CAUSES OF VOLCANIC ACTIVITY — DAY 265 



volcanoes which tend to confirm this reasoning. It has already been 

 stated that the lava lake at Kilauea has a temperature, usually, be- 

 tween 1,000° and 1,200°, and fluid lava overflowing the rim of the 

 basin at that temperature, still remains fluid at 700° or 800° (red 

 heat.) This is of course representative magma or lava which is in 

 process of crystallization. If samples of this lava be collected after 

 solidification and reheated in the laboratory, they can not be brought 

 again to molten condition below 1,300 degrees. It is, therefore, clear 

 that the fluidity of the lava in the basin is due primarily to the vola- 

 tile ingredients which it contained and has discharged during crys- 

 tallization, of which water is the chief. In complete accord with this 

 the great outbursts of gas and dust-laden steam from the more 

 violent volcanoes, like Vesuvius or Lassen Peak, are nothing other 

 than this volatile content of the magma below, discharged 

 explosively. 



It sometimes happens that volcanic magma either through pecu- 

 liarities of chemical composition or through sudden exposure, 

 whereby it is quickly chilled, solidifies without crystallization. Such 

 volcanic glasses are commonly called pitchstones or obsidians. A 

 very considerable mass of such obsidian is familiar to all tourists 

 in the Obsidian Cliff at Yellowstone Park. In smaller quantities 

 obsidian is frequently found in nearly all volcanic regions. Such 

 uncrystallized volcanic magma when analyzed is often found to 

 have retained much more of its water content than the 114 per cent 

 maximum which has just been cited for crystalline rocks.^- Here, 

 therefore, is direct confirmation — i. e., as direct as the inaccessible 

 character of the subject is ever likely to permit — of the conclusion 

 that the water content of magma before crystallization is greater 

 than the water content of the rock wliich forms from it. During the 

 process of crystallization, therefore, this water must in some manner 

 be discharged. 



If we apply these observations to Lassen Peak and suppose a body 

 of magma in process of slow crystallization at relatively low tem- 

 perature to exist beneath the mountain, and suppose further that the 

 water set free during the crystallization process is maintained under 

 pressure in a closed chamber, there is danger of a violent outbreak 

 whenever a structural weakness develops in the containing vessel. 

 Lassen Peak is situated in a region often visited by earthquakes. It, 

 therefore, requires no violent stretch of the imagination to believe 

 that one of these earthquakes did in fact weaken the structure early 

 in 1914 and that in consequence of this a small explosion through 

 the floor of the summit crater occurred. More or less in confirmation 

 of this supposition the forest rangers who first visited the mountain 



"Of. J. W. Judd, op. cit. 



