SHAPE AND STRUCTURE OF OCEAN BASINS 11 



associated structural features than in the oceanic portions. These 

 landward extensions, as shown in Fig. 4, include one from the 

 Carlsberg Ridge through the Gulf of Aden and into the East 

 African Rift Valleys; one from Easter Island into southern Chile; 

 one from Easter Island into the Gulf of California; one from the 

 vicinity of Spitzbergen into Siberia in the vicinity of the Verk- 

 hoyansk Mountains, and apparently three from the vicinity of 

 Macquarie Island into Tasmania, New Zealand, and the Ross Sea, 

 respectively. These landward extensions may be interpreted as 

 zones in which the crust has failed under tensional forces, accom- 

 panied by the extrusion of basic and ultrabasic lavas. 



There are a few areas in which no rift has been found (Dietrich, 

 1959) possibly owing to tilling by sediments or lavas, and there 

 are some indications that in the eastern Pacific the morphology of 

 the ridge may be different from that found elsewhere but, if so, 

 the boundary line between the two types is yet to be determined. 

 It is possible that in some areas there is a narrow median system 

 of an echelon of rifts rather than a single continuous rift. The 

 relations and contrasts between the seismic and aseismic ocean 

 ridges are gradually being discovered. But even now it is apparent 

 that the Mid-Oceanic Ridge system is a major morphologic 

 element of the ocean basins which also offers direct information 

 about fundamental tectonic processes. 



It has been established that the Mid-Oceanic Ridge system, of 

 which the Mid-Atlantic Ridge is the best known part, is a con- 

 tinuous, world-encircling system, about 40,000 miles long, which 

 is characterized by a median rift and a coincident narrow belt of 

 shallow-focus earthquakes. The significance of the differences in 

 structure between the Mid-Atlantic Ridge and typical land moun- 

 tain systems, and the significance of the striking tendency of the 

 ridge to bisect the oceans will be discussed in the next sections. 



Crystal Structure of Ocean Basins* 

 General Statement about Crust and Mantle 



For the purpose of this paper the crust (or silicic crust) is 



This section by Maurice Ewing. 



