Hidden magma layer discovered in Earth's mantle

Scientists have discovered an unexpected layer of liquified molten rock in Earth's mantle that may be responsible for the sliding motions of the planet's massive tectonic plates.

The magma layer was discovered at the Middle America trench off Nicaragua's shores.

The finding may carry far-reaching implications, from understanding basic geologic functions of the planet to new insights into volcanism and earthquakes, scientists said.

The research, reported in the journal Nature, was conducted by Samer Naif, Kerry Key, and Steven Constable of the Scripps Institution of Oceanography (SIO), and Rob Evans of the Woods Hole Oceanographic Institution.

"This new image greatly enhances our understanding of the role that fluids, both seawater and deep subsurface melts, play in controlling tectonic and volcanic processes," said Bil Haq, programme director in National Science Foundation's (NSF) Division of Ocean Sciences, which provided funding for the work.

Using advanced seafloor electromagnetic imaging technology pioneered at SIO, the scientists imaged a 25-kilometre-thick layer of partially melted mantle rock below the edge of the Cocos plate where it moves beneath Central America.

The new images of magma were captured during a 2010 expedition aboard the research vessel Melville.

After deploying a vast array of seafloor instruments that recorded natural electromagnetic signals to map features of the crust and mantle, the scientists realised they had found magma in a surprising place.

"This was completely unexpected. We went out looking to get an idea of how fluids are interacting with plate subduction, but we discovered a melt layer we weren't expecting to find," Key said.

For decades scientists have debated the forces that allow the planet's tectonic plates to slide across the Earth's mantle.

Studies have shown that dissolved water in mantle minerals results in a more ductile mantle that would facilitate tectonic plate motions, but for many years clear images and data required to confirm or deny this idea were lacking.

"Our data tell us that water can't accommodate the features we are seeing. The information from the new images confirms the idea that there needs to be some amount of melt in the upper mantle. That's what's creating this ductile behaviour for plates to slide," Naif said.