VOLCANO

plural volcanoes, or volcanos, any vent in the crust of the Earth or other planet or satellite, from which issue molten rock, pyroclastic debris, and steam. Volcanism (vulcanism) is the name given to the processes and phenomena associated with the surficial discharge of such material from volcanoes, geysers, and fumaroles. 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. One major 18th-century school of thought held that molten rock and volcanoes were simply accidents caused by burning coal seams. Geologists today agree that volcanism is a profound process resulting from the thermal evolution of planetary bodies. Heat does not easily escape from large bodies by conduction or radiation. Instead, partial melting and buoyant rise of magma are major contributors to the process of heat flux from the Earth's interior. Volcanoes are the surface manifestation of this thermal process, which has its roots deep inside the Earth and which hurls its ashes high into the atmosphere. The term volcano can either mean the vent from which magma erupts to the surface, or it can refer to the landform created by the solidified lava and fragmental volcanic debris that accumulate near the vent. One could say, for example, that large lava flows are erupted from Kilauea Volcano in Hawaii, the word volcano here signifying a vent. By contrast, one could say that Kilauea is a gently sloping volcano of modest size as Hawaiian volcanoes go, the reference in this case being to a landform. Broadly defined, all igneous rocks are the result of volcanism. If igneous rocks solidify from magmas that have not reached the surface, they are called intrusive igneous rocks, and this process in a more restricted sense is termed plutonism. Igneous rocks that cool and solidify at the Earth's surface are known as extrusive igneous rocks, and these are unequivocally products of volcanism. Volcanoes (and their products) are not the realm of any single scientific discipline. Rather, they require study by many scientists from several specialties: geophysicists and geochemists to probe the deep roots of volcanoes; geologists to decipher prehistoric volcanic activity; biologists to learn how life becomes established and evolves on barren volcanic islands; and meteorologists to determine the effects of volcanic dust and gases on the atmosphere, weather, and climate. This article discusses volcanism both by topics and by examples. Under the topic of types of volcanic activity, the 1980 eruption of Mt. St. Helens in southwestern Washington is used as an example of an explosive eruption that involves ejection of pyroclastic fragments. The 1984 eruption of Mauna Loa Volcano in Hawaii serves to illustrate a quiet eruption involving the effusion of lava. The widely varying composition of volcanic gases from fumaroles is explained by studies at Kilauea Volcano, and Yellowstone National Park in the western United States provides classic examples of hot springs and geysers. Volcanic landforms evolve from the cumulative effects of both constructive and destructive eruptions. Mt. Fuji exemplifies a stratovolcano and Mauna Loa typifies a shield volcano. Iceland provides fine examples of volcanic plateaus, and since it sits astride the Mid-Atlantic Ridge, it is also a logical starting place for a discussion of submarine volcanic structures. Volcanoes are closely associated with tectonic activity. Most of them occur on either the overriding or the diverging margins of the enormous lithospheric plates that make up the Earth's surface. The volcanoes of Japan provide an excellent example of the former, while those of the Mid-Atlantic Ridge the latter. Intraplate volcanoes such as those of the Hawaiian chain provide important evidence as to the direction and rate of plate motion. Volcanoes affect humankind in many ways. Their destructiveness is awesome, but the risk involved can be reduced by assessing volcanic hazards and forecasting volcanic eruptions. Volcanism provides fertile soils, valuable mineral deposits, and geothermal energy. Over geologic time volcanoes recycle the Earth's hydrosphere and atmosphere, and explosive eruptions can affect climate. plural Volcanoes, or Volcanos, any vent in the crust of the Earth or other planet or satellite (e.g., Jupiter's Io), from which molten rock, pyroclastic debris, and steam issue. Volcanoes are commonly divided into two broad types: fissure and central. Each type is associated with a different mode of eruption and surface structure. Fissure volcanoes are much more common than those of the central type. They occur along fractures in the crust and may extend for many kilometres. Lava, usually of basaltic composition, is ejected relatively quietly and continuously from the fissures and forms enormous plains or plateaus of volcanic rock. Submarine fissure eruptions are common along the crests of mid-ocean ridges and are pivotal in seafloor spreading (see seafloor spreading hypothesis). When molten rock is extruded under water, pillow lavapiles of sack-shaped rock masses measuring up to several metres in diameterare often formed. Central volcanoes have a single vertical lava pipe and tend to develop a conical profile. The volcanic cone is generally built up of a succession of lavas, ignimbrites, and welded tuffs (porous rock formed by the cementation of solidified volcanic ash and dust particles). Lava flows from the throat of a central volcano following the easiest path downhill, its flow pattern strongly influenced by the topography. The shape of any given volcanic landform depends on a variety of local circumstances and on the relative abundances of lavas, tuffs, and ignimbrites. This in turn depends on the composition of the magma arriving at the surface. The lower the viscosity, the more readily the lava flows away from the throat or fissure. There is, as a consequence, relatively little tendency to build up a steep-sided cone. The more viscous the magma, however, the greater the tendency to chill and solidify close to the source and to form a cone. In many cases, highly viscous lava also may clog the throat of the volcano, causing a pressure buildup that can only be relieved by violent explosions and nues ardentes. Such eruptions, exemplified by those of Mount St. Helens in southwestern Washington state during the early 1980s and Vesuvius in AD 79, may completely remove the top of a volcanic cone and occasionally part of the interior of the cone as well. The resultant roughly circular hollow is called a caldera. Further eruption may lead to the formation of a lava lake within the cone, and if the lava cools and solidifies, the inward drainage of rainwater may produce a water lake on the surface of the lava lake. A caldera may also form without an explosion by the collapse of the top of the cone into an underlying accumulation of magma. Kilauea on southeastern Hawaii Island is an excellent example of a large volcanic cone with a well-developed caldera produced by collapse. Additional reading Because volcanoes are studied by scientists in a wide range of specialties, discussions of volcanism appear in many diverse publications. This bibliography provides only the more general sources of information on volcanism. Many of these, however, contain extensive references that can lead the reader to more specialized data.Coverage at a nontechnical level can be found in Robert Decker and Barbara Decker, Volcanoes (1981), emphasizing physical processes and the relationship of volcanoes to plate tectonics; Peter Francis, Volcanoes (1976), concentrating on volcanic rock types; Fred M. Bullard, Volcanoes of the Earth, 2nd rev. ed. (1984), describing many volcanic eruptions on a worldwide basis; and Gordon A. MacDonald, Agatin T. Abbott, and Frank L. Peterson, Volcanoes in the Sea, 2nd ed. (1983), discussing Hawaiian volcanoes and the geology of Hawaii. On a fairly high technical level are Howell Williams and Alexander R. McBirney, Volcanology (1979); and Gordon A. Macdonald, Volcanoes (1972), with emphasis on descriptions of eruptions and the character of volcanic rocks. R.V. Fisher and H.-U. Schmincke, Pyroclastic Rocks (1984), is a comprehensive treatise on the origin and character of this important but diverse group of volcanic deposits. Grant Heiken and Kenneth Wohletz, Volcanic Ash (1985), provides a thorough discussion of the origin and character of airfall deposits. See also T. Simkin et al., Volcanoes of the World: A Regional Directory, Gazetteer, and Chronology of Volcanism During the Last 10,000 Years (1981).Useful popular books include Haroun Tazieff, Volcanoes, trans. from French (1961), an account of the study of volcanoes and volcanic eruptions from Greek and Roman to modern times; Volcano, rev. ed. (1982), a well-illustrated account of major volcanic eruptions with a chapter on monitoring active volcanoes, prepared by the editors of Time-Life Books; and Maurice Krafft and Katia Krafft, Volcano (1975; originally published in French, 1975), a book of colour photographs of volcanoes in action and a text on the mechanism of volcanoes.General research articles about volcanism are published in Science (weekly) and Nature (weekly); and more specialized articles in Journal of Geophysical Research: Solid Earth and Planets (monthly); Bulletin of Volcanology (bimonthly); and Journal of Volcanology and Geothermal Research (monthly). SEAN Bulletin (monthly), published by the Scientific Event Alert Network of the Smithsonian Institution, provides preliminary but up-to-date data on eruptions worldwide.Collections of articles can be found in National Research Council. Geophysics Study Committee, Explosive Volcanism: Inception, Evolution, and Hazards (1984); Robert Decker and Barbara Decker, Volcanoes and the Earth's Interior: Readings from Scientific American (1982); Peter W. Lipman and Donal R. Mullineaux (eds.), The 1980 Eruptions of Mount St. Helens, Washington (1981); and Robert W. Decker, Thomas L. Wright, and Peter H. Stauffer (eds.), Volcanism in Hawaii, 2 vol. (1987). Robert W. Decker Barbara B. Decker