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 miles per second, a speed that is more than15 times faster than the fastest bullet fired from a rifle, and on impact it would release an immense amount of energy. The devastation caused would reach far beyond the region of impact, the generation and spread of heat, fire, soot, ash, acid rain, would be cataclysmic and the toxic atmospheric fallout would be world wide.
Geologist Walter Alvarez, in his book “T.Rex and the Crater of Doom” published by Princeton University Press, writes of the Yucatan event, describing the meteorite impact there as being equivalent to the explosion of 100 million megatons of TNT, an explosion beyond comprehension. Similarly, geophysicist David Brez Carlisle, in his book “Dinosaurs, Diamonds, and Things from Outer Space” published by Stanford University Press, describes the same Yucatan event as releasing energy equivalent to the simultaneous explosion of all the nuclear arms now stockpiled in the world today. Also impossible to comprehend. Alvarez added that on impact, the energy released would have vaporized the entire six mile wide meteorite and the earth below it in about one second, creating a hole 24 miles deep, that would then quickly collapse leaving a shallower but still deep hole about 80 to100 miles wide. Such astounding facts as these are offered here in an effort to help convey a sense of the immense destruction caused when the paths of such large meteorites intersect with the orbit of Earth and crash with such fatal consequences to life on the planet.
In addition to meteorite impacts, several alternative theories have been suggested for the cause of extinctions, including: climate change, volcanism, tectonic activity – involving the Earth’s slowly shifting and deforming crust, changes in the atmosphere, increase in lethal radiation from space and a few others.
The once rejected view of the Earth being shaped by catastrophism seems to be making a comeback, or at least partly so, after so many years in which the guiding geological principal has been uniformitarianism, the process of gradual change over great lengths of geologic time.
As of January 2009, just 175 craters worldwide are known. These are listed in a registry called The Earth Impact Database, compiled, maintained and updated for the Canadian Government by the Planetary and Space Science Center at the University of New Brunswick, Canada, and whose stated purpose is “To provide a not-for-profit source of information to assist the scientific, industrial, government and public communities around the world in furthering our collective knowledge of impact structures on Earth”.
It is not easy to identify crater locations, many are from impacts that occurred millions of years ago and the Earth’s climate and geological activity can erode or bury craters or otherwise disguise their presence, and many may be covered by ocean waters.
But the search continues, with ground exploration and with the support of NASA and the space industry aided by computer technology, new sites are under investigation. As an example, in 2006 assisted by NASA’s space satellite, a discovery was made of a huge crater thousands of feet beneath the ice and snows of Antarctica. This find may not have been confirmed as an impact crater yet, but the site appears to be about 300 miles wide and is thought to be 250 million years old. If caused by an impacting body, it would be much larger than the object associated with the deaths of the dinosaurs and it would be larger than the currently known largest and oldest impact crater, the Vredefort Crater in South Africa, more than 150 miles in diameter and thought to be about two billion years old. The Antarctica Crater could also have caused the greatest ever mass extinction on Earth 250 million years ago, an exciting possibility for Earth Scientists.
Perhaps best known to the general public is the impact that may have caused the extinction of the dinosaurs and about 70% of other life forms 65 million years ago. An impact site has been identified on the coast of the Yucatan Peninsula, known as the Chicxulub Crater, that is about the correct age and is regarded by a many in the scientific community as being the most likely candidate to have been the cause of the extinction, although there are some contesting theories that invoke other possible causes such as volcanism and climate change which also coincided with the great dying of that time.
Several other possible 65 million year old impact sites have been identified and one of those, named the Shiva Crater, although not independently confirmed yet, is an underwater impact site larger than the Chicxulub Crater and is considered by some as a better candidate to be the cause of the dinosaur extinctions. Debate on this subject continues, with Chicxulub, Shiva and other craters having their supporters and detractors.
Now for the mother of all Earth impact events:
If a current widely accepted scientific hypothesis is correct, the Earth’s largest impact occurred at the birth of the Earth’s moon, when a Mars-sized body collided with the young Earth, perhaps only fifty million years after its formation 4.6 billion years ago. In so doing, planetary material was vaporized and ejected into space to later condense and become accreted to form the Moon. But that is another story.