DISCUSSION OF RESULTS 599 



which, if the expansion take place suddenly from a high pressure into the 

 air, might finally be extremely rapid. 



AN EXPLANATION OF THE FORMATION OF AA LAVA 



Parenthetically, it may be noted in passing that such rapid expansion 

 and consequent cooling when occurring suddenly at the surface may very 

 well be the sufficient cause of the Aa lava formations. Great blocks ap- 

 pear to have cooled in this way so rapidly that no opportunity was given 

 for the suddenly projected and rapidly expanding lava outbursts to "heal" 

 and resume liquid flow. The projected masses are cooled almost instantly 

 throughout their- mass and remain discrete blocks of the roughest and 

 most ragged outline (plate 25), which are pushed forward thereafter in a 

 manner which has been likened to a "moving stone wall," beneath which 

 th& advancing liquid can rarely be seen. This hypothesis of the manner 

 of formation of Aa lava has encountered no limitation from a field ex- 

 amination 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 nearly instant cooling throughout the mass of a very 

 large block, of lava. (Aa blocks are sometimes reported 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. 18 



The rate of cooling of gases expanding adiabatically has been especially 

 emphasized by Dary, 19 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 rapidly 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, 20 but this has serious difficulties because of the chemical com- 

 plications which would follow from it. 



CHEMICAL REACTION BETWEEN THE GASES 



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

 action between the gases thus set free in constantly increasing quantity as 



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

 Hitchcock : "Hawaii and Its Volcanoes," p. 282 ; Dana : "Characteristics of Volcanoes," 

 p. 241. 



19 R. A. Daly: "The Nature of Volcanic Action." Proc. Am. Acad. Sci., vol. 47. 1911. 

 p. 84. 



99 Daly has calculated n temperature gradient of 2,000° per :?7 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. 



