Editing Impact crater (section)

==History==
Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned, [[Meteor Crater]], was of cosmic origin. Most geologists at the time assumed it formed as the result of a volcanic steam eruption.<ref name=Levy/>{{rp|41–42}}

[[File:Eugene Shoemaker.jpg|thumb|upright|[[Eugene Shoemaker]], pioneer impact crater researcher, here at a crystallographic microscope used to examine meteorites]]

In the 1920s, the American geologist [[Walter H. Bucher]] studied a number of sites now recognized as impact craters in the United States. He concluded they had been created by some great explosive event, but believed that this force was probably [[volcano|volcanic]] in origin. However, in 1936, the geologists [[John D. Boon (geologist)|John D. Boon]] and [[Claude C. Albritton|Claude C. Albritton Jr.]] revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts.<ref>{{cite journal|last1=Boon|first1=John D.|last2=Albritton|first2=Claude C. Jr.|title=Meteorite craters and their possible relationship to "cryptovolcanic structures"|journal=Field & Laboratory|date=November 1936|volume=5|issue=1|pages=1–9}}</ref>

[[Grove Karl Gilbert]] suggested in 1893 that the Moon's craters were formed by large asteroid impacts. [[Ralph Baldwin]] in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960, [[Gene Shoemaker]] revived the idea. According to [[David H. Levy]], Shoemaker "saw the craters on the Moon as logical impact sites that were formed not gradually, in [[geologic time scale|eons]], but explosively, in seconds." For his [[Doctor of Philosophy|PhD]] degree at [[Princeton University]] (1960), under the guidance of [[Harry Hammond Hess]], Shoemaker studied the impact dynamics of Meteor Crater. Shoemaker noted that Meteor Crater had the same form and structure as two [[explosion crater]]s created from [[Nuclear weapon|atomic bomb]] tests at the [[Nevada Test Site]], notably [[Operation Buster-Jangle|Jangle U]] in 1951 and [[Operation Teapot|Teapot Ess]] in 1955. In 1960, [[Edward C. T. Chao]] and Shoemaker identified [[coesite]] (a form of [[silicon dioxide]]) at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite within [[suevite]] at [[Nördlinger Ries]], proving its impact origin.<ref name="Levy">{{cite book |last1=Levy |first1=David |url=https://archive.org/details/shoemakerbylevym00levy |title=Shoemaker by Levy: The man who made an impact |date=2002 |publisher=Princeton University Press |isbn=9780691113258 |location=Princeton |pages=59, 69, 74–75, 78–79, 81–85, 99–100 |url-access=registration}}</ref>

Armed with the knowledge of shock-metamorphic features, [[Carlyle S. Beals]] and colleagues at the [[Dominion Astrophysical Observatory]] in [[Victoria, British Columbia]], Canada and [[Wolf von Engelhardt]] of the [[University of Tübingen]] in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American [[Apollo program|Apollo]] Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the [[Moon]].<ref>Grieve, R.A.F. (1990) Impact Cratering on the Earth. ''Scientific American'', April 1990, p. 66.</ref> Because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.