Crater — A 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.
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 unmistakeable 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.
Daniel Barringer was one of the first to identify a geological structure as an impact crater, 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 Gene 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.
Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory in Canada, and Wolf von Engelhardt of the University of Tuebingen in West 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 a few thousand 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 continental drift.
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.
An asteroid falls onto the Earth at a speed of about 40,000 to 60,000 KPH. If the object weighs more than 1,000 tonnes, the atmosphere does not do much to slow it down, 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. They can destroy chondritic or carbonaceous chondritic bodies before they ever reach ground, but iron-metallic asteroids have more structural integrity and can strike the surface of the Earth in a violent explosion.
The result 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.
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 material in the impact regions. Relatively soft materials yield smaller craters than brittle materials. Erosion and other geological activities quickly hide impact craters on the Earth. The Barringer Crater is in superlative shape, but it is only about 50,000 years old.
Some volcanic features can resemble impact craters, and brecciated rocks are associated with other geological formations besides impact craters. 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, and “tektites”, or glassy spatters of molten rock. While rocks melted by the impact do 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.
– 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 or other carbon compounds, or “shistovite”, derived from quartz.