400-yucatan-impact-dixon – An asteroid 5-10 miles across impacts in the Yucatan to end the age of dinosaurs 65 million years ago. A fine layer of clay, rich in the element iridium -- more concentrated in meteorites than in terrestrial rocks -- marks the geological boundary between the Cretaceous and Tertiary eras. This clay layer is believed to have precipitated out of a planet-blanketing cloud of dust that was ejected into the atmosphere by the impact and subsequent fires. The dust darkened and cooled the earth so much that many species became extinct.
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