GEOPHYSICAL LABORATORY. 131 



volatile matter (mainly water) within the mountain appears to have found 

 vent on the 22d without actually blowing off the top of the mountain and 

 thereafter to have remained powerless to open the main crater conduit. It 

 was as though the lava plug with which the volcano had remained sealed for 

 so many years had been started upward by pressure from within, which had 

 then found release at the weakest point in the inclosing chamber. This 

 happened not to be through the plug itself, but below the plug on the northeast 

 side of the mountain, much as a gas explosion in a coal-burning stove might do, 

 raising the lid and escaping in a horizontal blast from beneath it. After the 

 emergence of these horizontal blasts the pressure was released and the lid 

 collapsed. Its present appearance indicates rupture by such forces at a great 

 many points of the exposed surface. 



The mechanism of this entire sequence of volcanic activity can, I think, be 

 inferred quite simply from the phenomena which have been described and 

 from an appropriate application of the laws of physical chemistry. The 

 great volcanic clouds which characterized the explosions were mainly of dust- 

 laden steam. There is extremely little evidence of the participation of sulphur 

 or other chemically active ingredients from beginning to end of this series of 

 outbreaks. Compared with other volcanoes which have been studied, the 

 volcanic phenomena at Lassen Peak occurred at comparatively low tempera- 

 tures, certainly not above red heat. It should be noticed also that Lassen 

 Peak is located in a region of considerable rainfall, that the summit crater 

 is almost never free from snow, and that several streams of considerable size 

 have their sources high on the flanks of the mountain. We are therefore 

 dealing with a cooling volcano hearth in a region of abundant water-supply. 



Considering such dynamic systems, it is usual to assume that temperature 

 and pressure rise and fall together. If the temperature were to rise, the 

 pressure would be increased and if it were to fall the pressure would fall with 

 it. In such reasoning, however, we overlook a vital factor in the considera- 

 tion of silicate systems in the presence of considerable quantities of water. 

 Laboratory measurements have shown that silicate solutions (rock magma, 

 lava) of high silica-content easily take up 10 to 15 per cent of water when in 

 liquid condition under pressure and carry it in solution. In accord with this 

 we find in nature occasional pitch-stones and obsidians, which have cooled 

 from the magma without crystallization, also containing 10 per cent or more 

 of water. On the other hand, crystalline rocks, as shown by the thousands 

 of analyses made of them, contain no more than 1.5 per cent of water. 

 In order to crystallize, therefore, a high-silica magma containing 10 per cent 

 of water must divest itself of more than 80 per cent of this water in the process 

 of crystallization. Such a large proportion of the water-content set free in 

 the hot volcano chamber will increase the pressure enormously, even though 

 the crystallizing operation is a cooling process. 



By way of illustration of this, silicate mixtures have been prepared in the 

 Laboratory, inclosed in bombs with a sufficient quantity of water, and the 

 pressure measured over a series of falling temperatures. It has been demon- 

 strated, for example, that a simple mixture of silica, potash, and water, which 

 begins to crystallize at 500° C, will discharge this water when the temperature 

 is lowered from 500° to 420°, and the water thus set free will immediately 

 develop a pressure appropriate to that high temperature. In this system 



