FORMATION OF PAHOEHOE LAVA 641 



with a curved front bulging outward. These slow-moving, viscous streams 

 crossed one another, but became more or less thoroughly intermingled. This 

 manner of progression, accompanied by many variations in detail, can be 

 plainly seen on the congealed surfaces now remaining, especially near the 

 sources of the lava stream." 



Hitchcock^ ^'' ascribed the hummocky character of the common pahoe- 

 hoe of Hawaii to the fact that the lava stiffens very quickly after ex- 

 posure. An illustration (plate 51) from the flow of 1880-1881, near 

 Hilo, shows typical pillowy forms in the right side of the view. 



"Standing by this variety of lava as it forms, one sees that it is a stream 

 of liquid material, and if a stick be thrust into it red lava will flow out. 

 . . . The crust is flexible and is modulated by the motion of the liquid 

 beneath." 



In these explanations of the formation of pahoehoe there is some ap- 

 parent contradiction. Thus Button, Green, and Hitchcock emphasize 

 the smooth liquid flow or the quickly formed restraining crust, while 

 Russell distinctly speaks of slow-moving viscous streams. A careful read- 

 ing of the full descriptions by these authors, however, will show that 

 while there is some contradiction of terms used in particular statements 

 there is substantial agreement as to the mode of formation of pahoehoe 

 and the characteristics of the lava that gives rise to it. The process is 

 that of short, quick forward impulses of lava so liquid that it spreads 

 rapidly in smooth-surfaced flows which, in turn, are quickly checked by 

 the rapid formation of a tough viscous membrane on the exposed surface. 

 This stretches under the pressure of the liquid interior until it stiffens 

 and finally breaks, and the process is repeated over and over as the flow 

 advances. The result is a smooth surface interrupted at shorter or longer 

 intervals by rolls, swells, and domes. 



Formation of Aa Lava 

 Button^ ^^ explains the formation of aa lava as follows : 



"The fields of aa are formed by the flowing of large masses of lava while 

 in a condition approaching that of solidification. Ordinarily they are very 

 thick and cover a very large area, representing therefore an enormous mass 

 comparable to that of a glacier. The movement is in some respects glacier- 

 like, but with this difference; instead of having a sensibly constant tempera- 

 ture throughout, it is hot within and more or less viscous and nearly cooled 

 on the surface. . . . During this slow glacier-like motion crushing strains 

 of great intensity are set up throughout the entire mass, and its behavior 



127 c. H. Hitchcock : Hawaii and its Volcanoes. Honolulu, 1909, p. 280, 

 ^ Loc. cit., p. 96, 



