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 . The fractured rock of buried impact craters (e.g., Chicxulub) may become a trap for oil and natural gas.
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
See also tektite .
See P. Hodge, Meteorite Craters and Impact Structures of the Earth (1994).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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