34 



OCEANOGRAPHY: 



oscillatory basins, which are areas of the ocean floor 

 extending outward from the land. The effects of in- 

 dividual basins can be likened to those on water in 

 any container. Contained water provided with a 

 constant rocking force will move from side to side or 

 end to end in a definite manner that is related to the 

 length, width, depth, and general shape of the con- 

 tainer. The amount of movement is dependent upon 

 the amount of energy applied and the size of the 

 basin. Since the amount of energy, or tidal pull, is 

 essentially constant upon different parts of our planet, 

 local tides are most dependent upon the character of 

 their oscillatory basins. The important feature of a 

 given basin is how long it takes water to move out- 

 ward and inward (oscillate). If the natural features 

 of the basin are such that normal outward movement 

 of water coincides with the period of low tide de- 

 velopment and inward movement with high tide de- 

 velopment, the tides will range from very low to 

 very high. Therefore, the closer the association be- 

 tween tidal and oscillatory basin cycles, the greater 

 the vertical displacement of waters; the more remote 

 the association, the less the vertical displacement. 

 For this reason, one can appreciate the fact that the 

 Bay of Fundy shows a close correlation between tide 

 and oscillatory basin cycles, and many oceanic is- 

 lands show direct antagonism between these two fac- 

 tors. 



Oscillatory basins also explain why the number of 

 daily tidal cycles varies in different parts of the world. 

 Some places never have more than one cycle, others 

 generally have two, and still others may have one or 

 two. This can be understood in terms of oscillatory 

 basins. Basins having cycles closely related to the 

 tidal cycle will have two tidal cycles per day. How- 

 ever, the more remote the oscillatory basin cycle from 

 that normally producing a tidal cycle, the greater is 

 the tendency for one potential tidal cycle to be can- 

 celled out by the oscillatory basin cycle. 



WAVES 



Ocean waters respond readily to air movements, 

 but it is the wave form and not the water that 

 moves (Figure 3.6). Only a 2 mph wind is needed to 

 produce a wave, and about one-fourth that speed to 

 produce ripples on water. However, three factors are 

 required to produce waves of much size; wind veloc- 

 ity, wind duration, and length of fetch (the distance 



Figure 3.6 Movement of a drop of water (at arrow point) illustrates 

 why wave form moves over the ocean but water does not move. 



water travels without being obstructed by land, 

 shoals, reefs, or other obstacles). Working together, 

 the factors produce an average maximum wave 40 feet 

 high. To form this wave takes the equivalent of a 

 60 mph gale lasting two days over an uninter- 

 rupted 900 miles of ocean. When this is considered, it 

 is not surprising that any wave over 80 feet high is 

 considered abnormal, and the very tallest waves are 

 about 125 feet high. 



If one wishes to be precise, three names should 

 be applied to waves. When a wave first forms in open 

 water and remains in wind-blown water, it is called a 

 sea. When a sea continues into relatively wind- 

 free and otherwise calm waters, it is called a swell. 

 Finally, when any wave reaches the land and breaks, 

 it is called surf. 



Surf visually emphasizes the tremendous energy 

 that can be stored within waves. When large waves 

 break, they may completely destroy man-made ob- 

 jects. Such things as jetties and sea walls, although 

 constructed to withstand the full ravages of surf, are 

 never permanent. In a few years, most are damaged 

 or destroyed. Destruction of this kind is possible be- 

 cause the force of a surf is known to reach 3 tons per 

 square foot, a force that will easily move 40-ton blocks 

 of cement. 



TIDAL WAVES 



Although tidal waves are waves in every sense, they 

 are not formed by wind and gravity, the factors pro- 

 ducing other waves or tides. These sometimes gigan- 

 tic waves are caused by submarine volcanic and 

 earthquake activity. However, they are large only as 

 they approach the shore. At sea, they are low and 



