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During the 1980s, researchers
believed the structure may have been responsible for the extinction
of the dinosaurs, as well as virtually all lifeforms of the Cretaceous
period. However, this theory has since been proven false for several
reasons. First, while the Manson impact structure is quite large,
scientists speculate that it is probably not large enough to have
caused a mass, global extinction. In addition, geological and
biological evidence suggests that the mass extinction occurred
in stages, resulting from multiple meteor impacts, not a single
impact. Scientists are also now of the belief that the Manson
event occurred approximately 9 million years before the extinction
of the dinosaurs took place, over 65 million years ago.
The Iowa Department of Natural Resources, Geological Survey Bureau
and the U.S. Geological Survey conducted further studies of the
structure in 1991 and 1992. This investigation involved the drilling
of 12 research cores and the examination of more than 4,000 feet
of impact rocks. Core studies confirmed that the Manson structure
is a well-preserved, complex impact structure that preserves six
primary types of impact rocks. These studies of impact melt rock
led to the determination that the impact occurred approximately
74 million years ago, during the late Cretaceous period.
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