SHAPE AND STRUCTURE OF OCEAN BASINS 



15 



DISTANCE IN NAUTICAL MILES 





18 

 z 



20- 

 r 

 1- 22- 



o 24- 



26- 



28- 



1,0- 



Low Velocity Sedimenis Volcanic Rocks 



Oceanic Layer 



16 



18 



20 



22 



24 



- 26 



-28 



J 30 



Fig. 5. Structure section across the Mid-Atlantic Ridge (Ewing and Ewing, 

 1959a). 



assumption of isostasy. An interesting alternative suggestion about 

 the appearance of a velocity of 7.3 km/sec so near the surface 

 arises from noting that the linear extrapolation of the outer mantle 

 velocity-depth curve of Fig. 7 to the surface gives also the velocity 

 of 7.3 km/sec. In other words, the observed velocity pattern could 

 conceivably arise from pulling or pushing the lithosphere aside and 

 allowing material of the outer mantle to rise toward the surface. 



A third view is that of Hess (1959), who suggests that the 

 elevation of the ridge results from a phase change from peridotite 

 to serpentine induced by the addition of water. This produces an 

 increase of volume, evolution of heat, and a suitable decrease in 

 elastic wave velocities. Additional measurements, like those of 

 Birch (1960), are needed for appraisal of this suggestion. 



Deep-sea trenches, island arcs, and marginal seas are associated 

 features, which will be discussed together as a class of anomalous 

 oceanic regions. The island arcs are chains of submarine mountains, 



