circular depression in a solid astronomical body formed by the impact of a smaller object From Wikipedia, the free encyclopedia
Impact craters are craters formed by meteorites or comets striking the Earth or other solid body. There is a system for recording and assessing craters.[1]
The prominent impact crater Tycho on the Moon. NASAphoto.Impact crater structurePosition of supposed Wilkes Land craterShocked breccia from the Azuara impact structure, Spain.Close-up of shatter cones developed in fine grained dolomite from the Wells Creek crater, USA.
There are some other suggested impact structures which are larger. It is a feature of the Earth that climate, weathering and plate tectonics removes most of the older features and events. In comparison, the Moon retains a nearly complete record of its past impact events. The implication is that the Earth once suffered a similar bombardment in its early history. This period is known as the Late Heavy Bombardment, because it occurred after the Moon was (presumably) knocked from the Earth.
The distinctive mark of an impact crater is the presence of rock which has undergone shock-metamorphic effects, shattered or melted rocks, and crystal deformations.[6] Examples:
A layer of shattered rock under the floor of the crater. This layer is called a 'breccia lens'.[7]
Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.[6][8][9][10]
High-temperature rock types, including laminated and welded blocks of sand, tektites, or glassy spatters of molten rock. They may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt.
Microscopic pressure deformations of minerals. These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, or varieties of shocked quartz.
Zeylik B. S.; Seytmuratova E. Yu, 1974: A meteorite-impact structure in central Kazakhstan and its magmatic-ore controlling role. Doklady Akademii Nauk SSSR: 1, Pages 167-170
French B.M. 1998. Traces of catastrophe: a handbook of shock-metamorphic effects in terrestrial meteorite impact structures. LPI Contribution #954. Houston, Texas, USA: Lunar and Planetary Institute.
Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a high-pressure shock.
