On video: Seafloor volcano spews magma

U. WASHINGTON—For the first time molten lava has been observed flowing from a deep ocean volcano. In video footage, clouds of milky-yellow sulfur gas billow, molten red lava explodes into the icy ocean water and turns almost instantaneously to black rock, while water vapor creates huge, glowing lava bubbles several feet across.

The video was recorded during a May 2009 expedition to West Mata volcano, about 125 miles southwest of Samoa. This is only the second seafloor volcanic eruption ever seen. The other, found in 2004, is near Guam. It sends explosions of debris, gases, and steam into the ocean, but no flows of molten lava have been observed.

“The whole ocean floor is created by this process of extruded lava but we’ve never seen it so we don’t know how rapidly this occurs, the volume of rock produced or how these eruptions build lava flows hundreds of meters thick,” says Robert Embly, a seafloor geologist with the National Oceanic and Atmospheric Administration and Oregon State University’s Hatfield Marine Science Center.

Scientists recorded the deepest erupting volcano yet discovered. Eruption of the West Mata volcano, documented in May, occurred nearly 4,000 feet below the surface of the Pacific Ocean, in an area bounded by Fiji, Tonga, and Samoa. (Courtesy: National Science Foundation and NOAA)

Scientists believe seeing such seafloor eruptions may be a way to better understand eruptions that occur on land because they can get so much closer to the ones on the seafloor, says Joe Resing, oceanographer at the University of Washington and chief scientist of the expedition when the volcano was observed 4,000 feet below the surface.

“Volcanic rocks, especially pillow basalts, are one of the most common rock forms on Earth, and yet no one has ever seen them forming in the deep ocean before,” Resing says. “More than 80 percent of volcanic eruptions take place underwater and we’ve made completely new observations to better understand fundamental processes shaping our Earth.”

The eruption occurred so deep that water pressures tamped down gas explosions and limited how far molten lava spewed. The streams of lava seen shooting up to 35 feet in the water and volcanic rock fragments thrown 150 to 300 feet from the eruption would have been even more expansive in air, Resing explains.

Also, oceanographers have unmanned, remotely operated vehicles that can maneuver near molten magma unlike anything available on land. During the expedition the remotely operated vehicle worked within eight feet of the eruption site to insert instruments into billowing water and gases.

Scientists found evidence that eruptions were taking place six months before the expedition where lava flows were seen. Resing’s colleague, Marv Lilley, professor of oceanography, was on that November 2008 expedition and recorded very high hydrogen concentrations in the water.

“I knew that these high levels could only be produced during an active eruption,” Lilley says.

The findings merited a return expedition. Scientists zeroed in on the summit of a small volcano and found eruptive activity in an area 325 feet long with big vents at either end. The most violently erupting vent with molten lava was named Hades. The other vent, Prometheus, was erupting with low-level, nearly continuous fire fountains and ejecting debris.

Scientists were surprised to find boninite lavas, believed to be among the hottest lavas that erupt on Earth. These had only been previously found long solidified at old, extinct volcanoes.

Resing and other scientists wonder what kind of volcano this is. One possibility is an arc volcano, which forms above regions in the ocean where one tectonic plate is sliding under another.

Arc volcanoes are most commonly found in the Pacific Ocean and have resulted in creation of such island-arc chains found in Hawaii, Aleutians, and Japan.

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