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Insights from the Ocean |
Despite the magnetic evidence that the continents had moved over geological history, proponents of the theory were still lacking proof of a mechanism. However, additional support was gathering on the high seas.
With the help of war-driven improvements in the technique of echo location, or sonar (for sound navigation and ranging), for submarine detection, Harry Hess, a geologist at Princeton University, aided the U.S. military with one hand and geoscience with the other. As the commander of an attack transport during the World War II, Hess had the most powerful model of an echo sounder available, and he ran it almost constantly during his Pacific missions, intent on expanding on what little was known about the configuration of the seafloor. By sending out pulses of sound and listening for return echoes from the seafloor, a sonar instrument can measure the distance between a ship and the ocean bottom. Hess combined measurements from several passages to produce a rough contour map of the seafloor and in the course of his war service discovered and mapped about 100 flat-topped underwater mountains. Later, back at Princeton, Hess theorized that these mountains originated as pointed-top volcanoes that had been subsequently flattened by erosion. This led him to consider the life cycle of underwater mountains, an interest he pursued throughout the 1950s.
Meanwhile, Columbia University had become the home base for a vigorous marine geology research program, headed by Maurice Ewing. During the early 1950s, the research ships of Columbia's Lamont Geological Observatory (now named Lamont-Doherty Earth Observatory) collected numerous depth soundings taken across the Atlantic Ocean, and in 1952 the Lamont researchers began assembling a map derived from these soundings.
One of the features of the Atlantic seafloor known since the mid-1870s was an undersea mountain range called the Mid-Atlantic Ridge. Rising from a broad flat plain to either side, the range had peaks that jutted up 10,000 feet from the ocean floor. The Lamont researchers discovered startling new information about it, however. Not only was the Mid-Atlantic Ridge high, it was long. Running some 9,000 miles, nearly the full length of the ocean from north of Greenland to south of Africa, the ridge stretched farther than the Rocky Mountains and the Andes combined. The Lamont researchers also found that the crest of the ridge system is virtually bare of sediments, as compared with the thick layer of sediment on the plains near the continental margins, which can reach a thickness of several miles. Perhaps the most unexpected feature of the Mid-Atlantic Ridge was the deep valley running down its spine. This rift, as it is called, descended an average of 6,000 feet from the ridge crest and ranged in width from 8 to 30 miles--dimensions that could comfortably contain the Grand Canyon of the Colorado River, which is at most about 18 miles wide. Bottom samples taken from the rift revealed the seafloor there to be made of extremely young, dark volcanic rock.
A map of the North Atlantic showing features of the midocean ridge was published by Lamont researchers Bruce Heezen, Marie Tharp, and Maurice Ewing in 1959. As it happened, echo soundings made elsewhere had produced similar profiles of the seafloor throughout the world, and now an extraordinary pattern emerged. The soundings revealed the entire midocean ridge system to be 37,200 miles in length, long enough to circle the equator one and one-half times. It was one of the planet's three dominant physical features, along with the continents and the oceans themselves. Researchers had also mapped a system of deep trenches--the deepest parts of the ocean basins--that nearly ring the Pacific Ocean and are found at the northeastern boundary of the Indian Ocean.
These were fundamental new discoveries, and Harry Hess, who had kept abreast of new data about the seafloor, was eager to explore their implications. In 1960, he gleaned from Bruce Heezen the idea that Earth was "coming apart at the seams"--that is, at the ridges. Given the youthful nature of the samples from the rift bottom, Heezen argued that volcanic rock--magma--was welling up from below the crust. From this hint of a mechanism that could explain the midocean ridges, Hess developed a new synthesis of earth science in his well-known 1962 paper, "The History of Ocean Basins." Although he characterized the essay as "geopoetry," as if to warn other scientists that not every concept could be proven, the work stimulated thinking in the field.
Echoing the insights of seismologists, Hess posited a multilayered interior for the planet. By this time researchers had refined their notions of Earth's innermost structure. Rather than a single iron core, they now described a solid inner core of iron and a fluid outer core of metal alloy, mostly iron. Surrounding this was the mantle, overlain by the outermost thin oceanic and thick continental crust. Hess then elaborated on the evolution of the planet's architecture. The crust was made up of iron-poor rock that had risen to the surface when radioactive decay heated and melted rocks in the interior of the newly condensed planet; this crust had once formed a single continental land mass. In the presence of continued heating in the planet's interior, a convection "loop" of rising and sinking material would be created in the mantle--just as Arthur Holmes had suggested in 1929.
Hess theorized that once the planet had formed, this mantle convection subdivided into numerous separate circulating loops extending from the core. Where the currents rise to the surface, molten material oozes out, building up the midocean ridges and forming new oceanic crust; as magma continues to flow, older seafloor is carried away in either direction from the ridge by mantle convection. Where the convection currents descend, cooled older oceanic crust plunges back into the mantle at deep ocean trenches.
Hess thus subordinated the configuration of the oceans and continents to the motions of the spreading, moving seafloor. While seafloor spreading was a compelling vision, it seemed untestable. Hess was proposing that it occurred at about the rate at which a fingernail grows. The proof would have to come indirectly--from magnetism, as it turned out.
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