SHAPE AND STRUCTURE OF OCEAN BASINS 7 



which is not displayed in a clear fashion in the hypsographic curve. 

 On a typical trans-Atlantic profile between New England and 

 Spanish Sahara (Fig. 3), the Mid-Atlantic Ridge appears as an 

 elevation about 1000 miles wide which reaches about 2 miles above 

 the basin floor. It occupies approximately one-third of the whole 

 Atlantic Ocean area, and, as has been shown recently, has an 

 extension which occupies about the same fraction of the area of 

 the Indian Ocean and then crosses the Pacific. Together with its 

 extensions, it forms the Mid-Oceanic Ridge system about which 

 the following facts have now been clearly established : 



1. The Mid-Oceanic Ridge system is continuous and can be 

 traced with certainty through a 40,000 mile world-encircling belt, 

 which passes through the Atlantic, Indian, Pacific, and Arctic 

 oceans, as shown in Fig. 4. 



2. A deep, narrow median valley or trench, from 20 to 80 miles 

 wide and H to 13^ miles deep, is a characteristic feature of the 

 ridge through the Atlantic and Indian oceans. Apparently the 

 valley is not conspicuous in the Pacific. 



3. A seismic belt, in which all shocks are shallow focus, is 

 continuous along the ridge and coincident with the median valley 

 (insofar as the valley position is known). 



4. The earthquake belt and an associated rift system extend 

 into continental areas in several places. 



5. In contrast to land mountain systems, this range of sub- 

 marine mountains consists of a thin upper layer, with seismic 

 velocities of about 4.5 to 5 km/sec, and a mass 30 to 40 km thick 

 in which the velocity is about 7.3 km/sec. 



6. The ridge shows a striking tendency to bisect the oceans 

 through which it passes, if we grant that the southeast Pacific is 

 an exception. 



The demonstration that the Mid- Atlantic Ridge may really be 

 extended, as a continuous tectonic and morphologic feature, 

 throughout the world-circling pattern shown in Fig. 4, has been 

 difficult, because many parts of the ocean are unsounded. Further, 

 there are a number of oceanic ridges which are not a part of the 

 system. Decisive evidence first came from the study of seismicity 

 of the oceanic areas of the earth. 



