SECT. 2] THE MID -OCEANIC RIDGE 395 



tinuous rifted mid-oceanic ridge which coincides with the epicenter belt (Ewing 

 and Heezen, 1960). Fig. 7 is the result of a preliminary study of the relation of 

 the epicenter belt to the major topographic features of the Indian Ocean. An 

 example of the very rough topography of the Mid-Indian Ocean Ridge is 

 shown in the sections of Fig. 35, Chapter 12, p. 274. 



The connection of the mid-oceanic seismic belt with the seismic belt of the 

 African rift valleys, and the similarity of the morphologies of the mid-oceanic 

 rift and the African rift valleys, give a clue to the structure and origin of the 

 rifted mid-oceanic ridge. The 800-mile-wide mid-oceanic ridge of the Arabian 

 (Sea narrows to less than 100 miles where it enters the 200-mile-wide Gulf of 

 Aden (Fig, 7). Its seismically active median rift trends west through the Gulf 

 of Tadjoura in French Somaliland. As the Gulf of Aden shallows and narrows 

 from east to west the rifted ridge maintains its median position, coinciding 

 with the epicenter belt. The coastal escarpments of the Gulf of Aden are formed 

 by rift faults, which converge toward the southwest and continue as the 

 boundary scarps of the Ethiopian rift valleys (Fig. 5). 



A. African Rift Valleys 



In Fig. 34, Chapter 12, p. 272, a profile across the Mid-Atlantic Ridge is 

 placed above two profiles across the East African rift valleys to illustrate their 

 nearly identical form. This similarity, indeed virtual identity, of morphology 

 and seismicity might still be considered superficial and accidental were it not 

 for the fact that the three profiles lie on the same seismically active morpho- 

 tectonic belt. The rifted mid-oceanic ridge seems to be closely similar in origin 

 and structure to the African rift valleys. 



Gregory (1921) gives evidence that the African rift valleys are tensional 

 grabens. Wayland (1930) and others attempted to explain the features in 

 terms of compression, but recent investigations have only added support for a 

 tensional origin. McConnell (1951), and others who favor a dominantly strike- 

 slip origin, attribute compressional, as well as tensional, features to minor 

 complications along the strike-slip faults. Bullard (1936), who offered a com- 

 pressional hypothesis to explain the negative gravity anomalies over the rifts, 

 has now abandoned compression in favor of tension (Bullard, in litt., 1959). 

 Vening Meinesz (in Heiskanen and Vening Meinesz, 1958) has offered an ex- 

 planation of the gravity anomalies in terms of simple extension. The com- 

 pressional theory came mainly from those who found it difficult to comprehend 

 large-scale extension on a shrinking earth; the shrinking, it was thought, being 

 demanded by the Earth's folded mountain ranges. 



5. Indian and South Pacific Oceans 



The mid-oceanic ridge continues southeast from Rodriguez Island, passing 

 near St. Paul Island, and then between Australia and Antarctica to the vicinity 

 of Macquarie Island (Fig. 5). Minor seismically active ridges diverge in this 

 region toward Tasmania, New Zealand and the Ross Sea. The grabens of 



