52 



THE FLOOR OF THE OCEAN 



< ? I 2 3 4 S 



FIGURE 25. SECTIONS TO ILLUSTRATE STAGES IN THE REDUCTION OF A 

 VOLCANIC ISLAND TO A SHOAL AT WAVE-BASE. 



rock by waves of the open ocean becomes almost negligible at 

 a depth no greater than about 300 feet. The level at this depth 

 is called technically the "wave-base," a term we shall find con- 

 venient for use later on. 



At the present time hundreds of island-crowned volcanic 

 piles are dotted over the ocean. Their study has to be super- 

 ficial in the literal sense of the word. The geologist can learn 

 something by direct inspection, but in every case only about the 

 top, a relatively minute fraction, of the whole volcanic struc- 

 ture. Even Hawaii, the most extensive, as well as the highest, 

 of the volcanic islands, rests on a submarine pedestal with a 

 volume about 20 times that of the visible island. In general the 

 ratio of island to submarine pedestal is much less than 1 to 100. 



Catastrophic Changes of Island Topography. — Both up- 

 growth of the structure and its erosion after emergence above 

 sealevel are long-enduring processes. Both are liable to be in- 

 terrupted by catastrophes. Sporadic, revolutionary alteration 

 of the form and extent of an island may be brought about by 

 two different causes, major explosion and landsliding on a big 

 scale. 



Toward the close of the eruptive stage, volcanic gas is col- 



