Meteor Impact Crater Found Beneath Iowa Town
July 11, 2013

Meteor Impact Crater Sits Below Iowa Town

Lee Rannals for - Your Universe Online

A meteor crater from 470-million-years-ago lies buried several hundred feet beneath the town of Decorah, Iowa, according to an airborne geophysical survey reported in EARTH Magazine.

Geologists first hypothesized the crater's existence while examining cuttings from water wells drilled near the town. During this examination they discovered evidence of a previously unknown shale deposit that was nearly a perfect circle, roughly 3.4 miles across. Another analyst revealed shocked quarters, which is a telltale sign of a meteor impact.

Scientists at the US Geological Survey (USGS) and the Iowa and Minnesota Geological Surveys conducted a high resolution geophysical survey of the region to determine water and mineral resources. They were mapping the Northeast Iowa Igneous Intrusive complex, which sits in the Midcontinent Rift System that formed about 1.1 billion years ago.

Data revealed a textbook signature of a low-density feature due to displaced bedrock in the crater. This corresponded with both the electromagnetic data and the borehole data that revealed the full extent of the circular shale layer.

"We now have three distinct datasets that all confirm the presence and geometry of an impact structure," Andy Kass, a geophysicist at USGS in Denver, Colo., who analyzed the new geophysical data, told EARTH Magazine.

He said they were lucky the shale layer was preserved within the crater, because it was eroded away nearly everywhere else.

Scientists believe the meteor that struck the area millions of years ago was about 820 feet in diameter due to the diameter of the crater. The crater joins a suite of Middle Ordovician impact craters in the Midwest that may, or may not, have resulted from the same impactor.

"It's a tantalizing possibility," Kass says. "Unfortunately, it's impossible to use dating techniques to see if all the impactors happened on a single day."

He said imprecators of this size in the Midwest should hit somewhere on Earth every 30,000 to 60,000 years. He will continue to analyze the data to create 3D computer model of the crater to better understand its structural features, the geometry and energy of the impact.

"This is a major find with both scientific and societal implications," says Douglas Howard, a planetary geologist and the associate program coordinator for StateMap and EdMap at the USGS National Cooperative Geologic Mapping Program, which funded the research.