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Primer: Plate Tectonics April 22, 2009

Posted by Will Thomas in EWP Primer.
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A map showing differing magnetic polarizations in rock on the Juan de Fuca plate with colors indicating age; from the United States Geological Survey.

During the 18th century, broad theories of the earth (such as that proposed by Jean-André de Luc, which I discussed a few weeks ago), attempted to account for a wide array of phenomenona, such as the origins of mountains, the origins of different rock strata, the presence of marine fossils on land, and so forth.  By the end of that century such wide-ranging and speculative theoretical “systems” had fallen into a degree of disrepute as a useful learned activity, and a more disciplined and less narratively ambitious geology slowly gained precedence.  Nevertheless, the questions asked by 18th-century savants remained valid, and varying theories of the earth’s history remained in circulation, with answers to many key questions remaining in flux until well into the 20th century.

Following World War II, the theory of the German meteorologist and paleoclimatologist Alfred Wegener (1880-1930), that continents drifted over the face of the globe, had fallen largely by the wayside.  Wegener had proposed his theory early in the 20th century to account for climatic changes in the earth’s distant past, for fossil similarities across continents, and for mountainous features of the earth’s crust, which required a new explanation after the decline of the cooling earth theory circa 1900.  Wegener’s theory—the most prominent of several proposed drift theories—had its sympathizers, some very well-respected, but the theory was not widely accepted, and geologists in North America were outright hostile to it, many assuming by mid-century that it had disappeared into the realm of crank science.

In fact, continental drift was never fully given up to the cranks, and in the 1950s and 1960s, new evidence and mechanisms for it were developed, which made a convincing enough case that it not only revived the fortunes of continental drift, but swiftly overcame all competition.  “Plate tectonics” arose at the confluence of a number of different (more…)