2 June 2006
Smoking Gun For Permian-Triassic Extinction Found
by Kate Melville 
Scientists have found evidence of a meteor impact in Antarctica that they believe caused the biggest mass extinction in Earth's history and led to the rise of the dinosaurs. The 300-mile-wide crater lies more than a mile beneath the East Antarctic Ice Sheet and the gravity measurements that reveal its existence suggest that it could date back about 250 million years - the time of the Permian-Triassic extinction, when almost all animal life on Earth died out. Located in the Wilkes Land region of East Antarctica, it's possible that it could also have begun the breakup of the Gondwana supercontinent, by creating the tectonic rift that pushed Australia northward. The new crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that many believe ultimately killed off the dinosaurs 65 million years ago.
The Chicxulub meteor is thought to have been 6 miles wide, while the Wilkes Land meteor could have been up to four or five times wider. "This Wilkes Land impact is much bigger than the impact that killed the dinosaurs, and probably would have caused catastrophic damage at the time," said Ralph von Frese, a professor of geological sciences at Ohio State University. Von Frese reported on his findings at the American Geophysical Union Joint Assembly meeting in Baltimore. Von Frese's team used gravity fluctuations to peer beneath Antarctica's icy surface and locate a 200-mile-wide plug of mantle material - a mass concentration, or "mascon" in geological lingo - that had risen up into the Earth's crust. Mascons are the planetary equivalent of a bump on the head. They form where large objects slam into a planet's surface. Upon impact, the denser mantle layer bounces up into the overlying crust, which holds it in place beneath the crater. "If I saw this same mascon signal on the moon, I'd expect to see a crater around it," said Von Frese. "And when we looked at the ice-probing airborne radar, there it was. There are at least 20 impact craters this size or larger on the moon, so it is not surprising to find one here," he continued. "The active geology of the Earth likely scrubbed its surface clean of many more." "On the moon, you can look at craters, and the mascons are still there," von Frese said. "But on Earth, it's unusual to find mascons, because the planet is geologically active. The interior eventually recovers and the mascon goes away." He cited the very large and much older Vredefort crater in South Africa that must have once had a mascon, but no evidence of it can be seen now. "Based on what we know about the geologic history of the region, this Wilkes Land mascon formed recently by geologic standards - probably about 250 million years ago," he said. "In another half a billion years, the Wilkes Land mascon will probably disappear, too." Von Frese said that the immediate effects of the impact would have devastated life on Earth. "All the environmental changes that would have resulted from the impact would have created a highly caustic environment that was really hard to endure. So it makes sense that a lot of life went extinct at that time." Source: Ohio State University
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