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An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body with the surface. In contrast to volcanic craters, which result from explosion or internal collapse,〔Basaltic Volcanism Study Project. (1981). Basaltic Volcanism on the Terrestrial Planets; Pergamon Press, Inc: New York, p. 746. http://articles.adsabs.harvard.edu//full/book/bvtp./1981//0000746.000.html.〕 impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain.〔Consolmagno, G.J.; Schaefer, M.W. (1994). ''Worlds Apart: A Textbook in Planetary Sciences;'' Prentice Hall: Englewood Cliffs, NJ, p.56.〕 Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is perhaps the best-known example of a small impact crater on Earth. Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Mercury, Callisto, Ganymede and most small moons and asteroids. On other planets and moons that experience more active surface geological processes, such as Earth, Venus, Mars, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or transformed by tectonics over time. Where such processes have destroyed most of the original crater topography, the terms impact structure or astrobleme are more commonly used. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth.〔French, B.M. (1998). ''Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures;'' Simthsonian Institution: Washington DC, p. 97. http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html.〕 The cratering records of very old surfaces, such as Mercury, the Moon, and the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been considerably lower, but it is appreciable nonetheless; Earth experiences from one to three impacts large enough to produce a 20 km diameter crater about once every million years on average.〔Carr, M.H. (2006) ''The surface of Mars;'' Cambridge University Press: Cambridge, UK, p. 23.〕〔Grieve R.A.; Shoemaker, E.M. (1994). The Record of Past Impacts on Earth in ''Hazards due to Comets and Asteroids,'' T. Gehrels, Ed.; University of Arizona Press, Tucson, AZ, pp. 417-464.〕 This indicates that there should be far more relatively young craters on the planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system. Formed in a collision 160 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate, perhaps causing the Chicxulub impact that may have triggered the extinction of the non-avian dinosaurs 66 million years ago.〔 Note that the rate of impact cratering in the outer Solar System could be different from the inner Solar System. Although Earth's active surface processes quickly destroy the impact record, about 170 terrestrial impact craters have been identified.〔Grieve, R.A.F.; Cintala, M.J.; Tagle, R. (2007). Planetary Impacts in ''Encyclopedia of the Solar System,'' 2nd ed., L-A. McFadden et al. Eds, p. 826.〕 These range in diameter from a few tens of meters up to about 300 km, and they range in age from recent times (e.g. the Sikhote-Alin craters in Russia whose creation were witnessed in 1947) to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters. They are also selectively found in the stable interior regions of continents.〔Shoemaker, E.M.; Shoemaker, C.S. (1999). The Role of Collisions in ''The New Solar System,'' 4th ed., J.K. Beatty et al., Eds., p. 73.〕 Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, and the subduction of the ocean floor into Earth's interior by processes of plate tectonics. Impact craters are not to be confused with landforms that in some cases appear similar, including calderas and ring dikes. ==History== Daniel Barringer (1860–1929) was one of the first to identify an impact crater, Meteor Crater in Arizona; to crater specialists the site is referred to as Barringer Crater in his honor. Initially Barringer's ideas were not widely accepted, and even when the origin of Meteor Crater was finally acknowledged, the wider implications for impact cratering as a significant geological process on Earth were not. In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the USA. He concluded they had been created by some great explosive event, but believed that this force was probably volcanic in origin. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts. The concept of impact cratering remained more or less speculative until the 1960s. At this time a number of researchers, most notably Eugene M. Shoemaker, (co-discoverer of the comet Shoemaker-Levy 9), conducted detailed studies of a number of craters and recognized clear evidence that they had been created by impacts, specifically identifying the shock-metamorphic effects uniquely associated with impact events, of which the most familiar is shocked quartz. Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory in Victoria, British Columbia, Canada and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American Apollo Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon.〔Grieve, R.A.F. (1990) Impact Cratering on the Earth. ''Scientific American,'' April 1990, p. 66.〕 Processes of erosion on the Moon are minimal and so craters persist almost indefinitely. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Impact crater」の詳細全文を読む スポンサード リンク
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