[meteorite-list] Scientists Say Giant Meteorite Struck Wisconsin Long Ago

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Apr 26 13:11:23 2004
Message-ID: <200404261711.KAA05823_at_zagami.jpl.nasa.gov>

http://www.usatoday.com/tech/news/2004-04-26-meteorite-rock-elm_x.htm

Scientists say giant meteorite struck Wisconsin long ago
By Juliet Williams
Associated Press
April 26, 2004

WAVERLY, Wis. - The muddy brown hills and rolling farmland here
look like others in Wisconsin. Tall grasses, cornfields and a
bubbling brook yield to rocky outcroppings and rows of trees.

But scientists years ago saw something different about those
rocks and concluded an ancient catastrophic event occurred here,
although what type of calamity remained a mystery.

They believe they have finally solved the puzzle: A 650- to
700-foot meteorite crashed into the earth at a speed possibly
reaching 67,500 mph.

The impact 450 million years ago dislodged rocks and created a
massive hole in a 4-mile area called Rock Elm about 70 miles
east of Minneapolis, three scientists said in an article published
in the Geological Society of America Bulletin.

Over time, shale, dirt and sediment filled the hole to make the
impact site virtually indistinguishable from the surrounding land.
A shallow sea covering Wisconsin at the time of the impact
likely blunted the meteorite's effect.

The report said the impact at Rock Elm released more than 1,000
megatons of explosive energy, lifted the earth at the center more
than 1,650 feet and sent shock waves through the rocks, crushing
them.

"They were at ground zero, so they got the brunt of it," said
William S. Cordua, of the University of Wisconsin-River Falls,
and one of the paper's authors.

The confirmation of what happened here millions of years ago is
significant to geologists seeking to trace geological patterns,
said Don Yeomans, an astronomer at NASA's Jet Propulsion
Laboratory in Pasadena, Calif.

Although they're not spectacular looking, to Cordua and other
scientists the rocks here have always appeared different than
those just a few miles away. They're tipped at an angle in many
places, reflecting the damage inflicted millions of years ago.

Worldwide, there are only about 200 such impact formations, and
only a couple dozen in the United States. They are believed
to have occurred only every few hundred thousand years.

The first modern indication of anything wrong here came in 1942,
when a UW-Madison graduate student spotted the differences in
soil and quartz and mapped out the area for more study.

"Mostly after its discovery it was pretty well ignored," said
Bevan M. French, a former NASA geologist who is a research
collaborator at the Smithsonian Institution. Even so, the area
has been known among amateur geologists and farmers as an
anomaly.

Since the 1980s, Cordua has trudged through grassy fields and
muddy bogs looking for answers about Rock Elm. He started
writing about the formation in 1985, and although he suspected
it was formed by a meteorite, he couldn't prove it.

"What I've been trying to do is hope that people who study
more of these things would get interested in it. And that
finally happened," Cordua said.
Received on Mon 26 Apr 2004 01:11:10 PM PDT