4 HISTORY OF THE OCEANS 



Depth- Frequency Relation 



We also take for granted acquaintance with the statistics about 

 area in relation to depth, as shown in the hypsographic and depth- 

 frequency curves, e.g., Dietrich, 1957, p. 6 (Fig. 1). There are two 

 predominant levels, one represented by a broad maximum of the 

 frequency curve at about —4950 m (2700 fm) and the other by a 

 sharper maximum at about +100 m (55 fm). 



The lower level is of fundamental structural importance. From 

 the depth-frequency curve we see that it represents the prevailing 

 water depth in deep sea basins, which occupy about 45% of the 

 total earth surface. It is the level for the typical oceanic column 

 of mantle, crust, and sediment. Its actual depth below sea level 

 depends simply on the amount of water in the oceans, but its 

 elevation with respect to the mean elevation of the continental 

 masses is a fundamental geophysical constant which is affected 

 only in a secondary manner by the weight of water in the oceans. 



The +100-m level represents the continental shelves, coastal 



500 Mill kn 



Fig. 1. H>psographic and deptli-frequenc\- cur\es (after Dietrich, 1957). 



