Geoscience Reference
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and reshaped their surfaces. It may have carved the moon from the
earth. It produced the most energetic event in the last 600 mil-
lion years of earth history, one that led directly to the K-T mass
extinction. Even though it is counterintuitive, our intellect forces us
to recognize that impact has happened thousands, indeed tens of
thousands of times, since the earth cooled (though few impacts
would have been the size of Chicxulub). Could the energy released
by myriad impacts throughout geologic time be the grand unifier of
geology?
R
ECOGNIZING
I
MPACT
Before we get too far out on a limb of speculation, let us ask first
whether there is hard evidence for impact at any mass extinction
horizon other than the K-T. Do any others show an iridium spike,
shocked minerals, and spherules, not to mention spinel, diamonds,
and soot? Do any others have an impact crater of corresponding
age? If the answer to these questions is no, we would have to set
aside the notion that impact, beyond its singular occurrence at the
K-T boundary, has played an important role in earth history. Luis's
prediction would have failed.
Of course, to be prepared to base a judgment on hard evidence
presumes that the indicators of impact, if once present, would re-
main around to be discovered and that they could be detected. Are
these fair presumptions? Not really. Recall that geologic boundaries
were defined, well over a century ago, primarily because they were
easy to spot in the field—they tend to be places where one rock
type abruptly gives way to another. But these are the very places
where erosion has done its work. Almost by definition then, geo-
logic boundaries are apt to be the location of gaps in the rocks: lev-
els at which erosion has removed whatever was present, including
any thin impact ejecta layers.
Another difficulty is that subduction has removed oceanic crust
older than about 125 million years. Any extinction older than that,
which includes four of the Big Five, cannot be found preserved in
cores drilled from the oceanic sedimentary layer, which offers the
most continuous and least disturbed sections. Instead we must seek
these older boundaries in continental rocks, where erosion is more
likely to have removed them.
What about our old friend iridium? If found in high concentra-
tions it is as good an indicator as ever, but the converse is not true:
Low iridium levels do not necessarily rule out impact. First, comets,
