Geoscience Reference
In-Depth Information
thousand years. On a geologic time scale, even these are instanta-
neous. Thus the boundary layer will be thin everywhere except, per-
haps, at sites closer to ground zero. The layer ought to be found
globally, though erosion might on occasion have removed it. If a thin
layer is found worldwide at the K-T boundary, it would be the first
universal geologic marker—rock formations ordinarily are no more
than regional.
FINDINGS
Around the world, the K-T boundary is marked by a thin clay layer,
almost always with high iridium levels. (As we will see, in North
America there are two boundary layers, with the thicker one on
the bottom.] No other rock unit extends over even a single conti-
nent, much less over all of them and the seafloors in between. The
very existence of this universal layer is evidence of a rare, perhaps
unique, geologic event, and is as strongly corroborative a piece of
evidence for the Alvarez theory as any.
PREDICTION 5:
The K-T boundary clays will contain shock
metamorphic effects.
Known markers of impact—shocked quartz grains; coesite or
stishovite; glassy, tektitelike spherules—will be found in the bound-
ary clays. The presence of these accepted indicators would provide
much stronger corroboration to doubting geologists than the irid-
ium spike, which prior to the Alvarez discovery was unrecognized as
an impact marker.
FINDINGS
In 1981, geologist Bruce Bohor of the U.S. Geological Survey
decided to look for shocked quartz at the K-T boundary and
applied for a Survey fellowship (ironically named in honor of G. K.
Gilbert). Turned down by the fellowship panel (which included a
specialist in shocked quartz), Bohor reapplied, only to be rejected
again. Showing admirable resolve, he went ahead on his own and
shortly did locate shocked quartz at the K-T boundary in a 1 cm
thick Montana claystone that also contained both a large iridium
spike and a pollen extinction.
9
Bohor's discovery was crucial in
making believers out of many geologists. First of all, one of their
own, rather than a know-it-all physicist, had made the discovery.
Second, instead of being based on an invisible element, shocked
quartz was a tried-and-true indicator that geologists had discovered
