Magma reservoir seen under ocean ridge.

Magma reservoir seen under ocean ridge

In the geophysical equivalent of a CAT scan, scientists have identified key parts of the molten-rock plumbing system underneath a mid-ocean ridge -- offering new insight into the process that forms two-thirds of the planet's surface.

Mark S. Burnett, David W. Caress and John A. Orcutt of the Scripps Institution of Oceanography in La Jolla, Calif., used computer tomographic techniques to analyze data from a 1982 experiment, called MAGMA, on the East Pacific Rise off the coast of Mexico. During MAGMA, researchers exploded thousands of charges near the ocean surface, generating seismic waves that traveled through the ocean crust. Instruments on the submerged ridge and the sea floor several kilometers to the east recorded waves that had refracted in rock layers underneath.

Researchers had previously only analyzed slices of the MAGMA data in a "trial and error fashion," says Orcutt. Tomography, used here for the first time to study a ridge, resembles the x-ray technique that produces three-dimensional medical images.

The new analysis helps resolve the structure of the magma--or molten rock--reservoir that feeds sea-floor spreading at the ridge, the researchers report in the May 18 NATURE. The seismic waves reveal that a long region of hot rock, only about 6 kilometers wide, sits underneath the ridge. Because of the way the seismic waves slow as they passed through the area, Orcutt says this hot zone is made of mostly solid rock, containing a small amount of melted material.

A more complete picture of the ridge emerged from combining these results with those from a different type of experiment, using reflected seismic waves. This project had shown that magma lies directly under the ridge at a depth of about 1.4 kilometers, but it could not resolve what sits beneath the magma. The composite results indicate that molten rock forms a thin "mushroom" capping the hot rock zone. The magma chamber itself may only be a few tens of meters thick, Orcutt says.

The new tomographic images in combination with other measurements "provide the best glimpse yet of the axial magma reservoir," says Ken C. Macdonald of the University of California, Santa Barbara. Researchers had overestimated the size of the melted pool in previous theories. Future studies will aim to determine the true size of the chamber.