Ch. 1] OCEANIC PROCESSES 19 



section, with the result that water tends to flow from the more dense 

 area to the less dense area. This process of adjustment of mass to- 

 gether with the wind causes the great ocean currents, such as the Gulf 

 Stream and the Japanese current. Owing to the rotation of the earth, 

 currents are deflected to the right in the northern hemisphere and to 

 the left in the southern hemisphere. 



The wind generates currents and tends to pile water up ahead of it, 

 thus overbalancing the water in front with respect to the water be- 

 hind. As a consequence the surface water in front sinks and the water 

 behind rises, bringing the deep water to the surface. This deep water 

 is rich in mineral nutrients needed for the microscopic plants, the 

 plankton, which are the basic source of food in the sea and of organic 

 matter in the sediments beneath. Upwelling of this sort is common 

 off the coast of California. 



The ocean currents convey warm water from the tropics toward the 

 poles and thus affect the temperature of the air, which in turn influ- 

 ' ences the movement of the wind, which then affects the motion of the 

 water. Thus the ocean, like a dog chasing its tail, is constantly trying 

 to attain a condition of equilibrium but never does. However, in many 

 parts of the ocean the rates of change on the average are fairly con- 

 stant; so a condition of dynamic, in contrast with static, equilibrium is 

 obtained. This general circulation of the water has profound effects 

 upon climate, the growth of organisms, the concentration of dissolved 

 substances in the sea, and thus in turn upon the deposition of sedi- 

 ments. 



Currents, however, are produced in the sea in several other ways. 

 Waves approaching the shore diagonally pile the water upon the 

 beach at an angle with the shore, and the water descending from the 

 beach reaches the sea at some point downwind from where it first 

 struck the beach (Munk and Traylor, 1947). Thus a longshore 

 current develops which may have a profound effect upon transporta- 

 tion and deposition of debris, particularly in areas in which the wind 

 blows mainly from the same general region. 



Tides produce currents in shallow water. The large tsunamis which 

 are reported to be caused by earthquakes conceivably could also set 

 up currents. In water less than 600 feet deep waves can stir up the 

 bottom and winnow the sediments (Stetson, 1938; Shepard, 1932, 

 1948). Internal waves, developed in layers of denser water beneath 

 the surface, conceivably could generate currents. Coarse sediments 

 in deep water in some areas may have resulted from such currents 

 (Revelle, 1944; Sverdrup Anniversary Volume, 1948, pp. 673, 683). 



