Understanding Earth: Chapter 5 Volcanoes PDF

CHAPTER 5 Volcanoes Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detai...

CHAPTER 5 Volcanoes Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Soufriere Hills is a stratovolcano composed of alternating layers of hardened lava, solidified ash, and rocks ejected by previous eruptions. The eruption of Soufriere Hills, Montserrat, Caribbean, began on Friday, January 8th, 2010. Residents said it was one of the largest eruptions they have witnessed at the volcano since its Copyright © 2006. W. H. Freeman & Company. All rights reserved. reawakening in 1995. Scientists don’t believe there was a major collapse of the dome, but a significant amount of material was lost. After the seventeenth century, the volcano experienced no recorded eruptions until 1995, when a series of major eruptions eventually forced the evacuation of Montserrat’s former capital, Plymouth. Learning Objectives After you have studied this chapter, you should be able to: Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  5.1 Describe how volcanoes transport magma from the Earth’s interior to its surface. 5.2 Differentiate between the major types of volcanic deposits and explain how the textures of volcanic rocks can reflect the conditions under which they solidified. 5.3 Summarize how volcanic landforms are shaped. 5.4 Discuss how volcanic gases can affect the hydrosphere and atmosphere. 5.5 Explain how the global pattern of volcanism is related to plate tectonics. 5.6 Illustrate the hazards and beneficial effects of volcanism. The northwestern corner of Wyoming is a geologic wonderland of geysers, hot springs, and steam vents—the visible signs of a vast active volcano that stretches across the wilderness of Yellowstone National Park. Every day, this volcano expels more energy in the form of heat than is consumed as electric power in the three surrounding states of Wyoming, Idaho, and Montana combined. This energy is not released steadily; some of it builds up in hot magma chambers until the volcano blows its top. A cataclysmic eruption of the Yellowstone volcano 630,000 years ago ejected 1000 km3 of rock into the air, covering regions as far away as Texas and California with a layer of volcanic ash. The geologic record shows that volcanic explosions nearly this big, or even bigger, have occurred in the western United States at least six times during the last 2 million years, so we can be fairly certain that such an eruption will happen again. We can only imagine what it might do to human civilization. Hot ash would snuff out all life within 100 km or more, and cooler but choking ash would blanket the ground more than 1000 km away. Ash thrown high into the stratosphere would dim the Sun for several years, dropping temperatures and plunging the Northern Hemisphere into an extended volcanic winter. The hazards volcanoes pose to human society, as well as the mineral resources and energy they provide, are certainly good enough reasons to study them. In addition, volcanoes are fascinating because they are windows through which we can look into Earth’s deep interior to understand the igneous and plate tectonic processes that have generated its oceanic and continental crust. Copyright © 2006. W. H. Freeman & Company. All rights reserved. In this chapter, we will examine how magma rises through Earth’s crust, emerges onto its surface as lava, and cools into solid volcanic rock. We will see how plate tectonic processes and mantle convection explain volcanism at plate boundaries and at “hot spots” within plates. We will see how volcanoes interact with other components of the Earth system, particularly the hydrosphere and the atmosphere. Finally, we will consider their destructive power as well as the potential benefits they can provide for human society. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Volcanoes as Geosystems The geologic processes that give rise to volcanoes and volcanic rocks are known collectively as volcanism. We had a glimpse of some of these processes when we examined the formation of igneous rocks in Chapter 4, but we will take a more detailed look at them here. Ancient philosophers were awed by volcanoes and their fearsome eruptions of molten rock. In their efforts to explain volcanoes, they spun myths about a hot, hellish underworld below Earth’s surface. Basically, they had the right idea. Modern researchers also see evidence of Earth’s internal heat in volcanoes. Temperature readings of rocks as far down as humans have drilled (about 10 km) show that Earth does indeed get hotter with depth. We now believe that temperatures at depths of 100 km and more—within the asthenosphere—reach at least 1300ºC, high enough for the rocks there to begin to melt. For this reason, we identify the asthenosphere as a main source of magma, the molten rock that we call lava after it rises to the surface and erupts. Portions of the solid lithosphere that ride above the asthenosphere may also melt to form magma. Because magma is liquid, it is less dense than the rocks that produce it. Therefore, as magma accumulates, it begins to float upward through the lithosphere. In some places, the magma may find a path to the surface by fracturing the lithosphere along zones of weakness. In other places, the rising magma melts its way toward the surface. Most of the magma freezes at depth, but some fraction, probably only 10 to 30 percent, eventually reaches the surface and erupts as lava. A volcano is a hill or mountain constructed from the accumulation of lava and other erupted materials. Taken together, the rocks, magmas, and processes needed to describe the entire sequence of events from melting to eruption constitute a volcanic geosystem. This type of geosystem can be viewed as a chemical factory that processes the input material (magmas from the Copyright © 2006. W. H. Freeman & Company. All rights reserved. asthenosphere) and transports the end product (lava) to the surface through an internal plumbing system. Figure 5.1 is a simplified diagram of a volcano, showing the plumbing system through which magma travels to the surface. Magmas rising buoyantly through the lithosphere pool together in a magma chamber, usually at shallow depths in the crust. This chamber periodically empties through a pipelike feeder channel to a central vent on the surface in repeated cycles of central eruptions. Lava can also erupt from vertical cracks and other vents on the flanks of a volcano. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.1 Volcanoes transport magma from Earth’s interior to its surface, where rocks are formed and gases are injected into the atmosphere (or hydrosphere, in the Copyright © 2006. W. H. Freeman & Company. All rights reserved. case of an underwater eruption). The cross-section shows an active volcano emitting lava and smoke. The volcano is labeled as follows: central vent, flow of the lava, and side vent. The horizontal and almost vertical sheets of magma, are labeled sill and dike. The bottom-most layer of the illustration is labeled lithospheric mantle with a magma chamber. An accompanying text reads: Magma, which originates in the asthenosphere rises through the lithosphere to form a magma chamber. Lava erupts from the magma chamber through a central vent and side vents. Lava accumulates on the surface to form a volcano and gases are injected into the atmosphere. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  As we saw in Chapter 4, only a small fraction of the asthenosphere melts in the first place. The resulting magma gains chemical components as it melts the surrounding rocks while rising through the lithosphere. It loses other components as crystals settle out during transport or in shallow magma chambers. And its gaseous constituents escape to the atmosphere or ocean as it erupts at the surface. By accounting for these changes, we can extract clues to the chemical composition and physical state of the upper mantle where the lavas originated. We can also learn about eruptions that occurred millions or even billions of years ago by using isotopic dating (see Chapter 8) to determine the ages of lavas. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Lavas and Other Volcanic Deposits Lavas of different types produce different landforms. The differences depend on the chemical composition, gas content, and temperature of the lavas. The higher the silica content and the lower the temperature, for example, the more viscous the lava is, and the more slowly it moves. The more gas a lava contains, the more violent its eruption is likely to be. Types of Lava Erupted lavas, the end products of volcanic geosystems, usually solidify into one of three major types of igneous rock (see Chapter 4): basalt, andesite, or rhyolite. Basaltic Lavas Basalt is an extrusive igneous rock of mafic composition (high in magnesium, iron, and calcium) and has the lowest silica content of the three igneous rock types; its intrusive equivalent is gabbro. Basaltic magma, the product of mantle melting, is the most common magma type. It is produced along mid-ocean ridges and at hot spots within plates, as well as in continental rift valleys and other zones of extension. The volcanic island of Hawaii, which is made up primarily of basaltic lava, lies above a hot spot. Basaltic lavas erupt when hot, fluid magmas fill up a volcano’s plumbing system and overflow (Figure 5.2). Basaltic eruptions are rarely explosive. On land, a basaltic eruption sends lava down the flanks of the volcano in great streams that can engulf everything in their path (Figure 5.3). When cool, these lavas are black or dark gray, but at their high eruption Copyright © 2006. W. H. Freeman & Company. All rights reserved. temperatures (1000ºC to 1200ºC), they glow in reds and yellows. Because their temperatures are high and their silica content low, they are extremely fluid and can flow downhill fast and far. Lava streams flowing as fast as 100 km/hour have been observed, although velocities of a few kilometers per hour are more common. In 1938, two daring Russian volcanologists measured temperatures and collected gas samples while floating down a river of molten basalt on a raft of colder solidified lava. The surface temperature of the raft was 300ºC, and the river temperature was 870ºC. Lava streams have been observed to travel more than 50 km from their sources. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.2 A central vent eruption from Kilauea, a shield volcano on the island of Hawaii, produces a river of hot, fast-flowing basaltic lava. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.3 A partly buried school bus in Kalapana, Hawaii. The village was buried by a basaltic lava flow from Kilauea. Basaltic lava flows take on different forms depending on how they cool. On land, they solidify as pahoehoe (pronounced pa-hoh-ee-hoh-ee) or aa (ah-ah) (Figure 5.4). Pahoehoe (the word is Hawaiian for “ropy”) forms when a highly fluid lava spreads in sheets and a thin, glassy, elastic skin congeals on its surface as it cools. As the molten liquid continues to flow below the surface, the skin is dragged and twisted into coiled folds that resemble rope. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.4 The two forms of basaltic lava are shown here: The jagged aa lava flow is moving over a pahoehoe lava flow on the island of Hawaii. Copyright © 2006. W. H. Freeman & Company. All rights reserved. “Aa” is what the unwary exclaim after venturing barefoot onto lava that looks like clumps of moist, freshly plowed earth. Aa forms when lava loses its gases and consequently flows more slowly than pahoehoe, allowing a thick skin to form. As the flow continues to move, the thick skin breaks into rough, jagged blocks. The blocks pile up in a steep front of angular boulders that advances like a tractor tread. Aa is truly treacherous to cross. A good pair of boots may last about a week on it, and the traveler can count on cut knees and elbows. A single downhill basaltic flow commonly has the features of pahoehoe near its source, where the lava is still fluid and hot, and of aa farther downstream, where the flow’s surface— Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  having been exposed to cool air longer—has developed a thicker outer skin. Basaltic lava that cools under water forms pillow lavas: piles of ellipsoidal, pillowlike blocks of basalt about a meter wide (Figure 5.5). Pillow lavas are an important indicator that a region on dry land was once under water. Scuba-diving geologists have actually observed pillow lavas forming on the ocean floor off Hawaii. Tongues of molten basaltic lava develop a tough, plastic skin on contact with the cold ocean water. Because the lava inside the skin cools more slowly, the pillow’s interior develops a crystalline texture, whereas the quickly chilled skin solidifies to a crystal-less glass. FIGURE 5.5 These bulbous pillow lavas, which were recently extruded on the Mid-Atlantic Ridge, were photographed from the deep-sea submersible Alvin. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Andesitic Lavas Andesite is an extrusive igneous rock with an intermediate silica content; its intrusive equivalent is diorite. Andesitic magmas are produced mainly in the volcanic mountain belts above subduction zones. The name comes from a prime example: the Andes of South America. The temperatures of andesitic lavas are lower than those of basalts, and because their silica Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  content is higher, they flow more slowly and lump up in sticky masses. If one of these sticky masses plugs the central vent of a volcano, gases can build up beneath the plug and eventually blow off the top of the volcano. The explosive eruption of Mount St. Helens in 1980 (Figure 5.6) is a famous example. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.6 Mount St. Helens, an andesitic volcano in southwestern Washington State, before, during, and after its cataclysmic eruption in May 1980, which ejected about 1 km3 of pyroclastic material. The collapsed northern flank can be seen in the bottom photo. Photo a shows Mount St. Helens before the eruption as viewed across Spirit Lake. Photo b shows its explosive eruption resulting in eruption column; and photo c shows the collapsed northern flank of Mount St. Helens as viewed across Spirit Lake. Some of the most destructive volcanic eruptions in history have been phreatic, or steam, explosions, which occur when hot, gas-charged magma encounters groundwater or seawater, generating vast quantities of superheated steam (Figure 5.7). In 1883, the island of Krakatau, an andesitic volcano in Indonesia, was destroyed by a phreatic explosion. This legendary eruption was heard thousands of kilometers away, and it generated a tsunami that killed more than 40,000 people. Copyright © 2006. W. H. Freeman & Company. All rights reserved. FIGURE 5.7 A phreatic eruption of an island-arc volcano spews out plumes of steam into the atmosphere. The volcano, about 6 miles off the Tongan island of Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Tongatau, is one of about 36 in the area. Rhyolitic Lavas Rhyolite is an extrusive igneous rock of felsic composition (high in sodium and potassium) with a silica content greater than 68 percent; its intrusive equivalent is granite. It is light in color, often a pretty pink. Rhyolitic magmas are produced in zones where heat from the mantle has melted large volumes of continental crust. Today, the Yellowstone volcano is producing huge amounts of rhyolitic magma that are building up in shallow chambers. Rhyolite has a lower melting point than andesite, becoming liquid at temperatures of only 600ºC to 800ºC. Because rhyolitic lavas are richer in silica than any other lava type, they are the most viscous. A rhyolitic flow typically moves about 10 times more slowly than a basaltic flow, and it tends to pile up in thick, bulbous deposits (Figure 5.8). Gases are easily trapped beneath rhyolitic lavas, and large rhyolitic volcanoes such as Yellowstone produce the most explosive of all volcanic eruptions. Copyright © 2006. W. H. Freeman & Company. All rights reserved. FIGURE 5.8 Aerial view of a rhyolite dome that erupted about 1300 years ago in Newberry Caldera, Oregon. The light-colored rhyolite flow stands out against the trees with Paulina Peak in the background. Its dome shape indicates that the lava was very viscous. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Textures of Volcanic Rocks The textures of volcanic rocks, like the surfaces of solidified lava flows, reflect the conditions under which they solidified. Coarse-grained textures with visible crystals can result if lavas cool slowly. Lavas that cool quickly tend to have fine-grained textures. If they are silica-rich, rapidly cooled lavas can form obsidian, a volcanic glass. Volcanic rock often contains little bubbles, created as gases are released during an eruption. As we have seen, magma is typically charged with gas, like soda in an unopened bottle. When magma rises toward Earth’s surface, the pressure on it decreases, just as the pressure on the soda drops when the bottle cap is removed. And just as the carbon dioxide in the soda forms bubbles when the pressure is released, the water vapor and other dissolved gases escaping from lava as it erupts create gas cavities, or vesicles (Figure 5.9). Pumice is an extremely vesicular volcanic rock, usually rhyolitic in composition. Some pumice has so many vesicles that it is light enough to float on water. Copyright © 2006. W. H. Freeman & Company. All rights reserved. FIGURE 5.9 A sample of vesicular basalt. Pyroclastic Deposits Water and gases in magma can have even more dramatic effects than bubble formation. Before magma erupts, the confining pressure of the overlying rock keeps these volatiles from escaping. When the magma rises close to the surface and the pressure drops, the volatiles may be released with explosive force, shattering the lava and any overlying solidified rock Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  and sending fragments of various sizes, shapes, and textures into the air (Figure 5.10). These fragments, known as pyroclasts, are classified according to their size. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17. Copyright © 2006. W. H. Freeman & Company. All rights reserved. FIGURE 5.10 An explosive eruption at Arenal volcano, Costa Rica, hurls pyroclasts into the air. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Volcanic Ejecta The finest pyroclasts are fragments less than 2 mm in diameter, which are classified as volcanic ash. Volcanic eruptions can spray ash high into the atmosphere, where ash that is fine enough to stay aloft can be carried great distances. Within two weeks of the 1991 eruption of Mount Pinatubo in the Philippines, for example, its ash was traced all the way around the world by Earth-orbiting satellites. Fragments ejected as blobs of lava that cool in flight and become rounded, or as chunks torn loose from previously solidified volcanic rock, can be much larger. These fragments are called volcanic bombs (Figure 5.11). Volcanic bombs as large as houses have been thrown more than 10 km by explosive eruptions. FIGURE 5.11 Volcanic bombs within a stratified pyroclastic deposit at Hawaii Volcanoes National Park. These explosive eruptions eject volcanic materials up into the atmosphere, which then fall down and accumulate as poorly sorted Copyright © 2006. W. H. Freeman & Company. All rights reserved. deposits in pyroclastic flows. Sooner or later, these pyroclasts fall to Earth, building the largest deposits near their source. As they cool, the hot, sticky fragments become welded together (lithified). Rocks created from smaller fragments are called tuffs; those formed from larger fragments are called breccias (Figure 5.12). Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.12 Welded tuff is a volcanic igneous rock that forms when still warm ash-flow deposits weld together under the weight of overlying deposits. Photo a shows a tuff depicted as a welded rock with small fragments and a penny placed next to it. Photo b shows a volcanic breccia depicted as a rock with large angular fragments embedded in a matrix. Pyroclastic Flows Pyroclastic flows, which are particularly spectacular and often deadly, occur when a volcano ejects hot ash and gases in a glowing cloud that rolls downhill at high speeds. The solid particles are buoyed up by the hot gases, so there is little frictional resistance to their movement. In 1902, with very little warning, a pyroclastic flow with an internal temperature of 800ºC exploded from the side of Mont Pelée, on the Caribbean island of Martinique. The avalanche Copyright © 2006. W. H. Freeman & Company. All rights reserved. of choking hot gas and glowing volcanic ash plunged down the slopes at a hurricane speed of 160 km/hour. In one minute and with hardly a sound, the searing emulsion of gas and ash enveloped the town of St. Pierre and killed 29,000 people. It is sobering to recall the statement of one Professor Landes, issued the day before the cataclysm: “The Montagne Pelée presents no more danger to the inhabitants of St. Pierre than does Vesuvius to those of Naples.” Professor Landes perished with the others. In 1991, Mount Pinatubo erupted and created a major pyroclastic flow that was captured on camera in this impressive image (Figure 5.13). Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.13 Pyroclastic flow emanating from Mount Pinatubo, Philippines. After being dormant for 611 years, Mount Pinatubo erupted with massive violence, destroying everything in its path and killing 847 people. The Mount Pinatubo eruption is considered the world’s most violent and destructive volcanic eruption of the 20th century. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Eruptive Styles and Landforms The surface features produced by a volcano as it ejects material vary with the properties of the magma, especially its chemical composition and gas content, the type of material (lava versus pyroclasts) erupted, and the environmental conditions under which it erupts—on land or under the sea. Volcanic landforms also depend on the rate at which lava is produced and the plumbing system that gets it to the surface (Figure 5.14). Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.14 The eruptive styles of volcanoes and the landforms they create are determined principally by the composition of magma. Illustration a is titled shield volcano. The magma chamber is at the center, and the lava flows from the two side vents and one central vent. The height of the mountain is 10 kilometers from sea level. The length is labeled 60 kilometers. Lava moves up from the magma chamber and flows through different layers and erupts out of the crater. The following parts are labeled: magma chamber, central vent, and crater. Text pointing to the layers of the mountain reads: Each layer represents many hundreds of thin flows of basaltic lava. Text pointing to the lava eruption reads: Lava can erupt on the flanks of a volcano as well as from the central vent. An accompanying photo shows Mauna Loa volcano in Hawaii.Illustration b is titled volcanic dome. The central vent is filled with lava, and a pile-up of the viscous felsic lava is above the vent. The following parts are labeled: central vent, volcanic dome, crater, and viscous felsic lavas pile up over the vent. An accompanying photo shows Mount St. Helens in Washington. The related text reads: A dome has been growing within the center of Mount St. Helens since its 1980 eruption.Illustration c is titled cinder-cone volcano. The central vent is filled with volcanic debris, and successive layers of ejected pyroclasts are depicted on either side of the central vent. The crater is labeled. Text pointing to the vent reads: The vent may become filled with volcanic debris. Text pointing to the layers on either side of the central vent reads: Successive layers of ejected pyroclasts dip away from the crater at the summit. An accompanying photo shows the volcanic eruption of Cerro Negro in Nicaragua. The related text reads: This eruption of Cerro Negro in 1968 built a cinder cone on an older terrain of lava flows.Illustration d is titled stratovolcano. The central vent is filled with lava, and the pyroclastic layers also filled with lava are on either side of the central vent. Several lava dikes are distributed around the volcanic mountain. The following parts are labeled: crater, pyroclastic layers, and lava flows. Text pointing to the central vent reads: Central vent filled with lava from the previous eruption. Text pointing to the lava dikes reads: Lava that has solidified in fissures forms radiating dikes that strengthen the cone. An accompanying photo shows Mount Fuji in Japan.Illustration e is titled caldera. The caldera lake is depicted at the center which resembles a large volcanic crater filled with water. Several side vents are depicted from the magma chamber and move toward the caldera. The caldera rim, caldera lake, and side vents are labeled. An accompanying photo shows Crater Lake in Oregon. The Copyright © 2006. W. H. Freeman & Company. All rights reserved. related text reads: Calderas result when a violent eruption empties a volcano’s magma chamber, which then cannot support the overlying rock. It collapses, leaving a large, steep- walled basin. Central Eruptions Central eruptions discharge lava or pyroclasts from a central vent, an opening atop a pipelike feeder channel rising from the magma chamber. The magma ascends through this channel to erupt at Earth’s surface. Central eruptions create the most familiar of all volcanic features: the Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  volcanic mountain, shaped like a cone. Shield Volcanoes A lava cone is built by successive flows of lava from a central vent. If the lava is basaltic, it flows easily and spreads widely. If flows are copious and frequent, they create a broad, shield-shaped volcano two or more kilometers high and many tens of kilometers in circumference, with relatively gentle slopes. Mauna Loa, on the island of Hawaii, is the classic example of such a shield volcano (Figure 5.14a). Although it rises only 4 km above sea level, it is actually the world’s tallest mountain: Measured from its base on the seafloor, Mauna Loa is 10 km high, taller than Mount Everest! It grew to this enormous size by the accumulation of thousands of lava flows, each only a few meters thick, over a period of about a million years. The island of Hawaii actually consists of the overlapping tops of several active shield volcanoes emerging from the ocean. Volcanic Domes In contrast to basaltic lavas, andesitic and rhyolitic lavas are so viscous they can barely flow. They often produce a volcanic dome, a bulbous, steep-sided mass of rock (see Figure 5.8). Domes look as though the lava has been squeezed out of a vent like toothpaste, with very little lateral spreading. Domes often plug vents, trapping gases beneath them (Figure 5.14b). Pressure can increase until an explosion occurs, blasting the dome into fragments. Cinder Cones When volcanic vents discharge pyroclasts, these solid fragments can build up to create cinder cones. The profile of a cinder cone is determined by the angle of repose of the fragments: the Copyright © 2006. W. H. Freeman & Company. All rights reserved. maximum angle at which the fragments will remain stable rather than sliding downhill. The larger fragments, which fall near the vent, form very steep but stable slopes. Finer particles are carried farther from the vent and form gentler slopes at the base of the cone. The classic concave-shaped volcanic cone with its central vent at the summit develops in this way (Figure 5.14c). Stratovolcanoes When a volcano emits lava as well as pyroclasts, alternating lava flows and beds of Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  pyroclasts build a concave-shaped composite volcano, or stratovolcano (Figure 5.14d). Lava that solidifies in the central feeder channel and in radiating dikes strengthen the cone structure. Stratovolcanoes are common above subduction zones. Famous examples are Mount Fuji in Japan, Mount Vesuvius and Mount Etna in Italy, and Mount Rainier in Washington State. Mount St. Helens had a near-perfect stratovolcano shape until its eruption in 1980 destroyed its northern flank (see Figure 5.6c). Craters A bowl-shaped pit, or crater, is found at the summit of most volcanic mountains, surrounding the central vent. During an eruption, the upwelling lava overflows the crater walls. When the eruption ceases, the lava that remains in the crater often sinks back into the vent and solidifies, and the crater may become partly filled by rock fragments that fall back into it. When the next eruption occurs, that material may be blasted out of the crater. Because a crater’s walls are steep, they may cave in or become eroded over time. In this way, a crater can grow to several times the diameter of the vent and hundreds of meters deep. The crater of Mount Etna in Sicily, for example, is currently 300 m in diameter. Calderas When great volumes of magma are discharged rapidly from a large magma chamber, the chamber can no longer support its roof. In such cases, the overlying volcanic structure can collapse catastrophically, leaving a large, steep-walled, basin-shaped depression much larger than a crater, called a caldera (Figure 5.14e). The development of the caldera that forms Crater Lake in Oregon is shown in Figure 5.15. Calderas can be impressive features, ranging in diameter from a few kilometers to 50 km or more. Owing to their size and high-volume eruptions, large caldera systems are sometimes called “supervolcanoes.” The Yellowstone supervolcano, which is the largest active volcano in the United States, has a caldera with an area greater than Rhode Island. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.15 Stages in the formation of the Crater Lake caldera. The Stage 3 collapse occurred about 7700 years ago. A map shows the Crater Lake in Oregon. The four stages in the formation of the lake are depicted below the map. Stage 1: Fresh magma fills a magma chamber and triggers a volcanic eruption in Mount Mazama. Stage 2: The eruption continues, and the magma chamber becomes partly depleted. Stage 3: The mountain summit collapses into the empty chamber, forming a caldera. Large pyroclastic flows accompany the collapse, blanketing the caldera and a surrounding area of hundreds of square kilometers. Stage 4: A lake forms in the caldera. As the residual magma in the chamber cools, minor eruptive activity continues in the form of hot springs and gas emissions. A small volcanic cone forms in the caldera. After some hundreds of thousands of years, enough fresh magma may reenter the collapsed magma chamber to reinflate it, forcing the caldera floor to dome upward again to create a resurgent caldera. The cycle of eruption, collapse, and resurgence may occur repeatedly over geologic time. Three times over the last 2 million years, the Yellowstone supervolcano has erupted catastrophically, in each instance ejecting hundreds or thousands of times more material than the 1980 Mount St. Helens eruption and depositing ash over much of what is now the western United States. Other resurgent calderas are Valles Caldera in New Mexico and Long Valley Caldera in California, which last erupted about 1.2 million and 760,000 years ago, respectively. Diatremes When magma from Earth’s deep interior escapes explosively, the vent and the feeder channel below it are often left filled with volcanic breccia as the eruption wanes. The resulting structure is called a diatreme. Shiprock, a formation that towers over the surrounding plain in New Mexico, is a diatreme exposed by erosion of the sedimentary rocks through which it erupted. To transcontinental air travelers, Shiprock looks like a gigantic black skyscraper in Copyright © 2006. W. H. Freeman & Company. All rights reserved. the red desert (Figure 5.16). Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.16 (a) The formation of a diatreme. (b) Shiprock, towering 5l5 m above the surrounding flat-lying sediments of New Mexico, is a diatreme that has been exposed by erosion of the softer sedimentary rocks that once enclosed it. Note the vertical dike, one of six radiating from the central vent. Copyright © 2006. W. H. Freeman & Company. All rights reserved. The illustration shows the cross-section of earth. The bottom layer is labeled asthenosphere. It contains the gas-charged magmas that ascends through the lithosphere and crust. The distance between the upper layer of crust and lower layer of lithosphere is 100 kilometers. The cross section of earth further depicts the formation of a diatreme in four stages. The accompanying illustrations show the magma channel in the crust over time. Stage 1: The gas- charged magma is depicted in the lithosphere layer. The corresponding text reads: Gas- charged magma from deep in the mantle forces its way upward, fracturing the lithosphere. Stage 2: The magma rapidly ascends and explodes. The corresponding text reads: Rapidly ascending magma breaks off and carries crust and mantle fragments as it explodes at Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  supersonic speed. Stage 3: The feeder channel forms a diatreme. The corresponding text reads: After the eruption, thefeeder channel forms adiatreme made up ofsolidi ed magma and theserock fragments, or breccia. Stage 4: Radiating dikes are formed on earth’s surface. The diatreme is made up of crust and magma fragments. The following parts are labeled: former volcanic cone, diatreme, dike, and mantle and crust fragments. The corresponding text reads: The softer sediments of the cone and surface of the crust erode, leaving the diatreme core and radiating dikes we see today. The eruptive mechanism that produces diatremes has been pieced together from the geologic record. The kinds of minerals and rocks found in some diatremes could have formed only at great depths—100 km or so, well within the upper mantle. Gas-charged magmas force their way upward from these depths by fracturing the lithosphere and exploding into the atmosphere, ejecting gases and solid fragments torn from the deep crust and mantle, sometimes at supersonic speed. Such an eruption would probably look like the exhaust jet of a giant rocket upside down in the ground blowing rocks and gases into the air. Perhaps the most exotic diatremes are kimberlite pipes, named after the fabled Kimberley diamond mines of South Africa. Kimberlite is a volcanic type of peridotite—an ultramafic rock composed primarily of olivine. Kimberlite pipes also contain a variety of mantle fragments, including diamonds that were pulled into the magma as it exploded toward the surface (see Figure 21.25). The extremely high pressures needed to squeeze carbon into the mineral diamond are reached only at depths greater than 150 km. From careful studies of diamonds and other mantle fragments found in kimberlite pipes, geologists have been able to reconstruct sections of the mantle as if they had had been able to drill down to 200 km or more. These studies provide strong support for the theory that the upper mantle is made primarily of peridotite. Fissure Eruptions The largest volcanic eruptions do not come from a central vent, but through large, nearly Copyright © 2006. W. H. Freeman & Company. All rights reserved. vertical cracks in Earth’s surface, sometimes tens of kilometers long (Figure 5.17). Such fissure eruptions are the main style of volcanism along mid-ocean ridges, where new oceanic crust is formed. A moderate-sized fissure eruption occurred in 1783 on a segment of the Mid- Atlantic Ridge that comes ashore in Iceland (Figure 5.18). A fissure 32 km long opened and, in six months, spewed out 12 km3 of basalt, enough to cover Manhattan to a height about halfway up the Empire State Building. The eruption also released more than 100 megatons of sulfur dioxide, creating a poisonous blue haze that hung over Iceland for more than a year. The resulting crop failures caused three-quarters of the island’s livestock and one-fifth of its human population to die of starvation. Sulphuric aerosols from the Laki eruption were transported by the prevailing winds across Europe, causing crop damage and respiratory Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  illnesses in many countries. FIGURE 5.17 A fissure eruption generates a “curtain of fire” on Kilauea, Hawaii, in 1992. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.18 (a) In a fissure eruption, highly fluid basaltic lava flows rapidly away from the fissures. (b) These volcanic cones lie along the Laki fissure in Iceland, which opened in 1783 and erupted the largest flow of lava on land in recorded history. The illustration shows fissures depicted as vertical cracks in the Earth’s surface. It also shows widespread layers formed by previous eruptions. The cinder cones, lava, and earlier flows are labeled. The related text reads: Highly fluid basalt erupting from fissures forms widespread layers rather than mountains. The photo shows volcanic cones lying along the Laki fissure in Iceland. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Flood Basalts Highly fluid basaltic lavas erupting from fissures on continents can spread out in sheets over flat terrain. Successive flows often pile up into immense basalt plateaus, called flood basalts, rather than forming a shield volcano as they do when the eruption is confined to a central vent. In North America, a huge eruption of flood basalts about 16 million years ago buried 160,000 km2 of preexisting topography in what is now Washington, Oregon, and Idaho to form the Columbia Plateau (Figure 5.19). Individual flows were more than 100 m thick and Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  some were so fluid that they traveled more than 500 km from their source. An entirely new landscape with new river valleys has since developed atop the lava that buried the old surface. Plateaus formed by flood basalts are found on every continent as well as on the seafloor. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.19 (a) The Columbia Plateau covers 160,000 km2 in Washington, Oregon, and Idaho. (b) Successive flows of flood basalts piled up to build this immense plateau, here cut by the Palouse River. The map shows the Columbia Plateau covering parts of eastern Washington, north-central Oregon, and northern Idaho. The photo shows the Palouse River depicting the flows of flood basalts in the Columbia Plateau. Ash-Flow Deposits Eruptions of pyroclasts on continents have produced extensive sheets of hard volcanic tuffs called ash-flow deposits. A succession of forests in Yellowstone National Park has been buried under such ash flows. Some of the largest pyroclastic deposits on the planet are the ash flows that erupted in the mid-Cenozoic era, 45 million to 30 million years ago, through fissures in what is now the Basin and Range province of the western United States. The amount of material released during this pyroclastic flare-up was a staggering 500,000 km3— enough to cover the entire state of Nevada with a layer of rock nearly 2 km thick! Humans have never witnessed one of these spectacular events. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  Interactions of Volcanoes with Other Geosystems Volcanoes are chemical factories that produce gases as well as solid materials. Courageous volcanologists have collected volcanic gases during eruptions and analyzed them to determine their composition. Water vapor is the main constituent of volcanic gases (70 to 95 percent), followed by carbon dioxide, sulfur dioxide, and traces of nitrogen, hydrogen, carbon monoxide, sulfur, and chlorine. Volcanic eruptions can release enormous amounts of these gases. Some volcanic gases may come from deep within Earth, making their way to the surface for the first time. Some may be recycled groundwater and ocean water, recycled atmospheric gases, or gases that were trapped in earlier generations of rocks. As we have seen, volcanic gases released at Earth’s surface have a number of effects on other geosystems. The emission of volcanic gases during Earth’s early history is thought to have created the oceans and the atmosphere, and volcanic gas emissions continue to influence those components of the Earth system today. Periods of intense volcanic activity have affected Earth’s climate repeatedly, and they may have been responsible for some of the mass extinctions documented in the geologic record. Volcanism and the Hydrosphere Volcanic activity does not stop when lava or pyroclastic materials cease to flow. For decades, or even centuries after a major eruption, volcanoes continue to emit steam and other gases through small vents called fumaroles (Figure 5.20). These emanations contain dissolved materials that precipitate onto surrounding surfaces as the water evaporates or cools, forming various encrusting deposits. Some of these precipitates contain valuable minerals. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.20 A fumarole encrusted with sulfur deposits on the Merapi volcano in Indonesia. Fumaroles are a surface manifestation of hydrothermal activity: the circulation of water through hot volcanic rocks and magmas. Circulating groundwater that comes into contact with buried magma (which may remain hot for hundreds of thousands of years) is heated and returned to the surface as hot springs and geysers. A geyser is a hot-water fountain that spouts intermittently with great force, frequently accompanied by a thunderous roar. The best-known geyser in the United States is Old Faithful in Yellowstone National Park, which erupts about every 65 or 90 minutes, sending a jet of hot water as high as 60 m into the air (Figure 5.21). We’ll take a closer look at the mechanisms that drive hot springs and geysers in Chapter 17. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.21 Old Faithful geyser, in Yellowstone National Park, erupts regularly about every 65 or 90 minutes. Copyright © 2006. W. H. Freeman & Company. All rights reserved. Hydrothermal activity is especially intense in the spreading centers at mid-ocean ridges, where huge volumes of water and magma come into contact. Fissures created by tensional forces allow seawater to circulate throughout the newly formed oceanic crust. Heat from the hot volcanic rocks and deeper magmas drives a vigorous convection current that pulls cold seawater into the crust, heats it, and expels the hot water back into the overlying ocean through vents on the rift valley floor (Figure 5.22). Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.22 Near spreading centers, seawater circulates through the oceanic crust, is heated by magma, and is reinjected into the ocean, forming black smokers and depositing minerals on the seafloor. The illustration shows gabbros which are coarse-grained igneous rocks at the bottom. Sheeted dikes are depicted above them, and pillow basalt lavas are on top of the dikes. The cold marine water is pulled into the crust by the convection current, and it is heated. The hot water is then expelled back into the overlying ocean through vents on the rift valley floor forming black smokers while the marine sediments are deposited on the floor. The area of black smokers is depicted in the illustration by vertical arrows pointing upward. The spreading center is represented as a vertical channel at the center. The following parts are labeled: gabbro, sheeted dikes, pillow basalt, marine sediments, area of black smokers, rift Copyright © 2006. W. H. Freeman & Company. All rights reserved. valley, and spreading center. The distance between the left end of the illustration and the magma channel running at the center is 2 kilometers. Given the common occurrence of hot springs and geysers in volcanic geosystems on land, the evidence for pervasive hydrothermal activity at spreading centers immersed in deep water should come as no surprise. Nevertheless, geologists were amazed once they recognized the intensity of the convection and discovered some of its chemical and biological consequences. The most spectacular manifestations of this process were first found in the eastern Pacific Ocean in 1977. Plumes of hot, mineral-laden water with temperatures as high as 350ºC were seen spouting through hydrothermal vents at the crest of the East Pacific Rise (see Chapter Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  22). The rates of fluid flow turned out to be very high. Marine geologists have estimated that the entire volume of the ocean’s water is circulated through the cracks and vents of Earth’s spreading centers in only 10 million years. Scientists have come to realize that the interactions between the lithosphere and the hydrosphere at spreading centers profoundly affect the geology, chemistry, and biology of the oceans in a number of ways: The creation of new lithosphere accounts for almost 60 percent of the energy flowing out of Earth’s interior. Circulating seawater cools the new lithosphere very efficiently and therefore plays a major role in the outward transport of Earth’s internal heat. Hydrothermal activity leaches metals and other elements from the new crust, injecting them into the oceans. These elements contribute as much to seawater chemistry as the mineral components dumped into the oceans by all the world’s rivers. Metal-rich minerals precipitate out of the circulating seawater and form ores of zinc, copper, and iron in shallow parts of the oceanic crust. These ores form when seawater sinks through porous volcanic rocks, is heated, and leaches these elements from the new crust. When the heated seawater, enriched with dissolved minerals, rises and reenters the cold ocean, the ore-forming minerals precipitate. The energy and nutrients at hydrothermal vents feed unusual colonies of strange organisms whose energy comes from Earth’s interior rather than from sunlight. Chemoautotrophic hyperthermophiles, similar to those that populate hot springs on land, form the base of complex ecosystems, providing food for giant clams and tube worms up to several meters long. Some scientists have speculated that life on Earth may have begun in the energetic, chemically rich environments of hydrothermal vents (see Chapter 22). Volcanism and the Atmosphere Volcanism in the lithosphere affects weather and climate by changing the composition and Copyright © 2006. W. H. Freeman & Company. All rights reserved. properties of the atmosphere. Large eruptions can inject sulfurous gases into the atmosphere tens of kilometers above Earth (Figure 5.23). Through various chemical reactions, these gases form an aerosol (a fine airborne mist) containing tens of millions of metric tons of sulfuric acid. Such aerosols may block enough of the Sun’s radiation from reaching Earth’s surface to lower global temperatures for a year or two. The eruption of Mount Pinatubo, one of the largest explosive eruptions of the twentieth century, led to a global cooling of at least 0.5ºC in 1992. (Chlorine emissions from Mount Pinatubo also hastened the loss of ozone in the atmosphere, nature’s shield that protects the biosphere from the Sun’s ultraviolet radiation.) Grotzinger, John, and Jordan Thomas. Understanding Earth, W. H. Freeman & Company, 2006. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/huji-ebooks/detail.action?docID=6235026. Created from huji-ebooks on 2025-01-09 22:16:17.  FIGURE 5.23 Satellite image of the huge ash cloud spewing from the erupting Cordón Caulle volcano in central Chile on June 13, 2011. The ash plume extends 800 km from the snow-covered Andes mountains (on left side of photo) to the Argentine city of Buenos Aires (center right of photo). This ash cloud encircled the planet, closing airports in Australia and New Zealand. The debris lofted into the atmosphere during the 1815 eruption of Mount Tambora in Indonesia resulted in even greater cooling. The next year, the Northern Hemisphere suffered Copyright © 2006. W. H. Freeman & Company. All rights reserved. a very cold summer; according to a diarist in Vermont, “no month passed without a frost, nor one without a snow.” The drop in temperature and the ash fall caused widespread crop failures. More than 90,000 people perished in that “year without a summer,” which inspired Lord Byron’s gloomy poem, “Darkness”: