The U.S. Geological Survey has confirmed the northeast Iowa town of Decorah is built on top of a meteorite crater created 470 million years ago.
Iowa Department of Natural Resources geologists first suspected the area was a meteorite crater in 2008 after studying well drill-cuttings and finding a unique type of shale. Shocked quartz was also found, which is considered strong evidence of a meteorite impact.
Recent airborne geophysical surveys included an airborne electromagnetic system, which can detect how well rocks conduct electricity. The data showed the crater was filled with electrically conductive shale.
“These data, when coupled with physical property measurements on drill core samples, will form the basis for modeling efforts to constrain the impact geometry and energy of the meteorite,” said Dr. Andy Kass, a USGS geophysicist.
The crater is about 5.5 kilometers wide and has been filled in with bedrock and sediments over the centuries.