Iridium and the Terminal Cretaceous Event
There have been a number of natural catastrophes in Earth’s past that wiped out huge numbers of plant and animal species- -in effect,”resetting the clock” and requiring a new start for much of life on Earth. One such catastrophe took place at the end of the Permian era, some 251 million years ago. But perhaps the most famous one occurred about 65 million years ago at the end of the Cretaceous, causing the dinosaurs to go extinct (except for a small number that evolved into modern birds). Evidence that the terminal Cretaceous event was triggered by the impact of an asteroid with Earth began to emerge in the early 1980s, when Luis and Walter Alvarez discovered a thin layer of iridium in rock strata (layers) 65 million years old at widely separated sites around the world. Iridium is very rare on Earth’s surface but is much more common in some kinds of meteorite. The two scientists calculated that the impact of such an object, about 10 kilometers across, could have spread the right amount of iridium around the globe in the cloud of debris (fragments) blasted out by the impact. The idea was greeted with skepticism at first, but more traces of debris from an impact occurring at the time of the death of the dinosaurs began to turn up as geologists searched for evidence for or against the hypothesis. To most people, the most decisive evidence came in the early 1990s, when a geological feature at Chicxulub in the Yucatan Peninsula of Mexico was identified as the remains of an impact crater in exactly the right place and with exactly the right age to connect it to the terminal Cretaceous event.
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