408 HEEZEN AND EWINCi [CHAP. 16 



9. Origin 



The origin and structure (Fig. 15) of the mid-oceanic ridge have been the 

 subject of much speculation by geologists and geophysicists. Some writers have 

 suggested that the mid-oceanic ridge represents a pile of debris left on the sea 

 floor when the continents drifted ajiart. Others have suggested that the ridge is 

 composed of sediments which filled the cracks between drifting continents; but 

 high heat flow, earthquake activity, and the results of the seismic-refraction and 

 reflection exploration reveal a modern active feature with crustal character- 

 istics not compatible with either of these hypotheses. Most current workers 

 believe that the mid-oceanic ridge is the result of tension, and that material 

 rising from the mantle beneath the crest of the mid-oceanic ridge is adding new 

 rock to the floor of the rift valley. However, there are two schools of thought 

 concerning the reason for this upflow of mantle rock. The one favored by most 

 workers is the convection-current hypothesis, which attributes the funda- 

 mental featiu'es of the ocean floor to the effect of convection currents which 

 rise beneath the crest of the mid-oceanic ridge and then flow laterally toward 

 each continent. It is supposed that the drag of these currents on the underside 

 of the continents causes the continents to be compressed and the oceans to be 

 stretched. The second school attributes the stretching of the ocean floors to a 

 general expansion of the interior of the earth. 



Since the mid-oceanic ridge is the longest mountain range on the surface of 

 the earth and covers an area almost equal to the area covered by the con- 

 tinents, its origin is of fundamental importance in considerations of the 

 deformation of the earth *s crust. 



References 



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