154 



Prof. J. Prestwich. 



gives the water aii elastic force sufficient to bar its further progress, 

 and fills the space n with vapour at an excessive tension, or with 

 water at the critical point. This tension has determined a state of 

 equilibrium which, so long as no change takes place in the opposing 

 surfaces of the vapour and the lava, is maintained without change. If, 

 then, anything occurs to weaken the resistance of one of the surfaces, 

 and equilibrium be destroyed, the surface, if elastic, will yield. Thus, 

 so long as a volcano is at rest, the pressure exercised by the column 

 of lava L against the sides of the duct counterbalances the tension of 

 the vapour at n. But, when the lava is forced upwards and flows 

 past n, the statical pressure at that point is so far lessened that -the 

 tension of the vapour now exceeds the resistance of the lava at the 

 same point, and the vapour is driven into the liquid lava L and rises 

 with and through it to the surface. At the same time the sudden 

 elastic expansion of the vapour lessens the pressure on the water at m, 

 which therefore becomes in turn explosive, and as this water is 

 replaced by a portion of that which is pressing forward from the 

 water- head in b, this action will be repeated so long as the water 

 supply lasts and the disturbing causes continue in operation. 



The influx of water into the volcanic duct will also be materially 

 influenced by the changes of level, or oscillation, which the column 

 of lava undergoes during eruptions. For the variation of pressure 

 caused by this state of changing equilibrium allows the vapour 

 pressure to predominate at one time, while the resistance to it by the 

 column of lava will be in excess at other times. 



The explosion of steam iato the lava involves, amongst other 

 effects, disturbances of the encasing strata whenever any portion of 

 the water or vapour explodes behind blocks of rock forming part of 

 the walls of the duct. In fissile or porous sandstones, where the water 

 cavities are small, it may be supposed that a minimum disJodgment of 

 the materials composing the strata takes place. But if, instead of soft 



Fig. 8. — Projection of blocks from the permeable Strata b into the ascending lava 

 (one side only is represented). 



