Monday, December 25, 2006

 

Volcanoes of the deep

Losing expensive scientific equipment is usually a cause for dismay. But when Maya Tolstoy*** couldn't retrieve eight seismometers from the bottom of the sea this April, the Columbia University geophysicist was thrilled.

The devices, she suspected, had been buried under a lava flow 1.2 miles below the surface of the Pacific Ocean. When scientists at the Woods Hole Oceanographic Institution* (WHOI) confirmed her suspicions a few months later, they also confirmed that her instruments had - for the first time ever - captured an undersea volcano while it erupted.

'We've been trying for decades to get instruments on top of a [deep-sea] eruption as it happens,' said Tolstoy, who published initial data from two recovered seismometers in the December 22 issue of the journal Science. 'We've never recorded this fundamental process on nearby seismic instruments before.'

From the new seismic data and a bevy of new scientific efforts launched at the eruption site, researchers expect to get an unprecedented look at the geological, chemical, and biological processes of deep-sea volcanic zones called mid-ocean ridges**.

Continued at "Volcanoes of the deep" (may require free registration)

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Based on the journal Science paper:

"A Sea-Floor Spreading Event Captured by Seismometers"

by M. Tolstoy et al.

Originally published in Science Express on 23 November 2006
Science 22 December 2006:
Vol. 314. no. 5807, pp. 1920 - 1922
DOI: 10.1126/science.1133950

Abstract

Two-thirds of Earth's surface is formed at mid-ocean ridges, yet sea-floor spreading events are poorly understood because they occur far beneath the ocean surface. At 9 degrees 50 minutes N on the East Pacific Rise, ocean-bottom seismometers recently recorded the microearthquake character of a mid-ocean ridge eruption, including precursory activity. A gradual ramp-up in activity rates since seismic monitoring began at this site in October 2003 suggests that eruptions may be forecast in the fast-spreading environment. The pattern culminates in an intense but brief (approx 6-hour) inferred diking event on 22 January 2006, followed by rapid tapering to markedly decreased levels of seismicity.

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*Excerpt from an earlier (but related) Woods Hole Oceanographic Institution news release ("Scientists "See" New Ocean Floor Just Before and After It Is Created" November 23 2006):

A multidisciplinary research team from six institutions has for the first time successfully measured a seafloor eruption at its source along the global mid-ocean ridge, the most active volcanic system on Earth. The event along the East Pacific Rise has provided researchers from Woods Hole Oceanographic Institution (WHOI) with a rare opportunity to observe what happens in the immediate aftermath of an eruption.

Ocean-bottom seismometers, which had been deployed by researchers from the Lamont-Doherty Earth Observatory (LDEO) of Columbia University, detected steadily increasing levels of earthquake activity over several years leading up to 2006. Research teams from WHOI and other institutions then used towed cameras and the submersible Alvin to make a visual confirmation of the seafloor eruptions, which decimated the well-studied communities of tubeworms, mussels, and other unique organisms that had been living at the hydrothermal vents in the area.

A research paper describing the seismic data was authored by Maya Tolstoy of LDEO and includes scientists from WHOI, the University of Hawaii, Brown University, the University of Washington, the University of Florida, and NOAA' s Pacific Marine Environmental Laboratory (PMEL). The findings will be published in an upcoming issue of Science, though a preliminary analysis was released on 23 November 2006 on the Science Express web site.

"We rarely get to observe a deep-ocean eruption just after the fact," said WHOI geologist Adam Soule, who helped confirm and map the seafloor eruption. "This will add substantially to our understanding of how mid-ocean ridges work."

Through research expeditions in May and June 2006 - as well as another that just departed on 22 November - WHOI researchers have been heavily involved in characterizing the extent of the eruptions and in observing its effects on the communities of unusual species that thrive in the mineral-rich fluids that spew from hydrothermal vents on mid-ocean ridges. More than two-thirds of Earth' s crust is created along seafloor spreading ridges, yet scientists rarely have a chance to observe the process is action.

In 2003, the LDEO team deployed a dozen seismometers on the ocean floor along the East Pacific Rise (EPR), near 9 degrees 50 minutes N latitude, about 400 miles south of Acapulco, Mexico. Each year when they returned to service the instruments, the researchers detected steadily increasing levels of earthquake activity as magma moved beneath the ridge axis. They suspected a major seismic event was imminent. Sometime between May 2005 (the last readout of the seismometers) and April 2006, a series of volcanic eruptions paved over areas of the ocean floor.

A scheduled expedition on the WHOI-operated Research Vessel (R/V) Knorr in April 2006 provided the first hint of an eruption. The LDEO team discovered that most of their instruments could not be recovered and surmised that a seafloor eruption had probably wiped them out.

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**Info on mid-ocean ridges from the New Millenium Observatory (NeMo):

The global mid-ocean ridge system is the largest single volcanic feature on the Earth, encircling it like the seams of a baseball. Here the Earth’s crust is spreading, creating new ocean floor and literally renewing the surface of our planet. Older crust is recycled back into the mantle elsewhere on the globe, typically where plates collide. The mid-ocean ridge consists of thousands of individual volcanoes or volcanic ridge segments which periodically erupt.

Beneath a typical mid-ocean ridge, mantle material partially melts as it rises in response to reduced pressure. This melted rock, or "magma", may collect in a reservoir a few kilometers below the seafloor, awaiting eruption. Much of the magma eventually freezes in place there within the crust, forming the bulk of the new oceanic crust without erupting at all. Average oceanic crust is about 10km thick, but only the upper 1 to 3 km are formed by eruption processes. [seismometer]

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***A May 2002 interview with Mara Tolkstoy from Dive and Discover:

...I received my undergraduate degree from the University of Edinburgh in geophysics in 1988. I spent the summer of 1987 at Scripps Institution of Oceanography as an undergraduate research fellow, and found I enjoyed research, and loved going to sea. After I graduated, I wanted to move back to the US and also wanted to travel. I figured that graduate work in marine geophysics would let me continue to do research in an area I liked, and would also give me opportunities to travel and go to sea. I was accepted at Scripps, and while I was there I went on 16 cruises in various parts of the Atlantic and Pacific Oceans. I specialized in marine seismology, and worked with both seismic data and seafloor instrumentation...

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