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Extinction upon impact?

Extinction upon impact?

Pieces of shocked quartz, foundaround the world, present strong evidence that a large meteorite or asteroid struck the earth 66 million years ago and generated a globe-girdling dust cloud that caused a round of mass extinctions and ultimately the demise of the dinosaurs, report researchers in the May 8 SCIENCE.

The grains of quartz, which were liftedfrom sediments at the Cretaceous-Tertiary (K-T) boundary, display microphysical features characteristic of grains found at known impact sites, write Bruce F. Bohor and his colleagues from the U.S. Geological Survey (USGS) in Denver. This resemblance indicates that the quartz was shocked, or fractured, by the impact of a large body, and that it was then lofted into the stratosphere and deposited around the world, says Bohor.

Minerals such as quartz and feldspardevelop these features when high-pressure shock waves exert shearing forces on their crystals, leaving behind parallel, fractured planes called lamellae.

The shocked quartz findings representthe latest volley in a series of debates over the cause of the extinctions that ended the Cretaceous period (SN: 4/18/87, p.248). Other geologists contend that an extended increase in volcanic activity at that time precipitated a range of damaging climatic effects--acid rain, atmospheric cooling, stratospheric ozone depletion--that caused the numerous extinctions.

Much of the recent debate has centeredon whether the discovery of shocked quartz in clay sediments at the K-T boundary proves the impact theory.

Last year, Neville Carter of Texas A&MUniversity in College Station reported finding shocked quartz grains at Toba, a 75,000-year-old volcanic eruption site in northern Sumatra. With this find, advocates of the volcanism theory claimed that the shocked minerals found by Bohor and others at the K-T boundary could be volcanic in origin.

In the SCIENCE paper, the USGS researcherscounter that the volcanically shocked quartz has only single sets of lamellae rather than the multiple lamellae seen in quartz from the K-T boundary and at impact sites. In addition, they say, 25 percent of the quartz at the K-T boundary is shocked, whereas much less than 1 percent of the quartz at Toba is shocked.

For Bohor, these results preclude thepossibility that volcanoes played a significant role in the activities at the K-T boundary, and they prove the impact theory. "We've shown that the material [from the K-T boundary] is shocked. We've shown that it's exactly the same as shocked material around craters . . . I don't think there's any doubt that there was an impact. There just can't be.'

However, Carter and others refuse toend the debate. They claim that volcanic explosions could also have produced the multiple lamellae in the grains that Bohor's group has found.

According to Yale paleontologist JohnOstrom, the extinctions of plant and animal life at the end of the Cretaceous were spread out in time. This indicates, he says, that the major cause of extinctions was not the impact of an extraterrestrial body, which would have ended life more quickly. "For dinosaurs, I think the majority of [paleontologists] probably subscribe to general climatic change rather than impact because there does seem to have been a general decline in both numbers and diversity,' he says. Both an impact and an increased period of volcanism would have contributed to these changes.

Photo: Shocked quartz grain from BrownieButte, Mont., displays multiple lamellae.
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Title Annotation:evidence that meteorite struck earth 66 million years ago and caused mass extinctions provided by discovery of shocked quartz
Author:Monastersky, Richard
Publication:Science News
Date:May 16, 1987
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