Geopoetry Becomes Geofact

It is apparent from a glance at the map: the earth's continents can be made to fit together like parts of a giant jigsaw puzzle. Yet for years most scientists stoutly resisted the conclusion that some time in the remote past, the earth's land masses began splitting up and drifting apart--a theory known as "continental drift." Sheer nonsense, they insisted. No known forces could possibly have propelled huge continents across the earth's dense, basaltic crust.

Discoveries of the past few years have now turned skeptics into believers. Scientists have not only uncovered the basic mechanism that moves continents, but are becoming increasingly convinced that it provides explanations of many fundamental mysteries about the earth. What causes earthquakes? How are mountain ranges formed? The answers have thoroughly jolted the once staid earth sciences. "It's a revolution," says Oceanographer Melvin Peterson of the Scripps Institution of Oceanography. "We are riding the crest of a breaking wave."

In making their case for continental drift, scientists used some obvious evidence: geological strata in South America that matched those of West Africa in areas where the continents might have been joined; fossils of plants and animals from one continent that were identical to those on another, even though they are separated by thousands of miles of ocean. Just off Antarctica's Ross Ice Shelf, for example, an Ohio State University expedition recently discovered the fossilized bones of a hippopotamus-like reptile called Lystro-saurus that had been thought to live only in prehistoric South Africa and Asia.

Missing Deposit. More subtle clues came out of the ocean depths, although few scientists at first recognized their importance. By 1959, Columbia University's marine geologists had completed charting a mysterious 47,000-mile-long chain of undersea mountains and ridges that twist through the middle of the major oceans like the seams of a giant basketball. Strangely enough, the water temperatures near these ridges are considerably higher than those elsewhere in the ocean. In addition, there are ocean-floor fault lines or cracks that were apparently caused by movement of the seabed itself. Although the oceans are billions of years old, sonar measurement of the depth of accumulated sediment indicated that the sea floor is no more than a few hundred million years old; the deposit of sediment should be much thicker.

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.

Using their newfound evidence to trace the drift of land masses, some scientists believe that the present continents resulted from the breakup of two su-percontinents. They call one Gondwanaland (after a region in India that is rich in African-type fossils); it presumably once consisted of the Indian subcontinent, Africa, South America, Australia and Antarctica. The other is Laurasia, the progenitor of North America, Greenland, Europe and Asia. About 250 million years ago, at the beginning of the age of dinosaurs, these land masses began to break up and drift apart. North America moved to the west, Europe and Asia toward the east. Africa remained largely stationary, while South America headed west and Australia edged northward away from Antarctica. Where it will all end, no one can be certain. The continents are still drifting --at rates of two or so inches a year --and show no signs of slowing down.

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