about 7.6 percent of the total ocean area. In this zone are the conti- 

 nental shelves, the shallow aprons surrounding continental land masses. 



Continental shelves vary greatly in width, from a few miles off the 



California coast to some 800 miles in the Siberian Arctic. Although 



the average slope of the shelf is gentle, within the shelf boundaries, 



terraces, ridges, hills, depressions, and steep-walled canyons may 

 occur. 



Next, a deeper zone of the continental margin from about 200 

 to 3,000 m deep takes up about 15.3 percent of the oceanic area. 

 Within this zone are the continental slopes of the oceanic floor, 

 regions in which continental land blocks drop off rapidly to the 

 deeper sea floor. A common feature of the slope is the occurrence of 

 extraordinarily large gullies, known as submarine canyons, which are 

 steep-walled fissures penetrating the slope and cutting across the 

 shelf. 



The continental slopes, with a typical gradient of about 4°, 

 drop smoothly to the incline of the continental rise which has a 

 gradient of about 0.3° to 0.05°. The continental rise then merges into 

 the floor of the ocean basins. 



The greatest portion, 75.9 percent of the sea floor, lies at 

 depths ranging from 3,000 to 6,000 m, the zone of large oceanic 

 depressions called basins. These are usually considered to be the 

 Atlantic, Pacific, and Indian. Other large areas also are sometimes 

 considered as separate units, such as the North Polar Basin, North 

 American Basin, and Pacific-Antarctic Basin. 



A major feature of the ocean floor is the mid-ocean ridge 

 about 1,500 to 2,000 km wide that extends for 60,000 km and covers 

 about 25 percent of the ocean floor, an area about equal to that of 

 all the continents. Throughout most of its length, the ridge is split 

 by a deep rift in which many earthquakes originate, the ridge being 

 the locus of a crack in the Earth's crust. The mid-ocean ridge runs 

 nearly twice around the Earth, occupying about the same position on the 

 hypsographic curve as continental slopes. 



Only 1.2 percent of sea floor lies at depths ranging from 6,000 

 to 11,000 m. This is the area of troughs, trenches, and deep basins. 

 The deepest trenches are found in the Pacific Ocean and exceed 10,000 m, 

 whereas the deepest point in the Atlantic Ocean is the Puerto Rico 

 Trench at 8,750 m. Note that the depth of the deepest trench is greater 

 than the height of the highest mountain. 



c. Bottom Sediments 



The crust of the ocean floor is coated by a sedimentary 

 blanket that has buried or modified the original igneous surface. The 

 morphology of the sea floor is, therefore, the result of generation of 

 igneous topography through processes of faulting and volcanism at the 

 crest of the mid-oceanic ridge, lateral motion away from the ridge 

 crest in a conveyor belt fashion, and slow burial by sedimentary 

 processes. 



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