A cataclysm 250 million years ago wiped out nearly all life in the Earth's oceans, and nearly three-quarters of the plants and animals on land vanished too. It was the greatest catastrophe the Earth has ever experienced - - but scientists who study such events are in sharp disagreement over what caused it. Was it the crash of a giant asteroid or meteorite that killed off so much life? Or was a violent surge of volcanism from deep within the Earth the deadly factor? The argument over sketchy evidence from the long-ago geologic time called the "End Permian" persists, and the contentious scientific debate will continue this week when the American Geophysical Union meets at Moscone Center, with nearly 11,000 scientists in attendance. Scientists do agree that the mass extinction was sudden: The Earth is known to be at least 4 billion years old, and "the Great Dying," as paleontologists call it, may have lasted less than 200,000 years from start to finish, a mere moment in geologic time. In the most recent stage of the controversy, teams of researchers have squared off in support of two opposing theories to explain what triggered the disaster. One international research group, led by Christian Koeberl of the University of Vienna and Kenneth Farley of the California Institute of Technology, is arguing that there is no hard evidence at all to support the impact theory. Newly discovered chemical signs in the Austrian Alps and the Italian Dolomites, where elements typical of asteroids or meteorites are almost nonexistent, is one key to their argument.

Koeberl and Farley, as well as many other geochemists, say the extinction was more likely due to an immense outpouring of lava in the northern part of a once huge super-continent known as Pangaea. The remains of that event can be found today in a vast surface region of basaltic rock in northern Russia known as the Siberian Traps. The volcanic violence would have induced abrupt global heating, throwing up a dark pall of hot ash, toxic gases and carbon dioxide that virtually no living plants or animals could survive, according to this theory. The darkness of the skies then would have caused a major period of global cold.

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