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Mass Extinctions on the Earth

Uploaded by jake69_ca on Feb 19, 2001


  • 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 herbivores.
  • Those smaller animals that could hide and hibernate until the sun came out again must have sruvived and formed the animals that there are today.
  • There is other evidence for this theory : there is a layer of iridium found on top of the fossils which can be found all over the world. this is important becouse there is a lot of iridium on asteroids while on earth it is only deep down inside the crater.

    Details of the Volcano Theory:
  • Another less popular theory to explain the disappearance of the dinosaurs suggests that a drop in the oxygen levels in...

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    Uploaded by:   jake69_ca

    Date:   02/19/2001

    Category:   Geography

    Length:   3 pages (570 words)

    Views:   2122

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