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Plate Tectonics

The saga of the plate tectonic revolution has been so oft cited that I will not take the time to repeat it here. It is the archetypal scientific revolution that had its roots back in Wegner's theory of continental drift in the 1920s. But plate tectonics was a concept that was poorly represented on the continents, and therefore there was little hope of getting the story straight before the post-World War II era of ocean exploration.

The decade of the 1950s was marked by a total lack of consensus on Earth history. Was the Earth expanding? Contracting? Did continents drift? Remain fixed? In 1959, Americans Harry Hess (from Princeton), Bill Menard, and Maurice Ewing were joined by the Canadian Tuzo Wilson and the British Sir Edward Bullard at an international oceanographic congress in New York City right after the end of the International Geophysical Year. All believed that the mid-ocean ridges were the source of some wholesale motion of Earth' s crust in a manner not compatible with continental drift. The data collected by Ewing and others showed that the mid-ocean ridges were clearly the youngest part of the seafloor. Wilson thought that Earth was expanding along the mid-ocean ridge system, whereas Ewing, Bullard, and Hess believed the ridges to be the rising limbs of thermal convection cells. Hess balanced the expansion with contraction at the trenches and mountain belts. Menard kept the continents in place while the seafloor recycled. After the congress, Hess and Robert Dietz wrote papers revising the notion of continental drift to include spreading seafloor. Most others were skeptical, citing the inability of rising and descending limbs of thermal convection to explain the fact that Antarctica is nearly entirely circled with mid-ocean ridges.

In 1963 came the breakthrough that would allow the concept of seafloor spreading to take a firm hold. Fred Vine and Drummond Matthews of Cambridge University became the first to publish the hypothesis that the puzzling magnetic anomalies in the ocean basins were the result of seafloor spreading combined with aperiodic reversals of Earth's magnetic field. In reaching this conclusion, they relied heavily on evidence just published by Allen Cox, Richard Doell, and Brent Dalrymple (Cox et al., 1964) for reversals of Earth's magnetic field globally recorded in volcanic rocks. This is one clear example of how advances in terrestrial Earth science research helped fuel a great discovery in MG&G. For the most part, however, it was an advantage not to have been too indoctrinated by the theories of terrestrial geologists in order to embrace the new paradigm.

Despite the attractiveness of the Vine-Matthews hypothesis, most Americans were still skeptical. George Backus published a paper in Nature in 1964 that proposed an elegant test of the Vine-Matthews hypothesis. He reasoned that the rate of seafloor spreading should increase from north to south in the Atlantic as a consequence of plate motion on a sphere. It should be simple enough to determine whether the pattern of magnetic stripes in the South Atlantic repeated that already found off Iceland, except with greater thickness to the stripes. His NSF proposal to fund just such an expedition was declined by a panel of his peers as being ''too speculative." NSF would soon prove the validity of the plate tectonic hypothesis, but not through deliberate forethought.

In 1965, J. Tuzo Wilson published a new explanation for the offset of the magnetic lineations across fracture zones. The lineations were offset because the ridge itself was offset (Figure 1). Earthquakes occurred only along the segment of the fracture zone between the two ridges where he predicted, based on seafloor spreading, that crust was moving in opposite directions. Later, Lynn Sykes at Lamont would go on to prove Wilson's hypothesis by showing that the first motions of earthquakes were consistent with this theory.

The tide turned in favor of the acceptance of seafloor spreading with the publication of the Eltanin-19 profile (Figure 2). The Eltanin was a southern ocean research ship owned by the National Science Foundation and operated by Lamont until she was retired in 1973. Walter Pitman, a student at Lamont, was the first, in December 1965, to take a careful look at that profile across the South Pacific and note the nearly perfect symmetry in the magnetic lineations. Eltanin -19 was fortuitous; it was collected in the Southern Ocean near the magnetic pole such that the magnetic anomalies were large and barely skewed. The seafloor spreading history had been steady to first order, with no major plate reorganizations back to 80 million years. Pitman began numbering the magnetic anomalies on a paper record, beginning at the left edge. By the time he got to the mid-ocean ridge, the numbers were large. He quickly realized that this would not do, erased his numbers, and began counting anew from the ridge outward. This original profile now hangs on the wall in John Mutter's office at Lamont. By this time, Cox et al. (1964) had firmed up the magnetic reversal time scale for the first few million years, and the correspondence with the spacing of the anomalies on the Eltanin profile was staggering. By February 1966, Pitman's colleagues at Lamont quickly embraced Vine-Matthews and the other tenets of the new theory. The institution with more than half of the existing magnetic and profiler records from the oceans and 80 percent of the deep-sea cores would from then on be working to help establish the evidence for seafloor spreading.

The conversion of Lamont came just before a National Aeronautics and Space Administration (NASA) conference at Columbia in 1966 on the "History of the Earth's Crust." The papers ultimately presented there bore in many cases little resemblance to the abstracts submitted months earlier. The field was moving too fast. At this meeting, Heirtzler presented the results of the Eltanin surveys. After his talk, Pitman recalls:

Menard from Scripps, who had opposed [continental drift] sat and looked at Eltanin-19, didn't say anything, just looked



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