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sections

1 Evolution of Earth’s Crust

2 Earthquakes

3 Earth’s Interior

4 Volcanoes

Lab A Case for Pacific PlateMotion

Lab Earthquake! Earthquake!Where’s the earthquake?

Italian FireworksSicily’s Mt. Etna is the largest volcano inEurope. Molten rock from deep in Earthrose through a weak spot in the crust anderupted many times onto Earth’s surface.Some of the exposed lava at the base ofMount Etna is nearly 300,000 years old.

Research the most recent eruptionand write a report about how it affected the surroundingenvironment.

Science Journal

Earth’s InternalProcesses

Alfio Scigliano/CORBIS

353353

Systems Make the followingFoldable to help you organizeinformation about types ofplate boundaries.

Fold one piece of paper lengthwiseinto thirds.

Fold the paper widthwise intofourths.

Unfold, lay the paper lengthwise,and draw lines along the folds. Labelyour table as shown.

Making a Table As you read the chapter,complete the table describing and illustratingdivergent, convergent, and transform plateboundaries.

PlateBoundary

TypeIllustrationDescription

Divergent

Convergent

Transform

STEP 3

STEP 2

STEP 1

Global Jigsaw PuzzleAlfred Wegener, a German scientist, noticedthat the shapes of continental coastlinesappeared as though they could match up.He suggested that the continents once weretogether as one giant landmass. Use a map ofthe world to test this idea. Can you see whathe saw?

1. Cut out continents from a copy of a worldmap provided by your teacher.

2. Try to arrange continents so that they fittogether.

3. Infer why the fit might not be perfect.

4. Think Critically What changes in proce-dure might demonstrate a better fit forthe continents?

Start-Up Activities

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354 CHAPTER 12 Earth’s Internal Processes

Continental DriftIn the early twentieth century, there was no single theory of

how Earth processes interrelated. Much geologic study was donelocally because transportation and communication were expen-sive. Based upon their observations, geologists developed theo-

ries that emphasized vertical changes, forexample, an erosion process that leveled highplaces, and a mountain-building process thatlifted them up again.

Then in 1915, Alfred Wegener (VEG nur)proposed a hypothesis that suggested thatEarth’s continents once were part of a largesuper-continent, shown in Figure 1, calledPangaea (pan GEE uh). Then, about 200 mil-lion years ago, the super-continent brokeinto pieces that drifted over the surface ofEarth like rafts on water. This revolutionaryidea of horizontal movement met with greatresistance among his peers. He was unable tofind the force capable of moving continents.It wasn’t until after his death in 1930, thatscientific advances finally justified hishypothesis.

Evolution ofEarth’s Crust

Reading Guide

■ Explain supporting evidence forthe continental drift hypothesis.

■ Discuss the failings of the conti-nental drift hypothesis.

Wegner’s continental drift hypothe-sis led to a unifying theory of Earthsciences known as plate tectonics.

Review Vocabularyhypothesis: statement proposed toexplain an observation or answer aquestion

New Vocabulary

• mid-ocean ridge

• rift valley

• divergent boundary

• convergent boundary

• subduction

• transform boundary

Figure 1 This illustration is anartist’s conception of whatPangaea may have looked like200 million years ago.

Mark Garlick/Photo Researchers

SECTION 1 Evolution of Earth’s Crust 355

Matching Coastlines The mostapparent match of continents is theeastern coastline of South Americawith the western coastline of Africa. Ifyou use your imagination, you can seethat the coastline of northwesternAfrica fits nicely with that of theeastern United States. When SouthAmerica and Africa are joined together,their southern tips fit very well into theWeddell Sea of Antarctica.

Wegener had to show that the con-tinents were actually joined. He usedthe analogy of a torn newspaper beingrepaired. Not only did you have to match the shapes, but alsojoin the lines of print. And the print had to match in terms of itscontent as well. What kind of content could this refer to incoastal regions of the continents? Wegener argued that youcould match rock types, fossils, erosion features, and mountainranges. If you found similar formations and structures on eachcontinent then the continents could have been joined togetherin that place.

Wegener’s opponents pointed out that the coastlines areconstantly wearing away due to wave action. How could some-one compare the present coastlines? Years later, during therevival of the hypothesis, oceanographers were able to show,using sonar, that the edges of the continental shelves matchedvery well, as shown in Figure 2.

Matching Fossils Wegener could notuse the remains of just any ancient livingthing to support the existence of Pangaea.For instance, animals that could fly orswim could appear in the fossil record inwidely separated places due to theirmobility, not because the places werenecessarily joined. Large land animalsprovided better evidence because theycould not have crossed oceans. Animalssuch as Lystrosaurus or Cynognathus, largeanimals that preceded the dinosaurs,supported a contiguous landmass.Glossopteris, a large fern with large, heavyspores also supported the idea of Pangaea.You can see in Figure 3 that these livingthings were widely distributed.

CynognathusMesosaurus

Lystrosaurus

Glossopteris

Africa

Australia

India

Antarctica

SouthAmerica

Figure 2 Weathering of the con-tinental edges does not affect thecontinental shelves (light blue).

Figure 3 Wegener chose fossilsof animals that could not swim orfly to prove Pangaea’s existence.Explain Why would being able tofly or swim eliminate a fossil organ-ism from Wegener’s proof?

NOAA/NGDC


Matching Rocks and Mountains Mountain ranges wereshown to be continuous in Pangaea, as shown in Figure 4.Once Pangaea broke apart, the mountain ranges became sep-arated. For decades, geologists studied and attempted toexplain the origin of these mountains as separate ranges;Wegener showed them to be one mountain range. Wegenerwas able to show that continents that were joined sharedunique rocks and minerals.

Wegener’s hypothesis was not accepted by his contempo-raries because he was unable to conceive of a force or mecha-nism that could drive continents apart. Wegener reasoned thatEarth’s rotation, the gravitational pull of the Sun and theMoon, and centrifugal force could move continents. Physicistsquickly showed that even combining these forces would notbe sufficient.

Why didn’t Wegener’s contemporaries accept hishypothesis?

Wegener used the analogy of continents moving over Earth’ssurface as ships moving through water. Skeptics argued that thisship would push a wall of water ahead of it and leave a wake.The continents didn’t leave a wake. Instead of the continentspushing up a wall of water, they were deformed. How could thisbe if they were thicker and stronger?

Seafloor Spreading HypothesisAfter World War II, Dr. Harry Hess revived Wegener’s ideas.

He used sonar, intended to detect submarines, to obtain accuratemaps of the seafloor. Using sonar data, astonishing three-dimensional seafloor models were created in 1960. Soon it was

apparent that a mid-ocean ridge system, orMOR, was continuous and wrapped aroundEarth. A MOR is shown in Figure 5.

Hess proposed a hypothesis of seafloorspreading, or divergence. He suggested thatmagma from the mantle is forced upwardbecause of its low density. This causes the crustto crack (fault) and move apart. The faultingcauses twin mountain ranges with a down-dropped rift valley between. This continuousprocess allows new rock to form as magma fillsin from below. See Figure 6.

0 3,000

NorthAmerica

SouthAmerica

Greenland Norway

GreatBritain

Africa

Cape FoldBelt

km

Appalachians

Figure 4 Wegener’s hypothesisshowed mountains on several con-tinents were once part of the samerange.

Figure 5 Discovery of a mid-ocean ridgeled Hess to hypothesize seafloor spreading.

The Floor of the Oceans by Bruce C. Heezen and Marie Tharp, ©1977 by Marie Tharp. Reproduced by permission of Marie Tharp


Ages of Sediment and Rocks In the early1960s, massive programs for drilling into theseafloor began. Extracted cores of seafloorshowed that sediments are thicker on top ofseafloor basalt near the continents. MOR sedi-ments, however, are thin. Cores of both sedi-ments found that near the continents the oldestsediments are at the bottom and young sedi-ments are at the top. MOR sediments are all ofrecent age. When the ages of rocks are meas-ured, the continental rocks are billions of yearsold, while seafloor rocks are less than 200 mil-lion years of age. Rocks of the oceanic crustincrease in age as their location extends fromthe MOR, and at the MOR they are new.

MagmaMagmaMagma

Oceanic crustOceanic crust

Rift valleyRift valley

Mid-ocean ridge

SPREADING DISTANCES The spreading rate along the mid-ocean ridge varies. In theAtlantic Ocean, it averages about 2.5 cm/year. About how far, in kilometers, wouldthe Atlantic seafloor have widened after 100 million years?

known values and unknown values

Identify the known values:

time � 100,000,000 years

spreading rate � 2.5 cm/year

Identify the unknown values:

spreading distance

the problem

Substitute the known values into the equation:

distance � rate � time

distance � 2.5 cm/year � 100,000,000 years � 250,000,000 cm

1 km/100,000 cm � 250,000,000 cm � 2,500 km

the answer

Does your answer seem reasonable? How wide is the Atlantic Ocean?

CHECK

SOLVE

IDENTIFY

Solve One-Step Equations

If the East Pacific Rise spreads at 12 cm/year, how wide will it be in 10 million years?

For more practice problems, go to page 879 and visit Math Practice at .gpescience.com

Figure 6 A rift valley forms alongthe mid-ocean ridge as plates diverge.


Magnetic Polarity of Rocks Studies show that Earth’smagnetic field repeatedly reverses itself, meaning that the mag-netic north pole becomes the south pole. Vine, Matthews,Wilson, et al discovered bands of reversed polarity in the seafloorrocks similar to those found on the continents. As magma crys-tals form, they take on the polarity of Earth at the time theyform. The pattern is identical on both sides of the MOR.

Theory of Plate TectonicsOriginating in the 1960s, the theory of plate tectonics is

relatively new. After seafloor spreading demonstrated thatEarth’s crust moved horizontally on a global scale, manyinvestigators were determined to understand such a whole-Earth system of movement. This system consists of about adozen major plates and many minor ones. Plates are com-posed of a rigid layer of uppermost mantle and a layer ofeither oceanic or continental crust above. Some plates arecomposed only of oceanic crust, and some are composed ofpart oceanic and part continental crust. J. T. Wilson is credited

with describing the cycle of repeated open-ing and closing of ocean basins throughEarth’s history.

There are three main kinds of platemotions. These are best visualized by con-sidering how plates interact along plateboundaries, where they meet. Plates canmove apart, move together, or slide pastone another. Although often visualized asnarrow boundaries, scientists now con-sider many boundaries to be wide zones ofinteraction.

Divergent Plate Boundaries Youlearned that at a mid-ocean ridge (MOR),magma rises along a faulted rift valley,spreads, and cools to form new oceanic crust.This spreading apart is what happens atdivergent boundaries. An MOR representsdivergence that is well-developed andthat has resulted in the production ofmajor ocean basins. In some locations onEarth today, divergent boundaries exist asrift valleys, where no mature ocean basinsexist yet, such as in East Africa, shown inFigure 7.


AfricanPlate

(Nubian)

ArabianPlate

IndianPlate

Red SeaNileriver

LakeVictoria

Equator

Plate boundariesEast African rift zone

Gulf of Aden

Gulf ofAden

Failedrift

Red Sea

AfricanPlate

(Somalian)

Figure 7 Large lakes and vol-canic mountains are characteristicsof a continental rift valley.

Sonar In the process ofrefining sonar’s capabili-ties, discoveries were madethat had peacetime bene-fits. For example, sonaroften is equipped on boatsto locate schools of fish. Arelated technique calledultrasound is used in medi-cine. Research current usesand applications of sonarand ultrasound.


Convergent Plate Boundaries Where plates collide, theycome together to form convergent boundaries. In some cases,less-dense, thick continental lithosphere moves toward denser,thin oceanic lithosphere. This results in the oceanic side bend-ing and being forced downward beneath the continental slab ina process called subduction. Heat along a subduction zone par-tially melts rock at depth and produces magma, which risestoward the surface. This magma feeds a volcanic arc that paral-lels this zone, shown in Figure 8. The region of collision also hasa deep-sea trench that parallels the zone. The Andes mountainrange in South America is an example.

Convergent plate boundaries also exist between two slabs ofoceanic lithosphere. In this case, the oceanic lithosphere that iscolder, and therefore denser, subducts. Magma erupted hereproduces chains of volcanic islands called island arcs. Japan is anexample of an ocean-ocean convergent boundary, also shown inFigure 8. As plates converge, stress builds, which could bereleased as tsunami-causing earthquakes.

Along some convergent plate boundaries, two continentalslabs of low density collide and tend not to subduct. Because ofthis resistance to subduction, the plates collide and buckleupward to form a high range of folded mountains. Volcanicactivity is noticeably absent and there is no trench. TheHimalaya of Asia are an example of folded mountains that occurwhere continental lithosphere collides.

What is formed when two continental platesconverge?

Figure 8 When plates collide,the more dense plate is subducted.The resulting features include vol-canoes, mountains, and deeptrenches.

ContinentalContinentalcrust

Continentalcrust

Lithosphere

Asthenosphere

Lithosphere

Asthenosphere

Oceanic crust

Oceanic crust

Volcanic arc Deep-seatrench

Topic: TsunamisVisit for Weblinks to information abouttsunamis.

Activity Research the mostrecent tsunami and its destruction.Record the epicenter and magni-tude of the earthquake that causedthe tsunami and post these dataon a world map. Write a shortreport that describes the tsunami’simpact on humans and theenvironment.

gpescience.com

LithosphereLithosphere

Oceanic crustOceanic crust

Lithosphere

Oceanic crust

AsthenosphereAsthenosphere

Oceanic crust

Oceanic crust

Oceanic crust

Island arcDeep-seatrench


Continentalcrust

Lithosphere

Asthenosphere

Lithosphere

Asthenosphere

Oceanic crust

Oceanic crust

Mountain range



Transform Plate Boundaries Some boundaries amongplates exist as large faults, or cracks, along which mostly hori-zontal movement is taking place, as shown in Figure 9. In thiscase, no new lithosphere is forming, as along a divergent bound-ary. In addition, old lithosphere is not being recycled, as along asubduction zone. The main result of transform boundaries ishorizontal motion of lithosphere.

Transform faults are extremely important where they cutperpendicular to the MOR. These fault systems allow movementaway from ridge crests to occur, as shown in Figure 10. If youobserve arrows that indicate net motion along these transformfaults, you will notice that this net motion trends away fromthe MOR.

What drives the plates?Research indicates that plates are driven by a combination of

forces. One such force is ridge push at the MOR. Because diver-gent boundaries are higher at the center of the ridge, gravityforces material down the slopes of the MOR.

When a plate subducts back into Earth at some convergentboundaries, the process of slab pull is thought to operate. Youprobably have experienced an analogy to slab pull when youfound your bed covers on the floor in the morning. During thenight, as you tossed and turned, the covers began to move off ofthe bed. Eventually, enough of the covers were over the side thatgravity took over and pulled the rest of the covers to the floor.Subducting plates may act in much the same way, as portions ofdescending plates are pulling the rest of a plate down with them.

Relative motion ofNorth American

Plate

San Andreasfault

Mexico

United States

East Pacific Rise

LosAngeles

SanFrancisco

Relative motionof Pacific Plate

Transform fault

Figure 9 Friction between platesmoving side by side causes cracksand breaks in the edges of theplates. This is the site of brief, butrapid energy release called anearthquake.

Figure 10 A transform fault cutsthrough the MOR, offsetting themountain range.


Self-Check1. Explain the processes of convergence and divergence.

2. Describe the key features of a divergent boundary.

3. Compare and contrast the three types of convergentplate boundaries.

4. Describe the possible driving mechanisms in the platetectonic theory.

5. Think Critically Predict what would happen if Earth’splates stopped moving.

6. Think Critically What would have to occur to stopEarth’s plate movement?

SummaryContinental Drift

• Wegener proposed that former super-continent Pangaea broke up into pieces,which drifted to their present positions.

• Evidence favoring continental drift includesmatching shorelines of continents and corre-lating rocks, fossils, and mountain ranges ofthose continents.

Seafloor Spreading Hypothesis

• Hess suggested that seafloor was created andspread apart at the mid-ocean ridge.

• Moving away from the MOR, rocks are olderand sediments are thicker.

• Magnetic reversals preserved in seafloor rocksare symmetrically distributed on either side ofthe MOR.

Theory of Plate Tectonics

• Earth’s rigid, outermost layers are composedof a dozen or so major plates and manysmaller ones.

• New lithosphere is created at the MOR andrecycled at convergent boundaries. Convectiveflow within the mantle drives the plates.

7. If two plates diverge at a rate of 1.3 cm/year, howmuch farther apart will the plates be after 200 millionyears?

8. How many times faster are plates moving at7.3 cm/year than those moving at 1.3 cm/year?

9. The average distance across an ocean is 16,000 km.Two continents on either side of the ocean are con-verging at a rate of 10 cm/year. How long will it takefor them to collide?

Friction between a plate and mantle material below the plateprobably is of major importance in relative plate motion. Forexample, plates that drag continental material along with themare noticeably slower than are purely oceanic plates. Scientiststhink that continental lithosphere has deep roots that causemore frictional force than would be expected at the base ofoceanic lithosphere.

What role does friction play in plate motion?

Thermal Energy Internal convection of mantle material isthe driving force for all mechanisms of plate motion. In turn,the main source of thermal energy that keeps Earth materialsconvecting comes from the decay of radioactive elements inEarth. Increased temperature due to pressure and frictionalheating produced as part of the mechanism itself probably areimportant. Conversion of secondary earthquake waves in theouter core may yet be another source of energy.

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Global Earthquake DistributionFor decades, scientists have known that earthquakes are not

distributed randomly, but rather, they occur in well-definedzones. These zones coincide with the edges of lithosphericplates. In fact, seismic data originating from earthquakes helpedto decipher the structure of Earth’s ocean floor and to infer thestructure and motion of Earth’s plates. Figure 11 shows the dis-tribution of large earthquakes.

EarthquakesReading Guide

■ Describe the causes and charac-teristics of earthquakes.

■ Explain how seismic waves affectEarth’s surface.

■ Describe how seismic waves areused to infer Earth’s internalstructure.

Earthquakes kill people and destroyproperty. Understanding earth-quakes may help minimize theireffects.

Review Vocabularyfriction: force that opposes thesliding motion between twotouching surfaces

New Vocabulary

• fault

• elastic rebound

• focus

• epicenter

Figure 11 Most earthquakesoccur along the edges of plates.Identify some other places earth-quakes occur.

USGS, National Earthquake Information Center

SECTION 2 Earthquakes 363

Depth of Focus Patternsdevelop when data about thefocus depths of earthquakesare plotted on a world map.Recall that divergent bound-aries are associated withtransform faulting thatallows plates to move inopposite directions. All ofthis faulting creates a narrowband of numerous, shallowearthquakes. In contrast,convergent boundaries havebroad zones of earthquakeswith the shallowest foci near the surface at the point of conver-gence, and the deepest foci located under volcanoes or moun-tains created in the collision area, as shown in Figure 12.

Causes of EarthquakesAn earthquake is any seismic vibration of Earth caused by

the rapid release of energy. Earthquake events can be either nat-ural or human-caused. Passing trains or large trucks and explo-sions can cause Earth to vibrate. As shown in Figure 13, sudden,virtually unpredictable, natural earthquakes that result in majordestruction are greatly feared.

Deformation Earth’s crust is composed of rigid, rocky mate-rial. Engineers would describe it as brittle. When a stress isapplied to a brittle material it shows little sign of strain, ordeformation, until it suddenly breaks. A strain is the manner ofdeformation in response to a stress. Stress is the force per unitarea that acts on a material. Stresses can be of four types:(1) compressive stress, in which a mass issqueezed or shortened, (2) a tensionstress, in which the mass is stretched orlengthened, (3) a shear stress, in whichdifferent parts of a mass are moved inopposite directions along a plane, or (4)torsion stress, in which a mass is sub-jected to twisting.

Figure 12 The depth of theearthquake focus (indicated bythe stars) is related to the activitycausing the earthquake.

Figure 13 This damage was caused whenthe buildings were shaken off their foundations.Explain How might this damage have beenprevented?

Oceanictrench

Divergentboundary Divergent

boundaryRiftvalley

Riftvalley

Convergentboundary

AP/Wide World Photos


Elastic Deformation Elastic deformation occurs when amaterial deforms as a stress is applied, but snaps back to its ori-gin shape when the stress is removed. Plastic deformation occurswhen a material deforms, or changes shape, as a stress is appliedand remains in the new shape when the stress is released.Modeling clay behaves plastically. You would expect all rocks toshow brittle deformation, which means breaking in response tostress. But rocks at depth, where temperatures are high enough,display plastic behavior. For example, you can break off an edgeof a wax candle when it is cold and brittle, but the wax bendsmore under stress—without breaking—when it warms up.

Energy Release Strain energy builds up along cracks inEarth’s crust in response to stress. When this strain energy isreleased suddenly, it causes rock to lurch to a new position. A faultis a crack along which movement has taken place. If no movementtakes place, the crack is a fracture. Earthquake-producing faultsoccur in broad zones in which rock is deformed in a brittle man-ner during the fault movements. These zones can be tens ofmeters wide. The sudden energy release that goes with faultmovement is called elastic rebound. Elastic rebound causes seis-mic vibrations, or earthquakes, like when you drag a table acrossthe floor and the legs catch and release making a rumbling sound.

Earthquake WavesEarthquake waves travel out in all directions from a point

where strain energy is released. This point is the focus, or pointof origin, of an earthquake. The point on Earth’s surface directlyabove the focus is the epicenter. When you throw a stone intowater you see concentric rings of waves move out across the sur-face from the point of impact. Earthquake waves are much thesame, except they move out from the focus in all directions, likea sphere of waves. These ideasare shown in Figure 14.

Earthquake waves can besorted broadly into two majortypes. Body waves travelthrough Earth. Surface wavestravel across Earth’s surface.

Demonstrating FourTypes of StressProcedure1. With palms facing down

at all times and your handsin contact with each other,clasp a large bar of taffywith both hands. First,push one hand forward2 cm while simultaneouslypulling the other backward2 cm. Return your hands tothe original position.

2. Still holding your hands incontact, twist your handsin opposite directions andreturn them to the originalposition.

3. Next move your handsabout 4 cm apart.

4. Finally push your handsback together to the origi-nal position.

Analysis1. Which type of stress did

you demonstrate in each ofsteps 1–4?

2. Describe the kinds ofdeformation you wouldexpect to result from eachof the four stresses.

Wavefronts

FocusEpicenter

Faults

Figure 14 Waves moving out fromthe focus may travel through the mantle.These may be picked up by seismographson the opposite side of Earth.

Body Waves One type of body wave is called a primary wave.Primary waves, also called P-waves, cause particles in a materialto undergo a push-pull type motion as shown in Figure 15.Because this motion is in the direction of wave travel, the waveenergy is transferred very quickly. The particles do not perma-nently change location. If there is matter around where particlescan bump into each other, then primary waves can movethrough it. Much like sound waves, P-waves travel through allkinds of matter.

Secondary waves (S-waves) are body waves that travel moreslowly than primary waves. They are sometimes called shearwaves, because of the relative motion of particles as energy istransferred. S-waves cause particles to move perpendicular tothe direction of wave travel. The farther body waves travel froman earthquake focus, the farther behind the S-waves get. It is thislag in time between the arrival of the first P-waves and the firstS-waves that is important in locating epicenters.

S-waves only can travel through solids. When one particlemoves, it moves its neighbors along with it. In gases and liquidsthe bonds are weak, or there are no bonds, so particles can moveindependently. The motions of S-waves are shown in Figure 16.

Figure 16 S-waves cause theearth to undulate creating surfacedamage.

Figure 15 P-waves arecompressional waves like thosemoving in a coiled spring.Describe How are P-waves likesound waves?


Spring at rest

P waves traveling along the surface

Compress spring

Wave direction

Wave direction

Compress

Expand

Particle motion

n

nnnnnnnPPPPPPPPPPPP

S waves traveling along the surface

Rope at rest

Shake rope

Particle motionWave directionn

Wave direction


Surface Waves Surface waves move in a more complexmanner, often causing a rolling motion much like ocean waves.As surface waves travel through material, they can exhibit an upand down rolling motion, and also a side-to-side motion thatparallels Earth’s surface. Foundations of human-built structuresoften are susceptible to the side-to-side rocking that mightresult from surface waves. These surface wave motions are illus-trated in Figure 17.

Figure 17 During an earth-quake the surface can roll like theocean and shift side to side at thesame time.

Table 1 Estimates of Earthquake Magnitude and Frequency

Richter Magnitude Description Estimated Occurrence Range Index per Year

� 2.0 recorded, but not generally felt 600,000

2.0–2.9 potentially felt 300,000

3.0–3.9 felt by some 49,000

4.0–4.9 felt by most 6200

5.0–5.9 damaging 800

6.0–6.9 destructive in densely 266 populated areas

7.0–7.9 potential to inflict major 18 damage

8.0 and above potential to destroy 1.4 communities near epicenter

Surface waves that are like ocean wavesSurface waves that are like S waves

367

Earthquake MeasurementTwo measurement schemes that have been used to charac-

terize earthquakes are the Modified Mercalli intensity scale andthe Richter magnitude scale. Intensity is a measure of groundshaking and the damage that it causes. The Modified Mercalliscale, Table 2, ranks earthquakes in a range from I–XII, XIIbeing the worst, and uses eyewitness observations and post-earthquake assessments to assign an intensity value. The Richtermagnitude scale, Richter scale for short, uses the amplitude ofthe largest earthquake wave. Richter magnitude is intended togive a measure of the energy released during the earthquake.Figure 18 shows a seismogram and how it is used to determinea Richter value. Table 1 shows the global frequency of differentmagnitude earthquakes.

Figure 18 A seismograph is aninstrument used to measure earth-quake waves. A seismogram is atracing of the seismograph’s pen.

Table 2 The Mercalli Scale of Earthquake Intensity

Level Description

I Rarely felt by people.

II Felt by resting people indoors; some hanging objects may swing.

III Felt indoors by several. Vibration like passing of a light truck.

IV Felt indoors by many. Vibration like passing of a heavy truck. Standing autos rock. Windows, dishes and doors rattle. Walls and frames may creak.

V Felt by nearly everyone indoors and outdoors. Small unstable objects upset. Some dishes and glassware broken. Swaying of tall objects noticed.

VI Felt by all. Walking is unsteady, many run outdoors. Windows, dishes, and glassware broken. Furniture overturned and plaster may crack.

VII Difficult to stand. Noticed by drivers of autos. Furniture and chimneys broken. Well built buildings hardly damaged. Poor structures considerable damage.

VIII People frightened. Ordinary buildings slightly damaged. Driving of autos affected. Tree limbs fractured. Damage to tall objects. Cracks in wet ground.

IX General panic. Damage great in substantial buildings. Some houses thrown off foundations. Underground pipes broken. Serious ground cracks.

X Most masonry and frame structures destroyed. Serious damage to dams, dikes, embankments. Water splashed out of rivers, canals, lakes. Rails bent.

XI Few structures remain standing. Bridges destroyed. Broad fissures in the ground. Slumps and landslides. Rails bent generally.

XII Damage nearly total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown into the air. Large rock masses displaced.

First Pwave

First Swave

Time(Later)(Earlier)

Surfacewaves

Figure 19

VISUALIZING EARTHQUAKE PROOF BUILDINGS


Because the most severe damage from an earthquakeis not caused when a structure shakes, but when itfalls down, engineers are developing ways to make

buildings safer. Their job is to prevent the energy of anearthquake from damaging a building’s structure.

What happens when the ground moves back and forthunder a building? The first floor moves back and forth, butthe energy is not transferred to the whole building. Theresult is that the bottom of the structure collapses.

One way to keep the build-ing stable is to design a sys-tem that allows the wholestructure to move as a unit.Base isolation systems usebearings that separate thebuilding from the ground.These bearings can be madeof large rubber pads or giantmetal springs and are placedbetween the ground and thebuilding support beams. Thestretchy rubber or metalspring absorbs the earth-quake energy.

Another way to protect buildingsin an earthquake is active damp-ing. Large blocks of metal or con-crete, weighing many tons slideback and forth as the buildingsways. The pendulum motion ofthe damper absorbs the energy ofthe earthquake, and reduces themovement of the building.

Buildings also can be protected by using structures that canbend. These diagonal braces, called unbonded braces, are madeof steel and concrete. The steel beam is shaped so that it canbend back and forth without breaking. The building moves butit does not collapse.

(tl)Bob Riha/Getty Images, (tr)Otto Greule Jr./Getty Images, (bl)Steven Powell, Star Seismic LLC, (br)Jerome Favre/AP/Wide World Photos


Self Check1. Describe the elastic rebound process.

2. Contrast primary and secondary seismic waves.

3. Compare and contrast the Richter scale with theMercalli scale.

4. Summarize the patterns of global earthquakedistribution.

5. Think Critically Why couldn’t you use the Mercalli scaleto measure an undersea earthquake?

SummaryGlobal Earthquake Distribution

• The majority of earthquakes occur at varyingdepths, and in zones that define the locationsof plate boundaries.

Causes of Earthquakes

• Earthquake waves are the result of elasticrebound in faults.

Earthquake Waves

• Body waves move throughout Earth.

• Surface waves move along Earth’s surface.Their motions cause the majority of earth-quake damage.

Earthquake Measurement

• The Modified Mercalli scale is a subjective dam-age scale that indicates earthquake intensity.

• The Richter magnitude scale measures ampli-tudes of waves generated by an earthquake.Energy released by an earthquake are esti-mated from amplitude data.

6. Calculate If a primary earthquake wave travels at arate of about 6 km/s through continental crust, howlong will it take it to reach a seismic station located1,200 km away?

7. Calculate If the secondary wave travels at 10 km/sto the same station in question 6, how much longerwill it take for the secondary wave to arrive after theprimary wave?

Levels of Destruction The level of destruction by earth-quakes is extremely variable. Research has shown that poorbuilding methods are the largest contributors to earthquakedamage and loss of life. In countries where there are poorlyconstructed buildings, it is not uncommon for tens of thou-sands of people to die in a single earthquake event. It is possibleto use high-technology building methods to make structuresearthquake resistant, but not earthquake proof. A large propor-tion of earthquake damage is secondary, such as damage bylandslides, fires, and tsunamis. Active earthquake zones are wellestablished, but predicting precise times for earthquakes inthose zones is not yet possible.

Earthquake Proofing Although no building can be madeentirely earthquake proof, scientists and engineers are findingways to reduce the damage to structures during mild or moder-ate earthquakes. Much damage occurs when older structures areshaken off their foundations, so securing a building to its foun-dation is important. Large masses that can move with the earth-quake absorb energy to make a building more secure. Figure 19shows some other possible methods for reducing the effects ofearthquakes and making buildings safer.




What’s inside?How is it possible to know anything about the interior of

Earth? In 1961 scientists drilled a 200 m deep hole into the oceaniccrust trying to reach the Mohorovicic discontinuity. The projectwas discontinued after Phase I. It is 6,371 km to the center of Earth.By human standards, this attempt was barely a scratch on Earth’ssurface. Imagine adding this feature to the diagram in Figure 20.

Seismologists, geologists who use seismic earthquake wavesto interpret characteristics of Earth, conceived of the idea to usethese waves to gather data. It is similar to a doctor using soundwaves to see inside a human body. As energy passes throughmatter it is scattered, absorbed, or unaffected. Observation ofseismic waves allows scientists to infer images of Earth’s interior.

Uniform Earth? If Earth was uniformin structure and composition, and youknew how fast earthquake waves traveledthrough its material, then it would be easyto calculate when earthquake wavesshould be detected on its opposite side.Observations show that seismic wavesarrive at different times than expected. Inorder for seismic waves to change speed,Earth must not be uniform throughout.If Earth is not uniform, then is there apattern to its interior structure?

Earth’s InteriorReading Guide

■ Explain how geologists infer thestructure of Earth’s deep interior.

■ Describe Earth’s internal structureand composition.

Using waves to indirectly determinethe internal structure of an objecthas many applications in thesciences and engineering.

Review Vocabularyrefraction: the bending of a wave asit changes speed in moving from onemedium to another

New Vocabulary

• discontinuity

• shadow zone

• asthenosphere


Continentalcrust

Upper MantleUpper Mantle

MohorovicicDiscontinuityDiscontinuityMohorovicic

Discontinuity

OceanicCrust

OceanicCrust

Figure 20 Also known as theMoho layer, the red line below wasthe target of the 1961 MoholeProject. Infer why they attempted to drillthrough oceanic crust instead ofcontinental crust.

SECTION 3 Earth’s Interior 371

Earthquake ObservationsAs seismic wave recording stations began to spread over

Earth, new discoveries were made as seismic wave data fromearthquakes were interpreted. Observations of refracted wavesshow that the waves do indeed bend as they encounter sharpchanges in density. A boundary that marks a density changebetween layers is called a discontinuity. One such discontinuityseparates the crust from uppermost mantle, and is known as theMohorovicic (moh huh ROH vee chihch) discontinuity, orMoho, illustrated in Figure 20.

Shadow Zones Observations show that, from a given epicen-ter, P-waves and S-waves travel through Earth for 105 degrees ofarc in all directions. Between 105 and 140 degrees from the epi-center, nothing is recorded. This “dead zone” is termed theshadow zone. From 140 degrees to 180 degrees (directly oppo-site the epicenter), only P-waves are recorded.

Shadow zones reveal two interesting facts about Earth’s inte-rior. Because S-waves seem not to appear on seismographslocated beyond 105 degrees from an epicenter, scientists thinkthat there is a layer of Earth that is absorbing them. In fact, thesewaves are thought to be converted to P-like waves in the outercore. Recall that S-waves only travel through solids. This sug-gests that the outer core is in a liquid state. See Figure 21.

If you move an energy source all around an object, youwould eventually be able to compile many different viewsand describe the three-dimensional shape of the object. Thisprocess is called tomography and is the way MagneticResonance Imaging (MRI) can show doctors the inside imagesof the human body. With earthquakes happening all overthe globe, seismologists have thousands of pointsources of energy, and can construct a tomographicview of the core.

Solid Inner Core The fact that P-waves passthrough the core, but are refracted along the way,indicates that the inner core is denser than theouter core and solid. The state of a particularmaterial depends on both pressure caused by theweight of overlying material and temperature.When pressure dominates, atoms are squeezedtogether tightly and exist in the solid state. If tem-peratures are high enough, atoms move apartenough to exist in the liquid state, even at extremepressures.

Figure 21 S-waves only travelthrough solids so they cannotpenetrate the liquid outer core,creating shadow zones.

Shadowzone

Shadowzone

105°105° 105°105°

P-waveP-waveS-waveS-wave

140°140° 140°140°

Earthquake epicenterEarthquake epicenter

MantleMantle

Outer coreOuter core

Innercore

Innercore


Composition of Earth’s LayersEarth’s internal layers, illustrated in Figure 22,

generally become denser with depth. The crust anduppermost mantle, which together form the litho-sphere, are made of rocky material—mostly silicates.The asthenosphere is a weaker, plasticlike layerupon which Earth’s lithospheric plates move. Muchlike the lithosphere, mantle below the asthenospherealso is composed of silicates, but the mineralspresent have different structures in response toconditions of higher pressure. The cores are mademostly of metallic material, such as iron and nickelwith noticeable amounts of oxygen and sulfur alsopresent. The core apparently has a compositionsimilar to some iron meteorites that have struckEarth throughout its history.

Astronomers hypothesize that early Earth may have formedfrom meteorite-like material that was forced together by gravityand heated to melting. Some of the material then was able tomigrate toward the core. Over billions of years, Earth’s matterhas melted and differentiated. The densest materials settledtoward the core, and relatively low-density materials floatedtoward the surface. This differentiation due to gravity is thoughtto have taken place in all of the planets.

Upper mantle660 km

Upper mantle660 km

Lower mantle2240 km

Lower mantle2240 km

Asthenosphere200 kmAsthenosphere200 km

00

Crust7-50 kmCrust7-50 km

00

Lithosphere100 kmLithosphere100 km

Self Check1. Describe the evidence used for subdividing Earth’s inte-

rior into layers.

2. Explain the following points, using seismic evidence foryour argument:

a. Earth has a non-uniform density.

b. Earth has a layered structure.

c. Earth has a liquid outer core.

3. Compare and contrast the inner and outer cores ofEarth.

4. Think Critically Explain why it is impossible to everreally know what materials compose Earth’s interior.

SummaryWhat’s inside?

• Earthquake-generated seismic waves provideinformation about Earth’s deep interior.

Earthquake Observations

• Earth’s interior has a layered structure.

• Earth’s layers become denser with depth.

• Changes in density occur at layer boundariescalled discontinuities.

Composition of Earth’s Layers

• Layers of crust and mantle are rocky, andcomposed mainly of silicates.

• The cores have a high density, metalliccomposition.

• Composition of Earth closely resembles thecomposition of meteorites.

5. Calculate What percent of the mantle are the uppermantle, lower mantle, and the asthenosphere?

Figure 22 Layering of Earth iscaused by heat and pressure. Themost dense materials are at thecenter and the less dense materialsare near the crust.



SECTION 4 Volcanoes 373

Origin of MagmaRecall that faults are weaknesses in Earth’s crust along which

movement takes place. This movement results in a local decreasein pressure, called decompression. With less pressure, the melt-ing point of rock material decreases, but the temperature canremain the same. Hot, nearly molten rock in Earth’s asthenos-phere, considered an important source for molten rock material,can change to a liquid by decompression melting. Rising magmacan become more fluid as it decompresses, particularly if its gascontent is high.

Any molten rock material has a lower den-sity than that of its solid counterpart. Becauseof this density difference, a buoyant force actson magma that forms from rock surroundingit. Rising magma may reach Earth’s surface ifpressure conditions allow and the rock hasconduits through which it can flow.

Imagine hot magma rising through thecrust, creating brittle deformation near thesurface in the form of fractures or faults. Thecracks in turn cause a drop in pressure, andmore paths are available through whichmagma can move toward the surface, asshown in Figure 23. This causes more defor-mation until magma reaches Earth’s surfaceas a volcanic eruption.

VolcanoesReading Guide

■ Describe the types and causesof different types of volcaniceruptions.

■ Explain the pattern of occurrenceof volcanoes and its link to platetectonics.

Volcanic eruptions have an impacton the composition of the atmos-phere and on climate.

Review Vocabularymelting point: temperature atwhich a solid begins to liquefy

New Vocabulary

• viscosity

• cinder cone volcano

• shield volcano

• composite volcano

Magma

Figure 23 Less dense, liquidrock rises to the surface throughcracks and fissures creating avolcano.


Magma on the Surface Two major physical settings onEarth produce most lava flows at the surface. Eruptions mostcommonly are found near boundaries that separate tectonicplates, above mantle plumes or hot spots on continents or in theocean basins. Figure 24 illustrates these volcanic settings.

Eruptive ProductsVolcanoes expel a wide variety of materials. These materials

can be sorted first by their state of matter. Volcanoes erupt lava,gases, and chunks of solid material.

Solids All solid materials expelled by a volcano are collectivelycalled pyroclasts. Often, lava is ejected into the air as globules.These globules cool and solidify as they fall to Earth. The small-est particles cool very quickly and form volcanic ash. Largerglobules form streamlined, volcanic bombs. In addition tochunks that cool as they fall, there often are chunks of alreadysolid material ripped away from the conduit of the volcano asmaterial travels through it. These chunks of rock are termed vol-canic blocks. The larger the size of a pyroclastic particle, thecloser it will fall to the volcano. Blocks fall back to ground on avolcano’s flanks. Ash can be picked up by wind and blown hun-dreds or even thousands of kilometers away.

Figure 24 Volcanoes are com-mon in subduction zones. Friction,conduction, and convection mayall play a role in creating fissuresthrough which a volcano mayerupt.

Continental Volcanoes

Subduction zone Hot spot Rift

Oceanic Volcanoes

Subduction zone Hot spot Rift

Topic: Huge EruptionsVisit for Weblinks to information about theworld’s most powerful volcaniceruptions.

Activity In your Science Journal,list information, including datesand locations, of ten of the world’smost powerful volcanic eruptions.On a copy of a world map, plot thelocations of these events. Is there apattern to their occurrence in placeor in time?

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Gases Volcanoes release a broad variety of superheated gases,the most common of which is water vapor. In addition carbondioxide and gases composed of sulfur compounds are expelledhigh into the atmosphere. There is strong evidence that volca-noes are major contributors of greenhouse gases that can affectclimate long after an eruption is over.

Liquids Magma from a volcano or fissure may remain a liquid,at least initially, and flow across the Earth’s surface as lava. Lavascan vary considerably in composition, which in turn affectstheir physical properties.

Viscosity is a measure of the resistance of a fluid to flow.The temperature of molten rock material influences its viscosity.You have experience with temperature control on viscosity whenyou try to pour cold pancake syrup. It has a high viscosity whenit first comes out of the refrigerator. But, let it warm up and itflows more easily because its viscosity decreases. Other factorsthat affect viscosity and flow are gas content and composition.

Low-viscosity lavas are generally basaltic in composition.Basaltic lavas are low in silica (SiO2) content, and high in certainchemical elements such as calcium, magnesium, and iron.Basaltic lavas flow from fissures—such as along the MOR andzones of continental rifting, and also from hot spot volcanoes.They tend to flow easily and form huge volcanic forms such asshield volcanoes and flood basalts, both of which cover largeareas on Earth’s surface.

If lavas have large quantities of gas dissolved in them, thenthe viscosity is lowered. High gas quantities allow magma toforcefully migrate through rock, sometimes spewing out explo-sively as a lava fountain that behaves much like a geyser!

Eruptive StylesVolcanoes can erupt in many different ways, depending on

viscosity. Thick, sticky, high-silica magmas are so viscous thatthey tend not to erupt, causing internal pressure within a vol-cano to rise. When Earth’s crust fails under such high pressureconditions, a violently explosive eruption occurs. This style oferuption is characterized by abundant pyroclasts. In contrast,the runny, low-silica, high-temperature basaltic lavas are so lowin viscosity that they erupt quite easily and often produce quieteruptions of freely flowing lava. Eruptive style is strongly linkedto temperature and composition, factors that are hard to meas-ure until after an eruption. Temperature and composition of amagma that ultimately erupts as lava can be linked to the typeof plate boundary associated with it.

Modeling LavaViscosityProcedure

1. Mix a small batch of batterfrom pancake mix accord-ing to directions.

2. Mix a second small batch,but use 25% more milk.

3. Hold your finger under asmall funnel and fill thefunnel with the first batchof batter.

4. Have a partner hold awatch with a secondhand. Remove your fingerand time how long it takesthe funnel to empty.Record your data.

5. Clean the funnel andrepeat step 4 using thesecond batch of batter.

6. Gently heat a small pan ona hot plate.

7. Pour part of the first batchof batter on the pan andobserve.

8. Repeat step 7 using thesecond batch of batter.

Analysis1. Which batter made the

flattest pancakes? Explainwhy.

2. Compare and contrast theproperties of the batters.

3. Which batter modeled low-silica lava?


Plate Boundary Setting Look at Figure 25. Most of Earth’svolcanoes are located along the Ring of Fire, which rims thePacific Ocean. They lie in subduction zones where continentaland oceanic materials are being mixed and partially melted. Thisplate motion and the associated melting create a wide variety ofmagma types that can potentially erupt. Large earthquakes andviolent volcanic eruptions often are located along these ocean-continent and ocean-ocean convergent boundaries.

Divergent plate boundaries also are volcanically active, butmost of the activity is underwater, along the MOR, and goesunnoticed by most people. There are places were divergence takesplace on land and you could witness its associated volcanic activ-ity. Iceland and the East African Rift Valley are examples of landareas that are part of divergent boundaries. Lava erupted in thesesettings is generally low-viscosity and basaltic in composition.

Hot Spots Hot spots are volcanically active sites that arise inplaces where large quantities of magma move to the surface inlarge, column-like plumes. Scientists think that plumes are posi-tioned according to internal convection patterns within themantle, and some may originate at the core-mantle boundary. Itseems that hot spots do not move much, but the plates moveover them. When a hot spot occurs under an oceanic plate, thisstable source of hot magma forms volcanic island chains. TheHawaiian islands are such a chain. Yellowstone National Park isan example of a hot spot under a continental plate. When a vol-cano moves off the hot spot it becomes inactive.

Hot spot volcanic eruptions produce lava somewhat similarto that formed along divergent boundaries, but they are not anexact match. These lavas tend to contain greater abundances ofalkali metals such as potassium and sodium. Like MOR lavas,hot spots, which can occur far from a plate boundary, tend togenerate fluid, basaltic lavas. But their compositions can changeas magmas penetrate rock material of changing composition,like in continental crust.

AFRICA

EUROPEASIA

AUSTRALIA

PACIFICOCEAN

INDIANOCEAN

ARCTIC OCEAN

ATLANTICOCEAN

NORTHAMERICA

SOUTHAMERICA

Active volcanoPlate boundary

Topic: Mt. PinatuboVisit for Weblinks to information the 1991eruption of Mt. Pinatubo in thePhilippines.

Activity Construct a time line forthis eruption. Begin with precur-sors, which are events that indi-cate volcanic activity is imminent,and end with long-term effectsafter the eruption.

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Figure 25 Many volcanoes occuron Earth along plate boundaries,over hot spots, or in rift valleys.



Types of VolcanoesVolcanoes are classified according to their size, shape, and

the materials that compose them. Recall that eruptive materialsthat form a volcano are related to the physical properties of itsmagma source. The temperature, composition, and gas contentof magma are important controls on the type of volcanic struc-ture that forms during an eruption. Table 3 summarizes thecharacteristics of main types of volcanoes.

Cinder Cone Volcanoes When eruption of gas-rich magmatakes place, eruptive products often are spewed into the airexplosively as large chunks. These large pyroclastic materialsmay pile up near the exit hole, or vent. When the primary erup-tive products are large fragments of solid material, cinder conevolcanoes form. They tend to be small, with most cones havingheights in the hundreds of meters range. When cinder conesoccur on the flanks of larger volcanoes, they are called parasiticcones. An example of a volcano with parasitic cones is MountKilimanjaro in the African rift valley.

What are the characteristics of cinder cones?

Shield Volcanoes Because they form from high-tempera-ture, fluid, basaltic lava, shield volcanoes erupt with abundantlava flows that can move for kilometers over Earth’s surfacebefore stopping. Shield volcanoes are broad, flat structuresmade up of layer upon layer of lava. Think of pancake batter. Ifthe batter is cold or thick, it piles up and you get thick pancakes.Add more milk and make a runny batter, and it flows easilyacross the skillet and makes thin pancakes. Volcanism in Hawaiiproduces shield volcanoes.

Table 3 Comparison of Melt Properties

Composition Silica Content Gas Content Viscosity Volcano Type

Basaltic lowest least (1-2%) lowest shield, fissure eruptions (such as MOR)

Andesitic intermediate intermediate (3-4%) intermediate composite

Rhyolitic highest highest (4-6%) highest volcanic dome

Volcano EruptionsVolcanic eruptions canthrow tons of ash intothe atmosphere wherewinds carry the asharound the world. Theash blocks sunlight andaffects plant growth.Scientists measurefossilized tree rings toinfer the affect ancientvolcanoes had on Earth.Research the amount ofash produced by MountPinatubo and how quick-ly it circled the globe.


Self Check1. Explain why most volcanoes are found at plate

boundaries.

2. Compare and contrast the physical settings for com-posite volcanoes, cinder cones, and shield volcanoes.

3. Describe causes for variation in eruptive style forvolcanoes.

4. Explain how magma that originates at depth can eruptas lava at the surface.

5. Describe how island chains form over a hot spot.

6. Think Critically List some possible consequences ifvolcanic activity on Earth were to slow down or stop.

SummaryOrigin of Magma

• Magma originates as molten rock materialbelow the surface and erupts at Earth’s sur-face as lava.

• When the density of magma is lower thansurrounding solid rock, it is forced towardEarth’s surface.

Eruptive Products

• Eruptive products can be solids, liquids, orgases.

Eruptive Styles

• The style of eruption, whether quiet or explo-sive, is related to its plate tectonic setting.

Types of Volcanoes

• Cinder cones are small, but they eruptviolently.

• Shield volcanoes are very large and mostlyexpel free-flowing lava quietly.

• Composite volcanoes are large and tend toerupt violently.

7. If a cinder cone is 540 m high and has a base diameterof 3 km, what is the volume of the volcanic cone incubic meters? Use the formula Vcone = (r2 h �)/3.

8. The dome in the caldera of the volcano has a height of 12 meters and a diameter of 50 m. What is its volume?


Composite Volcanoes When volcanoes occur along con-vergent plate boundaries, they tend to have magmas that arericher in silica content than those formed at hot spots or diver-gent boundaries. This is because as subduction takes place,water and sediment are forced down to regions of higher tem-perature. Partial melting of materials, in which the silica-richportion of rock and sediment melts first, produces viscousmagma. This produces volcanoes formed from alternatingexplosive events that produce pyroclastic materials, and lavaflows. These composite volcanoes, composed of alternatinglayers, are large, often thousands of meters high and tens ofkilometers across the base. Figure 26 shows all three types.

Figure 26 Shield volcanoes,like Mauna Loa, have createdsome of the largest mountainson Earth. Cinder cones, like SunsetCrater, are the smallest. Some ofthe famous volcanoes are thecomposites, like Mount Rainier.

Mauna Loa, Hawaii

Mount Rainier, Washington

Sunset Crater, Arizona

9 km

3 km

0.3 km


To measure motion you have to have a startingand an ending point. You must also know thetime it took to get from start to end. Volcanicactivity associated with a hot spot beneathHawaii gives geologists exactly that.

Real-World ProblemHow can scientists show that Earth’s plates aremoving?

Goal■ Infer a rate of movement for the Hawaiian

Islands over a hot spot

Materialsruler calculatorscale map of the Hawaiian Islands

Procedure1. Make a data table like the one shown below.

2. Measure and record the distances betweenthe island sets in the data table. Use the mapscale to convert measurements to km.

3. Refer to average ages given for each islandon the map. Calculate and record the age

differences for each set of islands in the datatable. Use the hot spot beneath the island ofHawaii as a starting reference point.

4. Calculate the rate of motion in km/year.Assume that the hot spot is stationary andthat the Pacific plate is moving over it.

Conclude and Apply1. Evaluate how meaningful your calculated

rate numbers are. Determine a better rateunit and convert your km/year rates to thesenew units.

2. Infer why the rates are not consistent usingwhat you know about plate movement.

3. Describe the overall motion of the PacificPlate based on your data.

4. Observe a map of the Pacific Ocean andinfer the location of a divergent zone thatcould be “pushing” the Pacific plate.

A Case for PacificPlate Motion

Share your findings with the class anddiscuss alternative interpretations.

LAB 379

Distance/Time Data for Hawaiian Islands

From/To

Distance (km)

Time Rate (years) (km/year)

Hawaii to Maui 161

Maui to Molokai

Molokai to Oahu

Oahu to Kauai

0

50

100

150

NiihauKauai

5.3 maOahu

2.1 ma

Molokai1.75 ma

Hawaiipresent

Maui1 maLanai

200km

Understanding earthquakes begins with locating them. To determinethe epicenter of an earthquake, scientists use a method called trian-gulation. If you know the locations of three points on a map, you candetermine a fourth point Diagram 1 shows how triangulation works.

Real-World ProblemYou are on vacation in City A and experience an earthquake. The radiostations are broadcasting information about the earthquake. Yourhome is in City B. Is your home near the epicenter?

Procedure1. Draw a 20-cm � 15-cm rectangle on a piece of plain white paper.

Orient the rectangle so that the 20-cm edge is vertical. This rectan-gle will serve as your map.

2. Using a scale of 1 cm � 200 km, draw a distance scale just belowthe rectangle on the white paper. Place an arrow parallel to one ofthe 20-cm vertical edges and label it North.

3. Within the rectangle, place City A 400 km from the north edge and400 km from the west edge. Locate City B 800 km from the northedge and 800 km from the east edge; and City C 1,200 km from thesouth edge and 1,200 km from the west edge of the rectangle.Your map will look similar to Diagram 2.

4. The earthquake happened at 08:37:00 PST. Copy and completeTable A on your own paper. Subtract the P-wave and S-wavearrival times to find the time differences for cities B and C. Use thetravel time graph and the time differences to complete the lasttwo columns.

Table A Earthquake Arrival Time Datafor Cities A, B, and C

City P-wave S-wave Time Difference Distance to Distance

Arrival Arrival (min/sec) Epicenter on Map

A 08:40:00 08:43:00 3 min/0 sec

B 08:41:15 08:45:00

C 08:39:40 08:42:10

Goals■ Examine a table of

seismic wave velocities.■ Analyze data from the

table.■ Determine the loca-

tion of an epicenter.

Possible Materialsplain white papergraph papercompassmetric ruler

Safety Precaution

Earthquake! Earthquake!Where’s the earthquake?


5. Estimate the distance from each cityfrom Table B below.

6. Use the Distance on Map measurementfor City A to set the compass width,and then draw the distance circlearound City A on your map. Your circlemay go off the map, but don’t worry.The epicenter is somewhere on the arcyou can draw on the map. Repeat thisprocess for City B and City C.

Analyze Your Data1. Estimate the time difference for

P-waves and S-waves coming from2,500 km.

2. Estimate the distance for a P-waveand S-wave pair that measure a timedifference of 3.3 minutes.

3. Determine how much closer to theepicenter City A is than City B. City Areceived waves 3.0 minutes apart. City B received waves 4.2 minutes apart.

Conclude and Apply1. Explain the relationship between body wave travel time differences and the

distance to an earthquake epicenter.

2. Evaluate the potential danger to your home City B. How far is your city fromthe epicenter?

3. Infer the arrival time difference for S-waves and P-waves at or very close to the epicenter.

4. List the possible sources of error in the methodsyou used to determine an epicenter. How canyou minimize errors?

Describe the location of the epicenter interms of distances from a reference pointon your map. Compare your descriptionswith others.

LAB 381

Table B Seismic Wave Arrival Times

Point

Time difference between Distance (km) traveled P-waves and S-waves (min) by waves

M 1.3 500

O 2.2 1,000

P 3.6 2,000

Q 4.5 3,000

R 5.4 4,000

A B

C

A

B C

200 km

cm

Diagram 1 Diagram 2

In 1815, people worldwide noticed unusuallybrightly colored sunsets. Then in 1816, theweather in many parts of the world was

colder than normal. In North America andEurope, 1816 was known as “the year without asummer.” Parts of New England had damagingfrosts in July and August. The fantastic sunsetsand the cold summer resulted from the mas-sive eruption of Tambora, a volcano inIndonesia.

Giant Weather MakersCommunication was slow in the early nine-

teenth century, so scientists immediately didnot connect Tambora’s eruption to weatherchange. Today, many researchers study theeffects of volcanic eruptions on weather. WhenMount Pinatubo in the Philippines eruptedin June 1991, weather stations around theworld—and even in space—recorded itseffects.

Mt. Pinatubo’s eruption was smaller thanTambora’s but was the second largest erup-tion in over 100 years. It blew ash and gasesinto the stratosphere, where they were car-ried by wind around the world. Because thesuspended particles and droplets absorbedand reflected sunlight, average temperaturestemporarily dropped by about 0.5°C in manyplaces. Other weather-related affectsincluded the increased strength of hurri-

canes in the Atlantic and Pacific Oceans and aswell as flooding rains in the U.S. Midwest.

Looking Back at VolcanoesThe effect of volcanoes on climate is so

important that some scientists even study vol-canic eruptions from thousands of years ago.They compare the dates of these ancient erup-tions with records of unusual weather. It hasbeen found that crop failures and disease epi-demics often occurred soon after large eruptions.Because changes in weather can affect thegrowth of trees, the growth rings of ancient treesalso are evidence that support the data fromwritten records. These ancient records helpscientist predict the weather impacts of futurevolcanic eruptions.

1020 mm Optical Depth

<10 -3 <10 -2 <10 -1

Mt. PinatuboMt. PinatuboMt. Pinatubo

Experiment Lay two sheets of black construction paper on a sunnywindowsill and place a thermometer on each sheet. Tape a large pieceof gauze or cheesecloth to the window so that it shades one of thesheets. Record the change in temperature for one half hour.

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Volcano Weather

SCIENCE AND

HISTORYSCIENCE

CAN CHANGE THE COURSE OF HISTORY!

Volcano Weather

Roger Ressmeyer/CORBIS


Evolution of Earth’s Crust

1. Earth’s crust and uppermost mantle,together called the lithosphere, consist ofabout seven large and numerous smallerplates.

2. Plate motion over the asthenosphere isinfluenced by forces that include ridgepush, frictional drag, and slab pull.

3. Mantle convectiondrives the systemof plates. Thermalenergy sources forconvection includeheat left over fromEarth’s formation,and decay ofradioactiveisotopes.

4. Plates meet alongdivergent, conver-gent, and trans-form boundaries.

Earthquakes

1. Earthquakes are vibrations in Earth causedby the sudden release of energy.

2. During an earthquake caused by movementalong a fault, strain energy is released in theform of seismic waves.

3. Most earthquake activity is confined toregions near plate boundaries.

4. Short-term prediction of earthquakes is notyet possible, but earthquakes can be ratedby their intensities and magnitudes.

Earth’s Interior

1. The interior of Earth consists of four mainlayers: crust, mantle, inner and outer core.

2. The crust and mantle are largely composedof solid silicates. The asthenosphere is alayer of mantle in a semi-solid state.

3. The two innermost layers, the inner andouter core, are mainly composed of iron,nickel, oxygen and sulfur. The outer core isin a liquid state, and the inner core is solid.

4. The crust is the thinnest layer, while themantle is the thickest.

Volcanoes

1. Most volcanic activity on Earth occurs inthe vicinity of plate boundaries or abovemantle plumes.

2. The viscosity ofthe magmafeeding volca-noes controlsthe volcanoes’eruptivecharacteristics.

3. Major controlson magma vis-cosity are: temperature, chemicalcomposition, and gas content.

4. Cinder cones, shield volcanoes, andcomposite volcanoes are three principlevolcanic landforms.

CHAPTER STUDY GUIDE 383

Use the Foldable that you made at the begin-ning of this chapter to help you review Earth’s internal process.

Interactive Tutor gpescience.com

0 3,000

NorthAmerica

SouthAmerica

Greenland Norway

GreatBritain

Africa

Cape FoldBelt

km

Appalachians

Gary Braasch/CORBIS


Complete each sentence with the correct vocabu-lary word or words.

1. The is the point of origin of anearthquake.

2. A zone of cracking in Earth’s crust alongwhich movement takes place is a .

3. A long, linear feature within a divergentplate boundary is a(n) .

4. A feature consisting of a relatively small pileof pyroclastic materials is a(n) .

5. A boundary marking an abrupt change indensity is a(n) .

Choose the word or phrase that best answers thequestion.

6. What process causes a material to breakdue to excess stress?A) plastic deformation.B) elastic rebound.C) elastic deformation.D) brittle deformation.

7. Which best identifies a shield volcano?A) sticky, silica-rich magmasB) great height compared to widthC) forms above hot spotsD) found mostly on continents

8. What characteristic was first used toidentify Earth’s layers?A) temperature. C) density.B) composition. D) thickness.

9. Which pair of plate tectonic boundaries isbest characterized by mostly shallow-focusearthquakes?A) divergent and transformB) divergent and continent-ocean

convergentC) continent-continent and continent-

ocean convergentD) transform and ocean-ocean convergent

zones

10. Which feature is common to and onlyfound in diverging regions?A) trenches C) volcanic arcsB) rift valleys D) island arcs

11. Which is NOT evidence used by Wegenerto support the continental drift hypothesis?A) matching magnetic patterns symmetrical

to the Mid-Ocean RidgeB) matching of continental marginsC) correlation of fossils among the

continentsD) mountain-range matching among the

continents

Use the illustration below to answer question 12.

12. Which volcano type has small height,small diameter and consists mostly ofpyroclasts?A) Hawaiian volcanoB) cinder cone volcanoC) composite volcanoD) shield volcano

384 CHAPTER REVIEW

asthenosphere p. 372cinder cone volcano

p. 377composite volcano p. 378convergent boundary

p. 359discontinuity p. 371divergent boundary

p. 358elastic rebound p. 364epicenter p. 364

fault p. 364focus p. 364mid-ocean ridge p. 356rift valley p. 356shadow zone p. 371shield volcano p. 377subduction p. 359transform boundary

p. 360viscosity p. 375

Vocabulary PuzzleMaker gpescience.com


CHAPTER REVIEW 385

13. Which earthquake waves travel throughmatter with a push-pull motion?A) secondary wavesB) surface wavesC) primary wavesD) body waves

14. Copy and complete the concept mapbelow summarizing characteristics ofdivergent, convergent and transform plateboundaries.

15. Copy and complete the table summarizingthree types of volcanoes.

16. Explain why the island of Kauai, oftenreferred to as the “Garden Isle,” has thickersoils and is better able to sustain agricul-ture than other Hawaiian Islands.

17. Infer what general depth of focus earth-quakes are likely to occur in the Himalaya.

18. Explain how the processes associated withplate tectonics maintain Earth’s recyclingof materials.

19. Calculating Lava Thickness Suppose thathot spot volcanism produces an aver-age of 76,000 m3 of lava per day anddoes so for 340 days. This lava flowsacross a region that has an area of10 km2. How thick will the resultinglava flow be?

20. Located on the island of Hawaii,Mauna Loa is the largest volcano onEarth and rises 17 km above its base.Using the thickness you found in ques-tion 19 as the annual thickness, howmany years did it take for Mauna Loato build from the sea bed?

21. Mauna Loa rises 4 km above sea level.How many years did it take to growfrom sea level to its present height?

22. An active volcano called Loihi lies justoff the coast of the island of Hawaii. Itis called a seamount because it still liesbelow the surface. If it grows at thesame rate you used in question 21 andlies 1250 m below the surface, how longwill it take for Loihi to break the oceansurface?

More Chapter Review gpescience.com

Interpreting Graphics

Magma Properties and Volcano Types

General Relative silica Relative Volcano Composition (SiO2) Content Viscosity Type

lowest

Andesitic intermediate

highest Lava Dome

Boundary

Tran

sfor

m

Conv

erge

nt

Dive

rgen

t


1. Who first proposed the hypothesis ofcontinental drift?

A. Matthews

B. Vine

C. Wegener

D. Wilson

Use the image below to answer question 2.

2. Which is the fern, pictured above, thatprovided support for Pangaea?

A. Antarctica

B. Glossopteris

C. Mesosaurus

D. Lystrosaurus

3. What did oceanographers show, usingsonar, which helped revive continental drift?

A. edges of the continental shelves matched

B. edges of the continents matched

C. fossils match similar fossils

D. rocks match similar rocks

4. Which forms when a less-dense plateconverges on a denser plate?

A. mid-ocean ridge

B. rift valley

C. subduction zone

D. transform boundary


5. The shadow zone, illustrated above, iscaused by which layer of Earth?

A. crust

B. mantle

C. outer core

D. inner core

CynognathusMesosaurus

Lystrosaurus

Glossopteris

Africa

Australia

India

Antarctica

SouthAmerica

386 STANDARDIZED TEST PRACTICE

For each question, double-check that you are filling in thecorrect answer bubble for the question number you arecompleting.

Record your answers on the answer sheet provided by your teacher or on a sheet of paper.

STANDARDIZED TEST PRACTICE 387Standardized Test Practice gpescience.com

6. If a divergent boundary separates at a rateof 2.5 cm/year, how much farther apartwould the two plates be after 230 years?

7. Compared to the two plates in question 7,how much farther apart would two platesbe after 230 years if the divergent boundarybetween them separated at 15 cm/year?

Use the illustration below to answer question 8.

8. The illustration shows reverse polarity bandson the seafloor. How does magnetic polarityof rocks support seafloor spreading?

9. What are the three different ways in whichtectonic plates can move?

10. What is different about what happensto plate edges at the different types ofconvergent boundaries?

11. How did Dr. Harry Hess obtain detailedmaps of the ocean floor?

12. What causes a rift valley to form atdivergent boundaries?

13. What is true about the thickness andage of sediments at different locationson the ocean floor that supports seafloorspreading?

14. What is the driving force for all mecha-nisms of tectonic plate movement?

15. What is the difference between a crack inEarth’s crust and a fault?

16. What causes the energy of an earthquake?

17. How are the focus and epicenter of anearthquake related?


18. Part A What is true about the depth ofearthquake foci and the locationof tectonic plate boundaries?

Part B Why are deep-focus earthquakesnoted only at ocean-oceanconvergent boundaries and ocean-continental convergent boundaries?

Divergentboundary Divergent

boundary

Convergentboundary


earth’s internal processes - wikispaces · seafloor spreading hypothesis ... section 1 evolution...

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