From the New York Times News Service by Walter Sullivan, New York Times Science Editor Art

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From the New York Times News Service by Walter Sullivan, New York Times Science Editor Article appeared in _The Oregonian_, Wednesday, July 14, 1993 Abstract: Scientists find that giant plumes of hot rock emerge to form strings of oceanic islands or to blanket continents with huge sheets of lava. ------------- Begin article ------------- Exploring Earth's Great Heat Engine Geophysicists have recently made striking progress in understanding the mechanism of the great heat engine within the Earth. Over the last several months they have reached a rough consensus that the vast plates that comprise the ocean floor sink deep into the Earth's mantle, only to be recycled into giant ascending plumes of hot rock. The plumes, driven, some believe, by heat from the Earth's core, emerge to form strings of oceanic islands such as the Hawaiian chain or to blanket continents with huge sheets of lava. The discoveries, said Michael E. Wysession, a professor at Washington University in St. Louis who is an authority on the Earth's interior, constitute "a second revolution." The revolution is comparable in scientific terms to the theory of plate tectonics, which came to be accepted three decades ago, showed that the continents had been in constant motion over the course of geological time, carried along by great plates. The new theory explains a lot of what is happening beneath the great plates. It reveals "the other half of the picure," Wysession said. The plate tectonic theory recognized that the slowly moving slabs of ocean floor eventually cool and plunge into the Earth's mantle, disappearing in deep trenches like those that lie along the west coast of South America and the southern arc of the Aleutian Islands. But the plates' fate thereafter was uncertain. The new theory holds that the plates, carrying a burden of oceanic sediment built up over many millions of years, get recycled within the mantle into a system of giant plumes of hot rock that burst to the surface millions of years later. Sixty plumes have been identified, rising under ocean islands and volcanoes in the hearts of continents. The depth from which those plumes originated remains uncertain, but it is being argued that they contain material from the slabs of ocean floor -- carbon and oxygen once associated with life on the surface. Analysis of oxygen in volcanic glasses that erupted from a plume in the South Pacific "leads to the virtually inescapable conclusion that they contain a component that was once at the surface of the Earth," according to a commentary in a recent issue of Nature by William M. White, a geophysicist at Cornell Universtiy. The glasses, dredged by the German research vessel Sonne from seamounts off Pitcairn Island, were analyzed by a team of Australian, British and German scientists. Although a plume had presumably carried the glasses up from deep in the Earth, it contained oxygen that showed contact long ago with the surface, the authors said. In other words, some of it had once been part of either the seafloor or a continent. White argued in his commentary that since it appears that the plumes carry carbon-rich remnants of ancient life, the resulting delivery of carbon dioxide to the atmosphere could affect levels of that greenhouse gas. "Deep mantle recycling and mantle plumes," he said, "may play a role in long-term climate regulation." When the hot plumes reach the surface, they may create strings of oceanic islands. On land a plume may produce a succession of volcanoes, such as the one now semidormant under Yellowstone National Park that 600,000 years ago blanketed North America with 350 cubic miles of ash. As the continent has drifted west, this plume has left a series of volcanic remnants across southern Idaho. Other plumes may blanket a vast region with lava, as occurred 15 million years ago when much of Washington and Oregon were covered, in some instances in little more than a single day. The plumes are slowly remolding the Earth's surface at the dictates of events in the mantle far beneath. At a recent meeting of the American Geophysical Union in Baltimore, scientists debated the details of the slabs' journeys as they plunged through the mantle and rose in plumes. Many participants argued that the slabs dropped through the entire thickness of the mantle, reaching down to the turbulent layer that separates the mantle form the Earth's moten core. Hints from earthquake recordings suggest that this layer is more uneven than the most mountainous part of the Earth's surface. Earthquake data make clear that the mantle is divided into an upper section and a lower one, although the difference between them is a matter of debate. Some scientists argue that the lower mantle is chemically different from the upper part and that the slabs of seafloor material only sink 400 miles to the boundary between those regions instead of 1,800 miles to the bottom. The plumes would then originate at that boundary region. Many geophysicists now believe that the only difference between the two sections is that the lower mantle is denser and that many, if not most, slabs sink all the way to the bottom. Widely discussed at the meeting was an intermediate theory according to which the sinking ocean slabs pile up at the bottom of the upper mantle until they are squeezed into a denser state and finally break through. Only then do the slabs sink all the way to the bottom of the mantle. Wysession proposes that the same process may affect rock rising in a plume. It presses against the bottom of the upper mantle until eventually penetrating it. The plume breaks through not as a continuous stream but as separate giant bubbles of hot rock. When the bubbles reach the ocean floor at the top of the mantle, thousands of years apart, each one creates a continental volcano or oceanic island. Since the seafloor may be drifting gradually over the hot plume, the bubbles' successive arrivals may be marked by a chain of islands. The ascent of the plumes can lift an entire region, such as the 3,000-mile-wide section of South Pacific floor where several "hot spots" have erupted, forming chains of Polynesian islands. Similar swells occur under Iceland, the Hawaiian chain and the Kerguelen Islands in the southern Indian Ocean. Besides seismic waves from earthquakes, other evidence about processes in the mantle comes from magnetic measurements and comparison of rocks tested under deep-earth pressures with those collected on midocean islands and ridges. The region between the mantle and the liquid core, where many believe the plumes originate, is described in a recent issue of Scientific American as "the most dramatic structure of the earth." The authors were Raymond Jeanloz of the University of California at Berkeley and Thorne Lay of the university's Santa Cruz campus. The region, they added, may be the planet's "most geologically active zone." The layer, mapped by dense clusters of earthquake detectors in Norway and elsewhere, has proved highly diverse, with "lumps" only a few miles wide and ranging in thickness from little more than a mile to 200 miles. The plate-plume recycling theory allows geophysicists to offer a complete theory of volcano formation, for which there now seem to be three sources. Plumes breaking through the Earth's crust or ocean floor are one. The compression of the slabs as they descend is another. This mechanism is believed to have created the chain of volcanoes in the Cascade Range in the Pacific Northwest and along western South America. The third source of volcanism is midocean ridges and other places where plates of the Earth's surface are pulling apart. Lava wells up to fill the gaps between parting slabs of ocean floor. The lava's composition shows that it originated in the upper part of the mantle and, contrary to earlier belief, may not be part of the deep circulation driving the plates. ------------- End article ------------- Alan Feuerbacher


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