Geoscience Reference
In-Depth Information
The Officer-Drake paper drew a strong rebuttal from Bevan
French of NASA, an expert on shock metamorphism and the co-
editor of a 1968 classic
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on the subject, who stated: "No shock-
metamorphic effects have been observed in undisputed volcanic or
tectonic structures."
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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.
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Bohor and two colleagues from the U.S. Geological Survey then
reported shocked quartz at seven additional K-T boundary sites.
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They also studied quartz grains in the Toba rocks and found that
1 percent show fractures, contrasted with the 25 percent to 40 per-
cent 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 deforma-
tion 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.
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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."
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Kerr noted that Carter found "no quartz
lamellae whatever in distant Toba ash falls."
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In his published re-
buttal with Officer, however, Carter appeared to reverse himself:
There, their evidence was said to "clearly repudiate
all
[italics theirs]
assertions of Alexopoulos et al."
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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
