist, Mohorovicic alsocontributed to the sciences of meteorologyand astronomy.

He established a station to follow thunder-storms, conducted climatic studies that lead tothe conclusion that temperature in the atmo-sphere decreases with an increase in altitude,and published widely on clouds, rainstorms,

and winds. After studying the seismic wavesfrom the October 8, 1909, earthquake in theKupa valley of Croatia, he made a very importantdiscovery. At a depth of approximately 50 km,there was a dramatic change in material withinEarth. This was based on his observation ofa change in velocity of seismic waves at thisdepth. This inconsistency became known asthe Moho, which is the boundary betweenthe crust and the mantle.

Facts and Figures


Section 8.4 Assessment

Reviewing Concepts1. List the major layers of Earth’s internal

structure based on physical properties. List thelayers in order from Earth’s center to the surface.

2. What is the composition of Earth’s core?

3. What evidence indicates that Earth’s outer coreis liquid?

4. What is the composition of the mantle?

Critical Thinking5. Comparing and Contrasting Compare the

physical properties of the asthenosphere andthe lithosphere.

6. Inferring Why are meteorites consideredimportant clues to the composition ofEarth’s interior?

Discovering Earth’s CompositionWe have examined Earth’s structure, so now let’s look at the composi-tion of each layer. Early seismic data and drilling technologyindicate that the continental crust is mostly made of lighter, graniticrocks. Until the late 1960s, scientists had only seismic evidence theycould use to determine the composition of oceanic crust. The recov-ery of ocean-floor samples was made possible with the development ofdeep-sea drilling technology. The crust of the ocean floor has abasaltic composition.

The composition of the rocks of the mantle and core is knownfrom more indirect data. Some of the lava that reaches Earth’s surfacecomes from the partially melted asthenosphere within the mantle. Inthe laboratory, experiments show that partially melting the rock calledperidotite produces a substance that is similar to the lava that eruptsduring volcanic activity of islands such as Hawaii.

Surprisingly, meteorites that collide with Earth provide evidenceof Earth’s inner composition. Meteorites are assumed to be composedof the original material from which Earth was formed. Their compo-sition ranges from metallic meteorites made of iron and nickel tostony meteorites composed of dense rock similar to peridotite.Because Earth’s crust contains a smaller percentage of iron than dometeorites, geologists believe that the dense iron, and other densemetals, sank toward Earth’s center during the planet’s formation.Lighter substances may have floated to the surface, creating the less-dense crust. Earth’s core is thought to be mainly dense iron andnickel, similar to metallic meteorites. The surrounding mantle isbelieved to be composed of rocks similar to stony meteorites.


Creative Writing Write a short fictionalstory about a trip to Earth’s core. Makesure the details about the layers of Earth’sinterior are scientifically accurate.

Discovering Earth’sComposition

ASSESSEvaluateUnderstandingAsk students to draw two cross sectionsof Earth: one where the layers aredefined by composition and one wherethe layers are defined by physicalproperties. Have students exchangepapers and check each other’s work.

ReteachUse Figure 15 to review the layersof Earth.

Stories will vary but students shouldinclude accurate information on thelithosphere, upper mantle, lower mantle,as well as the inner and outer core.

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5. The lithosphere is a cool, rigid shell formedfrom the crust and upper mantle. On averageit is 100 km thick. The asthenosphere is a soft,weak layer that experiences the conditionsneeded to produce a small amount of melting.6. Meteorites are thought to be made of thesame material from which Earth was formed.Therefore, when they are found, they cangive us an indication of the composition ofthe interior of Earth.

Section 8.4 Assessment

1. inner core, outer core, lower mantle,asthenosphere, lithosphere (upper mantle)2. The core is made of an iron-nickel alloy.3. the fact that S waves do not travel thoughthis layer4. The mantle is composed of peridotite. Answer to . . .

The Moho is theboundary between thecrust and the mantle.


Effects of EarthquakesAn earthquake is a shaking of the ground caused by sudden movements in theEarth’s crust. The biggest quakes are set off by the movement of tectonic plates.Some plates slide past one another gently. However, others get stuck, and the forces pushing the plates build up. The stress mounts until the plates suddenlyshift their positions and cause the Earth toshake. Most earthquakes last less than oneminute. Even so, the effects of an earth-quake can be devastating and long-lasting.

How the Earth Works

LANDSLIDEIn January 2001, an earthquake struck El Salvador. It causedthe landslide that left these Salvadoran women homeless.A landslide is a sudden drop of a mass of land down amountainside or hillside. Emergency relief workers fromaround the world often rush to the site of an earthquakedisaster like the one that occurred in El Salvador.

TSUNAMIIn 1755, an earthquake in Lisbon, Portugal, caused a

tsunami, as illustrated in this painting. A tsunami is ahuge sea wave that is set off by an undersea earthquake or

volcanic eruption. When tsunamis break on shore, theyoften devastate coastal areas. Tsunamis can race at speeds

of about 450 miles per hour and may reach heights ofabout 100 feet (30.5 m).

238 Chapter 8

238 Chapter 8

FOCUS

ObjectivesIn this feature, students will• explain what causes an earthquake.• describe the possible physical effects

of an earthquake.

Build VocabularyKey Terms Write the key terms on theboard. Ask volunteers to write definitionsbeside them. Then have the class worktogether to use the words in sentencesthat describe the causes and effects ofearthquakes.

INSTRUCTBellringerAsk students what comes to mind whenthey think of earthquakes. Discuss earth-quake experiences they may have hador heard about.Verbal

Use VisualsHave students read and examine thephotographs on this page and the next.Ask: What do you suppose people inthese regions had to do after theearthquake? (They had to find peoplewho were trapped under snow and rubble,rebuild buildings, and fix streets.)Visual

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Customize for Inclusion Students

Gifted Ask students to research a historicearthquake, like the one that hit Lisbon in1755. Have them imagine that they survived itand are writing a story about it for a foreignnewspaper. Encourage them to use factual

details and fictional interviews in their stories.Before they begin, remind them that the firstparagraph should answer these questions:Who? What? Where? When? and Why?


AVALANCHEEarthquakesmay trigger anavalanche—asudden fall of amass of ice andsnow. In 1970, asevere earthquakeoff the coast of Perucaused a disastrousslide of snow and rockthat killed some 18,000people in the valley below.

WHEN THE EARTH CRACKSMost people killed or injured by an earthquake arehit by debris from buildings. Additional damage canbe caused by aftershocks—tremors that can occurhours, days, or even months after an earthquake. Thescene above shows the city of Anchorage, Alaska,after a major earthquake. Extensive ground tremorscaused the street to break up as the soil below it col-lapsed. Buildings and cars were dropped more than10 feet (3 m) below street level.

1. Key Terms Define (a) earthquake, (b) tsunami, (c) landslide, (d) infra-structure, (e) avalanche, (f) aftershock,(g) seismic wave, (h) epicenter.

2. Physical Processes What physicalprocesses cause an earthquake tooccur?

3. Environmental Change How can an earthquake cause changes to thephysical characteristics of a place?

4. Natural Hazards (a) How can anearthquake change the human charac-teristics of a place? (b) How does theinternational community respond to adevastating earthquake?

5. Critical Thinking Solving ProblemsWhat can a community do to reducethe amount of earthquake damage thatmight occur in the future?

INFRASTRUCTURE DAMAGEWhen an earthquake occurred in Los Angeles in 1994, underground gas and water linesburst, causing fires and floods. Earthquakesoften cause tremendous damage to the infrastructure—the network of services that supports a community. Infrastructureincludes power utilities, water supplies, andtransportation and communication facilities.

SEISMIC WAVESAs tectonic forces build,rock beneath the surfacebends until it finallybreaks. The tectonic platessuddenly move, causingseismic waves, or vibra-tions, to travel throughthe ground. The wavesradiate outward from anunderground area calledthe focus, or hypocenter.Damage is usually greatestnear the epicenter, thepoint on the surfacedirectly above the focus.

When two tectonic plates suddenlymove past each other, waves ofbuilt-up energy are released.

Epicenter

As shock waves travel

away from the epicenter, the

destruction caused by the

earthquake decreases.Focus, or

hypocenter

Shock waves radiateoutward and upwardfrom the focus, orhypocenter.

239

ASSESSEvaluateUnderstandingWork with students to model how anearthquake happens. Encourage themto build models using everyday class-room materials, such as books fortectonic plates and paper strips forseismic waves.

ReteachGroup students in groups of four or five.Have each group create an earthquakesafety pamphlet. Tell students to findout the recommended ways to protectthemselves during an earthquake. Havethem create a pamphlet that explainsand illustrates safety instructions. Eachgroup should have researchers, an editorto compile the instructions, and anillustrator to create the pictures. Postthe completed pamphlets on a bulletinboard.

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an earthquake; (g) vibrations that travelthrough the ground; (h) the point on thesurface directly above the focus2. sudden movements in Earth’s crust3. Earthquakes can change the level of theland, cause fires and floods, and cover areaswith snow and ice.4. (a) Sample answer: Damage from anearthquake can destroy buildings and makea place uninhabitable; (b) Emergency reliefworkers from around the world may arrive

Assessment

1. (a) a shaking of the ground caused bysudden movements in Earth’s crust; (b) ahuge sea wave that is set off by an underseaearthquake or volcanic eruption; (c) a sud-den drop of a mass of land down a moun-tainside or hillside; (d) the network of servic-es that supports a community; (e) a suddenfall of a mass of ice and snow; (f) tremorsthat can occur days, or even months, after

to help with rescue and cleanup efforts.People around the world may donatemoney to help victims survive until theyrebuild their lives.5. Sample answers: Design buildings towithstand the shock of an earth quake. Buildvillages, towns, and cities far from fault lines.


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