The impact of solid bodies is the most fundamental of all processes that have taken place on the terrestrial planets.' 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 fossil record is periodic, which the evidence strongly suggests it is, geologists 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 operated 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 century ago and abandoned, but geologists apotheosized Lyell's uni-formitarianism of process and natural law in the words of James
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