Airguns gauge undersea volcanoes

A mechanism that counters established thinking on how the rate at which tectonic plates separate along midocean ridges controls processes such as heat transfer in geologic materials, energy circulation, and even biological production has been revealed by a Georgia Institute of Technology, Atlanta, study. The research also has pioneered a new seismic technique--simultaneously shooting an array of 20 airguns to generate sound--for studying the Earth's mantle, the layer beneath the 10- to 40-kilometer-deep crust on the seafloor.

"Midocean ridges produce most of the volcanism on the Earth, releasing a lot of heat--in some places enough to support large biological communities on the seafloor," notes Daniel Lizarralde, assistant professor in the School of Earth and Atmospheric Sciences. "There are large variations in the amount of ridge volcanism worldwide that are probably controlled by processes deep in the mantle. Those processes leave behind an imprint in the crust and mantle that have moved away from ridge. In this study, we did something new. We went well away from ridge where things have cooled down and looked at those imprints."

Previous research has demonstrated that slow rates of plate separation, or spreading, correlate to dramatic changes in various processes occurring at midocean ridges. However, geologists have not had a thorough understanding of this cause and effect relationship. Hoping to reveal that connection, Lizarralde and his colleagues chose to study an extreme case that occurred over a 35,000,-000-year period along an 800-kilometer line southwest of Bermuda in the western Atlantic Ocean.

They found that, as the spreading rate changes, the ability of molten rock, or melt, to get out of the mantle is hindered. "It's like air getting swept up into the atmosphere, water droplets forming and then not being able to fall out as rain," Lizarralde explains. "That's a weird system, and that's what's happening along slow-spreading ridges. The melt gets stuck there, and that changes the thermal balance of things and the buoyancy of the mantle."

This finding differs from the established idea that a slow spreading rate at a midocean ridge cools geologic materials and does not produce much melt. "We found that it's probably not as cold in the melt zone as we thought," Lizarralde surmises. "The same amount of melt is produced, but it gets trapped. The implication of the differences between the old notion and ours is that the mechanisms we propose can explain variations in the chemistry of rocks that come out at midocean ridges worldwide"

Some scientists believe these geochemical variations are best explained by heterogeneity of the mantle. Still others point to evidence indicating the mantle is generally uniform, which is consistent with material mixing caused by heat transfer in convection, Lizarralde points out. "Now, our mechanism explains this geochemical variability, while still having a uniform mantle. If the mantle retains some of the melt--like we're saying--it's likely that it would preferentially keep some chemicals in that melt and let others out."

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