Crater

crater, circular, bowl-shaped depression on the earth's surface. (For a discussion of lunar craters, see moon.) Simple craters are bowl-shaped with a raised outer rim. Complex craters have a raised central peak surrounded by a trough and a fractured rim.

Many of the largest craters are formed by the impact of meteorites. Impacting at speeds in excess of 10 mi/sec (16 km/sec), a meteorite creates pressures on the order of millions of atmospheres, producing shock waves that blast out a circular hole and often destroy the meteorite. Meteor, or Barringer, Crater, near Winslow, Arizona, c. 3-4 mi (1 1-5 km) in diameter and 600 ft (180 m) deep, is probably the best-known crater of this type. Of the more than 160 impact craters identified on earth, the largest are at Manicouagan, Quebec; Vredefort, South Africa; and Chicxulub (off the coast of the Yucatán peninsula), Mexico. Others include the Chesapeake Bay impact crater, Virginia; Chubb Crater, Quebec; Lake Bosumtwi, Ghana; Brent Crater, Ontario; and Kebira, SW Egypt. Two sizable impact events occurred in the 20th cent., both in Siberia. In 1908 in the Tunguska Basin near Lake Baykal one occurred that caused vast destruction of timber from its blast, and the other in 1947 at Sikhote-Alin also caused great damage. Craters that have been obliterated by erosion over thousands of years, leaving only a circular scar on the earth's surface, are called astroblemes.

Craters are also commonly formed at the surface opening, or vent, of erupting volcanoes, particularly of the type called cinder cones, where the lava is extruded rather explosively. Virtually all volcanoes display a crater, called a sink, around the vent; this is believed to be a collapse feature caused by molten lava subsiding as an eruption phase diminishes. Volcanic craters formed in these ways are relatively small, usually less than 1 mi (1.6 km) in diameter, and represent only a small fraction of the cone's diameter at the base. A

caldera is a much larger crater, typically ranging from 3 to 18 mi (5–30 km) in diameter, and represents a considerable fraction of the volcano's basal diameter. In a few instances, however, tremendous volcanic eruptions have left calderas 50 mi (80 km) or so, such as that that forms much of Yellowstone National Park or the basin of Lake Toba, Sumatra, Indonesia. Most calderas are formed by the collapse of the central part of a cone during great eruptions. A few small calderas have been formed by explosive eruptions in which the top of a volcano was blown out. Some volcanic craters are created by a combination of these events. Formed thousands of years ago, the caldera that contains Crater Lake, Oreg., is 6 mi (9.7 km) in diameter. In recent times, caldera-producing eruptions occurred at Krakatoa, Indonesia, in 1883 and Katmai, Alaska, in 1912.

See also tektite.

Bibliography

See P. Hodge, Meteorite Craters and Impact Structures of the Earth (1994).

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crater

Circular depression in the surface of a planetary body. Most craters are the result of impacts of meteorites or of volcanic explosions. Meteorite craters are more common on the Moon and Mars and on other planets and natural satellites than on Earth, because most meteorites either burn up in the Earth's atmosphere before reaching its surface or erosion soon obscures the impact site. Craters made by exploding volcanoes (e.g., Crater Lake, Ore.) are more common on the Earth than on the Moon, Mars, or Jupiter's moon Io, where they have also been identified.

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crater

1. the bowl-shaped opening at the top or side of a volcano or top of a geyser through which lava and gases are emitted

2. a similarly shaped depression formed by the impact of a meteorite or exploding bomb

3. any of the circular or polygonal walled formations covering the surface of the moon and some other planets, formed probably either by volcanic action or by the impact of meteorites. They can have a diameter of up to 240 kilometres (150 miles) and a depth of 8900 metres (29 000 feet)

4. a large open bowl with two handles, used for mixing wines, esp in ancient Greece

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crater [′krād·ər]

(geology)

A large, bowl-shaped topographic depression with steep sides.

A rimmed structure at the summit of a volcanic cone; the floor is equal to the vent diameter.

(mechanical engineering)

A depression in the face of a cutting tool worn down by chip contact.

(metallurgy)

A depression at the end of the weld head or under the electrode during welding.

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Crater [′krād·ər]

(astronomy)

A constellation, right ascension 11 hours, declination 15°S. Abbreviated Crt. Also known as Cup.

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Crater 

a bowl- or funnel-shaped depression with a diameter measuring between dozens of meters and several kilometers and a depth from a few meters to hundreds of meters. Craters form during volcanic eruptions, as a result of the impact of meteorites, and as a result of large man-made explosions. Volcanic craters are usually located on the peaks or slopes of volcanoes. At the bottom of the crater are one or several vents through which lava and other volcanic products rising along an outlet channel from the magmatic center reach the surface. Sometimes the bottom of the crater is covered with a lake of lava or a small, newly formed volcanic cone.

The craters on the moon and Mars are round depressions surrounded by circular ridges; their diameters are up to 100–200 km and they are up to several kilometers deep. A distinction is made among them between craters formed during the impact of meteorites and asteroids and craters of volcanic origin (similar to the earth's volcanic craters and calderas).

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Crater 

(the Cup), a constellation of the southern hemisphere; its brightest star has a visual stellar magnitude of 3.6. The constellation is most easily viewed in March; it is visible from the central and southern regions of the USSR. (SeeSTELLAR SKY.)