The catastrophe that signalled the end of the Cretaceous period 65 million years ago may not have been a single event, say geologists in the US. Instead, the Earth may have been bombarded by comets or asteroids for hundreds of thousands of years. The dinosaurs and other species would then have been wiped out in steps rather than all at once.
Evidence is mounting that the principal event was the impact of an asteroid on the Yucatan coast, creating the Chicxulub crater. However, new studies indicate that the smaller Manson crater in Iowa was created at nearly the same time. The structure, 35 kilometres across and buried under 60 metres of soil, was created by a body too small to have caused mass extinctions at the end of the Cretaceous.
Mick Kunk of the US Geological Survey in Reston, Virginia, used argon-argon isotope dating to clarify the picture. He found ...
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