64. What is the continental shelf? 



Officially, United States laws define the continental shelves as the 

 seaward extention of the coast to a depth of 600 feet; this limit is set 

 for the purpose of granting mineral rights, including oil drilling. The 

 edge of the continental shelf, where the bottom begins to slope steeply, 

 most commonly is found at depths between 360 and 480 feet. 



At the time the shelf received its name, it was thought to be essen- 

 tially flat; now geologists know that the continental shelf has basins, 

 ridges, and deep canyons. Compared to the deeper ocean floor, how- 

 ever, the relief is gentle; hills and basins on the shelf usually do not ex- 

 ceed 60 feet. 



The continental shelf width varies from practically nothing to several 

 hundred miles. The shelf along the east coast of the United States is 

 many times wider than that along the west coast. If all the continental 

 shelves of the world are included, the average width is approximately 

 40 miles. 



The shelf slopes gently, at an average drop of 12 feet per mile, from 

 the shore to the continental slope. In contrast, the grade of continental 

 slopes is 100 to 500 feet per mile. 



Sediment Layer 



About? percent of the ocean is underlain by continental shelves. These 

 are the areas where intensive mineral exploration is now being con- 

 ducted. 



Cromie, William J. 



Exploring the Secrets of the Sea, Prentice-Hall, 1962. 

 Engel, Leonard, and Editors of LI FE 



The Sea, Life Nature Library, Time, Inc., 1961. 

 Stewart, Harris B., Jr. 



Deep Challenge, Van Nostrand, 1966. 



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