Scientists discover how rate of tectonic plate separation controls geologic processes

12/06/04



The image is a map of the study area southwest of Bermuda. The image is copyrighted by researcher Daniel Lizarralde and is being used with his permission.
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A new study has revealed a mechanism that counters established thinking on how the rate at which tectonic plates separate along mid-ocean ridges controls processes such as heat transfer in geologic materials, energy circulation and even biological production.

The study also 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. The research, led by the Georgia Institute of Technology with funding from the National Science Foundation (NSF), will be reported in the Dec. 9, 2004 issue of the journal Nature.

"Mid ocean 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," said Daniel Lizarralde, lead author of the Nature paper and an assistant professor in the Georgia Tech 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," he explained. "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 shown that slow rates of plate separation, or spreading, correlate to dramatic changes in various processes occurring at mid-ocean ridges. But researchers 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-million-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 explained. "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 mid-ocean ridge cools geologic materials and doesn't produce much melt. "We found that it's probably not as cold in the melt zone as we thought," Lizarralde said. "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 mid-ocean ridges worldwide."

Some scientists believe these geochemical variations are best explained by heterogeneity of the mantle. But others point to evidence indicating the mantle is generally uniform, which is consistent with material mixing caused by heat transfer in convection, Lizarralde explained.

"Now, our mechanism explains this geochemical variability, while still having a uniform mantle," he added. "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."

The research team reached its conclusion with data gathered using a new technique devised by Lizarralde, an active-source seismologist, who generates and measures his own sounds to study the Earth's crust and mantle. Passive-source seismologists deploy seismometers and rely on earthquakes to produce the acoustic waves they need to study the crust and mantle. Until now, scientists have not used active-source seismic techniques to study the mantle because of a technical difficulty -- sound travels much slower in water than in the earth, interfering with signal reception.

"After a certain window during which you can record the energy traveling through the earth, you find a lot of energy rattling around in the water," Lizarralde explained. "It tends to block the signals. So we changed the way we were shooting an array of airguns we used to generate sound. As a result, we were able to study the mantle and look at the residual products of melting and flow that happened at a mid-ocean ridge."

Lizarralde and his colleagues simultaneously released compressed air from an array of 20 airguns – each a meter in length and 8 inches in radius -- straight down from the ocean surface at different points along their study area. Seismometers on the seafloor recorded the energy traveling through the earth.

"The energy goes down and then turns when it gets into the earth and heads back toward surface in a long arc," Lizarralde explained. "The deeper you want to see into the earth, the farther the separation needed between your sound source and your receiver to make the arc longer and longer. The technical challenge is to record sound at these long offsets of receiver distances of 300 to 400 kilometers. That's what we were able to do in this study."

Lizarralde and 17 other scientists -- including researcher Jim Gaherty and Georgia Tech graduate student Sangmyung Kim, co-authors of the Nature paper -- gathered their data during a month-long research cruise in the summer of 2001.The ship they used is operated by Lamont-Doherty Earth Observatory of Columbia University, where Gaherty, a former Georgia Tech assistant professor, now conducts research.

Source: Eurekalert & others

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