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Animals seared by deep-sea eruptions.

In a stunning case of serendipity, oceanographers conducting a submarine survey have discovered a scene of underwater carnage--a site in the Pacific Ocean where recent lava eruptions sizzled a community of tubeworms and other animals living on the ocean floor. The investigators reached the site so soon after the volcanic outpourings -- possibly even during the eruptions--that they spotted a few partially covered animals still clinging to life.

The identification of this site, located about 1,000 kilometers southwest of Acapulco, Mexico, marks the first detection of such recent deep-sea lava flows. Although underwater ridges generate about 75 percent of all eruptions on Earth, scientists have never before witnessed the immediate effects of such an event, says Rachel M. Haymon, a geologist at the University of California, Santa Barbara. Haymon co-directed the survey with Daniel J. Fornari of Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y. The scientists reported the discovery their observations in and will describe their observations in detail at next week's meeting of the American Geophysical Union in San Francisco.

The new lava flows lie 2.5 kilometers underwater on the midocean ridge -- a submerged volcanic mountain chain wrapped around the globe like the seam on a baseball. Scientists know that volcanic activity along this ridge creates the ocean crust through a process of seafloor spreading, but until now they lacked a detailed picture of such eruptions.

"Now, at least in one place, we know the scale of an eruption, we know what it could do and what it looks like," says Micheal R. Perfit of the University of Florida in Gainesville, who participated in the survey.

Oceanographers ordinarily have no way to determine the precise age of the lavas covering midocean ridges. Some flows may be centuries old, while others date back only a decade. But when Haymon and her colleagues began their dives in the submersible Alvin early last April, they realized the seafloor had recently erupted because the scenery did not match pictures taken there 15 months earlier. Over the next few days, they found evidence indicating that they might be witnessing an actual eruption or its immediate aftermath.

Suspended particles had turned the water near the seafloor extremely murky, and prodigious streams of superhot water--up to 403[degress]C--poured from the rocks. At a spot doubled "Tubeworm BBQ" by Haymon and others, the divers found lava flows covering animal colonies, with freshly scorched tubeworms that had not yet decayed. In late April, the region was devoid of scavengers. But four weeks later, Perfit and his colleagues discovered hordes of crabs feeding on the dead animals. Analysisof radioactive isotopes in the lava suggests the eruptions occurred between March 26 and April 6.

The discovery of recent lava flows now gives researchers the opportunity to track post-eruptive changes, says Jean-Christophe Sempere of the University of Washington in Seattle. "People are very excited, and there is a big push from investigators from varius disciplines to work together to study the evolution of this piece of midocean ridge," he says. For instance, some scientists will check to see how long it takes organisms to establish colonies or newly formed lava.

This region of the Pacific midocean ridge draws considerable scientific traffic. In January, the international Ocean Drilling Program will attempt to bore into a ridge section just 25 kilometers north of the part that erupted. Scientists are also planning to dive at the eruption site in March.

Although Haymon and her colleagues didn't expect to find an eruption in progress, she says previous work by others had hinted that the site was active. In the 1970s, sonary surveys suggested that magma had risen close to the seafloor there, causing this section of the ridge to bulge upward.

"We're wising up to ways that cna pinpoint the regions most likely to be active," she says.
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Author:Monastersky, Richard
Publication:Science News
Date:Dec 7, 1991
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