WASHINGTON -- 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 in Washington 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," Mr. 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, Mr. French said.
This shale was the first clue that certain Iowans may be unknowingly living in a crater.
Jean Young, an amateur geologist in northern Iowa, noticed the shale about a dozen years ago, when inspecting gravel pulled up by well-drilling machines. It looked like no other rock she had seen in the region.
Ms. Young sent samples to Robert McKay, a geologist at the Iowa Department of Natural Resources. Mr. McKay then pulled from his files "churn gravel" from dozens of other wells drilled in a four-township area centered on Decorah.
He found the same shale in some of the other samples, too. When plotted on a map, the shale-rich borings described a "nice circular basin" about 31/2-miles wide, neatly bisected by the Upper Iowa River and almost completely encompassing Decorah, he said.