Location of the impact structure in Iowa, United States
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Manson impact structure (Iowa)
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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 74Ma.[1] It was one of the largest known impact events to have happened in North America.[2] Previously it was thought to have led to the extinction of the dinosaurs until isotopic ages proved that it was too old.[3]
No surface evidence exists due to comparatively recent coverage by glacial till, and the site where the impact structure lies buried is now a flat landscape. But, hidden about 20 to 90m (66 to 295ft) below the surface is a buried structure about 38km (24mi) in diameter. It lies under the southeast corner of Pocahontas County and extends under portions of three adjoining counties. That an anomalous structure underlaid the area was known from unusual water well drill cuttings in 1912 of deformed rock, "crystalline clast breccia with a melt matrix" as a later report described it.[5] A research investigation was started in 1955, and it was labeled a "cryptovolcanic structure" (a hypothetical volcanic steam explosion). Further investigation was undertaken by Robert S. Dietz who proposed an impact origin in 1959 and by Nicholas Short in 1966 who produced evidence of shocked quartz grains which confirmed the impact origin of the structure.
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 40 Ar/39 Ar isotope ratio dating of the core from the impact structure gave an age of about 74Ma, or about 10Ma older than the K–T boundary.[6]
↑"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.
↑Hartung, J. B., Kunk, M.J. and Anderson, R.R., Geology, geophysics, and geochronology of the Manson impact structure. Global Catastrophes in Earth History, Geological Society of America, Special Paper 247, pp. 207-221. 1990.
Christian Koeberl and Raymond R. Anderson, eds; 1996, The Manson Impact Structure, Iowa: Anatomy of an Impact Crater, Geological Society of America Special Paper 302, ISBN0-8137-2302-7
