Astronomy

About the Asteroid Theory of Cretaceous Extinction



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The Asteroid Theory is one of the theoretical explanations for the extinction of the dinosaurs at the end of the Cretaceous period, about 65 million years ago, and is currently the most widely accepted explanations. Essentially, the Asteroid Theory states that a large asteroid, about ten kilometres across or more, slammed into the Earth with sufficient force to cause an extinction-level event, devastating the biosphere with a combination of heat, tsunamis, and clouds of dust which obscured the sunlight and caused temporary but dramatic global cooling. The leading candidate for the Asteroid Theory is the Chucxulub crater on the Yucatan Peninsula, in Mexico.

- About the Cretaceous-Tertiary Extinction Event -

That the Cretaceous-Tertiary Extinction, or "K-T extinction," actually happened is not disputed by any scientists. Roughly 65 million years ago, following a cataclysmic event, a large number of species suddenly went extinct. Plant species were especially hard hit, presumably because the event in question disrupted the regular levels of sunlight and therefore disrupted photosynthesis. However, the best-known victims of the extinction event were the dinosaurs - all of them. Other reptile species survived, but no evidence of a surviving dinosaur species has ever been uncovered from layers following the K-T extinction. If they did, they would almost certainly have been isolated populations of small species.

In contrast, the K-T extinction marked the opportunistic rise of new species, in particular mammals. Those species which survived were, in general, the ones with the greatest variety of food sources, and the ones least dependent on living prey: in other words, insect-eaters, carrion-eaters, and omnivores were best able to survive the brief period of starvation and deprivation, and then to recover afterwards. Others, such as the ancestors of today's crocodiles, would have been aided as well by their ability to survive for extremely long periods of time without food.

- About the Asteroid Theory -

The currently most-accepted theory to explain this sudden extinction is that a large asteroid slammed into the Earth's surface about 65 million years ago. An asteroid ten kilometres wide, or more, would have unleashed destruction greater than millions of nuclear weapons simultaneously: a shockwave of fire racing around the surface of the Earth, tsunamis potentially thousands of feet high, and clouds of dust and debris flung into the atmosphere, which would have taken several years to settle, during which time much less sunlight would have made it to the Earth's surface. This combination of fire, flood, and cooling would be devastating to all life. Indeed, an asteroid impact event is still considered one of the greatest threats to the long-term survival of the human species, although the chances of an impact occurring in any single given year are negligible.

The most likely candidate for the asteroid impact is an enormous crater formation lying on the coast of the Yucatan Peninsula in Mexico, the Chicxulub crater. The Chicxulub crater is about 110 miles wide, large enough to have been caused by a 10-kilometre-wide object, and has been dated to the same rough period of time as the extinction event itself. Its size, coupled with this dating, make it the most likely candidate for the extinction that wiped out the dinosaurs. However, there are also other possibilities. Since most of the Earth's surface is covered in water, it is statistically most likely that an asteroid impact would have occurred underwater. If so, the evidence could have been obscured by millions of years of ocean currents stirring up the sea floor, or it might be the cause of the 600-kilometre-long Shiva formation off the coast of India, which has been dated to the same period but has not been definitively linked to an asteroid impact. Still, for the moment, the Chicxulub crater is the most likely suspect.

- Alternative Explanations -

The Asteroid Theory is widely but not universally accepted. The largest extinction event ever, the Permian-Triassic extinction of 250 million years ago, is believed to have been caused by an unprecedentedly enormous volcanic eruption at the Siberian Traps, in Russia, and wiped out as many as 90% of the species then living on Earth. For this reason, there is continuing debate that the extinction of the dinosaurs was caused either partially or wholly by something other than an asteroid impact. Even among the large majority which accept the Asteroid Theory, a minority currently argue that the extinction of the dinosaurs was caused by the cumulative effects of several asteroid impacts happening over a relatively short period of time, rather than a single catastrophic impact.

A number of other hypotheses exist, however, though most supporters have abandoned them in recent years in favour of the asteroid theory. One possibility is a supermassive volcanic eruption comparable to the Siberian Traps, such as the Deccan Traps eruption, which occurred around the same time as the extinction and continued for a period of several hundred thousand years. It is also known that sea levels fell during the late Cretaceous, perhaps due to a collapse or regression of large areas of the sea floor. This would have had considerable effects on saltwater life, and possibly major global ramifications as well.

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