Seabed seismic scan shows melt zone.

Most of the Earth's crust forms when partially molten rock oozes from the planet's interior, emerging at mid-ocean ridges where the sea floor Is spreading. This Internal process has been largely hidden from the eyes of researchers. Using seismic sensors, however, scientists now have produced detailed images of magma under an ocean ridge.

Results of the Mantle Electromagnetic and Tomography Experiment, reported in the May 22 Science, offer the first evidence that molten rock, called melt, occupies a wider and deeper area under the East Pacific Ridge than some researchers had predicted.

The seismic readings, together with data from electromagnetic sensors, will help researchers develop new models of how seabed crust forms, says Donald W. Forsyth, a geologist at Brown University in Providence, R.I. He coordinated the imaging project, one of the largest marine geophysical studies.

Researchers from seven institutions deployed 51 seismic monitors on the ocean floor traversing the East Pacific Ridge about 4,000 kilometers west of South America. For six months ending in May 1996, the detectors recorded waves from a series of earthquakes around the world. The waves tended to slow down as they passed through regions of melt, which is less dense than surrounding rock. This allowed scientists to map the melt, the seismic equivalent of medicine's CAT scan.

The imaging results suggest the melt area is as much as 600 km wide, at least 170 Ian deep, and skewed to the west of the ridge. The finding contrasts with theories that predicted the melt was located either in a broad but shallow horizontal plane or in a thin, vertical plane falling directly beneath the ridge. Some researchers have suggested that melt might rise from deep in the earth's mantle, but the findings tend not to support that view, Forsyth says.