Rocks from the Depths

The most accessible part of the earth's interior is at the ocean's bottom, where the crust is thin. Project Mohole, the U.S. attempt to reach the boundary layer between the earth's crust and mantle by drilling off the coast of Mexico, so far has penetrated only ordinary, surface-type rocks. Last week, the Woods Hole Oceanographic Institution reported far better success with another method. From the fractured north wall of the Puerto Rico Trench, its research ship Chain has dredged up the first samples of "third layer" rock ever gathered by man.

When geophysicists tag the rock strata under the ocean, they call the ocean water the first layer. On the bottom is the second layer: sediment and sedimentary rock averaging 1 km. thick. Below it lies the third layer, which seismic waves have proved to be made of unusually heavy rock. The third layer is normally unreachable, but scientists making a seismic survey in 1959 got hints that it might be exposed on the sides of the Puerto Rico Trench. In 1960 Dr. Earl Hays of Woods Hole took photographs showing fractured rock on the trench's north wall.

Misplaced Pacific. To geophysicists, the Puerto Rico Trench is one of the most interesting places on earth. Lying north of Puerto Rico, it is something like the Grand Canyon sunk under three to four miles of water. Like other deep ocean trenches, it is believed to be a place where the earth's crust is sinking into the interior, perhaps carried down by slow, enormous currents in the plastic mantle. Since trenches are characteristic of the Pacific Ocean, where they abound, some geophysicists consider the Puerto Rico Trench a part of the Pacific that has bulged into the Atlantic between North and South America. Another bit of bulging Pacific may be the trench-bordered Scotia Sea south of South America.

When Woods Hole scientists took a closer look at the Trench, they found by echo sounding that its north wall is scored by fractures where deep-down rock seemed to be freshly exposed. Photographs showed the rock too, but bringing it to the surface was no easy task. Any sort of dredging in deep water is difficult; pulling a dredge among rocks and crags at the end of many miles of cable looked almost impossible.

Trick Dredge. The problem was solved by Woods Hole's Andrew Nalwalk, who designed a special dredge that would flip itself free if it got snagged on a boulder. Three hundred feet up its cable it carried a "pinger," whose sound could be detected by the Chain four miles above. The interval between the pinger's sound and its reflection from the bottom told the scientists when the dredge was on the bottom and moving with its cable at a proper angle. This eliminated "kiting" (sailing above the bottom) and snarl-ups caused by letting out too much cable.

After many tries, the trick dredge brought up chunks of strange, heavy rock from four miles down. Some of the surfaces were dark brown, showing that they had been exposed to the iron and manganese oxides that slowly deposit from sea water. Other surfaces were fresh and light green. Dr. John B. Hersey, chief scientist of the cruise, believes that the chunks with fresh faces were broken by the dredge out of the mysterious third layer. If so, they may show what the crust of the earth was like billions of years ago, before the infant ocean rained sediment on it.

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