The Ocean 



11 



East Indian negative gravitational anomaly. There are similar shaped deeps in the 

 European Mediterranean, in the Gulf of Mexico and in other places, though not of 

 the same depth or extent. Amongst these may be reckoned the comparatively small but 

 very deep Romanche deep which divides the mid-Atlantic Ridge in two, at about 

 18-19° W. on the equator. The corresponding lowering of the mid- Atlantic ridge 

 is as low as 4500-4800 m. The great significance of this deep connection between the 

 eastern and the western troughs for the hydrographic structure of the water masses of 



-100 



Isost Anomaly 



600 



Fig. 18. Gravity profile from Benkulen (Sumatra) towards the Indian Ocean (see Fig. 17). 



the eastern trough will be discussed later (see Chap. Ill, 5, b). The greatest depth 

 measured in the Romanche deep is 7230 m. A bathymetric chart of the area has 

 been given by Stocks and Wtisx (1935). 



While the slope of the deep-sea bottom is in general slight and only reaches larger 

 values at the continental slope, occasionally very steep gradients occur near islands, 

 submarine banks and reefs. As on land there has often been major volcanic activity on 

 the sea bottom, partly in extended zones associated with the deep-sea trenches and 

 partly more widely spread. The steepest slopes are always those of the purely oceanic 

 islands which are all of volcanic origin; these slopes are of the same order of magnitude 

 as those of land volcanoes. The slope of the island St Helena, for example, over short 

 distances is as much as 38-40° and the Atlantic island St Paul has slopes of 62°. 



In numerous cases the volcanic forces have been insufficient to build an island cone 

 up to the surface. They form submarine peaks, whose summits may still be some 

 hundreds of metres below the surface and seldom come up to normal anchorage 

 depths. These submarine volcanic cones were only occasionally found by wire sound- 

 ing, which allows them to be quickly and accurately charted. In this connection there 

 might be mentioned the surveys of the area of the Bogoslov volcano (Bering Sea) by 

 the United States Coast and Geodetic Survey (Smith, 1937) and the survey of the 

 "Altair" peak (Defant, 1939). 



4. Arrangement of the General Bottom Topography of the Individual Oceans 



For an elucidation and abbreviation of the following discussion Plate 2 is presented, 

 and it shows all the main characteristic features of sea-bottom topography in a clear 

 manner. For each ocean there is a list of the principal features which have been desig- 

 nated by letters and numbers on the plate. The capital letters show the deep-sea basins 

 (troughs) in succession for each ocean, the small letters denote the ridges and rises 

 that separate these basins, and the numbers indicate the deep-sea trenches. 



