How and Where Volcanoes FormOne of the most dramatic activities associated with plate tectonics is theeruption of a volcano. The term volcano refers both to the opening inEarth’s crust through which molten rock, gases, and ash erupt and to thelandform that develops around this opening.

Magma FormationA volcanic eruption occurs when magma—molten rock that has formeddeep within Earth—rises to the surface. Most of the asthenosphere is Solidbecause of the pressure exerted by the lithosphere above it. This pressureraises the melting temperatures of materials in the asthenosphere. Yet formagma to form, some of these materials must melt. The following threeconditions allow magma to form:• A decrease in pressure can lower the melting temperatures ofmaterialsin the asthenosphere. Such a decrease takes place along the rift valleyat a mid-ocean ridge, where the lithosphere is thinner and exerts lesspressure.

• An increase in temperature can cause materials in the asthenosphereto melt. Such an increase occurs at a hot spot.

• An increase in the amount of water in the asthenosphere can lowerthe melting temperatures ofmaterials there. Such an increase occursat subduction boundaries.

Conditions at both divergent and subduction boundaries are ideal formagma formation. Most volcanoes are found along mid-ocean ridges,where plates are moving apart, and at subduction boundaries, where platesare being forced under other plates.

Once magma forms, it tends to rise to the surface because its density islower than that of the solid materials surrounding it. The characteristics ofa magma and the rates at which it rises depend upon the amount of silicait contains.

I9.1KEY IDEAVolcanoes form where magmareaches Earth’s surface.

KEY VOCABULARY• volcano• hot spot

Cj.1 LAZONE LO

How Are Volcanoes Related toPlate Tectonics? Plot volcanolocations. Examine maps and 3Ddiagrams to explore volcanism atplate boundaries.Keycode: E50901

VOLCANO The diagram (inset)shows magma rising inside avolcano. Rising magma can lead toan explosive eruption like this one atAlaska’s Mt. Augustine in 1986.

At 5 Auction Boundariesyou may recall from Chapter 8, subduction boundaries are places where

Ithospheric plates collide. The diagrams below show the two processes byich volcanoes form: first, when an oceanic plate is forced beneath a0ntital plate, and second, when one oceanic plate is forced beneathanother oceanic plate.

At a subduction boundary, volcanoes always form on the overridingplate. Where an oceanic plate collides with a continental plate, thevolcanoes form on the overriding continental plate. The Cascade Range,which extends along the Pacific coast from northern California to BritishColumbia, Canada, is made up ofvolcanoes that have formed at anoceaniccontinental subduction boundary,Where two oceanic plates collide, the volcanoes create a chain of

olcanic islands, called a volcanic island arc, on the overriding plate.the Mariana Islands in the Pacific Ocean are an example of a volcanicisland arc formed at such a subduction boundary.

(jTft‘ CLASSZONECOM

Observe an animation of volcanism ata subduction zone.Keycode: ES0902

BETWEEN AN OCEANIC PLATE AND A CONTINENTAL PLATE

Some of the magmareaches Earth’s surface,and volcanoes formon the overridingcontinental plate.

0 The magma is less densethan its surroundings,so it rises.

BETWEEN OCEANIC PLATESThe process by which magma forms at an oceanic-oceanic subductionboundary is similar to the process at an oceanic-continental boundary.Notice that the difference between the two processes occurs at step 4.

—

llthosphere

\

3Magma that reaches Earth’ssurface is underwater. Thus,an arc of volcanic islandsforms underwater on theoverriding oceanic plate.

Water in the

is releasedintosubducted rock

the asthenosphere. Ciere

0 The water lowers melting temperaturesof materials in the asthenosphere,leading to the formation of magma.

Oceaniclithosphere

Chapter 9 Volcanoes I )5

(jUh1Luv4imIiilcsszoNEC0Observe an animation of volcanismalong a rift zone.Keycode: ES0903

jt1frTiTgEfl

COMMUNICPTEthe information to present a

summary of the risks and benefitsassociated with living in an areawith Iceland’s level of volcanicactivity.

ANALYZEVolcanic activity in Iceland cancause problems. but it alsoprovides a nearly inexhaustiblesource of energy, such as thatproduced by the geothermalplant below. Find informationabout the uses of Iceland’sgeothermal resources.

At Divergent BoundariesBelow a rift, mantle material rises from deeper, hotter regions within EarthAlso, because of the rift, the pressure is lower than it is elsewhere in themantle. This decrease in pressure lowers melting temperatures. Thecombination of high temperature and low pressure causes large amountsof magma to form. Like the magma formed at subduction boundaries, themagma formed at divergent boundaries is less dense than the materialsaround it. The magma therefore rises through the rift to the surface.

Rift

Most of the magma that reaches Earth’s surface does so at divergentboundaries, along the mid-ocean ridges. Because most mid-ocean ridgesare underwater, you can understand why most of Earth’s volcanic activitytakes place beneath its oceans.

One place where a mid-ocean ridge rises above sea level is Iceland.Here the rifts associated with the Mid*AtlantiC Ridge mark the surface ofthe land. Much of Iceland is volcanically active, and sometimes thisvolcanic activity is harmful to living things. In 1783 the Icelandic volcanoLaid erupted releasing lava, ash, and poisonous gases including sulfurdioxide. This eruption killed over threefourthS of Iceland’s livestock. It also.caused flooding. The destruction of crops and livestock resulted in thedeaths of nearly 10.000 people from starvation or disease.

MAGMA FORMATION AT A MIDOCEAN RIDGE

ve ot Spotsotau volcanic activity occurs at plate boundaries. Hot spots refer to areasfolcanic activity that result from plumes ofhot solid material that have

sen from deep within Earth’s mantle. As the material rises, it melts at areas

flower pressure. Some scientists estimate that the sources ofmagmaoduciflg such hot spots are anywhere from 670 kilometers to nearly

300 kilometers beneath the surface.A hot spot remains in the same place even as a lithospheric plate

ioves across it. As the diagram below shows, the Hawaiian Islands have

)rmed as the Pacific plate has moved northwest over a hot spot. Theagest and youngest island, Hawaii, currently lies above the hot spot. As

riu might expect, this island is volcanically active. Extending northwest is

chain of extinct volcanoes, many ofwhich have sunk below sea level asrOSiOfi has worn them away Each volcano in that chain formed over theot spot that now lies below Hawaii. The chain also includes underwaterolcanoes called seamounts which are not shown in the illustration below.

(0 4.9 MILLION YEARS AGOhe island of Kauai formed asolten rock hardened and built upn the seafloor over the hot spot.s the plate moved northwest, itarried the island away from theOt spot.

.8 TO 2.5 MILLION YEARS AGOhe island of Oahu formed afterw volcano on Kauai had movedvax trom the hot spot. Again,molten rock hardened and built upntil Qahu rose above sea level.

ODAY The island oflawajj now sits over theOt spot. The hot spot fuelsriree active volcanoes,cluding one underwater.riot shown here) Thiseamount is located tohe east of the island.

1wflmincLASSZONE COM

Observe an animation of volcanicislands forming over a hot spot.Keycode: E50904

Kauai

KauaOahu

KauaiOahu

Molokal

—

Chapter 9 Volcanoes

Island formation continues about 30 kilometers off the coast of theisland of Hawaii, where scientists are studying a young, very activeunderwater volcano called Loihi Sea,nount. These scientists predict thattens of thousands of years from now, the peak of Loihi will reach sea leveland become a new Hawaiian island. In the meantime, studies of Loihi arerevealing important information about hot spots, deep-sea geology, andthe effects of seamounts on marine life.

L.OIHI SEAMOUNTThis computer-generated image shows LoihiSeamount and the island of Hawaii.

7!ç4.

ii

Hawaii

Monitoring an underwater volcano involves unique difficulties.Although Lothi is taller than Mount St. Helens was before its eruption in1980, its peak is still about 900 meters below sea level. Since 1996, scientistshave been using submersibles to study the site. In 1997, an underseaobservatory was deployed at Loihi’s summit. This observatory allowsscientists to monitor eruptions and earthquakes with an underwatermicrophone, a seismometer, and a pressure sensor.

3,1 iection Review

Describe how magma forms at a subduction boundary.

0 Describe the volcanic activity that occurs at a divergent boundary.

CRITICAL THINKING Make a concept map or other graphicorganizer that shows how the following geologic features arerelated: subduction boundary, divergent boundary, hot spot,overriding plate, subducting plate, volcano. You may add otherwords to your concept map if you wish.

O Explain the difference between the volcanic activity that occurs at ahot spot and the volcanic activity that occurs at a subductionboundary between an oceanic plate and a continental plate.

O GEOGRAPHY Use the map of plate boundaries on pages 712—713.Identify the plates associated with the development of the CascadeRange and those associated with the development of the MarianaIslands.

Loihi Seamount

0

198 Unit 3 Dynamic Earth

Ma,11a and Erupted MateriakgjIauea a volcano on the island of Hawaii, has spewed molten rock for

decade5 In contrast, Mount St. Helens exploded violently in 1980 after

more than a century ofbeing a quiet, snow-capped mountain, Differencesthe volcanic activity at these two sites result partly from differences in

the magruas that rise to the surface there.

Types of Magmasilica, a principal ingredient in all magmas, determines a magmasyjos1ty or resistance to flow. Magmas high in silica resist flow, whereasmagmas with lower silica content flow more easily. Basaltic magmascontain the least silica; as a result, they flow most easily. Andesitic andrhyolitic rnagmas contain more silica and are more resistant to flow.

Magmas also contain gases—mainlywater vapor and carbon dioxide.The gases dissolved in the magmas at the depths where they form, mayI,ubble out of solution as the magrnas rise. Most gases escape easily frombasaltic magmas. When these magmas reach the surface, any remaininggases usually produce relatively harmless fountains and floods. Gases inmore viscous andesitic and rhyolitic magmas, however, cannot escape aseasily. As such magmas rise, the gases expand and propel the magmarapidly to the surface. The result can be an explosive eruption of gas anddebris, such as the one at Mount St. Helens.

Each type ofmagma tends to form at specific locations. Basalticmagmas form at rifts and at oceanic hot spots. Andesitic magmas tend toform at subduction boundaries. Rhyolitic magmas generally form wherehot spots underlie continental plates.

3.2KEY IDEAThe composition of magmalargely determines how explosivea volcanic eruption will be.

KEY VOCABULARYa viscositya lavapahoehoeaa• pillow lavaa pyroclastic material• pyroclastic flow

Basaltic Magma Andesitic Magma

Intermediate (about 60%)Least (about 50%)

Least

Least viscous

on Rarely explosive

t ‘r Highest

Rifts, oceanic hot spots

Rhyolitic Magma

Intermediate

Most (about 70%)

Intermediate

Most

Sometimes explosive

Most viscous

Intermediate

Usually explosive

Subduction boundaries

Lowest

Continental hot spots

Mount St. Helens

Chapter 9 Volcanoes 199

Lava FlowsMagma that reaches Earth’s surface is called lava. Like magma, lava isprimarily molten rock. The composition of lava may differ from that ofmagma, however, because materials may be added to or removed fromthe magma as it rises to the surface.

ti h,s n LandBasaltic lava flows are usually associated with less-explosive eruptions. Asthese flows cool, they form basaltic rock. The temperature and speed of abasaltic flow affect the appearance of its hardened surface. Volcanologistsuse two Hawaiian terms to describe solidified lava flows on land,palioehoe (puh-HOH-eh-HOH) and aa (AH-ah).

Basaltic lava at a high temperature flows quickly out of vents, formingpahoehoe, lava with smooth, ropelike surfaces. Cooler basaltic lava movesmore slowly. It cools quickly into aa, with rough, jagged surfaces.

L) Fill an eyedropper with water.Squeeze the water into thestraw. Time how long it takesfor all of the liquid to exit thestraw into the cup.) Repeat step 2, using vegetable

oil. Then use syrup. Use anew straw for each liquid.

Compare the three liquids to thetypes of magma described in thechart on page 199.Which of the liquids is most likebasaltic magma? Most likeandesitic magma? Most likerhyolitic magma? Explain.

Whether it comes from an underwater eruption or flows from land into thesea, lava that cools underwater has a distinctive shape—a rounded,pillowlike form with a hard crust. Pressure builds up inside the lava untilits crust cracks, and more lava pours out, forming yet another pillow shape.The resulting mass of rounded lumps is called pillow lava.

Ash and Rock Fragmentso- comes More explosive eruptions usually involve magmas which contain trapped

from a Greek word meaning fire” gases. When these gases are released, solid fragments called pyroclasticand -clastic from a Greek word (PY-roh-KLAS-tihk) material may be ejected. A combination of pyroclasticmeaning ‘broken”. material and fluid lava is common in most volcanic eruptions.

ir

‘, cjJa1ofljModeling ViscosityaLr .s.2 disposable cups• water• stopwatch• vegetable oil

• 3 straws• eyedropper• syrup

?roce(Ura) Poke a hole in a cup andposition a straw through thehole and down to the secondcup as shown, The strawshould be tilted about 5O.

PAHOEHOE Smooth, ropelike surfacescharacterize this cooling pahoehoe.

Underwater Lava Flows

AA Sharp, jagged surfaces characterizethis cooling aa.

Unit 3 Dynamic Earth

pyroclastic materials are classified by size. The smallest pieces are

called ash, pieces of intermediate size are called lapilli. and the largest

fragments are called blocks and bombs. The classification chart below

compares pyroclastic materials.

In some violent eruptions, pyroclastic material combines with hot gases to

form a pyroclastic flow—a dense, superheated cloud that travels downhillwith amazing speed. The cloud may follow the course of a valley, movingfaster than 100 kilometers per hour. The eruption ofMount Vesuvius inD. 79 produced a pyroclastic flow that buried the Roman city of Pompeiiunder pumice and ash.

Cj,ø/ C14S5!ONE COM

Examine video clips of eruptedmaterials.Keycode: ES0905

How do basaltic, rhyolitic, and andesitic magmas differ?

Describe pahoehoe, aa, and pillow lava.

Explain why rhyolitic and andesitic magmas are associated withmore-explosive eruptions, whereas basaltic magma is associatedwith less-explosive eruptions.

TCAL rH1NKING Use the information in this section to inferwhether a volcano that erupts explosively would be more likely toform at a subduction boundary or at a divergent boundary.

TiEMAflCS One of the fastest recorded lava flows in Hawaiitraveled over land at an average speed of 2.7 meters per second.Work with a partner to estimate your running speed. Predict howsuccessful you would be if you tried to outrun such a lava flow.

apnh

Diameter less than 2 mm Diameter from 2 to 64 mm Diameter greater than 64 mm

Chapter 9 Volcanoes 01

Volcanic LandforrnsThe term volcano refers not only to a volcanic vent, but also to thelandform that develops as the materials from a volcanic eruption harden.The shape and structure of a volcano are determined by the nature of itseruptions and the materials it ejects.

Shield VolcanoesBecause of its low viscosity, basaltic lava tends to flow long distances beforehardening. In some cases, the lava builds up in layers, forming shieldvolcanoes with broad bases and gently sloping sides. The broad base of ashield volcano can support a mountain of enormous height. For example,Mauna Loa, a volcano on the island of Hawaii, rises 4170 meters abovesea level and its base is 5000 meters below sea level; thus, its total height is9170 meters.

Because shield volcanoes discharge basaltic lavas, they tend to be lessexplosive than other types of volcanoes. Basaltic lava flows, however,may be frequent and copious, causing damage to homes, highways, andother propertv

Cinder ConesA cinder cone, perhaps the simplest type of volcano, forms when moltenlava is thrown into the air from a vent. As it falls, the lava breaks intofragments that harden before hitting the ground. These fragments

accumulate, forming a cone-shaped mound withan oval base. Cinder cones, which tend to besmaller than other types of volcanoes, typicallyform in groups and on the sides of larger volcanoes.

9*3KEY IDEAThe shape of a volcanic landformis determined by the materialsproduced during an eruption.

KEY VOCA2IJLAY• shield volcano• cinder cone• composite volcano• lahar• caldera• lava plateau

MAUNA LOA is a shield volcano on the island of Hawaii.

con . site Volcanoesflip0Site volcanoes develop when layers of materials from successive

e,tplosiVe eruptions accumulate around a vent. The materials include

harde1 lava flows and other pyroclastic material.The 1980 eruption ofMount St. Helens illustrates some of the forces

and events involved in shaping a composite volcano. Except for a few

minor erupflOflS, Mount St. Helens had been quiet since 1857. In thespring of 1980, however, earthquake activity increased, a bulge in the north

face of the volcano’s peak grew larger, and small eruptions of steam and

ash occurred.Eventually, an earthquake burst the bulge that had been forming on

the volcano. Magma, water, and gases exploded in a massive cloud ofsuperheated ash and stones. Some of the hot ash mixed with the snowand ice on the mountain to form a fast-moving mudflow called a laharUAH-hahr). In some areas, the debris from the lahar and landslidesreached a depth of about 46 meters.

Xfter a violent eruption, a composite volcano may remain relativelyquiet for a long period of time. Beneath the surface, however, gas-richmagma may again be building up pressure, eventually leading to anotherexplosive eruption.[ he illustrations on these pages

show three volcanic landforms andthe processes which formed each type.

MOUNT SHASTA is a composite volcanolocated in California.

CAPULIN is a cinder conelocated in New Mexico.

Sometimes magma beneath a volcano is released after the top of thevolcano collapses, forming a large crater-shaped basin called a caldera(kal-DAIR-uh). Many active volcanoes have calderas at their summits.In some cases, a caldera fills with water to form a lake.

Crater Lake in Oregon is an example of a caldera. About7700 years ago, a volcano exploded in a cataclysmic eruption.The collapse of the cone’s top created a caldera. Gradually, thecaldera filled with water to form Crater Lake, shown at left.

Some calderas, such as the one in Yellowstone National Parkin the western United States, are still active. Beneath Earth’ssurface, magma superheats the water which feeds the hotsprings and geysers atYellowstone. Geologists estimate that theeruption that produced the Yellowstone caldera may have been1000 times more powerful than the 1980 Mount St. Helens blast.

Lava Plateaus

0 Overtime, rainwater filled thecaldera, forming Crater Lake.Additional volcanic activity formedthe small cone in the center.

Sometimes plate tectonics results in the formation of along, narrow crackor fissure in Earth’s surface. Basaltic lava pouring from the fissure spreadsacross the land, forming a lava plateau. The basaltic lava that formed theColumbia Plateau in the northwestern United States is over one kilometerthick in some places and covers an area of about 164,000 square kilometers.

115etboui ReviewCompare and contrast the ways in which shield volcanoes and cindercones are formed.

0 CRITICAL THINKING Describe the formation of a composite volcano.

WRITING The eruption of Mount Rainier, a composite volcano, couldpose a serious threat to local residents. Write a description of thepotential hazards that people living near Mount Rainier might face.

CRATER LAKE is a caldera that hasfilled with water.

0 About 7700 years ago, a volcaniceruption partially emptied anunderground magma chamber.

The top of the cone collapsedinward, forming a basin known asa caldera.

04 Unit 3 Dynamic Earth

A,hen Mount Pinatubo, aVV volcano in the Philippines,rumbled to life in April 1991, scientistsquickly set up a station to monitor itsactivity. They also collected samples ofthe gas escaping from the summit ofthe mountain. Toward the end ofMay, they detected an increase insulfur dioxide emissions, which ledthem to conclude that magma wasrising. In early June, they observedthat the side of the volcano wasswelling. Meanwhile, seismic activitywas increasing. All evidencesuggested that a large eruption wasabout to occur.

From June 9 through June 15,Mount Pinatubo erupted in a series ofincreasingly violent explosions.Fortunately, scientists and publicofficials had worked together to setup a plan for educating and alertingthe population. Some 58,000 peoplehad been evacuated from the areaspredicted to be in the greatestdanger. Eventually, as many as200,000 people were forced toevacuate their homes. The evacueeswere spared from the dangerouslahars the eruption produced.Thousands of lives had been saved

from what proved to be one of themast destructive eruptions of the20th centuty S

i;sio;—SCIENCE NOTEBOOK

i Consider ways in whichI predictions made by scientistspromote safety. Explain howscientific predictions might affectyour life

predicting Eruptions, Saving LivesConsider two of the deadliest eruptions of the past century:

1902 An eruption of Mount Pelée on Martinique killsnearly 30,000 people.

1985 Lahars triggered by an eruption of Colombia’sNevado del Ruiz volcano sweep through the townof Armero, killing over 20,000 people.

Can such tragedies be prevented? Is it possible to predicta volcanic eruption in time to save lives?

DEVASTATION The 1985 eruption ofNevada del Ruiz, in Colombia, causedextensive damage and casualties,

CIASSZONECOM

How Fast Do Gases from VolcanicEruptions Travel?Use satellite images of MountPinatubo to calculate the speedof volcanic ash and gases movingthrough the atmosphere.Keycode: ES0906

EVACUATINGPeople carry theirbelongings awayfrom a dangerousarea near MountPinatubo.

Chapter 9 Volcanoes 205