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In 1960, the late Princeton geologist Harry Hess provided an imaginative answer to the puzzle. Refining a rough idea suggested a generation earlier, Hess proposed that the earth's mantle is really a giant convection system. Like hot air in a room, he suggested, material heated by radioactive elements in the earth's interior slowly rises out of a relatively fluid layer of the earth's mantle called the asthenosphere. This lava surfaces at the mid-ocean ridges; hence, the higher water temperatures. As it flows away from the ridges, it hardens into a more rigid layer called the lithosphere. When new lava oozes out, it attaches itself to the older lithosphere and continues to move laterally from the mid-ocean ridges. Millions of years and thousands of miles later, the moving lithosphere plunges back into the earth, carrying its sediment with it and forming the deep ocean trenches found at the edge of continents. Hess's theory at last had provided a workable mechanism needed by continental-drift theorists. It was the sea floor itself that moved. Like giant conveyor belts, the ocean bottoms transported the earth's huge land masses on top of them and spread them apart.
The theory was so unorthodox and tenuous that Hess cautiously called it "geopoetry." It was soon to become geo-fact. After studying Hess's work, a 24-year-old Cambridge University graduate student, Frederick J. Vine, proposed an ingenious test. The iron in the lava from the mid-ocean ridges, he suggested, should be imprinted with the direction of the earth's magnetic field prevailing at the time that the lava cooled off. But patterns in land rocks had already shown that the magnetic field has inexplicably reversed itself as many as 171 times in the past 76 million years. Thus, successive bands of sea floor, gradually spreading out from the mid-ocean ridges, should have recorded each reversal almost as accurately as a huge magnetic tape recorder.
Building the Andes. Vine's recorder provided almost instant playback. Surveying the seabed with sensitive magnetometers towed by an oceanographic vessel, he and other investigators found a zebra-striped pattern of magnetism, its direction repeatedly reversing as their ship moved farther away from the mid-ocean ridges. Seismologists quickly followed with proof of their own. If the sea floor was actually rising from the ridges and dropping back into the earth through the trenches, they reasoned, there should be more seismic shocks in these regions than in surrounding areas. Tests proved them right. The U.S. oceanographic vessel Glomar Challenger has provided even more persuasive evidence. All 135 cores of sediment it has collected from the ocean floor fit into a neat pattern: the farther from the mid-ocean ridge they were drilled, the older they proved to be.
As the evidence piles up, so do the refinements of the Hess theory. Geophysicists are now convinced that the lithosphere conveyor-belt system actually consists of six separate plates that drift on top of the earth's mantle. When they collide, they can build mountains; the Andes were probably created when the Pacific plate wedged under the Atlantic plate, throwing up vast amounts of the overlying continent. When they move apart, they produce quake-prone schisms like California's San Andreas Fault.