Cocos Plate has magma layer underneath it.

The orange colored area enclosed by a dashed line denotes a 
magma layer that scientists believe is facilitating the motion of 
the Cocos plate off Nicaragua. The blue areas represent the Cocos 
plate sliding across the mantle and eventually diving beneath the 
Central American continent, while the black dots signify 
earthquake locations. 

Scientists find unexpected magma  layer helping Cocos to move

By the Scripps Institution of Oceanography news staff Scientists have found a layer of liquefied molten rock in Earth’s mantle that may be acting as a lubricant for the sliding motions of the planet’s massive tectonic plates. The discovery may carry far-reaching implications, from solving basic geological functions of the planet to a better understanding of volcanism and earthquakes. The scientists discovered the magma layer at the Middle America trench offshore Nicaragua. Using advanced seafloor electromagnetic imaging technology, the scientists imaged a 25-kilometer (15.5-mile-) thick layer of partially melted mantle rock below the edge of the Cocos plate where it moves underneath Central America. The Cocos plate and its interaction with the lighter Caribbean plate are responsible for many of Costa Rica's earthquakes. The scientists are from the Scripps Institution of Oceanography at the University of California at San Diego The discovery is reported in the Wednesday issue of the journal Nature by Samer Naif, Kerry Key, and Steven Constable of Scripps, and Rob Evans of Woods Hole Oceanographic Institution. The new images of magma were captured during a 2010 expedition aboard the U.S. Navy-owned and Scripps-operated 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 realized they found magma in a surprising place. “This was completely unexpected,” said Key, an associate research geophysicist in the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics at Scripps. “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.” For decades scientists have debated the forces and circumstances 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,” said Naif, a Scripps graduate student and lead author of the paper. “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.” The researchers say their results will help geologists better understand the structure of the tectonic plate boundary and how that impacts earthquakes and volcanism. “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,” said Key. The researchers are now seeking to find the source that supplies the magma in the newly discovered layer.