World renowned University of the Witwatersrand (Wits) paleo-anthropologist Philip Tobias told a public lecture on human evolution here that the Vredefort crater, which is between 250 and 300 kilometers (150 and 190 miles) in diameter, was long thought to be of volcanic origin.

However, geologists, led by Wits professor Uwe Reimold, had recently shown that the structure was caused by the impact of an extra-terrestrial object such as an asteroid or comet, Tobias said Wednesday.

"It is the largest impact structure that has yet been identified on planet Earth. It exceeds even the Sudbury crater in Ontario, Canada, which is about 200 kilometers (125 miles) in diameter," he said.

"Not only is the Vredefort crater the biggest so far identified on Earth, but it is the oldest. It has been dated to 2.1 billion years."

The town of Vredefort is situated at the centre of the crater, which is bigger than the Chicxulub depression made by an asteroid or comet at Yucatan, in Mexico. The Chicxulub collision is thought to have caused the extinction of most of the dinosaurs.

"Like Chicxulub, Vredefort may well have been a major catastrophe whose worldwide consequences had an enormous impact on the history of life on Earth," Tobias said.

Reimold told AFP that the way minerals in the crater had been deformed indicated the indentation could not have been caused by volcanic activity.

Initial estimates, he added, indicated that the crater could have been caused by a comet or asteroid five to 10 kilometers (three to six miles) in diameter, travelling at a speed of between 40,000 and 250,000 kilometers (25,000 and 150,000 miles) an hour.

According to Reimold, the impact on the Earth's atmosphere could well have caused a setback of a few million years in the evolution of life as it existed at the time.

The crater is so old and eroded, however, that it is difficult, except in a few places, to see the effect of the impact with the naked eye.

Scientists at both Wits and Potchefstroom, southwest of Johannesburg, who are working jointly on the project, say many studies remain to be undertaken at Vredefort.

The site has been known since the turn of the century, but only dated in 1996, and only in the past few years has it been suspected that it was caused by a collision.

Copyright 1999 AFP. All rights reserved. The material on this page is provided by AFP and may not be published, broadcast, rewritten or redistributed.

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