Terrible Turbidity

One of the mysteries of the ocean is the long, deep gorges that wind across the continental shelves like submerged river valleys. Oceanographers thought at first that they were really valleys cut by ancient rivers when sea level was lower, and flooded by the rising water when ice-age glaciers melted. This theory went out of fashion when improved sounding methods showed that some of the streamlike channels lead down to the ocean floor itself three miles below the surface. The level of the ocean could never have fallen as low as that.

A more recent theory has it that the gorges were cut by "turbidity currents," i.e., rivers of mud on the bottom. When a slope of loose material is disturbed--by an earthquake, for example--mud and sand get mixed with the water. Since the turbid mixture is heavier than clear water, it flows down the slope, eroding a valley just as a river does on land. This was known to happen in lakes, and many oceanographers believed that the same thing happened deep under the ocean.

Cable Trouble. In a recent American Journal of Science, Bruce C. Heezen and Maurice Ewing of Columbia University buttress this theory with a neat bit of historical research. In 1929 a strong earthquake shook the continental shelf 450 miles east of Nova Scotia. It cut a whole sheaf of telegraph cables in a peculiar way. Six cables went out at the same time, but others did not fail until many hours later.

Heezen and Ewing appealed to Western Union and other cable companies. Just as they hoped, the companies had made careful studies of the costly disaster 24 years ago and had kept all the records. As each cable failed, the exact second of its failure was recorded by instruments on land. Other instruments determined accurately the position of each break. More information came from the repair crews. Long sections of some of the cables had been carried away and lost. Other cables were buried deep under mud and sand.

Bit by bit, Heezen and Ewing reconstructed what must have happened on that day of undersea commotion. The sea bottom near the epicenter of the quake is rather irregular with many comparatively steep slopes of loose material. The quake must have jolted this detachable stuff, starting slumps and landslides that cut the nearest cables at about the same time.

Racing Mud. The trouble did not stop there. The stirred-up mud and sand got mixed with water, and the heavy turbid fluid raced down the continental slope like an enormous river more than 100 miles wide, cutting cable after cable. By plotting the time and place of each cable break, the oceanographers could estimate closely how fast the turbidity current flowed. On the sloping continental rise (at the foot of the continental slope), it raced at 50 knots (57.6 m.p.h.). More than 13 hours later, when it cut the last cable 300 miles to the southeast, the mud was still flowing at 12 knots (13.8 m.p.h.). It must have spread for hundreds of miles over the flat ocean floor.

This reasoning is supported by the fact that many cables were not merely broken but buried by the mud flow. Heezen and Ewing believe that many such flows, racing for hundreds of miles through the still depths of the ocean, have carved the gorges on their slopes. The "deltas," where their finest material finally settles down, are the flat plains that form the floors of many deep ocean basins.

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