Geoscience Reference
In-Depth Information
young, that the continents have never been in the same place twice—
all proved hard for those raised on uniformitarianism to accept.
After a generation to get used to plate tectonics, geologists have
incorporated it into uniformitarianism. Because we can measure with
laser beams, satellites, and global positioning systems the almost im-
perceptible movement of continents and the spreading of the sea-
floors, and can use the data to project backward to determine what
the surface of the earth used to look like, the present can still be said
to be the key to the past. Indeed, the way in which plate tectonics
shows how older crust is buried in the mantle and recycled into
new crust is reminiscent of Hutton's endless cycles. But Hutton and
Lyell would certainly have rejected continental drift as impossibly
antiuniformitarian. In any event, to say that one can infer the past
positions of continents from their present positions and measured
rates of motion is to appeal only to methodological uniformitarian-
ism, which, as we have seen, is only to say that geologists proceed
scientifically.
But meteorite impact as a force on the earth takes us into a new
realm. Since we have never observed a large meteorite striking the
earth, yet the existence of terrestrial craters tells us that they have,
we cannot understand earth history by relying solely on processes
that we can observe today. In short, the present is
not
a reliable key
to the past. Just the opposite: To understand the role of impact cra-
tering, we have to invert Hutton's aphorism and realize that, in the
case of an event so rare as to fall outside human experience, the past
must provide the key to understanding the present and the future.
By the time the Alvarez theory appeared in 1980, the space age
had brought overwhelming evidence that impactors of every size
had hit every object in the solar system countless times. We could
of course stretch definitions to recognize the ubiquity of impact
and claim that it amounts to a kind of uniformity, but this is equiv-
alent to saying, "catastrophism is uniformitarian," an abominable
oxymoron that would empty both words of meaning. As Ursula
Marvin has pointedly said, "To regard the cataclysmic effects of
impact as uniformitarian is an exercise in 'newspeak.'"
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Walter Alvarez drew the right conclusion about the proper place
for uniformitarianism in geological thinking: "Perhaps it is time to
recast uniformitarianism as merely a sort of corollary to Ockham's
Razor, to the effect that if a set of geological data can be explained by
common, gradual, well-known processes, that should be the explana-
tion of choice, but that when the evidence strongly supports a more
sudden, violent event, we will go where the evidence leads us."
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