R.R. Anderson

Iowa Academy of Science
104th Session
April 24-25, 1992, The University of Northern Iowa, Cedar Falls, IA
1992 Program Abstracts, p. 21


In 1991 the first of a series of core holes were completed, sponsored by the Iowa Department of Natural Resources - Geological Survey Bureau and U.S. Geological Survey, and dedicated to investigating the Manson Impact Structure. The M-1 core sampled rocks on the edge of the Central Peak, and the M-2 core penetrated the rocks of the Crater Moat. Preliminary investigation of these cores has already provided valuable insight into the formation of this well-preserved, complex crater. The current model begins with the impact of a stoney asteroid into the center of North America at the end of the Cretaceous Period, 65.7 million years ago. The asteroid, about 2 km in diameter and traveling about 20 km/sec, exploded with the energy equivalent of 750,000 megatons of TNT, vaporizing the asteroid and 7 km3 of Earth materials, melting 37 km3 of rocks, ejecting 617 km3 of solid materials, and displacing 1,047 km3 of materials downward, forcing the rim of the crater to be uplifted at least 0.5 km above pre-impact surface 25 seconds after impact. The uplifted rim then collapsed as a sequence of large blocks, forcing the Central Peak upward to a temporary height approximating the rim. At this time, several minutes after impact, large volumes of crater rim and ejecta materials slumped into the crater and moved as a high-velocity debris flow, across the crater floor and up onto the Central Peak. Within weeks the region had stabilized and a lake began to form in the crater moat. Eventually the lake was filled with sediment and the moat area was covered by a sequence of fanglomerates that prograded inward from the crater rim. An extended period of erosion and burial of the area by till from a number of continental ice sheets in the Pleistocene produced the Manson Impact Structure as we observe it today.