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First look at the base of the plates.

First look at the base of the plates

Geophysicists have long used explosions to study what lies below Earth's surface. By sending seismic waves down into the ground and measuring the reflected waves that return, they can see structures within Earth's lithospheric plates, which form the hard outer shell of the planet. Now for the first time, researchers have succeeded in looking at the deepest section of the plates -- opening up new possibilities for understanding how plate tectonics really works.

A team of researchers headed by J.E. Lie at the University of Oslo in Norway collected data with the research vessel, Mobil Search. The ship carried a large air gun and towed hundreds of microphones strung out along a 4.5-kilometer-long streamer. As it plied the waters south of Norway, the ship shot off the air gun and recorded teh reflected waves -- a technique called seismic-reflection profiling.

In the July 12 NATURE, Lie's group reports finding several abnormally reflective patches located at depths of 100 to 110 km. They cannot yet tell what the "reflectors" represent, but in the shallower portion of the plates, reflectors usually signify faults or interfaces between two different types of rock.

The lithospheric plates include both the crust and the uppermost section of the mantle. Profiling work on land, using vibrating trucks, has rarely detected features in the mantle. But a British ocean experiment has recently detected significant mantle structures. Pushing the bounds of profiling even deeper, the Norwegians report they can see reflectors at the lowest mantle portion of the lithosphere.

"It it is correct, this is a first time, extraordinary result," comments John Mutter of the Lamont-Doherty Geological Observatory in Palisades, N.Y. Mutter, who leads Lamont's research effort in seismic profiling, says geophysicists studying plate tectonics have had to infer how the lower portion of the lithosphere behaves when plates collide or stretch. Now they have a means of testing those inferences.
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Title Annotation:Earth's lithospheric plates
Publication:Science News
Date:Jul 28, 1990
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