Chapter 18 - Volcanoes
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Model Magma Movement Magma is molten rock beneath bottom of the beaker. Squeeze Earth’s surface that rises because it is the dropper and keep the bulb less dense than the surrounding rock. depressed as you slowly pull the In this activity, you will model how dropper back out of the cold water. magma moves within Earth. 1. Fill a 250-mL beaker with 175 mL of ice-cold water. 2. Carefully fill a 100-mL beaker with very hot tap water. Add 2-3 drops of food coloring to the water and e B d CAUTION: Always wear safety goggles and an apron in the lab. What You’ll Learn * How magma forms. * What kinds of features form as the result of i Observe In your science journal, &“5 describe what happened to the col- 3 ored water when it entered the igmeans actimity Wit ity beaker. How might this be similar ' Earth. 3. Carefully fill a dropper with the to what happens to magma o i * How volcanoes form hot, colored water. beneath Earth’s surface? Infer | and how they can be 4. Slowly insert the full dropper into what would have happened if you | classified. the 250-mL beaker until the tip had released the hot water at the of the dropper is 1 cm from the surface of the cold water. . Why It's Important - Many of Earth’s internal }‘ ‘ processes help to shape ‘ our planet’s surface. Igneous activity deep within Earth and at its surface produce many of | the mountains and rock Magma OBJECTIVES Volcanic eruptions are spectacular events. The ash that spews from some volcanoes can form billowy clouds that travel around the world * Describe factors that Jastors the before raining back down to Earth. The red-hot lava that erupts from formations on Earth. j Zl]]?f;;;;egormatzon other volcanoes, such as the Hawaiian volcano Kilauea shown on the facing page, can destroy everything in their paths. In the last 10 000 { Compare and contrast years, more than 1500 different volcanoes have erupted—providing | the different types evidence that Earth is indeed geologically active. Where do ash, lava, of magma. and other types of volcanic debris come from? ‘ ‘ L | EARTY.SCIENCE % e ‘ A _ ' viscosity All volcanoes are fueled by magma deep beneath Earth’s surface. To find out more about vol- canic activity on Earth, visit the Glencoe Science Web Site at science.glencoe.com Recall from Chapter 5 that magma is a mixture of molten rock, sus- pended mineral grains, and dissolved gases deep beneath Earth’s surface. Magma forms when temperatures are high enough to melt the rocks involved. Depending on their composition, most rocks begin to melt at temperatures between 800°C and 1200°C. Such i = | i > - 18.1 Magma 471
Figure 18-1 Both pressure and the presence of water affect the melting tempera- Y ture of minerals and thus, rocks. Figure 18-1. How does the melting point of wet albite compare to that S albite veiung curveswfi!;Vt""' of dry albite at a depth of 3 km? At a depth of 12 km? Dry albite melting curve TYPES OF MAGMA - 100 Recall from Chapter 5 that the three major igneous rock types are basalt, andesite, and granite. These rocks form from three major types of magma: basaltic magma, andesitic magma, and rhyolitic magma. The term rhyolitic is used to describe the magma that solidifies to form granite because magmas are named after extrusive rocks. Basaltic magma has the same composition as basalt. Locate the Hawaiian Islands in Figure 18-2, which shows some of Earth’s active volcanoes. The volcanoes that make up the Hawaiian Islands, which L i include Kilauea and Mauna Loa, are made of basalt. Surtsey, which 1 ! formed south of Iceland in 1963, is another basaltic volcano. 800 1o 1200 Andesitic magma has the same composition as andesite. Mount St. Temperature (C) Helens in Washington State and Tambora in Indonesia are two andesitic volcanoes. You will find out more about Tambora in the Science & the Environment feature at the end of this chapter. Rhyolitic magma has the same composition as granite. The dormant volcanoes in Yellowstone National Park in the western United States were fueled by rhyolitic magma. ( -1 200 Wet albite melting curve Depth (km) Pressure (MPa) -1 300 temperatures exist at the base of the lithosphere and in the astheno- ' sphere, the plasticlike portion of the mantle directly beneath the lithosphere. Recall that temperature increases with depth beneath Earth’s surface. If rocks melt at temperatures found in the astheno- ‘ sphere, and temperature increases with depth, then why isn’t the 1‘ entire mantle liquid? What other factors, besides temperature, affect ‘j the formation of magma? Some Active Volcanoes of the World Pressure Pressure is one factor that determines whether rocks will ‘ melt to form magma. Like temperature, pressure increases with depth | because of the weight of overlying rocks. Laboratory experiments have shown that as pressure increases, the temperature at which a ‘; substance melts also increases. Figure 18-1 shows two melting curves ‘ j" for a variety of feldspar called albite. Find the line that represents the =0 ‘ dry melting curve. Note that at Earth’s surface, dry albite melts at \ Pifo‘agubf ( i about 1100°C, but at a depth of about 12 km, the melting point of dry AN o | albite is about 1150°C. At a depth of about 100 km, the melting point %'& "~ B Galapagos—=e ‘ . of dry albite increases to 1440°C. The effect of pressure explains why Krakatau " ®ee w‘"%"-.. Islands most of the rocks in Earth’s lower crust and upper mantle do not melt WBihbors i\ ' % to form magma, even though the temperatures are high enough. i’£< — Mt. Fuji e T ] Kilauea = [ 2 : Paricutin 8 2 e— Mariana Islands Mauna Loa Kilimanjdre f: 2 . Water The presence of water also influences whether a rock will Y ¥l ;E:;::,e(; v | | | melt. Recall that water can be found in the pore spaces of some rocks B g : ‘ | | and can be bound into the crystal structure of some minerals. Even EE::Etion\. g | a small amount of water can have a significant effect on a mineral’s, : ‘ | and thus a rock’s, melting point. At any given pressure, a wet mineral : | or rock will melt at a lower temperature than the same mineral or Figure 18-2 Compare this map of some of Earth’s active volcanoes to the map shown rock under dry conditions. Locate the melting curve of wet albite in in Figure 17-13 on page 455. Where are most active volcanoes located? ‘ 472 CHAPTER 18 Volcanic Activity i . : . 7 18.1 Magma 473
How does silica affect lava flow? Model the changes in lava viscosity with the addition of silica.: a2 B4 caurion: Always wear safety goggles and an apron in the lab. Procedure 1. Pour 120 mL of dishwashing liquid into a 250-mL beaker. 2. Stir the liquid with a stirring rod. Describe the viscosity. _ 3. Add 30 g of NaCl (table salt) to the liquid. Stir well. Describe what happens. 4. Repeat step 3 three more times. Analyze and Conclude 1. What do the liquid and NaCl represent? 2. How does an increase in silica affect lava viscosity? 3. Basaltic eruptions are called flows because of the way they move across Earth’s sur- face. What can you infer about the silica content of a basaltic flow? Magma Composition What accounts for the different types of magma? A number of factors determine the composition of magma, as shown in Table 18-1. One of these factors is viscosity, the internal resis- tance to flow. Substances such as honey, liquid soap, and motor oil have a higher viscosity than water, vinegar, and gasoline. Refer to Table 18-1. What kind of magma has a viscosity similar to that of honey? You can model the effect of silica content on vis- cosity in the MiniLab on this page. Basaltic Magma Basaltic magma typi- cally forms when rocks in the upper mantle melt. Most basaltic magma rises relatively rapidly to Earth’s surface and reacts very little with crustal rocks because of its low viscosity. Because basaltic magma contains small amounts of dissolved gases and silica, the volcanoes it fuels erupt relatively quietly. Andesitic Magma Andesitic magma is found along continental margins, where oceanic crust is subducted into Earth’s man- tle. The source material for this magma can be either oceanic crust or oceanic sediments. As shown in Table 18-1, andesitic magma contains about 60 percent silica. This high silica content results in its having an interme- diate viscosity. Thus, the volcanoes it fuels are said to have intermediate eruptions. Rhyolitic Magma Rhyolitic magma forms when molten material rises and mixes with the overlying silica- and water-rich continental crust. The high viscosity of rhyolitic magma inhibits its movement. This resistance to flow, along with the large volume of gas trapped within this magma, makes the volcanoes fueled by rhyolitic magma very explosive. VISCOSITY The viscosity of magma and of its surface counterpart, lava, depends on both temperature and composition. The hotter the magma or lava, the lower the viscosity. The temperatures of basaltic lavas are generally between 1000°C and 1250°C. Rhyolitic lava temperatures are usually between 700°C and 900°C. Which type of lava, basaltic or rhyolitic, has a greater viscosity as a result of its temperature? What do you think happens to viscosity as magma or lava cools? The amount of silica in magma or lava increases the viscosity, as you discovered in the MiniLab on the previous page. Thus, magmas and lavas high in silica have higher viscosities than magmas and lavas low in silica. As shown in Table 18-1, rhyolitic magmas have the highest silica content, basaltic magmas the lowest, and andesitic magmas have silica contents between these two extremes. Based on composition, which type of lava, basaltic lava or andesitic lava, has a lower viscosity? Basaltic lavas, because of their low silica content, have a lower viscosity than andesitic lavas. The basaltic lava flows that often erupt from Mauna Loa in Hawaii, which is shown in Figure 18-3, have been clocked at 16 km/h! " SECTION, ASSESSMENT - Figure 18-3 Basaltic lava has a low viscosity, and thus, flows relatively quickly from a volcano. The basaltic volcano shown here is Mauna Loa, one of the many volcanoes that make up the Hawaiian Islands. 1. Describe three factors that affect the formation of magma. 2. How does the presence of water affect the lava flow? the melting temperature of a rock? witnessed the eruption, what do you think they were able to observe about | P! ' SKILL REVIEW ‘ 4 3. Compare and contrast the properties of 7. Concept Mapping Use the following terms | % T e ; the three types of magma. ! | Table 18-1 Magma Composition and Characteristics i yp 9 N to construct a concept map to organize | " " silica Licaticion 4. Refer to Table 18-1. Where does andesitic the major ideas in this section. For more | | C iti Moturc_el Viscosit Con::nt Co:1t§nt Explosiveness Magma psegms formid What is the sslirce materiat ‘help, refer to the Skill Handbook. @ omposition ateria scosity p g of this type of magma? Ll I Basaltic Upper mantle Low 1-2% about 50% Least Both oceanic and 5. Explain the relationship between the vis- moderate R , : . g eruption ti : | | magma continental crust cosity of a magma and its temperature. ~een Sruption [i‘ | Andesitic Oceanic crust Intermediate 3-4% about 60% Intermediate Contine_ntal margins 6. Thinking Critically A volcano violently intermediate f ! . magma and oceanic associated with erupted in Indonesia in 1883. What can silica ‘ i sedfincims subduction 20iEe you infer about the composition of the E‘ . Rhyolitic Continental High 4-6% about 70% Greatest Continental crust magma that fueled the volcano? If people magma magma magma ; ? k|| magma crust I R R TR I R R T R IR TV R, i J (1S 474 CHAPTER 18 Volcanic Activity 18.1 Magma 475
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OBJECTIVES * Explain how magma affects overlying crustal rocks. - Compare and contrast intrusive igneous rock bodies. VOCABULARY pluton batholith stock laccolith sill dike Figure 18-4 Magma can enter fissures in rocks (A). Magma can also cause blocks of rock to break off the overlying rock into which the magma intrudes. These blocks of rock become part of the magma body (B). Magma can cause the rocks with which it comes in contact to melt (C). Intrusive Activity Magma, because it is molten, is less dense than surrounding rocks. This density difference, which you modeled in the Discovery Lab, forces magma to move upward and eventually come into contact with, or intrude, the overlying crust. Intruding magma can affect the crust in several ways, as shown in Figure 18-4. Magma can force the overlying rock apart and enter the newly formed fissures. Magma can also cause blocks of rock to break off and sink into the magma, where the rocks may eventually melt. Finally, magma can melt the rock into which it intrudes. But what happens deep in the magma chamber as the magma slowly cools? PLUTONS Recall from Chapter 5 that when magma cools, minerals form. Over a very long period of time, these minerals will combine to form intrusive igneous rock bodies. Some of these rock bodies are thin, ribbonlike features only a few centimeters thick and several hundred meters long. Others are very large, ranging in size from about 1 km? | to hundreds of cubic kilometers. These intrusive igneous rock bod- ies, called plutonms, can be exposed at Earth’s surface as a result of uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks. Batholiths and Stocks The largest plutons are called batholiths. These irregularly shaped masses of coarse-grained igneous rocks cover at least 100 km? and take millions of years to form. Batholiths are common in the interiors of major mountain chains. Many batholiths in North America are composed primarily of granite, the 476 CHAPTER 18 Volcanic Activity Volcano Lava flow is 1 P ”\// Stock Figure 18-5 Igneous activity results in the formation of bodies of rock both at the surface and deep within Earth. most common rock type found in plutons. However, gabbro and diorite, the intrusive equivalents of basalt and andesite, are also found in batholiths. The largest batholith in North America, the Coast Range Batholith in British Columbia, is more than 1500 km long. Irregularly shaped plutons that are similar to batholiths but smaller in size are called stocks. Both batholiths and stocks, as shown in Figure 18-5, cut across older rocks and generally form 10-30 km beneath Earth’s surface. Laccoliths Sometimes, when magma intrudes into parallel rock lay- ers close to Farth’s surface, some of the rocks bow uRwérd as a result of the intense heat and pressure of the magma body. When the magma solidifies, a laccolith forms. As shown in Figure 1 85, a laccolith is a mushroom-shaped pluton with a round top and flat bottom. Compared to batholiths and stocks, laccoliths are relatively small; they are, at most, up to 16 kilometers wide. Laccoliths exist in the Black Hills of South Dakota, the Henry Mountains of Utah, and the Judith Mountains of Montana, among other places. Sills and Dikes A sill is a pluton that forms when magma intrudes parallel to layers of rock, as shown in Figure 18-5. A sill can range from only a few centimeters to hundreds of meters in thick- ness. The Palisades Sill, which is exposed in the cliffs above the Hudson River near New York City, is about 300 m thick. What effect do you think this sill, shown on the next page, had on the sedimen- tary rocks into which it intruded? M. Laccolith 18.2 Intrusive Activity 477
layers of parallel rock. Figure 18-6 The Palisades Sill in New York State formed over 200 million years ago when magma forced its way into Figure 18-7 Note that, unlike a sill, this dike, which is in Arizona, cuts across the rocks it intrudes. Unlike the sill shown in Figure 18-6, which is parallel to the rocks it intrudes, a dike is a~pluton that cuts across preexisting rocks, as shown in Figure 18-7. Dikes often form when magma invades cracks in surrounding rock bodies. Most dikes are a few centimeters to several meters wide and up to tens of kilometers long. The Great Dike in Zimbabwe, Africa, however, is an excep- tion: it is about 8 km wide and 500 km long. While the textures of sills and dikes vary, many of these plutons are coarse grained. Recall from Chapter 5 that grain size is related to the rate of cooling. Coarse-grained sills and dikes are thought to have formed deep in Earth’s crust, where the magma cooled relatively slowly to yield large mineral grains. PLUTONS AND TECTONICS Many plutons are formed as the result of mountain-building processes. In fact, batholiths are found at the cores of many of Earth’s mountain ranges. Where did the enormous volume of magma that cooled to form these igneous bodies come from? Recall from Chapter 17 that many major mountain chains formed along continental- continental convergent plate boundaries. Scientists hypothesize that these collisions might have forced continental crust down into the upper mantle, where it melted, intruded into the overlying rocks, and eventually cooled to form batholiths. 478 CHAPTER 18 \Volcanic Activity Batholiths are also thought to have formed as a result of oceanic- oceanic convergence. Again, recall from Chapter 17 that when two oceanic plates converge, one plate is subducted into the mantle. Parts of this subducted plate melt to form magma. The Sierra Nevada batholith, which has been exposed at Earth’s surface as a result of uplift and erosion, formed from at least five episodes of igneous activity beneath what is now California. The famous gran- ite cliffs found in Yosemite National Park, some of which are shown in Figure 18-8, are relatively small parts of this extensive batholith. The plutons that form deep beneath Earth’s surface represent the majority of igneous activity on our planet. Nevertheless, most peo- ple think of volcanoes when they hear the words igneous activity. These often-spectacular examples of igneous activity at Earth’s sur- face are discussed in the next section. 1. Discuss three ways in which magma affects the crust into which it intrudes. 2. What are plutons, and how are they classified? 3. How are sills and dikes similar? How do SKILL REVIEW Figure 18-8 The granite cliffs that tower over Yosemite National Park in California are parts of the Sierra Nevada batholith that have been exposed at Earth’s surface. found along the margins and coarser grains are found toward the middle of the pluton. What might cause this difference in texture? they differ? Give an example of each. 4. What is a laccolith? 5. Thinking Critically Sometimes, the texture in the same sill varies: finer grains are 6. Making a Table Make a table in which you compare and contrast the different types of intrusive igneous bodies. For more help, refer to the Skill Handbook. 18.2 Intrusive Activity 479
OBJECTIVES * Describe the major parts of a volcano. » Compare and contrast shield, cinder-cone, and composite volcanoes. « Contrast the volcanism that occurs at plate boundaries. - Explain the relationship between volcanism and hot spots. VOCABULARY vent crater caldera shield volcano cinder-cone volcano composite volcano tephra pyroclastic flow hot spot What comes to mind when you hear the word volcano? Do you pic- ture clouds of ash and jagged rocks being thrown violently into the air? Or do you envision rivers of reddish-orange lava flowing down the slopes of a steep volcanic peak? Both of these represent volcanic activity on Earth’s surface. Volcanism produces various features that alter Earth’s landscape. In this section, you will examine some of these features, beginning with the one created at the point where magma reaches the surface: the vent. ANATOMY OF A VOLCANO At the beginning of this chapter, you learned that magma chambers deep within Earth fuel the volcanoes that erupt at the planet’s sur- face. Also recall that when magma reaches Earth’s surface, it is called lava. Lava erupts through an opening in the crust called a vent. As lava flows out onto the surface, it cools and solidifies around the vent. Over time, the lava can accumulate to form a mountain known as a volcano. At the top of a volcano, around the vent, is a bowl- shaped depression called a crater. The crater is connected to the magma chamber by the vent. Locate the crater of the volcano shown in Figure 18-9. Figure 18-9 A crater is the bowl-shaped depression that surrounds the central vent at a volcano’s summit. The volcano shown below is one of many that dot the northern Arizona landscape near Flagstaff. e oy are T T e R N ‘.n‘ )’4_:, i in Oregon. This caldera formed about 6600 years ago as a result of numerous volcanic eruptions of Mount Mazama. Volcanic craters are usually less than 1 km in diameter!Larger depressions called calderas, which can be up to 50 km in diameter, however, can form when the summit or the side of a volcano col- lapses into the magma chamber that once fueled the volcano. The caldera now known as Crater Lake formed in this way, as shown in figure 18-11. The caldera walls, which are visible in the photograph in Figure 18-10, form cliffs that tower nearly 600 m above the water’s surface. Wizard Island, which is located in the center of the lake, is actually a small volcanic cone that formed after the caldera collapsed. TYPES OF VOLCANOES The appearance of a volcano depends on two factors: the type of material that forms the volcano and the type of eruptions that occur. Based on these two criteria, three major types of volcanoes have been identified: shield volcanoes, cinder-cone volcanoes, and composite volcanoes. Each differs in size, shape, and composition. Shield Volcanoes A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base. Shield volcanoes form when layer upon layer of basaltic lava accumulates during Figure 18-11 Crater Lake formed as the result of many eruptions. Mount Mazama erupted many times. Mount Mazama - ag' . chamber | The top of partially empty magma chamber collapsed. Pyroclastic Pyroclastic flow Caldera eventually filled with water to form lake. Wizard Island Crater Lake 18.3 Volcanoes 481
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Figure 18-12 Mauna Loa, shown in the distance, is a shield volcano in Hawaii. A small cinder-cone volcano on the flank of Mauna Kea is visible in the foreground. = NATIONAL GEOGRAPHIC To learn more about volcanoes, go to the National Geographic Expedition on page 886. nonexplosive eruptions. Recall that eruptions involving basaltic lava are less explosive than other eruptions. This is because basaltic lava has a low viscosity as a result of the relatively small amounts of gases and silica it contains. The shield volcanoes that make up the Hawaiian Islands are made of basalt. Mauna Kea, which is shown in Figure 18-12, is one such volcano. Cinder-Cone Volcanoes A cinder-cone volcano forms when material ejected high into the air falls back to Earth and piles up around the vent. Cinder-cone volcanoes have steep sides, as shown in Figure 18-13, and are generally small; most are less than 500 m high. The magma that fuels cinder-cone volcanoes contains more water and silica than the magma that fuels shield volcanoes. This more vis- cous magma also contains large volumes of gases, which make cinder- cone volcanoes more explosive in nature than shield volcanoes. Composite Volcanoes Composite volcanoes form when layers of volcanic fragments alternate with lava. As with cinder-cone volca- noes, the magma that forms composite volcanoes commonly con- tains large amounts of silica, water, and gases. Composite volcanoes are much larger than cinder-cone volcanoes, and, because of their violently explosive nature, they are potentially dangerous to humans and the environment. Two composite volcanoes of the Cascade Range in the western United States, Mount St. Helens and Mount Rainier, are shown in Figure 18-14. 482 CHAPTER 18 Volcanic Activity - h) 4 . Cinder cone R ey e Figure 18-13 The lzalco volcano in El Salvador shows the typical profile of a cinder-cone volcano. Size and Slope Look at the small sketches that show the relative sizes of the three types of volcanoes in Figures 18-12 through 18-14. These diagrams are drawn to scale. As you can see, shield volcanoes are by far the largest. The smallest volcanoes are cinder-cone volca- noes, which often form on or very near larger volcanoes. Notice, too, that cinder-cone volcanoes have the steepest slopes, while shield vol- canoes have the gentlest slopes. The slopes of cinder-cone and com- posite volcanoes are concave, and the slopes of shield volcanoes are straight. These differences in both size and slope are the result of many factors, including the different kinds of materials that make up each volcano, the vegetation that grows on the volcano’s slopes, local climate, and the eruptive history of the volcano. VoLcANIC MATERIAL Rock fragments thrown into the air during a volcanic eruption are called tephra. Tephra can be newly cooled and hardened lava, min- eral grains that started to crystallize prior to the eruption, or pieces of the volcanic cone. Tephra are classified by size. The smallest frag- ments, called dust, are less than 0.25 mm in diameter. Ash, another kind of tephra, is larger than dust but less than 2 mm in diameter. Somewhat larger fragments of tephra are called lapilli, an Italian word that means “little stones.” Lapilli are larger than 2 mm but less than 64 mm in diameter. The largest tephra thrown from a volcano Figure 18-14 Two months before it erupted in 1980, Mount St. Helens, foreground, displayed the steeply sloping sides of a typical composite volcano. Mount Rainier, another composite volcano, is shown in the background. 18.3 Volcanoes 483
Figure 18-15 More than 29 000 people died as a result of the pyroclastic flow that accompanied the 1902 eruption of Mount Pelée on the island of Martinique. Note that much of the city of St. Pierre was destroyed. EARTH SCIENCE % To find out more about damage caused by volca- noes, visit the Glencoe Science Web Site at L science.glencoe.com can be the size of a car or a small building. When these large volcanic fragments are angular, they are called volcanic blocks. Volcanic blocks as large as houses have been ejected more than 10 km into the air during some eruptions. When blobs of lava are forcefully ejected from a volcano, they may cool to form rounded or streamlined tephra called volcanic bombs. Volcanic bombs may harden in the air or they may flatten and solidify after they hit the ground. Pyroclastic Flows Some tephra cause tremendous damage and kill thousands of people. Violent volcanic eruptions can send clouds of gas, ash, and other tephra down a slope at incredible speeds. This rapidly moving volcanic material, which is called a pyroclastic flow, can travel at speeds of nearly 200 km/h and may contain hot, poiso- nous gases. The temperature at the center of a pyroclastic flow can exceed 700°C. One of the most widely known and deadly pyroclastic flows occurred in 1902 on Mount Pelée, on the island of Martinique in the Caribbean Sea. More than 29 000 people suffocated or were burned to death. What little was left of the town of St. Pierre after the eruption is shown in Figure 18-15. WHERE DO VOLCANOES OCCUR? The distribution of volcanoes on Earth’s surface is not random. Most volcanoes form at plate boundaries. In fact, about 80 percent of all volcanoes are found along convergent boundaries, and about 15 per- cent are found along divergent boundaries. Only about 5 percent of extrusive igneous activity occurs far from plate boundaries. 484 CHAPTER 18 \Volcanic Activity Convergent Volcanism Recall from Chapter 17 that plates come together along convergent boundaries. Also recall that conver- gence involving oceanic plates creates subduction zones, places where slabs of oceanic crust descend into the mantle and eventually melt. The magma generated is forced upward through the overlying plate and forms volcanoes when it reaches the surface. The volcanoes associated with convergent plate boundaries form two major belts, as shown in Figure 18-16. The larger belt, the Circum-Pacific Belt, is also called the Pacific Ring of Fire. It stretches along the western coasts of North and South America, across the Aleutian Islands, and down the eastern coast of Asia. Volcanoes in the Cascade Range of the western United States, and Mount Pinatubo in the Philippines are some of the volcanoes in the Circum-Pacific Belt. The smaller belt, which is called the Mediterranean Belt, includes Mount Etna and Mount Vesuvius, two composite volcanoes in Italy. Divergent Volcanism Volcanic activity is also common along divergent plate boundaries, where two plates are moving apart. Magma is forced upward into the fractures and faults that form as the plates separate. These areas of major faults and fractures are called rift zones. Most of the world’s rift volcanism occurs under water along ocean ridges. Recall from Chapter 17 that this type of volcanism results in the formation of new ocean floor during the o Vesuvius = Krakatau === Circum-Pacific Belt == Mediterranean Belt EARTH SGIENCE —6n me—— 9 Update For an online update of recent volcanic eruptions, visit the Glencoe Science Web Site at science. glencoe.com and select the appropriate chapter. The Circum-Pacifi EN Meditrranean Volcanic Belts Deception Island‘—y ; — J Figure 18-16 Most of Earth’s volcanoes form two distinct volcanic belts: the larger Circum-Pacific Belt and the much smaller Mediterranean Belt. - Nevado del Ruiz 18.3 Volcanoes 485
Using Numbers Look at Figure 18-17. Note that the distance from Daikakuji Sea- mount to Hawaii is about 3500 km. Daikakuji is 43 mil- lion years old. What is the average speed of the Pacific Plate? averaging 0.4 km/h. Making and Using Graphs Calculate and graph how fast lava flows On June 8, 1783, the Laki fissure zone in Iceland began to erupt in what would become the largest flood basalt in recent history. A flood basalt forms when lava flows 2. Plot the data on a graph: from fissures to create a vast plain or plateau. The Laki erup- tion resulted in a total volume . of 14.73 km?® of basalt, which cov- Thinking Critically ered 565 km?. The lava erupted from fissures located 45 km from the coast, and flowed at speeds process of seafloor spreading. One of the few places where rift vol- canism can be observed above sea level today is in Iceland. This island is a part of the Mid-Atlantic Ridge, and consequently, several active volcanoes dot the landscape. Hot Spots Some volcanoes are located far from plate boundaries. These volcanoes form as the result of hot spots, which are unusually hot regions of Earth’s mantle where high-temperature plumes of mantle material rise toward the surface. Plumes originate deep within the mantle, or perhaps even near the core-mantle boundary. The intense heat of the plumes melts rock, which is then forced upward toward the crust as magma. The magma, in turn, melts through the crust to form volcanoes. While a plume does move ver- tically, it does not move laterally. As a result, a trail of progressively older volcanoes forms as a plate moves over a hot spot. Some of Earth’s best known volcanoes formed as a result of hot spots under the Pacific Ocean. The Hawaiian Islands, for example, continue to rise above the ocean floor as the Pacific Plate moves slowly over a hot spot. The volcanoes on the oldest island, Kauai, are inactive because the island no longer sits above the hot spot. The world’s most active volcano, Kilauea, is on the big island of Hawaii and is currently located over the hot spot. Another volcano, Loihi, is forming on the seafloor east of the big island of Hawaii and may eventually break the ocean surface to form a new island. - Analysis 1. Design a data table to show the distance traveled by the lava over a five-day period. Calculate the dis- tance every 12 hours. put time on the x-axis and distance on the y-axis. 3. How long did it take the lava to reach the coast? 4. How many kilometers did the lava travel in three days? 486 CHAPTER 18 Volcanic Activity The chains of volcanoes that form over hot spots provide important information about plate motions. The rate and direction of motion can be calculated from the positions of these volcanoes. Even changes in plate motion that occurred in the distant past can be determined. Look at Figure 18-17. Note that the Hawaiian Islands are at one end of the 5800-km Hawaiian-Emperor volcanic chain. The oldest sea- mount, Meliji, is at the other end of the chain and is about 75-80 million years old, which indicates that this hot spot has existed for at least that many years. =) The bend in the chain at Daikakuji Seamount records ag }Q{Daikak”ji a change in the direction of the Pacific Plate tha % %% occurred about 43 million years ago. ' g In addition to seamount chains, hot spots can result in the formation of flood basalts. Flood basalts erupt from fissures rather than a central vent and 00 500 Km form flat plains or plateaus rather than volcanic e —@UEVEIELR UG TR [T e T ‘-North America Pacific Ocean Sev Hawaii mountains. The volume of basalt in these eruptions can be tremendous. The Columbia River Basalts in the northwestern United States, for example, contain 170 000 km? of basalt. The volume of basalt in the Deccan Traps in India is estimated to be 512 000 km?’. The volume of basalt in the Laki eruption in Iceland, which you can analyze in the Problem-Solving Lab on page 486, is small by compari- son at 14.73 km’, Volcanic activity is proof that Earth is a dynamic planet. And, while many volcanic eruptions can be spectacular events, these geologic phe- nomena can pose risks to humans and their environment. In the Internet GeoLab that follows, you will research and rank some of Barth’s potentially dangerous volcanoes. Figure 18-17 The Emperor Seamounts and the Hawaiian Islands continue to form as the Pacific Plate moves over a stationary hot spot in the mantle. 1. What is a volcanic crater, and how does it differ from a caldera? 2. Describe the different kinds of tephra. 3. Explain why volcanic blocks would be uncommon on shield volcanoes. SKILL REVIEW 6. What are hot spots? 7. Thinking Critically The slopes of compos- ite volcanoes are notoriously unstable and prone to landslides. Why? 4. What is-a pyroclastic flow? What are the characteristics of a pyroclastic flow that make them so dangerous? 5. Where are Earth’s major volcanic belts located? 8. Comparing and Contrasting Compare and contrast the characteristics of the three major types of volcanoes. For more help, refer to the Skill Handbook. 18.3 Volcanoes 487
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f | - Science €4— — the Environment | it was June 10. Tambora What Jerome and other New Englanders expe- rienced during the cold summer of 1816 was directly linked to an event that had occurred one year earlier and thousands of kilometers away. On April 5, 1815, Mount Tambora, a dormant volcano in Indonesia, came alive in a series of explosive eruptions. Tambora’s Direct Impact Historic reports attest to Tambora’s explosive power. The eruption was heard on Jakarta, which is more than 1200 km away. Over the course of that April week, the volcano ejected an estimated 150 km? of tephra into the ocean and onto sur- rounding islands. By contrast, the volume of debris erupted during the 1980 explosion of Mount St. Helens was only 1 km?®. Volcanic ash from Tambora hung thickly in the sky and caused a three-day period of darkness within 800 km of the island. By the time the erup- tions stopped, more than a third of the 3900-m mountain had been blown off. Some 12 000 peo- ple were Killed directly by volcanic fallout, and more than 80 000 died soon after from famine and disease. The disaster, however, was not over. Its global effects would be felt around the world the following year—the year without a summer. Indirect Consequences Tambora spewed an immense amount of vol- canic dust and gases such as sulfur dioxide into the atmosphere. These particles prevented sun- 490 CHAPTER 18 Volcanic Activity The Year Without a Summer ’n 1816, Chauncey Jerome, a Connecticut clockmaker, reported that the clothes his wife had hung out to dry the day before had frozen overnight. This would not have been significant, except for the date— light from reaching Earth’s surface. In effect, the short wavelengths of incoming sunlight, which are similar in size to particles of dust and gas, collided with the particles and were reflected back into space. The problem was worsened when heat radiated from Earth’s surface, which takes the form of longer wavelengths, escaped into space. The net result was wildly fluctuating weather on a global scale. A snowfall in southern ltaly, unusual in itself, caused widespread alarm because the snow was tinted red from the vol- canic ash. In New England, summer temperatures dipped and soared from about 2°C to over 31°C “within a matter of days. Crops were devastated. At the time, the cause of the climatic changes was not understood; no one linked the changes to the eruption of Tambora. Today, how- ever, we know that volcanic gases can linger in the atmosphere for years after an eruption and wreak havoc on the weather. Not all volcanic eruptions have negative effects. Go to science.glencoe.com for links to information on the eruptions that occurred in northern Arizona some 1000 years ago. How did these eruptions affect the Sinagua? What positive impacts did the eruptions have? Summary Section 18.1 Main Ideas = Temperature, pressure, and the presence of water are factors that affect the formation of magma. ® As pressure increases, the temperature at which a substance melts also increases. At any given pressure, the presence of water will cause a substance to melt at a lower temperature than the same substance under dry conditions. ® There are three major types of magma: basaltic magma, andesitic magma, and rhyolitic magma. These magmas differ in the source rock from which they form, viscosity, silica content, gas content, and explosiveness. Basaltic magma is the least explosive magma; rhyolitic magma is the most explosive. Vocabulary viscosity (p. 474) SECTION 18.2 Intrusive Activity ¥ i j Main Ideas = Magmatic intrusions affect the crust in several ways. Magma can force overlying rock apart and enter the newly formed fis- sures. Magma can also cause blocks of rock to break off and sink into the magma chamber. Magma can melt the rock into which it intrudes. Batholiths, stocks, sills, dikes, and laccoliths are plutons that are classified according to their size, shape, and relationship to sur- rounding rocks. Batholiths are the largest plutons and often form the cores of many of Earth’s major mountain chains. Vocabulary batholith (p. 476) dike (p. 478) laccolith (p. 477) pluton (p. 476) sill (p. 477) stock (p. 477) SECTION 18.3 Volcanoes Main Ideas Lava flows onto Earth’s surface through a vent. Over time, multi- ple lava flows may accumulate to form a volcano. A crater is a depression that forms around the vent at the summit of a volcano. A caldera is a large crater that forms when a volcano collapses during or after an eruption. = There are three types of volcanoes: shield volcanoes, cinder-cone volcanoes, and composite volcanoes. ' * Rock fragments ejected during eruptions are called tephra. * Most volcanoes form along convergent and divergent plate boundaries. Volcanoes also form over hot spots, which are unusually hot areas in the mantle that are stationary for long periods of time. @ Flood basalts form when lava flows from fissures to form flat plains or plateaus. Vocabulary caldera (p. 481) cinder-cone volcano (p. 482) composite volcano ~ (p.482) crater (p. 480) hot spot (p. 486) pyroclastic flow (p. 484) shield volcano (p. 481) tephra (p. 483) vent (p. 480) Study Guide 491
h"‘n‘;l-_“-._— L e ML ENG LR ETNE G EEY 1. Which of the following does NOT play a role in magma formation? a. temperature b. pressure c. presence of water d. tephra type 2. Which of the following is true? a. An increase in pressure results in a higher melting temperature of a dry substance. b. A decrease in pressure increases the temperature at which a dry substance melts. ¢. The addition of water increases the melting temperature of a substance. d. An increase in pressure decreases the melting temperature of a dry substance. 3. Which of the following melts to form rhyolitic magma? a. continental crust b. oceanic crust c. oceanic sediment d. the upper mantle 4. Which type of pluton is completely parallel to the rock layers into which it intrudes? | a. dike ¢. laccolith b. sill d. stock 5. The Hawaiian volcanoes formed as a result of which of the following? a. divergence b. a hot spot ¢. subduction d. subsidence 6. Which of the following is NOT true? a. An increase in silica increases the viscosity of a magma. b. Andesitic magma has both an intermediate gas content and explosiveness. ¢. An increase in temperature increases a magma’'s viscosity. d. Basaltic magma has a low viscosity and contains little gas. 492 CHAPTER 18 Volcanic Activity . What is the largest type of tephra? a. ash ¢. dust b. volcanic blocks d. lapilli . Which of the following has broad, gently sloping sides and a circular base? a. hot spot b. cinder-cone volcano €. composite cone _ d. shield volcano . What is the Ring of Fire, and why does it exist? 10. Explain the relationship between hot spots and « volcanism. Use the table to answer questions 11-15. Private | State Local Sector Federal Forestry 168.0 | 218.1 63.7 — Clean-up. 307.9 9.7 5.0 41.3 Property 43.6 44.8 25 16.0 Agriculture — 39.1 —_— — Income ——— 8.9 —_— ] Transportation 2.1 11. 12. 13. 14. 15. _need to skip a question, make sure you skip the * corresponding bubble on the answer sheet also. What was the total economic cost of cleaning up after the eruption? ‘What was the total economic loss from this eruption? What percent of the total loss was caused by property damage? Which sector suffered the smallest loss? Which sector suffered the greatest economic loss? What percent of the total was this? ) THE . ' PRINCETON REVIEW STANDARDIZED TEST FORMS Fill in one answer bubble as you answer each question. If you ICSQELGT R ~ Applying Main Ideas 16. As rhyolitic magma rises to Earth’s surface, 17. 18. 19. 20. 21. 22. 23. 24, 25. 26. pressure decreases and water escapes from the magma. What effect does this have on the melting temperature? How might this cause the magma to solidify before reaching the surface? How does magma affect the rocks into which it intrudes? Hawaiian lava flows can travel great distances through underground passageways called lava tubes. Why would lava flow faster through a lava tube than it would above ground? Describe batholiths and explain where and how they form. What is a laccolith and how does it form? Explain the relationship among a vent, a crater, and a caldera. . Thinking Critically Soils that form from volcanic debris are very productive. What are some reasons for the high fertility of volcanic soils? Pumice, a volcanic glass that contains such a large percentage of holes that it floats in water, is almost never basaltic in composition. Why? Which type of volcano would you expect to produce the largest volume of tephra? Explain. Why do shield volcanoes have gentle slopes and large bases? Geothermal energy associated with magma chambers close to Earth’s surface can be used to produce electricity. Name several places in the United States where the use of this energy might be possible. '};RHIFT\ICETON Test P i B ERINCE] ~ Test Practice INTERPRETING DIAGRAMS Use the diagram below to answer the following questions. 1. What kind of volcano is shown in the diagram? a. cinder-cone volcano b. composite volcano ¢. shield volcano d. hot-spot volcano 2. What kind of volcanic feature is designated by the letter A? a. the vent b. the magma chamber ¢. the crater d. the sill 3. What type of material makes up the layer designated by the letter B? a. lava ¢. tephra b. flood basalts d. volcanic gases 4. What type of material makes up the layer designated by the letter C? a. lava ¢. tephra b. flood basalts d. volcanic gases 5. Which of the following is NOT true of this type of volcano? a. It erupts violently. b. The magma that fuels it is rich in silica. c. It forms over a hot spot in Earth’s mantle. d. It has concave slopes. Assessment 493 Tl AL