242 VULCANISM 



herited from its birth are doubtless leading factors in the explana- 

 tion of vulcanism. 1 The explanation includes (i) the origin of 

 lavas, and (2) the forces by which they are expelled. 



The current explanations of vulcanism fall into two general 

 classes: (i) those which assume that lavas are residual portions of 

 an original molten mass, and (2) those which assign lavas to the local 

 liquefaction of rock. The first of these views prevailed formerly, 

 but it encounters grave difficulties because of the independent action 

 of adjacent vents. When lava columns vary thousands of feet in 

 height on the same mountain mass, as in the Hawaii volcanoes, 

 even a resort to the hypothesis of local residual reservoirs is un- 

 satisfactory. 



Another view which has had much currency supposes that 

 surface water and Us absorbed gases penetrate to heated rock and are 

 absorbed by it, rendering the whole liquid, and that the lava thus 

 formed is forced to the surface. It does not appear, however, that 

 surface water penetrates below the zone of fracture, and hence is far 

 from reaching highly-heated rocks. Relief of pressure lowers the 

 melting point of rock, and when felt by rocks already hotter than 

 their melting temperatures at lowered pressures, has been held to 

 be a possible cause of vulcanism. The necessary relief of pressure 

 is assigned to faulting and denudation; but many volcanoes are 

 located in the bottom of the ocean, where denudation does not take 

 place, and faulting that would give relief of pressure is not always 

 related to vulcanism in any clear way. Melting by crushing has been 

 suggested, but in the deeper parts, crushing involves increase of 

 pressure, which opposes melting. Sinking to the zone of high tem- 

 perature under the weight of accumulated sediments, is also assigned 

 as a cause of melting, but there is very little sedimentation in the 

 ocean far from land where many volcanoes are situated. 



If the earth grew up by slow accessions of matter, and if its 

 interior heat is due chiefly to the internal compression resulting 

 from growth, the distribution of internal temperature would be such 

 that, with like conductivity, the flow of heat from the deep interior 

 to a thick outer zone (about Vs of the radius) of the earth would 

 be greater than the loss from this zone to the superficial shell. 

 The deeper parts of the outer zone might thus rise in temperature. 

 This zone is, under this view, supposed to be composed of various 



1 For fuller statement see the authors' larger work, Vol. I, pp. 395-607, Vol. II, 

 pp. 99-106, 116-118, 120, 130. 



