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Last updated:
September 13, 2002

Rock Elm Asteroid Crater Site of Speculation, Study

A long, long time ago, more than 400 million years, an asteroid about a half mile in diameter traveling at about 12 miles per second smacked into the earth at Rock Elm, Wisconsin, leaving a hole about four miles in diameter and 200 yards deep.

The crater, known as the Rock Elm Disturbance in eastern Pierce County, is one of more than 100 known impact craters on our planet, according to UW-River Falls Geology Professor Bill Cordua. There may be many more, as yet undiscovered, in countries that have not been explored for craters as well as beneath the sea.

In Wisconsin, the Rock Elm Disturbance is the largest of three known sites. The other two are near Pepin, on the Mississippi River in west-central Wisconsin, and Glovers Bluff in the central part of the state.

Though most of the Rock Elm crater is on private property, part of it falls within the boundaries of Nugget Lake County Park. Visitors to the park occasionally pan for gold or visit Blue Rock, a geological formation of dark blue rock about 150 feet wide and 60 feet high.

Park Manager Scott Schoepp says there is a small interpretive sign by Blue Rock written by Cordua to explain the significance of the area, but it is very difficult to identify the disturbance without some assistance.

Nevertheless, the site has generated its share of attention over the years, not only from students of geology but also from prospectors seeking gems and minerals, people who speculate about UFO visits to the area, and those who wonder when another asteroid will drop out of the sky.

Chris Peters, a recent UW-RF graduate from Stillwater, Minn., who majored in geology, conducted a study on the disturbance for his senior research experience and solved one riddle: he was able to determine the age of the crater in Rock Elm.

Peters painstakingly used a telescope to examine sediment from cores that a commercial company had drilled while prospecting for gold and diamonds at the disturbance. He found fossils from two types of tiny, soft-bodied creatures, conodonts and scolecodonts, and matched them with other fossils of known ages. With this information, he was able to identify the era that the asteroid hit as the Ordovician Period, roughly 430 million to 455 million years ago when ancient Wisconsin was located just north of the Equator.

Both gold and diamonds have been found in the crater, in the sediment of its streams. About a dozen diamonds were found in the 1880s, according to Cordua, who said they were identified by George F. Kunz, a gemologist. Kunz described the diamonds as pale yellow to blue in color and weighing as much as two carats.

A hundred years later, in the 1980s, EXMIN out of Bloomington, Ind., and the Superior Oil Company from Tucson, Ariz., explored the area for gold, finding only some placer gold.

"The relationship between gold and diamonds is not known," said Cordua, "but it is interesting from a geological point of view that they have both been found in the crater. They probably washed out of the glacial material in that spot."

Though there have been numerous reports of UFO sightings in the area surrounding the Rock Elm Disturbance‹the nearby town of Elmwood claims to be the UFO capital of Wisconsin‹Cordua is skeptical. "I believe those sightings are illusions caused by swamp gas," he said. "The sediment that filled in the crater is clay rich, so there is a lot of heavy, boggy soil that generates swamp gas. The gas is methane, and can make you disoriented," he said. "It can also flare if it is ignited by a lightning strike."

As for the likelihood of another asteroid hitting the earth in this lifetime, Cordua said it is certainly possible but it is unlikely. Over a thousand years, the chances get higher. "An impact of this size comes along about every 100 million years," he said.

"There are hundreds of asteroids in orbits that cross the earth. We don't think any constitute a danger to us now, but of course we don't know about all of them. There is a torino scale that is used to measure the danger of asteroids on a scale of one to 10, with 10 being the most dangerous, but to my knowledge none of the asteroids we know about are rated above a one or two."

Cordua suggested that those who are interested in knowing more about the torino scale visit the Web site at http://neo.jpl.nasa.gov/torino_scale.html.

There is evidence that in 1908 a comet blew up in the air in Siberia, Cordua said. The trees that it blew down for many miles were radially distributed from ground zero, and the force behind it was somewhat equivalent to the largest hydrogen bomb that we have. Fortunately, the area of impact was not inhabited.

The most famous asteroid in history may be the one that hit the Yucatan Peninsula in the Gulf of Mexico about 65 million years ago, forming the chicxulub crater. Scientists say the event created tidal waves that were more than 300 feet high, fires that raged through continents and a dust cloud that blocked the sun for nine months.

More than 50 percent of the species became extinct within a relatively short period of time after it hit the earth. A lot of people think it was this event that led to the extinction of the dinosaurs.

Many of the small mammals were better survivors. There was an empty ecological niche, so the mammals filled it. From the mammals, humans eventually evolved.

"Things happened that wouldn't have happened otherwise because of the asteroid," Cordua said. "Catastrophe and recovery. This is part of the development of life on our planet."

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