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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 developments in the study of the sea floor, in theorization about the magentization of the earth, in gravitational and geomagnetic measurement, and in the study of magnetism in rocks.

The first pieces fell into place in the first half of the 1950s as British physicists Patrick Blackett, Teddy Bullard, and Keith Runcorn undertook investigations of the nature of the earth’s magnetic field.  Suspecting that the field was transitory, they sought evidence of shifts in the field over time preserved in the magnetic fields of volcanic rocks.  Finding fields from similarly aged rocks on different continents pointed in different directions, they concluded that continents had drifted.  The evidence, however, was inconclusive, as some rocks produced incoherent patterns, while others seemed to be magnetized in the opposite direction.

In the early 1960s, groups at Berkeley and the Australian National University determined that “normally” and oppositely magnetized rocks could be placed in coherently dated groups, confirming suggestions made decades earlier by researchers in France and Japan, and again in the 1950s in Iceland.  This finding allowed a paleomagnetic history of the earth to be assembled, charting points in time when the earth’s magnetic field reversed itself.

In 1963, to explain maps of sea-floor magnetization off the northwest coast of the United States produced two years earlier by Scripps Institute of Oceanography scientists (see the illustration above, with age data superimposed), Canadian geophysicist Lawrence Morley—who could not get the theory published—and, independently, Cambridge geophysicists Fred Vine and Drummond Matthews suggested the striped patterns were produced by a spreading sea floor.  As volcanic activity produced new rock, it recorded the polarity of the earth’s magnetic field at that time.

While the evidence lining up in favor of continental drift theory was producing converts, in general the Vine-Matthews-Morley theory was not immediately well-received.  Over the next several years, however, new supporting theories and evidence would add up to convert the reluctant, such as the researchers at the Columbia University-based Lamont Geological Observatory.  Further physical and magnetic mapping of the Atlantic sea floor by Lamont researchers compiled further global evidence of sea floor spreading.  Canadian geophysicist Tuzo Wilson interpreted faults cutting across oceanic ridges as the jagged edges (“transform” faults) of larger faults (identified as thick dotted lines in the diagram above) rather than as the lateral displacement (a “transcurrent” or “strike-slip” fault) along a once-straight ridge.  Seismic analysis performed by Lamont’s Lynn Sykes in 1967 lent credence to Wilson’s interpretation consistent with spreading sea floors.

Beginning in the last few years of the 1960s, the system of the earth’s plates and their “tectonics” were worked out, quantitative and predictive models were developed, and plate tectonics became a central explanatory mechanism in understanding the history of the earth’s geology and climate, the nature of many geological phenomena, and the patterns of habitation of the life forms that have lived on the earth.

The history of the acceptance of continental drift and plate tectonics represents a substantial reconceptualization of geological knowledge at a late date in history.  Scholars have written a number of book-length histories on the subject in the past twenty years, all of which use the history as a window onto broader philosophical and sociological problems of rejecting and accepting theories.   The history makes clear how important it is for historians and philosophers to take care in considering at what point it becomes “unfair” or “ignorant” to doubt or reject a theory, as well as whether there is a “proper” moment to undergo a conversion from one scientific point of view to another.

See Naomi Oreskes’ The Rejection of Continental Drift: Theory and Method in American Earth Science (1999), sociologist John Stewart’s Drifting Continents and Colliding Paradigms: Perspectives on the Geoscience Revolution (1990), and Homer LeGrand’s Drifting Continents and Shifting Theories (1988).  Also see the volume Plate Tectonics (2001), a collection of recollections by participants edited by Oreskes with LeGrand, and with a short historical introduction by Oreskes.  The books differ in their styles, emphases, and in the characterization of details, but the broad outlines of the history are well-known, and offer more proper names and episodes in a complex history than can be fit in here.



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