In 1980, Walter Alvarez and his group at the University of California–Berkeley, discovered a thin layer of clay in the geologic record, which contained an unexpected amount of the rare element iridium.
They proposed that the iridium-rich layer was evidence of a massive comet hitting the Earth 66 million years ago at the time of the extinction of the dinosaurs. The Alvarez group suggested that the global iridium-rich layer formed as fallout from an intense dust cloud caused by the impact. The cloud of dust covered the Earth, producing darkness and cold. In 1990, the large 100-mile diameter crater from the impact was found in Mexico’s Yucatan Peninsula.
The timing of this impact, together with the fossil record, have led most researchers to conclude that this collision caused the mass extinction of the dinosaurs and many other forms of life. Subsequent studies found evidence of other mass extinctions in the geologic past, which seem to have happened at the same time as pulses of impacts, determined from the record of impact craters on the Earth. And these coincidences occurred every 30 million years.
Why do these extinctions and impacts appear to happen within an underlying cycle? The answer may lie in our position in the Milky Way Galaxy.
