S Hocked M Inerals

Bruce Bohor's '98' discovery of shocked quartz (previously found only at known impact craters and at the sites of nuclear explosions, in the K-T boundary clay in the Hell Creek area of Montana, home of T. rex) convinced many geologists that impact was a reality. Glenn Izett of the U.S. Geological Survey, who wrote the definitive paper on the K-T section in the Raton Basin, spoke for them: "I started off as a nonbeliever. What got me was the appearance of these shocked minerals at the K-T. In the impact bed, you see grains everywhere that have these features in them. Just a millimeter or two below, you'll never see any of those features."25

Unshocked quartz has no fracture planes; quartz deformed in other geologic settings than impact sites sometimes has single sets. The multiple sets of crisscrossing planes illustrated in Figure '2, however, are diagnostic of great shock. The planes actually are closely set layers of glassy material precisely oriented to the crystal structure of quartz. Officer and Drake took exception to the claim that shocked quartz was diagnostic of impact: "The presence of lamellar quartz features [the parallel planes] does not in and of itself demonstrate a meteor impact origin."26 As evidence, they stated: "Lamellar features . . . are also a characteristic of both normal tectonic [mountain building] metamorphism and shock metamorphism, although the normal tectonic features are quite different from the

FIGURE I 2 (Top) Unshocked quartz from an explosive volcanic rock in the Jemez Mountains, New Mexico. Note the absence of shock planes. (Right) Shocked quartz from the K-T boundary in the Raton Basin, Colorado, showing two sets of shock planes. [Photo courtesy of Glenn Izett.27]
Shocked Quartz Definition

shock features," meaning that shocked quartz is produced not only by meteorite impact, but by such familiar geologic processes as metamorphism."

Officer and Drake cite as evidence the presence of shocked minerals at the giant, ancient structures of Sudbury and Vredefort, which they say are of internal, nonimpact origin, and conclude on that basis that shock features do not "demonstrate a meteor impact origin."29 They state two premises and use them to draw a conclusion: (1) Sud-bury and Vredefort were not formed by impact but are of internal origin; (2) both contain shocked minerals; therefore (3) shocked minerals are not diagnostic of impact. If this technique seems familiar, it is. It has a history among debaters and rhetoricians extending all the way back to Aristotle. If the original statement in such a three-step syllogism is itself false, however, then the chain of logic breaks down and the final conclusion may be false. If Sudbury and Vredefort are of impact origin, then the conclusion in step 3 could be false (it could also be true but there is no way of knowing from this logic). Based on several lines of evidence, geologists now believe that meteorite impact did create both structures. At least it is sufficiently likely that Sudbury and Vredefort were formed by impact that they cannot be cited as evidence that the recognized impact markers, such as shocked quartz, are not diagnostic of impact.

The Officer-Drake paper drew a strong rebuttal from Bevan French of NASA, an expert on shock metamorphism and the coeditor of a 1968 classic30 on the subject, who stated: "No shock-metamorphic effects have been observed in undisputed volcanic or tectonic structures."31 Officer and Drake and their co-workers soon responded, pointing to the giant caldera of the ancient volcano Toba, on the island of Sumatra, near Krakatoa but 50 times larger, which last erupted about 75,000 years ago. In 1986, Neville Carter of Texas A & M, an expert on shocked quartz, along with Officer and others, reported that feldspar and mica in the Toba volcanic ejecta contain microstructures resembling those produced by shock, although the structures were rare in quartz from Toba, and were not the multiple, crisscrossing sets known as planar deformation features.32

Bohor and two colleagues from the U.S. Geological Survey then reported shocked quartz at seven additional K-T boundary sites.33 They also studied quartz grains in the Toba rocks and found that 1 percent show fractures, contrasted with the 25 percent to 40 percent of quartz grains in a typical K-T boundary clay that show the fractures. More importantly, the deformed quartz from Toba exhibits only single sets of parallel planes, not the multiple planar deformation features characteristic of impact shock.

A group of Canadian geologists compared quartz grains from Toba, a known impact site, the K-T boundary clay, and two sites known to have undergone tectonic deformation. They found that the appearance and orientation of planar features from the known impact structure and those observed in samples from the K-T boundary were essentially identical. They concluded that although other lamellar deformational features in quartz can result from other geologic processes, these features only superficially resemble those from the K-T boundary and those believed to have resulted from impact.34

According to an article by Richard Kerr, a reporter for Science magazine who has covered the meteorite impact debate from its inception, Neville Carter agreed, saying that "there is no question that there is a difference."35 Kerr noted that Carter found "no quartz lamellae whatever in distant Toba ash falls."36 In his published rebuttal with Officer, however, Carter appeared to reverse himself: There, their evidence was said to "clearly repudiate all [italics theirs] assertions of Alexopoulos et al."37

Such claims did not persuade Kerr, who summed up: "Try as they might, advocates of a volcanic end to the Cretaceous have failed to find the same kind of so-called shocked quartz grains in any volcanic rock. Because shocked quartz continues to maintain its exclusive link to impacts, the impact hypothesis would seem to be opening its lead over the sputtering volcanic alternative."38

In '99', Officer and Carter published a lengthy review paper on what they called "enigmatic terrestrial structures."3' Although they did admit that some of the cryptoexplosion structures studied by Walter Bucher and others were due to meteorite impact, they concluded that the Sudbury and Vredefort structures, and several others whose origin had been disputed, were of "relatively deep-seated," that is, internal, origin. One of the features presented as of internal origin was the buried structure at Manson, Iowa, which as we will see became a prime contender for the K-T impact site and is now firmly regarded as caused by impact. After an extensive review of shock metamorphism, Officer and Carter wound up their argument:

Perhaps the widely held, but erroneous, belief that only multiple sets of these features are diagnostic of shock deformation has resulted from their many recent illustrations at the Cretaceous/Tertiary boundary. However, surprisingly, this myth has also been promulgated recently by workers with extensive experience with dynamic deformation of quartz. The experimental work summarized above should be sufficient to convince the critical, unbiased reader that single sets of planar features are just as diagnostic of shock deformation as multiple sets. This information combined with the observation that single sets are just as common as are multiple sets in naturally shocked quartz should finally put this nontrivial matter to rest.40

It appears to be true that single sets of deformation planes indicate some level of shock. As the debate went on, single sets were found in other volcanic rocks, including ash from Mount St. Helens. But it is also true that the multiple, crisscrossing sets of deformation lamellae—the planar deformation features—found repeatedly in quartz at the K-T boundary, occur only at known impact structures, in high-pressure laboratory experiments, and at the sites of nuclear explosions. Volcanic rocks do not contain them, and therefore the multiple sets of planes remain diagnostic indicators of a higher level of shock than produced by volcanism—or, as far as we yet know, by any internal process. To quote three experts: Impact-shocked quartz and quartz altered by other terrestrial processes "are completely dissimilar . . . due to the vastly different physical conditions and time scales. . . . Well characterized and documented shock effects in quartz are unequivocal indicators of impact."4'

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  • russell
    Are planar deformation features found only in impact sites?
    9 years ago

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