A volcano forms when magma beneath the Earth's crust forces its way to the surface. Alternating elipsis

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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National preserve, southeastern shore of Hawaii island, U.S. Established in 1916, it occupies an area of 358 sq mi (927 sq km) and includes the active volcanoes Mauna Loa and Kilauea, 25 mi (40 km) apart. Other highlights are Kau Desert, an area of lava formations near Kilauea, and a tree-fern forest that receives nearly 100 in. (2,500 mm) of annual rainfall.

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