644 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



Development of spheeoidal axd Pillow-like Fokms 



Green^^^ observed the clevelopmeut of domical surface forms on pahoe- 

 hoe, as described in a previous quotation, and adds : 



"The domes we look upon ... as hydrostatic effects on a cooling sphe- 

 roidal mass of lava. In the first place the lava tends to form great flattened 

 spheroids, not so much because it is all viscous like porridge, which tends to 

 pour in spheroidal masses, but because the instant it is exposed to radiation 

 a tough skin or pellicle forms on the surface and holds the molten lava inside, 

 whilst the constant new supply expands the whole equally in every direction." 



The great flow of 1859 reached from near the top of Mauna Loa on 

 the north side to the sea, a distance of 38 miles. After it had flowed into 

 the sea continuously for six months, Green writes :^^^ 



"The red-hot molten lava was quietly tumbling into the sea over a ledge 

 perhaps 6 to 8 feet high and 500 to 600 feet long. The lava did not seem to 

 be quite so liquid or of such a bright color as it did when it ran out of the 

 openings in the side wall of the aa stream up in the mountains some months 

 before. It ran more like porridge in great flattened spheroids, which were 

 sometimes partially united together and sometimes almost separate. . . . 

 It was not until after each spheroidal mass had disappeared for a second or 

 two under the water that puffs of steam came to the surface." 



It is suggested that the tendency to form spheroids in the molten state 

 may be in part the origin of basaltic columns, which under compression, 

 cooling, and contraction, while still semifluid, tend to take on a six-sided 

 columnar form. The same principle is thought to account for spheroidal 

 weathering in lavas. 



Anderson^ ^^ saw secondary cones composed of masses like pillow lava, 

 evidently of subaerial construction, on the surface of a recent flow in 

 Iceland, where it occupied a river valley. 



Flett^^^ compared the light spong}' lava pillows of Port Isaac, Corn- 

 wall, with the bombs projected off Pantelleria in 1891, which rolled on 

 the surface of the water in clouds of steam and finally exploded with a 

 loud noise. Presumably any such that sank without exploding formed 

 light cavernous pillows. 



In 1910 Dr. Tempest Anderson^^^ described his observations on pillow- 

 like forms in process of formation at the volcano of Matavanu, in Savaii, 



i»* W. L. Green : Loc. cit, pp. 172, 173. 

 "5 W. L. Green : Loc. cit, p. 277. 



136 Tempest Anderson : Quar. Jour. Geol. Soc. London, vol. 64. 1908, p. 271. 

 13^ J. S. Flett : Quar. Jour. Geol. Soc. London, vol. 64, 1908. p. 270. 

 138 Tempest Anderson : The volcano of Matavanu, in Savaii. Quar. Jour. Geol. Soc. 

 London, vol. 66, 1910, pp. 621-639. 



