Scientists have long puzzled over why certain volcanoes, including those ringing the Pacific Ocean, spew forth lava that doesn't match with their chemical predictions. Now it appears they may finally have an answer, reported today in the journal Nature.

Standard theory holds that the shifting of Earth's plates sends sea floor sediments lying on top of the Pacific Plate down about 60 miles into the planet. "That material then leaks up into the Earth's upper mantle, which causes the mantle to melt and dark basalts to come out of the rock as lava," team member Jonathan M. Lees explains. "What we found makes us think something different is also happening in some places." Previously researchers believed that the slab was too cold to melt and flow to the surface. But when Lees and his colleagues analyzed pieces of hardened lava called adakites from easternmost Russia's Kamchatka province, Adak Island in Alaska'a Aleutian chain and elsewhere, they found that the adakites contain not only mantle but slab, indicating that pieces of the plate had also melted.

"There appears to be a very large tear in the slab where the Aleutians and Kamchhatka intersect so that the edge of the slab is exposed to mantle," Lees says. "When that happens, as it also does in some other places like California, it allows the mantle to erode the slab so that we see this very interesting and unexpected geochemical signature--or combination of chemicals--near some volcanoes. It's a kind of contamination, or mixing, of mantle and slab."

"The more seismic and geophysical evidence we come up with in our experiments, the more we seem to find that our model is correct," Lees reports. "For scientists like us, that's fun and pretty exciting. It's showing us how the Earth works, explaining its plumbing system."