MELT Data Sheds New And Surprising Light On Birth Of Oceanic Plates

Forcesthat shape these young oceanic plates have come into clearer focusthrough research conducted by scientists at the Woods HoleOceanographic Institution, Brown University and the Japan Agency forMarine-Earth Science and Technology.

The research represents thefirst time that seismic and electromagnetic data were analyzed intandem from 1995 Mantle Electromagnetic and Tomography, or MELT,Experiment. MELT employed 51 ocean-bottom seismometers and 30magnetotelluric receivers two miles under the sea to measure soundwaves and magnetic fields along the East Pacific Rise, making it one ofthe largest marine geophysical experiments ever conducted.

In apaper published in Nature, the team notes that in rock down to a depthof about 60 kilometers below the ocean floor, electrical currentsconduct poorly and sound waves travel rapidly. Deeper down, beyond 60kilometers, there is a dramatic increase in electrical conductivity,and sound waves travel at their slowest.

A switch in seismic andelectrical properties with depth was expected. Researchers weresurprised, however, at how close to the East Pacific Rise thisstructure develops and how little it changes with increasing distancefrom the rise.

Brown marine geophysicist Donald Forsyth said theteam, led by Robert Evans from the Woods Hole OceanographicInstitution, has a theory about the cause of the sudden compositionalchanges at 60 kilometers: dehydration.

As magma migrates to thesurface to form crust at the rise, it leaves behind a dry, residuallayer about 60 kilometers thick. This change from “dry” surface rock to“damp” rock below it increases electrical conductivity and slowsseismic velocity, the researchers write.

Here is what they didnot expect: These changes occur, the team found, less than 100kilometers away from the highest point on the ridge. And the seismicand electrical measurements remained nearly constant at least about 500kilometers away from the crest.

Separating seafloor guides magmaup to mid-ocean ridges such as the East Pacific Rise, where the moltenrock erupts, fans out along the ocean floor and cools to form newcrust. Cooling allows sound waves and electrical currents to travelfaster. But scientists thought this cooling – and the resulting changesin the rock – would be gradual.

“About two-thirds of the Earth’ssurface is oceanic crust – and it is all formed at ridges,” Forsythsaid. “So this work helps us better understand the basic processes ofhow this crust is formed.”

The National Science Foundation funded MELT and the latest research.