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Volcano

Volcano

A volcano is a hole in Earth's surface through which magma (called lava when it reaches Earth's surface), hot gases, ash, and rock fragments escape from deep inside the planet. The word volcano also is used to describe the cone of erupted material (lava and ash) that builds up around the opening.

Volcanic activity is the main process by which material from Earth's interior reaches its surface. Volcanoes played a large part in the formation of Earth's atmosphere, oceans, and continents. When Earth was new, the superheated gases within it (including carbon dioxide) streamed out through countless volcanoes to form the original atmosphere and oceans.

Volcanoes are found both on land and under the oceans (where they are called seamounts). Geologists label volcanoes by their periods of activity. If a volcano is erupting, it is called active. If a volcano is not presently erupting but might at some future date, it is called dormant. If a volcano has stopped erupting forever, it is called extinct. Generally, volcanoes are labeled extinct when no eruption has been noted in recorded history.

Words to Know

Caldera: Large circular depression formed when an empty magma chamber causes the collapse of the volcano above it.

Chemosynthesis: Process by which the energy from certain chemical reactions, rather than light (as in photosynthesis), is used by some organisms to manufacture food.

Hot spot: An upwelling of heat from beneath Earth's crust.

Ignimbrite: Rock formation that results from a large pyroclastic flow.

Lava: Magma at Earth's surface.

Magma: Molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals.

Photosynthesis: Process by which light energy is captured from the Sun by pigment molecules in plants and algae and converted to food.

Pyroclastic flow: A dense wave of superheated air and rock that moves as a fluid from an erupting volcano, sometimes crossing thousands of square miles of landscape.

Seafloor spreading: Spreading of the seafloor outward at ridges where two oceanic plates are diverging.

Seamount: Large, submarine volcano.

Tuff: Fused hard rock formed from a large pyroclastic flow.

How volcanoes form

According to the geologic theory called plate tectonics, Earth's crust is broken into various rigid plates that "float" on the surface of the planet. The plates move in response to intense pressure created underneath by the movement of currents carrying heat energy from the center of the planet to the surface. This pressure causes plates to move toward or away from each other (and also past each other in a horizontal motion).

Volcanoes form on land near coastal areas when a continental (land) plate and an oceanic plate converge or move toward each other. Since the oceanic plate is denser, it subducts or sinks beneath the continental plate. As the rock of this subducted oceanic plate is pushed farther and farther beneath the continent's surface, extremely high temperatures and pressure melt the rock. This creates hot, buoyant magma that then rises toward the surface. When the magma reaches the crust, it collects in a magma reservoir or chamber. When pressure inside the reservoir exceeds that of the overlying rock, magma is forced upward through cracks in Earth's crust.

Hydrothermal Vents

Hydrothermal vents are cracks in the ocean floor or chimney-like structures extending from the ocean floor up to 150 feet (45 meters) high. Due to nearby volcanic activity, these vents release hot mineral-laden water into the surrounding ocean. Temperature of this fluid is typically around 660°F (350°C).

Often, the fluid released is black due to the presence of very fine sulfide mineral particles (iron, copper, zinc, and other metals). As a result, these deep-ocean hot springs are called black smokers. Hydrothermal vents usually occur at midocean ridges where new seafloor is created.

Hydrothermal vents are surrounded by unusual forms of sea life, including giant clams, tube worms, and unique types of fish. These organisms live off bacteria that thrive on the energy-rich chemical compounds transported by hydrothermal fluids. This is the only environment on Earth supported by a food chain that does not depend on the energy of the Sun or photosynthesis. The energy source is chemical, not solar, and is called chemosynthesis.

Seamounts (underwater volcanoes) form when oceanic plates both converge (move toward each other) and diverge (move away from each other). When oceanic plates converge, one sinks beneath the other, creating a deep-sea trench. Rising magma from the subducted plate then rises to form volcanoes along the trench. When oceanic plates diverge, magma seeps upward at the ridge between the plates to create new seafloor (a process called seafloor spreading). Volcanoes form on either side of the ridge.

Hot spots are special areas where volcanoes form apart from plates converging or diverging. Hot spots are a common term for thermal plumes of magma welling up through the crust far from the edges of plates. As a plate drifts over a hot spot, magma from Earth's interior rises and volcanic activity takes place. Some famous hot spots are Hawaii, Yellowstone National Park (United States), Iceland, Samoa, and Bermuda.

Volcanic eruptions

Volcanoes erupt different material, and they each have their own style of erupting. These varied eruptions result from the differences in magma that each volcano contains. Magma that is low in gas and silica (silicon dioxide, a compound found widely in rocks and minerals) yields a gentle flow of thin, quickly spreading lava. In contrast, magma that is rich in gas and silica gives rise to violent explosions: the thick, tarlike magma may plug up the volcanic vent, blocking the upward movement of the magma until built-up pressure blows away the overlying rock. Geologists classify volcanic eruptions according to four chief forms or phases: Hawaiian, Strombolian, Vulcanian, and Peleean.

In a Hawaiian phase, runny lava gushes out in a fountain without any explosive eruptions. In a Strombolian phase (named after the Stomboli volcano on an island north of Sicily), thick lava is emitted in continuous

but mild explosions. Lava arcs and steam-driven clouds of ash shower the dome with molten drizzle. A Vulcanian phase occurs when a magma plug has blocked the volcanic vent. The resulting explosive eruption hurls tons of almost solid magma into the sky, and a vapor cloud forms over the crater. The most violent eruption is the Peleean, named after Mount Pelee on the Caribbean island of Martinique. Fine ash, thick lava, and glowing, gas-charged clouds are emitted, traveling downhill at a tremendous speed.

Fierce rains often accompany eruptions because of the release of steam from the volcano, which then condenses in the atmosphere to form clouds. Volatile gases in the magma also fly into the atmosphere upon eruption. These include hydrogen sulfide, fluorine, carbon dioxide, and radon. A dense wave of ash, superheated gases, and rock that moves as a fluid from an erupting volcano is known as a pyroclastic flow. Flows travel downhill at speeds more than 60 miles (100 kilometers) per hour, filling existing valleys with the fluid mixture. This material deflates as it cools. The rock formation that results is called an ignimbrite (pronounced IG-nim-bright), and the fused rock is called tuff. Ignimbrites can cover hundreds of square miles of landscape, such as the Mitchell Mesa Tuff of West Texas.

When a volcano erupts such a large volume of material, often emptying its magma chamber, the central part of the cone is left unsupported. As a result, the crater and walls of the vent collapse into the hollow chamber, creating a large circular depression known as a caldera across the summit. The famous Crater Lake in southern Oregon formed in this way.

Volcanic structures

The size and shape of a volcano is dependent on the history and type of its eruptions. Based on this, geologists classify volcanoes into four shapes: cinder cones, composite cones, shield volcanoes, and lava domes.

Cinder cones are built of lava fragments. They have slopes of 30 to 40 degrees and seldom exceed 1,640 feet (500 meters) in height. Sunset Crater in Arizona and Parícutin in Mexico are examples of cinder cones.

Composite cones (or stratovolcanoes) are made up of alternating layers of lava, ash, and solid rock. They are characterized by slopes of up to 30 degrees at the summit, tapering off to 5 degrees at the base. Mount Fuji in Japan and Mount St. Helens in Washington are composite cone volcanoes.

Shield volcanoes are built primarily by a series of lava flows that pile one on top of another. Their slopes are seldom more than 10 degrees at the summit and 2 degrees at the base. The Hawaiian Islands are clusters of shield volcanoes. Mauna Loa (on the island of Hawaii) is the world's largest active volcano, rising 13,680 feet (4,170 meters) above sea level. Kenya's Mount Kilamanjaro, the tallest mountain in Africa, is a shield volcano.

Lava domes are made of thick, pasty lava squeezed like toothpaste from a tube. Examples of lava domes are Lassen Peak and Mono Dome in California.

Volcanic catastrophes

Numerous volcanoes erupt around the world every century, usually in sparsely populated areas. Even so, volcanoes have threatened human civilization throughout history and will do so as long as people live on Earth's often violent surface.

An ash fall from Mount Vesuvius buried the Roman city of Pompeii in a.d. 79. The volcano, which sent a column of hot ash 12 miles (19 kilometers) into the sky, struck down the people where they lived, preserving the shapes of their bodies where they fell in the ash. The nearby city of Herculaneum was covered by a pyroclastic flow that destroyed it in seconds. Pompeii remained buried until 1748, when construction workers first unearthed parts of the ancient citymuch of it appearing as it did on the morning Vesuvius erupted.

On August 27, 1883, the volcanic island of Krakatoa in Indonesia erupted, blowing an ash cloud 50 miles (80 kilometers) high then collapsing into a caldera. The collapse was heard almost 2,500 miles (4,020 kilometers) away. Resulting tidal waves reaching 130 feet (40 meters) killed 36,000 people in coastal Java and Sumatra. Spectacularly weird sky phenomena from this eruption included brilliant green sunrises and moon-rises in the equatorial latitudes, followed by day-long blue sunlight and bright green sunsets.

On the morning of May 18, 1980, Mount St. Helens in Washington erupted with the force of more than 500 atomic bombsone of the largest volcanic explosions in North American history. The blast, which sent a mushroom-shaped ash plume 12 miles (20 kilometers) high, reduced the summit (peak) by more than 1,300 feet (400 meters). Sixty people and countless animals were killed, and every tree within 15 miles (24 kilometers) was flattened. Ensuing landslides carried debris for nearly 20 miles (32 kilometers).

Volcanic benefits

The eruption of volcanoes through geologic time built the continents. The soil of some of the world's richest farmland draws its fertility from minerals provided by nearby volcanoes. The heat of magma boils water into steam that spins the turbines of geothermal power stations. Geothermal stations now light electric power grids in Iceland, Italy, New Zealand, and a other places. Enough heat flows from the world's volcanic regions and midoceanic ridges to power industrial civilization for several hundred million years. This power source awaits only the development of feasible geothermal technology.

[See also Island; Ocean; Plate tectonics; Rocks ]

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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; 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 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. Pelée, is more violent, emitting fine ash; hot, gas-charged fragments of lava; and a characteristic superheated pyroclastic flow that travels downhill at great speed. Plinian, or Vesuvian eruptions, derives its name from Pliny the Younger, who described the eruption of Vesuvius in AD 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 (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), Iztaccíhuatl 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 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).

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Volcano

Volcano

Volcanoes are vents or fissures in Earth's crust through which lava , gases, and pyroclastic debris are released. More commonly, the term volcano refers to the landform built up from the accumulation of lava and/or pyroclastic debris. Based on the timing of their last eruption, volcanoes are classified as active (having erupted during historic time), dormant (having no recent eruptions, but with the potential to erupt again), or extinct (having no historic eruptions and showing no evidence of future eruptions). There are currently over 500 active volcanoes on Earth's surface, including famous examples such as Mt. Fuji, Mt. St. Helens, and Mauna Loa. Mt. Vesuvius, which last erupted in a.d.79, is an example of a dormant volcano; Mt. Kilimanjaro is an extinct volcano.

Fueled by Earth's internal processes, volcanoes occur primarily along plate boundaries but also form above hot spots. Eruptive activity may include lava flows, lateral blasts, ash flows, lahars, the release of volcanic gases, or any combination of these. Different types of volcanoes, each with a unique set of characteristics and eruptive styles, include shield volcanoes, composite volcanoes, lava domes, calderas, and cinder cones. Different types of magma form under different plate tectonic settings, and the type of magma present determines the type of volcano that will form in a given area .

Shield volcanoes, with their gentle slopes and curved profile, are the largest of all volcanoes. They are built up from repeated basaltic flows, often beginning at the ocean floor. Basaltic magma has a relatively low silica content, allowing it to flow readily. As a result, shield volcanoes are characterized by lava flows rather than explosive pyroclastic activity. Shield volcanoes are most commonly formed above hot spots under basaltic oceanic crust. They are also formed in areas where the mid-ocean ridge intersects with land, as in Iceland, or in areas of active rifting , like east Africa . In these areas, as the magma is rising to the surface, it mixes with only basaltic rocks, allowing it to preserve its mafic composition and flow readily. Probably the most famous shield volcanoes, Mauna Loa and Mauna Kea, currently rest above the Hawaiian hotspot. Measured from its base on the ocean floor to its summit, Mauna Kea is 5.6 mi (9 km) tallslightly taller than Mt. Everest.

Composite volcanoes, also known as stratovolcanoes, have steep sides and a characteristic cone shape. They are built up from alternating layers of lava and pyroclastic debris. Lava associated with composite volcanoes generally has an intermediate composition, and is more resistant to flow than basaltic lava. This results in the mixture of flows and explosions. Composite volcanoes occur above subduction zones, where rising magma mixes with both oceanic and continental crust raising the overall silica content. They are ubiquitous along the subduction zones of the Pacific Rim, and some famous examples include Mt. Fuji in Japan and Mt. Rainier in Washington. Their ability to erupt explosively, as demonstrated by Mt. St. Helens in 1980, makes these some of the most dangerous volcanoes on Earth.

Lava domes are steep-sided, rounded domes, formed because of pressure exerted by rising viscous magma. Rhyolite , a felsic magma, is usually associated with lava domes. Its felsic composition makes it highly viscous, forcing it to move slowly, building up pressure and deforming the ground surface above. Lava domes are generally associated with composite volcanoes, although they can occur on their own. They are capable of causing deadly eruptions as tremendous amounts of built-up pressure are suddenly released in giant explosions. Eruption of a lava dome was responsible for the death and destruction caused by the 1902 eruption of Mt. Pelée on Martinique.

Calderas are massive depressions created by rare, violent explosions. Also associated with rhyolitic magma, caldera eruptions are capable of expelling enormous amounts of ash and debris in a single explosion. Calderas form where hotspots occur under continental crust. As magma rises, it mixes with the felsic continental crust, resulting in a high silica content. As is the case with lava domes, the resultant viscous magma cannot flow, and explodes when sufficient pressure has built up. Although there have been none in recent geologic history, about 600,000 years ago a large caldera eruption occurred at what is presently the site of Yellowstone National Park in Wyoming and Montana. The famous hot springs and geysers of the area are the legacy of that eruption, and it is believed that the site has the potential to produce another eruption in the future.

Cinder cones are steep-sided, cone-shaped, relatively small volcanoes that are formed by the accumulation of pyroclastic debris. They are not associated with any one particular lava type, and occur in a number of settings. They are commonly found on the flanks or inside the summit craters of larger volcanoes, and form when pyroclastic debris ejected by the main volcano accumulates to form the smaller cone. Perhaps the most famous cinder cone, Parícutin volcano in Mexico, grew suddenly out of a farmer's cornfield and within one month had risen to a height of almost 1,000 ft (305 m). Cinder cones tend to have short life spans; lava flows released by Parícutin eventually covered an extensive area, but within 10 years the volcano became dormant.

See also Convergent plate boundary; Nuee ardent

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Volcanic Eruptions

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

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Crater, Volcanic

Crater, volcanic

A crater is a steep-sided roughly circular to elliptical depression in the earth caused either by volcanic activity or by the impact of an extraterrestrial body. Volcanic craters are formed by explosive events, and/or by the collapse of part of a volcano following withdrawal of magma . Impact craters are the result of collisions between Earth and extraterrestrial bodies such as meteors or comets .

Large volcanic craters are known as calderas among vulcanologists. There are two often-complementary processes involved in their formation; violent eruptions of ash and magma, and/or the collapse of a volcanic surface following withdrawal of a large body of magma from the subsurface. An example of the first type may be Crater Lake in Oregon, thought to have been produced by a violent explosion that destroyed a volcano the size of Mount St. Helens. The caldera at Kilauea, in contrast, is thought to be the result of magma drainage from beneath the summit. There is still significant discussion about whether volcanic calderas are formed directly by explosion, indirectly by collapse of the surface following magma ejection or withdrawal, or by both.

Impact craters are the result of collisions of extraterrestrial bodies with the earth. Only recently have scientists begun to understand the importance of impact processes in shaping the planet and life on it. Exploration of our solar system has revealed that essentially all planetary bodies are cratered. The density of craters on the older surfaces of the Moon indicates an intense bombardment from approximately 4.6 to 3.9 billion years ago. The Moon itself is likely the result of a collision of a Mars size object with a young Earth. The earth experienced the same bombardment as the other planetary bodies. In fact, Earth is subject to about twice as many impacts as the moon because of the difference in gravity . This is not obvious because tectonic and erosion activity on the earth have removed evidence of most of the impacts that have occurred. Nevertheless, approximately 150 craters have been identified, with more recognized every year.

Perhaps the most well-known impact crater on Earth is Chicxulub, a buried crater in the Yucatan, Mexico, that is 110 miles (180 kilometers) in diameter. Most geoscientists now believe that this impact event was responsible for the great mass extinction of the dinosaurs and many other species at the Cretaceous/Tertiary (K-T) boundary, 65 million years ago. Impacts this size occur infrequently, on the order of one every 100 million years. However, impacts that could cause damage similar to a nuclear winter , occur at time scales estimated as two or three every million years. This estimate is significant because the most recent known event, Zhamanshin in Kazaksthan, occurred about a million years ago.

See also Meteoroids and meteorites; Volcanic eruptions

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Volcanic Vent

Volcanic vent

Volcanic vents are openings in Earth's crust where molten lava and volcanic gases escape onto the land surface or into the atmosphere. Most volcanoes have a circular central vent near their summit crater that serves as a conduit for ongoing volcanic construction. Basaltic lavas that cool to form oceanic crust, oceanic plateaus, and continental flood basalts erupt from large, elongate, planar vents called fissures. New oceanic crust is created at axial fissures along the globe-encircling ocean ridge system. Small cracks and ducts in volcanic and hydrothermal provinces serve as vents for escaping lava, gas, and water that create smaller-scale volcanic features like gaseous fumaroles, hot springs , geysers, and rootless splatter cones called hornitos.

Each of the three main types of volcanoescinder cones, shields, and composite volcanoesforms by eruption of lava, volcanic ash and gases from a central vent. A cinder cone, like Volcan Parícutin in Mexico, begins with an eruption from a vent in the land surface and grows into a steep-sloped, circular mountain as cinders from successive eruptions form a cone around the vent. Shield volcanoes, like the Hawaiian Islands, are composed of low-viscosity basaltic lava that flows easily and rapidly from a central vent. Though sometimes very large, shield volcanoes have a simple structure of stacked, low-angle lava flows around the central vent.

Composite volcanoes, or stratovolcanoes, are very large volcanic edifices composed of alternating layers of volcanic ash, volcanic ejecta and lava flows. Mt. Rainier in Washington, Cotopaxi in Ecuador, Mt. Etna in Sicily, and Mt. Fuji in Japan are stratovolcanos. Extremely large, pyroclastic eruptions of gas-charged, viscous lava issue from a central vent, or group of vents, in the summit crater of a composite volcano . However, because the andesitic and rhyolitic lava that composes a stratovolcano is so viscous, the central vent system is often plugged between large eruptions. Lava fills

fissures on the flanks of the mountain creating radial dikes. Gases and fluids also escape from secondary vents, creating fumaroles and hot springs on the slopes of a stratovolcano. When a composite volcano becomes dormant, erosion wears away the volcano, leaving the vertical column that cooled in the feeder duct beneath the volcanic vent. Shiprock in New Mexico and Devil's Tower in Wyoming are examples of volcanic necks that formed this way.

See also Mid-ocean ridges and rifts; Volcanic eruptions

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Volcanoes

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 .

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volcanic cone

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.

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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.

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volcano

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.

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volcano

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.

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Volcanoes

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.

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volcano

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

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volcanic plug

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.

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volcano

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

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volcano

volcanoMano, piano •Arno, boliviano, Bolzano, Carnot, chicano, guano, Kano, llano, Locarno, Lugano, Marciano, Marrano, meccano, oregano, Pisano, poblano, Romano, siciliano, soprano, SukarnoRenault, steno, tenno •techno • Fresno • Pernod •ripieno, volcano •albino, bambino, beano, Borodino, Borsalino, cappuccino, casino, chino, Comino, concertino, Filipino, fino, Gino, keno, Ladino, Latino, Leno, maraschino, merino, Monte Cassino, Navarino, neutrino, Pacino, palomino, pecorino, Reno, San Marino, Sansovino, Torino, Trevino, Valentino, vino, Zenominnow, winnow •Llandudno • Gobineau • domino •Martineau •lino, rhino, wino •tonneau • Grodno •Livorno, porno •Mezzogiorno •cui bono?, kimono, Mono, no-no, phono •Bruno, Gounod, Juneau, Juno, Uno •Huguenot • pompano •Brno, inferno, journo, Salerno, Sterno

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