C. Koeberl and R.R. Anderson

The Geological Society of America
Special Paper 302, 1996, p. 1-30
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.


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 mechanism, and that about 150 impact structures have been recognized on earth. To put the studies of the Manson crater, which are reported in this volume, in a proper framework, we review here 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 of 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 characteristics of impact and do not occur in natural materials formed by any other processes. For comparison with Manson, we give an overview of our current knowledge of impact structures in the United States of America. We discuss 27 structures, which include confirmed, probable, and possible structures, in detail, and briefly mention a few other features, for which an impact origin has been suggested. Unfortunately, we have to conclude that about half of these structures are not well studied, even though they are relatively accessible.