58 VOLCANOES 



Park. Even more extensive are the lava plains of the Deccan in 

 India, and much smaller, but still impressive, fields occur in Ire- 

 land and Scotland. 



The Causes of Volcanic Activity. — Many theories have been 

 advanced to explain the causes of volcanic activity, but none are 

 satisfactory. In an elementary work, like the present, no ade- 

 quate discussion of this most difficult problem can be given, but 

 merely a brief sketch of some of the ways in which its solution 

 has been attempted. 



The problem is to account (i) for the intense heat of the 

 ejected materials, (2) for the presence of the steam, (3) for 

 the ascensive force of the lava, and (4) for the intermittency 

 of the action, and the past and present distribution of the 

 vents. 



(1) The high temperature has been accounted for in two prin- 

 cipal ways. By some it is supposed that it is due to the original 

 heat of the earth, not yet lost by radiation. Of those who accept 

 this opinion, some believe that larger or smaller portions of the 

 earth's interior have never solidified and that these form the 

 reservoirs of lava which supply the volcanic vents. Others again 

 assume that the interior of the globe is exceedingly hot, but solidi- 

 fied by pressure ; when, by fracturing or folding of the overlying 

 rocks, this pressure is partially relieved, the highly heated masses 

 become liquefied along that line of reduced pressure. 



In the second class of hypotheses on this subject of tempera- 

 ture, it is supposed that the proper heat of the earth's interior is 

 no longer sufficient to produce fusion, and that it must be sup- 

 plemented from some other source. Many are the attempts to 

 determine where this additional source of heat supply is to be 

 found. One of the most celebrated of these attempts (Mallet) 

 seeks the additional heat in the friction produced by the folding 

 and crushing of rocks deep within the crust of the earth, due 

 to the shrinkage of the earth as it cools. Others have sought to 

 show that the heat is generated chemically, by the oxidizing effect 

 of descending waters upon the unoxidized interior of the globe, 

 or by the combustion of hydrogen gas. 





