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And shocked mineral grains.

. . . and shocked mineral grains

If there were a "smoking gun' for the impact theory, proposed to explain the mass extinctions at the Cretaceous-Tertiary (K-T) boundary, it would be an impact crater. But no such crater has been found. One possible reason for this is that the proposed meteorite or comet landed in the oceans, as has been suggested by many scientists. But Glen A. Izett and Charles L. Pillmore at the U.S. Geological Survey (USGS) in Denver now believe they have evidence of a land-based impact.

Izett and Pillmore found shocked quartz and feldspar gains in the Raton (N.M.) Basin. These grains have been fractured along their crystal axes in the same way that quartz grains found near craters and nuclear explosion sites have been fractured when the shock waves from such events ripped through the crust. Izett and Pillmore's find confirms discoveries by Bruce Bohor, also at USGS, of shocked quartz at the K-T boundary in Denmark, Spain, Montana and New Zealand (SN: 3/31/84, p. 197).

But their find also adds the shocked feldspar grains as well as some compound quartz grains, or fragments of quartz particles that have been fused together, to the impact proponents' arsenal. Moreover, the composition of the feldspar and the occurrence of the shocked compound quartz (rarely found in oceanic crust) strongly suggest that the meteorite or comet smashed into a continent and not the ocean, argues Izett.

Based on a model of how ash is distributed from a volcano, the researchers also use the size of the grains to speculate that North America was the continent hit. Says Izett: "What impresses us is that these shocked metamorphic minerals in the western United States, from Canada all the way to Mexico, are fairly large --0.5 millimeter in diameter--which suggests that the impact may have been fairly close, within a few thousand kilometers of Raton Basin and of Montana and Canada.'
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Title Annotation:shocked grains found to sustain crater impact theory
Author:Weisburd, Stefi
Publication:Science News
Date:Nov 9, 1985
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