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 Parícutin 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 emitted in recurring explosions; the ejection of incandescent material produces luminous clouds. A more explosive volcanic eruption is the Vulcanian, where the magma (lava before emission) accumulates in the upper level of the vent but is blocked by a hardened plug of lava that forms between consecutive explosions. When the explosive gases have reached a critical pressure within the volcano, masses of solid and liquid rock erupt into the air and clouds of vapor form over the crater. The Peleean, derived from Mt. Pelée, is the most violent, emitting fine ash; hot, gas-charged fragments of lava; and superheated steam in an incandescent "cloud" that travels downhill at great speed. Eruptions are often accompanied by torrential rains caused by the condensation of steam. The erupted fragments vary in size, including minute particles of volcanic dust and ash, lapilli (cinders or pellets), bombs (rounded or ellipsoidal masses of hardened magma), and huge masses called blocks.

Historical Volcanoes

Notable eruptions within historic times have been those of Vesuvius, in Italy (A.D. 79, 1906, and other times); Tambora, in Indonesia, where between 30 and 50 cu mi (125-210 cu km) of molten and shattered rock were blown into the air (1815); Krakatoa, near Java, material from which was sent 17 mi (27 km) into the atmosphere (1883); Parícutin, in Mexico, the volcano that began in a cornfield (1943); Hibok Hibok, on Camiguin island in the Philippines, which killed 84 people (1948); Besymianny, in Kamchatka, where 2 cu mi (8 cu km) of material were hurled into the air (1956); the peak of Tristan da Cunha, whose eruption caused the entire settlement to be evacuated (1961); Agung, in Bali, which killed 1,100 people (1963); Mt. St. Helens in Washington, which exploded with an energy equivalent to 10 million tons of TNT, killing 35, with 25 missing (1980); El Chichon in Mexico, which expelled about 500 million tons of ash and gas (1982); and Mt. Pinatubo in the Philippines, which killed over 500 people and ejected over 2 cu mi (8 cu km) of material (1991). Other notable volcanoes are Cotopaxi and Chimborazo (Ecuador), Iztaccihuatl and Popocatépetl (Mexico), Lassen Peak and Katmai (United States), and Etna (Sicily). Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 ft (4,170 m) above sea level and over 29,000 ft (8,850 m) above the ocean floor; from its base below sea level to its summit, Mauna Loa is taller than Mt. Everest. In 1963 the birth of the volcanic island Surtsey near Iceland was observed. In November of that year events began with a submarine eruption along the Mid-Atlantic Ridge. Eruption followed eruption until June, 1967, by which time the island stood 492 ft (150 m) above sea level and covered an area of almost 2 sq mi (3 sq km). The island has not grown since the last eruption, and it is presently volcanically quiet.

Volcano Islands, Jap. Kazan-retto, island group, c.11 sq mi (30 sq km), W Pacific. The group consists of three islands, of which Iwo Jima (Iwo To) is the most important. The highest peak (3,181 ft/970 m) is on Minami-iwo-jima. There are sugarcane plantations and sulfur mines on the Volcano Islands. The inhabitants are Japanese and Koreans. Japanese fishermen and sulfur miners arrived in 1887, and Japan annexed the islands in 1891. Captured by U.S. forces in World War II, the islands were placed under U.S. administration from 1945 until 1968, when they were restored to Japan.

Sunset Crater Volcano National Monument: see National Parks and Monuments (table).

Capulin Volcano National Monument: see National Parks and Monuments (table).

Vent in the crust of the Earth from which molten rock, hot rock fragments, ash, gas, and steam issue. Most volcanoes are found on the boundaries of the enormous plates that make up the Earth's surface. Some of the most violent eruptions take place along convergent boundaries where one plate margin is forced beneath another. The most famous such boundary is the circum-Pacific belt bordering the Pacific Ocean; the island arcs and mountain ranges of this “Ring of Fire” have seen gigantic explosions, among them the eruptions of Mount Pinatubo in the Philippines in 1991, Mount Saint Helens in the U.S. state of Washington in 1980, and Krakatoa (Krakatau) in Indonesia in 1883. Volcanic activity is also common at divergent boundaries, where two plates slowly pull apart and allow molten rock to escape to the surface; the most prominent example is the Mid-Atlantic Ridge, site of volcanic islands such as Iceland, the Azores, Ascension, Saint Helena, and Tristan da Cunha. Yet another type of volcanic activity is found on the island of Hawaii, located on a “hot spot” near the middle of the Pacific Plate where plumes of partially melted rock rise from below the Earth's crust; there the volcanoes Kilauea and Mauna Loa frequently eject streams and fountains of lava. Some of the best-known volcanism takes place around the Mediterranean Sea, where the eruptions of Mount Etna, Vesuvius, the islands of Stromboli and Vulcano, and other volcanoes have been observed for millennia. Some volcanoes have cultural or religious significance for the peoples around them; these include Misti Volcano in Peru, Mount Fuji in Japan, and Ol Doinyo Lengai in Tanzania.

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Japanese Kazan-rettō or Iō-rettō

Group of three small volcanic islands, western Pacific Ocean. Administratively part of Japan, they lie south of the Bonin Islands. After they were visited by Japanese fishermen and sulfur miners in 1887, the three islands of Kita Iō, Iō (Iwo Jima; the largest), and Minami Iō were claimed by Japan in 1891. Iō was the scene of heavy fighting during World War II. After the war Japan retained residual sovereignty over the islands, but the U.S. administered them from 1951 until they were returned to Japan in 1968.

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Preserve, north-central Arizona, U.S. Established in 1930, the monument covers 5 sq mi (13 sq km) and contains the brilliant-hued cinder cone of an extinct volcano that erupted circa 1064. It rises 1,000 ft (300 m) and has a crater 400 ft (120 m) deep and 1,280 ft (390 m) in diameter. The tract contains numerous lava flows, fumaroles, and lava beds.

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Spanish Volcán Misti

Volcano, Andes Mountains, southern Peru. It is flanked by Chachani and Pichupichu volcanoes and rises to 19,098 ft (5,821 m) above sea level, towering over the city of Arequipa. Its pristine snowcapped cone is thought to have had religious significance for the Incas, and it has inspired legends and poetry. Now dormant, it last erupted during an earthquake in 1600.

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