that extends seaward for some fifty miles. This plateau, known as 

 the continental shelf, slopes gently downward away from the coast, 

 and on the whole it is remarkably flat at depths between fifty 

 fathoms and a hundred fathoms. As far as we can tell, its rocks and 

 sediments are similar to those on the continents. We find areas 

 sometimes of sand and mud, sometimes of coarse gravel and shells, 

 and occasional mounds indicating that solid rock must be sticking 

 up through the sediment. We are not sure whether the flatness of 

 the continental shelves is the work of currents and waves cutting 

 into the edge of the continents and leveling it out, or whether the 

 shelves are great dumping grounds where the eroded products of 

 the land are accumulating, as they do in a delta at a river mouth. 



As we reach the outer edge of the shelf, quite suddenly we find 

 the ocean depth increasing and we descend the continental slope. 

 The slope continues for forty or fifty miles, plunging down to the 

 deep-sea floor. Some of the slopes drop at a rate of one in twenty, 

 others at a rate of one in four, comparable with the sides of the 

 mountain peaks on the land. It is the continental slope, not the 

 coast line, that marks the true boundary of each continent. It is 

 here that the thickness of the Earth's crust - that thin shell of hard 

 rocks lying above the world-encircling mantle — changes abruptly 

 from a thickness of about thirty miles under the continent to about 

 five miles under the ocean. The continents, then, appear as great 

 "rafts" of lighter rock floating in the heavier mantle rock, whereas 

 the oceanic crust is a mere layer of scum. 



Each continental slope is under constant attack from erosive 

 and other destructive forces in the sea. Many slopes are incised 

 with steep-walled canyons that serve as passageways through which 

 the continental sediments flow out into the ocean basins. For many 

 years we thought that these canyons were cut by ancient rivers 

 that flowed at a time when the sea level was much lower and the 

 edges of the shelves were exposed to the air. But it does not now 

 seem likely that the sea level could ever have dropped as much as 

 six hundred feet below its present level; furthermore, we still have 

 to explain how those parts of the canyons that lie thousands of 

 fathoms below the surface could have been cut. 



Most submarine geologists today think that these giant canyons 

 — however they were formed originally — are kept scoured out by 

 currents of sediment-laden water hurtling at great speeds down the 

 continental slopes. These turbidity currents, as they are called, may 

 be set in motion when an earthquake or some other violent disturb- 

 ance creates sHding and slumping in the sediments heaped on the 

 edges of the continental shelves. When this happens, some of the 

 sediments are stirred up into the water and create a local body of 

 water that is denser than the surrounding water. The denser, sedi- 

 ment-laden water begins to flow down the slope, picking up more 

 sediment along the way. As it becomes still denser it travels faster, 

 and by the time it reaches the base of the slope it may be moving 

 at enormous speeds - up to fifty or sixty miles an hour. Such a 

 turbidity current south of Newfoundland was touched off by an 

 earthquake in 1929 and broke several submarine telegraph cables 

 along its path. The exact time when each cable was broken could 

 be worked out accurately from the interruption of services, and so 

 it was possible to measure the speed of the current as it passed. 



A turbidity current begins to deposit part of its sediment load as 



The destructive force of a turbidity current 

 can be enormous. In 1929 an earthquake started 

 a turbidity current south of Newfoundland. 

 As the mud-laden water rushed down the slope 

 it broke several telegraph cables along its 

 path, as shown in this chart. Core samples 

 taken near the lower end of the turbidity 

 current revealed graded layers of silt (laid 

 down by the current) in the upper part of the 

 core, and clays of the abyssal plain in the 

 bottom part of the core. 



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