642 J. V. LE"\VIS ORIGIX OF PILLOW LAVAS 



conforms strictly to that of viscous bodies. The superficial portions in part 

 yield plastically to the strains, in part yield by crushing, splintering, and 

 fissuring. The result is a chaos of angular fragments. The aa streams are 

 always thicker than the pahoehoe and rise from 50 to 80 feet above them, 

 being bounded by a margin of low cliffs and talus. . . . The same lava 

 stream may exhibit pahoehoe or aa, according to the cii'cumstances attending 

 the flow, and the final form which the stream takes is quite independent of 

 the chemical constitution of the lava." 



Dana^^^ described the rugged aa lavas of Kilauea as looking 'T.ike 

 ploughed-up lava streams on a majestic scale" and, as far as visible, con- 

 sisting of detached masses of very irregular shapes and confusedly piled 

 np to nearly a common level. In some of the aa streams there are occa- 

 sional bomb-like masses, in striking contrast with the t^^jical aa, smooth- 

 ish on the outside and more or less rounded and boulder-like, and ranging 

 from a few inches to 10 feet or more in diameter. Some of these have a 

 laminated structure of alternating concentric layers of hard, slightly 

 vesicular lava, with softer scoriaceous material. Some with a hard out- 

 side shell seemed to be filled inside with fragments of reddish or grayish 

 scoria. Common sizes are 3 to 5 feet in diameter, and they lie in the 

 midst of the blocks of aa, "proving that all had a common origin," and 

 that the rounded masses are not true bombs. Dana also observed that the 

 same stream may change from pahoehoe to aa and back again in differ- 

 ent parts of its course, and he ascribed the origin of aa lavas to the pres- 

 ence of ground moisture, the ascending vapors of which chill the flow and 

 cause it to break up.^^^ He characterizes the typical aa as 



"roughly cavernous, horridly jagged, with projections often a foot or more 

 long that are bristled all over with points and angles. . . . The reader's 

 conception of it will be feeble at best if he has not already had a view of 

 chaos." 



RusselP^^ accounted for the formation of aa as follows : 



"On the rate of flow, or, more definitely, on the ratio of rate of flow to 

 rate of surface cooling, depend certain marked contrasts in the resultant 

 surface features. When motion was slow and continued after the lava had 

 become viscous, the stiffened crust was left with either a generally smooth, 

 flat surface, or acquired oval stream-like ridges, bulging mounds and dome- 

 like swells, while the crust formed where motion was more rapid or had 

 continued after the surface had passed to a rigid condition became broken 

 and the blocks were variously displaced and heaped up so as to produce 

 excessive roughness." 



129 .L D. Dana: History of changes in Kilauea. Am. .Tour. Sci. (3). vol. xxxiy. 1881 

 pp. .362-364. 



130 .1. D. Dana : Characteristics of Volcanoes. New York. 1890. pp. 241-245, 

 131 1. C. Russell ; Bull. U. S. Geol. Survey, No. 199, 1902, p. 90. 



