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Crater Tied to Extinctions

<i> From staff and wire reports</i>

New, more precise dating of the Manson impact crater in Iowa indicates that it was formed at the same time the dinosaurs and many other species became extinct about 65 million years ago. The new dating provides greater credibility to the hypothesis that an asteroid crashed into Earth, creating a shroud of smoke and dust that plunged the planet into global winter.

Researchers believe that an asteroid caused the extinctions because they have found relatively high concentrations of iridium, a metal that is rare on Earth but common in asteroids, in sediments deposited at the time the dinosaurs died.

The Manson crater, in northwestern Iowa about 18 miles west of Ft. Dodge near the town of Manson, is the largest impact crater in the United States, with a diameter of about 22 miles.

Researchers from the U.S. Geological Survey reported last week in Science that they measured the age of the crater by determining the ratio of two isotopes of argon in the rocks, a common dating technique. They determined that the asteroid hit Earth 65.7 million years ago, a time that is “indistinguishable from recent estimates” of the date of dinosaur extinction.

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