104 



EWaNG 



[chap. 7 



lying at an average depth of 1 2 km below sea-level are of the same material as 

 those underneath the continental M discontinuity, \\hich is at an average 

 depth of 30-40 km. He further notes that it is somewhat surprising to expect 

 that a material A\'ould have the same compressional-wave velocity under the 

 widely different conditions of temperature and pressure found at 12 km depth 

 under oceans and 35 km under continents, despite the fact that increasing 

 temperature and pressure change the seismic velocity in oj)posite directions. 

 Still further uncertainties about the mantle composition are apparent in the 

 light of recent hypotheses that the velocity change at the M discontinuity is 



25 - 



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7.5 8.0 8.5 9.0 



COMPRESSIONAL WAVE VELOCITY (KM/SEC) 



Fig. 1. Graph of observed sub-crustal velocities versus number of observations. 



caused by a phase change rather than by a change in material. In view of the 

 foregoing, therefore, the definition of mantle rocks here employed is based on 

 8.1 km/sec as typical of the compressional-wave velocity in them. Thus, a 

 lower velocity is assumed to represent mantle rocks which are either partially 

 contaminated by crustal material, or to represent a zone where fractional 

 phase change has occurred. Velocities higher than S.l km/sec then must 

 represent mantle material which has been more thoroughly differentiated 

 from crustal material or in which, possibly as a result of mineral inhomogeneity, 

 the phase change has produced an imusually high -velocity material. It is 

 possible, of course, that all velocities different from 8.1 km/sec are the result 

 of inaccurate measurement, but most of those who have analyzed any ap- 

 preciable number of profiles would not accept that explanation. The relatively 

 wide variation of velocities around 8.1 km/sec is entirely consistent with 



