640 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



*1 



"The superficial crust of cooled lava undergoes rupture at numberless points 

 and little rivulets of lava are shot out under pressure. Preserving their 

 liquidity for a short time they spread out very thin and are very quickly 

 cooled, forming pahoehoe. Scarcely is one of these little offshoots of lava 

 cooled when it is overflowed by another and similar one, and this process is 

 repeated over and over again. In a word, pahoehoe is formed by small off- 

 shoots of very hot and highly liquid lava from the main stream driven out 

 laterally or in advance of it in a succession of small belches. These spread 

 out very thin, cool quickly, and attain a stable form before they are covered 

 by succeeding belches of the same sort." 



Green^^* has written of the Hawaiian pahoehoe as follows : 



"This class of lava we saw flowing from the aa stream of 1859, showing 

 that it is not a different kind of lava, but merely a different form taken under 

 different circumstances of flowing. The lava seems to form pahoehoe when it 

 is not in too great quantity and runs out quietly. For instance, the stream 

 that broke out from the aa stream of 1859, while we were watching it, seemed 

 flrst to form into a flattened spheroidal mass, the crust of which was con- 

 stantly cooling and the fresh supply of molten lava was constantly increasing 

 its size. At last a limit seemed to be arrived at between the retaining power 

 of the cooling crust and the pressure of the column of liquid lava inside it, 

 when the molten lava would break away from the lower edge of the dome 

 and form another just like it and close to it, and so on, the molten lava from 

 the aa stream continually keeping up the supply and running from one to 

 another, forming a succession of domes, most of which were hollow and in 

 many the top has disappeared. . . . The domes we look upon, as explained 

 above, as hydrostatic effects on a cooling spheroidal mass of lava.'* 



Dana^^^ observed that pahoehoe shows "by the fine and coarse flow, 

 lines over it that it cooled as it flowed . . . often wrinkled, twisted, 

 ropy, billowy, hnmmocky, knobbed, and often much fractnred.^' In places 

 it was found to have a glassy crust one-half an inch or less in thickness. 



From the study of the well preserved lava surfaces on the Snake Eiver 

 plains of Idaho, EusselP^® described the origin of pahoehoe as follows : 



"The origin of the peculiar and highly characteristic pahoehoe surface is 

 due to the flow of viscous lava, which in consistency resembled asphaltum or 

 pitch. The onward motion was not a continuous flow, as in the case of 

 thoroughly liquid substance, but the surface and front of the advancing stream 

 stiffened and bulged upward, and the more thoroughly molten material within 

 broke through the tenacious but still plastic surface portion and advanced 

 as a well-defined stream for perhaps a few yards or rods, and in turn stiffened 

 at the surface and expanded on account of pressure from within and halted 



124 W. L. Green : Vestiges of the Molten Globe, pt. ii : The Earth's Surface Features 

 and Volcanic Phenomena. Honolulu, 1887, pp. 172, 173. 



125 J. D. Dana : Characteristics of Volcanoes. New York, 1890, p, 9. 



^« I. C. Russell : Geology and water resources of the Snake River plains of Idaho, 

 Bull, U. S, Geol Survey, No. 199, 1902, p. 91. 



