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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.
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