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; an example of a continental hot spot is found at Yellowstone National Park .

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 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. Minute dust and ash and aerosols carried high into the earth's atmosphere can have a cooling effect on the climate; 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 (AD 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), Etna (Sicily), and Hekla , Katla , and Laki (Iceland). 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.

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).

Volcanic eruptions

A volcanic eruption is the release of molten rock and volcanic gases through Earth's crust to the surface. Molten rock within the earth, or magma , is driven to erupt by buoyancy because it is lighter than the surrounding rock. Dissolved gases within the magma are under great pressure and force magma upwards. The upward migrating magma takes advantage of preexisting zones of weaknesses such as fractures or established volcanic necks until it eventually breaks through the surface.

An eruption may last for a few minutes or many hours and days. An eruption may be only a discharge of steam and gases through a small vent, a relatively mild oozing of lava from a fissure in a shield volcano , or a spectacular explosion that shoots huge columns of gases and debris into the sky. The explosiveness of an eruption depends to a great extent on the composition of the molten rock. Magma high in silica will be more viscous than one low in silica. A high-viscosity magma (such as a rhyolite ) will tend to trap dissolved gases. The pressure of the gases can build up to the point where they are released in a spontaneous explosive eruption. A less viscous magma (such as a basalt ) allows volcanic gases to bubble through more easily, preventing great build-ups of pressure, and resulting in calmer outpourings of lava.

The length an eruption is described as an eruptive pulse, eruptive phase, or eruptive episode. An eruptive pulse is a very short event lasting a few seconds to minutes. An eruption that lasts a few hours to days and consists of numerous eruptive pulses is called an eruptive phase. Eruptions that involve repeated pulses and phases over days, months, or years is an eruptive episode.

Volcanic eruptions are described according to explosivity, lava type, and other constituents such as ash, gas, and steam content or the nature of rock fragments produced. Some common eruption types are named for classic types of volcanoes that characterize the eruption. These include Hawaiian, Plinian (Vesuvian), Strombolian, and Vulcanian. Some types of eruptions have more descriptive names, such as effusive and phreatic.

A Hawaiian-type eruption consists of a highly fluid basaltic lava that tends to flow effusively from linear fissures or from a central vent in the production of shield volcanoes. The release is not generally explosive as lava gently flows in streams or through lava tubes. Sometimes the lava accumulates in lava lakes . Occasionally, however, more spectacular fountains of lava spurting out from a vent do occur.

A Plinian, or Vesuvian, eruption is a more explosive and potentially destructive event where large amounts of ash, dust, and gas are blown out of a central source at a high velocity. The eruptive cloud often forms a large column extending high into the air above the volcano. Avalanches of hot ash, rock, and gas, called nuee ardentes, can travel down the side of the volcano at up to 100 mph (160 kph) are possible, such as the one that covered the Italian city of Pompeii. Rhyolitic to dacitic compositions are common. The name is derived from the historian Pliny, who recorded the eruption of Vesuvius in a.d.79.

Strombolian eruptions are characterized by discrete episodic explosions or fountains of basaltic lava from a single vent or crater. The eruptive pulses are caused by the release of volcanic gases, and are separated by periods of a few seconds to hours. Lava fragments consisting of partially molten volcanic bombs that become rounded as they fly through the air are commonly produced.

Vulcanian, or hydrovolcanic eruptions are explosive events that release a combination of ash and steam into the air, producing an eruptive column. Fragments of lava are ejected, but owing to a high viscosity or previous cooling, the fragments do not form aerodynamic bombs. The composition of the lava is generally andesitic to dacitic.

An effusive eruption is a general term for any non-explosive release of lava. The lava gently wells up from the ground and overflows, cooling on its way down the slope. Effusive eruptions are common in a Hawaiian type event. When a basaltic effusive eruption occurs on the ocean floor, pillow lavas often form. As the name suggests, pillow basalts are rounded elongate shapes the lava takes due to extrusion under the pressure of the ocean. As pillow lavas continually erupt, they form stacked mounds of pillows. Effusive eruptions may occur with a range of compositions, although they are most common in low viscosity lavas such as basalt.

If cool ground water or surface water comes in contact with magma below the surface, a phreatic eruption may occur. This is caused by water that is heated into pressurized steam, creating an explosive eruption driven solely by the steam. Because the eruption is driven by steam, no new rock is formed.

See also Extrusive cooling; Fumerole; Hawaiian island formation; Hotspots; Lahar; Nuee ardent; Pipe, volcanic; Tuff; Volcanic vent

VOLCANOES

VOLCANOES are mountains with a vent from which molten material from deep within the earth can spew under the appropriate conditions. Volcanoes have existed for geologic eons, but many are no longer active. The number of volcanoes worldwide that earth scientists consider active—those that can erupt—was about five hundred in the

mid-1990s. Volcanoes are usually located at the junction of the earth's lithospheric plates. In the United States most active volcanoes are located in Alaska or in Hawaii, which consists of a group of islands formed by earlier volcanic eruptions. The West Coast of the continental United States also has a relatively inactive volcanic zone.

The two principal volcanoes in the United States are Mauna Loa and Kilauea, both in the Hawaiian island chain. Mauna Loa, the world's largest volcano, erupted most recently in 1975 and 1984. Kilauea is in almost continual eruption. Alaskan eruptions occurred in 1989, when Mount Redoubt, along Cook Inlet, southwest of Anchorage, erupted; in 1992, when Mount Spurr erupted; and in 1996, when an unnamed volcano on Augustine Island (also in Cook Inlet) erupted. Although not in the United States, Mount Pinatubo in the Philippines projected enough ash into the stratosphere during its eruption in 1991 to have a significant cooling effect on the U.S. climate for several years. Eruptions in the lower forty-eight states are rare but certainly not unknown: for example, the widely publicized eruption of Mount St. Helens in Washington State in 1980. Despite dire predictions and a minor eruption in 1990, the area surrounding Mount St. Helens had largely recovered from the effects of the 1980 eruption by 2000.

There are two volcanic observatories in the United States. One, established on Kilauea in 1912, is the second oldest in the world, ranking behind only one in Italy, on Mount Vesuvius. Following the eruption of Mount St. Helens in 1980, an observatory was established there.

BIBLIOGRAPHY

Decker, Robert W., and Barbara B. Decker. Mountains of Fire: The Nature of Volcanoes. New York: Cambridge University Press, 1991.

Scarth, Alwyn. Volcanoes: An Introduction. College Station: Texas A&M University Press, 1994.

Nancy M.Gordon/c. w.

See alsoAlaska ; Geology ; Hawaii ; Paleontology .

volcanogenic earthquakes On a global scale the association of earthquakes, particularly those with deep foci, and volcanicity is very close, as is indicated by the fact that both occur at destructive and constructive plate boundaries. In contrast, the earth tremors that are the direct consequence of volcanic activity are small, minor shocks that are spread over a relatively large area.

Before the commencement of a volcanic eruption quite local shocks are commonly registered which are caused by the opening of fissures in the underlying rocks or in the volcanic structure itself. These are due to the sluggish movement of very hot viscous magma several kilometres below the volcanic vent from one storage chamber to another. This motion takes place under great steam pressure through a network of tubes and pipes. In the process, various parts of the surrounding rock become hotter and more strained as the magma pushes through them. This results in fractures in the neighbouring rocks, the strain being relieved by elastic rebound, which is felt as an earthquake.

After a major eruption has ceased, more-or-less circular volcano-tectonic basins appear, termed calderas. These form by collapse of the surface rocks owing to partial evacuation of the underlying magma chamber. This caldera collapse induces very complex ground movements that are accompanied by earthquakes.

Harold G. Reading

volcanic cone A conical mound of volcanic ejecta accumulated around an eruptive vent. Cones have outer slope angles of about 30° and are topped by a depression or crater over the site of the vent. The type of material which accumulates to form the cone can be used to name the type of cone. For example, alternate layers of lava with beds of ash and other pyroclastic material characterize a strato-volcano (composite volcano), and spatter ejected from a vent during a Hawaiian-type eruption would accumulate to form a spatter cone around the vent. Scoria ejected from a vent during a Strombolian-type eruption would accumulate to form a scoria cone around the vent. See VOLCANO.

volcano A naturally occurring vent or fissure at the Earth's surface through which erupt molten, solid, and gaseous materials. The viscosity, gas content, and rate of extrusion of the magma probably determine the shape of the mountain built by the eruptions. The magma may reach the surface either through a single channel (see CENTRAL VENT VOLCANO), or through a series of vertical fractures (see FISSURE VOLCANO). Types of eruptions are named after volcanoes associated with them. See HAWAIIAN ERUPTION; PELÉEAN ERUPTION; PLINIAN ERUPTION; STROMBOLIAN ERUPTION; SURTSEYAN ERUPTION; VESUVIAN ERUPTION; and VULCANIAN ERUPTION.

vol·ca·no / välˈkānō; vôl-/ • n. (pl. -noes or -nos) a mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapor, and gas are or have been erupted from the earth's crust. ∎ fig. an intense suppressed emotion or situation liable to burst out suddenly: what volcano of emotion must have been boiling inside that youngster.

volcano a mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapour, and gas are or have been erupted from the earth's crust; in figurative usage, an intense suppressed emotion; a situation liable to burst out suddenly. Recorded from the early 17th century, the word comes from Italian, from Latin Volcanus Vulcan.

411. Volcanoes

See also 179. GEOLOGY ; 283. MOUNTAINS .

volcanism

the phenomena connected with volcanoes and volcanic activity. Also vulcanism. —volcanist, n.

volcanology

Geology. the scientific study of volcanoes and volcanic phenomena. Also vulcanology. —volcanologist, n. —volcanologic, volcanological, adj.

volcano Vent from which molten rock or lava, solid rock debris, and gases issue. Volcanoes may be of the central vent type, where the material erupts from a single pipe, or of the fissure type, where material is extruded along an extensive fracture. Volcanoes are usually classed as active, dormant or extinct. See also volcanism

volcanic plug (volcanic neck) The cylindrical filling of an ancient volcano which, due to its greater resistance, may be preserved after the volcanic edifice has been eroded away. See also PUY.

volcano XVII. — It. — L. Volcānus, Vulcānus Rom. god of fire.
So volcanic XVIII. — F. volcanique.

earthquakes, volcanogenic see volcanogenic earthquakes