A wealth of evidence has forced my colleagues and me to conclude that 65 million years ago a mountain‐sized object hit Earth and caused the extinction of most of the then existing species, bringing a close to the Cretaceous period of geological history and opening the Tertiary period. Much of the evidence for this lies in the unusual layer of clay that separates those periods in the geological record, shown in figure 1. For example, this stratum contains anomalously high concentrations of iridium, an element whose abundance in the crust of the Earth is only one ten‐thousandth that in meteorites and, presumably, in other “bolides,” or large pieces of Solar System debris.
Mass Extinctions Caused by Large Bolide Impacts
Luis W. Alvarez; Mass Extinctions Caused by Large Bolide Impacts. Physics Today 1 July 1987; 40 (7): 24–33. https://doi.org/10.1063/1.881078
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