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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 (11-5 km) in diameter and 600 ft (180 m) deep, is probably the best-known crater of this type. Of the 190 impact locations identified on earth, the largest craters by diameter (80 mi/130 km or greater) are at Vredefort, South Africa; and Chicxulub (off the coast of the Yucatán peninsula), Mexico. Others include Acraman Crater, South Australia; Brent Crater, Ontario; the Chesapeake Bay impact crater, Virginia; Chubb Crater, Quebec; Lake Bosumtwi, Ghana; and Manicouagan, Quebec; and Popigai Crater, Siberia. 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 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; the pressure that a caldera collapse produces on a magma chamber helps drive such 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.
See P. Hodge, Meteorite Craters and Impact Structures of the Earth (1994).
SCO(The SCO Group, Inc., Lindon, UT, www.unxisco.com) A vendor of Unix operating systems for the x86 platform. After SCO declared bankruptcy in 2007, UnXis was formed in 2011 to acquire all its assets and intellectual property.
The SCO Group was the combination of two companies: Utah-based Caldera, Inc. and Santa Cruz-based The Santa Cruz Operation.
Caldera was a software company founded in 1994 by Ransom Love and Bryan Sparks. Its primary products were the OpenLinux and DR-DOS operating systems (see DR-DOS). In 2001, Caldera acquired the products and people from The Santa Cruz Operation, widely known as SCO, and a year later changed the company name to The SCO Group.
The Santa Cruz Operation was founded in 1979 as a custom programming house. In 1984, it introduced SCO XENIX, its first operating system, which ran on the Apple Lisa, PC XT and the DEC Pro 350. It later developed only for the Intel platform, and more than two million licenses of its Unix client and server products were sold. In 1995, SCO purchased UnixWare and all the AT&T source code for Unix System V from Novell. It merged UnixWare and its OpenServer product into a single operating system, which was released in 1998 as SCO UnixWare 7. SCO had also offered Linux, but abandoned the line in the spring of 2003. See UnixWare and OpenServer.