WATER AND VOLCANIC ACTIVITY DAY AND SHEPHERD. 299 



jected masses are cooled almost instantly throughout their mass and 

 remain discrete blocks of the roughest and most ragged outline (pi. 

 9), which are pushed forward thereafter in a manner which has been 

 likened to a " moving stone wall," beneath which the advancing 

 liquid can rarely be seen. This hypothesis of the manner of forma- 

 tion of Aa lava has encountered no limitation from a field examina- 

 tion of Aa flows at the point of outbreak, and enjoys still further 

 confidence from the fact that this is almost the only conceivable 

 method of bringing about a nearl}^ instant cooling throughout the 

 mass of a very large block of lava. (Aa blocks are sometimes re- 

 ported to reach the size of a small house.) Any manner of cooling 

 from the outside inward in such masses must have resulted in much 

 mechanical deformation during the forward movement after the 

 surface had " set," causing rupture and outbursts of imprisoned 

 liquid, none of which were found in the field.^ 



The rate of cooling of gases expanding adiabatically has been espe- 

 cially emphasized by Daly,- who has contended that when the liquid 

 lava finds exit through a long and rather narrow pipe, like the vent 

 at Halemaumau, the pressure must diminish rapidlj'^ as the lava rises, 

 and the temperature must fall rapidly in accordance with the law of 

 adiabatic expansion. In order to maintain such an exposed surface 

 basin in the liquid state, it is then necessary to postulate a very high 

 temperature for the lava far below the surface,^ but this has serious 

 difficulties because of the chemical complications which would follow 

 from it. 



CHEMICAL REACTION BETWEEN THE GASES. 



The second consequence of the gradual release of gases is the in- 

 terreaction between the gases thus set free in constantly increasing 

 quantity as the surface is approached. These reactions are accom- 

 panied by evolution of heat, which obviously operates to raise the 

 temperature of the surrounding lava so long as the reacting gases 

 remain in contact with it. The heat generated by these gas reactions, 

 in the region near the surface where the amount of gas is large, may 

 well be much more than sufficient to counteract the cooling effect of 

 the expansion within the rising lava column, which may thus become 

 hotter and not cooler as it approaches the surface. 



Precise figures can hardly be given for the difference in magnitude 

 between the two forces which have been assumed to oppose each other 



1 For other explanations of the formation of the Aa lava see Green, loc. cit., p. 171 ; 

 Hitchcock : " Hawaii and its volcanoes," p. 282. Dana : " Characteristics of volcanoes," 

 p. 241. 



3R. A. Daly: "The nature of volcanic action." Proc. Amer. Acad. Sci., vol. 47. 1911. 

 p. 84. 



3 Daly has calculated a temperature gradient of 2,000° per 37 meters of depth for the 

 rate of cooling of the gas alone, but the calculation takes no account of the relatively 

 enormous mass of adjacent lava which must be cooled by the gas. 



