VOLCANOES AND IGNEOUS INTRUSIONS 335 



them. The theories may be classed under three heads : (I) those 

 which assume a molten interior; (II) those based upon the assump- 

 tion that the earth is solid from the surface to the center; (III) those 

 holding that a few miles below the surface a zone of rock exists which 

 is either molten, or at any rate in a non-crystalline condition. 



/. Theory Based upon the Assumption that the Interior is Molten 



The theory of a molten interjor is now held by few geologists because of the many 

 objections to it (p. 273). In the earlier days of geology when this theory had general 

 acceptance, the difficulty of accounting for the independence of volcanic eruptions 

 brought forth much discussion, and a number of modifications to the theory were sug- 

 gested. If all lavas came from one great reservoir, it is evident that according to the 

 law of hydrostatics eruptions would be simultaneous, or in two adjacent vents, from 

 the lowest one. 



77. Theories Based upon the Assumption that the Earth is Solid 



{a) Heat by Friction. — This theory is based on the fact that heat is developed by 

 friction when rocks grind and crush each other. It is held that when great earth 

 blocks (segments) move past each other, the pressure and friction along the lines of 

 movement develop heat on a large scale. If fluxes (rocks which upon uniting with 

 others produce a substance that will melt readily) are present to lower the melting 

 point of the rock silicates, the heat may be sufficient to produce molten rocks and 

 volcanoes. Since all rocks contain more or less water, steam under immense pressure 

 will be developed upon the fusion of the rock. Explosions of the steam developed in 

 this way are believed to be competent to drill channels to the surface, and to eject the 

 molten rock through the chimneys thus formed. The intermittent action and extinc- 

 tion of volcanoes, according to this theory, are dependent upon the movement of the 

 earth's segments. It should be noted in this connection that no observations have 

 been made of faults the walls of which are fused as a result of slipping. 1 



(b) Formation of Lava Reservoirs by Relief of Pressure. — This theory rests on 

 the assumption that at moderate depths the heat of the earth is so great that the 

 solid state of rocks is maintained only by the pressure of overlying rocks. If this 

 assumption is correct, it is only necessary to show that the pressure of highly heated 

 rocks can be relieved. This is thought by the advocates of the theory to be accom- 

 plished when deeply buried, sedimentary strata are folded. If a stratum strong 

 enough to sustain the weight of the overlying rocks is arched, and the underlying rocks 

 are thus relieved of some of the pressure, the latter may melt. A volcano or fissure 

 eruption may then occur if a crack to the surface is present through which the lava can 

 force its way. The supply of lava would depend upon the amount of molten rock 

 under the arch, and the extinction of the volcano would result from its exhaustion. 



Two strong objections to the theory are : (1) the difficulty of accounting for a tem- 

 perature in sedimentary rocks high enough to- fuse them, and (2) the difficulty of ex- 

 plaining the presence of sedimentary rocks of basaltic composition (p. 331) of suffi- 

 cient thickness under an arch to be a source of the lava of massive plateaus. 



1 Schwartz, E. H. L., — Causal Geology, p. 241. 

 CLELAND GEOL. — 22 



