10/3/12 - mass extinction on earth

In today's excerpt - there are five known periods of mass extinction in the Earth's history. One of these occurred when the Chicxulub asteroid slammed into the Earth 65 million years ago, a blast that sent debris halfway to the moon and ended the era of the dinosaurs:

"The diversity of dinosaurs and the flourishing of Mesozoic plants reached their zenith during the Cretaceous period, when huge birds flew in vast forests and flowering plants spread across the world. It was warmer even than it is today; polar and mountaintop ice were long gone and dinosaur species roamed from Alaska to Antarctica. Small mammals managed to survive in special niches, but the day belonged to the giant reptiles whose only enemy, it seemed, would be a sudden return to the frigid conditions of the Permian. 

"But the age of the dinosaurs came to a much more dramatic end -- not with a glacial whimper but with an extraterrestrial bang. One day about 65 million years ago, a comet or asteroid only about 10 kilometers (6 miles) in diameter streaked toward Earth at a speed of 90,000 kilometers per hour (55,000 mph) on a collision course. It approached from the southeast at a low angle, striking Earth in what is today the area of the Yucatan Peninsula of Mexico. When you get off a boat at the small port of Progreso, there is a small, hand-painted sign that points to Chicxulub, a Maya name for a local village. But to geographers, Chicxulub means the end of one era and the start of another. Here the asteroid's impact produced an explosion equivalent to about 100 trillion tons of dynamite, forming a crater approximately 180 kilometers (110 miles) in diameter, 65 kilometers (40 miles) deep, and encircled by a geological fault 30 kilometers (about 20 miles) beyond, all of it buried today by later sediments.

"It is possible that the Chicxulub asteroid was one of a swarm, and that smaller ones struck the Earth elsewhere, including the ocean. In any case, the impact's devastation reached around the planet, and was at its worst in North America. The impact area was a shallow sea with soft, deep sediments, and the blast sent a mass of debris hurtling thousands of miles into the heart of the continent and high into the atmosphere and beyond. Researchers David King and Daniel Durda calculate that some of it reached halfway to the Moon before falling back to Earth. And when it did fall back, it rained red-hot rocks on the rotating planet, setting fires to forests almost everywhere. The atmosphere was heated enough to evaporate entire lakes, incinerate whole ecosystems, and extinguish most life over large low-latitude regions. 

"The Chicxulub impact ended the Cretaceous and marked the beginning of a new geologic-calendar period, the Tertiary. Popularly, the transition is called the K/T Boundary, but its significance is hard to overstate, because this was one of the three greatest known mass extinctions ever. While it is possible that some dinosaurs survived the original blast, notably in higher latitudes, food chains had been fatally disrupted and they, too, died out. Some small mammals were better equipped to outlive the crisis, perhaps keeping cool in high-latitude caves and burrows, depending less on the luxuriant vegetation and reptilian life the dinosaurs had needed. But the faunal and floral exuberance of the Mesozoic era came to a sudden, irrevocable end.

"The K/T blast had long-term effects on global environments. Much of the enormous volume of pulverized, ejected rock remained in orbit around the Earth, choking the atmosphere and blocking the sun. It may be that the asteroid's impact shook volcanoes around the planet into action, adding eruptions to the toxic mix. The smoke from worldwide fires darkened the skies across the globe. Eventually the overheated atmosphere cooled, and the blockage of the sun sent temperatures plummeting still more, creating colder global conditions than had been experienced for 185 million years -- since the Permian Ice Age. Now it becomes important to be familiar with the epochs of the Tertiary period, because the first of these epochs, the Paleocene, witnessed major climactic reversals, and the next one, the Eocene, saw the beginnings of a new ice age that probably would have come whether the K/T impact occurred or not."


Harm de Blij


Why Geography Matters: More Than Ever


Oxford University Press


Copyright 2012 by Oxford University Press


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