WHILE analyzing data from seismic stations designed to monitor underground nuclear tests, geologists made a serendipitous discovery: In Earth's lower mantle, 900 to 1,250 miles below the surface of the Gulf of Mexico and western Caribbean Sea, lies a giant blob of confined matter that speeds up seismic waves and changes their direction.The Blob
"The results are surprising, as scientists had considered the lower mantle to be nearly homogenous," says Ileana Tibuleac of Southern Methodist University in Dallas. The blob is the biggest anomaly ever seen in the lower mantle-roughly 375 miles long, 90 miles wide, and of unknown thickness but Tibuleac and her SMU colleague Eugene Herrin can only speculate as to what it is: Part of a tectonic plate diving into the mantle? Material from the base of the mantle floating upward? Or material from the upper mantle that is sinking? In any case, the geologists assume the region must be either colder, dense or composed of different elements than its surroundings to account for the deviations it produced in seismic waves emanating from earthquakes near the Windward Islands.
Further research may lead to a better understanding of dynamic processes within the mantle. "Some of the biggest natural disasters, like earthquakes and volcanic eruption: are closely related to the composition of the mantle and the way it moves,' Tibuleac says.
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Science, May 2000
Author: Steve Nadis
Artist: John
MacNeill
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Edited by: William Hutton, 11/27/00
