Geoscience Reference
In-Depth Information
CHAPTER
13
GEOLOGY'S GOLDEN AGE
The impact of solid bodies is the most fundamental of all
processes that have taken place on the terrestrial planets. 1
Eugene Shoemaker
A SECOND
REVOLUTION?
In the last three decades, geologists have been asked to accept, in
order, that continents are not fixed in place but, carried on giant
plates, roam over the surface of the earth; that impact is ubiquitous
in our solar system; that thousands of meteorites, some of them
huge, have struck the earth in its history; and that one impact formed
the Chicxulub crater and caused the K-T mass extinction. If the fos-
sil record is periodic, which the evidence strongly suggests it is, geol-
ogists will also be asked to consider the likelihood that several mass
extinctions, and not just one, are due to extraterrestrial impact. Thus
to a greater extent than even the pro-impactors could have imagined
when the Alvarez theory broke, there is strong evidence that major
events in earth history are controlled by forces from outside the
earth. Where do these advances leave what has been the key concept
of geology for a century and a half: uniformitarianism?
Recall from Chapter 2 that the awkward term uniformitarianism
was coined by Whewell to describe Lyell's conception of the earth.
Lyell believed that the only processes that have ever operated are
those that we can observe operating today, which have always oper-
ated at the same rate. As a result, the earth has always looked as it
does now; its history reveals no evidence of directional change. His
uniformitarianism of rate and state was disproven more than a cen-
tury ago and abandoned, but geologists apotheosized Lyell's uni-
formitarianism of process and natural law in the words of James
2 I I
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