It is round and about a mile wide. It’s long been there, in the center of the state. But, says Wake Dort, no one knew what it is: a meteorite crater.
To Native Americans living in the central Great Plains, the spectacle must have been awesome. A fireball brighter than 50 suns plunges blazing through the atmosphere, explodes, and ignites the prairie for miles around. The blinding flash gives way to a dark, silent mushroom cloud. A minute later comes the stupendous roar and a blast wave that knocks trees and people to the shaking ground. Most living things within ten miles die.
If University of Kansas geologist Wakefield Dort is correct, that’s what happened 3,000 years ago right in the middle of present-day Nebraska, about 12 miles west of the small town of Broken Bow. In 1990 Dort was examining a topographical map of central Nebraska when he noticed a round, shallow depression, about a mile wide. I took one look at it and said, ‘Ye gods! This doesn’t fit anything,’ Dort recalls. My colleague Ed Zeller came over and looked at the map, and he said, ‘Good God, Wake, you’ve got a meteorite crater!’
Dort was initially, but only initially, skeptical. We’ve tried everything we could think of to deny a meteorite impact, since it just seemed too exotic, he says. But every bit of evidence kept coming back to that--we could not explain the depression in any other way. There are no volcanoes in the vicinity, Dort says, the rock formations aren’t the sort to collapse and form sinkholes, and the depression is too round and steep- sided to have been scoured by the wind.
What’s more, in the surrounding farmlands Dort’s group has found glassy plates and small, spherical blobs rich in iron and titanium. These rock fragments are unlike anything found ordinarily in central Nebraska. Dort thinks they may be pieces of the exploded meteorite that rained down as molten droplets.
Dort estimated the age of the crater by comparing the soil layer where the glassy fragments were found with a virtually identical layer nearby that has been radiocarbon-dated as being 3,000 years old. Because he has yet to actually date the layer where the glass was found, Dort admits that the crater’s age is uncertain. But since it lies above a layer of silt that is 12,000 years old, it must be younger than that--which means it was formed after humans had already occupied the area.
In contrast, Arizona’s famous Meteor Crater--one of only two confirmed impact craters visible at the surface in the United States--was created before human settlement, roughly 50,000 years ago. With a diameter of about a mile, the Nebraska crater is slightly larger than Meteor Crater, but it is not nearly as distinctive. Whereas Meteor Crater was dug out of slow-eroding hard rock in a desert, the Nebraska depression lies in loose soil that erodes with every rainstorm, says Dort. It is now only 65 feet deep; one can drive through it and easily miss it.
And in fact Dort hasn’t convinced all the experts that his depression really is an impact crater. Eugene Shoemaker of the U.S. Geological Survey cautions that Dort has not yet found some of the key diagnostic features of an impact, such as structural deformations in the crater’s rim and a shocked, compressed layer of bedrock underneath it. I’ll give ten-to-one odds that it’s not an impact crater, says Shoemaker. For every impact crater, there are at least 10,000 crater-shaped features. Retorts Dort: I’m totally positive--that’s POSITIVE with capital letters.
Dort hopes to drill into the bedrock under the crater this summer and thereby gather enough evidence to silence the skeptics. Meteorite expert Hal Povenmire of the Kennedy Space Center, who has examined the crater with Dort, is already convinced: he thinks it was dug by a stony meteorite slightly denser than the one now thought to have caused the 1908 explosion over the Tunguska River in Siberia--the most recent large impact on Earth. That meteorite released about as much energy as a hydrogen bomb, and the nighttime glow was bright enough to read by in England. The Nebraska impactor, if there was one, would have been considerably larger-- about 500 feet across--and would have released hundreds of times more energy. The Native Americans who lived in and around Nebraska would have seen more than a distant glow.