| Mass extinctions in the past 542 million years of | | | | miles deep, that would then quickly collapse leaving a |
| Earth's history | | | | shallower but still deep hole about 80 to100 miles wide. |
| In the world of today we are constantly reminded of | | | | Such astounding facts as these are offered here in an |
| the perils faced by life on Earth as this current century | | | | effort to help convey a sense of the immense |
| unfolds and if something is not done soon to face up | | | | destruction caused when the paths of such large |
| to and actually take appropriate steps to combat the | | | | meteorites intersect with the orbit of Earth and crash |
| climate crisis, it is possible that the very survival of | | | | with such fatal consequences to life on the planet. |
| humanity is in jeopardy. The world will survive but we, | | | | In addition to meteorite impacts, several alternative |
| as a species, may not, or if we do, our civilizations may | | | | theories have been suggested for the cause of |
| collapse in the ensuing struggles of individuals and | | | | extinctions, including: climate change, volcanism, tectonic |
| groups in the fight to preserve their lands and | | | | activity - involving the Earth's slowly shifting and |
| resources essential to their very existence. | | | | deforming crust, changes in the atmosphere, increase |
| But that is now, in the present, but in the past, for | | | | in lethal radiation from space and a few others. |
| reasons quite unrelated to human activities, in fact | | | | The once rejected view of the Earth being shaped by |
| millions of years before humans came into existence, | | | | catastrophism seems to be making a comeback, or at |
| even in their most primitive form, perhaps as long ago | | | | least partly so, after so many years in which the |
| as six million years ago, still a minute passage of time | | | | guiding geological principal has been uniformitarianism, |
| compared to that of the Earth's existence, life on Earth | | | | the process of gradual change over great lengths of |
| has suffered from catastrophic events that have | | | | geologic time. |
| decimated both land and marine species to such an | | | | As of today, December 14, 2009, just 176 craters |
| extent that we can say that probably more than 95 | | | | worldwide are known, or at least confirmed as such. |
| percent of all life forms that every existed have | | | | These are listed in a registry called The Earth Impact |
| become extinct. We, as humans, are just a small | | | | Database, compiled, maintained and updated for the |
| fraction of that life still around to speculate on such | | | | Canadian Government by the Planetary and Space |
| events of the past. | | | | Science Center at the University of New Brunswick, |
| For those of us interested in the Earth sciences and | | | | Canada, and whose stated purpose is "To provide a |
| related subjects dealing with the history of the Earth it | | | | not-for-profit source of information to assist the |
| is inevitable that attention will turn to the matter of | | | | scientific, industrial, government and public communities |
| what has become known as "Mass Extinctions" of | | | | around the world in furthering our collective knowledge |
| which there have been many over the course of the | | | | of impact structures on Earth". |
| approximately 550 million year history of life forms for | | | | It is not easy to identify crater locations, many are |
| which we have fossil evidence on Earth. While it is | | | | from impacts that occurred millions of years ago and |
| known that life existed for billions of years before that | | | | the Earth's climate and geological activity can erode or |
| time, evidence in the form of fossils is rare or | | | | bury craters or otherwise disguise their presence, and |
| non-existent because of lack of skeletal or body parts | | | | many may be covered by ocean waters. |
| enabling preservation. | | | | But the search continues, with ground exploration and |
| Impact Craters | | | | with the support of NASA and the space industry |
| In the consideration of causes of mass extinctions, | | | | aided by computer technology, new sites are under |
| there is growing interest on the part of geologists, and | | | | investigation. As an example, in 2006 assisted by |
| other earth science specialists, in the study of impact | | | | NASA's space satellite, a discovery was made of a |
| craters. Impact craters are residual evidence on the | | | | huge crater thousands of feet beneath the ice and |
| surface of the Earth that shows that the Earth has | | | | snows of Antarctica. This find may not have been |
| been struck by asteroids and comets from outer | | | | confirmed as an impact crater yet, but the site |
| space at times throughout the long history of the Earth. | | | | appears to be about 300 miles wide and is thought to |
| Increased interest is due, in part, to the possibility that | | | | be 250 million years old. If caused by an impacting |
| such impacts are responsible for the periodic mass | | | | body, it would be much larger than the object |
| extinctions of life that have occurred throughout the | | | | associated with the deaths of the dinosaurs and it |
| history of planet Earth. | | | | would be larger than the currently known largest and |
| Collisions of the Earth with other minor bodies of the | | | | oldest impact crater, the Vredefort Crater in South |
| Solar System, asteroids, comets, and debris from their | | | | Africa, more than 150 miles in diameter and thought to |
| past collisions, have been occurring since the birth of | | | | be about two billion years old. The Antarctica Crater |
| our planet approximately 4.6 billion years ago. At first, | | | | could also have caused the greatest ever mass |
| the rate of impact occurrences was high but over time | | | | extinction on Earth 250 million years ago, an exciting |
| their frequency gradually diminished as the regions of | | | | possibility for Earth Scientists. |
| the Earth's orbit were cleared of debris left over from | | | | Perhaps best known to the general public is the impact |
| the accretion of the main planetary bodies as the | | | | that may have caused the extinction of the dinosaurs |
| Solar System came into existence. Much of that | | | | and about 70% of other life forms 65 million years ago. |
| clearing would have been by the absorption of material | | | | An impact site has been identified on the coast of the |
| through collision with the Earth and Moon. | | | | Yucatan Peninsula, known as the Chicxulub Crater, |
| It is the more recent several hundred million years that | | | | that is about the correct age and is regarded by a |
| is of special interest, the time since the blossoming and | | | | many in the scientific community as being the most |
| explosion of multi-celled life that first appeared on | | | | likely candidate to have been the cause of the |
| Earth about 540 million years ago. Geologists have | | | | extinction, although there are some contesting theories |
| divided the great length of time of Earth's history into a | | | | that invoke other possible causes such as volcanism |
| series of intervals based on major geological and | | | | and climate change which also coincided with the great |
| biological events, such as periods of mountain building | | | | dying of that time. |
| or mass extinction episodes, including five major | | | | Several other possible 65 million year old impact sites |
| extinction events that have occurred in the last 250 | | | | have been identified and one of those, named the |
| million years. | | | | Shiva Crater, although not independently confirmed yet, |
| All impact craters are of interest, but particularly so are | | | | is an underwater impact site larger than the Chicxulub |
| those relics of craters that can be dated to match the | | | | Crater and is considered by some to be a better |
| age of any of the extinction episodes. Finding these, | | | | candidate as the cause of the dinosaur extinctions |
| may enable the determination of whether or not the | | | | than is Chicxulub. Debate on this subject continues, |
| more massive impacts are the cause of, or are | | | | with Chicxulub, Shiva and other craters having their |
| perhaps linked to, any of the extinction events. Also, | | | | supporters and detractors. |
| additional examination and research may help discover | | | | As mentioned earlier, there are several other |
| whether there is a direct connection of impact events | | | | non-impact events conjectured as possible causes or |
| with climate change or episodes of volcanism, as has | | | | contributors leading to mass extinction occurrences, |
| been proposed by some scientists. | | | | some of which may be related to each other, including |
| From data that can be obtained and with information | | | | climate change, excessive lava outpourings, falling sea |
| deduced from impact craters, calculations can be | | | | levels, and geological instabilities of the dynamic Earth |
| made that show, in the cases of the largest impacts, | | | | such as plate tectonics. |
| that the impacting objects, asteroids or comets, have | | | | In addition to those, it has also been suggested that |
| delivered truly cataclysmic life ending blows to Earth. A | | | | one or more extinctions could have been caused by |
| meteorite of several miles in diameter, when | | | | events from beyond the Solar System, such as |
| approaching earth on a collision course, would be | | | | nearby, in astronomic terms, supernovae or |
| traveling at the very high rate of speed of 15 or more | | | | gamma-ray bursts. A supernova is the explosion that |
| miles per second, a speed that is more than15 times | | | | occurs at the death of a massive star as it collapses |
| faster than the fastest bullet fired from a rifle, and on | | | | to form a neutron star when it has used up much of |
| impact it would release an immense amount of energy. | | | | its hydrogen and other nuclear fuel. From that |
| The devastation caused would reach far beyond the | | | | cataclysmic event, lethal radiation spreads out at high |
| region of impact, the resultant generation and spread | | | | speed in all directions. Similarly, gamma ray bursts, also |
| of heat, fire, soot, ash, acid rain, would be cataclysmic | | | | known as GRB's, refer to the radiation emanating from |
| and the toxic atmospheric fallout would be world wide. | | | | super-massive explosions in distant galaxies. But those |
| Geologist Walter Alvarez, in his book "T.Rex and the | | | | are topics worth discussing elsewhere while our |
| Crater of Doom" published by Princeton University | | | | concentration in this article has been on crater |
| Press, writes of the Yucatan event, describing the | | | | evidence of past periodic impacts and I would like to |
| meteorite impact there as being equivalent to the | | | | finish up with reference to just one more terrestrial |
| explosion of 100 million megatons of TNT, an explosion | | | | impact. |
| beyond comprehension. Similarly, geophysicist David | | | | The mother of all Earth impact events: |
| Brez Carlisle, in his book "Dinosaurs, Diamonds, and | | | | If a current widely accepted scientific hypothesis is |
| Things from Outer Space" published by Stanford | | | | correct, the Earth's largest impact occurred at the birth |
| University Press, describes the same Yucatan event | | | | of the Earth's moon, when a Mars-sized body collided |
| as releasing energy equivalent to the simultaneous | | | | with the young Earth, perhaps only fifty million years |
| explosion of all the nuclear arms now stockpiled in the | | | | after its formation 4.6 billion years ago. In so doing, |
| world today. Also impossible to comprehend. Alvarez | | | | planetary material was vaporized and ejected into |
| added that on impact, the energy released would have | | | | space to later condense and become accreted to |
| vaporized the entire six mile wide meteorite and the | | | | form the Moon. But that is another story. |
| earth below it in about one second, creating a hole 24 | | | | |