A previously unknown layer of liquefied molten rock in Earth's mantle may be "lubricating" the sliding motions of massive tectonic plates, U.S. researchers say.

Scientists at Scripps Institution of Oceanography at the University of California, San Diego, said the discovery could help explain basic geological functions of the planet and perhaps lead to better understanding of volcanism and earthquakes.

"This was completely unexpected," Scripps geophysicist Kerry Key said. "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 at all -- it was pretty surprising."

The scientists discovered the magma layer at the Middle America trench off the shore of Nicaragua, a Scripps release reported Wednesday.

Using advanced seafloor electromagnetic imaging technology, they imaged a 15.5-mile-thick layer of partially melted mantle rock below the edge of the Cocos plate where it moves underneath Central America.

"The information from the new images confirms the idea that there needs to be some amount of melt in the upper mantle and that's really what's creating this ductile behavior for plates to slide," Scripps graduate student Samer Naif said.

The finding could yield a better understanding of the structure of tectonic plates where they meet each other, the researchers said.

"One of the longer-term implications of our results is that we are going to understand more about the plate boundary, which could lead to a better understanding of earthquakes," Key said.