vent in the crust of the Earth or other planet or satellite, from which issue eruptions of molten rock, hot rock fragments, and hot gases. A volcanic eruption is an awesome display of the Earth's power. Yet while eruptions are spectacular to watch, they can cause disastrous loss of life and property, especially in densely populated regions of the world. Sometimes beginning with an accumulation of gas-rich magma (molten underground rock) in reservoirs near the surface of the Earth, they can be preceded by emissions of steam and gas from small vents in the ground and by a swarm of small earthquakes. In some cases, magma rises in conduits to the surface as a thin and fluid lava, either flowing out continuously or shooting straight up in glowing fountains or curtains. In other cases, entrapped gases tear the magma into shreds and hurl viscous clots of lava into the air. In more violent eruptions, the magma conduit is cored out by an explosive blast, and solid fragments are ejected in a great cloud of ash-laden gas that rises tens of thousands of metres into the air. One feared phenomenon accompanying some explosive eruptions is the nuée ardente, or pyroclastic flow, a fluidized mixture of hot gas and incandescent particles that sweeps down a volcano's flanks, incinerating everything in its path. Great destruction also can result when ash collects on a high snowfield or glacier, melting large quantities of ice into a flood that can rush down a volcano's slopes as an unstoppable mudflow.

Strictly speaking, the term volcano means the vent from which magma and other substances erupt to the surface, but it can also refer to the landform created by the accumulation of solidified lava and volcanic debris near the vent. One can say, for example, that large lava flows erupt from Mauna Loa volcano in Hawaii, referring here to the vent; but one can also say that Mauna Loa is a gently sloping volcano of great size, the reference in this case being to the landform. Volcanic landforms (continental landform) have evolved over time as a result of repeated volcanic activity. Mauna Loa typifies a shield volcano, which is a huge, gently sloping landform built up of many eruptions of fluid lava. Mount Fuji (Fuji, Mount) in Japan is an entirely different formation. With its striking steep slopes built up of layers of ash and lava, Mount Fuji is a classic stratovolcano. Iceland provides fine examples of volcanic plateaus, while the seafloor around Iceland provides excellent examples of submarine volcanic structures.

Volcanoes figure prominently in the mythology of many peoples who have learned to live with eruptions, but science was late in recognizing the important role of volcanism in the evolution of the Earth. As late as 1768, the first edition of Encyclopædia Britannica gave voice to a common misconception by defining volcanoes as “burning mountains, which probably are made up of sulphur and some other matter proper to ferment with it, and take fire.” Today geologists agree that volcanism is a profound process resulting from the thermal evolution of planetary bodies. Heat does not easily escape from large bodies such as the Earth by the processes of conduction or radiation. Instead, heat is transferred from the Earth's interior largely by convection—that is, the partial melting of the Earth's crust and mantle and the buoyant rise of magma to the surface. Volcanoes are the surface sign of this thermal process. Their roots reach deep inside the Earth, and their fruits are hurled high into the atmosphere.

Volcanoes are closely associated with plate tectonic (plate tectonics) activity. Most volcanoes, such as those of Japan and Iceland, occur on the margins of the enormous solid rocky plates that make up the Earth's surface. Other volcanoes, such as those of the Hawaiian Islands, occur in the middle of a plate, providing important evidence as to the direction and rate of plate motion.

Major volcanoes of the world Major volcanoes of the worldThe study of volcanoes and their products is known as volcanology, but these phenomena are not the realm of any single scientific discipline. Rather, they are studied by many scientists from several specialties: geophysicists (geophysics) and geochemists (geochemistry), who probe the deep roots of volcanoes and monitor signs of future eruptions; geologists, who decipher prehistoric volcanic activity and infer the likely nature of future eruptions; biologists, who learn how plants and animals colonize recently erupted volcanic rocks; and meteorologists, who determine the effects of volcanic dust and gases on the atmosphere, weather, and climate.

The root zone of volcanoes is found some 70 to 200 km (40 to 120 miles) below the surface of the Earth. There, in the Earth's upper mantle, temperatures are high enough to melt rock and form magma. At these depths, magma is generally less dense than the solid rocks surrounding and overlying it, and so it rises toward the surface by the buoyant force of gravity. In some cases, as in the undersea zones where the tectonic plates of the Earth's crust are separating, magma may move directly up to the surface through fissures that reach as deep as the mantle. In other cases, it collects in large underground reservoirs known as magma chambers before erupting to the surface. Molten rock that reaches the surface is called lava.

Common types of volcanic rockMost magma formed by partial melting of the mantle is basaltic in composition, but, as it ascends, it assimilates silica, sodium, and potassium from the surrounding host rocks (rock). Volcanic rocks (extrusive rock) found where magma erupts to the surface are classified into four major types, or “clans”— basalt, andesite, dacite, and rhyolite. These rocks are ranked, as can be seen in the table (Common types of volcanic rock), mainly by their silica content, which ranges from approximately 50 percent for basalt to approximately 75 percent for rhyolite. As silica content increases, rock types generally become more viscous.

If the vast, unseen undersea lava flows of the oceanic ridge system are considered, lava flows are the most common products of the Earth's volcanoes. There are two major types of lava flow, referred to around the world by their Hawaiian names: pahoehoe, a more fluid flow with a smooth to ropy surface; and aa (or a'a), a more viscous flow whose surface is covered by thick, jumbled piles of loose, sharp blocks. Both types have the same chemical composition; the difference seems to be in the eruptive temperature and the speed of movement of the flow. As much as 99 percent of the island of Hawaii is composed of aa and pahoehoe flows. Indeed, Kilauea volcano has erupted continuously since 1983, its lava flows covering more than 100 square km (40 square miles) of land and adding more than 2 square km (0.8 square mile) to the island where the lava has poured into the ocean. In the Mediterranean region, Mount Etna (Etna, Mount) has issued lava more than 150 times since its first recorded activity in 1500 BC.

Massive volcanic explosions are caused by the rapid expansion of gases, which in turn can be triggered by the sudden depressurization of a shallow hydrothermal system or gas-charged magma body or by the rapid mixing of magma with groundwater. The ash, cinders, hot fragments, and bombs thrown out in these explosions are the major products observed in volcanic eruptions around the world. These solid products are classified by size. Volcanic dust is the finest, usually about the consistency of flour. Volcanic ash is also fine but more gritty, with particles up to the size of grains of rice. Cinders, sometimes called scoriae, are the next in size; these coarse fragments can range from 2 mm (0.08 inch) up to about 64 mm (2.5 inches). Fragments larger than 64 mm are called either blocks or bombs (bomb). Volcanic blocks are usually older rock broken by the explosive opening of a new vent. Large blocks ejected in such explosions have been hurled as far as 20 km (12 miles) from the vent. Volcanic bombs, in contrast, are generally incandescent and soft during their flight. Some bombs take on strange, twisted shapes as they spin through the air. Others have a cracked and separated crust that has cooled and hardened in flight; they are called “breadcrust bombs.”

Ash falls from continued explosive jetting of fine volcanic particles into high ash clouds generally do not cause any direct fatalities. However, where the ash accumulates more than a few centimetres, collapsing roofs and failure of crops (agriculture, origins of) are major secondary hazards. Crop failure can occur over large areas downwind from major ash eruptions, and widespread famine and disease may result, especially in poorly developed countries. In the long run, however, the decomposition of nutrient-rich volcanic fallout is responsible for some of the world's best soils.

Mudflows (mudflow), or lahars (lahar), are common hazards associated with stratovolcanoes and can happen even without an eruption. They occur whenever floods of water mixed with ash, loose soil, or hydrothermal clay sweep down valleys that drain the sides of large stratovolcanoes. The huge mudflows generated by meltwater from the ice cap of Mount Ruiz (Ruiz, Mount), Colombia, in 1985 are classic examples of mudflows associated with eruptions. Heavy rainfall or earthquake-induced avalanches of ice or hydrothermal clay also can cause mudflows on steep volcanoes during periods of repose between eruptions.

climate, too, is subject to the effects of volcanic activity. High ash clouds, especially if they are rich in sulfur dioxide, can inject much fine dust and aerosol droplets of sulfuric acid into the stratosphere, above tropospheric (troposphere) rain clouds. Their height greatly increases the residence time of these fine particles in the atmosphere—they are not washed quickly back to Earth but spread slowly into haze layers that can blanket a hemisphere or even the entire Earth.

In more-detailed classification schemes based on character of eruption, volcanic activity and volcanic areas are commonly divided into six major types, shown schematically in the diagram-->. They are listed as follows in order of increasing degree of explosiveness:

● Icelandic

● Hawaiian

● Strombolian

● Vulcanian

● Pelean

● Plinian

The Plinian type is an intensely violent kind of volcanic eruption exemplified by the outburst of Mount Vesuvius (Vesuvius) in Italy in AD 79 that killed the famous Roman scholar Pliny the Elder and was described in an eyewitness account by his nephew, the historian Pliny the Younger. In this type of eruption, gases boiling out of gas-rich magma generate enormous and nearly continuous jetting blasts that core out the magma conduit and rip it apart. The uprushing gases and volcanic fragments resemble a gigantic rocket blast directed vertically upward. Plinian eruption clouds can rise into the stratosphere and are sometimes continuously produced for several hours. Lightning strikes caused by a buildup of static electricity are common close to Plinian ash clouds, adding one more element of terror to the eruption.

Earthquakes and steam explosions announced the reawakening of Mount Pinatubo in 1991, surprising many geologists because Pinatubo was not even listed in catalogs of world volcanoes. This mountain (at that time having an elevation of 1,745 metres, or 5,725 feet) lacked the classic conical shape of a volcano because erosion had carved its summit into a ragged ridge with steep jungle-covered slopes, and there was no written record of any eruptions. Nevertheless, scientists at the Philippine Institute of Volcanology and Seismology (PHIVOLCS) took the awakening of Pinatubo very seriously, knowing that the longer the repose between eruptions, the more dangerous a volcano may be. The area surrounding the volcano included densely populated regions. Clark Air Base, a major U.S. Air Force base in the Philippines, also abutted the volcano.

The climactic eruption began in the early afternoon of June 15. Visible observations were impossible because of winds and rain brought by the typhoon, but seismograph, barograph (barometer), and satellite observations recorded the second largest eruption of the 20th century—exceeded only by the giant 1912 eruption of Novarupta near Mount Katmai in Alaska. The huge Plinian-type eruption of Pinatubo lasted about nine hours. By mid-afternoon, conditions around the volcano included pitch darkness, falling ash and pumice lumps as large as 4 cm (1.6 inches) in diameter, high winds and heavy rain, lightning flashes, and earthquakes. Major ground tremors were felt about every 10 to 15 minutes. Satellite images showed that a giant, umbrella-shaped eruption cloud had formed that was 400 km (250 miles) in diameter and 34 km (21 miles) high at its apex. The ash fall from this cloud covered an area of 7,500 square km (2,900 square miles) to a depth of 1 cm (0.4 inch) or more with wet, gray ash and pumice; the maximum thickness was about 50 cm (20 inches) a few kilometres southwest of the vent area. The volume of ash was about 5 cubic km (1.2 cubic miles). Most of the 300 deaths caused directly by the eruption were the result of roofs and buildings collapsing from the weight of wet ash.

Kilauea, a smaller and younger volcano on the southeast side of Mauna Loa, has been erupting lava from 1983 to the present. Its output of lava has averaged about 400,000 cubic metres (14 million cubic feet) per day, in sharp contrast to the 12 million cubic metres (424 million cubic feet) per day during the first week of the 1984 eruption of Mauna Loa. It is this slow but steady effusion of molten lava that has allowed the eruption of Kilauea to continue so long. Apparently, magma from depth is replacing the amount being erupted at a balanced rate. In contrast, the effusion of lava at Mauna Loa in 1984 was at a much more rapid rate than that at which magma could be resupplied from depth, and the eruption was soon exhausted. Both Mauna Loa and Kilauea were erupting at the same time in 1984. Even though the difference in elevation between the vents on Mauna Loa and Kilauea was only 2,000 metres (6,600 feet), there was no apparent effect of one eruption upon the other. This indicates that, although both volcanoes have the same general source region of magma about 60 km (37 miles) below the surface, their conduits and shallower magma chambers are separate.

The second largest eruption of the 19th century also occurred in Indonesia. Krakatoa (Krakatau), a composite volcano on a small uninhabited island between Sumatra and Java, erupted explosively on August 26–27, 1883. The eruption was similar to the Tambora outburst but smaller, involving about 18 cubic km (4.3 cubic miles) of magma erupted in Plinian ash clouds and pyroclastic flows. Krakatoa was a smaller volcano than Tambora, and, when the eruption had emptied part of its magma chamber, it collapsed to form a caldera that was partly below sea level. Twenty-three square km (8.9 square miles) of the island disappeared, and where a volcanic peak 450 metres (1,475 feet) high once stood was water as deep as 275 metres (900 feet). The largest explosion on the morning of August 27 produced an ash cloud that was reported to have reached a height of 80 km (50 miles), and the detonation was heard as far away as Australia. A tsunami over 30 metres (100 feet) high followed the explosion and apparent caldera collapse, killing about 36,000 people on the adjacent shores of Java and Sumatra.

On May 8, 1902, there occurred a violent eruption of Mount Pelée (Pelée, Mount), a stratovolcano on the island of Martinique in the Caribbean Sea. Although less than 1 cubic km (0.24 cubic mile) of magma was erupted, much of it formed a high-velocity pyroclastic flow that swept down a steep valley to the port of Saint-Pierre. Within minutes the town and virtually all of its inhabitants (some 29,000 people) were incinerated.

The second worst volcanic disaster of the 20th century occurred on November 13, 1985, when a relatively small eruption of Mount Ruiz (Ruiz, Mount), a stratovolcano in the Andes Mountains of Colombia, killed 25,000 people. This volcano is tall enough, at an elevation of 5,400 metres (17,700 feet), to have a glacial ice cap. When a brief explosive eruption dumped several million cubic metres of hot pyroclastic fragments onto the ice surrounding the summit crater, a sudden surge of meltwater sent massive mudflows down canyons on both the east and west sides of the volcano. Much of the town of Armero, built on a low plain beside the Lagunilla River 50 km (30 miles) east of and nearly 5 km (3 miles) in elevation below the summit of Ruiz, was buried by the mudflows. Twenty-two thousand of its inhabitants were killed.

most common volcano types of the worldThe common mental image of a volcano is that of a steep symmetrical cone sweeping upward in a concave curve to a sharp summit peak. Mount Fuji (Fuji, Mount) in Japan is the archetype of this image, but in reality only a few volcanoes attain this ideal shape. Each of the more than 1,500 potentially active volcanoes or volcanic areas around the world has a distinct form, though most can be generalized into nine categories (see table (most common volcano types of the world)). These categories are described below in order of their numerical importance.

Stratovolcanoes such as Mayon Volcano in the Philippines, Mount Momotombo in Nicaragua, and Ol Doinyo Lengai in Tanzania are steep cones built by both pyroclastic and lava-flow eruptions. The cone-shaped form slopes up gradually and becomes steeper (up to 35°) toward the summit, which generally contains a crater. Stratovolcanoes are composed of volcanic rock types that vary from basalt to rhyolite, but their composition is generally andesite. They may erupt many thousands of times over life spans of millions of years. A typical eruption begins with ash explosions and ends with extrusion of thick, viscous lava flows. The alternating layers (strata) of ash and lava are not continuous, blanketlike deposits; rather, they are overlapping lobes or tongues of ash and lava. For this reason many geologists refer to stratovolcanoes as composite volcanoes.

These structures occur in various forms, but many are cone-shaped seamounts (seamount). Some ancient island volcanoes were eroded flat or covered with a coral cap at sea level before they sank below the sea surface as they and the crust supporting them cooled and became denser. These flat-topped seamounts are called guyots (guyot). Most of the active submarine volcanoes that are known occur at shallow depths beneath the sea. They are recognized because their explosive eruptions can be detected and located by hydrophones (hydrophone). Active submarine volcanoes at depths of a few thousand metres are probably common, particularly along oceanic spreading centres, but the water pressure at these depths reduces explosive boiling, and so the eruptions are difficult to detect. One exception to this is the submarine volcano Loihi, a seamount whose summit caldera is 1 km (0.6 mile) below sea level and 30 km (19 miles) southeast of the island of Hawaii. Although eruptions of this youngest volcano of the Hawaiian chain have not been directly observed, seismographs detected swarms of earthquakes at shallow depths beneath the summit of Loihi in 1971–72, 1975, and 1996. Observers in a submersible research vessel from the University of Hawaii dove to Loihi as the 1996 earthquakes waned; they discovered a new collapse crater at the summit and fine white “dust” that greatly reduced visibility but saw no red lava or erupting vents. A submarine eruption has not yet been directly observed in progress.

Most calderas—large circular or oval depressions more than 1 km (0.6 mile) in diameter—have been formed by inward collapse of landforms after large amounts of magma have been expelled from underground. Many are surrounded by steep cliffs, and some are filled with lakes. The terms crater and caldera are often used synonymously, but calderas are larger than craters. A crater can occur inside a caldera, as at Taal Lake in the Philippines, but not the reverse. Calderas are often associated with large eruptions (those producing volumes of 10 cubic km 【2.4 cubic miles】 or more) of dacitic or rhyolitic magma that form pyroclastic plateaus.

Whether a volcano is designated a caldera, shield volcano, or stratovolcano with a caldera depends on the principal landform feature. For example, Crater Lake in Oregon in the northwestern United States is designated a caldera, but Kilauea in Hawaii is designated a shield volcano even though it has a large summit caldera.

Pyroclastic cones (also called cinder cones (cinder cone) or scoria cones) such as Cerro Negro in Nicaragua are relatively small, steep (about 30°) volcanic landforms built of loose pyroclastic fragments, most of which are cinder-sized. The fragments cool sufficiently during their flight through the air so that they do not weld together when they strike one another. Generally, the crater from which the cinder fragments were ejected is located in the centre of the cone. In areas with strong prevailing winds, however, the crater may be upwind of the cone. The rock type involved in pyroclastic cones is generally basalt or basaltic andesite, and the eruption type is either the moderately explosive Vulcanian or the gentler Hawaiian, which produces high lava fountains.

pumice cones are structures similar to cinder cones, but they are made up of volcanic glass fragments so riddled with gas-bubble holes (vesicles) that they resemble a sponge and are very lightweight. Less common pyroclastic landforms include maars, low-relief craters often filled with water and surrounded by a rim of ejected material that was probably formed by explosive interaction of magma and groundwater; and tuff rings and tuff cones, which are landforms built of compacted pyroclastic deposits. Tuff rings and cones resemble maars, but they have higher rims and are not filled with water. Tuff rings are only about 5 metres (16 feet) high, with craters roughly at ground level. Tuff cones are higher and steeper, with craters above ground level. Punchbowl and Diamond Head on Oahu island, Hawaii, are famous examples of tuff cones.

Landforms of this sort consist of steep domal mounds of lava so viscous that the lava piles up over its vent without flowing away. The rock types that form lava domes are generally andesites (andesite), dacites (dacite), or rhyolites (rhyolite). Somehow these viscous lavas have lost much of their gas content in prior eruptions or during a slow rise to the surface. Even so, it is not unusual for an actively growing lava dome to have an explosive eruption that disrupts all or part of the dome. Many lava domes grow by internal intrusion of lava that causes swelling and oversteepening of the dome. Rockslides build up an apron of talus blocks around the lower sides of the dome. Lava domes can form mounds several hundred metres high with diameters ranging from several hundred to more than 1,000 metres (3,300 feet). Thick lava flows sometimes move short distances from the dome and distort its generally circular or oval shape. A good example of a lava dome is the one in the explosion crater at Mount St. Helens (Saint Helens, Mount).

● The volume of volcanic products

● The interval length between eruptions

● The composition of volcanic products

● The variety of volcanic eruption types

● The geometry of the vent

● The environment into which the volcanic products are erupted

Common types of volcanic rockThe chemical composition of magma affects its physical properties, which in turn have a major influence on the landform built by a volcanic eruption. Four common volcanic rock types are listed in the table (Common types of volcanic rock): basalt, andesite, dacite, and rhyolite. As the silica content increases, these rock types generally become more viscous; as the magmatic gas content increases, they become more explosive. Other physical properties are, however, important in determining the character of lava flows. For example, hot basaltic lava produces flows with smooth to ropy surfaces. These flows, known as pahoehoe, tend to flow farther than the cooler aa flows of the same chemical composition that have rough, broken surfaces. (See the section Lava flows (volcano).)

Yellowstone National Park in the United States is one of the most famous areas of hot springs and geysers in the world. The total heat flux from these thermal features is estimated to be 300 megawatts (300 million watts (watt)). The last great eruption at Yellowstone occurred about 630,000 years ago when some 1,000 cubic km (240 cubic miles) of rhyolitic (rhyolite) pumice and ash were ejected in huge pyroclastic flows and resulted in the formation of a caldera—a large circular or oval depression caused by collapse of the surface following magma removal—approximately 45 by 75 km (28 by 47 miles) in size. Yellowstone Lake now occupies part of this giant caldera. Since that last great outburst, about 1,200 cubic km (288 cubic miles) of rhyolite lava flows and domes have erupted in numerous smaller events. The cooling roots of such past eruptions, or possibly the new intrusions of magma at shallow depth, are the heat sources for the Yellowstone hot springs and geysers.

Geysers are hot springs that intermittently spout a column of hot water and steam into the air. This action is caused by the water in deep conduits beneath a geyser approaching or reaching the boiling point. At 300 metres (about 1,000 feet) below the surface, the boiling point of water increases to approximately 230 °C (450 °F) because of the increased pressure of the overlying water. As bubbles of steam or dissolved gas begin to form, rise, and expand, hot water spills from the geyser's vent, lowering the pressure on the water column below. Water at depth then momentarily exceeds its boiling point and flashes into steam, forcing additional water from the vent. This chain reaction continues until the geyser exhausts its supply of boiling water.

After a geyser stops spouting, the conduits at depth refill with groundwater, and reheating begins again. In geysers such as Yellowstone's Old Faithful, the spouting and recharge period is quite regular. This famous geyser has gushed to heights of 30 to 55 metres (100 to 180 feet) about every 90 minutes for more than 100 years. If Old Faithful's eruption lasts only a minute or two, the next interval will be shorter than average, while a four-minute eruption will be followed by a longer interval. Other geysers have much more erratic recharge times.

Taking a more distant view of volcanic landforms from space, one can see that most volcanoes group together to form linear to arcuate belts across the Earth's surface. It is now clear that these volcanic chains are closely related to global tectonic activity. Many active volcanoes are located in the so-called “Ring of Fire” made up of island arcs (island arc) and mountain ranges bordering the Pacific Ocean (see the map-->). The concept of seafloor spreading and, more broadly, the theory of plate tectonics offer a logical explanation for the location of most volcanoes.

Topographic maps reveal the locations of large earthquakes and indicate the boundaries of the 12 major tectonic plates (plate tectonics). For example, the Pacific Plate is bounded by the earthquake zones of New Zealand, New Guinea, the Mariana Islands, Japan, Kamchatka, the Aleutian Islands, western North America, the East Pacific Rise, and the Pacific-Antarctic Ridge.

most common volcano types of the worldConceptual models of how subduction and rift volcanoes may form are shown schematically in the diagram--> and in the video. Of the 1,450 volcanoes listed in the table (most common volcano types of the world) of landform types, 80 percent occur along subduction zones, and 15 percent occur along rift zones. These percentages are somewhat misleading, however, because most of the Earth's rift zones are about 2 to 3 km (1.2 to 1.9 miles) below sea level, where volcanic activity is hard to detect. At those depths active submarine volcanoes have yet to be observed, though many hydrothermal areas have been found along submarine rift zones by research submersibles. Iceland, a segment of the Mid-Atlantic Ridge that emerges above sea level, has 70 volcanoes that have erupted during the past 10,000 years. If this is a typical number for a rift system, there may be several thousand potentially active volcanoes along the oceanic ridges that are the surface expressions of the world rift system.

Rift volcanoes form when magma rises into the gap between diverging plates. They thus occur at or near actual plate boundaries. Measurements in Iceland suggest that the separation of plates is a continuous process but that the fracturing is intermittent, analogous to a rubber band that is slowly stretched until it snaps. Earthquake swarms and volcanic eruptions occur when the stretching exceeds the strength of the near-surface rocks, which then fracture along steeply dipping cracks parallel to the rift. Basaltic magma rising along these fractures causes Icelandic-type fissure eruptions.

The 5 percent of known volcanoes in the world that are not closely related to plate margins are generally regarded as intraplate, or “hot-spot,” volcanoes. A hot spot is believed to be related to the rising of a deep-mantle plume, which is caused by very slow convection of highly viscous material in the Earth's mantle. As hot but solid mantle rock moves upward, partial melting may occur from the lowering of its pressure-dependent melting temperature. Where a lithospheric plate moves over a hot spot, a chain of volcanic islands may be created. As the plate moves, the older volcanoes are transported away from the magma source and become extinct. The younger, active volcanoes are clustered at the end of the chain over the hot spot. It is not known how a volcanic hot spot maintains its position for millions of years while a plate passes over it. Detailed seismic sounding of the mantle should increase the understanding of the mechanism controlling hot spots.

Hawaiian (Hawaii) volcanoes are the best examples of hot-spot volcanoes. The five volcanoes that form the island of Hawaii at the southeast end of the Hawaiian chain are all less than one million years old. Two of these, Kilauea and Mauna Loa, are two of the most active volcanoes in the world. Northwestward along the Hawaiian chain each island is progressively older. The extinct volcano or volcanoes that formed the island of Kauai are about five million years old. Topographic maps show a major submarine continuation of the Hawaiian Ridge to the northwest of the Hawaiian Islands and then a dogleg bend into the Emperor Seamounts, which comprise an entirely submarine ridge continuing northward to the edge of the Pacific Plate.

On an introductory level, Robert Decker and Barbara Decker, Volcanoes, 4th ed. (2005), emphasizes physical processes and the relationship of volcanoes to plate tectonics. Volcanoes, eruptions, hazards, and the benefits of and myths surrounding volcanoes are described in Richard V. Fisher, Grant Heiken, and Jeffrey B. Hulen, Volcanoes: Crucibles of Change (1997). A well-illustrated account of major volcanic eruptions with a chapter on monitoring active volcanoes is given in Time-Life Books, Volcano (1982). A history of the myths and science of volcanoes is presented in Maurice Krafft and Katia Krafft, Volcanoes: Fire from the Earth, trans. by Paul G. Bahn (1993; originally published in French, 1992). Haraldur Sigurdsson, Melting the Earth: The History of Ideas on Volcanic Eruptions (1999), discusses the evolution of thought from 5th-century-BC Greece to the present.Volcanic hazards and how humans cope with them are presented in Stephen L. Harris, Agents of Chaos: Earthquakes, Volcanoes, and Other Natural Hazards (1990); and Kerry Sieh and Simon LeVay, The Earth in Turmoil: Earthquakes, Volcanoes, and Their Impact on Humankind (1998). R. Scarpa and R.I. Tilling (eds.), Monitoring and Mitigation of Volcano Hazards (1996), discusses tools for reducing the risks of volcanic activity.Kenneth Wohletz and Grant Heiken, Volcanology and Geothermal Energy (1992), presents the relationship between volcanism and geothermal exploration.

Articles on the 1991 eruption of Mount Pinatubo are collected in Christopher G. Newhall and Raymondo S. Punongbayan (eds.), Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (1996). The world's largest volcano is described in J.M. Rhodes and John P. Lockwood (eds.), Mauna Loa Revealed: Structure, Composition, History, and Hazards (1995). Hawaiian volcanoes and the geology of Hawaii are discussed in Gordon A. MacDonald, Agatin T. Abbott, and Frank L. Peterson, Volcanoes in the Sea, 2nd ed. (1983).A description of 15 famous volcanoes from Stromboli to Pinatubo and how humans have coped with them is given in Alwyn Scarth, Vulcan's Fury: Man Against the Volcano (1999, reissued 2001). A monumental work by prominent volcanologists is presented in Haraldur Sigurdsson (ed.), Encyclopedia of Volcanoes (2000). An excellent catalog of world volcanoes and their past eruptions is given in Tom Simkin and Lee Siebert, Volcanoes of the World: A Regional Directory, Gazetteer, and Chronology of Volcanism During the Last 10,000 Years, 2nd rev. ed. (1994).

Popular books about volcanoes and the scientists who study them include Joseph Cone, Fire Under the Sea: The Discovery of the Most Extraordinary Environment on Earth—Volcanic Hot Springs on the Ocean Floor (1991); Richard V. Fisher, Out of the Crater: Chronicles of a Volcanologist (1999), on the personal adventures of a well-known scientist; and Dick Thompson, Volcano Cowboys: The Rocky Evolution of a Dangerous Science (2000, reissued 2002), on the trials and tribulations of scientists studying two catastrophic eruptions, Mount St. Helens and Pinatubo.Robert W. Decker Barbara B. Decker