Geoscience Reference
In-Depth Information
be false—certainly if all did—the Alvarez theory would have to be
abandoned. On the other hand, if most or all are met, the theory
would be strongly corroborated.
An explosion of research effort followed publication of the ini-
tial Alvarez paper as geologists around the world set to work, some
seeking to confirm its predictions while others tried to refute them
(in principle, intent does not matter as long as the rules are fol-
lowed). Key events in the refinement of the theory were the confer-
ences held at the Snowbird ski resort in Utah in 1981 and 1988, and
in Houston in 1994.
4
The conferences brought together the leading
workers in the new field of impact studies and a variety of other
specialists, proponents of the theory and opponents alike, and pro-
vided a forum for papers and for debate that went on into the wee
hours. For tracing the evolution of the Alvarez theory, the reports
from these conferences are indispensable.
Here are six of the most important predictions made by the
Alvarez theory, followed in each case by the corresponding findings.
PREDICTION
1: Impact effects will
be seen
worldwide at
the
K-T
boundary.
A global catastrophe would leave global evidence. Most if not all
K-T boundary sites around the world will contain an iridium anom-
aly, though the concentration might be greater at sites closer to the
ground zero of meteorite impact. At some locations, however, sub-
sequent geologic processes might have removed iridium or even
eroded the boundary layer entirely away, leaving a gap in the rock
record. Thus although the absence of iridium from a few K—T
boundary clay sites might not falsify the Alvarez theory, were irid-
ium found nowhere other than in Italy and Denmark, the theory
would be in trouble.
FINDINGS
By the time of the first Snowbird Conference in 1981, only a year
after the original paper in
Science,
the number of sites with confirmed
iridium anomalies had risen to 36. By the end of 1983, it had reached
50; by 1990 it had climbed to 95; today it is well over 100. Iridium
concentrations in the boundary clays are the highest ever measured
in terrestrial materials. Only a few K-T sections lack iridium.
One site was of critical importance, for it was the first in which
the rocks studied had been deposited not in seawater but in fresh.
Some had claimed that impact of a meteorite was not the only way
to get iridium into a rock layer. Seawater contains trace amounts of
the element; perhaps there were processes that could somehow
