For some years, scientists have wondered about the grand collisions of continents at the edge of the "ring of fire" -- the earthquake- and volcano-prone area around the Pacific. They are curious about what happens to the continental plates that are forced back into the Earth.

The plates meet one another with massive but excruciatingly slow force, and material from the plate that lies under the ocean dives beneath the Asian continental plate.

There have been competing theories about what happens in the collision. One postulates that the material from the ocean plate sinks more than 1,500 miles to the molten core of the Earth. A competing theory holds that the material sinks only a short distance, melts and reemerges to make new ocean floor along the mid-ocean ridges.

Geophysicists from Caltech reported last week the discovery of huge slabs of continental material about 400 miles beneath the Asian continent. The researchers said the slabs appear to be remnants of the Pacific plate, and instead of sinking down through the Earth's mantle, they have moved horizontally. The findings apparently support the theory of melting and reemergence.

Hua-Wei Zhou and Robert Clayton of Caltech made the discoveries using a new technique called "seismic tomography." The method takes data from 3,000 stations around the world to make a new, computerized image of the interior of the Earth.

These new images showed the cool, slab-like material beneath the Asian continent. The scientists suggested the existence of such shallow slabs helps explain why earthquakes appear to extend only to this depth and no deeper on seismic maps.