VOLCANOES AND 



ash, pumice, etc., over the bottom of the Pacific and 

 Atlantic oceans, even at distances remote from land, as 

 has been made known by the voyage of the Challenger, 

 may indicate the prevalence and persistence of submarine 

 volcanic action." 



It is fairly clear, therefore, that the sea bottom is 

 leaky, and that volcanoes which are a consequence of 

 it are scattered freely over the deep ocean floor. In some 

 places, of course, very few eruptions occur, either because 

 the underlying rocks are less leaky or the sea is too 

 shallow for pressure. 



We have pictured the water of the oceans thus sinking 

 down into the hot rocks. It will not always cause an 

 explosion at once. The steam may not immediately 

 become free, but will become absorbed in the hot rocks 

 till the lava grows so fully saturated by the hot vapour 

 that it swells and requires more space. When the ten- 

 sion becomes great enough the crust begins to shake and 

 the paroxysm continues till the steam-saturated lava 

 moves along the nearest break or " fault " or vent. 

 When the underlying molten rock has thus obtained 

 more space the agitation ceases till the tension again 

 becomes too powerful for the crust to withstand, when 

 another readjustment takes place. A familiar illustra- 

 tion of this process is seen in the lid of a tea-kettle when 

 the steam pressure accumulates till it sets the lid quiver- 

 ing. As the steam escapes at the sides the agitation 

 slowly dies down and the lid then remains quiet till the 

 accumulating pressure again requires relief, when the 

 shaking is renewed. Thus the process is periodic, and 



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