'Anti-plume' found off Pacific Coast

07/14/04

CORVALLIS, Ore. – The gradual subduction of the Juan de Fuca plate beneath the North American plate puts tremendous stress on the seafloor, creating cracks and fissures, hydrothermal vents, seafloor spreading, and literally hundreds of small earthquakes on a near-daily basis.

Now a North American team of scientists has documented for the first time a new phenomenon – the creation of a void in the seafloor that draws in – rather than expels – surrounding seawater.

They report their discovery in the July 15 issue of the journal Nature.

Oregon State University oceanographer Robert Dziak said the discovery is important because it adds a new wrinkle to scientific understanding of seafloor spreading, the fundamental process of plate tectonics and the creation of ocean crust. Dziak has a dual appointment with the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory.

"Just when you think you're beginning to understand how the process works, there's a new twist," Dziak said. "There was an episode of seafloor spreading on a portion of the Juan de Fuca Ridge that was covered with about a hundred meters of sediment and what usually happens in that case is that lava erupts onto the ocean floor and hot fluid is expelled into the water.

"In this case, though, it actually drew water down into the subsurface, which is something scientists have never before observed," he said.

The research team included Earl Davis, of the Geological Survey of Canada's Pacific Geoscience Centre; Keir Becker, from the Rosenstiel School of Marine and Atmospheric Science at the University of Florida; Dziak; and John Cassidy, Kelin Wang and Marvin Lilley of the University of Washington.

Dziak said the researchers think the seafloor spreading caused the ocean crust to dilate, increasing the pore space much like a sponge. "It's like an anti-plume," he said. "Instead of sending materials from within the Earth to the ocean floor, it simply sucks down the surrounding seawater."

The researchers aren't sure exactly what causes the dilation, but it has multiple implications. First, it changes how scientists view seafloor spreading since there isn't an automatic outpouring of lava, or hot liquid via hydrothermal vents previously associated with tectonic plate theory.

The size of these potential "voids" also intrigues scientists, who wonder how much seawater can be subsumed. If large, or frequent, they could affect surrounding water temperatures and chemical composition, Dziak said.

Finally, water migrating downward through the Earth may be enough to trigger the growth of bacteria at startling depths. Last year, in an unrelated study, OSU oceanographer Martin Fisk and a team of researchers found bacteria in a hole drilled 4,000 feet through volcanic rock. Basalt rocks have all of the elements required for life, Fisk pointed out, including carbon, phosphorous and nitrogen. Only water is needed to complete the formula.

Dziak is able to monitor offshore activities from his laboratory at OSU's Hatfield Marine Science Center in Newport, where he uses an array of undersea hydrophones through a unique arrangement with the U.S. Navy. During the past dozen years, Dziak and his research team have recorded more than 30,000 earthquakes in the Pacific Ocean off the Northwest coast – few of which have ever shown up on land-based seismic equipment.

The earthquakes, most having a magnitude of 2.0 to 4.0, originate along the Juan de Fuca Ridge, a submarine mountain range 300 miles west of the Oregon coast that was formed by seafloor spreading or the movement of oceanic plates away from one another.

"It is the only real-time hydrophone system in the world available for civilian research," Dziak said. "It allows us to listen to the earthquakes as they occur and when something unusual happens, we can send out a group of scientists to study the events as they unfold."

The hydrophone system – called the Sound Surveillance System, or SOSUS – was used during the decades of the Cold War to monitor submarine activity in the northern Pacific Ocean. As the Cold War ebbed, these and other unique military assets were offered to civilian researchers performing environmental studies, Dziak said.

SOSUS also pointed the researchers to the activities leading to the "anti-plume" discovery outlined in Nature.

The number of earthquakes offshore initially stunned researchers because they weren't being detected on land – even by the most sensitive seismometers. The scientists also discovered that these quakes occurred daily, but every so often there would be a "swarm" of as many as a thousand quakes in a three-week period.

"In the last 10 years, I've seen seven of these swarms," Dziak said. "The plate doesn't move in a continuous manner and some parts move faster than others. Every four years or so, a section of the Juan de Fuca Ridge exhibits a large earthquake swarm and lava breaks through onto the seafloor.

"Usually, the plate moves at about the rate a fingernail might grow – say three centimeters a year. But when these swarms take place, the movement may be more like a meter in a two-week period."

On Monday, July 12, the region was jolted by a 4.9 magnitude quake just offshore from Dziak's Newport lab – one that was felt more than 50 miles inland at the main OSU campus.

"There's a lot of activity going on out there," Dziak said of the offshore quakes. "That was one of the few that did show up on conventional seismic equipment and drew the attention of the public. There are hundreds, even thousands more that do not."

Source: Eurekalert & others

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