Chapter 15 Volcanic Eruptions, Hazards, and Mitigation

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Volcanic eruptions are spectacular, violent, and often quite dangerous expressions of Earth’s dynamic internal processes (Figure 15-1). More than 80% of the earth’s surface above and below sea level is of volcanic origin, and gaseous emissions from volcanoes helped form the earth’s oceans and atmosphere.²² On average, about 60 to 70 eruptions occur worldwide each year; half of these are continuations of previously erupting volcanoes, and the others are new eruptions.^63,69 There are approximately 600 active volcanoes in the world and probably another 800 that have erupted at least once during the past 10,000 years.⁶⁹ Some volcanoes erupt only once in their lifetime, whereas others erupt repeatedly or even continuously. The largest explosive eruptions occur infrequently; in general, the longer the time interval between eruptions, the larger the next eruption tends to be. Some of the worst volcanic catastrophes in history have occurred at volcanoes believed to be extinct, including the famous eruption of Vesuvius in AD 79, which destroyed the cities of Pompeii and Herculaneum (see later).⁶⁹ In fact, in the past two centuries, 12 of the 17 largest eruptions were the first eruptions known in historical times.^57,63 Table 15-1 lists some notable historical eruptions, the most recent of which occurred at Chaitén Volcano in southern Chile, coming back to life after being inactive for 9400 years.³⁷ The Chaitén eruption, which began in May 2008, has continued nonexplosively into June 2010. This eruption has special volcanologic significance, because it is the world’s first major rhyolitic eruption since the 1912 eruption of Novarupta, Alaska—the largest in the 20th century (Figure 15-2; see Table 15-1).

FIGURE 15-1 Eruption of Mt St Helens on May 18, 1980.

(Courtesy Robert M. Krimmel, U.S. Geological Survey.)

TABLE 15-1 Notable Historical Eruptions

FIGURE 15-2 A, Satellite view of an ash plume produced during the explosive phase of the ongoing eruption of Chaitén Volcano, southern Chile; plume is drifting downwind across Argentina and dissipating over the Atlantic Ocean. B, Aerial oblique view in January 2010 of the actively growing 2008-2010 lava dome within Chaitén’s 3-km (1.9-mile)-wide summit caldera.

(A MODIS/NASA image, May 3, 2008; B courtesy John Pallister, U.S. Geological Survey.)

Volcanic eruptions have been responsible for the deaths of approximately 300,000 people in the past 400 years. In recent years, the average is two to four fatal volcanic events per year.⁶⁴ Causes of death and number of fatalities are not well documented.⁶⁴ Most deadly of the volcano hazards, pyroclastic flows have claimed the most lives, whereas tephra is the most common killer (Box 15-1). Long after the eruption, famine and disease epidemics are responsible for up to one-third of the total fatalities attributed to explosive eruptions (Tables 15-2 and 15-3).⁶⁴

TABLE 15-2 Causes of Fatalities From Notable Volcanic Disasters Since 1000 AD

TABLE 15-3 Fatalities From Volcanic Eruptions, 1783-2000

Compared with other natural disasters, volcanic eruptions occur infrequently, affect few people, and are responsible for only a small percentage of fatalities. Only some 2% of all natural disasters are from volcanic activity.^33,65 The deadliest volcanic eruption in history, Tambora, Indonesia, in 1815, killed approximately 60,000 people, whereas 1 million people were killed in the worst hurricane (Ganges Delta in Bangladesh, 1970) and 830,000 were killed in the worst earthquake (Shaanxi earthquake in China, 1556).^36,62,69 Nevertheless, volcanoes have the potential to unleash one of the most destructive forces on Earth. Approximately 74,000 years ago, an Indonesian volcano named Toba exploded and ejected 2800 km³ (672 cubic miles) of ash, dust, and volcanic gases high into the stratosphere, where it was carried around the world by high-altitude winds. The particulate matter interfered with solar radiation and is thought to have led to a 10° C (22° F) temporary global cooling of the earth’s surface.^62,77 Such a degree of cooling today would be catastrophic on a global scale.

We can expect more fatalities from volcanic eruptions as human settlements inexorably encroach on areas with high-risk volcanoes.⁴ Current estimates suggest that about 500 million people, or 10% of the world’s population, live within 100 km (62 miles) of a volcano that has been active in the historical record.^4,53 Auckland, New Zealand, occupies an area of young volcanoes. Seattle–Tacoma, Washington, is on land that could be devastated by mudflows from an eruption of Mt Rainier. Naples, Italy, is built on the flanks of Vesuvius, where in the first minutes of a major eruption more than 100,000 people could be killed (Figure 15-3).^4,9,57 Latin America and the Caribbean are areas of particularly high risk because of population density. During the 20th century, 76% of all fatalities from volcanic eruptions occurred in this region, and in the past 10 years, one-half of the most powerful eruptions occurred in this area. In general, most fatalities occur in densely populated, less developed countries (Table 15-4).⁵³

FIGURE 15-3 A, Mt Eden, one of 50 geologically young but dormant volcanoes in the Auckland Volcanic Field, within the city of Auckland, New Zealand. B, Mt Rainier sits just 137 km (85 miles) southeast of Seattle–Tacoma, Washington. C, Bay of Naples, Italy, with Mt Vesuvius in the background.

(A courtesy Lloyd Holmer, Institute of Geological and Nuclear Sciences, New Zealand; B courtesy Lyn Topinka, U.S. Geological Survey; C courtesy Robert I. Tilling, U.S. Geological Survey.)

TABLE 15-4 Fatalities From Volcanic Eruptions by Region, 1600-1982

Although volcanic eruptions constitute a significant natural hazard, other processes and products of volcanism can be highly beneficial to society, explaining in part why so many people live on or near volcanoes.⁶² Volcanic ash rejuvenates soil and can prevent the loss of phosphorus, resulting in highly productive agricultural land (Figure 15-4, online). Volcanic ore deposits supply diamonds, copper, gold, silver, lead, and zinc. Products of volcanic activity are used as building materials, as abrasive and cleaning agents, and for many chemical and industrial uses.²² Geothermal heat and steam can drive turbines to generate electricity, heat homes and industries directly, and be enjoyed by people at hot spring resorts (Figure 15-5, online). The beauty of volcanoes and volcanic activity also generates income for communities through tourism.

FIGURE 15-4 Farmer plowing a lush rice paddy in central Java, Indonesia. Sundoro Volcano looms in the background. The most highly prized rice-growing areas have fertile soils formed from the breakdown of young volcanic deposits.

(Courtesy Robert I. Tilling, U.S. Geological Survey.)

FIGURE 15-5 Blue lagoon in Iceland, with Svartsengi power plant in the background. Clean geothermal energy heats 87% of the homes in Iceland, including all of the capital city, Reykjavík. Excess geothermal water (which is absolutely clean) is ejected into the lagoon, and where there is swimming, the temperature averages about 40° C (104° F). The lagoon is a very popular tourist destination.

(Courtesy Mary Dagold.)

Vesuvius, AD 79

The best-known volcanic eruption was Vesuvius on August 24, AD 79. This eruption killed thousands of people, devastated the surrounding countryside, and destroyed at least eight towns, most notably Pompeii and Herculaneum. Before this eruption, Vesuvius was seen as a benign mountain with lush vineyards planted on the slopes and human settlements on the mountain’s flanks. Unlike the nearby often-erupting volcanoes Etna and Stromboli, Vesuvius was not considered volcanic. Even a series of preeruption earthquakes, as is typical before eruptions, did little to raise concern that Vesuvius was an active volcano. The eruption was witnessed and documented by Pliny the Younger. In two letters written to the Roman historian Tacitus, Pliny wrote that the first explosion began in the early afternoon and ended in the evening of the following day. At first, Vesuvius produced an enormous eruption cloud that ejected ash, pumice, and volcanic gases vertically up to 30 km (19 miles) high. Then, from a darkened sky, ash and pumice rained down on the towns of Pompeii and Straide, causing roofs to collapse under the weight of the ash fall and burying the towns (Figure 15-6, online).

FIGURE 15-6 View from the main square of the well-preserved ruins of Pompeii, with Vesuvius in the background.

(Courtesy Harvey E. Belkin, U.S. Geological Survey.)

The town of Herculaneum, lying at the foot of Mt Vesuvius on a cliff overlooking the sea, was initially spared from burial. The prevailing wind blew away from the town and toward Pompeii. But in the early hours of August 25, Vesuvius exploded again, this time ejecting hot gases, ash, and pumice down the mountain’s slopes as a pyroclastic flow. Herculaneum was destroyed, buried beneath more than 20 m (66 feet) of pumice and ash. Whatever remained of Pompeii was also destroyed at that time.⁵⁶ At about that time, Pliny the Younger wrote of the death of his uncle, Pliny the Elder, who probably died of asphyxiation from being caught too close to a pyroclastic flow.⁴

The remains of more than 2000 people found amid the ruins offer clues as to the causes of death. People unwilling or unable to flee their homes were instantaneously suffocated as hot volcanic ash and gases entered buildings. Others had multisystem trauma as the force of the explosion and pyroclastic flows scooped up rocks and building materials and smashed them into anything in their path. Skeletons recently uncovered in beach caves in Herculaneum suggested that the cause of death was the intense heat, roughly 500° C (932° F) (Figure 15-7).^12,44,67

FIGURE 15-7 The “Garden of the Fugitives,” in Pompeii, Italy, reveals 13 adults and children huddled together in a futile attempt to shield themselves from pyroclastic flows from the eruption of Vesuvius in AD 79. The human casts are obtained by filling cavities left in hardened ash with liquid chalk.

(Courtesy Lancevortex.)

Volcanoes and Their Global Distribution

Different images are associated with the word volcano—for example, a violently erupting Mt St Helens, a peaceful-looking snow-capped Volcán Villarrica in Chile, or rivers of lava flowing down the flanks of Kilauea Volcano in Hawaii (Figure 15-8). Volcano also means the opening, or vent, in the earth’s crust through which molten rock, ash, and gases are ejected. Molten rock, while underground, is called magma. It becomes lava once it reaches the surface. Whether a volcano erupts explosively (like Vesuvius or Mt St Helens) or nonexplosively (like the lava flows of Kilauea) depends on the magma—its composition, temperature, gas content, viscosity, and crystal content.⁶² Magma that is fluid and hot tends to erupt frequently (every few years) and generally nonexplosively. This type of volcano most commonly produces fountains or rivers of red-hot lava.⁷² In contrast, magma that is less fluid and cooler, and that contains trapped gases, tends to rise sluggishly and can plug up the volcanic vent. If enough pressure builds up as trapped gases expand during ascent, the pent-up pressure can blow the plug, abruptly unleashing the expanding gases and producing a violent eruption.³⁵

FIGURE 15-8 A, Violently erupting Mt St Helens on May 18, 1980. B, Villarrica Volcano, Chile, looks peaceful and even has a ski lift on its flanks, but the mountain is actually the most active volcano in Chile. C, A long river of lava flowing downhill from Kilauea Volcano in 1983. It ultimately enters the ocean 12 km (7.5 miles) away.

(A courtesy Keith Ronnholm; B courtesy Robert I. Tilling, U.S. Geological Survey; C courtesy J. D. Griggs, U.S. Geological Survey.)

The most volcanically and seismically active zone in the world, called the Ring of Fire, coincides roughly with the borders of the Pacific Ocean (Figure 15-9).³⁵ The volcanically active countries in this zone include Russia, Japan, the Philippines, Indonesia, Papua New Guinea, New Zealand, and the countries on the Pacific coasts of North, Central, and South America. Volcanoes are also scattered in the Atlantic and Pacific Oceans—in Hawaii, Iceland, and the Galapagos.

FIGURE 15-9 The Ring of Fire, a zone of frequent earthquakes and volcanic eruptions encircling the Pacific Ocean. Where the plates come together, they form deep oceanic trenches (green).

(From Kious WJ, Tilling RI: This dynamic Earth: The story of plate tectonics, Washington, DC, 1996, U.S. Government Printing Office.)

Theory Of Plate Tectonics

The global distribution of volcanoes, as well as the origin and distribution of mountain ranges and earthquakes, are explained by plate tectonics.^54,69 This theory suggests that the earth’s outermost layer is broken into a number of rigid plates that move relative to one another as they float atop the hotter, semisolid, more mobile material of the mantle. The nature and distribution of volcanic activity depend on the formation, movement, and destruction of these plates at their margins (Figure 15-10).

FIGURE 15-10 Cross section of the main types of plate boundaries.

(From “This Dynamic Planet,” a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)

When two tectonic plates move away from each other, or diverge, new crust is created as molten rock pushes up from the mantle and oozes out onto the earth’s surface or the seafloor. One well-known, mostly oceanic, divergent boundary, the Mid-Atlantic Ridge, is a submerged mountain range in the Atlantic Ocean that extends from the Arctic Ocean to beyond the southern tip of Africa. Along the Mid-Atlantic Ridge are numerous volcanoes, including those that rise above sea level in Iceland (Figure 15-11). Another prominent divergent boundary, entirely continental, is the 11,000-km-long (6835-mile-long) Great African Rift. About 75% of Earth’s volcanism occurs unseen along divergent boundaries, deep on the ocean floor.

FIGURE 15-11 A, Cross section of an oceanic divergent boundary. B, Map of the Mid-Atlantic Ridge, along which the Eurasian and African plates are pulling away from the North American and South American plates. Note the ridge rising above sea level in Iceland. C, Exposed segment of the Mid-Atlantic Ridge at Thingvellir, Iceland. Left of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (right of the fissure).

(A from “This Dynamic Planet,” a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory; B and C from Kious WJ, Tilling RI: This dynamic Earth: The story of plate tectonics, Washington, DC, 1996, U.S. Government Printing Office.)

Two plates can also move toward one another, or converge. How two converging plates interact depends on the type of crust—continental or oceanic. Continental parts of plates, composed largely of granitic rocks, are relatively lightweight compared with the much denser and heavier oceanic parts of plates, which are composed of basalt. When two continental plates collide, both buckle upward to form a mountain range. The Himalaya Mountains are the result of the Indian plate colliding against the Eurasian plate (Figure 15-12). Few volcanoes are located in zones of continental collisions.

FIGURE 15-12 A, Cross section of two continental parts of plates converging, resulting in the formation of a mountain range. B, The snow-capped Himalayas and the Tibetan Plateau are produced by the collision of the Indian-Australian Plate (left) against the Eurasian Plate (right).

(A from Kious WJ, Tilling RI: This dynamic Earth: The story of plate tectonics, Washington, DC, 1996, U.S. Government Printing Office; B courtesy NASA, photo STS41G-120-22. http://eol.jsc.nasa.gov/sseop/EFS/printinfo.pl?PHOTO=STS41G-120-22.)

Many of the world’s volcanoes are located at the convergent boundary between a continental plate and an oceanic plate. When these plates converge, the heavier oceanic plate dives, or subducts, below the lighter continental plate. This produces tremendous pressure and heat, melting the rock deep in the mantle. This molten rock, or magma, traps gases, such as carbon dioxide (CO₂) and sulfur dioxide (SO₂), making it buoyant in the surrounding denser, solid rock, and it begins to rise through the surrounding solid rock. Magma movement causes earthquakes that can be recorded with sensitive volcano-monitoring equipment. If the rising magma finds an area of weakness at the earth’s surface, a volcano is created either just inland from the coast—for example, the volcanoes in eastern Russia, the Cascades in North America, and the Andes in South America—or just off the coast, such as the volcanic island arcs of the Aleutian Islands in Alaska, and Indonesia (Figure 15-13). Because of their proximity to a continent’s coastline or to an island, subduction volcanoes are often near populated regions and thus can have a significant human impact. Eruptions tend to be violent and explosive, such as the 1980 eruption of Mt St Helens or the 1991 eruption of Mt Pinatubo in the Philippines.⁶²

FIGURE 15-13 A, Cross section showing subduction of an oceanic plate resulting in the formation of continental volcanoes and mountains. B, Kronotsky Volcano, at 3525 m (11,570 feet) above sea level, lies at the margin of the Pacific Ocean on the Kamchatka Peninsula of Russia. It was formed from the subduction of the Pacific Plate (right) under the Eurasian Plate (left). C, Cross section illustrating subduction of an oceanic plate with formation of volcanic island arcs. D, View of steep-sided, symmetric Carlisle volcano on Carlisle Island in the central Aleutian Islands. The 1620-m (5315-foot)-high stratovolcano has erupted several times since the late 1700s.

(A and C from Kious WJ, Tilling RI: This dynamic Earth: The story of plate tectonics, Washington, DC, 1996, U.S. Government Printing Office; B modified from NASA, photo STS61A-45-0098. http://eol.jsc.nasa.gov/sseop/EFS/photoinfo.pl?PHOTO=STS61A-45-98; D courtesy M. Harbin, University of Alaska Fairbanks, U.S. Geological Survey.)

Some volcanoes are found far from plate boundaries. The Hawaiian volcanoes are more than 3200 km (1988 miles) from the nearest plate boundary, and Yellowstone, with over 10,000 geysers, hot springs, and boiling mud pools, is located in the interior of the North American plate. At both of these locations, an inferred hot spot below the plate melts overriding rock to produce magma that can rise toward the surface and ultimately erupt onto the seafloor or land. It has been assumed that the hot spot is stationary, whereas the tectonic plate above it moves. However, recent studies have questioned the fixedness of hot spots, prompting substantial debate among Earth scientists.

Examination of oceanic hot spots shows that in a series of islands created by plate movement, the islands farthest from the hot spot are the oldest. For example, a 6000-km (3728-mile) chain of volcanoes stretches from the older Emperor Seamounts (underwater sea mountains) off Alaska to the younger Hawaiian Islands. These were all created by passage of the Pacific Plate over the Hawaiian hot spot. This hot spot, currently located under the Big Island of Hawaii, has remained stationary for about 45 million years, whereas the Pacific Plate has been slowly moving to the northwest. According to the stationary-hot-spot model, as the plate continues to move over the hot spot, a new Hawaiian island will eventually emerge above sea level. In fact, just 35 km (22 miles) southeast of the Big Island there is an underwater volcano, Loihi, that has risen 3 km (1.9 miles) above the seafloor and is within 1 km (0.6 miles) of the ocean surface (Figure 15-14). At hot-spot volcanoes, eruptions tend to be nonexplosive and are rarely life threatening.³⁵ Huge volumes of fluid lava pour out, often creating large volcanic mountains, such as Mauna Loa, the largest single volcano in the world, standing 8851 m (5.5 miles) above the sea floor.

FIGURE 15-14 A, Cross section of the Hawaiian hot spot. B, Map of the 6000-km (3728-mile)-long Emperor Seamount–Hawaiian Ridge formed by the passage of the Pacific Plate over the Hawaiian hot spot over a span of 70 million years.

(A from Kious WJ, Tilling RI: This dynamic Earth: The story of plate tectonics, Washington, DC, 1996, U.S. Government Printing Office; B from “This Dynamic Planet,” a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)

Types of Volcanoes

As erupted material accumulates around a volcanic vent, a volcano is formed and progressively grows. Classified by structure, the most common types of volcanoes are composite volcanoes, calderas, shield volcanoes, subglacial volcanoes, and flood basalts. These can be roughly grouped as either explosive or nonexplosive volcanoes. However, volcanoes can show both explosive and nonexplosive behavior during their lifespan. For example, Kilauea typically erupts nonexplosively, spewing out fountains and rivers of lava, yet a 1924 eruption was spectacularly explosive, and new studies show that Kilauea’s explosive eruptions are as frequent as those of Mt St Helens.

Generally Explosive Volcanoes

Composite Volcanoes

Composite volcanoes, or stratovolcanoes, have steep sides and are typically associated with subduction zones at converging plate boundaries. Mt Fuji in Japan, Mt Pinatubo in the Philippines, and Mt Rainier in the United States are classic examples. However, composite volcanoes can also be formed at divergent plate boundaries (e.g., Hekla in Iceland and Mt Kilimanjaro in Tanzania) or at hot spots (e.g., Mt Erebus in Antarctica and Tenerife of the Spanish Canary Islands).^56,57 The steep-sided shape results from the deposition of viscous lava flows alternating with pyroclastic flows and ashfall deposits. The explosive nature of composite volcanoes comes from the high viscosity and volatility of the magma (Figure 15-15).

FIGURE 15-15 A, Cross section of a composite volcano (also called a stratovolcano). B, Mt Fuji, Japan, is a perfect example of a composite volcano.

(Courtesy Thomas C. Pierson, U.S. Geological Survey.)

Calderas

Formed by the collapse of a volcanic structure, calderas are circular or elliptical depressions, generally over 1 km (0.62 mile) in diameter. Crater Lake in the U.S. Cascade Range is a large, partially filled caldera 10 km (6.2 miles) in diameter and 600 m (1970 feet) deep, formed when Mt Mazama exploded 7700 years ago. Krakatau, in Indonesia, was created by an 1883 eruption that involved rapid emptying of the magma chamber and subsequent collapse of the volcano.⁵⁶ Kilauea Crater is another well-known caldera (Figure 15-16).

FIGURE 15-16 A, Cross section of the formation of a caldera. B, Crater Lake, Oregon, was formed by the caldera-forming eruption of Mt Mazama 7700 years ago. C, When that mountain erupted cataclysmically, the summit collapsed, forming a caldera that eventually filled with water to form Crater Lake. D, Kilauea Caldera, Hawaii, in 1954 after an eruption within the caldera (dark area is the new lava).

(B courtesy Peter Dartnell, U.S. Geological Survey; C courtesy Crater Lake Natural History Association; D courtesy U.S. Geological Survey.)

Very large calderas fed by huge active magma chambers have been called supervolcanoes and are capable of producing enormously explosive eruptions. Yellowstone Caldera, 85 km (53 miles) long by 45 km (28 miles) wide, the largest and most worrisome in the United States, has a magma chamber that is 40 km (25 miles) long, 20 km (12 miles) wide, and 10 km (6 miles) deep.⁵⁷ Yellowstone has produced explosive eruptions 1000 times larger than the 1980 Mt St Helens eruption. Two other geologically young, large calderas located in the United States are Long Valley Caldera in California and Valles Caldera in New Mexico. Outside the United States, very large calderas include Campi Flegrei in Italy, Kamari Caldera on the island of Kos in the eastern Aegean, and Rabaul Caldera in Papua New Guinea. Fortunately, very large caldera-forming eruptions were rare events, occurring approximately once in hundreds of thousands of years (Figure 15-17, online).⁵⁷

FIGURE 15-17 A, Map of Yellowstone National Park and the 650,000-year-old caldera (outlined in red) that is 45 km (28 miles) wide and 85 km (53 miles) long. B, View from space of Rabaul Caldera, Papua New Guinea. The caldera is 9 km (5.6 miles) wide by 14 km (8.7 miles) long and filled by the sea.

(A courtesy U.S. Geological Survey; B courtesy NASA.)

Generally Nonexplosive Volcanoes

Shield Volcanoes

Shield volcanoes have gentle slopes and are formed almost exclusively of layers of lava that have often flowed great distances from the eruptive vents. The magma, unlike that of explosive volcanoes, is predominantly of low viscosity and low volatility. Shield volcanoes are mostly formed at hot spots, although some are located at convergent or divergent plate boundaries—for example, Erta Ale in Ethiopia and Mt Etna in Italy. The largest volcanoes on Earth, Mauna Loa and Mauna Kea of Hawaii, are classic examples of shield volcanoes (Figure 15-18).

FIGURE 15-18 A, Cross section of a shield volcano. B, Snow-capped Mauna Loa as seen from the Hawaiian Volcano Observatory on the Big Island of Hawaii.

(B courtesy Robert I. Tilling, U.S. Geological Survey.)

Subglacial Volcanoes

Historically active volcanoes located under glaciers, called subglacial volcanoes, are known only in Iceland and Antarctica. They can erupt explosively, but thick ice cover generally inhibits material from being ejected high into the atmosphere. Instead, lava flows out, often melting ice and creating subglacial lakes and rivers. If this subglacial water suddenly escapes onto the glacier’s surface (a glacial burst, or jökulhlaup), a huge river can form and destroy anything in its path. Grímsvötn volcano, in Iceland, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by an ice shelf 200 m (656 feet) thick, and only the southern rim of the 6- by 8-km (3.7- by 5-mile) caldera is exposed (Figure 15-19).⁵⁷ Eyjafjallajökull, a subglacial volcano 120 km (75 miles) from Reykjavik, Iceland, began to erupt on March 20, 2010, after being dormant since 1823. During the eruption, hot lava melted the overlying glacial ice and generated jökulhlaups (see Figure 15-19, C) that caused destructive flooding and prompted evacuation of more than 800 inhabitants. Then, on April 14, the eruption entered a much more explosive phase and propelled an enormous ash plume more than 8 km (5 miles) into the atmosphere.⁶¹ The easterly drift of this plume over northern Europe provides an illustrative example of the potential hazards of volcanic ash for aviation safety (see Volcanic Ash, later). As of June 2010, the Eyjafjallajökull eruption is continuing, although at a reduced level of activity.

FIGURE 15-19 A, Cross section of a subglacial volcano. B, Subglacial volcano Grímsvötn in Iceland lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200-m (656-foot)-thick ice shelf. A volcanic plume rises, during the November 2004 eruption, from the exposed southern rim of the 6- by 8-km (3.7- by 5-mile) caldera. C, Aerial view of jökulhlaups from the 2010-to-present eruption at Eyjafjallajökull, Iceland (see text).

(B courtesy Freysteinn Sigmundsson, Nordic Volcanological Center.)

Flood-Basalt Plateaus

Flood-basalt plateaus are massive areas of hardened lava produced from the largest volcanic events known on Earth.²⁹ Found on all continents and ocean floors, flood-basalt plateaus are created when basaltic magma erupts rapidly from fissures to form sheets of lava flows, typically tens of meters thick and covering tens of thousands of square kilometers.²⁹ Good examples are the Columbia River Plateau (Figure 15-20, online) and the Deccan Plateau of India. The rate of erupting lava can be 20 times the average eruption rate of a hot-spot volcano, and the eruption is on average 1000 times bigger than that of a supervolcano.⁵⁷ Not surprisingly, the copious release of volcanic gases during the eruptions of flood basalts can severely affect the climate, and there is a strong correlation between mass extinction on Earth and the eruption of flood basalts (see Figure 15-20).⁵⁷

FIGURE 15-20 The Columbia River Flood-Basalt Plateau (green) covers approximately 163,000 km² (63,000 square miles) of the Pacific Northwest. In some areas, it is up to 1800 m (5900 feet) thick.

(Courtesy U.S. Geological Survey.)

Undersea Volcanoes

Most undersea volcanoes are found at oceanic divergent plate boundaries. Others are found at undersea hot spots or near island arcs at convergent plate boundaries. Where two plates diverge, magma is injected along the space created. If the magma reaches the seafloor, pillow lava (from slow effusions) or sheets of lava (from more rapid effusions) form. Sometimes, if the magma supply to a single point is sufficiently large and persistent, a cone-shaped volcano, called a seamount, rises from the seafloor. Seamounts found at undersea hot spots become island shield volcanoes, like the Hawaiian volcanoes, if they rise above sea level. Seamounts at subduction zones become island-arc volcanoes, like the volcanoes of Indonesia, above sea level. It is not uncommon for hot-spot seamounts to emerge above sea level and then later sink as the seamount moves away from the hot spot and the seafloor subsides under the weight of the lava. Atolls, found in equatorial regions, are the coral reefs around drowning island volcanoes, after they become inactive and sink below sea level (Figure 15-21, online).⁵⁷

FIGURE 15-21 A, Scuba divers filming undersea volcanic activity offshore of Kilauea Volcano. B, Formation of an atoll. C, Bora Bora (top right), Society Islands, French Polynesia. The island of Bora Bora includes the remnants of an extinct volcano surrounded by a barrier coral reef. Tupai Atoll (bottom left) is all that remains of a sunken volcano.

(A courtesy Sharkbait Productions, Kona, Hawaii; C courtesy NASA, photo STS068-258-042. http://eol.jsc.nasa.gov/sseop/EFS/photoinfo.pl?PHOTO=STS068-258-42.)

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