Volcano

VOLCANO [volcano] vents or fissures in the earth's crust through which gases, molten rock, or lava , and solid fragments are discharged. Their study is called volcanology. The term volcano is commonly applied both to the vent and to the conical mountain (cone) built up around the vent by the erupted rock materials. Volcanoes are described as active, dormant, or extinct. The soil resulting from decomposition of volcanic materials is extremely fertile, and the ash itself is a good polishing and cleansing agent.

Occurrence

Volcanoes are found in association with midocean ridge systems (see seafloor spreading ) and along convergent plate boundaries, such as around the Pacific Ocean's "Ring of Fire" (see plate tectonics ), the ring of plate boundaries associated with volcanic island arcs and ocean trenches surrounding the Pacific Ocean. Continental volcanoes are also associated with converging plate boundaries, such as the volcanoes of the Cascade Range along the W coast of the United States. Isolated volcanoes also form in the midocean area of the Pacific apparently unrelated to crustal plate boundaries. These sea mounts and volcanic island chains, such as the Hawaiian chain, may form from rising magma regions called hot spots.

Volcanic Cones and Craters

Shapes of volcanoes include composite cones, or stratovolcanoes, with steep concave sides such as Mt. St. Helens in the W United States; shield cones have gentle slopes and can be relatively large such as the Hawaiian Islands; and cinder cones as Parнcutin in Mexico, with steep slopes made of cinderlike materials. Explosive eruptions build up steep-sided cones, while the nonexplosive ones usually form broad, low lava cones. Cones range in height from a few feet to nearly 30,000 ft (9 km) above their base. Usually the cone has as its apex a cavity, or crater, which contains the mouth of the vent. Such craters are typically less than 1 mi (1.6 km) across, but larger craters, called calderas, ranging in diameter from 3 mi to--in a few instances--50 mi (5-80 km), are formed by particularly large eruptions (see crater ).

Volcanic Eruptions

More than 500 volcanoes are known to have erupted on the earth's surface since historic times, and many more have erupted on the ocean floor unobserved by humans. Fifty volcanoes have erupted in the United States, which ranks third, behind Indonesia and Japan, in the number of historically active volcanoes. Of the world's active volcanoes, more than half are found around the perimeter of the Pacific, about a third on midoceanic islands and in an arc along the south of the Indonesian islands, and about a tenth in the Mediterranean area, Africa, and Asia Minor.

Evidence of extraterrestrial volcanic activity also has been found. Space probes have detected the remnants of ancient eruptions on earth's moon, Mars (which has the largest volcano in the solar system, Olympus Mons, 340 mi/550 km across and 15 mi/24 km high), and Mercury; these probably originated billions of years ago, since these bodies are no longer capable of volcanic activity. Triton (a satellite of Neptune), Io (a satellite of Jupiter), and Venus are the only bodies in the solar system besides earth that are known to be volcanically active. The volcanic processes that occur in the outer portion of the solar system are very different from those in the inner part. Eruptions on earth, Venus, Mercury, and Mars are of rocky material and are driven by internal heat. Io's eruptions are probably sulfur or sulfur compounds driven by tidal interactions with Jupiter. Triton's eruptions are of very volatile compounds, such as methane or nitrogen, driven by seasonal heating from the sun.

Terrestrial volcanic eruptions may take one or more of four chief forms, or phases, known as Hawaiian, Strombolian, Vulcanian, and Peleean. In the Hawaiian phase there is a relatively quiet effusion of basaltic lava unaccompanied by explosions or the ejection of fragments; the eruptions of Mauna Loa on the island of Hawaii are typical. The Strombolian phase derives its name from the volcano Stromboli in the Lipari, or Aeolian, Islands, N of Sicily. It applies to continuous but mild discharges in which viscous lava is