Mass Extinctions on the Earth

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  • Topic: Cretaceous–Tertiary extinction event, Dinosaur, Cretaceous
  • Pages : 2 (638 words )
  • Download(s) : 37
  • Published : October 8, 1999
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Intro:
Sixty-five million years ago, some phenomenon triggered mass extinctions on the lands and in the oceans so profound that they define the geological boundary between the older Mesozoic Era, often called the "Age of Reptiles," and the modern Cenozoic Era, the "Age of Mammals." On a finer scale, the extinctions define the boundary between the Cretaceous (geological symbol, "K"), and Tertiary ("T") periods. This mass extinction is usually referred to as the K-T extinctions. The dinosaurs became extinct during the K-T mass extinction. To examine how the K-T extinctions fit into a broader perspective, please see the The cause of the K-T extinctions is one of the great mysteries in science, and many scientists have proposed theories to account for it. Theories span a vast spectrum of causes including: sea level change, supernova explosions, climate change, and on and on. Beginning in the 1980s, two new theories became the topic of an intense scientific debate. They are the K-T impact extinction theory originated by the Nobelist physicist, Luis Alvarez, and his team, and the K-T Deccan Traps extinction theory, which, for short, is called volcano-greenhouse theory.

Details of the Impact Theory
In the mid-late 1970s, Luis Alvarez and his impact team began searching for the cause of the K-T extinctions. His evidence of impact was enrichment of the chemical, iridium, in a thin layer of clay a few centimenters thick at the K-T boundary. His original theory held that a giant asteroid struck earth 65 million years ago, blasting so much dust into the atmosphere that it blocked out sunlight, and plunged earth into the blackness and cold of a sudden, short-duration, "impact winter." According to Alvarez theory, the global blackout triggered extinctions among the plant kingdom, and then among herbivores that depended upon plants for food, and then among the carnivores that ate the...
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