by James Robinson

Created on: February 03, 2009

There appears to be a growing interest in the study of terrestrial impact craters on the part of geologists and other Earth scientists. Impact craters are residual evidence on the surface of the Earth that shows that the Earth has been struck by asteroids and comets from outer space at times throughout the long history of the Earth. Increased interest is due, in part, to the possibility that such impacts are responsible for the periodic mass extinctions of life that have occurred throughout the history of planet Earth.

Collisions of the Earth with other minor Solar System bodies, asteroids and comets, have been occurring since the birth of our planet approximately 4.6 billion years ago. At first, the rate of impact occurrences was high but over time their frequency gradually diminished as the regions of the Earth's orbit were cleared of debris left over from the accretion of the main planetary bodies as the Solar System came into existence. Much of that clearing would have been by the absorption of material through collision with the Earth and Moon.

It is the more recent several hundred million years that is of special interest, the time since the blossoming and explosion of multi-celled life that first appeared on Earth about 540 million years ago. Geologists have divided the great length of time of Earth's history into a series of intervals based on major geological
and biological events, such as periods of mountain building or mass extinction episodes, including five major extinction events that have occurred in the last 250 million years.

All impact craters are of interest, but particularly so are those relics of craters that can be dated to match the age of any of the extinction episodes. Finding these may enable the determination of whether or not the more massive impacts are the cause of, or are perhaps directly linked to, any of the extinction events. Also, additional examination and research may help discover whether there is a direct connection of impact events with climate change or episodes of volcanism, as has been proposed by some scientists.

From data that can be obtained and with information deduced from impact craters, calculations can be made that show, in the cases of the largest impacts, that the impacting objects, asteroids or comets, have delivered truly cataclysmic life ending blows to Earth. A meteorite of several miles in diameter, when approaching earth on a collision course, would be traveling at the very high rate of speed of 15 or more

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