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Plates in Motion |
Scientists now held the key to a whole new way of understanding planet Earth. Tuzo Wilson, in an effort to explain seafloor fault lines, was the first to tackle the far-reaching implications of seafloor spreading.
Around the globe, researchers had noted faults--fractures perpendicular to the mid-ocean spreading ridges that cross whole oceans and break the ridges up into segments. When Wilson took up the question, the favored interpretation was that the faults were evidence of the tearing of the ocean crust from edge to edge. The ridges were assumed to have started out as continuous features that were later fragmented and offset by the faults. Wilson disagreed. Yes, the faults were evidence of crustal tearing, but only between the spreading ridge segments, segments that had always been offset. This new view suggested that active deformation is concentrated at the ridges and along their connecting faults and that the rest of the ocean crust simply drifts along, unbroken. Wilson gave the name "plate" to these large masses of moving rock. He further proposed that Earth's surface was divided into about seven large crustal plates and several smaller ones.
Wilson's ideas about oceanic faults and plates were easily tested by the emerging earthquake location data set and Lynn Sykes, working at Lamont, was quick to try this test. Wilson's theories passed with flying colors. Sykes found that oceanic earthquakes were, indeed, concentrated along the mid-ocean ridges and their connecting faults, and that the interiors of the oceanic "plates" were nearly aseismic, or earthquake-free.
Earthquake studies also supplied a crucial step for our understanding of subduction zones. By the 1940s, Kiyoo Wadati in Japan and Hugo Benioff at the California Institute of Technology observed that deep earthquakes were on a plane dipping beneath the ocean floor and were concentrated in areas around the edges of oceans close to volcanoes on land. Studies in the 1950s showed that those ocean areas were also the location of deep trenches, which Harry Hess invoked in his model of seafloor spreading. The deep trenches and the quakes associated with them had puzzled seismologists. Some of these earthquakes occurred very deep in the mantle, where high temperatures should have softened anything rigid so that rocks would flow rather than being so hard and brittle that they fracture easily in earthquakes.
 What changed that view was work by Lamont's Jack Oliver, Bryan Isacks, and Lynn Sykes, who examined earthquake activity in a trench near the South Pacific island of Tonga. Beginning in 1964, they collected seismic data to identify the subterranean sources, or foci, of earthquakes there. They observed, as had Benioff and Wadati, that the foci outlined a plane tilting down from the ocean floor at about 45 degrees. But the Lamont team was the first to recognize that this plane was a slab of descending material cool and hard enough to sustain earthquakes and, moreover, that the slab-containing the seafloor itself-was being bent down into the trench, creating the earthquake zone. The descending slab of seafloor, they determined, had considerable thickness, some 60 miles. What was moving was not just the surface of the seafloor, or the crust alone, but a thicker block. It seemed fair to call this moving block what Wilson had--a plate.
As the 1960s drew to a close, Xavier Le Pichon at Lamont, Dan McKenzie at Scripps, and W. Jason Morgan at Princeton University went on to define the shapes of the contiguous plates and how their movement and location on the globe could be described by elementary spherical geometry, not only for the present but for the past and future, too. In a speech before a convention of his colleagues in 1967, Tuzo Wilson declared that seafloor spreading and plate tectonics "could be as important to geology as Harvey's discovery of the circulation of the blood was to physiology or evolution to biology." |
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