24 



The Ocean 



long. It divides the Atlantic Ocean into two parts: the eastern and the western At- 

 lantic troughs. These two elongated depressions are further divided into basins by 

 transverse ridges. The peculiar relief features of the Atlantic Ridge which forms the 

 axis of the Atlantic and runs roughly parallel to the continental coast on both sides is 

 regarded by many as the beginning of a mountain fold, but it could also be the rump 

 of an old one (Kossmat, 1931). 



The Indian Ocean shows a similar division. Here also there is an Indian Ocean 

 Ridge dividing it into an eastern and a western half, though these two halves appear to 

 be less subdivided. The Pacific Ocean, on the other hand, is largely a single basin (see 

 p. 29). 



Amongst the most prominent features of the oceanic bottom topography are the 

 narrow elongated arcs of marginal deeps that lie near the surrounding mountain 

 chains (or island chains) of the Pacific basin and contain the greatest ocean depths. 

 These remarkable depressions are confined exclusively to the margins of the Pacific 

 Ocean; they can also be found in the Sunda arc in the eastern Indian Ocean, in the 

 Caribbean, in the middle Atlantic Basin and in the south Sandwich marginal deep in 

 the western part of the South Atlantic. They are usually termed "deep-sea trenches" 

 or "troughs". This has reference only in a morphological sense and not to its origin. 

 They are very closely connected with folding processes in the earth's crust, and to some 

 extent are the counterpart of the mountain chains of the land, and have a related origin. 

 As an example, the Mariana marginal deep is shown in Fig. 16 both on an isobathic 

 chart and in a profile perpendicular to its longitudinal extension (Sigematsu, 1933). 

 Its topographical form is typical of all well-developed marginal deeps. On the side 

 towards the land, towards the submarine ridge which runs alongside the deep and is 

 always of mountainous character, the slope is steep, on the ocean side of the deep the 

 slope is more gentle. On the landward side the angle of the slope may be as much as 

 20° or more; according to Schott the mean value for a large number of Pacific deeps 

 is 6-3°. They are always long and narrow. 



Table 7. The most important trenches 

 (With reference to soundings up to 1954) 



