132 CARNEGIE INSTITUTION OF WASHINGTON. 



the pressure may increase sevenfold (from 50 atmospheres to 340 atmospheres) 

 in cooling through this interval of 80°. 



Without going further into the physical-chemical details, this process finds 

 direct application at Lassen Peak. The magma is an andesite high in silica, 

 and so far as we have evidence of the heat conditions it is in process of crystal- 

 lization at a comparatively low temperature. The available water in the 

 region is abundant to support the assumption that the percentage of water 

 carried in the magma is large. The enormous quantities of water given off 

 in the 300 or more explosions of record, some of which continued for several 

 hours, still further supports this assumption. 



It remains to consider how such a process can produce a succession of 

 explosions of gradually increasing intensity up to a culminating point and 

 then gradually subside. 



Suppose that the temperatures within the mountain were relatively low, 

 as every indication during the period of explosive activity seems to establish. 

 Then crystallization was probably proceeding at a very slow rate in a magma 

 of high viscosity which very probably was much undercooled. Suppose, 

 further, that into this inert mass there was suddenly introduced, through 

 cracks at the top, a body of water, under a pressure-head appropriate to the 

 height of the mountain, which was speedily transformed to steam, which, 

 under such circumstances, we know is quickly absorbed by the magma. The 

 first effect of such an intrusion is to cause a considerable increase in the rate 

 of crystallization. Water is coming in at a low pressure from one source, and 

 in consequence of it water is released from another source (out of solution) 

 at a high pressure. An increased rate of crystallization means an increased 

 rate of setting free of the water-content already impounded in solution in the 

 inert lava. The resulting pressure increase might be expected to seek an 

 outlet through the opening which had thus been broached in the crater floor, 

 and a small explosion might result. An undercooled lava system, though 

 inert if cool enough, is unstable, and once it is set in action through increase 

 of fluidity in this manner the tendency to reach equilibrium (through crystalli- 

 zation) is strong. Consequently, the action here indicated is likely to proceed 

 rapidly and the rapidly accumulating pressure to find relief in successive 

 explosions depending for their magnitude and frequency directly upon the 

 rate of advancing crystallization and the increasing amount of participating 

 lava as the successive explosions expose greater masses to the new and favor- 

 able conditions. This seems to account well for the low intensity of the 

 initial outbreak and the gradual increase in volume and intensity as the 

 crater floor was shattered by successive explosions and greater quantities of 

 water were admitted to the volcano hearth, bringing an increasing quantity 

 of crystallizing magma into an action which eventually becomes more or 

 less self-perpetuating until all the available uncrystallized lava is participating. 

 Then follows the culmination and subsidence. 



This analysis, if correct, might lead to two alternative results. The break- 

 ing down of the resistance of the containing envelope, that is, the mountain 

 itself, might happen either from without or from within. If the release had 

 come solely through accumulated pressure within, which eventually became 

 adequate to overcome the normal resistance of the mountain, then a terrific 



