Geoscience Reference
In-Depth Information
ing of billions of diamonds at impact sites and K-T locations must
rank as the most surprising and important of the unexpected dis-
coveries triggered by the Alvarez theory.
T
HE
F
IRST
H
URDLE
How well, then, has the Alvarez theory done in meeting its first hur-
dle: being tested against the six predictions? It can be summed up as
follows:
l . The K-T iridium anomaly is found worldwide.
2
. With a few exceptions, the iridium enrichment is not found at
other geological horizons.
3.
Impact does produce distant ejecta deposits enriched in iridium.
•4.
Almost everywhere the K-T boundary itself can be located
around the earth, the boundary clay layer is present. Except for
a few sites the layer is thin.
5
. Accepted indicators of impact—quartz with planar deformation
features, coesite and stishovite, and spherules that resemble
microtektites—are present at many K-T locations.
6.
The impact crater may have been found (see Chapter
7).
In addition, the seventh prediction, of unexpected discoveries,
has not only been met, the surprising findings of soot, amino acids,
meteoritic osmium isotopes, spinel, and finally diamonds, some of
which are difficult or impossible to explain by terrestrial causes, help
to corroborate the theory.
A neutral observer examining this evidence would have to con-
clude that the Alvarezes had a strong initial case for the impact half
of their theory. But when a theory has potentially revolutionary con-
sequences, few observers are neutral. Opponents of the theory imme-
diately began to attack in earnest, arguing that either the tests were
not valid, or, if they were, had failed. In this view, the iridium anom-
aly is not restricted to the K-T horizon; indeed, opponents claimed,
iridium is not a true marker of impact at all. The boundary clay
shows no sign of a meteoritic component; besides, shocked quartz
and spherules are not diagnostic indicators of impact. The critics
claimed repeatedly to have falsified the theory or to have found an
alternative that fulfilled the predictions at least as well and that was
