Oceanography: Seafloor Spreading

OCEANOGRAPHY: SEAFLOOR SPREADING

Liquid Layer

In 1960 the theory of plate tectonics was given a huge boost by the oceanographer Harry H. Hess, who demonstrated his theory that the seafloor was spreading. The Earth is composed of layers—the outer crust, beneath that the mantle, and below that the core. Hess suggested that the mantle, which is eighteen hundred miles thick, has two layers. The deeper layer is solid, he said, which accorded with generally accepted theories about the mantle. But the upper mantle was more like a hot liquid, according to Hess. It pushed up from below. In the deep seafloor rifts, the mantle is close to the surface. It therefore actually comes to the surface (the seabed) and pushes the Earth's crust aside on either end of the rift.

Mohorovicic Discontinuity

The easiest way to prove Hess's theory was to drill a hole in the seafloor to pierce the interface of the crust and mantle, called the Mohorovicic Discontinuity, after the Yugoslavian seismologist who discovered it. Mohorovicic was shortened by scientists to Moho; thus the idea was to drill a hole in the crust of the ocean floor until the mantle was reached—a "mohole." Moholes would have to be twenty-five to forty-five miles deep on land but are under three miles deep in some ocean regions.

Cuss I.

The ocean rifts were known to contain mantle under only a few miles of crust. Thus came the Cuss I, a converted drilling barge named for the oil companies Continental, Union, Shell, and Superior that paid to have the former navy barge converted for Project Mohole, sponsored by the National Academy of Sciences under the direction of Dr. Gordon Lill. The vessel was 260 feet long with a 98-foot derrick. Cuss I would drill six miles into the ocean floor to reach the mantle, taking samples that provided both scientific information and evidence of locations for oil beneath the sea.

Samples of Layers

The ocean floor starts with about five hundred feet of clay and soft sediment that drops from above. Then comes up to two miles of rocks and lava, then a relatively thin four miles of oceanic crust, mostly basalt rock. Finally there is the high-pressure mantle. The core samples from Cuss I gave indirect proof of seafloor spreading by the mantle pushing up in the ocean rifts. The closer to a rift the sample was, the younger the crust rock was found to be.

Glomar Challenger.

This work continued through the decade and was facilitated by the introduction of the "floating doughnut," the $12.6 million Glomar Challenger. This ship could drill in water 20,000 feet deep. It had a doughnut shape, with a drill in the middle that had a 142-foot tower. It could drill up to 2,500 feet beneath the ocean floor. The problem with deep-sea drilling is keeping the ship steady on the surface while drilling miles down below. Too much ship movement can snap the drill. The Glomar Challenger was a technological wonder that had sonar on either side of the drill, feeding information to a computer that controlled the roll of the ship on the surface. The ship was capable of staying within 3 percent of its specified drilling depth. The drill string was made of 38,000 feet of five-inch pipe in 30-foot sections. This made it strong but flexible.

Dating Rock Samples

Rock samples from the ocean floor were dated by a radioactive-decay process. The small amount of uranium in seawater attaches to sediments, and its breakdown products, protoactinium 231 and thorium 230, also attach to sediments and sink over time. The protoactinium decays much faster than the thorium. Measuring the ratio of the two in a sample can determine the age of the sample accurately.

Proof of Spreading

The Glomar Challenger data showed that the seafloor was spreading evenly and gave further proof that the continents drift on a viscous layer of mantle.

Sources:

"Birth Date of Man," Time, 76 (11 July 1960): 53;

Wallace Cloud, 'The Ship That Digs Holes in the Sea," Popular Mechanics, 131 (March 1969): 108-111, 236;

"Did It Break? Is It Lost?," Life, 50 (7 April 1961): 37-40;

Robert S. Dietz, "The Spreading Ocean Floor," Saturday Evening Post, 234 (21 October 1961): 34-35, 94-96;

John Steinbeck, "High Drama of Bold Thrust through Ocean Floor," Life, 50 (14 April 1961): 110-122;

"Time for a Theory," Science News, 95 (10 May 1969): 449-450.