The space rock smashup showered Earth with up to 100 times more meteorites than today's rate (a rock the size of a football field hits the planet about every 10,000 years). Yet, only a dozen or so impact craters have been found from the ancient bombardment 470 million years ago, during the Ordovician Period. Most are in North America, Sweden and western Russia. There are only about 185 known impact craters on Earth of any age, while the moon has more than 100,000.
But the number of Ordovician craters may soon take off. That's because it's easier and cheaper than ever to hunt down evidence that confirms an impact. The clinchers include shocked minerals, deformed rocks and structural features that match other craters.
"Google Earth images are not good enough to identify an impact structure," noted planetary geologist Christian Köeberl on Oct. 22, at the Geological Society of America's annual meeting in Vancouver, British Columbia. During the Vancouver meeting, researchers presented new clues that bring suspected craters in Wisconsin, Kentucky and Tennessee closer to official listings as Ordovician impact craters.
The three enigmatic structures retain their circular shape, but have lost most of their original features through erosion. In the last century, quarrying has also slowly dismantled the Wisconsin crater. Only the central uplift seems to persist. When a meteorite hits, the impact's force causes the underlying rock to rebound upward, leaving a topographic high in the center of the crater.
In each state, researchers looked for traces of minerals shattered or heated by the impact. So far, no one has found one of the smoking guns in crater research: shatter cones, the finely fractured rocks created when the shock wave travels through the ground. The fractures are often arranged in a conical shape, like an ice cream cone.
Three little craters
But even without a smoking gun, at Brussels Hill in Door County, Wisconsin, a meteorite impact is the best explanation for the perfectly round, 130-foot-tall (40 meters) hill, said Emily Zawacki, an undergraduate at Lawrence University in Appleton, Wisconsin. The flat-topped peak is filled with fractured blocks of Cambrian sandstone that should lie some 1,300 feet below the younger carbonate rocks. The fragmented rocks all tilt toward the center of the hill, and a series of faults radiate outward from its center.
The evidence all points to a deeply eroded impact crater, Zawacki said. "This is a highly disturbed area in otherwise flat-lying stratigraphy," Zawacki said. "It very clearly is anomalous and we feel a meteoritic impact best explains it."
In the middle of Tennessee, the Howell Structure has confounded geologists for decades. The bowl-shaped basin is about the same diameter as Brussels Hill (about 1.2 miles, or 2 km). In this case, however, the suspected crater is weaker than the surrounding rocks, creating a depression. A pile of fragmented carbonate and other craterlike features suggests an impact origin.
Keith Milam, a professor at Ohio University in Athens, recently uncovered a rare trove of rock cores drilled at Howell in the 1960s. John Bensko, a retired lunar geologist from NASA's Marshall Space Flight Center, provided the 15 segments. Bensko oversaw the testing of drilling equipment intended for the canceled Apollo 18 program. The first tests on the rock cores suggest the fragmented carbonate rocks were shocked by a meteorite impact, Milam reported at the Vancouver meeting.
Finally, the Jeptha Knob structure in Kentucky is a site that stands out on Google Earth and just needs the right mineral evidence to certify its impact origin. "I don't think you can say for sure this is an impact structure yet," said Eric Gibbs, an undergraduate at Ohio University in Athens. Gibbs is testing the X-ray diffraction pattern produced by minerals from the crater. The pattern shortens and widens with increasing shock, he said.
The initial tests, presented at the Vancouver geology meeting, support an impact origin for the hill. Jeptha Knob is the highest point in Kentucky's Bluegrass Region, rising some 300 feet (90 m) above the surrounding farms. The round crater is ringed by faults and busted-up Ordovician limestone, but topped by flat layers of younger carbonate rocks.
The apparent alignment of many of these craters makes it seem that some coincidence favored Earth's tropical latitudes during the big Ordovician bombardment.
Read more at Discovery News
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