December 13, 2007 by TMO
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body. In most common usage, the term is used for the approximately circular depression in the surface of a planet, moon or other solid body in the solar system, formed by the hyper-velocity impact of a smaller body with the surface. Impact craters typically have raised rims, and they 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 the Earth.
In the early Solar System, rates of impact cratering were much higher than today. The large multi-ringed impact basins, with diameters of hundreds of kilometres or more, retained for example on Mercury and the Moon, record a period of intense early bombardment in the inner Solar System that ended about 3.8 billion years ago. Since that time, the rate of crater production on Earth has 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. This indicates that there should be far more relatively young craters on the planet than have been discovered so far.
Although the Earthâ€™s active surface processes quickly destroy the impact record, about 170 terrestrial impact craters have been identified. These range in diameter from a few tens of metres up to about 300 km, and they range in age from recent times (e.g. the Sikhote-Alin craters in Russia witnessed in 1947) to more than two billion years, though most are less than 200 million years old because geological processes tend to obliterate older craters. They are also selectively found in the stable interior regions of continents. Few under sea 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 Earthâ€™s interior by processes of plate tectonics.