Magazine article Science News

Craters and Extinctions: Time of Reckoning

Magazine article Science News

Craters and Extinctions: Time of Reckoning

Article excerpt

Like fashion, geology goes through fads. In the 18th century, natural historians believed that a primeval ocean sculpted the landscape. These days, extraterrestrial impacts are all the rage as an explanation for major events in Earth's history.

In keeping with the current style, a team of scientists reports evidence that a giant body slammed into South Africa 145 million years ago, at the close of the Jurassic period. The crash may explain a surge of extinctions among reptiles and marine life at the time, says Christian Koeberl of the University of Vienna.

The impact carved a large crater, now hidden beneath Kalahari Desert sands near Morokweng, in the northwest part of South Africa. Koeberl's group and a South African team, working independently, found the crater while studying gravity and magnetic measurements of the region. They reported their discovery last year but could not tell the crater's age and size at that time.

Koeberl and his colleagues from Australia and South Africa have now dated rocks drilled from the crater. They used two methods, one that measures the radioactive decay of uranium and another that charts the decay of thorium. Their analyses peg the crater's age at 142.8 million to 147.7 million years, they report in the August Geology.

"That is indistinguishable from the currently determined age of the boundary between the Jurassic and Cretaceous periods," says Koeberl. The correspondence raises the question of whether the impact caused the moderate extinctions at that time, he says.

The Morokweng crater ranks as one of the largest on Earth. Judging from the available evidence, Koeberl says that the circular structure measures at least 120 kilometers across. Some data hint that it may be 340 km in diameter, which would make it the biggest crater known. …

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