Volcanic Eruptions, Hazards, and Mitigation

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).





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 15-3 Fatalities From Volcanic Eruptions, 1783-2000














































  Fatalities
Volcanic Hazard No. %
Posteruption famine and disease epidemics 75,000 30
Pyroclastic flows 67,500 27
Lahars 42,500 17
Volcanogenic tsunamis 42,500 17
Debris avalanches 10,000 4
Volcanic ash 10,000 4
Volcanic gases 1750 <1
Lava flows 750 0.3
TOTAL 250,000

Modified from Baxter PJ: Impact of eruptions on human health. In Sigurdsson H, Hougthon BF, McNutt SR, et al, editors: Encyclopedia of volcanoes, San Diego, 2000, Academic Press.


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



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


































  Fatalities
Region No. %
Indonesia 161,000 67
Caribbean 31,000 13
Japan 19,000 8
Iceland 9400 4
Everywhere else 19,000 8
TOTAL 239,400 100

Modified from Blong RJ: Volcanic hazards: A sourcebook on the effects of eruptions, Orlando, Fla, 1984, Academic Press.


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.





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).



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




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



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.




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).



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.



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.



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



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.




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




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).



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




Generally Nonexplosive Volcanoes




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.




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




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


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Sep 7, 2016 | Posted by in EMERGENCY MEDICINE | Comments Off on Volcanic Eruptions, Hazards, and Mitigation
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