12 HISTORY OF THE OCEANS 



considered as all the solid earth above the mantle, being separated 

 from it by the Mohorovicic discontinuity. The crust extends to 

 depths of 35 to 55 km below sea level under the continents, and 

 to 10 to 12 km below sea level (4 to 6 km below the ocean floor) 

 under the oceans. 



There is a great discontinuity in the velocity of elastic waves, 

 and probably also in composition, at the Mohorovicic discon- 

 tinuity. xAlmost everywhere the uppermost part of the mantle is 

 considered to be peridotite, with compressional (P) and shear {S) 

 wave velocities of 8.1 and 4,6 km/sec, respectively. The continental 

 crust, in many continental areas, has been divided into an upper 

 and lower part, separated by the Conrad discontinuity. The upper 

 part consists principally of granitic, volcanic, and sedimentary 

 rocks, many of which have been strongly metamorphosed. Elastic 

 body wave velocities in the upper part of the crust range up to 

 6.5 and 3.6 km/sec for P and S, respectively (Katz, 1955). The 

 lower part of the crust, both under continents and under oceans, 

 is believed to be basaltic rock, in which the seismic velocities are 

 about 6.7 and 3.8 km/sec. Data for typical continental and 

 oceanic crustal sections are given in Table I. 



Crustal Section for Ocean Basins 



In the earlier reports of seismic refraction results for oceanic 

 crust, only water, sediment, and a crustal or "oceanic" layer were 

 mentioned. But later, in many areas, an additional layer has been 

 found between the oceanic layer and the sedimentary layer. The 

 velocity in it is 4.5 to 5.5 km/sec, the average being about 5 

 km/sec. The average thickness of this layer is about 1-2 km. It 

 is probably present in many areas where previously not reported, 

 owing to masking of its refraction arrival by the sedimentary and 

 oceanic layers. 



The sedimentary column for typical basins in the Atlantic 

 averages about 500 m, whereas in many parts of the Pacific the 

 average is 300 m. This difference may be accounted for by the 

 remoteness from land of large parts of the Pacific, the relative 

 scarcity of rivers which bring large sediment loads, and by the 

 presence of sediment traps (the deep-sea trenches which occur 

 along a large part of the border of the Pacific). 



