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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.”
