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Point of impact: Manson, Iowa?

Point of impact: Manson, Iowa?

Since 1980, scientists have debated whether a meteor is responsible for the extinctions of the dinosaurs and many other forms of life at the end of the Cretaceous period. But if such an apocalyptic impact did occur, where is the big hole in the ground?

According to some scientists at the U.S. Geological Survey (USGS), the answer is in Iowa. In the cornfields of the western part of the state, buried under 30 meters of glacial gravel, there is a 35-kilometer-wide crater called the Manson, Iowa Impact Structure. In 1985, scientists tested the rock in the crater and determined that the crater's maximum age is about 70 million years, putting the impact in the ballpark of the Cretaceous-Tertiary (K-T) boundary, which is 66 million years old. In the sedimentary record, this boundary is a thin layer of clay and mineral fragments, which some say was formed by a globe-circling cloud of dust that would have followed a large impact.

Using a refined dating technique, USGS researchers Michael Kunk and John Sutter of Reston, Va., and Glen Izett of Denver have now determined the Manson crater is 66 million years old. Within the resolution of this technique, the Manson impact was contemporaneous with the K-T boundary, says Kunk.

Did the Iowa impact produce that huge cloud blocking out the sunlight and extinguishing much of the life on earth? The Manson crater is too small to satisfy most planetary geologists who study craters; they estimate that the K-T impact would have produced a crater over 100 km wide. But Eugene Shoemaker of the USGS in Flagstaff, Ariz., resolves this by invoking a currently popular idea that at least two meteors hit earth at the end of the Cretaceous.

The Manson crater's revised birthday makes it an attractive possibility as a site of one of the impacts. "It's the best candidate I know of so far,' says Kunk. Several researchers have formed a consortium to study the structure of the crater and the kinds of rocks created by the heat of that impact. By comparing them with the fragments found at the K-T boundary and by refining the date of that boundary, they hope to determine whether the crash in Iowa created at least some of the evidence found at the K-T boundary.
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Title Annotation:meteor that may have caused extinction of dinosaurs
Author:Monastersky, Richard
Publication:Science News
Date:Dec 19, 1987
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