Section 10.1 10.1 The Nature of Volcanic Eruptions 1 FOCUSmrbakerearth.weebly.com/uploads/3/0/8/0/30809369/ch10_book__es_.… · the section, rewrite the green topic headings as questions

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Volcanic eruptions are more than spectacular sights. They are win-dows to Earth’s interior. Because volcanoes eject molten rock thatformed at great depth, they provide opportunities to observe theprocesses that occur deep beneath Earth’s surface.

On May 18, 1980, one of the largest volcanic eruptions to occur inNorth America changed a scenic volcano into the smoldering wreckshown in Figure 1. On this date, Mount St. Helens erupted with tremen-dous force. The blast blew out the entire north flank of the volcano,leaving a gaping hole. The eruption ejected nearly a cubic kilometer of ashand rock debris. The air over Yakima, Washington, 130 kilometers to theeast, was so filled with ash that noon became almost as dark as midnight.Why do volcanoes like Mount St. Helens erupt explosively, while otherslike Kilauea in Hawaii are relatively quiet?

10.1 The Nature ofVolcanic Eruptions

Key ConceptsWhat determines the typeof volcanic eruption?

What materials are ejectedfrom volcanoes?

What are the three maintypes of volcanoes?

What other landforms areassociated with volcaniceruptions?

Reading StrategyPreviewing Copy the table. Before you readthe section, rewrite the green topic headingsas questions. As you read, write the answers tothe questions.

Vocabulary◆ viscosity◆ vent◆ pyroclastic material◆ volcano◆ crater◆ shield volcano◆ cinder cone◆ composite cone◆ caldera

The Nature of Volcanic Eruptions

What factors affect a.an eruption?

?

Figure 1 A Mount St. Helensbefore the May 18, 1980,eruption. B After the eruption,Spirit Lake filled with debris.

A B

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FOCUS

Section Objectives10.1 Explain the factors that

determine the type of volcaniceruptions that occur.

10.2 Describe the various types ofvolcanic materials that areejected from volcanoes.

10.3 List the three main types ofvolcanoes.

10.4 Distinguish how the differenttypes of volcanic landformsform.

Build VocabularyWord Parts Explain to students thatthe prefix pyro- is Latin and Greek for“fire” or “heat.” Clastic means “madefrom fragments of preexisting rocks.”Pyroclastic materials are hot fragmentsof preexisting rocks that are blown fromthe vent of a volcano.

Reading Strategya. viscosity and dissolved gasesb. What are the types of volcanicmaterials? lava flows; pyroclasticmaterial, such as ash; volcanic gasesc. What are the types of volcanoes?shield volcanoes, cinder cones,composite conesd. What are some other volcaniclandforms? calderas, pipes, lava plateaus

INSTRUCTUse VisualsFigure 1 During the eruption of MountSt. Helens, the height of the volcanowas lowered by 400 meters. Ask: Whatwould have caused this damage?(Force built up within the volcano andblew the top off.) Infer where the debrisfrom this blast went. (Some of the finedebris particles remained in the air for atime before settling; some of the materialflowed down the side of the volcano in theform of mud; some of the material simplytumbled down the side of the volcano.)Verbal

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Reading Focus

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Basaltic Least (~50%) Least Least (1–2%) Least Shield VolcanoesBasalt PlateausCinder Cones

Andesitic Intermediate Intermediate Intermediate Intermediate Composite Cones (~60%) (3–4%)

Rhyolitic Most (~70%) Greatest Most (4–6%) Greatest Pyroclastic FlowsVolcanic Domes

Tendency to FormComposition Silica Content Viscosity Gas Content Pyroclastics Volcanic Landform

(ejected rock fragments)

Table 1 Magma Composition

Why are somevolcanoesexplosive?

Procedure1. Obtain two bottles of

noncarbonated water andtwo bottles of club soda.

2. Open one bottle of thenoncarbonated water andone bottle of the clubsoda. Record yourobservations.

3. Gently shake each of theremaining unopenedbottles. CAUTION: Wearsafety goggles and pointthe bottles away fromeveryone.

4. Carefully open eachbottle over a sink oroutside. Record yourobservations.

Analyze andConclude1. Observing What

happened when thebottles were opened?

2. Inferring Which bottlerepresents lava with themost dissolved gas?

Volcanoes and Other Igneous Activity 281

Factors Affecting EruptionsThe primary factors that determine whether a volcano erupts

violently or quietly include magma composition, magma tempera-ture, and the amount of dissolved gases in the magma.

Viscosity Viscosity is a substance’s resistance to flow. For example,maple syrup is more viscous than water and flows more slowly. Magmafrom an explosive eruption may be thousands of times more viscousthan magma that is extruded quietly.

The effect of temperature on viscosity is easy to see. If you heatmaple syrup, it becomes more fluid and less viscous. In the same way,the mobility of lava is strongly affected by temperature. As a lava flowcools and begins to harden, its viscosity increases, its mobilitydecreases, and eventually the flow halts.

The chemical composition of magmas has a more important effecton the type of eruption. The viscosity of magma is directly related toits silica content. In general, the more silica in magma, the greater is itsviscosity. Because of their high silica content, rhyolitic lavas are veryviscous and don’t flow easily. Basaltic lavas, which contain less silica,tend to be more fluid.

Dissolved Gases During explosive eruptions, the gases trapped inmagma provide the force to eject molten rock from the vent, an open-ing to the surface. These gases are mostly water vapor and carbondioxide. As magma moves nearer the surface, the pressure in the upperpart of the magma is greatly reduced. The reduced pressure allows dis-solved gases to be released suddenly.

Very fluid basaltic magmas allow the expanding gases to bubbleupward and escape relatively easily. Therefore, eruptions of fluidbasaltic lavas, such as those that occur in Hawaii, are relatively quiet.At the other extreme, highly viscous magmas slow the upward move-ment of expanding gases. The gases collect in bubbles and pockets thatincrease in size until they explosively eject the molten rock from thevolcano. The result is a Mount St. Helens.

Factors AffectingEruptions

Why are somevolcanoes explosive?ObjectiveAfter completing this activity, studentswill be able to explain why trappedgases cause explosive reactions involcanoes.

Skills Focus Observing, Inferring,Predicting

Prep Time 5 minutes

Materials 2 bottles of noncarbonatedwater, 2 bottles of club soda, papertowels

Class Time 20 minutes

Safety Be sure that students point theopen bottles away from everyone.

Teaching Tip Have paper towelsavailable for students to use to cleanup after the lab.

Expected Outcome Students willobserve dissolved gases and fluid“explode” from the bottle ofcarbonated liquid.Kinesthetic, Logical

Analyze and Conclude1. Answers may vary but should statethat the bottles with non-carbonatedwater opened without any escapinggases or fizzing. The bottles of clubsoda, when opened, fizzed with theescaping carbon dioxide. After the clubsoda was shaken, the gases escapedmore explosively.2. the shaken bottle of club soda

For EnrichmentHave students research the violenteruption of Krakatau in 1883. Havestudents prepare a newspaper articledetailing the events surrounding theeruption of this volcano. The articleshould be written as if the volcano haderupted recently.

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Customize for English Language Learners

Have students work in pairs to make a chartshowing the facts about factors affectingeruptions, volcanic material, types ofvolcanoes, and other volcanic landforms.

Students may want to illustrate their facts withdrawings to further their understanding of theconcepts. Students can use this chart as astudy aid for quizzes and tests.


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Volcanic MaterialLava may appear to be the main material extruded from a volcano, butthis is not always the case. Just as often, explosive eruptions eject hugequantities of broken rock, lava bombs, fine ash, and dust. All volcaniceruptions also emit large amounts of gas.

Lava Flows Hot basaltic lavas are usually very fluid because of theirlow silica content. Flow rates of 10 to 300 meters per hour arecommon. In contrast, the movement of silica-rich (rhyolitic) lava isoften too slow to be visible. When fluid basaltic lavas harden, they com-monly form a relatively smooth skin that wrinkles as the still-moltensubsurface lava continues to move. These are known as pahoehoe(pah HOH ee hoh ee) flows and resemble the twisted braids in ropes,as shown in Figure 2. Another common type of basaltic lava called aa(AH ah) has a surface of rough, jagged blocks with dangerously sharpedges and spiny projections.

Gases Magmas contain varied amounts of dissolved gases held inthe molten rock by confining pressure, just as carbon dioxide is held insoft drinks. As with soft drinks, as soon as the pressure is reduced, thegases begin to escape. The gaseous portion of most magmas is onlyabout 1 to 6 percent of the total weight. The percentage may be small,but the actual quantity of emitted gas can exceed thousands of tonseach day. Samples taken during a Hawaiian eruption consisted of about70 percent water vapor, 15 percent carbon dioxide, 5 percent nitrogen,5 percent sulfur, and lesser amounts of chlorine, hydrogen, and argon.Sulfur compounds are easily recognized because they smell like rotteneggs and readily form sulfuric acid, a natural source of air pollution.The composition of volcanic gases is important because they have con-tributed greatly to the gases that make up the atmosphere.

Figure 2 Lava Flows A Typicalpahoehoe (ropy) lava flow,Kilauea Hawaii. B Example of aslow-moving aa flow.Drawing Conclusions Which ofthe flows has more viscous lava?

A B

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Volcanic Material

Students may have the misconceptionthat earthquakes shaking the regionaround the volcano are the only reasonfor volcanic eruptions. Earthquakes arecommon triggers of volcanic eruptions,but are not the only factors involved.Explain to students that volcanoes canerupt whenever magma builds upenough force to erupt from undergroundto the surface. The factors that determinethe violence of the eruption are magmacomposition, magma temperature, andthe amount of dissolved gases themagma contains.Verbal

Use VisualsFigure 2 Have students look closely atthese photographs. Ask: How can youtell the aa flow is slow moving? (It isrough and jagged rather than smooth.)Visual

Observing ViscosityPurpose Students will observe fluidsthat have different viscosities.

Materials 2 large beakers, hot plate,water, 2 large test tubes, test-tubeclamp, ice, corn syrup

Procedure Pour corn syrup into thetwo large test tubes in advance. Put onetest tube into a large beaker filled withice. Put the other test tube into a largebeaker half filled with water on a hotplate. Heat the syrup in a hot-water bathuntil it is very hot. Boiling the syrup isnot necessary. Slowly pour the contentsof each test tube into another beakerone at a time to demonstrate the natureof fluids with differing viscosities.

Expected Outcome Students willobserve that the cold syrup is veryviscous and flows very slowly—similarto silica-rich lava. The hot syrup is notviscous and flows very fast—similar tosilica-poor lava.Visual, Verbal

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Section 10.1 (continued)

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Pyroclastic Materials When basaltic lava is extruded, dissolvedgases propel blobs of lava to great heights. Some of this ejected materialmay land near the vent and build a cone-shaped structure. The windwill carry smaller particles great distances.Viscous rhyolitic magmas arehighly charged with gases. As the gases expand, pulverized rock and lavafragments are blown from the vent. Pyroclastic material is the namegive to particles produced in volcanic eruptions. The fragmentsejected during eruptions range in size from very fine dust and volcanicash (less than 2 millimeters) to pieces that weigh several tons.

Particles that range in size from small beads to walnuts (2–64 mil-limeters) are called lapilli or more commonly cinders. Particles largerthan 64 millimeters in diameter are called blocks when they are madeof hardened lava and bombs when they are ejected as glowing lava.Because bombs are semimolten upon ejection, they often take on astreamlined shape as they hurtle through the air.

Types of VolcanoesVolcanic landforms come in a wide variety of shapes and sizes. Eachstructure has a unique eruptive history. The three main volcanictypes are shield volcanoes, cinder cones, and composite cones.

Anatomy of a Volcano Volcanic activity often begins when afissure, or crack, develops in the crust as magma is forced toward thesurface. The gas-rich magma moves up this fissure, through a circularpipe, ending at a vent, as shown in Figure 3. Repeated eruptions of lavaor pyroclastic material often separated by long inactive periods even-tually build the mountain called a volcano. Located at the summit ofmany volcanoes is a steep-walled depression called a crater.

What is a volcanic bomb?

Lava

VentCrater

Pyroclasticmaterial

Conduit(pipe)

Figure 3 Anatomy of a“Typical” Volcano This cross section shows a typicalcomposite cone. Interpreting Diagrams Howwas the volcano in the diagramformed?

For: Links on volcanic eruptions

Visit: www.SciLinks.org

Web Code: cjn-3101

Types of VolcanoesBuild Reading LiteracyRefer to p. 278D in this chapter, whichprovides the guidelines for identifyingmain ideas and details.

Identify Main Idea/Details Havestudents read Types of Volcanoes onpp. 283–286. Ask them to identify themain idea of each paragraph. Point outthat the main idea is usually within thefirst or second sentence of a paragraph.Encourage students to include thisexercise in the notes they use to study.Verbal

Build Science SkillsInterpreting Diagrams/Photographs Have students studyFigure 3. Ask: Why do you think theterm parasitic cone is given to thisfeature in the diagram? (This cone doesnot have its own lava source but gets itslava from another conduit, or pipe.)Visual, Logical

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Answer to . . .

Figure 2 The lava in the aa lava flowis more viscous.

Figure 3 The volcano was formed aslayers of pyroclastic material and lavaflows were built up around the vent.

a large streamlinedchunk of pyroclastic

material that is larger than 64 mmin diameter

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


The form of a volcano is largely determined by the compositionof the magma. As you will see, fluid lavas tend to produce broad struc-tures with gentle slopes. More viscous silica-rich lavas generate coneswith moderate to steep slopes.

Shield Volcanoes Shield volcanoes are produced by the accu-mulation of fluid basaltic lavas. Shield volcanoes have the shape of abroad, slightly domed structure that resembles a warrior’s shield, asshown in Figure 4. Most shield volcanoes have grown up from thedeep-ocean floor to form islands. Examples of shield volcanoes includethe Hawaiian Islands and Iceland.

Cinder Cones Ejected lava fragments the size of cinders, whichharden in the air, build a cinder cone. These fragments range in sizefrom fine ash to bombs but consist mostly of lapilli, or cinders. Cindercones are usually a product of relatively gas-rich basaltic magma.Although cinder cones are composed mostly of loose pyroclastic ma-terial, they sometimes extrude lava.

Cinder cones have a very simple shape as shown in Figure 5A. Theshape is determined by the steep-sided slope that loose pyroclasticmaterial maintains as it comes to rest. Cinder cones are usually theproduct of a single eruption that sometimes lasts only a few weeks andrarely more than a few years. Once the eruption ends, the magma in thepipe connecting the vent to the magma chamber solidifies, and the vol-cano never erupts again. Because of this short life span, cinder conesare small, usually between 30 meters and 300 meters and rarely exceed700 meters in height.

Oceanic crust

Flank eruption

Central vent

Summit caldera

Magma chamber

Figure 4 Shield VolcanoesShield volcanoes are built mainlyof fluid basaltic lava flows. Theycontain only a small amount ofpyroclastic materials. These broad,slightly domed structures are thelargest volcanoes on Earth. Anexample is Kilauea in Hawaii.

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Integrate PhysicsGeothermal Energy Hot magma nearthe surface of Earth can be beneficial.Geothermal energy takes advantage ofEarth’s internal energy and uses it as anenergy source. Have students researchthis renewable source of energy. Studentsshould prepare a short report about thisnatural energy source. The report shouldinclude an illustration showing anexample of how geothermal energycan be used in a specific application.Verbal

Use VisualsFigure 4 Have students comparethe photograph to the drawing. Ask:Why might photographs of shieldvolcanoes make them look not as tallas they really are? (Because shieldvolcanoes are so broad, they often give theimpression of being lower than they are.)How would you describe the viscosityof the lava at a shield volcano? (lowviscosity) What is the origin of theother islands in the diagram? What doyou think they would look like underthe sea level? (They are shield volcanoes,or parts of shield volcanoes. Beneath thesurface, the rocky formations likely flareoutward either as individual shieldvolcanoes or as portions of a volcanoshared by oneor more of the other islands.)

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Parícutin is an active volcano in Mexico. It isone of the youngest volcanoes on Earth. OnFebruary 20, 1943, Parícutin began eruptingfrom a fissure in a cornfield. By the end of thefirst year, the cone had reached an elevation of450 m. Volcanic eruptions finally ended in 1952.

The resulting fire, ash, and lava destroyedtwo villages. In one of the villages, a localchurch is still standing at the edge of the lavaflow. The top of the church and the bell towerare visible, but the lower portions of thechurch are buried in lava.

Facts and Figures



Cinder cones are found by the thousands all around Earth. Some,like the one shown in Figure 5B, near Flagstaff, Arizona, are located involcanic fields. This field consists of about 600 cones. Others form onthe sides of larger volcanoes. Mount Etna, for example, has dozens ofcinder cones dotting its flanks.

Composite Cones Earth’s most beautiful and potentially dan-gerous volcanoes are composite cones, or stratovolcanoes. Most arelocated in a relatively narrow zone that rims the Pacific Ocean, appro-priately called the Ring of Fire. The Ring of Fire includes the largecones of the Andes in South America and the Cascade Range of thewestern United States and Canada. The Cascade Range includes MountSt. Helens, Mount Rainier, and Mount Garibaldi. The most activeregions in the Ring of Fire are located along curved belts of volcanicislands next to the deep ocean trenches of the northern and westernPacific. This nearly continuous chain of volcanoes stretches from theAleutian Islands to Japan, the Philippines, and New Zealand.

A composite cone is a large, nearly symmetrical structure com-posed of layers of both lava and pyroclastic deposits. For the most part,composite cones are the product of gas-rich magma having anandesitic composition. The silica-rich magmas typical of compositecones generate viscous lavas that can only travel short distances.Composite cones may generate the most explosive eruptions that ejecthuge quantities of pyroclastic material. Compare the shape and heightof composite cones with other types of volcanoes in Figure 6.

CraterPyroclastic material

Central vent filledwith rock fragments

Figure 5 Cinder Cones A A typical cinder cone has steepslopes of 30–40 degrees. B Thisphotograph shows SP Crater, acinder cone north of Flagstaff,Arizona. Inferring What feature is shown in the lower part of thephotograph?

A B

0 10

4 km

Sunset Crater, Arizona,a large cinder cone

Mount Rainier, Washington,a large composite cone

Sea level

Mauna Loa, Hawaii, a large shield volcano 20 km

Figure 6 Profiles of VolcanicLandforms A Profile of MaunaLoa, Hawaii, the largest shieldvolcano in the Hawaiian chain. B Profile of Mount Rainier,Washington, a large compositecone. C Profile of Sunset Crater,Arizona, a typical steep-sidedcinder cone.

A

B C

Observing anExplosive EruptionPurpose Students will observe theexplosive nature of gases trapped inan enclosed container.

Materials 2-L soda bottle, rubberstopper, white vinegar, baking soda,paper towel, rubber band, thin bathor kitchen towel, scissors

Procedure Pour 150 mL of vinegarinto the empty soda bottle. For safety,fold the towel around the bottle. Securethe towel at the neck of the bottle witha rubber band. Cut an 8-cm squarepiece of paper towel. Put about 5 mL ofbaking soda in the center of the papertowel. Fold the paper towel around thebaking soda to make a packet. Drop thepacket into the bottle. Put the stopperinto the bottle. Do not put the stopperin too tight. Have everyone stand a safedistance away from the bottle.

Expected Outcome Students willobserve the explosive forces that arecreated when trapped gases are released.Point out to students that this is similarto gases trapped inside an active volcano.When enough force is built up, trappedgases can blow the top off of the volcano.Gases, magma, and pyroclastic materialsthen flow from the volcano through thenew opening.Visual

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Answer to . . .

Figure 5 a lava flow


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Figure 7 Composite ConeMount Shasta, California, is oneof the largest composite cones inthe Cascade Range. Shastina isthe smaller cone that formed onthe left flank of Mt. Shasta.

Fujiyama in Japan and Mount Shasta in California show the clas-sic shape you would expect of a composite cone, with its steep summitand gently sloping flanks, as shown in Figure 7. About 50 such volca-noes have erupted in the United States in the past 200 years. On aglobal scale, numerous destructive eruptions of composite cones haveoccurred during the past few thousand years. A few of these have hada major influence on human civilization.

Dangers from Composite Cones One of the most devas-tating features associated with composite cones are pyroclastic flows.They consist of hot gases, glowing ash, and larger rock fragments. Themost destructive of these fiery flows are capable of racing down steepvolcanic slopes at speeds of nearly 200 kilometers per hour. Somepyroclastic flows result when a powerful eruption blasts material outthe side of a volcano. Usually they form from the collapse of tall erup-tion columns that form over a volcano during an explosive event. Oncegravity overcomes the upward thrust provided by the escaping gases,the material begins to fall. Massive amounts of hot fragments, ash, andgases begin to race downhill under the influence of gravity.

Large composite cones may also generate mudflows called lahars.These destructive mudflows occur when volcanic debris becomes satu-rated with water and rapidly moves down steep volcanic slopes, oftenfollowing stream valleys. Some lahars are triggered when large volumesof ice and snow melt during an eruption. Others are generated whenheavy rainfall saturates weathered volcanic deposits. Lahars can occureven when a volcano is not erupting.

What is a lahar?

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Integrate Social StudiesMount Pelée Living in the shadowof a composite cone can be particularlydangerous. In 1902, Mount Pelée eruptedin a fiery pyroclastic flow of hot gasesinfused with incandescent ash andlarger rock fragments. The mostdestructive of pyroclastic flows, a nuéeardente (burning cloud), destroyed theport town of St. Pierre on the Caribbeanisland of Martinique. The destructionhappened in moments. All of the 28,000inhabitants of the town were killed withthe exception of one person who wasbeing held in a dungeon on the outskirtsof town. A few people that were onships in the harbor also were spared.

Shortly after this eruption, scientistsarrived on the scene. They discoveredmasonry walls almost one meter thickknocked over like dominoes. Large treeswere uprooted, and cannons were tornfrom their mounts. Have students usethe Internet to research this volcaniceruption and prepare a short reporton it. Ask: What name is given toeruptions that are similar to the onethat destroyed St. Pierre? (a peelean-type eruption, which is named after MountPelée)Verbal

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The five deadliest volcanic eruptions knownare (1) Tambora, Indonesia, which occurredin 1815. There were 92,000 deaths, primarilythe result of starvation. (2) Krakatau, Indonesia,which occurred in 1883. There were 36,000deaths, primarily the result of a tsunami.(3) Mount Pelée, Martinique, which occurred

in 1902. There were 28,000 deaths, primarilythe result of pyroclastic flows. (4) Nevado delRuiz, Colombia, which occurred in 1985.There were 25,000 deaths, primarily the resultof mudflows. (5) Unzen, Japan, which occurredin 1792. There were 14,000 deaths, primarilythe result of a volcano collapse and a tsunami.

Facts and Figures

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Other Volcanic LandformsCalderas A caldera is a large depression in a volcano. Mostcalderas form in one of two ways: by the collapse of the top of a com-posite volcano after an explosive eruption, or from the collapse of thetop of a shield volcano after the magma chamber is drained. CraterLake, Oregon, is located in a caldera. This caldera formed about 7000years ago when a composite cone, Mount Mazama, violently eruptedand collapsed, as shown in Figure 8.

Necks and Pipes Most volcanoes are fed magma throughconduits, called pipes, connecting a magma chamber to the surface.Volcanoes are always being weathered and eroded. Cinder cones areeasily eroded because they are made up of loose materials. When therock in the pipe is more resistant and remains standing above the sur-rounding terrain after most of the cone has been eroded, the structureis called a volcanic neck, as shown in Figure 9A on page 288.

The best-known volcanic pipes are the diamond-bearing pipes ofSouth Africa. The rocks filling these pipes formed at depths of at least150 kilometers, where pressure is high enough to form diamonds. Theprocess of moving unaltered magma through 150 kilometers of solidrock is unusual, resulting in the rarity of diamonds.

Eruption ofMount Mazama

Partialy emptiedmagma chamber

Collapse ofMount Mazama

Figure 8 Crater Lake in Oregonoccupies a caldera about 10 kilometers in diameter. About7000 years ago, the summit offormer Mount Mazama collapsedfollowing a violent eruption thatpartly emptied the magmachamber. Rainwater then filledthe caldera. Later eruptionsproduced the cinder cone calledWizard Island.

Caldera Formation

A

B

C

Formation of Crater Lake and Wizard Island

D

E

Other VolcanicLandformsUse VisualsFigure 8 Have students study thediagrams in Figure 8. Tell students thatthe word caldera means “a cookingpot.” Ask: Why is an eruption thatempties or partially empties themagma chamber an important firststep for a caldera to form? (Themagma chamber must be emptied orpartially emptied to create a void. Thenthe volcano collapses into the newlycreated void to create a deep depressionin the landscape.) Why is the namecaldera a good description of thistype of landform? (When the volcanocollapses, a large well is created thatresembles a cooking pot.)Verbal, Logical

Use CommunityResourcesMany U.S. Geological Survey (USGS)offices have educational outreach staffand programs. Contact your regionaloffice and ask a USGS scientist to speakto your class about plate tectonics andvolcanic activity.Interpersonal

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Answer to . . .

a mudflow down theslope of a volcano


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Section 10.1 Assessment

Reviewing Concepts1. What factors determine the type of

volcanic eruption?

2. List the materials ejected from volcanoes.

3. Describe the three types of volcanoes.

4. What is a caldera?

Critical Thinking5. Comparing and Contrasting Compare the

formation of a lava plateau with the formationof a cinder cone.

6. Applying Concepts What type of eruptionproduces a viscous magma containing53 percent silica and a gas content of2 percent?

7. Calculating If a pyroclastic flow wastraveling 145 kilometers per hour, how longwould it take to reach a town 2.5 kilometersfrom the volcano’s crater?

Lava Plateaus You probably thinkof volcanic eruptions as building amountain from a central vent. But thegreatest volume of volcanic material isextruded from fissures. Rather thanbuilding a cone, low-viscosity basalticlava flows from these fissures, covering awide area, as shown in Figure 9B. Theextensive Columbia Plateau in the north-

western United States was formed this way. Here, numerous fissureeruptions extruded very fluid basaltic lava, shown in Figure 9C.Successive flows, some 50 meters thick, buried the landscape, buildinga lava plateau nearly 1.6 kilometers thick.

Summary Research a volcanic eruption.Write a paragraph describing the eruption.Make sure to classify what type of volcanoerupted.

Lavafountaining

Fissure

Basalticlava flows

Figure 9 Other VolcanicLandforms A Ship Rock, NewMexico, is a volcanic neck. ShipRock consists of igneous rock thatcrystallized in the pipe of avolcano that then was erodedaway. B Lava erupting from afissure forms fluid lava flowscalled flood basalts. C These dark-colored basalt flows are nearIdaho Falls, Idaho.

A

B

C

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Use VisualsFigure 9 Have students study Figure 9.Ask: Infer why the volcanic neck is stillin place while the surrounding terrainhas eroded away. (The rock in thevolcanic neck is more resistant to erosionthan the surrounding terrain.)Verbal, Logical

ASSESSEvaluateUnderstandingHave students play a quiz game toreview the material in this section. Askeach student to write three questions onthree separate sheets of paper. Collectthe questions. Divide the class into twoteams. To play the game, alternategiving a member of each team aquestion from the collected papers.Give each team a point for each correctresponse. The team with the mostpoints wins the game.

ReteachSet aside any questions that areanswered incorrectly from the quizgame above. After the game, give eachteam the stack of missed questions. Letthe entire team work together to givethe correct response to the questions.

Answers will vary, but should accuratelyclassify the volcano and give a cleardescription of the eruption.

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composed of layers of lava flows and pyroclas-tic material from more explosive eruptions.4. A caldera is a large, collapsed depressionin a volcano.5. A lava plateau is formed by repeated erup-tions from a long, narrow fissure that canbuild up to form a thick deposit of volcanicrock over a large area. A cinder cone is asmall volcanic cone that forms from cinders,usually from a single eruption.6. The eruption would most likely be explosive.7. The pyroclastic flow would reach the townin just over 1 minute (1.03 minutes).


1. The type of volcanic eruption is determinedby the magma composition, magma tempera-ture, and amount of dissolved gases.2. The materials ejected from volcanoesinclude lava, gases, and pyroclastic materials,such as ash, dust, cinders, volcanic blocks,and volcanic bombs.3. Cinder cones are small, steep cones,composed mainly of loose cinders. Shieldvolcanoes are large, gently sloping volcanoescomposed of layers of mainly quiet lava flows.Composite cones are large, steep cones,



10.2 Intrusive Igneous Activity

Reading StrategyComparing and Contrasting After youread the section, compare the types ofplutons by completing the table.

Key ConceptsHow are intrusive igneousfeatures classified?

What are the majorintrusive igneous features?

What is the origin ofmagma?

Vocabulary◆ pluton◆ sill◆ laccolith◆ dike◆ batholith◆ geothermal

gradient◆ decompression

melting

Types of Plutons Description

Sill a.

Laccolith b.

Dike c.

Batholith d. ?

?

?

?

A lthough volcanic eruptions are among the most violent and spec-tacular events in nature, most magma cools deep within Earth. Thestructures that result form the roots of mountain ranges and some ofthe most familiar features in the landscape.

PlutonsThe structures that result from the cooling and hardening of magmaat depth are called plutons. Because all plutons form beneath Earth’ssurface, they can be studied only after uplift and erosion have exposedthem. Plutons occur in a great variety of sizes and shapes. Intrusiveigneous bodies, or plutons, are generally classified according to theirshape, size, and relationship to the surrounding rock layers.

Sills and Laccoliths Sills and laccoliths are plutons thatform when magma is intruded close to the surface. Sills and laccolithsdiffer in shape and often differ in composition. A sill forms whenmagma is injected along sedimentary bedding surfaces, parallel to thebedding planes. Horizontal sills, like the one shown in Figure 10, arethe most common.

For a sill to form, the overlying sedimentary rock must be lifted to aheight equal to the thickness of the sill.Although this is a not an easy task,at shallow levels it often requires less energy than forcing the magma upto the surface. Because of this, sills form only at shallow depths, wherethe pressure exerted by the weight of overlying rock layers is low. Asshown in Figure 11A on page 290, sills look like buried lava flows.

Figure 10 Sills This dark,horizontal band is a sill of basaltthat intruded into horizontal layersof sedimentary rock in Salt RiverCanyon, Arizona. Inferring How could youdetermine if a horizontal igneousrock layer was a lava flow or a sill?

FOCUS

Section Objectives10.5 Classify intrusive igneous

features.10.6 Describe the major intrusive

igneous features.10.7 Describe the origin of magma.

Build VocabularyWord Parts List on the board thefollowing word parts and meanings:lakkos, “reservoir”; lithos, “stone”;bathos, “depth.” Have students identifythese word parts in the vocabulary terms.Discuss the terms’ meanings withstudents.

Reading Strategya. pluton formed parallel withsedimentary rocks, commonly horizontalb. similar to a sill, but forms a lens-shapedmass that pushes the overlying strataupwardc. pluton that cuts across the preexistingrocksd. largest intrusive igneous body with asurface exposure of over 100 sq km

INSTRUCT

PlutonsUse VisualsFigure 10 Have students study Figure10. Ask: Why do sills only form atshallow depths? (The overlying sedimen-tary rock must be lifted to a height equalto the height of the sill, so the weight ofthe rock cannot be more than the magmacan lift.) Why does the sill form belowthe sedimentary rock instead of at thesurface? (because it requires less force toraise the sedimentary rock than to forcethe magma to the surface)Verbal

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Section 10.2

Answer to . . .

Figure 10 The upper surface of alava flow would not show evidence ofcontact with another rock layer aboveit, while the upper surface of a sillshows evidence that it was intrudedinto preexisting layers of sedimentaryrocks. The sedimentary rock layersabove the sill could also show evidenceof heating and contact metamorphism.


290 Chapter 10

Cindercones

Compositecones

Fissureeruption

Magmachamber

Implacement of magma

Laccolith

Volcanicnecks

Dike

Crystallization of igneousplutons and erosion

Batholith

Sills

A

B

Batholith

Extensive uplift anderosion exposes batholith

Batholith

Types of Igneous Plutons

C

Figure 11 A This diagram shows the relationship betweenvolcanism and intrusive igneous activity. B This view showsthe basic intrusive igneous structures, some of which havebeen exposed by erosion long after their formation. C Aftermillions of years of uplift and erosion, a batholith isexposed at the surface.

Laccoliths are similar to sills because theyform when magma is intruded between sedi-mentary layers close to the surface. However,the magma that generates laccoliths is moreviscous. This less-fluid magma collects as alens-shaped mass that pushes the overlyingstrata upward. Most laccoliths are not muchwider than a few kilometers.

Dikes Some plutons form when magma isinjected into fractures, cutting across preexist-ing rock layers. Such plutons are called dikes,as in Figure 11B. These sheetlike structureshave thicknesses ranging from less than a cen-timeter to more than a kilometer. Most dikes,however, are a few meters thick and extend lat-erally for no more than a few kilometers.

Some dikes radiate, like spokes on a wheel,from an eroded volcanic neck. The movementof magma probably formed fissures in the vol-canic cone from which the magma flowed toform the dikes. Many dikes form whenmagma from a large magma chamber invadesfractures in the surrounding rocks.

Batholiths The largest intrusive igneousbodies are batholiths. The Idaho batholith,for example, covers an area of more than 40,000 square kilometers and consists of manyindividual plutons. Indirect evidence fromgravity and seismic studies indicates thatbatholiths are also very thick, possibly extend-ing dozens of kilometers into the crust.

Compare and contrastsills and laccoliths.

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Build Reading LiteracyRefer to p. 474D in Chapter 17, whichprovides guidelines for monitoring yourunderstanding.

Monitor Your Understanding Havestudents read the passages Plutons andOrigin of Magma (pp. 289–292). Whenthey reach the bottom of p. 289, havethem stop and write down the mainideas in the passages. Have them askthemselves, “Did I have any troublereading this passage? If so, why?”Then, have them come up with theirown strategies to improve theirunderstanding. Have students use thisstrategy as they continue reading.Interpersonal, Verbal

Integrate Social StudiesThe Henry Mountains Thesemountains, located in southeasternUtah, are largely composed of severallaccoliths believed to be fed by a muchlarger magma body nearby. Themountain range is named for JosephHenry, an American scientist. Henry wasthe first secretary of the SmithsonianInstitution. Have students find the HenryMountains on a map or an atlas.Verbal

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Customize for Inclusion Students

Learning Disabled Make concept mapsfor each section and cover them with clearcontact paper. Then, cut the maps into puzzlepieces. Provide students with the pieces andhave them put the puzzle together. Afterstudents complete the puzzle, have themmake flashcards with concept connections andadded notes. For example, students may have

cards with the names of plutons on one sideand a definition or example on the other.Students also may have key concept cardswith an important word missing. For example,A ________ is a pluton formed when magmais injected along sedimentary beddingsurfaces, parallel to the bedding planes. (sill)



An intrusive igneous bodymust have a surface exposure greaterthan 100 square kilometers to be con-sidered a batholith. Smaller plutonsare called stocks. Many stocks appearto be portions of batholiths that arenot yet fully exposed. Batholiths mayform the core of mountain ranges, asshown in Figure 12. In this case, upliftand erosion have removed the sur-rounding rock, exposing the batholith.

Origin of MagmaThe origin of magma has been contro-versial in geology for a long time. Basedon available scientific evidence, Earth’scrust and mantle are composed primarily of solid, not molten, rock.Although the outer core is a fluid, its iron-rich material is very denseand stays deep within Earth. What is the source of magma that pro-duces igneous activity? Geologists conclude that magmaoriginates when essentially solid rock, located in the crust and uppermantle, partially melts. The most obvious way to generate magmafrom solid rock is to raise the temperature above the level at whichthe rock begins to melt.

Role of Heat What source of heat is sufficient to melt rock?Workers in underground mines know that temperatures get higher asthey go deeper. The rate of temperature change averages between 20°Cand 30°C per kilometer in the upper crust. This change in temperaturewith depth is known as the geothermal gradient. Estimates indicatethat the temperature at a depth of 100 kilometers ranges between1400°C and 1600°C. At these high temperatures, rocks in the lower crustand upper mantle are near, but not quite at their melting point tem-peratures. So they are very hot but still essentially solid.

There are several ways that enough additional heat can be gener-ated within the crust or upper mantle to produce some magma. First,at subduction zones, friction generates heat as huge slabs of crust slidepast each other. Second, crustal rocks are heated as they descend intothe mantle during subduction. Third, hotter mantle rocks can rise andintrude crustal rocks. All of these processes only form relatively smallamounts of magma. As you’ll see, the vast bulk of magma forms with-out an additional heat source.

What is a geothermal gradient?

Figure 12 Batholiths MountWhitney in California makes upjust a tiny portion of the SierraNevada batholith, a hugestructure that extends forapproximately 400 kilometers.

Origin of Magma

Some students may have themisconception that all mountains arevolcanoes (either extinct, dormant, oractive). Explain to students that mostmountains are the result of crustaldeformation. Mountain building isdiscussed in the next chapter. Ask: Whatclues could scientists use to determineif a mountain is the result of volcanicactivity? (Scientists could look for signs ofvolcanic activity, such as the presence ofigneous rock or plutons.)Verbal

Build Science SkillsObserving Frictionand Heat Gather twoflat rocks. Havestudents feel thetemperature of the rocks before thefollowing activity begins. If an infraredthermometer is available, take thetemperature of the rocks. Simulate themotion of two plates at a subductionzone by rubbing and grinding two flatrocks together. After a few minutes, feelthe rocks again or take the temperatureof the rocks with the infrared thermo-meter. Students will observe a temp-erature increase in rocks due to thefriction between the two rocks. This issimilar to the activity that occurs at asubduction zone. However, the rocks ata subduction zone are much larger andare forced together with a great deal offorce, resulting in a great deal of frictionand heat. Friction is not a factor in themelting of magma beneath subductionzones.Kinesthetic, Logical

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American geologist Ferdinand VandiveerHayden lived from 1829 to 1887. Haydenexplored and documented information aboutthe American West for over 30 years. The“Hayden surveys” provided scientificinformation on the geology, botany, andzoology of the American West. In 1867, he

was placed in charge of the newly establishedU.S. Geological and Geographical Survey ofthe Territories. This department was theprecursor of the U.S. Geological Survey, whichis now part of the United States Departmentof the Interior.

Facts and Figures

Answer to . . .

Both sills and laccolithsare plutons formed by

magma intrusions close to the surface,but they differ in shape and usuallydiffer in composition.

the change in tempera-ture with depth


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Reviewing Concepts1. How are intrusive features classified?

2. List the major intrusive igneous bodies.

3. What are the three major ways thatmagma forms?

4. What is a pluton?

Critical Thinking5. Comparing and Contrasting Describe the

difference between a sill and a dike.

6. Relating Cause and Effect What effectdoes a decrease in confining pressure have onthe melting temperature of rocks in the uppermantle?

Role of Pressure If temperature were the onlyfactor that determined whether or not rock melts,Earth would be a molten ball covered with a thin,solid outer shell. This is not the case because pres-sure also increases with depth. Melting, which causesan increase in volume, occurs at higher temperaturesat depth because of greater confining pressure. Inthis way, an increase in confining pressure causes anincrease in the rock’s melting temperature. Theopposite is also true. Reducing confining pressurelowers a rock’s melting temperature. When confining

pressure drops enough, decompression melting is triggered. Thisprocess generates magma beneath Hawaii where plumes of hot rockmelt as they rise toward the surface.

Role of Water Another important factor affecting the meltingtemperature of rock is its water content. Water causes rock to melt atlower temperatures. Because of this, “wet” rock buried at depth has amuch lower melting temperature than does “dry” rock of the samecomposition and under the same pressure. Laboratory studies haveshown that the melting point of basalt can be lowered by up to 100°Cby adding only 0.1 percent water. In addition to a rock’s composition,its temperature, depth (confining pressure), and water content deter-mine if it is a solid or liquid.

In summary, magma can be formed in three ways. First, heat maybe added when a magma body from a deeper source intrudes andmelts crustal rock. Second, a decrease in pressure (without the additionof heat) can result in decompression melting. Third, water can lowerthe melting temperature of mantle rock enough to form magma.

Convection Currents Recall what youlearned about convection currents inChapter 9. Explain how convection cur-rents could affect the depth at whichmolten rocks are found.

Figure 13 Basaltic Magma atthe Surface Lava extruded alongthe East Rift Zone, Kilauea,Hawaii.Observing Does this lava appearto have a high viscosity or a lowviscosity? Explain.

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Integrate PhysicsKinetic Theory and Pressure Thekinetic theory can help students visualizethe role that pressure plays in themelting of rock. Have students recallthat the particles in a solid are closelypacked and are bonded to the particlessurrounding them. When a substance isheated or gains thermal energy, thekinetic energy of the individual particlesincreases. Have students recall thattemperature is the average kineticenergy of the individual particles in asubstance. A substance melts when theparticles have enough kinetic energyto overcome the bonds between theparticles in a solid. If a substance isunder pressure, the particles must gainmore thermal energy to overcome thebonds between the particles and theforce (pressure) holding the particles inplace. Therefore, the substance, in thiscase the rock, must absorb more thermalenergy to overcome the additionalforce. This gives the substance a highermelting temperature. Have studentsexplain why reducing confining pressurelowers a rock’s melting temperature.(The particles no longer have to overcomethe additional force.)Verbal, Logical

ASSESSEvaluateUnderstandingHave students write three reviewquestions from the chapter. Then havestudents work with a partner to ask eachother their questions.

ReteachUse Figure 11 to review the differenttypes of igneous plutons.

Sample answer: Convection currentswithin the mantle bring hot mantlematerial closer to the surface.

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4. the structure that results from the coolingand hardening of magma at depth5. A sill is a pluton that forms when magma isinjected along bedding surfaces and parallel tothe bedding planes. A dike is a pluton thatforms when magma is injected into fractures,cutting across preexisting rock layers.6. A decrease in confining pressure willdecrease the melting temperature, causingdecompression melting to occur.


1. Intrusive features are classified by theirshape, size, and relationship to the surround-ing rock layers.2. batholiths, laccoliths, sills, and dikes3. Magma forms by (1) heat being added tocrustal rocks when hotter, deeper mantlerocks rise into the crust; (2) by a decrease inpressure without an increase in temperature;(3) by the addition of water, which can lowerthe melting point enough to form magma.

Answer to . . .

Figure 13 It appears to have a lowviscosity because it is flowing relativelyeasily from a fissure.

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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. ?

?


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

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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


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.

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

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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.


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.

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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


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.


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.



Hot spot

Continentalcrust


Risingmantleplume

Continentalcrust

Riftvalley

Magmachamber

Asthenosphere

Oceaniccrust

Divergent platevolcanism

Divergentplate volcanism

Intraplate volcanism


Floodbasalts




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

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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.

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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.

L1

L2

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(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.


Documents

Section 10.1 10.1 The Nature of Volcanic Eruptions 1 FOCUSmrbakerearth.weebly.com/uploads/3/0/8/0/30809369/ch10_book__es_.… · the section, rewrite the green topic headings as questions