Manson and company: impact structures in the United States
Christian Koeberl, Raymond R. Anderson, 1996. "Manson and company: impact structures in the United States", The Manson impact structure, Iowa; anatomy of an impact crater, Christian Koeberl, Raymond R. Anderson
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Impact cratering is a geological process that is still rather unappreciated by the geological community, despite the fact that on all other planets and satellites with a solid surface impact cratering is the most important surface-modifying process. About 150 impact structures have been recognized on Earth to date. To put the studies of the Manson crater in a proper framework, we review some fundamental principles of impacts and how to recognize impact craters. The formation conditions of impact craters lead to pressure and temperature conditions in the target rocks that are significantly different from those reached during any internal terrestrial processes. Among the most characteristic changes induced by the impact-generated shock waves are irreversible changes in the crystal structure of rock-forming minerals, such as quartz and feldspar. These shock metamorphic effects are characteristic of impact and do not occur in natural materials formed by any other process. For comparison with Manson, we give an overview of our current knowledge of impact structures in the United States of America, which include confirmed, probable, and possible structures and a few other features for which an impact origin has been suggested. Based on the discovery of remnants of meteoritic matter and/or shock metamorphic effects in the crater rocks (which we accept as criteria to confirm an impact origin), we classify 20 craters in the United States of America as confirmed impact structures. Unfortunately, we have to conclude that about half of these structures are not well studied, even though they are relatively accessible.