Counterpoint in impact debate.

Counterpoint in impact debate

Scientists who believe that powerful volcanoes, rather than a gigantic asteroid impact, led to the extinction of the dinosaurs have a new argument for their side of the debate. It concerns the tiny pieces of shocked quartz and feldspar that have been considered evidence for the impact theory. These minerals are found in the layer of clay that was laid down all over the world 65 million years ago at the boundary of the Cretaceous and Tertiary periods, when the dinosaurs and about three-fourths of all animal species became extinct. The bits of quarts and feldspar contain microfractures that suggest they withstood a sudden and very intense shock--a greater shock than is created by most volcanoes. Therefore, impact theorists have said, they must have been created by an impact (SN:6/7/86, p.356).

But could the microfractures have been caused by exceptionally powerful volcanoes? That's a question nobody ever asked before, according to Charles B. Officer of Dartmouth College in Hanover, N.H., a proponent of the volcano theory. So Officer and three other geologists studied minerals from the depression left by Toba, the monstrous volcano that erupted on Sumatra 75,000 years ago. Toba had 400 times the volume of Krakatoa -- the volcano between Java and Sumatra that erupted in 1883--and it left a caldera more than 50 times larger. In this depression, the geologists found minerals with shock features like those in the Cretaceous-Tertiary boundary layer, according to their report in the May GEOLOGY.

Officer and his colleagues do not conclude that their findings at Toba prove volcanoes deposited the Cretaceous-Tertiary boundary clay. But they say their work casts doubt on the mineral evidence for the impact theory. "This says you can create shock fractures in minerals from an impact or from a large volcanic eruption," Officer says. "So you cannot say, a priori, that if you have shock features, you have an impact."