Catching subduction in the act.

Catching subduction in the act

As the Pacific plate inches northwest, its leading edge dives into the mantle of the earth, forming some of the deepest trenches in all of the world's oceans. This recycling process, called subduction, has been going on for tens of millions of years at these sites. Now, in another part of the Pacific, some earth scientists think they are beginning to see the earliest stages in the birth of a subduction zone that will eventually form a trench to the north and west of New Guinea.

Researchers from the University of Hawaii first proposed this idea two years ago when they noticed that a supposedly quiet area of the Pacific was actually experiencing major earthquakes (SN: 1/11/85, p.25). The earthquakes seemed to be clustered in a line that ran through Micronesia, and they were originating at shallow depths in the crust. These observations led the Hawaii scientists to postulate that this band of earthquakes was signaling the initial stages of subduction.

The bathymetry of the area is not detailed enough to tell if there is, in fact, a trench along this line. But other researchers from Hawaii, the University of South Carolina at Columbia and the Air Force Geophysical Lab in Hanscom, Mass., have recently used satellite data to look at the gravity near small sections of the proposed subduction zone. They found that over the proposed trench, gravity was lower than normal, which is the expected reading over a trench. They also detected a bulge in the ocean surface to the south of the proposed trench, meaning that there is some excess matter beneath the bulge attracting the water to that spot. Since there are no underwater mountains in that area, this excess mass must be below the oceanic crust and could be the edge of a slab of crust that is beginning to be thrust down into the earth.

While such findings are consistent with the existence of the proposed subduction zone, scientists have examined only a tiny portion of the zone. Still, they have some time to do further work on this area. If subduction is really starting, scientists believe it will be 5 million to 10 million years before this process really gets going.