II 



77?^ Ocean 



includes not more than 6% of the surface of the Earth, and this percentage is being 

 decreased rather than increased by the results of echo sounding. These figures empha- 

 size that the deep-sea basins are not just chance depressions in the crust of the Earth. 

 This division of the structure of the Earth is one of the most important of geophysical 

 phenomena and requires a special explanation that must be very closely connected 

 with the history of the Earth. 



Charting the sea bottom by means of isobaths and measurement of the areas of the 

 different depth-intervals makes it possible to calculate the volume of each ocean and 

 of the total ocean. The quotient of the volume and the surface area gives the mean 

 depth. The volume of the ocean (including all the adjacent seas) amounts to 1370-6 

 million km^ and the mean depth is therefore around 3800 ± 100 m. The volume 

 and mean depth can also be worked out for parts of the ocean and for the adjacent 

 seas: the values for most areas according to Kossinna are given in Table 4. The 

 Atlantic, Indian and Pacific Oceans have the mean depths 3930, 3960 and 4280 m 

 respectively. These figures are not very different; the mean deviation is little more than 

 4^0. In addition to this general agreement, the figures for the depth-intervals in all 

 three oceans, as shown in Table 5, demonstrate a very similar morphological structure 

 of the Earth crust. This is further proof of a uniform structure in different parts and 

 an indication that the existence of the two favoured levels of the Earth's crust repre- 

 sented by the continents and the deep-sea bottom is a universal phenomenon prevailing 

 over all parts of the Earth's crust. If the average density of sea water, taking the com- 

 pressibility into account, is as 1-037, the total mass of the ocean will be 1-42 x 10^^ = 

 1-42 trillion tons which is only 1/4200 part of the mass of the Earth. 



Table 5. Morphological structure of the three oceans (exchiding mediterranean seas). 



Areas of the different depth-intervals given in percentage of the total Earth surface 



(Atlantic Ocean according to Stocks 1938; otherwise according to Kossinna 1921) 



Depth-interval 

 in km 



0-0-2 



Atlantic Ocean 

 Indian Ocean 

 Pacific Ocean 



All oceans 



5-8 

 3-2 

 1-7 



3-1 



5-6 



23-3 

 19-9 

 28-8 



26-2 



6-7 



0-7 

 0-4 

 1-8 



1-2 



0-3 

 0-3 

 0-1 



Sum 



100-0 

 100-0 

 100-0 



100-0 



3. Special Characteristics of Sea-bottom Topography 



The larger and smaller oceans and parts of the oceans are usually considered as more 

 or less extended volumes sunk into the solid crust of the Earth. From this one is guided 

 to assume that the sea bottom taken as a whole is concave inward. In reality this is so 

 only in exceptional cases; in general the sea bottom arches upward and follows the 

 surface of a sphere with a somewhat larger radius than that of the surface of the Earth. 



Expressed in another way the radius of curvature of the sea bottom points towards 

 the centre of the Earth almost all the time and differs little from the radius of curva- 

 ture of the Earth. If large areas are considered, really concave basins occur very in- 

 frequently and are limited to the margins of the deep ocean trenches, to crater- 

 shaped basins and especially to individual adjacent seas. Bathymetric charts of the 



