10.3 plate tectonics and igneous activity...

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10.3 Plate Tectonics and Igneous Activity Reading Strategy Outlining After you read, make an outline of the most important ideas in the section. Key Concepts What is the relationship between plate boundaries and igneous activity? Where does intraplate volcanism occur? Vocabulary intraplate volcanism More than 800 active volcanoes have been identified worldwide. Most of them are located along the margins of the ocean basins, mainly within the circum-Pacific belt known as the Ring of Fire. A second group of volcanoes is found in the deep-ocean basins, including on Hawaii and Iceland. A third group includes volcanic structures that are irregularly distributed in the interiors of the continents. Until the late 1960s, geologists had no explanation for the distribution of volcanoes. With the development of the theory of plate tectonics, the picture became clearer. Convergent Plate Boundaries The basic connection between plate tectonics and volcanism is that plate motions provide the mechanisms by which mantle rocks melt to gen- erate magma. At convergent plate boundaries, slabs of oceanic crust are pushed down into the mantle. As a slab sinks deeper into the mantle, the increase in temperature and pressure drives water from the oceanic crust. Once the sinking slab reaches a depth of about 100 to 150 kilometers, the fluids reduce the melting point of hot mantle rock enough for melt- ing to begin. The magma formed slowly migrates upward forming volcanoes such as Mount St. Helens shown here. As you read about the relation- ships between plate tectonics and igneous activity, refer to Figure 17 on pages 296–297, which sum- marizes the relationships. I. Plate Tectonics and Igneous Activity A. Convergent Plate Boundaries 1. 2. ? ? Volcanoes and Other Igneous Activity 293 Figure 14 Convergent Boundary Volcano Mount St. Helens emitting volcanic ash on July 22, 1980, two months after the huge May eruption. Mount St. Helens is located at a convergent boundary between the Juan de Fuca plate and the North American plate. FOCUS Section Objectives 10.8 Explain the relationship between plate tectonics and volcanism. 10.9 Explain where intraplate volcanism occurs. Build Vocabulary Definitions Have students write a definition for continental volcanic arc and intraplate volcanism in their own words. After students read the section, ask them to draw a diagram that illustrates the definitions. Reading Strategy 1. Ocean-Ocean 2. Ocean-Continent B. Divergent Boundaries C. Intraplate Igneous Activity INSTRUCT Convergent Plate Boundaries Build Science Skills Interpreting Diagrams/ Photographs In the caption for Figure 14, it states that Mount St. Helens is located on the convergent boundary of the Juan de Fuca plate and the North American plate. Have students find these plates on a map of Earth’s plates. Ask: How do the sizes of the two plates compare? (The Juan de Fuca plate is much smaller than the North American plate.) Which plate is subducting? (the Juan de Fuca plate) Verbal L2 2 L2 L2 Reading Focus 1 Volcanoes and Other Igneous Activity 293 Section 10.3

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10.3 Plate Tectonics andIgneous Activity

Reading StrategyOutlining After you read, make an outline ofthe most important ideas in the section.

Key ConceptsWhat is the relationshipbetween plate boundariesand igneous activity?

Where does intraplatevolcanism occur?

Vocabulary◆ intraplate

volcanism

More than 800 active volcanoes have been identified worldwide.Most of them are located along the margins of the ocean basins, mainlywithin the circum-Pacific belt known as the Ring of Fire. A secondgroup of volcanoes is found in the deep-ocean basins, including onHawaii and Iceland. A third group includes volcanic structures that areirregularly distributed in the interiors of the continents. Until the late1960s, geologists had no explanation for the distribution of volcanoes.With the development of the theory of plate tectonics, the picturebecame clearer.

Convergent Plate BoundariesThe basic connection between plate tectonics

and volcanism is that plate motions provide themechanisms by which mantle rocks melt to gen-erate magma. At convergent plate boundaries, slabsof oceanic crust are pushed down into the mantle.As a slab sinks deeper into the mantle, the increasein temperature and pressure drives water from theoceanic crust. Once the sinking slab reaches a depthof about 100 to 150 kilometers, the fluids reduce themelting point of hot mantle rock enough for melt-ing to begin. The magma formed slowly migratesupward forming volcanoes such as Mount St.Helens shown here. As you read about the relation-ships between plate tectonics and igneous activity,refer to Figure 17 on pages 296–297, which sum-marizes the relationships.

I. Plate Tectonics and Igneous ActivityA. Convergent Plate Boundaries

1.

2. ?

?

Volcanoes and Other Igneous Activity 293

Figure 14 ConvergentBoundary Volcano Mount St.Helens emitting volcanic ash onJuly 22, 1980, two months afterthe huge May eruption. Mount St.Helens is located at a convergentboundary between the Juan deFuca plate and the NorthAmerican plate.

FOCUS

Section Objectives10.8 Explain the relationship

between plate tectonicsand volcanism.

10.9 Explain where intraplatevolcanism occurs.

Build VocabularyDefinitions Have students write adefinition for continental volcanic arc andintraplate volcanism in their own words.After students read the section, ask themto draw a diagram that illustrates thedefinitions.

Reading Strategy1. Ocean-Ocean2. Ocean-ContinentB. Divergent BoundariesC. Intraplate Igneous Activity

INSTRUCT

Convergent PlateBoundariesBuild Science SkillsInterpreting Diagrams/Photographs In the caption forFigure 14, it states that Mount St.Helens is located on the convergentboundary of the Juan de Fuca plateand the North American plate. Havestudents find these plates on a map ofEarth’s plates. Ask: How do the sizes ofthe two plates compare? (The Juan deFuca plate is much smaller than theNorth American plate.) Which plate issubducting? (the Juan de Fuca plate)Verbal

L2

2

L2

L2

Reading Focus

1

Volcanoes and Other Igneous Activity 293

Section 10.3

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Bezymianny

KrakatoaTambora

Mt. Mayon

Fujiyama

Mariana Is.

Mauna Loa

Tonga Is.

PavlofShishaldin

Katmai(“Valley of

10,000 Smokes”)

Kilauea

Parícutin

Popocatepetl

Galapagos Is.

Cotopaxi

Easter Is.

Surtsey

HeklaLaki

Canary Is.Pelée

South Sandwich Is.

Deception Is.

VesuviusMt. St. Helens

Misti

Etna

Santorini

Kilimanjaro

PinatuboMt. Unzen

Nevado del Ruiz

294 Chapter 10

Ocean-Ocean Volcanism at a convergent plate where one oceanicslab descends beneath another results in the formation of a chain ofvolcanoes on the ocean floor. Eventually, these volcanic structures growlarge enough to rise above the surface and are called volcanic islandarcs. Several volcanic island arcs border the Pacific basin, including theAleutians.

Ocean-Continent Volcanism associated with convergent plateboundaries may also develop where slabs of oceanic lithosphere aresubducted under continental lithosphere to produce a continental vol-canic arc. The mechanisms are basically the same as those at islandarcs. The major difference is that continental crust is much thicker andis composed of rocks with a higher silica content than oceanic crust.As the silica-rich crustal rocks melt, the magma may change compo-sition as it rises through continental crust. The volcanoes of the AndesMountains along the western edge of South America are an example ofa continental volcanic arc, as shown in Figure 15.

Divergent Plate BoundariesMost magma is produced along the oceanic ridges during seafloorspreading. Below the ridge axis where the plates are being pulled apart,the solid yet mobile mantle rises upward to fill in the rift where theplates have separated. As rock rises, confining pressure decreases. Therock undergoes decompression melting, producing large amounts ofmagma. This newly formed basaltic magma is less dense than themantle rock from which it was formed, so it buoyantly rises.

Partial melting of mantle rock at spreading centers producesbasaltic magma. Although most spreading centers are located alongthe axis of an oceanic ridge, some are not. The East African Rift inAfrica is a site where continental crust is being rifted apart.

Major Volcanoes

Figure 15

Location Note theconcentration of volcanoesencircling the Pacific basin,known as the Ring of Fire.Inferring How are thevolcanoes in the middle of theAtlantic Ocean related to aplate boundary?

For: Links on predicting volcanicactivity

Visit: www.SciLinks.org

Web Code: cjn-3103

294 Chapter 10

Section 10.3 (continued)

Customize for English Language Learners

Select and copy an appropriate paragraphfrom one of the sections, such as the lastparagraph on p. 293. Leave the first and lastsentences intact, since they are usually theintroductory and concluding sentences. Forthe sentences in the middle, remove key wordsand replace them with a blank. For example,

leave blanks for convergent in the secondsentence of this paragraph, mantle in the thirdsentence, and the last use of melting in thefourth sentence. Have students read theparagraph and fill in the blanks with theappropriate words.

Divergent PlateBoundaries

Observing PlateMovementPurpose Students will observeconvergent plate movements.

Materials 9 student textbooks,2 pieces of poster board, thin cardboardor 1-cm stack of notebook paper

Procedure Stack eight textbooks intwo equal stacks. Leave about 5 cmbetween the textbook stacks. Theremaining textbook will represent acontinental crustal plate. The posterboard will represent the subductingoceanic lithosphere. Give the posterboard a slight curve so that it willsubduct downward. Place the textbookon one of the stacks and the posterboard on the other stack. Ask: What doyou predict will happen when thesetwo plates collide? (The less rigid platewill subduct under the rigid plate.) Startmoving the “plates” toward each other.The oceanic lithosphere should subductunder the continental plate. Repeat thisprocedure using two pieces of posterboard. Before moving the plates together,ask: What do you predict will happenwhen these two plates collide? (Thetwo slabs of crust will form a trench asthey descend into the mantle.) One pieceof poster board needs to be curved so itwill form a trench as it subducts.

Expected Outcomes Students willobserve how the oceanic lithospheresubducts under the crustal plate andhow two oceanic plates form a trench.Visual, Kinesthetic

Answer

Inferring They occur at divergentboundaries for continental plates and atocean ridges for oceanic plates.

L2

Download a worksheet on volcanicactivity for students to complete,and find additional teacher supportfrom NSTA SciLinks.

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Section 10.3 Assessment

Reviewing Concepts1. How are the locations of volcanoes related

to plate boundaries?

2. What causes intraplate volcanism?

3. Where is most of the magma produced onEarth on a yearly basis?

4. What is the Ring of Fire?

Critical Thinking5. Comparing and Contrasting What are the

differences between volcanic island arcs andcontinental volcanic arcs?

6. Predicting Would it be more likely for amajor explosive eruption to occur at an oceanridge or at a convergent ocean-continentalboundary? Explain your answer.

Intraplate Igneous ActivityKilauea is Earth’s most active volcano, but it is in the middle ofthe Pacific plate, thousands of kilometers from a plate bound-ary. Intraplate volcanism occurs within a plate, not at a plateboundary. Another site of intraplate volcanism is YellowstoneNational Park.

Most intraplate volcanism occurs where a mass ofhotter than normal mantle material called a mantle plumerises toward the surface. Most mantle plumes appear to formdeep within Earth at the core-mantle boundary. These plumesof hot mantle rock rise toward the surface in a way similar to theblobs that form within a lava lamp. Once the plume nears thetop of the mantle, decompression melting forms basalticmagma. The result may be a small volcanic region a few hun-dred kilometers across called a hot spot. More than 40 hot spotshave been identified, and most have lasted for millions of years.By measuring the heat flow at hot spots, geologists found thatthe mantle beneath some hot spots may be 100–150°C hotterthan normal.

The volcanic activity on the island of Hawaii, shown in Figure 16,is the result of a hot spot. Where a mantle plume has persisted for longperiods of time, a chain of volcanoes may form as the overlying platemoves over it. Mantle plumes are also thought to cause the vast out-pourings of lava that create large lava plateaus such as the ColumbiaPlateau in the northwestern United States.

Volcanoes and Other Igneous Activity 295

Explanatory Paragraph Write a para-graph to explain how magma is formed inthe crust without adding heat.

Figure 16 Intraplate VolcanoAn eruption of Hawaii’s Kilaueavolcano. The Hawaiian hot spotactivity is currently centeredbeneath Kilauea and is anexample of intraplate volcanicactivity.

Intraplate IgneousActivityBuild Reading LiteracyRefer to p. 186D in Chapter 7, whichprovides the guidelines for relating textand visuals.

Relate Text and Visuals Havestudents compare the drawings of theplates and volcanic activity in Figures 15and 16 to the text explanations in thissection.Visual

ASSESSEvaluateUnderstandingHave students create a ten-questioncrossword puzzle or word scrambleusing the concepts from this section.Have students exchange papers andwork the puzzles.

ReteachUse Figure 15 to reteach the concepts inthis section.

Magma can form by decompressionmelting if the rock begins to rise and thepressure decreases. This causes thetemperature at which melting occurs todecrease. If water is added, thetemperature at which the rock meltsdecreases. A body of hotter rock mayrise and trigger melting in the crust.

L1

L2

3

L1

Volcanoes and Other Igneous Activity 295

3. Most of the magma produced each yearon Earth is produced at ocean ridges duringseafloor spreading.4. The Ring of Fire is a chain of volcanoesthat are located around the edge of thePacific Basin.5. A volcanic island arc is formed when twooceanic plates converge and form a subduc-tion zone. The magma produced is ofbasaltic composition. A continental volcanicarc is formed by subduction of an ocean

Section 10.3 Assessment

1. Most volcanoes are located at either diver-gent or convergent plate boundaries, whereplate motions provide the mechanisms toform magma.2. Intraplate volcanism is caused by hotmantle plumes rising up from the core-mantle boundary, causing decompressionmelting and forming small areas of volcanicactivity on the surface.

plate beneath a continental plate. Themagma produced is more silica rich thanthat formed at a volcanic island arc.6. An explosive eruption would be more likelyat a convergent ocean-continental boundary,because the magma produced is more silicarich, more viscous, and contains more water.

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296 Chapter 10

Hot spotHawaii

Oceaniccrust

Decompressionmelting

Risingmantleplume

Continentalcrust

Mantle rockmelts

Continentalvolcanic arc

Trench

Oceaniccrust

Continentalcrust

Mantle rockmelts

Marginalsea

Volcanicisland arc

Trench

Asthenosphere

Oceaniccrust

Water drivenfrom plate

Convergent platevolcanism

Convergent platevolcanism

Water drivenfrom plate

Intraplate volcanism

Subducting oceanic lithosphere

Subducting oceanic lithosphere

Three Zones of Volcanism

Figure 17

Regions The three zones of volcanism areconvergent plate volcanism,divergent plate volcanism,and intraplate volcanism.Two of these zones areplate boundaries, and thethird is the interior area ofthe plates. Drawing Conclusions Inwhich zones do volcanoesoccur on both continentalplates and oceanic plates?

296 Chapter 10

AnswerDrawing Conclusions Volcanoesoccur on both continental and oceanicplates in all the zones—convergent platevolcanism, divergent plate volcanism,and intraplate volcanism.

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Volcanoes and Other Igneous Activity 297

Hot spot

Continentalcrust

Decompressionmelting

Risingmantleplume

Continentalcrust

Riftvalley

Magmachamber

Asthenosphere

Oceaniccrust

Divergent platevolcanism

Divergentplate volcanism

Intraplate volcanism

Decompressionmelting

Floodbasalts

Decompressionmelting

Volcanoes and Other Igneous Activity 297

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How the Earth WorksEffects of VolcanoesA volcano is an opening in the Earth’s crust from which lava,or molten rock, escapes to the surface. The impact of powerfulvolcanic eruptions is both immediate and long-lasting. Burningrocks are flung out in all directions. Huge clouds of scorchingash and fiery gases billow high into the sky. As a result, the land-scape and even the weather can be changed. Soil may becomemore fertile when enriched with nutrients from volcanic ash.Islands, mountains, and other landforms may be created fromthe material emitted by volcanoes.

The Devil’s Towerin Wyoming

DUST AND GASExplosive volcanoes,like Mount St. Helensin Washington (right),spit clouds of ash andfumes into the sky.The debris can com-pletely cover humancommunities.Another hazardis that volcanicgases may bedeadlypoisons.

ERUPTING LAVARed-hot lava is hurled into the air during an eruption of a volcanoon Stromboli, an island off the coast of southern Italy. The Strombolivolcano is one of only a few volcanoes to display continuous eruptiveactivity over a period of more than a few years.

DRAMATIC ROCK FORMATIONSLava flows can form amazing rock forma-tions. Columnar rocks are volcanic rocksthat split into columns as the lava cools.The Devil’s Tower in Wyoming (below) isone example of a columnar rock. Anotherexample is the Giant’s Causeway (left). Thisrock formation in Northern Ireland is theresult of a lava flow that erupted millionsof years ago.

The Giant’s Causewayin Northern Ireland

298 Chapter 10

298 Chapter 10

FOCUS

ObjectivesIn this feature, students will• explain what a volcano is.• describe the immediate effects of a

volcanic eruption.• identify some long-term effects of a

volcanic eruption.

Build VocabularyClassify Terms Draw a four-columnchart on the board. Label the columnsas follows: Volcanic Effect, Definition ofEffect, Immediate or Long-Term Effect, andLocal or Worldwide Effect. Have studentsuse information on these two pages tocomplete the chart.

INSTRUCTUse VisualsAsk students to read the captions on thispage and the next. Have them make alist of places in the United States and inother countries where volcanoes are orhave been active.Visual

BellringerHave students list ten effects of avolcanic eruption. Examples may includeclouds of smoke, lava trails, and ascorched landscape.Logical

L2

L22

L2

Reading Focus

1

The ancient Roman city of Pompeii wasencased in lava when Mount Vesuvius eruptedin A.D. 79. The volcano destroyed the city, andmost people were buried in ash and lava. Rainhardened the ash, forming perfect molds ofpeople and preserving articles of everyday life.Pompeii’s ruins were first discovered in the latesixteenth century. Since 1748, archaeologists

have excavated materials that provide adetailed picture of life in a busy Romanport town. In addition to houses, bakeries,restaurants, and factories, scholars haveuncovered inscriptions on buildings, tombs,and statues. Even the graffiti on Pompeii’swalls gives us clues about the values andconcerns of this ancient society.

Facts and Figures

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1. Key Terms Define (a) volcano, (b) lava, (c) columnar rock, (d) plume,(e) crater lake.

2. Natural Resources How can soilbecome more fertile as a result of volcanic eruptions?

3. Environmental Change (a) How canvolcanic activity create new landforms?(b) How can explosive volcanic erup-tions affect the atmosphere and weath-er around the world?

4. Natural Hazards What are some of theways in which a volcanic eruption candevastate nearby human settlements?

5. Critical Thinking SequencingStudy the diagram of the HawaiianIslands and the caption that accompa-nies it. (a) Which island on the diagramis probably the oldest? Why do youthink so? (b) What will happen to thevolcanoes on the island of Hawaii as aresult of plate movement?

A few lichens find ahome on the lava.

Plants take root in thebeginnings of topsoil.

LIFE RETURNS TO THE LAVAIn time, plant life grows on lava. Lichen and moss often

appear first. Grass and larger plants slowly follow. The uppersurface of the rock is gradually weathered, and the roots of

plants help break down the rock to form soil. After manygenerations, the land may become lush and fertile again.

A crater lake in Iceland

A STRING OF ISLANDSThe Hawaiian Islands are the tops of

volcanic mountains. They have developedover millions of years as a plume, or a veryhot spot in the Earth’s mantle, erupted greatamounts of lava. As the Pacific Plate moves

over the stationary plume, it carries olderislands in the chain to the northwest. Today,

active volcanoes are found on the island ofHawaii and the newly forming island of Loihi.

Plate moving across plume

Magmachamber

Stationaryplume

Volcanic pipe

KauaiOahu Maui Hawaii

A satellite image shows the global spread of emissions from the 1991eruptions of Mount Pinatubo in the Philippines.

AFFECTING THE WORLD’S WEATHERPowerful eruptions emit gas and dust that can rise high into the atmosphere andtravel around the world. Volcanic material can reduce average temperatures inparts of the world by filtering out some of the sunlight that warms the Earth.

A CRATER LAKEA crater lake is a body of waterthat occupies a bowl-shapeddepression around the openingof an extinct or dormant volcano.An eruption can hurl the waterout of the crater. The water canthen mix with hot rock anddebris and race downhill in adeadly mudslide.

299

ASSESSEvaluateUnderstandingHave students review the information inthe charts they have created. Ask: Whatare some positive effects of volcanoes?(They create islands, fertilize soil, andcreate beautiful rock formations.)

ReteachHave students compare and contrast theformation and eruptions of Mount St.Helens shown in the photograph withthe Hawaiian Islands shown in thediagram.

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L2

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Volcanoes and Other Igneous Activity 299

(b) Gas and dust from an eruption may risehigh into the atmosphere, travel around theworld, and filter out sunlight.4. Volcanic debris can completely coverhuman communities, and volcanic gases aredeadly poisons.5. (a) Kauai is the oldest island because ofthe direction in which the plate is moving.(b) The volcanoes on the island of Hawaii willbecome extinct as plate movement causes theisland to move away from the stationaryplume.

Assessment

1. (a) an opening in Earth’s crust from whichlava escapes to the surface; (b) molten rock;(c) volcanic rocks that split into columns asthe lava cools; (d) a very hot spot in Earth’smantle; (e) a body of water that occupies abowl-shaped depression around the openingof an extinct or dormant volcano2. Soil becomes enriched with nutrients fromvolcanic ash.3. (a) Underwater plumes erupt great amountsof lava over millions of years, building thetops of underwater volcanic mountains.

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