30 



HISTORY OF THE OCEANS 

 12 3 4 



35 kr 



200 kr 



900 kr 



2900 km 



Fig. 11. Density in the mantle in grams per cubic centimeter, reduced to 

 surface temperature and pressure (after Birch, from Heiskanen and Vening 

 Meinesz, 1958). 



It is also to be noted that no deep focus earthquakes have ever 

 been found at these depths, indicating that a rate of strain accumu- 

 lation sufficient to produce earthquakes occurs only above 800- 

 900 km. 



Between depths of 900 km and 200 km, all the recent seismic, 

 thermal, geochemical, and electrical studies have been interpreted 

 to indicate a broad zone of transition between the homogeneous 

 lower mantle, and the low-velocity region of the upper mantle, as 

 indicated by the change in reduced densities in Fig. 11. Ringwood 

 (1958) has shown that this transition zone is quite likely to be a 

 region in which iron and magnesium silicates change from their 

 high-pressure spinel lattice structure to the low-pressure olivine 

 crystal lattice structure found at shallower depths. 



Benioff (see Fig. 8) has shown that the rate of strain accumu- 

 lation is remarkably constant in this region between depths of 300 

 and 800 km. 



