[meteorite-list] 1.85 Billion Year Old Meteorite Debris Found in Minnesota

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 16 Jul 2007 13:48:02 -0700 (PDT)
Message-ID: <200707162048.NAA06588_at_zagami.jpl.nasa.gov>

http://wcco.com/local/local_story_196120304.html

1.85 Billion Year Old Meteorite Debris Found In MN
Associated Press
July 15, 2007

(AP) Grand Marais, Minn. A meteorite that slammed into Earth 1.85
billion years ago at the present site of Sudbury, Ontario, is now making
news 500 miles away in northeastern Minnesota.

When the Ham Lake fire burned more than 118 square miles in northeastern
Minnesota and Ontario in May, geologists had a field trip scheduled
along the Gunflint Trail for the annual meeting of the Institute of Lake
Superior Geology.

The fire closed access to most areas the group had planned to visit, so
Minnesota Geological Survey geologist Mark Jirsa went up the trail to
scout new locations.

In a spot he had never visited, with fire in three directions, Jirsa
found evidence of a cataclysm that probably no one but a geologist would
have noticed -- debris from the Sudbury impact.

That blast created a crater more than 150 miles across, scattering rock
and dust over nearly a million square miles.

"It's fairly dark rock," Jirsa said. "They look like concrete, but in
this concrete you would throw pieces of rock of all sizes and shapes and
in all possible orientations."

The rock includes balls the size and shape of a large taconite pellet
which Jirsa believes formed in the impact's huge, hot cloud of dust,
much as hailstones form in a storm cloud.

Jirsa says the Gunflint area probably was in or near a shallow sea when
the meteorite stuck. He says there are indications that a huge tsunami
may have ripped up the sea bottom and seashore, mixing them with rocks
fallen from the sky into the concrete-appearing geological mess he found.

University of Toronto geology professor James Mungall, who has
researched and written about the Sudbury meteorite for the scientific
journal Nature, said the meteorite was probably traveling between 12 and
37 miles per second when it hit Earth with a force equal to several
billion Hiroshima-sized atomic bombs. Temperatures soared above 10,000
degrees, 6,500 cubic miles of rock melted and the huge crater formed.

"The object was probably between 10 and 20 kilometers (6 and 12 miles)
in diameter, and some of us think it was more likely to have been a
comet than an asteroid, but there is no definitive evidence," Mungall said.

Previously, material thrown out by the meteorite's impact had been found
as far from Sudbury as Hibbing. But the Hibbing materials -- tiny
fragments of shocked quartz and small ejecta -- were found in core
samples from 800 to 1,000 feet below the Earth's surface, while the
Gunflint site lies exposed.

Jirsa said there's still much work to be done in the field to determine
what secrets the Gunflint site holds that other sites don't.

"That's the critical thing. This is a different geological setting; it's
a little farther away from the impact, the rocks are altered
differently. It may reveal some secrets about the impact that other
discoveries haven't yet. That's what we're hoping," Jirsa said.

"I think the excitement for the people of Minnesota is that we are one
place in the world where you can see evidence of an ancient meteorite
impact," said University of Minnesota geology professor emeritus Paul
Weiblen, who is studying the debris. "This is the second-oldest and
second-largest impact crater in the world."
Received on Mon 16 Jul 2007 04:48:02 PM PDT