Signs of an ancient worldwide wallop.

Signs of an ancient worldwide wallop

Years, perhaps decades, will pass before scientists agree on a basic theory to explain why a vast number of species went extinct some 66 million years ago. Now entering the debate is one more piece of evidence suggesting an extraterrestrial body hit the Earth at that time, which is known as the Cretaceous-Tertiary (K-T) boundary.

John McHone and his colleagues at Arizona State University in Tempe report in the March 3 SCIENCE that they have detected a mineral called stishovite in samples collected from the K-T boundary at Raton, N.M. McHone says natural stishovite has not been found anywhere on Earth except at sites connected with impacts, and its presence at the K-T boundary proves that at least one body, such as a comet or meteorite, struck the planet then. "Stishovite just clinches it," he says. "There was an impact."

The researchers detected the minute amounts of stishovite through two techniques, nuclear magnetic resonance and X-ray diffraction. In the past, scientists studying the K-T boundary have found only hints of this mineral.

Luis and Walter Alvarez started the modern K-T debate in 1979 when they first proposed an impact caused the mass extinctions that extinguished the dinosaurs, among others. They raised the theory to explain high concentrations of the element iridium they had found in a thin layer of clay at the K-T boundary. Iridium is rare in the crust but is concentrated in the deep earth and extraterrestrial objects.

The Alvarez group suggested the colliding body would have vaporized on impact, and hurled up iridium-rich dust that blacked out the world. Settling back to the surface, the dust would have formed the global clay layer seen at the K-T boundary. Researchers have since found more impact evidence at the boundary, such as mineral grains deformed by a high-pressure shock wave.

Another group argues instead that an intense period of volcanic eruptions altered the Earth's climate and caused species to disappear, some abruptly and some perhaps gradually over millions of years. As evidence for the volcanic theory, these scientists have maintained that volcanoes can bring iridium-rich rock from the Earth's mantle to the surface. Violent eruptions may also produce limited kinds of shocked mineral grains (SN: 4/18/87, p.248).

Many scientists believe, however, that volcanoes cannot produce stishovite, a dense form of silica formed by extreme pressures. The important thing about stishovite is that it breaks down when heated. Even temperatures as low as 300[deg.]C, if prolonged, will make the mineral revert to a less dense form of silica, so it cannot survive in a volcanic environment, McHone says.

The stishovite discovery has convinced some scientists but not all. "I think McHone's find is very important. But to take it as a confirming nail in the coffin is far too preliminary," says Neville Carter, an expert in shocked minerals at Texas A&M University in College Station. In any case, the new information does not address how an impact affected life. Much evidence suggests an impact did not act alone in causing the extinctions.