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 city—much 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 bombs—one 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 ]

Volcanic Eruptions

Volcanic eruptions in different parts of the world have caused immense and unquantifiable catastrophes and loss of life and property. Such disasters have paralyzed global, regional, national, and local trade for varying periods, and transport infrastructures such as seaports, ships, roads and vehicles, railroads, coaches, and airports and airplanes have been destroyed. Traffic has had to be diverted elsewhere until the structures are completely rebuilt. In disaster areas, scarcity of consumer goods has led to higher costs of production, distribution, and consumption while new markets elsewhere formed.

Volcanologists, climatologists, and historians have made various estimates for the number of volcanic eruptions since 1450. The World Almanac and Book of Facts (2002) lists 152 eruptions of active volcanoes between 1702 and 2001. Tom Simkin and Lee Seibert estimate 2,600 eruptions between 1956 and 1993, whereas others allege between 10 and 60 eruptions worldwide on a daily basis. The Smithsonian Institute's catalog recognizes 539 volcanoes with historic eruptions and 529 others that have not erupted in historic times, notable and memorable even though they have convincing evidence of eruptions in the past 10,000 years. Given the wide discrepancies, further research is clearly needed to ascertain the real number of eruptions since 1450. The volcanic events that this article highlights are some of the most revealing typical in terms of their general characteristics and their emissions of different types lava, ash, gas, and magma.

VESUVIUS, ITALY (1779 AND 1794)

Mount Vesuvius lies east of Naples, in southern Italy. It erupted eight times between 1631 and 1944 alone. During the eruptions of 1779 and 1794, showers of ash, cinder, and stone fell on neighboring communities, killing thousands. Further emissions of sulphur caused additional respiratory problems. On the sides of Vesuvius whole forests and vineyards were destroyed, and most of the villages at the foot of the mountain were covered by lava. Much damage was also done by torrents of boiling water that flowed down its sides. Maritime trade by the Dutch, French, and British was diverted for a few months in both 1779 and 1794 to ports southeast and northwest of Naples, Salerno, Corsica, Sardinia, and Sicily. A few months later, locals returned and resettled along the base of Mount Vesuvius, farming grapes, oranges, lemons, vegetables, and walnuts, and hoping to take advantage of the port's strategic geography in terms of trade.

SUMBAWA, INDONESIA (1815)

Sumbawa, an island in Indonesia, lies east of Lombok and is best known for the Tambora volcano, which erupted in 1815. This, the largest eruption in modern history, destroyed the kingdoms of Tambora and Papeyat and caused crop failures in neighboring Bali and Lambok.

Ash from the volcano fell as far away as 800 miles, in central Java and Kalimantan. Dust particles in the atmosphere caused a global cooling the following year, which was declared the "year without summer." Historians and climatologists have attributed this phenomenon to the unparalleled eruption and emission of sulphur dioxide into the atmosphere. That year, there were abnormally low daily minimum temperatures in Europe and North America from late spring to early autumn, and communities around the world experienced crop failures, famine, hunger, and food riots.

An estimated 92,000 people were killed in the 1815 eruption: 10,000 direct deaths caused by bomb impacts, tephra fall, and pyroclastic flows, and 82,000 indirect deaths from starvation, disease, and hunger brought on by the eruption. Ships in the Celebes, crossing the Flores Sea, Timor Sea, and Banda Sea were diverted to other safe ports for several years. Rice, maize, beans, cattle, cophra, tobacco, and coffee were procured from other areas as the local community struggled to regain its footing.

SOUFRIÈRE, ST. VINCENT, WEST INDIES (1902)

Previous eruptions of the Soufrière volcano in St. Vincent, in the eastern Caribbean, occurred in 1792 and 1851. The eruption of 1902 was accompanied by an outburst of a great flood of hot mud down the slope of the mountain that wiped out about 1,600 people. The calamity was aggravated by a violent asphyxiating blast that caused ships to burst into flames and sink into the boiling sea. An accompanying series of violent earthquakes led to the boiling and overflowing of the crater lake of Soufrière. Masses of lava flowed from the summit in all directions and destroyed all in its path.

Internal shipping and coastwise and inter-island fishing collapsed when the harbor was shut down for months. Inbound transatlantic vessels, their passengers, and cargoes were diverted to the other Windward Island—St. Lucia, Barbados and Grenada, Trinidad and Tobago—and communication with the outside world did not recommence for months.

MOUNT ST. HELENS, WASHINGTON, UNITED STATES (1980)

On May 18, 1980 St. Helens erupted and sent up a dense cloud of volcanic ash that fell on vast stretches of Washington State, northern Idaho, and western and central Montana. Several mudflows and floods raced down the valleys of the north and south forks of the Toutle River and nearly destroyed numerous bridges. Heavy masses of material were dumped into the Columbia River, complicating shipping and transport on that thoroughfare. In addition, the damage to more than 400-square kilometers of land induced chaos on the area's interstate highway system.

In addition, the rapid spread of ash cloud created significant air pollution in several major U.S. cities. The progressive spread around the world of this same ash adversely affected the atmosphere of distant regions. Most of the sixty victims died by asphyxiation from inhaling hot volcanic ash and by thermal and other injuries. Commercial buildings and civil works around Spirit Lake and Skamania and Cowlitz Counties suffered serious damage, and many residents lost their jobs and homes. The cost of the eruption was estimated at $1.1 billion. The disaster dealt a crippling blow to U.S. tourism as well as road, rail, and air transport for weeks.

The debris that blocked Spirit Lake, the north fork of the Toutle River, Swift Creek, Pine Creek, and Muddy River handicapped water transport and the haulage services in the regional trade. Timber could not be floated downstream to supply the wood-related industries. Because of the ash cloud and air pollution more than 1,000 commercial flights were cancelled and several airports closed. It became impossible for Washingtonians to conduct business with passengers from New Zealand, Canada, Australia, and Hawaii. The closure of area highways and railroads from Seattle to Spokane disrupted interstate trade and compounded traffic elsewhere. Exports of wheat, apples, potatoes, and alfalfa, traditionally products of the Pacific Northwest, were seriously disrupted.

SEE ALSO Agriculture; Climate; Harbors;Ships and Shipping.

BIBLIOGRAPHY

Berresheim, H., and Jaeschke W. "The Contribution of Volcanoes to the Global Atmospheric Sulphur Budget." Journal of Geophysics Research Vol. 88 (1983): 3732.

Blaikie, Piers; Canmon, Terry; Davis, Lan; and Wisner, Ben. At Risk: Natural Hazards, People's Vulnerability, and Disasters. London and New York: Routledge, 1994.

Bullard, F. M. Volcanoes of the Earth. Austin: Texas University Press, 1976.

Decker, R. W., and Decker, Barbara. Volcanoes. New York: W. H. Freeman, 1989.

Green, J., and Short, N. M. Volcanic Landforms and Surface Features. New York: Springer-Verlag, 1971.

Holmes, Arthur. Principles of Physical Geology, revised edition. London: Nelson, 1965.

MacDonald, G. A. Volcanoes. London: Prentice-Hall, 1972.

Adebayo A. Lawal

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) tall—slightly 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

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

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