Craters and extinctions: time of reckoning.

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.

"I think it's really exciting," comment C. Wylie Poag of the U.S. Geological Survey in Woods Hole, Mass. "it will give people an incentive to look more carefully at the Jurassic-Cretaceous boundary."

Poag notes that the connection between the crater and the extinctions is far from airtight. Geologists do not have a firm idea of exactly when the Jurassic ended because they have not yet dated the boundary using radiometric techniques. Instead, they have estimated an age of 145 million years, using dates of rocks above and below the boundary. That means the impact could have preceded or postdated the extinctions by several million years.

Geologists have much better evidence that a comet or meteorite knocked life for a loop 65 million years ago, at the boundary between the Cretaceous and Tertiary periods. The crash left a global layer of debris and a 180- to 280-km-wide crater beneath the Yucatan Peninsula. The age of this impact coincides with the disappearance of the last dinosaurs and half of the genera alive in the Cretaceous (SN: 3/1/97, p. S20).

At other times in Earth's history, impacts seem to have had little effect. Last week, Canadian and Russian geologists reported that two large bodies hit nearly simultaneously in the late Eocene epoch, but they apparently did not decimate life.

In the July 24 Nature, Richard Bottomley of Canadian Union College in College Heights, Alberta, and his colleagues dated a Siberian crater called Popigai at 35.7 million years old. That time falls within a few hundred thousand years of the age of a crater beneath the Chesapeake Bay. "Right now, there doesn't appear to be any dying out associated with these impacts," says Bottomley. The nearest major extinction came some 2 million years later, at the end of the Eocene epoch.