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February 3, 1997

Drilling cruise probes ocean trench, studies the origins of volcanoes

By Theobolt Leung

A floating team of scientists poking into the Pacific Ocean floor off Costa Rica has unearthed mud that raises puzzling questions about what fuels the volcanoes 125 miles away on shore.

Researchers aboard Leg 170 of the Ocean Drilling Program explored the origins of Central American volcanoes and other geological mysteries during a two-month expedition from October to December. The international team was led by UCSC geologist Eli Silver and Gaku Kimura of the University of Osaka in Japan.

A patchwork of cooled, hardened plates of rock is all that separates the oceans from the earth's hot upper mantle. But molten material constantly seeps out of ridges between the plates, hardening and adding to the plates as they drift apart. Only after creeping for eons and accumulating marine sediments does each section of oceanic plate sink--or "subduct"--back into the underlying stew.

The Cocos plate dives under Central America, carrying into the Middle America Trench more than a thousand feet of clay and organic slime. This cold, gray ocean mush, scientists believe, is converted into red-hot magma 80 miles below the surface. The less-dense magma then rises, violently breaching the surface through volcanoes. Several active volcanoes, some of them quite dangerous, dot the Costa Rican landscape.

Scientists have used a telltale radioactive element, beryllium-10, to clock the mush-to-magma recycling process. A natural product of the atmosphere, beryllium-10 settles in high concentrations on the seafloor and immediately begins decaying. In a process similar to carbon dating, geologists can determine the age of the resulting lava.

Volcanoes in neighboring Nicaragua have more beryllium-10 than anywhere else, researchers have found. But the element is conspicuously absent from Costa Rica's lava. One explanation for the discrepancy is that Costa Rican magma spends 1.5 million more years underground. Another possibility is that several hundred feet of beryllium-rich ocean silt is scraped off by the jutting toe of the continent. Earlier studies predicted that the top 250 feet of the incoming 1,300 feet of sediment piled up on the seafloor.

To test these hypotheses, the team used the Ocean Drilling Program's sophisticated JOIDES Resolution vessel to drill holes up to a third of a mile deep, piercing the edge of the Caribbean plate to reach the subducting Cocos plate. Using a new tool, the researchers sampled the physical and chemical properties of the plates at five different sites.

Silver was surprised by what they found. "Essentially, almost everything goes down," he says, except for the uppermost 15 to 30 feet of sediment. This material remains basically unchanged as it slides under the overriding plate, compressing only slightly. "It's really mind boggling," says Silver, that sediments made of 75 percent water can subduct without being deformed. "You'd think it's just fluff."

The crew returned with many unanswered questions, but with mounds of data to analyze. One thing Silver and his colleagues will soon know is whether the ocean sludge off Costa Rica contains as much beryllium as it does off Nicaragua. If this is true, they will have to rethink their theories of how the sediments are recycled into volcanic lava.

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