Geology of Iowa

This article is about Iowa geology. For the study of buried human cultural remains in Iowa, see Iowa archaeology.

The geography of Iowa includes the study of bedrock, landforms, geology, and paleontology of the state of Iowa.

Iowa's bedrock geology generally increases in age from west to east. In northwest Iowa Cretaceous bedrock is ca. 74 million years old, in eastern Iowa Cambrian bedrock dates to ca. 500 million years ago.

Seventy-four million years ago, a large asteroid crashed into what is now southeast Pocahontas county creating the Manson crater. Probably a mile in diameter, it would have killed most animals within 650 miles, roughly an area from modern Denver to Detroit. This was originally thought to have been one of the causes of the dinosaur extinction, but recalculation of the impact's age indicates it occurred some 12 million years before the mass extinction. Although glaciation has erased all surface evidence of the impact, the bedrock associated with this impact is unique in Iowa.

A much older meteorite strike created the Decorah crater during the Middle Ordovician Period, 470 million years ago. The crater is estimated to be 3.5 miles (5.6 km) in diameter, covered by Winneshiek Shale. There is no surface evidence of the impact, as the Winneshiek Shale is more than 50 feet below the bottom of the Upper Iowa River. The impact, equivalent to 1,000 megatons of TNT, did not appear to penetrate the Earth's mantle, but it did push down the underlying Ordovician and Cambrian bedrock several hundred feet. It may be one of several Middle Ordovician meteors that fell roughly simultaneously 469 million years ago, part of a proposed Ordovician meteor event.

Buried deeply within Iowa's bedrock, the Midcontinent Rift System can be seen clearly in magnetic anomaly maps of Iowa. This is a billion-year-old tectonic plate scar that extends from Kansas through Lake Superior. This rift is not seismically active.

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