14 HISTORY OF THE OCEANS 



corresponding point on the upward path, have been reported in 

 several papers (Nafe and Drake, 1957; Ewing and Ewing, 1959a). 

 The situation here is similar to that described for reflection studies, 

 in that conversion more commonly occurs at some level above the 

 oceanic layer than at its upper surface. Here again there is the 

 possibility for increase and refinement of our knowledge about the 

 superficial layers and their contact with the oceanic layer. 



It is only very rarely that either the reflections or the conversions 

 give positive information about the M (Mohorovicic) discon- 

 tinuity. Our positive information about it comes principally from 

 refraction measurements. Almost everywhere in the ocean basins, 

 the M discontinuity, as judged by seismic refraction measurements, 

 is a sharp, smooth boundary between rocks of the contrasting 

 types listed in Table I. These rocks are usually taken to be basalt 

 and peridotite. There are, however, exceptional areas in which 

 the transition is not sharp, as described above, and in which the 

 familiar rock types are not identifiable. These exceptional areas 

 are the island arcs and deep sea trenches, the continental margins, 

 and the Mid-Oceanic Ridge system. 



Mid-Oceanic Ridge System 



The Mid-Oceanic Ridge system aff^ords an outstanding example 

 of an extensive area in which the crustal structure dift'ers widely 

 from that typical of the ocean basins. The Ridge is up to 1000 

 miles wide and rises 1 to 2 miles above the floors of the basins 

 which commonly occur symmetrically on both sides of it. From 

 seismic refraction measurements (Ewing and Ewing, 1959a) we 

 know that it is commonly capped by a rock layer 3 to 5 km thick 

 in which the velocity of P waves is 4.5 to 5.5 km/sec, appropriate 

 for volcanic rocks as in Fig. 5. 



Beneath this cap, to a depth which probably ranges up to 35 to 

 40 km, is the main body of the ridge, with a compressional wave 

 velocity of about 7.3 km/sec. It has been suggested (Ewing and 

 Ewing, 1959a) that this velocity which is intermediate between 

 those for typical oceanic crust and mantle, may represent a mixture 

 of crustal and mantle rocks. The depth of 35 to 40 km, at which a 

 velocity of 8.1 km/sec would be found, was estimated on the 



