SECT. 2] 



TOPOGRAPHY OF THE DEEP-SEA FLOOR 



271 



ocean. The portion of this feature lying in the Atlantic Ocean — the Mid- 

 Atlantic Ridge — can be divided into distinctive physiographic provinces, 

 identifiable on most trans-Atlantic profiles (Fig. 31) (see Chapter 16). 



Fig. 32. Earthquake epicenters in the North Atlantic and the mid-oceanic rift. (After 

 Elmendorf and Heezen, 1957.) 



a. Crest provinces 



The rift mountains and high fractured plateau, which constitute this category, 

 form a strip 50-200 miles wide. In some places, particularly the Xorth Atlantic, 

 the rift valley is bounded by the iuAvard-facing scarps of the rift mountains. 

 The floor of the rift valley lies 500-1500 fm below the adjacent peaks of the 

 rift mountains. The rift mountains drop abruptly to the high fractured plateau, 

 which lies in depths of 1600-1800 fm on either side of the rift mountains. An 

 earthquake belt follows the rift valley (Figs. 32 and 33). Heat-flow measurements 



