November 07, 2001

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LINCOLN, Neb. — A nearly mile-wide depression in the middle of Nebraska, once heralded as the site of a large meteorite's impact, is really just another hole in the ground, University of Nebraska scientists say.

The Bartak Depression, named for the family on whose Custer County land it's located, was created thousands of years ago by wind, not a meteorite, the NU researchers announced this week.

The NU research counters the conclusions of University of Kansas researchers, who said in 1992 that the depression, which they renamed the Merna Crater after the nearby town, likely was created by the explosion of a large meteorite with the force of several hydrogen bombs between 3,000 and 500 years ago. In the absence of any large meteorite fragments, the KU researchers' original theory, proposing a meteorite impact, was revised a few years later to include a meteor that exploded about three miles in the air above the site.

The KU team's conclusion caused a media stir and some skepticism among scientists at the time because it challenged existing timetables on meteorite impacts, including the likelihood of future impacts.

Articles on the team's findings ran in national science magazines such as National Geographic, Earth and Discover and in a number of newspapers. Entrepreneurial residents embraced the depression's newfound fame as a potential tourism boon with a community festival known as Merna Crater Days.

"This thing hit the news because it pointed to higher rates of meteor bombardment just as people were becoming concerned about the risk of a large meteor that could hit the Earth some day, but probably a long ways down the road," said Jim Swinehart, one of three NU Conservation and Survey Division researchers who set out in 1998 to further study the site.

That NU research, which included drilling test holes in and next to the depression, found that the "crater" had the same origin as similar, though less impressive, depressions in the region: Relentless winds that scoured out hollows during very dry periods thousands of years ago.

"We didn't want folks to start thinking like Chicken Little and say the sky is falling," Swinehart said. "Part of the reason for the original investigation was to bring tourist dollars to the area, but it's based on poor science."

The NU team reported its findings this week at the annual meeting of the Geological Society of America, the same organization that initially heard the KU team's conclusions nine years ago. Swinehart was joined by NU researchers Mark Kuzila, lead author of the study, director of CSD and its head soil scientist; and geologist Joe Mason.

"This is the kind of thing that makes scientists either salivate or froth at the mouth," said Swinehart of the meteorite theory. "But it just isn't so."

The KU researchers' findings were based on relatively shallow drilling that turned up microscopic metal-rich fragments and glass shards they said were unusual for the region and pointed to a meteorite origin.

The NU team's more extensive drilling, 200 to 400 feet deep, however, failed to turn up the debris around the rim of the crater or the dramatic deformation and compression of underlying sediments that a meteorite would have caused.

"The first most obvious discrepancy was that there were not relatively large pieces of glassy debris in the area," Swinehart said. "These would be many tens of feet thick around the crater. You can imagine. You blow it all up and it falls to the ground." Instead, the NU team found, the Bartak Depression has more in common with a similar, previously studied depression near Ong, Neb., about 120 miles southeast of Merna, than with the most recent, large impact crater in the United States, the 50,000-year-old Meteor Crater near Winslow, Ariz., also nearly a mile in diameter.

Both the Ong and Merna sites reflect an ancient landscape formed more than 25,000 years ago in wind-blown silt called loess that is covered in more recent loess.

The NU researchers also had an explanation for the presence of magnetic minerals and glass shards that their colleagues at KU attributed to a meteorite. Such material is common in and under the landscape of central and western Nebraska, likely having been reworked by wind and water after having blown in from ancient volcanoes thousands of miles away, Swinehart said.

"You give me a loess sample from central Nebraska, and I guarantee you I'll find a few percent to 15 percent glass shards," he said.

The meteorite theory also seemed highly unlikely to Bill Shoemaker, a meteor expert with the U.S. Geological Survey who had said in Discover magazine in 1993 that the odds were 10 to 1 against the Bartak Depression being caused by a meteorite.

For every genuine meteorite impact site, Shoemaker said, there are thousands of depressions that have some crater-like features but were not created by meteorites.

The Conservation and Survey Division is part of NU's Institute of Agriculture and Natural Resources.

11/07/01-CF

James Swinehart - Ph.D.
Conservation & Survey Division
Professor/Research Geologist
(402)472-7529

Mark Kuzila - Ph.D.
School of Natural Resources
Director
(402) 472-7537

Charlie Flowerday
Editor/ Publications Officer - Conservation and Survey Division
(402) 472-3471

Department: Conservation & Survey