U.S. Scientists, Colleagues Solve Volcanic Mystery, Learn Tears Occur In Vast Plates

The question has been why analysis of hardened lava there, also on Adak Island in Alaska's Aleutian chain and elsewhere around the globe reveals a chemical makeup different from what researchers predict it should be. Pieces of the lava are named adakites because of where the primitive rocks first were discovered two decades ago.

According to the standard model of how vast pieces of the earth's surface move in relation to one another, the Pacific Plate is continuously thrusting underneath the Eurasian Plate so that North America and Asia are drawing closer. The process takes water-laced sediments on the sea floor lying atop the Pacific Plate to depths of about 60 miles deep, changing their nature through intense heat and pressure.

"That material then leaks up into the earth's upper mantle, which causes the mantle to melt, and dark basalts to come out in volcanoes as lava, the model suggests," said Dr. Jonathan M. Lees, associate professor of geology at the University of North Carolina at Chapel Hill. "What we found makes us think something different also is happening in some places."

Dr. Gene Yogodzinki of Dickinson College, a geochemist, and colleagues conducted chemical analyses indicating that pieces of the plate, or slab, also have melted, an observation contradicting the earlier belief that the slab is too cold to melt and flow to the surface as lava. Lees, a volcanologist who has been recording seismic activity on Kamchatka for the past three years, said his data suggest the current model should not be abandoned but rather changed.

A report on the work appears in the Jan. 25 issue of the journal Nature.

"There appears to be a very large tear in the slab where the Aleutians and Kamchatka intersect so that the edge of the slab is exposed to the mantle," he said. "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."

Scientists do not find the unusual lava in Hawaii and the Azores, which are not near the edge of any slab's subduction zone. These exemplify a different kind of volcano, one arising from what researchers call hot spots deep in the earth beneath the oceanic crust. Volcanoes formed by one slab sliding under another -- rather than just burning holes through them -- include Mt. St. Helens, Mt. Rainer, Mt. Pinatubo, Mt. Vesuvius and Mt. Fuji.

"What we've done is to explain a mysterious chemical signature in rocks with a very simple new model that has important implications for our understanding of the mantle," Lees said. "Before, the origin of adakites was the subject of much controversy because we did not have a satisfactory model for how they got ocean crust in them. Now we believe the cold slab melts because it's torn and exposed to the hot mantle."

The mantle lies about 18 miles below the continental crust and about three to five miles beneath oceanic crust. Lava comes from molten mantle rock about 60 miles beneath the surface.

Co-authors of the paper work at the Institute for Volcanic Geology and Geochemistry in Petropavlovsk-Kamchatsky in Russia and the Geochemical Institute in Goettingen, Germany.

Lees said that many geochemists and geophysicists he has discussed it with like the research because it appears to solve the mystery. Others disagree but have not been able to refute the new model.

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