FORMATION OF AA LAVA 643 



Hitchcock^^- said of the Hawaiian aa: "The roughness of it beggars 

 any possible description." Sheets of it, somewhat compressed and modi- 

 fied, can be recognized among the older beds, but "they must not be con- 

 founded with the spherical or rounded masses analogous to the columnar 

 structure/^ He concluded that the study of aa lava has not revealed its 

 nature and origin. 



Day and Shepard"^^^ attribute the formation of aa lava to the rapid 

 expansion of escaping gases and the consequent cooling. 



"Great blocks appear to have cooled in this way so rapidly that no oppor- 

 tunity was given for the suddenly projected and rapidly expanding lava out- 

 bursts 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, 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 for- 

 mation of aa lava has encountered no limitation from a field examination 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." 



The total lack of agreement as to the cause of aa lava is obvious. 

 Among the authors quoted — Button : large viscous flow ; Dana : influence 

 of ground water ; Russell : rapid flow of stiff lava ; and Day and Shepard : 

 cooling effect of escaping gases — there is apparently a wide range of hy- 

 pothesis ; but they naturally fall into two groups : ( 1 ) those that depend 

 on the brittleness of highly viscous lavas, and (2) those that invoke the 

 chilling effect of volatile substances. Day and Shepard's criticism of the 

 first group would seem to be valid at least in regard to the coarser varie- 

 ties of aa. To a certain extent the theoretical differencsas may be due to 

 disagreement as to the facts in the field. Thus Dutton and Dana state 

 that the same flow may change from aa to pahoehoe and back again in 

 different parts of its course, while Eussell finds that aa is particularly 

 characteristic of the middle and lower portions of flows which exhibit 

 typical pahoehoe in the upper parts of their courses. 



132 c. H. Hitchcock : Loc. cit, pp. 281, 283. 



133 A. L. Day and E. S. Shepard : Water and volcanic activity. Bull. Geol. Soc. Am., 

 vol. 24, 1913, p. 599. 



