270 HEEZEN AND MENARD [CHAP. 1 2 



dredge can break off only weathered material unless the dredger has extra- 

 ordinary luck. If present sani])ling is considered to be adequate, the ocean- 

 basin floor is no older than Cretaceous. However, further sampling may indicate 

 greater age and it seems more logical to assume that the basin is very old. 



As to faulting, it can only be concluded that the ocean-basin floor was 

 formerly more seismically active. The only alternative is to assume that the 

 oceanic type of crust can be faulted without eartli quakes and this is not entirely 

 acceptable because large earthquakes occur in a few places within the oceanic 

 crust. 



When viewing the physiographic provinces of the North Atlantic or the 

 South Atlantic, one sees a strong parallelism not only between the continental 

 margins and the mid-oceanic ridge, but also between all the other province 

 boundaries of the ocean basin. It has been pointed out that, in the Pacific, and 



25°N 24°N 



^9°W ^^^^ 45°30W 



-1600- 



-2000- 





^^_y^^__^^^^^ 



-^%Sl 



'2&"''"^"^ v V sv 2400 fathoms- 



Step I Upper Step | High Fractured Plateau Rift Mountains k/alleJ ^''^* Mountains 

 ■* ^200 nautical miles *■ 



Fig. 31. Tracing of a Precision Depth Recorder record showing crest and western flank of 

 Mid-Atlantic Ridge. (After Heezen, Tharp and Ewing.) 



to a certain extent in the Atlantic, the minor relief of the continental margins 

 parallels the continental slope. This is certainly true in those few areas where 

 surveys are sufficiently detailed to permit a study of the minor relief of the 

 abyssal hills and the lower steps of the mid-oceanic ridge. Here again, the 

 minor rehef features are parallel both to the continental margins on the one hand 

 and the crest of the mid-oceanic ridge on the other. To a remarkable degree, 

 the topography of the sea floor resembles a piece of expanded-metal grill-work 

 in which the major tojoographic lines are all roughly parallel. 



C. Mid-Oceanic Ridge (see Chapter 16) 



The greatest topographic feature of the sea floor is an essentially continuous 

 median elevation extending through the Atlantic, Indian, Antarctic and South 

 Pacific Oceans for a total distance of over 30,000 miles. The relief of the eleva- 

 tion above the adjacent ocean-basin floor is 1-3 km, and the width in most 

 places is more than 1000 km. This median elevation, known as the Mid-Oceanic 

 Ridge, is essentially a broad, fractured swell occupying the center third of the 



