Section 10.1 10.1 The Nature of Volcanic Eruptions 1 FOCUSvmearthscience.weebly.com/uploads/5/8/8/5/58854521/10.1.pdf · dows to Earth’s interior. ... 10.1 The Nature of Volcanic

280 Chapter 10

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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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).

Section 10.1 Assessment

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,


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Section 10.1 10.1 The Nature of Volcanic Eruptions 1 FOCUSvmearthscience.weebly.com/uploads/5/8/8/5/58854521/10.1.pdf · dows to Earth’s interior. ... 10.1 The Nature of Volcanic