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First evidence of mantle plume.

According to the theory of plate tectonics, the Hawaiian islands and many other volcanic chains represent rows of geologic blisters, formed as the Earth's ever-shifting surface plates pass over stationary hot patches in the mantle. Geoscientists believe such hot spots are fed by cylindrical plumes of sweltering rock rising from deep in .the mantle, but attempts to locate such plumes have failed to deliver convincing evidence. Two seismologists now say they have found the first clear signs of a plume rising from the deep mantle toward a seamount off the west coast of Canada.

Henri-Claude Natal of the Ecole Normale Superieur in Paris and John C. VanDecar of Utrecht University in the Netherlands focused their search beneath the Bowie seamount, the youngest in a line of submerged volcanoes running toward the northwest. To probe the deep mantle, they analyzed records of Alaskan earthquakes, looking specifically at how long it took seismic waves to travel under the seamount and reach recording stations in Washington state. Because seismic vibrations slow down when passing through hot rock, this technique can reveal temperature variations in the mantle.

The travel-time data suggest the existence of a plume about 150 kilometers in diameter at a depth of 700 km below the ocean floor, the researchers say, They were surprised to find that the suspected plume lies 150 km east of the seamount, rather than directly beneath. To explain the offset, they suggest that the plume could take a tilted path upward or that the hot spot's position may not coincide exactly with the seamount.

Natal and VanDecar may find themselves in a hot spot of their own as their controversial claim arouses debate among geophysicists. Thorne Lay, a seismologist at the University of California, Santa Cruz, credits the two for making one of the best attempts so far at detecting the subtle signature of a plume. But he says the data are far from conclusive, and he remains unconvinced that they have indeed located a plume.
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Title Annotation:cylindrical plumes of hot rock believed to be related to the formation of volcanic island chains
Author:Monastersky, Richard
Publication:Science News
Article Type:Brief Article
Date:Jun 12, 1993
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