THE OCEANS, THEIR STRUCTURES AND FEATURES 



29 



SALINITY 



The salt content of the seas is fairly constant at 

 about 34.4 parts in 1000 parts of water. The most 

 important physical effect of this salt is to depress the 

 freezing point about four degrees below that of fresh 

 water. This 28° F. freezing point bears an interesting 

 relationship to the deep-water temperature of 35 °F. 

 The difference of 7°F. (about 4°C.) from freezing 

 means that deep water is at its greatest possible den- 

 sity. Any water that might become colder than 35 °F. 

 becomes lighter, hence rises and mixes with warmer 

 water. Naturally, this serves to prevent deep water 

 from freezing. 



PRESSURE 



The pressure in sea water increases approximately 

 one atmosphere for every 33 feet of depth. There- 

 fore, at 99 feet the four atmospheres pressure is due 

 to the weight of water above that particular depth. 



OCEAN-FLOOR TOPOGRAPHY 



The bottom of the ocean is more rugged than the 

 surface of the land. Under the waters are valleys, 

 gorges, cliffs, plateaus, volcanoes, and mountain 

 ranges. Moreover, all of these physical features 



occur on a gigantic scale, perhaps because there is 

 less erosion within the sea than upon the land. Ex- 

 cept for the mild sculpturing of deep ocean currents 

 and the more abrasive undersea landslides, no major 

 erosion factor is known to affect the ocean floor. 



DEEP-SEA FLOOR 



The differences between land and ocean-floor 

 topography are emphasized when consideration is 

 limited to that half of the earth comprising the deep- 

 sea fioor (Figure 3.2). Such a comparison emphasizes 

 the error in calling the deep-sea area a "floor." 

 Rather, it is ribbed, corrugated, and grooved by 

 pronounced ridges and trenches. Superimposed 



upon this pattern are mountain ranges. A relief map 

 of the ocean floor would show that it would be impos- 

 sible to walk very far in a straight line — in fact, travel 

 of any kind would be virtually impossible. 



Covering the bottoms of ocean basins are sedi- 

 ments, the marine oozes, that are sometimes streaked 

 with sand of unknown origin. The oozes are some- 

 times white, but some are yellow, green, pink, red, or 

 brown. They are composed of remains of shells and 

 skeletons of plants and animals, of volcanic rocks, and 

 of some meteorite fragments. 



Closer to the land at the base of the continental 

 slopes are sediments of land origin. Erosion of the 



OCEAN 



volcanic 

 island 



CONTINENT 

 coastal plain 



MANTLE 



Figure 3.2 Mojor crustal features of continents, ond especially oceon basins, as portrayed above 

 tfie earth's mantle. Differential tfiickness in the crust and its component parts are suggested in 

 relation to various land forms. 



