Iowa Crater Points to Early Hit From Asteroid

Tuesday, 19 Feb 2013 01:51 PM

By Cyrus Afzali

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In an event that dwarfs Friday’s asteroid explosion over Russia, a Smithsonian geologist Tuesday said an asteroid as large as a city block hit northern Iowa 470 million years ago.

Bevan French, an adjunct scientist at the National Museum of Natural History, told The Washington Post the space rock left a crater nearly four miles wide beneath what is now Decorah, Iowa.

French said the asteroid’s collision with bedrock created such force that it shattered into tiny grains of minerals. The “shock quartz” was found in gravel beneath the town.

The discovery is especially unique because traces of impact craters tend to disappear with the effects of erosion and the shifting of tectonic plates. If the find is accepted by other scientists, the Decorah crater would become the 184th impact crater discovered, according to the University of Brunswick, which maintains an international database on craters.

The crater went undetected for so long, scientists say, because the vast majority of it lies underground, having been filled by an unusual shale that formed after an ancient seaway deposited sediment and sea creatures that hardened into fossils, French said.

Jean Young, an amateur geologist, was among the first to notice the shale some 12 years ago after inspecting gravel unearthed by well-drilling equipment. Young sent samples to geologist Robert McKay at the Iowa Department of Natural Resources. Young compared the sample to gravel unearthed during other well drillings and discovered a basin about 3.5 miles wide divided by the Upper Iowa River.

News of the black rock was first published in a scientific paper in 2006, in which it was called Winnesheik Shale. However, it was only after French identified the shock quartz pulled from beneath the shale that an impact from a giant asteroid seemed likely. Fossils in the shale led scientists to date the crater’s creation to about 470 million years ago, part of a period known as the Middle Ordovician that was marked by a large increase in early ocean life.

In 2004, a group of Swedish astronomers suggested that a massive collision in the asteroid belt beyond Mars about 469 million years ago resulted in fragments bombarding the Earth.

Scientists say that 20 percent of all meteorites on Earth originated during that period.

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