Decorah Rocks

Decorah, Iowa (detail)Decorah, Iowa is a quiet sort of midwestern town. With a population of just over 8,100, the seven square miles of Decorah make up the Winneshiek County seat, right about where Route 52 crosses Highway 9. Once upon a time, the Winnebago tribe lived here after being driven from Wisconsin; the town is named after Waukon Decorah, who led Ho-Chunk Winnebago warriors alongside American forces in the Black Hawk War of 1832, and lived to see his people once again uprooted so that white American settlers could have the land. The good people of Decorah named their town after him, and then dug up what they believed was his body in 1859 in order to build over the gravesite. In 1876, they dug up the bones again, in order to renovate the courthouse.

The town is also home to Luther College and the Vesterheim Norwegian-American Museum. Each year, the town is flooded by thousands of tourists celebrating Norwegian heritage during Nordic Fest.

It is a picturesque little place in the American heartland.

And it also sits on a 470 million year-old cosmic impact crater:

An asteroid as big as a city block smashed into what is now northern Iowa about 470 million years ago, says a Smithsonian geologist, supporting a theory that a giant space rock broke up and bombarded Earth just as early life began flourishing in the oceans.

The impact dug a crater nearly four miles wide that now lies beneath the town of Decorah, said Bevan French, one of the world’s foremost crater hunters and an adjunct scientist at the National Museum of Natural History.

The asteroid that carved it would have dwarfed the estimated 55-foot-wide space rock that exploded over southern Russia on Friday.

The Decorah object smashed into bedrock with such force that it shattered tiny grains of minerals. French found this “shock quartz” in gravel from beneath the town, he told two dozen colleagues during a seminar at the museum last week.

Finding impact craters is rare, as erosion and the shifting of tectonic plates tend to erase them. The Decorah crater, if accepted by other scientists, would be just the 184th known, according to an international database at the University of New Brunswick.

But spying the evidence of the Earth’s most dramatic explosions requires only humble equipment — a simple black microscope. As sun streamed into French’s office above Constitution Avenue one recent afternoon, he placed a glass slide under the microscope’s lens and invited a reporter to peer in. A thin slice of rock from beneath Decorah sat on the slide.

Three white circles — quartz crystals no bigger than mustard seeds — popped into view. Dozens of parallel lines striped each circle: evidence of a rock-crushing pulse.

“They’re shattered,” French said of the crystals. Geologists consider shock quartz near-definitive evidence of an extraterrestrial impact.

The Decorah crater lay undiscovered until now because almost none of it peeks above ground. Instead, it is filled by an unusual shale that formed after an ancient seaway sluiced into the crater, depositing sediment and an array of bizarre sea creatures that hardened into fossils, French said.

This shale was the first clue that certain Iowans may be unknowingly living in a crater.

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A Tale of Two Rocks

It has been quite a day for a falling sky. As murmurs rippled around the world that the end may be near, and astronomers worked to reassure everyone that asteroid 2012 DA14 would not strike the Earth today, despite its historic pass inside our satellite orbital ring, an unrelated meteorite chose to scare people in Russia this morning.

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