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SUMMIT_DOWN_LOW
Cameras catch underwater volcano in the act

Seafloor eruption in the South Pacific is the deepest and most violent yet seen

SAN FRANCISCO — An unmanned submersible toting high-definition cameras has captured video of a volcano erupting more than a kilometer below the surface of the South Pacific. One unexpected bonus: Researchers have grabbed samples of a chemically distinct, water-rich type of lava never before seen at an actively erupting volcano.

“It was an underwater Fourth of July,” said Bob Embley, a marine geologist at NOAA’s Pacific Marine Environmental Laboratory VENTS program in Newport, Ore. ”It was extremely exciting, and we’re ecstatic,” he reported December 17during the annual meeting of the American Geophysical Union.

The West Mata undersea volcano lies about 200 kilometers, or a half-day’s cruise, southwest of Samoa, and its summit lies about 1,165 meters below the ocean surface. Immense water pressure at that depth helped suppress the violence of the eruption’s explosions and allowed oceanographers to steer the submersible to within three meters or so of the fuming peak, Embley said. There, gas violently spewing from the summit inflated the pools of viscous lava like chewing gum blown into bubbles, each more than one meter across. In the 4° Celsius water at the seafloor, those bubbles of molten rock quickly chilled and then shattered to form volcanic shrapnel.
While the larger chunks of rock quickly fell back onto the sides of the peak, smaller bits remained suspended in the water — one of the many clues that oceanographers used to find the eruption in the first place. Also, images taken from the submersible during other parts of the expedition revealed medium-sized bits of fresh ash on the ocean floor as many as 30 kilometers from the undersea peak, said Joseph Resing, a chemical oceanographer at the University of Washington in Seattle.

Sulfurous gases emitted by the eruption render the seawater extremely acidic, with a pH somewhere between battery acid and stomach acid, says Julie Huber, a microbiologist at the Marine Biological Laboratory in Woods Hole, Mass. Microbial diversity near the volcanic vent is very low, possibly because of the ferocity of the ongoing eruption. As with hydrothermal vents around the world, vents spewing warm water on the slopes of the volcano host microbial mats. Further analyses will reveal whether the small shrimp found grazing on the mats are the same species that live around other seamounts and hydrothermal vents elsewhere in the Pacific.

Rock samples snatched from the maw of the volcano include boninite lavas, a magnesium- and water-rich type of lava previously found only around extinct volcanoes that erupted more than 1 million years ago, says Resing. Boninites were previously presumed to result only during certain parts of a volcano’s life cycle, and the new finding provides scientists with an unprecedented opportunity to study how tectonic activity recycles Earth’s ocean crust.

During one three-hour stretch of the submersible’s visit to the volcanic peak, lava added about three meters of height to some parts of the peak, although much of that material probably tumbled away later in an undersea landslide, says Resing.

“Watching the world form before your eyes is pretty amazing,” says Huber. “The entire ship was on their toes, watching television at 3 a.m.as we saw these spectacular images coming back from the ocean floor.”

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