What is it?
The Manson Impact Structure is one of the largest known meteorite craters in the continental United States. Measuring more than 24 miles in diameter, the crater is believed to have been created by the impact of a stony meteorite that seared through the Earth's atmosphere and collided with our planet some 74 million years ago. The meteorite spanned approximately 1.5 miles in diameter, weighed close to 10 billion tons and was traveling at a rate of 45,000 miles per hour when it crashed into the Earth. The crater was named after the small town of Manson, Iowa, which lies near its center.
The unusual geology of the land surrounding the Manson structure was discovered in 1912, when a town water well was drilled. Rock samples were discovered to be unlike other rocks in the region. Instead of the bedrock typically found in north-central Iowa, drillers encountered thick shale directly above granitic rock.
The mysterious geology was originally thought to be a river-channel deposit. In 1953, the Iowa Geological Survey and the U.S. Geological Survey obtained two drill cores from the structure to investigate the site further. They concluded that the crater was a cryptovolcanic structure produced by a giant blast of volcanic gas. It was not until 1959 that evidence was found by Robert Dietz, which proposed the Manson structure to be of meteor-impact origin. In 1966, Nicholas Short confirmed Dietz's proposal.
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.