The Alvarez hypothesis, based on work by Alvarez et al at U.C. Berkeley in 1980, is a widely accepted explanation for the Cretaceous-Tertiary (K-T) extinction event. This is the research that proposed that a large extraterrestrial impact was responsible for the mass extinction.
Alvarez et al found that a significant layer of iridium lining the K-T boundary from 100 sites all over the world: everywhere from the ocean floor to the continental shelf in Europe to North American floodplains. Iridium is an extremely rare metal that hardly ever occurs on earth–but it is found in much greater proportions in meteorites. This suggests that it was a large impact caused the extinction, scattering iridium across a vast expanse of the earth.
We have even found the impact site: the enormous crater is called Chicxulub, and it is located on Mexico’s Yucatán peninsula. Going down through the rock layers, we found a heavy layer of iridum dating to ~65 MYA (the K-T boundary) lying beneath Chicxulub. Fascinating? I think so.
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