THE OCEANS, THEIR STRUCTURES AND FEATURES 



31 



slopes. They are believed to contain rocks inter- 

 mediate between the lighter continental and heavier 

 deep-sea floor types, and they contain sediments that 

 are thinner (because of the steep slope) but other- 

 wise similar to those of the continental shelfs. Their 

 upper limit generally represents the zone of maximum 

 sunlight penetration. Therefore, both the continental 

 slopes and ocean deeps are often areas of perpetual 

 darkness. 



CURRENTS 



In a previous consideration of layering of the 

 earth, it was mentioned that ocean waters have five 

 layers. The top layer, often consisting of about 100 

 feet of surface waters, is most directly influenced by 

 localized wind action but is still dominated by the 

 general pattern of world winds and earth rotation. 

 The second layer is one in which a number of swift 

 underwater currents are found. In general, these cur- 

 rents act to replace waters removed by surface cur- 

 rents. For example, in the Pacific Ocean there is the 

 Cromwell Current, about 200 miles wide, between 

 100 and 1000 feet below the surface, and about 7000 

 miles long. It moves at a rate of approximately 2)^2 

 knots eastward along the equator, and terminates at 

 the Galapagos Islands. Below this zone of swift un- 

 derwater currents is a third zone of slow-moving 

 waters that, in turn, caps at least two zones of deep- 

 sea waters. 



The swift second-layer currents generally move in 

 a direction opposite to, but in a path related to, a 

 surface current. For this reason, these underwater 

 currents act to replace waters removed by surface 

 flow. This phenomenon is best indicated by the 

 northward-moving Gulf Stream at the surface and 

 the associated, underwater, southward-flowing Gulf 

 Stream Countercurrent. These two currents also 

 serve as examples of the fact that the relationship of 

 depths in the two surface layers can be much dif- 

 ferent from that involving the Cromwell Current. In 

 the case of the Gulf Stream, the current is large, 

 3000 feet deep and 40 miles wide. Obviously, there- 

 fore, the underlying current cannot start at the 100- 

 foot level. In fact, the Countercurrent does not 

 even start immediately below the Gulf Stream at the 

 3000-foot level. Actually, the upper layer of the 

 Countercurrent 's dense, cold Arctic waters is about 

 6000 feet below the surface. Therefore, layering of 



ocean waters does occur, but it is not a precise depth 

 zonation. Rather, there is great local variation in 

 the depth, thickness, and width of currents that 

 characterize individual layers. 



The general movements of ocean currents are 

 governed by factors somewhat related to those pro- 

 ducing the winds over the world (Figure 3.3), In 



ROTATION) 

 (Coriolis Force) 



SOLAR HEAT 



WINDS 



CURRENTS 



Figure 3.3 Three major factors forming the over-all pattern of ocean 

 currents in the northern and southern hemispheres. 



fact, winds are one of the forces contributing to 

 major water movements. However, in addition to 

 winds, the primary factors causing ocean currents 

 are earth rotation, solar heat, and changing water 

 density. Secondarily, currents are affected by the 

 distribution of land masses, by climate, and by 

 gravity. 



When the atmosphere was discussed, it was stated 

 that the rotation of the earth partially controls the 

 winds. The same rotation and winds work together to 

 produce ocean currents of a pattern similar to that 

 of the air currents. The trade winds push the equa- 

 torial currents westward and, with the earth's rota- 



