32 



HISTORY OF THE OCEANS 



1500 2000 



Depth, km 



2500 



3000 



Fig. 13. Electrical conductivity distribution of McDonald for the mantle, with 

 Lahiri and Price's d and e distributions for comparison (Tozer, 1959). 



velocities, will probably show an even closer approach of the 

 melting point curve to the temperature distribution in this part 

 of the mantle. The closer approach of the low-velocity zone to the 

 surface of the earth beneath the oceans might explain the well- 

 known paradox that the nonradioactive oceanic crust shows, 

 despite some variability, average values of heat flow similar to 

 those found for the radioactive crust in continental areas (Bullard 

 et al., 1956; Maxwell and von Herzen, 1959; von Herzen, 1959). 

 Above the low-velocity zone lies the high-speed subcrustal region, 

 which is quite likely to be the lower limit of the lithosphere beneath 

 continental and oceanic regions. 



