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The Manson impact structure is an impact structure near the city of Manson, Iowa where an asteroid or comet nucleus struck the Earth during the Cretaceous Period, approximately 74 Year. It was one of the largest known impact events to have happened in North America."The Manson impact was the biggest thing that has ever occurred on the mainland United States. Of any type. Ever," Bill Bryson, A Short History of Nearly Everything 2003:238. Previously it was thought to have led to the extinction of the until isotopic ages proved that it was too old.
In 1991 and 1992 the U.S. Geological Survey along with others including the Iowa Geological Survey conducted detailed research in part to test the possible connection of the Manson impact structure with the Cretaceous–Paleogene extinction event. The / isotope ratio dating of the core from the impact structure gave an age of about 74 Ma, or about 10 Ma older than the K–T boundary.
The impactor is considered to have been a stony meteorite about in diameter. The site at the time was the shore of a shallow inland sea,Bryson 2002:237. the Western Interior Seaway. The impact disrupted granite, gneiss, and of the Precambrian basement as well as sedimentary formations of Paleozoic age, Devonian through Cretaceous. Limestone layers that give the rest of Iowa hard water were instantaneously vaporized down to the basement rocks, giving Manson the anomalous soft water that it has today.Bryson 2002:237.
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