Geoscience Reference
In-Depth Information
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."
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In 1991, Officer and Carter published a lengthy review paper
on what they called "enigmatic terrestrial structures."
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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 experi-
mental work summarized above should be sufficient to convince the criti-
cal, 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 nat-
urally shocked quartz should finally put this nontrivial matter to rest.
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It appears to be true that single sets of deformation planes indi-
cate 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 dis-
similar . . . due to the vastly different physical conditions and time
scales. . . . Well characterized and documented shock effects in
quartz are unequivocal indicators of impact."
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