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- "Meteor crater" redirects here. For the crater of that name, see Meteor Crater.
An impact crater (impact basin, astrobleme or sometimes crater) is a circular or oval depression on a surface, usually referring to a planet, moon, asteroid, or other celestial body, caused by a collision of a smaller body (meteor) with the surface. The Meteor Crater (Barringer Crater) is a prime example.
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. Ancient planetary craters whose original relief has disappeared leaving only a "ghost" of a crater are sometimes known as palimpsests. On Earth, where no topographic crater remains, the term impact structure is more commonly used.
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. Impact craters are found on nearly all solid surface planets and satellites. As the number of impact craters increases on a surface, the appearance of the surfaces changes; this can be used to establish the age of extraterrestrial terrain. After a period of time, however, an equilibrium is reached in which old craters are destroyed as quickly as new craters form.
Daniel Barringer (1860-1929) was one of the first to identify a geological structure as an impact crater, the Barringer Meteorite Crater (or the "Meteor Crater") in Arizona, but at the time his ideas were not widely accepted, and when they were, there was no recognition of the fact that Earth impacts are common in geological terms.
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 quartz.
Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory, (Victoria, British Columbia, Canada), and Wolf von Engelhardt of the University of Tübingen in Germany 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.
 Formation and structure
An object falling from open space hits the Earth with a minimum velocity of 11.6 km/s (7 mi/s). Since the energy from motion grows as the square of the velocity, this gives moving rock more energy per kilogram than ordinary chemical explosives. Massive objects can easily cause kiloton explosions that resemble nuclear explosions. Seismographs record about one multikiloton impact somewhere on the Earth each year, usually in mid-ocean.
If the object weighs more than 1,000 tons, an atmosphere does not slow it down much, though smaller bodies can be substantially slowed by atmospheric drag, as they have a higher ratio of surface area to mass. In any case, the temperatures and pressures on the object are extremely high. These temperature and pressure extremes can destroy chondritic or carbonaceous chondritic bodies before they ever reach ground, but metallic iron-nickel meteorites have more structural integrity and can strike the surface of the Earth in a violent explosion.
When the object hits, it compresses a column of air, water and rock into an extremely hot plasma. This plasma expands violently, and cools rapidly (i.e. it explodes). The plasma and other ejecta splashes at orbital or near-orbital speeds. It can be thrown off into space, or can travel several times around the planet before re-entering as secondary meteors. Airless planets usually preserve stains of the ejecta around impact craters as a pattern of "rays". Other non-impact theories for crater-ray formation have been suggested, in the scientific literature.
Very energetic chemistry occurs in the plasma. In an Earth impact, powerful acids can be formed from saltwater and air. The vaporized rock of the plasma condenses into characteristic cone-shaped droplets of glass called tektites, and these are widely distributed by the high speeds. Tektites are found in isolated strewnfields on Earth. Note: Several researchers reject the popular impact-origin theory of tektites based on comparisons to bonafide impactite glasses. These researchers point out that the largest and youngest (700,000 years ago) tektite strewnfield, known as the Australasian field, is not associated with any known impact crater.
Oceanic impacts can be considerably more damaging than those on land. Large objects will invariably penetrate or displace the water to impact the seabed, causing huge tsunamis over a large area. The impact that created the Chicxulub Crater is believed to have produced tsunamis 50 to 100 metres (150-300 feet) high which deposited debris many miles inland.
The result of an impact on land or at sea is a crater. There are two forms, "simple" and "complex". The Barringer crater in Arizona is a perfect example of a simple crater, a straightforward bowl in the ground. Simple craters are generally less than four kilometers across and are generally circular except when the impact angle is less than 20 degrees.
Complex craters are larger, and have uplifted centers that are surrounded by a trough, plus broken rims. The uplifted center is due to the "rebound" of the earth after the impact. It is something like the ripple pattern created by a drop of water into a pool, frozen into the Earth when the melted rock cooled and solidified.
In either case, the size of the crater depends on the size of the impactor and the material in the impact regions. Relatively soft materials yield smaller craters than brittle materials. The size of craters invariably changes over time; in the short term, craters shrink as a result of slumping, and over the longer term erosion and other geological processes quickly hide impact craters on the Earth. The Barringer Crater is one of the best-preserved on the planet, but it is only about 50,000 years old. There are almost no signs of the 65 million year-old Chicxulub crater on the Earth's surface, despite it being one of the largest known on the planet.
Some volcanic features can resemble impact craters, and brecciated rocks are associated with other geological formations besides impact craters. Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and the association of volcanic flows and other volcanic materials. An exception is that impact craters on Venus often have associated flows of melted material.
The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.
Impacts produce distinctive "shock-metamorphic" effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:
- A layer of shattered or "brecciated" rock under the floor of the crater. This layer is called a "breccia lens".
- Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.
- High-temperature rock types, including laminated and welded blocks of sand, spherulites and tektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: it is reported in the scientific literature that some "shock" features, such as small shatter cones, which are often reported as being associated only with impact events, have been found in terrestrial volcanic ejecta.
- 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 stishovite and coesite, varieties of shocked quartz.
Craters can also be created from underground nuclear explosions. One of the most crater-pocked sites on the planet is the Nevada Test Site, where a number of craters were purposely made during its years as a center for nuclear testing (see, for example, Operation Plowshare).
 Crater categorisation
In 1978, Chuck Wood and Leif Andersson of the Lunar & Planetary Lab devised a system of categorisation of lunar impact craters. They used a sampling of craters that were relatively unmodified by subsequent impacts, then grouped the results into five broad categories. These successfully accounted for about 99% of all lunar impact craters.
The LPC Crater Types were as follows:
- ALC — small, cup-shaped craters with a diameter of about 10 km or less, and no central floor. The archetype for this category is 'Albategnius C'.
- BIO — similar to an ALC, but with small, flat floors. Typical diameter is about 15 km. The lunar crater archetype is Biot.
- SOS — the interior floor is wide and flat, with no central peak. The inner walls are not terraced. The diameter is normally in the range of 15-25 km. The archetype is Sosigenes crater.
- TRI — these complex craters are large enough so that their inner walls have slumped to the floor. They can range in size from 15-50 km in diameter. The archetype crater is Triesnecker.
- TYC — these are larger than 50 km, with terraced inner walls and relatively flat floors. They frequently have large central peak formations. Tycho crater is the archetype for this class.
Beyond a couple of hundred kilometers diameter, the central peak of the TYC class disappear and they are classed as basins.
 Lists of craters
 Notable impact craters on Earth
- Main article List of impact craters on Earth
 Some extraterrestrial craters
- Caloris Basin (Mercury)
- Hellas Basin (Mars)
- Mare Orientale (Moon)
- Petrarch crater (Mercury)
- Skinakas Basin (Mercury)
- South Pole-Aitken basin (Moon)
- Herschel crater (Mimas)
 Largest craters in the Solar System
- South Pole-Aitken basin - Moon - Diameter: 2,500 km
- Hellas Basin - Mars - Diameter: 2,100 km
- "Skinakas Basin" - Mercury - Diameter: ~1,600 km
- Caloris Basin - Mercury - Diameter: 1,350 km
- Mare Imbrium - Moon - Diameter: 1,100 km
- Isidis - Mars - Diameter: 1,100 km
- Mare Tranquilitatis - Moon - Diameter: 870 km
- Argyre - Mars - Diameter: 800 km
- Mare Serenitatis - Moon - Diameter: 700 km
- Mare Nubium - Moon - Diameter: 700 km
- Beethoven - Mercury - Diameter: 625 km
- Valhalla - Callisto - Diameter: 600 km, with rings to 4,000 km diameter
- Hertzsprung - Moon - Diameter: 590 km
- Apollo - Moon - Diameter: 540 km
- Huygens - Mars - Diameter: 470 km
- Schiaperelli - Mars - Diameter: 470 km
- Menrva - Titan - Diameter: 440 km
- Korolev - Moon - Diameter: 430 km
- Dostoevskij - Mercury - Diameter: 400 km
- Tolstoj - Mercury - Diameter: 390 km
- Goethe - Mercury - Diameter: 380 km
- Mare Orientale - Moon - Diameter: 350 km, with rings to 930 km diameter
- Epigeus - Ganymede - Diameter: 340 km
- Gertrude - Titania - Diameter: 320 km
- Asgard - Callisto - Diameter: 300 km, with rings to 1,400 km diameter
- Vredefort - Earth - Diameter: 300 km
- Mead - Venus - Diameter: 270 km
There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars (see http://planetarynames.wr.usgs.gov/).
- ^ http://www.straightdope.com/mailbag/mcraters.htm
- ^ http://www.unb.ca/passc/ImpactDatabase/essay.html
- Charles A. Wood and Leif Andersson, New Morphometric Data for Fresh Lunar Craters, 1978, Proceedings 9th Lunar and Planet. Sci. Conf.
- Bond, J. W., "The development of central peaks in lunar craters", Moon and the Planets, vol. 25, Dec. 1981.
- Melosh, H.J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, 245 p.
 See also
 External links
- Photographs of terrestrial impact craters.
- a study of a South Carolina crater
- The Geological Survey of Canada Crater database, 172 impact structures
- A recent news report about tektites
- Aerial Explorations of Terrestrial Meteorite Craters
- Google Earth Placemarker based on the Geological Survey of Canada Crater database (KML)
- All 172 confirmed meteor impact sites on earth, viewable in Google Earth (Largest, Most recent, Per continent, Including size indicator)
- Impact sites, with individual bibliographies
- Solarviews: Terrestrial Impact Craters
- Earth Impact Database
- Giant Crater Found: Tied to Worst Mass Extinction Ever
- WA Geological Survey Meteorite Impacts in Western Australia