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Where Earth's insides ooze out.

Earth Sciences

Where Earth's insides ooze out

Drilling into a recently discovered form of underwater seamount, oceanographers have found a spot where material from Earth's mantle oozes onto the seafloor in cold eruptions.

Most seamounts that dot the ocean floor arise when hot, molten basaltic rock spews out of volcanoes and builds layer upon layer. But scientists several years ago identified a new type of submerged mountain. Dredged rock samples and dives in the submersible Alvin have revealed that these seamounts consist of serpentinite, a nonvolcanic rock formed when water reacts with minerals in the mantle (SN: 11/19/88, p.333).

Now, to get a better look, researchers have drilled into underwater mountains near the Izu-Bonin and Mariana trenches south of Japan. Reaching more than a mile above the seafloor, these seamounts sit above a subduction zone in which the Pacific plate slides under the Philippine plate, creating the famous trench system that includes the lowest point on Earth's surface.

The drilling cores, taking during a recent leg of the Ocean Drilling Program, show that serpentinite not only covers the tops of the seamounts but also fills their interiors. While the Izu-Bonin seamount is extinct, the Mariana seamount remains active, and the researchers pulled up soft, plastic serpentinite there. In fact, the drill could penetrate only 150 meters into the center of the seamount before sticking, says, Patricia Fryer of the University of Hawaii at Manoa in Honolulu, a co-chief scientist on Leg 125.

On the basis of holes drilled into the seamounts' centers and flanks, the researchers believe they can now explain the processes that create these structures. Fryer says the origin relates directly to the subduction going on underneath. The serpentinite forms when water escapes out of rocks on the sinking Pacific plate. Because serpentinite is lighter than the surrounding mantle rock, it rises toward the surface. Reaching the crust, the serpentinite "mud" oozes upward along fractures that lead to the seamount summit. As it erupts, the cool mud flows down the flanks like lava from a volcano. Flow textures in some of the drill cores support this idea, Fryer says.
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Title Annotation:Earth Sciences
Publication:Science News
Date:Jul 1, 1989
Words:354
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