142 PROCEEDINGS OF THE AMHERST MEETING 



SURFACE FUSION OF LAVA 



BY J. S. DILI.EK x 



Some time ago there came to the U. S. Geological Survey in Washington, 

 through the U. S. Department of Agriculture, from Dr. Calderon, of San Sal- 

 vador, Central America, samples of lava erupted in that region June 7, 1917. 

 from the crater of El Pinar, San Salvador. Central America. They illustrate 

 in a striking manner the refusion of lava by included gases escaping during 

 eruption, after coming into contact with the air at the surface. 



The samples of lava were collected near the surface of the lava-flow, in 

 pockets of scoria which Dr. Calderon says were formed by the escape of gases, 

 especially on all steep slopes where the lava formed cascades. 



The largest piece of lava received from the eruption of 1917 is approximately 

 7 inches in length, breadth, and thickness, and its two portions are quite 

 unlike, as shown in the illustration, figure 1. One portion is red, bounded by 

 surfaces of fractures. It is of light weight, wholly vesicular, much like 

 pumice ; vesicles generally fine and irregular, seldom large. The other, or 

 lower, portion of the specimen is black, generally very vesicular, but the 

 vesicles are round and vary in size. The striking feature of this portion is 

 its flow surface. Much of it is in the icicle-shaped bodies hanging from the 

 other portion like stalactites. The molten lava dropped in small streams from 

 some of these bodies and coiled on the bottom of the pocket. Both portions 

 are basaltic lava and under the microscope show numerous minute plagioclase 

 crystals in a globulitic base which contains much clear glass. 



The contact between the two portions is well marked. It is irregular and 

 evidently, as it is the limit of the flow lines, it marks the limit of fusion. 



Judging from the composition and relation of the two portions, there seems 

 to be good reason for believing that the black, lower portion of the specimen 

 was formed by the fusion of the red. upper portion by hot gases in a lava 

 pocket. This opinion is corroborated by the fact that a piece of the red por- 

 tion fused before a blast lamp becomes black. 



Judging from the statement of Dr. Calderon. the specimens were collected 

 in the lava-flow only where it descended in cascades. The included gases thus 

 liberated from the breaking, cascading flow mixed with air in the lava pockets 

 and apparently produced sufficient heat to melt the adjoining lava of the same 

 flow. This phenomena is similar to that observed by Day and Shepherd at 

 Kilauea. in Hawaii, and described before this Society in 1913. 



In the small lava-flow from the great eruptions of Lassen Peak, California. 

 May 1!) and 22, 1915. there are samples of surface fusion and flowage (figure 2) 

 finite analogous to that just noted from San Salvador. The lava from Lassen 

 Peak, however, is dacite and much more siliceous than the basalt of San Sal- 

 vador. Therefore it was much more difficult to melt, and when melted was 

 much less fluent. Only a thin surface coating of the dacite was melted locally 

 and flowed among the unfused portions. 



The surface melting of the dacite took place about the close of the great 

 eruption, in May. 1915, when the body of rising volcanic gases pushed up the 

 more or less softened lava plug that occupied the throat of the volcano until, 



1 Publication approved by the Director of the U. S. Geological Survey 



