24 



The Evolution of the Universe 



rocks become hotter and thus less dense as a result of thermal 

 expansion and that these rocks then move plastically upwards 

 toward the crust where they cool and sink again. In this way con- 

 vection currents in the mantle would develop. Urey (1952) sug- 

 gested that the separation of iron from nickel compounds at the 

 boundary between the core and the mantle releases heat and that 

 this energy may initiate convection currents in the mantle. 



Several writers since 1952 have disclaimed the existence of con- 

 vection currents in the mantle, but Menard (1960) demonstrated 

 that many lines of evidence concerning the East Pacific Rise are 

 explainable on the basis of such currents. The East Pacific Rise 

 is a vast low bulge of the ocean floor, about 2,000 to 4,000 km 

 wide with its crest arising northward off the coast of British 

 Columbia and Washington, cutting under the western edge of the 

 North American continent to about midwestern Mexico, from there 

 extending across the Pacific first south and then irregularly to near 

 New Zealand. In a lucid, remarkable synthesis, Menard pointed 

 out how the convection hypothesis ( Fig. 10 ) explained the observed 



PLAN OF ARCHED BUT UNDISPLACED BLOCK 



PLAN OF ARCHED $ DISPLACED BLOCK 



COMPRESS\Oj}^-^- 

 I 



f-RXNSLAljON 



FLOW 



Fig. 10. A diagram of the crustal features associated with the East Pacific 

 Rise and their supposed relation to the convection-current hypothesis. The 

 circle in the upper right depicts block faulting and rifting caused by the sup- 

 posed up-welling of the crust. (After Menard.) 



