Geoscience Reference
In-Depth Information
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
]
shock features," meaning that shocked quartz is produced not only
by meteorite impact, but by such familiar geologic processes as
metamorphism.
2
8
Officer and Drake cite as evidence the presence of shocked min-
erals 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."
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9
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 min-
erals 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.
