Volcano

Volcano:Geological landform, consisting of a fissure in the earth’s crust, above which a cone of volcanic material has accumulated. At the top of the cone is a bowl-shaped vent called a crater. The cone is formed by the deposition of molten or solid matter that flows or is ejected through the vent from the interior of the earth. The study of volcanoes and volcanic phenomena is called volcanology. Most volcanoes are composite landforms built up partly of lava flows and partly of fragmental materials. Italy’s Mount Etna, in Sicily, and Mount Vesuvius, near Naples, are examples of composite cones. In successive eruptions, the solid materials fall around the vent on the slopes of the cone, while lava streams issue from the vent and from fissures on the flanks of the cone. Thus, the cone is built up of layers of fragmental materials and flows of lava, all inclined outward away from the vent. Some enormous, craterlike basins, called calderas, at the top of long-dormant or extinct volcanoes, are eventually occupied by deep lakes, such as Crater Lake in Oregon (see Crater Lake National Park). Some calderas are the result of cataclysmic explosions that destroy the erupting volcano; the volcanic islands of Thíra, Greece, and Krakatau, Indonesia, and Crater Lake are in this category. Others form when the subterranean magma chamber, emptied by repeated eruptions, can no longer support the weight of the volcanic pile above it and so collapses.

Many volcanoes are born underwater, on the seafloor. Mount Etna and Mount Vesuvius began as submarine volcanoes, as did the vast cones of the Hawaiian Islands and many other volcanic islands in the Pacific Ocean. States of Volcanic Activity : Some volcanoes are much more active than others. A few may be said to be in a state of permanent eruption, at least for the geological present. Stromboli, in the Lipari Islands near Sicily, has been constantly active since ancient times. Izalco, in El Salvador, has been constantly active since it first erupted in 1770. Other constantly active volcanoes are found in a belt, called the Ring of Fire, that encircles the Pacific Ocean. A similar belt in Central and South America includes Miravalles Volcano in Costa Rica, and Sangay and Cotopaxi in the Andes of Ecuador. Many other volcanoes, such as Mount Vesuvius, continue in a state of moderate activity for longer or shorter periods and then become quiescent, or dormant, for months or years. Atitlán in Guatemala was active for some 300 years before 1843 and has since been inactive. The eruption that succeeds prolonged dormancy is usually violent, as was the 1980 eruption of Mount Saint Helens in the state of Washington, after 123 years of quiescence. The massive eruption of Mount Pinatubo in the Philippines during June 1991 came after six centuries of dormancy. The danger to life posed by active volcanoes is not

limited to eruption of molten rock or showers of ash and cinders. Disastrous mudflows are an equally serious hazard. One triggered by an eruption that melted ice and snow on Colombia’s Nevado del Ruiz volcano in 1985 claimed more than 25,000 lives. Eruption: In a violent eruption of a volcano, the lava is highly charged with steam and other gases, such as carbon dioxide, hydrogen, carbon monoxide, and sulfur dioxide, which continuously escape from the lava’s surface with violent explosions and rise in a turbid cloud. This cloud frequently discharges showers of rain. Large and small portions of the lava are shot upward, forming a fiery fountain of incandescent drops and fragments, which are classified as bombs, cinders, or ash, depending on their size and shape. These objects or particles fall back in showers on the external slopes of the cone or into the crater, from which they are again and again ejected. Lightning often plays through the cloud, especially if the cloud is heavily charged with dust particles. The lava rises in the vent and finally flows over the rim of the crater or oozes, as a pasty mass, through a fissure in the side of the cone. This may mark the so-called crisis, or crucial point, of the eruption; after a final ejection of fragmental material, the volcano may then return to a quiescent state. The enormous energy expended during an explosive

eruption is shown by the heights to which rocks and ash are projected. The fine ashes of Krakatau in Indonesia reportedly were carried by the uprush of gas and vapors to a height of 27 km (17 mi) when that volcano exploded in 1883. The vapor and dust clouds thus produced can cause long-lasting atmospheric and climatic effects. For example, scientists have tentatively linked the globe-encircling dust clouds thrown up by the explosion of Mexico’s relatively small El Chichón volcano in 1982 with the widespread destruction caused by the 1982-83 El Niño disturbance. The entire summit of Papandayan in Java was blown off during a great eruption in 1772, as was the summit of Mount Saint Helens in 1980. The cone of Mount Vesuvius has frequently been altered, and the explosion of Krakatau destroyed most of the island formed by the volcano. Cooling Stage : For a long period after it has ceased to erupt either lava or fragmental materials, a volcano continues to emit acid gases and vapor in what is called the fumarolic stage. After this phase, hot springs may arise from the volcano. An example of this type of activity may be seen in the geysers of Yellowstone National Park in Wyoming and in the hot springs of the North Island of New Zealand (see Geyser). Eventually, the last traces of volcanic heat may disappear, and springs of cold water may issue from the volcano and from the ground in its vicinity. Volcanoes in the U.S. that have become inactive in recent geological time are Crater

Lake, Mount Shasta in California, Mount Hood in Oregon, and Mounts Rainier and Baker in the state of Washington. Inactive Period: After becoming inactive, a volcano undergoes progressive reduction in size through erosion caused by running water, glaciers, wind, or waves. Finally, the volcano may become completely obliterated, leaving only a volcanic pipe, that is, a chimney filled with lava or fragmental material and extending from the earth’s surface to the former lava reservoir. The rich diamond mines of South Africa are found in volcanic pipes. Lava Flows : Under certain circumstances, instead of issuing from a central vent, lava pours out along vertical fissures that may extend for several kilometers across the land surface. Flows of this sort have created thick sheets of basalt covering hundreds of square kilometers. The results of some of these inundations of lava can be seen in western North America, as in the great lava plain of the Snake River in Idaho. In modern times, fissure eruptions, generally on a smaller scale, have been observed in Iceland and Hawaii. Theories of Volcanism:Geologists once thought that the chief cause of

volcanic action was the introduction of water into the highly heated interior of the earth. In recent years, however, as the mechanics of interaction of the earth’s crustal plates have become better understood, geologists have succeeded in incorporating volcanism into the theory of plate tectonics. Wherever they occur, active volcanoes ultimately derive their energy from processes associated with movements of crustal plates. They tend, moreover, to be spatially associated with major plate boundaries. Volcanoes form at two kinds of plate boundaries: convergent and divergent. At the former, where one plate plunges (is subducted) beneath the other, material on the upper surface of the subducted plate is dragged downward on an oblique path into the earth’s crust until it reaches a depth where it becomes molten. It then rises along vertical fissures and is ejected at the surface through a volcanic vent. At divergent plate boundaries, such as the MidAtlantic Ridge, where oceanic crust is being stretched and rifted apart, a linear zone of weakness (spreading center) forms; this serves as an outlet for eruption of magma (deeplying molten rock material) brought upward by giant convection currents in the mantle. Volcanologists entertain several theories to account for the action of volcanic gases in producing an eruption. The simplest theory is that the action is similar to that of the gas in a carbonated beverage in producing a fountain of soda water, as when a soda bottle is shaken. The birth of a volcano and the building of its cone and crater were

observed firsthand in 1943, when the volcano of Paricutín erupted in the middle of a central Mexican cornfield, providing geologists with an opportunity to observe the sequence of ejected material. The district had previously been shaken by earthquakes over a two-week period; then, on February 20, a vent was observed to open and emit steam and volcanic dust, then hot fragments, and later molten rock. The 8-month eruption built up a cone about 450 m (about 1500 ft) high. The accompanying lava flows buried the village of Paricutín and nearby settlements.