volcano

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volcano,

vents or fissures in the earth's crust through which gases, molten rock, or lavalava
, molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava.
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, 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 spreadingseafloor spreading,
theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H.
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) and along convergent plate boundaries, such as around the Pacific Ocean's "Ring of Fire" (see plate tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
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), 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 RangeCascade Range,
mountain chain, c.700 mi (1,130 km) long, extending S from British Columbia to N Calif., where it becomes the Sierra Nevada; it parallels the Coast Ranges, 100–150 mi (161–241 km) inland from the Pacific Ocean.
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 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; an example of a continental hot spot is found at Yellowstone National ParkYellowstone National Park,
2,219,791 acres (899,015 hectares), the world's first national park (est. 1872), NW Wyo., extending into Montana and Idaho. It lies mainly on a broad plateau in the Rocky Mts., on the Continental Divide, c.
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.

Volcanic Cones and Craters

Shapes of volcanoes include composite cones, or stratovolcanoes, with steep concave sides such as Mt. St. HelensSaint Helens, Mount,
volcanic peak, 8,363 ft (2,549 m; 9,677 ft/2,950 m before its 1980 eruption) high, SW Wash., historically the most active volcano in the Cascade Range. Dormant since 1857, Mt. St.
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 in the W United States; shield cones have gentle slopes and can be relatively large such as the Hawaiian Islands; and cinder cones as ParícutinParícutin
, active volcano, c.8,200 ft (2,500 m) high, Michoacán state, W central Mexico. In one of the most spectacular eruptions of modern times, Parícutin burst forth from a cornfield on Feb.
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 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 cratercrater,
circular, bowl-shaped depression on the earth's surface. (For a discussion of lunar craters, see moon.) Simple craters are bowl-shaped with a raised outer rim. Complex craters have a raised central peak surrounded by a trough and a fractured rim.
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).

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 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, and there are apparently volcanoes on Pluto that erupt ice consisting of water and nitrogen, ammonia, or methane, possibly driven by heat from radioactive decay in the dwarf planet's core.

Terrestrial volcanic eruptions may take one or more of five chief forms, or phases, known as Hawaiian, Strombolian, Vulcanian, Peleean, and Plinian. 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. PeléePelée
, volcano, 4,429 ft (1,350 m) high, on N Martinique, in the West Indies. On May 8, 1902, the day after the eruption of Soufrière on St. Vincent, Pelée also erupted, engulfing Saint-Pierre at its base in a pyroclastic flow and killing c.
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, is more violent, emitting fine ash; hot, gas-charged fragments of lava; and a characteristic superheated pyroclastic flowpyroclastic flow,
turbulent, fluidized mixture of rock, volcanic ash, and hot gas that moves like an avalanche away from a volcanic eruption. A pyroclastic flow may contain a mix of rock fragments ranging up to the size of boulders, with the heavier fragments moving along closer
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 that travels downhill at great speed. Plinian, or Vesuvian eruptions, derives its name from Pliny the Younger, who described the eruption of VesuviusVesuvius
, Ital. Vesuvio, active volcano, S Italy, on the eastern shore of the Bay of Naples, SE of Naples. The only other active volcano on the European mainland is the Campi Flegrei (se Phlegraean Fields) caldera on the Gulf of Pozzuoli to the east.
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 in A.D. 79. The Plinian eruption is similar to Strombolian and Vulcanian eruptions with significant ash and pumice and pyroclastic flows, but it also produces a characteristic massive, sustained eruptive column of hot ash that can reach 28 mi (45 km) in height.

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. Minute dust and ash and aerosols carried high into the earth's atmosphere can have a cooling effect on the climate, and significant amounts of chlorine and bromine gases ejected in large eruptions can reach the stratosphere and deplete the ozone layer. The dust and ash can also be a hazard to air travel. The 1783 eruption of Laki, S Iceland, had devastating effects on local livestock and, as result, the populace; the resulting sulfur dioxide haze that spread over parts of Europe is believed to have negatively affected the health of the inhabitants.

Historical Volcanoes

Notable eruptions within historic times have been those of Vesuvius, in Italy (A.D. 79, 1906, and other times); TamboraTambora
, active volcano, N Sumbawa, Nusa Tenggara Barat prov., Indonesia, rising to 9,255 ft (2,821 m). The volcano's 1815 eruption was one of the most destructive in historical times, killing an estimated 50,000–90,000 people, destroying the kingdoms of Tambora and
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, in Indonesia, where between 30 and 50 cu mi (125–210 cu km) of molten and shattered rock were blown into the air (1815); KrakatoaKrakatoa
or Krakatau
, volcanic island, c.5 sq mi (13 sq km), W Indonesia, in Sunda Strait between Java and Sumatra; rising to 2,667 ft (813 m). A momentous volcanic explosion on Aug.
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, 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 CunhaTristan da Cunha
, group of volcanic islands in the S Atlantic, about midway between S Africa and S America, part of the British Saint Helena overseas territory. The only habitable island of the group is Tristan da Cunha (1999 pop. 286), formed by a volcano rising to c.
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, 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 Chichón in Mexico, which expelled about 500 million tons of ash and gas (1982); and Mt. PinatuboPinatubo, Mount
, active volcano, 5,840 ft (1,780 m), central Luzon island, the Philippines, c.55 mi (90 km) NW of Manila. Dormant for 600 years, it began erupting on Apr. 2, 1991.
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 in the Philippines, which killed over 500 people and ejected over 2 cu mi (8 cu km) of material (1991). Other notable volcanoes are CotopaxiCotopaxi
, active volcano, 19,347 ft (5,897 m) high, N central Ecuador. A symmetrical snowcapped cone in the Andes, it is one of the highest volcanoes in the world. It is continuously active, and frequent eruptions have caused severe damage.
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 and ChimborazoChimborazo
, inactive volcano, 20,577 ft (6,272 m) high, central Ecuador; the highest in Ecuador. Its summit is always snowcapped. First explored by Alexander von Humboldt in 1802, it was first scaled in 1880 by Edward Whymper.
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 (Ecuador), IztaccíhuatlIztaccíhuatl
, Ixtacíhuatl,
or Ixtaccíhuatl
[Aztec,=white woman], dormant volcano, 17,160 ft (5,230 m) high, central Mexico, on the border between Puebla and Mexico state.
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 and PopocatépetlPopocatépetl
[Aztec,=smoking mountain], volcano, 17,887 ft (5,452 m) high, in the Cordillera de Anáhuac, central Mexico, on the Puebla-Mexico state border; the second highest peak in Mexico.
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 (Mexico), Lassen Peak and Katmai (United States), EtnaEtna
or Aetna
, volcano, 10,958 ft (3,340 m) high, on the east coast of Sicily, S Italy. One of the most active volcanoes in the world, it also is the highest active volcano in Europe. The shape and height of its central cone have often been changed by eruptions.
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 (Sicily), and HeklaHekla
, volcano, 4,892 ft (1,491 m) high, SW Iceland. Since the early 11th cent. more than 20 eruptions have been recorded; the worst occurred in 1766 and the most recent in 1947. Hekla emits steam and has several crater lakes.
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, KatlaKatla,
volcano, 4,961 ft (1,512 m) high, SW Iceland. One of Iceland's most active volcanoes, it is partly buried under the Mýrdalsjökull glacier, which covers its eruptive vents.
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, and LakiLaki,
volcano, 2,684 ft (818 m) high, S Iceland, at SW edge of the Vatnajökull glacier. Its eruption in 1783 was one of the more devastating on record, leading to the deaths of a quarter of Iceland's inhabitants (mainly due to a famine that resulted from the eruption's
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 (Iceland). Mauna LoaMauna Loa
, mountain, 13,680 ft (4,170 m) high, in the south central part of the island of Hawaii, in Hawaii Volcanoes National Park. Its many craters include Kilauea and Mokuaweoweo, two of the world's largest active craters.
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 (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 SurtseySurtsey,
volcanic island, c.1.25 sq mi (3.2 sq km), S of Iceland in Vestmannaeyjar (Westman Islands). The island was formed by the eruption (Nov., 1963) of Sutur, an underwater volcano named for a giant of Icelandic legend. For four months the fissure, estimated to be c.
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 near Iceland was observed. In November of that year events began with a submarine eruption along the Mid-Atlantic Ridge. Eruption followed eruption until they ended in 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 diminished in size since then due to erosion.

Bibliography

See S. Van Rose and I. Mercer, Volcanoes (2d ed. 1991); F. Martin, Volcano (1996); H. Sigurdsson, Melting the Earth: The History of Ideas on Volcanic Eruptions (1999); H. Sigurdsson et al., ed., Encyclopedia of Volcanoes (1999); C. Oppenheimer, Eruptions that Shook the World (2011).

What does it mean when you dream about a volcano?

A dream about an erupting volcano often represents an emotional eruption that is building up within the dreamer’s psyche, and hence is a warning to the individual to let off steam before a blowup.

volcano

[väl′kā·nō]
(geology)
A mountain or hill, generally with steep sides, formed by the accumulation of magma erupted through openings or volcanic vents.
The vent itself.

volcano

1. an opening in the earth's crust from which molten lava, rock fragments, ashes, dust, and gases are ejected from below the earth's surface
2. a mountain formed from volcanic material ejected from a vent in a central crater

Volcano

(dreams)
Volcanic eruptions in dreams usually represent our erupting emotions. Feelings that you may be harboring during the day might take the form of a volcanic eruption in a dream state. The unconscious psyche may be releasing positive or negative feelings in the safety of a dream (i. e., strong sexual feelings, passion, anger, rage, and fear). Superstitionbased dream interpretations say that pouring lava is a warning about poor health.