The volcano, a 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 molten or solid matter that flows through the vent from the interior of the earth. The study of volcanoes is called volcanology. Most volcanoes are composite landforms built up partly of lava flows and partly of fragmental materials. Italy's Etna, in Sicily, and 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 come up from the vent on to 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. Some calderas are the result of huge explosions that destroy the erupting volcano; the volcanic islands of Thera, 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. Etna and Vesuvius began as submarine volcanoes, as did the vast cones of the Hawaiian Islands and many other volcanic islands in the Pacific Ocean. (Source:Savage earth)
States of volcanic activity
Some volcanoes are much more active than others. A few are in a state of permanent eruption, at least for the time being. 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 Cuilapa Miravalles in Costa Rica, and Sangay and Cotopaxi in the Andes of Ecuador. Many other volcanoes, such as Vesuvius, continue in a state of moderate activity for longer or shorter periods and then become 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 Nevada del Ruiz volcano in 1985 claimed more than 25,000 lives.
Volcanic arcs and subduction
Dynamic problems unique to Pacific-type coasts were recognized as early as the 1930s by American seismologists, who showed that earthquakes associated with these belts are at shallow depths near the outer (ocean) side of volcanic island arcs, but that the depth of seismic shocks increases until it reaches a maximum of about 700 km (about 430 mi) at a distance of 700 km landward from the front of the arc. By close analysis of a single instance, the American seismologist Hugo Benioff concluded that this geometry represented a fault plane extending through the crust into the upper mantle and inclined downward, toward land, at an angle of about 45°. A similar underthrusting, of the Southern Alps beneath the Northern Alps, had been proposed in 1906, and in the 1950s the process was named subduction. The existence of similar subduction planes has now been demonstrated along almost all Pacific-type coasts. (Those where the zone is absent possess geologic evidence to show that a zone of this type formerly existed, but that it is simply inactive today.) Most of these belts disclose a major fault system that runs parallel to the general mountain system. At long intervals, the movement on the fault changes from gradual to abrupt, and a shift of about 1 to 5 m (about 3 to 15 ft) may be produced by just a single earthquake. Such faults are found in Chile, Alaska, Japan, Taiwan, the Philippines, New Zealand, and Sumatra. During subduction, ocean crust is constantly being drawn down into the mantle and melted. Because it is continually recycled, no part of the modern ocean crust is more than 200 million years old. Indeed, crustal blocks are constantly moving and jostling as they are carried by the various plates. An important effect of the melting of subducted ocean crust is the production of new magma. When subducted ocean crust melts, the magma that forms rises upward from the plane of subduction, deep within the mantle, to erupt on the earth's surface. Eruption of magma melted by subduction has created long, arc-shaped chains of volcanic islands, such as Japan, the Philippines, and the Aleutians. Where an oceanic tectonic plate is subducted beneath continental crust, the magma produced by subductive melting erupts from volcanoes situated among long, linear mountain chains, such as the Cordillera, up to 100 km inland from the zone of subduction. (The zone itself is located along a submarine trench offshore of the continent.) In addition to creating and feeding continental volcanoes, melting of subducted ocean crust is responsible for the formation of certain kinds of ore deposits of valuable metallic minerals.