FORCES AND PROCESSES IN OCEAN BASINS 45 



and not from a boulder. Naturally a means of drilling really deep 

 holes would be of the first importance, but a drill that could 

 penetrate only a few feet would also be most welcome. 



If good collections of reasonably fresh submarine volcanic rocks 

 can be made, it should be possible to date them by potassium- 

 argon or rubidium-strontium ratios. This would give us the ages 

 of the great systems of ridges and of the groups and chains of sea- 

 mounts. Nothing would do more to reduce the range of possible 

 speculation. 



The great majority of the innumerable seamounts that occur in 

 all the oceans are volcanos, and it seems reasonable to suppose 

 that they were fed from a pipe leading to molten basaltic material 

 either above or below the Moho. The ridges, on the other hand, 

 may be chains of overlapping volcanos or the result of fissure 

 eruptions. The topography strongly suggests faults parallel to the 

 axis of the ridge, and it may be that faults have controlled the 

 outflow of lava. 



The first impression that vulcanism is more common at sea 

 than on land may be an illusion since we do not know how long 

 a time span is represented by the volcanos we see. If the oceanic 

 volcanos lasted for much longer periods than volcanos on land, 

 there would naturally be more of them recognizable at a given 

 time. Little is known about the erosion of submarine rocks, and 

 the very decayed state of the large collection made in 1958 by 

 M. N. Hill and D. H. Matthews came as something of a surprise. 

 Matthews has made a study of these rocks and has shown that 

 they have been broken into pieces a few centimeters across and 

 that iron and manganese minerals have been deposited in the 

 cracks. This work has explained the origin ot the masses of angular 

 debris frequently seen on photographs and in dredge hauls taken 

 near seamounts. We do not at present know whether this breaking 

 up of the rock takes place immediately on its coming in contact 

 with sea water or whether it is a process that goes on slowly over 

 a long period. If the rock is split and attacked by sea water while 

 still hot, the whole thickness of the rock in a seamount or ridge 

 may be broken up and decayed. The rather low seismic velocities 

 found on the ridges would be consistent with this. 



The seismic results suggest that material giving velocities around 



