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Major geological discovery comes during Ham Lake fire

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Farmington, 55024

Farmington Minnesota P.O. Box 192 / 312 Oak St. 55024

The recent Ham Lake fire was impressive.

But it was nothing compared to what happened when a meteorite slammed into Earth at present-day Sudbury, Ontario, 1.85 billion years ago.

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The blast created a crater more than 150 miles across, scattering rock and dust over nearly a million square miles. A superheated cloud darkened the sky.

Although separated by 500 miles and 1.85 billion years, the two events are connected: The Ham Lake fire forced geologists to alter plans for a scheduled field trip. That change led to the discovery of debris from the Sudbury impact near the Gunflint Trail.

"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."

Wieblen and Minnesota Geological Survey geologist Mark Jirsa were planning a May field trip along the Gunflint Trail for the annual meeting of the Institute of Lake Superior Geology when the Ham Lake fire broke out, closing access to most of the planned trip. Hoping to salvage something, Jirsa went up the trail to scout new locations.

And in a spot he had never visited, with fire in three directions, he discovered evidence of a cataclysm.

It was, however, evidence that no one but a geologist probably would have noticed.

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

Destruction didn't just rain from the sky that day. The Gunflint area probably was in or near a shallow sea when the meteorite stuck. Jirsa found 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.

"When the meteorite hit, it's very likely that the seas went out and then the seas came back in with a vengeance," Jirsa said.

It was vengeance driven by the force of a meteorite that was probably traveling between 12 and 37 miles per second, said University of Toronto geology professor James Mungall, who has researched and written about the Sudbury meteorite for the scientific journal Nature.

"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," he said.

Whether it was a fast-moving hunk of rock or a faster-moving dirty snowball, the meteorite hit the 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 impact literally turned part of the Earth inside out -- causing the Earth's upper crust to be buried under several miles of melted rock thrown up out of the lower crust. The collision is the likely reason for the region's rich nickel deposits.

It may also have caused widespread extinctions, just as a meteorite hitting near the Yucatan 65 million years ago may have ended the reign of dinosaurs. Whereas rocks laid down in shallow seas before the Sudbury impact contain fossils of simple life forms, rocks formed after the impact contain higher levels of carbon, which may represent a mass die-off, Jirsa said.

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

Jirsa found the site late in the day and with fire burning in three directions. At the time, he was unable to do more than give the area a quick once-over.

"There's a lot of work that needs to be done in the field to see what this deposit tells us that other sites don't," Jirsa said. "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."

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