IMPACT CRATERS

The term impact crater is used for 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 approximately circular depressions in the surface of a planet, moon or other solid body in the Solar System, formed by the hypervelocity impact of a smaller body with the surface. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims, and floors that are lower in elevation than the surrounding terrain.  Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. The Barringer Crater in US (Arizona) is the first and perhaps the best-known example of an impact crater on Earth.

Impact craters are the dominant landforms on many solid Solar System objects including the Moon, Mercury, Callisto, Ganymede and most small satellites and asteroids. On other solar system bodies which experience more-active surface geological processes, such as Earth, Venus, Mars, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or otherwise obliterated by tectonic processes. Where such processes have destroyed most of the original crater topography, the terms impact structure or astrobleme are more commonly used.

Very old surfaces, such as those of Mercury, the Moon, and the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. Since that time, the rate of crater production on Earth has been considerably lower, but still appreciable.   The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system.  The Earth experiences from one to three impacts large enough to produce a 20 km diameter crater about once million years or so.  This suggests that there should be far more relatively young craters on the planet than have been discovered so far.

Although the Earth’s vigorous surface processes quickly destroy the impact record, about 183 terrestrial impact craters have been identified.  These range in diameter from a few tens of meters up to about 300 km, and range in age from recent times (e.g. the Sikhote-Alin craters in Russia whose creation were witnessed in 1947) to more than two billion years.  Most impact craters are less than 500 million years old because geological processes tend to obliterate older craters. They are also selectively found in the stable interior regions of continents.  Few undersea 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.  Impact craters are not to be confused with other landforms that in some cases appear similar, including calderas and ring dikes.

(Ref: http://en.wikipedia.org/wiki/Impact_crater).

It is widely believed that large meteorite impacts have significantly influenced the environment of the earth in the geological past.  The devastation caused by these events include short-term effects like fire-storms and tsunamis fissuring of the earth at the antipodal parts of impact setting the stage for large scale volcanic eruptions lasting for millions of years, causing global warming, changing the chemistry of the seas, and killing almost all animal and plant life in the seas and on land.  For an interesting visualization of what might happen if a 40 km diameter meteorite were to hit the earth see this video: http://www.youtube.com/watch?v=vdSTE4jWJmk.

An inventory of confirmed impact structures is maintained by the Earth Impact Database, which is an authoritative source of information.  The database, initiated in 1955 by the Dominion Observatory, Ottawa, currently (as of December 2012) lists 183 confirmed impact sites on earth.  This database can be accessed at:

http://www.passc.net/EarthImpactDatabase/index.html.

Explore fifty of earth’s most obvious asteroid impact sites with satellite images and maps at http://geology.com/meteor-impact-craters.shtml.   For some interesting information on the mechanism of formation of impact craters follow this link:

http://en.wikipedia.org/wiki/Impact_crater.  

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