SCIENCE AND THE SEA 



at a horizontal distance from the drill hole not to exceed five per- 

 cent of the depth. This requirement was satisfied by measuring by 

 sonar the distances from the platform to sonar buoys moored on the 

 ocean bottom, feeding these data into a computer which regulated 

 the direction and pitch of the platform's propellers to keep its posi- 

 tion within allowable limits. 



Figure 1. Submersible DEEPSTAR-iOOO. capable of diving to iOOOfoot depths 

 for periods up to 2i hours. Built by Westinghouse Electric Corporation. 



Vertical 

 propulsion motor 



Main ballast tank 

 Aft trim tank 



Main propulsion 

 motor 



Bow th raster 



Viewing ports \ 



Batteries \ 



Mechanical arm 



Figure 2. Cutaway drawing of submersible STAR III equipped with external 

 mechanical arm that has interchangeable "hands". 



Courtesy General Dynamics Corporation. 



THE CONTINENTAL SHELF 



The Continental Shelf is the shallow platform adjacent to the 

 continents or some large islands, stretching out from the shore to a 

 depth where a marked increase of slope to greater depths is noted. 

 Its special feature, the shallowness, dampens the swell which rolls 

 in from the deeper parts of the ocean, and makes the establishment 

 of harbors along its edge feasible. Because most of the fish in the 

 ocean is on the shelf, fishermen have been busily engaged in this 

 area. 



This platform has been better explored and surveyed than the 

 remainder of the ocean floor because of the shelf's importance to 

 the mariner who avoids the shoals and obstructions, and uses sound- 

 ings and bottom samples as aids to navigation. However, investi- 

 gation of the shelf is an endless task, because tectonic activity fre- 



quently changes the bottom contours considerably as was experi- 

 enced when the U.S. Coast and Geodetic Survey took soundings in 

 an area in Alaska affected by the earthquake occurring in 1963. 

 Moreover, erosion and deposition cause alteration in the bottom 

 relief. 



In recent years the shelf has gained added importance for the 

 large amount of minerals found on and beneath its floor. In 1946 

 the United States took possession of the mineral rights on the con- 

 tinental shelf adjacent to its shores; the 100-fathom curve was de- 

 fined as its outer limit. 



In many other parts of the world, however, the outer limit of 

 the shelf, where a marked increase in slope is noted, lies at a different 

 depth. According to F. P. Shepard, the average slope of the shelf in 

 the world is 0° 1', its average width is about 42 miles, and the aver- 

 age depth of the outer margin is 72 fathoms. It has been estimated 

 that the shelves cover about 7.6 percent of the earth's surface. 



Formerly, the shelf was believed to be always a gentle sloping 

 plain, but this is only true in specific locations. Often the shelf is 

 hilly with many irregularities. An example of shelves with a rough 

 bottom is found adjacent to the land masses which had been covered 

 by glaciers. Deep depressions, such as the fjords in Norway and 

 British Columbia, or deep troughs, such as the Gulf of St. Lawrence 

 and the Straits of Juan de Fuca, penetrate far into the land, and 

 often extend across the shelves, but with shallower depths seawards. 

 Those deep bays frequently have many basins containing muddy 

 sediments combined with grave! and sand. Many banks, rising close 

 to sea level, extend along the outer shelf and are covered by sand 

 and gravel, among which are the Grand Banks of Newfoundland 

 and Georges Bank off New England. They furnish the best fishing 

 grounds in the world. Numerous hills, mostly mantled with rock or 

 covered with boulders or gravel, are sometimes located on the inner 

 shelf. 



Shelves with a smoother bottom are those with elongated sand 

 banks and depressions. This bottom relief is found on the shelf 

 adjoining the shore of New Jersey and resembles the topography on 

 land, the plains with their sand ridges. These shelves are mostly 

 covered with sand or a combination of mud and sand. In areas sub- 

 ject to severe storms or strong tidal currents, such as the North Sea 

 or off Nantucket, elongated banks are shifting constantly and have 

 to be sounded frequently to warn the mariner of changing condi- 

 tions. A narrow shelf which is subjected to strong currents is found 

 on the east coast of Florida and off Cape Hatteras. 



On the Pacific coast the shelves located off young mountain 

 ranges are mostly narrow and deeper than the average or are lack- 

 ing entirely. An interesting example of a missing shelf is on the 

 west coast of South America where the land declines with a 5-degree 

 slope from the crest of the Andes Mountains to the bottom of a 

 trench offshore. 



The shelves which are the widest and the shallowest are found 

 off river deltas and in areas where coral growth is plentiful. On the 

 northern coast of Siberia, where large rivers carry loads of sedi- 

 ments, the shelves are the widest in the world. Wide shelves can 

 also be found at the mouth of the Amazon River, in the Yellow and 

 Bering Seas, in the Gulf of Siam, in the Persian Gulf, and in the Bay 

 of Bengal. Shelves where coral growth is extensive are found off 

 Australia. The shelf off Queensland is about 170 miles wide and very 

 shallow. The Great Barrier Reef of active coral growth lies on its 

 outer part. 



Interesting features in the bottom relief of the shelf are the 

 submarine valleys. They are sometimes loaded with sediments but 

 others which are scoured by tidal currents can be easily detected, 

 as is the case of the valleys off the coast of Brittany. 



A discontinuous submarine valley lying slightly below the sur- 

 rounding area is located between Java, Sumatra, Malay Peninsula, 

 and Borneo. Hurd Deep with a depth of 94 fathoms and the Ouessant 



