18 HISTORY OF THE OCEANS 



Crustal Summary 



There is a strong contrast between typical crustal sections for 

 continents and for ocean basins. Of the ocean areas where anom- 

 alous crustal structure is found, particular attention is directed to 

 the Mid-Oceanic Ridge system. In this region the lithological 

 composition of the crust and the thermal fiux through it suggest 

 that matter and heat have been brought toward the surface on a 

 large scale more recently than in other major divisions of the 

 earth's surface. 



Mantle Structure of Ocean Basins* 



We have discussed the great difference between continental and 

 oceanic areas in regard to crustal structure. It is necessary to know 

 whether this difference extends into the upper mantle. If so, there 

 are important implications about origin of the continents, possi- 

 bility of continental drift, and most of the major tectonic problems. 



Body Wave Studies 



The transmission of earthquake body waves, of both compres- 

 sional and shear types, through the earth has been the most 

 fruitful means of investigation of the earth's interior. The ray 

 paths and wave fronts in the mantle and core are shown in Fig. 6. 



Information about upper mantle structure was formerly derived 

 exclusively from travel times and amplitudes of body waves at 

 epicentral distances less than 25°. It was therefore restricted to 

 continental areas in which there were numerous earthquakes and 

 also numerous and well-distributed seismographs, out to epicentral 

 distances of about 25°. Compressional and shear wave velocities 

 of about 8.1 and 4.5 km/sec for P and S, respectively, were 

 generally found at shorter distances, but beyond 10° the interpre- 

 tation of the arrivals became obscure. Between 13° and 20°, 

 depending perhaps on the region investigated, Gutenberg (1948), 

 Lehmann (1954), and others have shown that there is a large 

 change in amplitude, some changes in wave character including 



This section by Maurice Ewing and Mark Landisman. 



