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A crater (basin or impact crater) is a circular depression on the surface of a planet, moon, asteroid, or other celestial body. Craters are typically caused by meteorite impacts, although some are caused by volcanic activity (see volcano for more on these). In the center of craters on Earth a crater lake often accumulates, and in craters formed by meteorites a central island (caused by rebounding crustal rock after the impact) is usually a prominent feature in the lake.
Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests. Although it might be assumed that a major impact on the Earth would leave behind absolutely unmistakable evidence, in fact the gradual processes that change the surface of the Earth tend to cover the effects of impacts. Erosion by wind and water, deposits of wind-blown sand and water-carried sediment, and lava flows in due time tend to obscure or bury the craters left by impacts. Simple slumping of weak crustal material can also play a role, especially on outer solar system bodies such as Callisto which are covered in a crust of ice.
However, some evidence remains, and over 150 major craters have been identified on the Earth. Studies of these craters have allowed geologists to find the remaining traces of other craters that have mostly been obliterated.
In the 1920s, the American geologist Walter H. Bucher studied a number of craters in the US. He concluded they had been created by some great explosive event, but believed they were the result of some massive volcanic eruption. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded the craters he studied were probably formed by impacts.
The issue remained more or less speculative until the 1960s. A number of researchers, most notably Eugene M. Shoemaker, conducted detailed studies of the craters that provided clear evidence that they had been created by impacts, identifying the shock-metamorphic effects uniquely associated with impacts, of which the most familiar is Shocked quartzShocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz will be deformed along planes inside the crystal. These planes.
Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory , ( Victoria, British ColumbiaVictoria is a Canadian city, and the provincial capital of British Columbia. Victoria also refers to Greater Victoria including this municipality and those immediately surrounding it. It is also the seat of the Capital Regional District. Location It is ne, CanadaCanada historically the Dominion of Canada is the second-largest, and northernmost, country in the world. It is a decentralized federation of 10 provinces and 3 territories, governed as a constitutional monarchy, and formed in 1867 through an act of Confe), and Wolf von Engelhardt of the University of Tübingen in GermanyThe Federal Republic of Germany ( German: Bundesrepublik Deutschland is one of the world's leading industrialized countries, located in the middle of the European Union. It is bordered to the north by the North Sea, Denmark and the Baltic Sea, to the east began a methodical search for "impact structures". By 1970, they had tentatively identified more than 50.
Their work remained controversial, but the American Apollo Moon landings, which were in progress at the time, provided evidence of the rate of impact cratering on the Moon. 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.
The age of known impact craters on the Earth ranges from about a thousand (e.g. the Haviland crater in Kansas) to almost two billion years, though few older than 200 million years have been found, as geological processes tend to obliterate older ones. They are also selectively found in the stable interior regions of continents. Few underwater 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 the Earth's interior by processes of plate tectonics.
Current estimates of the rate of cratering on the Earth suggest that from one to three craters with a width greater than 20 kilometers are created every million years. This indicates that there are far more relatively young craters on the planet than have been discovered so far.
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