26 



The Ocean 



Table 7 gives a list of the marginal deeps and the greatest depths that have so far been 

 measured in each. Without doubt these marginal deeps contain the deepest fissures 

 in the Earth's crust, and in their neighbourhood are the greatest vertical differences in 

 height that are to be found within a short horizontal distance on the Earth's 

 crust. 



The marginal deeps are conspicuously associated with the volcanic belt which 

 stretches along the landward side (on island chains or submarine ridges) parallel 

 with the line of deep-sea trenches and with the earthquake belt which is also present 

 in the immediate neighbourhood of the trenches, especially on the landward side. This 

 connection with seismic and volcanic activity is always present and indicates a causa- 

 tive connection between these phenomena. Another phenomenon closely associated 

 with the marginal deeps is the strong negative gravitational anomaly occurring along 

 a very narrow line. The investigations of Vening-Meinesz (1932, 1934) on the gravi- 

 tational field in the East Indies and later the investigation of Hess (1938) in the West 

 Indies have clarified this connection. The belt of abnormal gravity does not coincide 

 exactly with the line of deep-sea trenches, but is displaced towards the adjacent moun- 

 tain ridge. There exists in all cases a parallelism with the deep-sea trenches, but the 

 relationship to the topography is more complicated than this. In the Philippine trench 

 the line of negative anomaly lies directly underneath the trench (see Fig. 17) but it is 



800 



Fig. 17. Gravity profile over the Philippine Trench at Surigao (isostatic anomaly; observed 



values indicated by black dots; the bottom profile shown schematically with a vertical 



enlargement by 1:15) (according to Vening-Meinesz). 



weak, although the trench is particularly deep; in the Java trench the gravitation 

 anomaly is very pronounced but lies at the side of the trench (Fig. 18). Since a line of 

 negative gravitational anomaly is present wherever there is a deep-sea trench, there 

 must undoubtedly be some connection between the two phenomena. This is also indi- 

 cated by the relationship of seismic activity and the distribution of volcanoes mentioned 

 above. For the explanation of this relationship, see especially Vening-Meinesz 

 (1940). 



In addition to the deep-sea trenches there are also the differently shaped, nearly 

 circular depressions. It cannot yet be decided whether these should be regarded as 

 deformed marginal deeps but those between the Sunda Islands, the Moluccas, and the 

 Philippines (Celebes, Sulu, Banda and other deeps) occur in close connection with the 



I 



