CAUSES OF VOLCANIC ACTIVITY 59 



Similar divergences of opinion obtain with regard to the nature 

 and origin of the lavas ejected by volcanoes. The view most 

 commonly held is that they are, for the most part, the original, 

 unaltered material of the globe, whether this has always remained 

 fluid, or has been remelted by release of pressure, or otherwise. 

 According to another opinion, volcanic products are formed from 

 the fusion of sedimentary material which was laid down under 

 water, but has been deeply buried within the crust of the earth by 

 subsidence. A third view recognizes both sources of supply. 



(2) The problem as to the origin of the steam which plays so 

 important a part in volcanic eruptions, is likewise very differently 

 solved by different investigators. One opinion is that the steam, 

 like the lava itself, is primordial and was absorbed from the atmos- 

 phere (which then contained all the waters of the sea) when the 

 surface of the globe was still molten. Melted substances will, it 

 is known, absorb many times their own volume of steam and 

 gases, when in contact with them under pressure. From this it is 

 inferred that the lava has contained the steam ever since the first 

 cooling of the surface crust. A second opinion derives the water 

 from the surface of the earth, supposing that it descends partly 

 through fissures and partly through the pores of the overlying 

 rocks by capillarity. The nearness of volcanoes to the sea is 

 looked upon as favouring this view. Others, again, employ both 

 methods of explanation, regarding the ordinary steam which im- 

 pregnates all lavas as primordial, but believing that the violently 

 explosive eruptions are caused by the sudden access of large 

 bodies of water to the lava rnasses. 



(3) The causes of the ascensive force of the lava column are 

 likewise very differently explained by various writers. Some find 

 an all-sufficient cause in the steam pressure, while others maintain 

 that some other force must be at work and find this in the unequal 

 contraction of the earth, and consequent pressure upon the molten 

 or plastic layer beneath. It has been calculated that a radial con- 

 traction of one millimetre " would suffice to supply matter for 

 five hundred of the greatest known volcanic eruptions" (Prest- 

 wich) . 



