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).

crater A bowl-shaped depression on a planetary surface. Craters are usually of either impact or volcanic origin; however, some craters on the satellites of the outer planets may be the result of ice volcanism in which internal heating melts and vaporizes the ice to produce gas explosions and water flows. Volcanic craters tend to be less circular than impact ones, and may be formed either by explosion or collapse. Volcanic craters of purely explosive origin are normally under 1 km in diameter and are usually found at the top of a cone of ejected debris; collapse craters can be more than 100 km across, and may have no outer slope at all. Impact craters have been found on all bodies in the Solar System photographed by space probes except Jupiter's volcanic satellite Io, but volcanic craters do not seem to exist on the smaller Solar System bodies.