Geoscience Reference
In-Depth Information
CHAPTER
7
To CATCH A CRATER
It is of the highest importance in the art of detection
to be able to recognize out of a number of facts which
are incidental and which are vital.
1
Sir Arthur Conan Doyle
he Alvarezes and their supporters had recognized from the start
that finding the K-T impact crater would clinch the impact half of
their theory, but they also knew that the odds against finding it were
high. For one thing, an incoming meteorite has about a 67 percent
chance of striking in the ocean, where, if the resulting crater did not
lie in the 20 percent of the seafloor that has disappeared down the
deep-sea trenches since K-T time, it is likely to have been covered
by younger sediments. If the meteorite struck near one of the poles,
the crater might now be covered with ice. In short, it was far easier
to think of reasons why the K-T crater, if it existed, should not be
found than reasons why it should. Clever detective work, and even
more importantly, good luck, would surely be required.
CLUES
Fortunately, the crater detectives had the benefit of some important
clues. At first, the clues seemed to point toward an oceanic location
for the crater, a worrisome possibility given the likelihood that in
that case it would no longer be visible. As the K-T boundary clay
continued to be studied, however, other evidence began to come to
light that suggested a continental landing site. For example, the
boundary clay contains a small but persistent fraction of broken rock
fragments, the majority of which are granitic, and granite is only
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