Volcano. I INTRODUCTION Volcano, mountain or hill formed by the accumulation of materials erupted through one or more openings (called volcanic vents) in the earth's surface. The term volcano can also refer to the vents themselves. Most volcanoes have steep sides, but some can be gently sloping mountains or even flat tablelands, plateaus, or plains. The volcanoes above sea level are the best known, but the vast majority of the world's volcanoes lie beneath the sea, formed along the global oceanic ridge systems that crisscross the deep ocean floor (see Plate Tectonics). According to the Smithsonian Institution, 1,511 above-sea volcanoes have been active during the past 10,000 years, 539 of them erupting one or more times during written history. On average, 50 to 60 above-sea volcanoes worldwide are active in any given year; about half of these are continuations of eruptions from previous years, and the rest are new. Volcanic eruptions in populated regions are a significant threat to people, property, and agriculture. The danger is mostly from fast-moving, hot flows of explosively erupted materials, falling ash, and highly destructive lava flows and volcanic debris flows (see Volcano Hazards below). In addition, explosive eruptions, even from volcanoes in unpopulated regions, can eject ash high into the atmosphere, creating drifting volcanic ash clouds that pose a serious hazard to airplanes. II VOLCANO FORMATION All volcanoes are formed by the accumulation of magma (molten rock that forms below the earth's surface). Magma can erupt through one or more volcanic vents, which can be a single opening, a cluster of openings, or a long crack, called a fissure vent. It forms deep within the earth, generally within the upper part of the mantle (one of the layers of the earth's crust), or less commonly, within the base of the earth's crust. High temperatures and pressures are needed to form magma. The solid mantle or crustal rock must be melted under conditions typically reached at depths of 80 to 100 km (50 to 60 mi) below the earth's surface. See also Earth; Magma. Once tiny droplets of magma are formed, they begin to rise because the magma is less dense than the solid rock surrounding it. The processes that cause the magma to rise are poorly understood, but it generally moves upward toward lower pressure regions, squeezing into spaces between minerals within the solid rock. As the individual magma droplets rise, they join to form ever-larger blobs and move toward the surface. The larger the rising blob of magma, the easier it moves. Rising magma does not reach the surface in a steady manner but tends to accumulate in one or more underground storage regions, called magma reservoirs, before it erupts onto the surface. With each eruption, whether explosive or nonexplosive, the material erupted adds another layer to the growing volcano. After many eruptions, the volcanic materials pile up around the vent or vents. These piles form a topographic feature, such as a hill, mountain, plateau, or crater, that we recognize as a volcano. Most of the earth's volcanoes are formed beneath the oceans, and their locations have been documented in recent decades by mapping of the ocean floor. See also Ocean and Oceanography. III VOLCANIC MATERIALS Three different types of materials may erupt from an active volcano. These materials are lava, tephra (rock fragments), and gases. The type and amount of the material that erupts from an active volcano depends on the composition of the magma inside the volcano. A Lava Lava is magma that breaks the surface and erupts from a volcano. If the magma is very fluid, it flows rapidly down the volcano's slopes. Lava that is more sticky and less fluid moves slower. Lava flows that have a continuous, smooth, ropy, or billowy surface are called pahoehoe (pronounced pah HOH ee hoh ee) flows, while aa (pronounced ah ah) flows have a jagged surface composed of loose, irregularly shaped lava chunks. Once cooled, pahoehoe forms smooth rocks, while aa forms jagged rocks. The words pahoehoe and aa are Hawaiian terms that describe the texture of the lava. Lava may also be described in terms of its composition and the type of rock it forms. Basalt, andesite, , and rhyolite are all different kinds of rock that form from lava. Each type of rock, and the lava from which it forms, contains a different amount of the compound silicon dioxide. Basaltic lava has the least amount of silicon dioxide, andesitic and dacitic lava have medium levels of silicon dioxide, while rhyolitic lava has the most. See also Lava; Igneous Rock. B Tephra Tephra, or pyroclastic material, is made of rock fragments formed by explosive shattering of sticky magma (see Pyroclastic Flow). The term pyroclastic is of Greek origin and means 'fire-broken' (pyro, "fire"; klastos, "broken"). Tephra refers to any airborne pyroclastic material regardless of size or shape. The best-known tephra materials include pumice, cinders, and volcanic ash. These fragments are exploded when gases build up inside a volcano and produce an explosion. The pieces of magma are shot into the air during the explosion. Ash refers to fragments smaller than 2 mm (0.08 in) in diameter. The finest ash is called volcanic dust and is made up of particles that are less than 0.06 mm (0.002 in) in diameter. Volcanic blocks, or bombs, are the largest fragments of tephra, more than 64 mm (2.5 in) in diameter (baseball size or larger). Some bombs can be the size of a small car. C Gases Gases, primarily in the form of steam, are released from volcanoes during eruptions. All eruptions, explosive or nonexplosive, are accompanied by the release of volcanic gas. The sudden escape of high-pressure volcanic gas from magma is the driving force for eruptions. Gases come from the magma itself or from the hot magma coming into contact with water in the ground. Volcanic plumes can appear dark during an eruption because the gases are mixed with dark-colored materials such as tephra. Most volcanic gases predominantly consist of water vapor (steam), with carbon dioxide (CO2) and sulfur dioxide (SO2) being the next two most common compounds along with smaller amounts of chlorine and fluorine gases. IV ERUPTION Volcanoes erupt differently depending on the composition of the magma beneath the surface, the amount of gas in the magma, and the type of vent from which it erupts. In general, the more viscous, or stiffer, the lava, the more explosive the eruptive activity. During explosive eruptions, the lava erupted is torn into shreds, forming a variety of fragmental or pyroclastic materials depending on the physical state of the lava and on the force of the explosions. Explosive eruptions can eject a large amount of material into the air. Nonexplosive eruptions produce lava flows and eject very little pyroclastic material into the air. A Explosive Eruptions Explosive eruptions can eject liquid and semisolid lava as well as solid fragments of volcanic or nonvolcanic rock that have been carried along by the rising magma before eruption. Very violent explosive eruptions are called Plinian eruptions, after Roman naturalist Pliny the Elder. These eruptions can last for several hours to days and eject a large amount of pyroclastic material. Some volcanoes can produce much more energetic eruptions that eject materials farther from the vents because of their andesitic and dacitic composition. Andesitic and dacitic lava is generally thicker than basaltic lava. Stiff lava generally produces more-explosive eruptions. B Nonexplosive Eruptions If the eruption is nonexplosive, as is typical for Hawaiian volcanoes, lava flows are produced. The lava comes out of rifts in the sides of the volcano, or vents in a rift. Tephra is rarely ejected during a nonexplosive eruption. Nonexplosive eruptions are characterized by basaltic lava and by the type of volcanoes they form, called shield volcanoes. V TYPES OF VOLCANOES Volcanoes come in different shapes and sizes, depending on the makeup of the magma, the style of the eruption, and how often they erupt. The major types of volcanoes, roughly in order of increasing size, are cinder cones, composite volcanoes (also called stratovolcanoes), shield volcanoes, calderas, and plateaus. Calderas and plateaus are shaped differently than traditional volcanoes--neither has a mountain-like shape. A Cinder Cones and Composite Volcanoes Cinder cones and composite volcanoes have the familiar conelike shape that people most often associate with volcanoes. Some of these form beautifully symmetrical volcanic hills or mountains such as Parícutin Volcano in Mexico and Mount Fuji in Japan. Although both cinder cones and composite volcanoes are mostly the results of explosive eruptions, cinder cones consist exclusively of fragmental lava. This fragmental lava is erupted explosively and made up of cinders. Cinder cones are typically much smaller than composite volcanoes for two reasons: (1) they involve only weakly explosive, small-volume eruptions of basaltic cinder that does not travel far from the vent; and (2) they usually have a short life--often only a single eruptive burst before becoming extinct. In contrast, composite volcanoes can grow much larger because they represent the accumulated products of repeated eruptions from the same vent(s) over a long time. Composite volcanoes are composed of explosively erupted pyroclastic materials layered with nonexplosively erupted lava flows and deposits of volcanic debris. They are mostly built from materials that come from andesitic or dacitic lava. In some composite volcanoes that undergo a major explosive eruption, such as Mount Saint Helens, nonexplosive extrusions of lava within the summit crater can later construct a bulbous mound of accumulated lava. This mound is called a lava dome or a volcanic dome. B Shield Volcanoes Shield volcanoes (also called volcanic shields) get their name from their distinctive, gently sloping mound-like shapes that resemble the fighting shields that ancient warriors carried into battle. Their shapes reflect the fact that they are constructed mainly of countless fluid basaltic lava flows that erupted nonexplosively. Such flows can easily spread great distances from the feeding volcanic vents, similar to the spreading out of hot syrup poured onto a plate. Volcanic shields may be either small or large, and the largest shield volcanoes are many times larger than the largest composite volcanoes. The classic examples of shield volcanoes are the Hawaiian volcanoes Mauna Loa and Kilauea. C Caldera A caldera is a round or oval-shaped low-lying area that forms when the ground collapses because of explosive eruptions. An explosive eruption can explode the top off of the mountain or eject all of the magma that is inside the volcano. Either of these actions may cause the volcano to collapse. Calderas can be bigger than the largest shield volcanoes in diameter. Such volcanic features, if geologically young, are often outlined by an irregular, steep-walled boundary (a caldera rim), which reflects the original ringlike zone, or fault, along which the ground collapse occurred. Some calderas have hills and mountains rising within them, called resurgent domes, that reflect volcanic activity after the initial collapse. Good examples of calderas can be seen at Yellowstone National Park (Wyoming) and Long Valley (eastern California). These were formed by explosive eruptions in the geologic past that were thousands of times larger than any historical eruption. Some calderas are filled with water, forming lakes such as Crater Lake in Oregon. Such powerful caldera-forming eruptions, whose ash deposits can be traced thousands of kilometers from their sources, potentially pose the greatest volcanic hazards to society; luckily, they are very rare geological events. D Volcanic Plateaus Some of the largest volcanic features on earth do not actually look like volcanoes. Instead, they form extensive, nearly flat-topped accumulations of erupted materials. These materials form volcanic plateaus or plains covering many thousands of square kilometers. The volcanic materials can be either very fluid basaltic lava flows or fartraveled pyroclastic flows. The basaltic lava flows are called flood or plateau basalts and are erupted from many fissure vents. The Columbia Plateau in the states of Oregon, Washington, and Idaho is an example of flood basalts. The pyroclastic flows, or ash flows, are from huge explosive caldera-forming eruptions. The Yellowstone Plateau of Wyoming and Montana is built of pyroclastic flows. VI VOLCANO DISTRIBUTION The magma-forming regions of the earth and the volcanoes built above them are not randomly scattered but instead are confined to several zones and special places. While these volcanically active areas have long been known, the scientific reason for their distribution was not understood until the emergence of the theory of plate tectonics in the late 1960s. According to this theory, the earth's surface is broken into a dozen or so large solid slabs (called plates). These plates consist of both crustal and rigid upper mantle material. They are 50 to 150 km (30 to 95 mi) thick and ride upon hotter, more free-flowing mantle. The plates are moving relative to one another at average rates of several centimeters a year. The vast majority of the world's active volcanoes, above and below the sea, are found along or near the boundaries between these shifting plates. Volcanoes can also be found in the middle of tectonic plates, although midplate volcanoes are relatively rare. The Hawaiian Islands are the exposed part of a midplate volcanic chain. A Volcanoes at Plate Boundaries There are three main types of plate boundaries: divergent (spreading), convergent (coming together), and transform (moving horizontally). Divergent boundaries occur where plates are moving apart. The volcanoes that form along such boundaries are generally nonexplosive and have new basaltic magma filling the widening separation or feeding lava flows. Even though most of the earth's volcanism occurs along divergent boundaries, the eruptions often occur unobserved because divergent boundaries are covered by the oceans, except those in Iceland and East Africa. Convergent boundaries separate plates that are moving toward each other. Most of the world's above-sea volcanoes are located along such boundaries, which are also called subduction zones. Although composite volcanoes near subduction zones produce only about 15 percent of global volcanism, they account for more than 80 percent of documented historical eruptions, mostly explosive. Transform boundaries are areas where one plate is grinding horizontally past another. These boundaries are often zones of frequent earthquakes, but they are not volcanically active. B Midplate Volcanoes Some volcanoes are located thousands of kilometers from any active plate boundary. These midplate (or intraplate) volcanoes sometimes form long, well-defined volcanic chains. Scientists believe they are formed by magma from a partly melting plate overriding a stationary heat source, or hot spot, in the mantle. The best example of such midplate hot-spot volcanism is the Hawaiian Ridge-Emperor Sea Mounts chain within the Pacific plate. Hot-spot volcanism within oceanic plates typically is nonexplosive and constructs basaltic shield volcanoes such as Mauna Loa and Kilauea in Hawaii. Hot-spot volcanism within the continental regions can be either explosive or nonexplosive. Explosive volcanism forms calderas and ash-flow plateaus or plains. Nonexplosive volcanism forms plateaus composed of basaltic lava flows such as the Columbia Plateau. VII VOLCANO HAZARDS Eruptions pose direct and indirect volcano hazards to people and property, both on the ground and in the air. Direct hazards are pyroclastic flows, lava flows, falling ash, and debris flows. Pyroclastic flows are mixtures of hot ash, rock fragments, and gas. They are especially deadly because of their high temperatures of 850° C (1600° F) or higher and fast speeds of 250 km/h (160 mph) or greater. Lava flows, which move much more slowly than pyroclastic flows, are rarely life threatening but can produce massive property damage and economic loss. Heavy accumulations of volcanic ash, especially if they become wet from rainfall, can collapse roofs and damage crops. Debris flows called lahars are composed of wet concretelike mixtures of volcanic debris and water from melted snow or ice or heavy rainfall. Lahars can travel quickly through valleys, destroying everything in their paths. Pyroclastic and volcanic debris flows have caused the most eruption-related deaths in the 20th century. Indirect hazards are usually nonvolcanic effects that accompany or follow eruptions. Examples are earthquakes, tsunamis, rainfall-caused debris flow, and posteruption disease and famine. Tsunamis are large seismic sea waves generated by sudden movement of the seafloor. This sudden seafloor movement can be caused by a large earthquake or by the collapse of an island volcano during or after an eruption. Tsunamis can devastate low-lying coastal areas and can be deadly if people living in such areas are not evacuated. Indirect hazards also include volcanic deposits from large eruptions. These deposits can blanket farm fields and grazing lands, leading to the loss of crops and livestock and ultimately to the starvation of people dependent on them for life. During the period from the 17th century to the 19th century, tsunamis and posteruption starvation and disease caused most eruption-related deaths. Starting in the early 1980s, another indirect volcanic hazard began to attract increasing attention: jet aircraft encounters with airborne volcanic ash. More than 60 airplanes, mostly commercial jetliners, have been damaged by such encounters. VIII VOLCANOLOGY - THE STUDY OF VOLCANOES Volcanology is a branch of geology, the study of the earth. Volcanology emphasizes studies of the processes, products, hazards, and environmental impacts of volcanic eruptions. Volcanologists are geologists who specialize in studies of 'young' volcanism. They focus on eruptions within the past 10,000 years, especially on those within recorded history. Volcanologists also study currently active or potentially active volcanoes. They use conventional geologic methods, including geologic mapping and age determination of the deposits of past eruptions. They also use field and laboratory studies of volcanic products, geophysical surveys, and drilling studies. The information they gather provides clues about the volcano's eruptive style (explosive vs. nonexplosive; eruption sizes), eruption frequency, underground structure, and magma reservoir. Volcanologists use this information to evaluate the likelihood of future eruptions (long-term forecasts) and other hazards. They also try to construct maps that show the most vulnerable areas on and around the volcano. All eruptions are accompanied by geophysical and (or) geochemical changes, including earthquake activity, deformation of the volcano, and increased release or change in volcanic gases. To make regular measurements of such changes, scientists install sensors on active and restless volcanoes. Information from these instruments is sent to a volcano observatory for analysis and interpretation by volcanologists. There, they make short-term forecasts of possible eruptions or changes in the course of an on-going eruption. Since the advent of space technology, volcanologists have been using satellite-based systems in addition to ground-based methods to study volcanoes. IX PREDICTING ERUPTIONS A major challenge of volcanology is to predict the next eruption of an active or dormant volcano. Scientists generally consider a volcano active if it has erupted one or more times in historical time. This guideline is poor, however, because written history is much longer for volcanoes in some parts of the world, for instance in Japan and Italy, than in other parts, such as the United States and New Zealand. Dormant volcanoes are currently inactive but considered by scientists to have potential for future eruption. Long-dormant volcanoes believed to lack potential for renewed activity are defined as extinct. The distinction between dormant and extinct is based on the amount of knowledge about a given volcano and is not absolute. Scientists try to predict eruptions by taking measurements of events leading up to possible activity, such as earthquakes, ground movement, and the release of gases. Despite several encouraging successes, including the 1991 eruption of Mount Pinatubo, Philippines, and several recent eruptions of Sakurajima Volcano, Japan, the prediction of explosive eruptions still eludes volcanologists. The success rate is better for prediction of nonexplosive eruptions at well-monitored volcanoes. For example, nearly all of the lava dome-building eruptions at Mount Saint Helens since May 1980 have been predicted successfully, days to weeks in advance. The biggest obstacle to improving eruption prediction is that only a tiny fraction of over 500 active volcanoes in the world are adequately monitored by modern instruments and well-trained volcanologists. X RESOURCES FROM VOLCANOES Volcanic eruptions can obviously cause serious human, economic, and environmental impacts, but volcanoes can also be rich in natural resources. Perhaps the greatest resource from volcanoes is the land formed by the materials they erupt. Volcanic activity has created some of most scenic and fertile regions on earth. Since the early 20th century, harnessing the natural heat of volcanic systems has provided a nearly pollution-free source of thermal and electric energy (see Geothermal Energy). For example, the steam from The Geysers (Northern California), the world's largest geothermal field, is sufficient to meet the electricity needs of the city of San Francisco (see Geothermics). Many valuable ore deposits, such as copper, lead, zinc, gold, and silver, are contained in volcanic rocks, or in magmatic rocks that form the deep roots of volcanoes. Crushed lava rock, pumice, cinders, and other eruptive products are a source of raw materials for the road-building, construction, manufacturing, and landscaping industries. XI EXTRATERRESTRIAL VOLCANOES Space exploration in recent decades has documented compelling evidence of volcanoes and their products on several other planets and moons in our solar system (see Planetary Science). Photos taken by satellites orbiting the Earth's moon show huge lava fields called mares. Samples from these mares collected by the astronauts in the Apollo program were analyzed to be basalts of similar, but not identical, composition to those found on Earth. Also, satellite imagery of the surfaces of Mars and Venus revealed volcanoes and volcanic features similar to, but larger than, features found on Earth. For example, Olympus Mons, a gigantic (about 600 km/375 mi in diameter) shield volcano on Mars, is larger across than the length of the Hawaiian Islands strung together. The Mars Pathfinder Mission of 1997 returned data that Martian volcanic rocks appear to be similar to those found on Earth, including some evidence of the rock andesite. The volcanism on the Earth's moon, Mars, Mercury, and Venus mostly occurred billions of years ago; these planetary bodies are now cold and dead. However, scientists have found evidence in Martian meteorites that indicates volcanic activity on Mars may have occurred as recent as 150 million years ago. Well-documented evidence of active extraterrestrial volcanism has come from the imagery from the Voyager 2 space satellite. Such activity, although poorly understood, appears to be quite different from Earth volcanism. In 1979 this satellite captured dramatic images of volcanic plumes, predominantly of sulfur dioxide (SO2), shooting as high as 280 km (nearly 175 mi) above the surface of Io, one of the moons of Jupiter. Between April 1997 and September 1997, the Galileo spacecraft also returned images showing dramatic volcanic activity on Io. Other images of Io show fresh-appearing lava and probable pyroclastic deposits. Io's volcanic activity indicates the presence of molten material located in its core, or interior, but the cause of heat inside Io is different from that on Earth. Planetary scientists believe that Io is heated internally by friction that results from the moon being pulled in different directions by Jupiter and the neighboring moons Ganymede and Europa. Stereo images taken by Voyager 2 in 1989 of Triton, a moon of Neptune, revealed a plume of gas, mostly likely of nitrogen, rising about 8 km (nearly 5 mi) above its surface and extending almost 150 km (about 93 mi) downwind. This gas plume suggests that Triton may also have a hot core capable of causing volcanic activity. In 2006 planetary scientists announced the dramatic discovery of liquid-water geysers on Enceladus, a moon of Saturn. The Cassini spacecraft first detected the geysers in the moon's south polar region in January 2005. Cameras onboard the spacecraft returned images of giant plumes reaching 418 km (260 mi) into space. Other instruments onboard the spacecraft identified oxygen atoms in the plumes. Scientists theorized that tidal forces created by the gravitational fields of Saturn and other nearby moons cause rocks in the interior of Enceladus to rub against each other. The resulting heat warms pockets of liquid water just below the icy surface. When cracks form in the icy crust, the heated water escapes in the form of gigantic plumes. Sunlight then breaks down the water molecules into oxygen and hydrogen atoms. Contributed By: Robert I. Tilling Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
