46 HISTORY OF THE OCEANS 



4.5 to 5 km/sec occurs not only on the ridges and seamounts but 

 very widely under the areas covered by sediment. It is not certain 

 that the material is the same as in the places where decayed 

 basalt is exposed, but it seems possible and even likely that most 

 of the sea floor is underlaid by such material. 



For several kilometers above the IN I oho, the seismic velocities 

 are usually between 6 and 7 km/sec, and the material is usually 

 supposed to be basalt. We have no evidence whether this is an 

 original part of the ocean basin or whether it has been formed by 

 some long continuing process. It may be a continuous body of 

 solid basalt or it may be a series of flows analogous to the plateau 

 basalts of the continents and perhaps with sediments lying between 

 the flows. 



There would be considerable difficulty in accounting for the 

 melting of material above the oceanic Aloho. A heat flow of 

 2 X 10"^ cal/cm- sec, which is probably above the average, would 

 give a temperature of under 200°C at the INIoho even if all the 

 heat came from greater depths. The best hope of explaining the 

 widespread oceanic vulcanism and the relati\'ely high heat flows 

 seems to be to suppose that both the heat and the lava come from 

 below the Moho. It also seems likely that the radioactivity of the 

 upper part of the mantle must be nearer to that of basalt than to 

 that of dunite. (For a more detailed discussion see BuUard et al., 

 1956.) Whether the basalt is formed from the material of the 

 mantle by difl^erentiation or by a phase change is unknown. 



Clearly many of the uncertainties of the processes occurring in 

 the ocean basins center round the extrusive rocks. The reason for 

 this is largely the difficulty of collecting them. It is to be hoped 

 that the proposed attempts to drill deep holes at sea will succeed 

 and will provide the samples that are so desirable. 



The Larger Pattern 



The pattern of the ocean floor is dominated by linear features: 

 ridges, chains of islands, fracture zones, and trenches. The major 

 features of continental structure, the fold mountains and their 



