T72 ORIGIN OF VOLCANIC HEAT. 



sions are themselves produced along parallel axes, or over definite 

 areas, by the more rapid contraction, from cooling, of hotter material 

 of the earth's mass beneath the crumpled shell. For when the mass 

 of a mountain, already forced up by lateral pressure, presses down- 

 ward upon the resolved vertical force, so as to equal the resistance to 

 crushing of the rocks on either side, then the further force of lateral 

 pressures must be used up in crushing the rock between them to 

 powder, or in developing such heat as shall render the mass plastic 

 and so displace it. The contraction is necessarily greatest along lines 

 or planes of weakness in the crust ; and the heat developed in the 

 rocks thus squeezed is necessarily greatest along those lines or planes 

 or places where movement and pressure are greatest. If one bed of 

 rock is less compressible than another, it will become more heated by 

 compression. Thus quartz would become nearly three times as hot 

 as clay, and communicate its heat to the adjacent beds. In such 

 positions the temperature may rise locally to a red heat, or even to 

 the point of fusing the rocks which are crushed and pressed together. 

 Hence heat is produced beneath the places where it is exhibited in 

 volcanic vents ; and the heat produced locally is consumed locally in 

 originating physical and chemical changes in the rock substance, and 

 in the mechanical work of ejection. 



This being the theory, the question arises, Is the compression 

 competent to produce the results which are inferred ? And upon this 

 point Mr. Osmond Fisher has discussed some theoretical doubts in 

 his " Physics of the Earth's Crust," to which reference may be made. 



Evidence in support of Mr. Mallet's Views. Mr. Mallet observes 

 that under the ordinary conditions of experiment at the earth's 

 surface, such a rock as granite or porphyry crushes under a pressure 

 of 14 tons to the square inch. On the hypothesis of a contracting 

 crust one mile thick, it is calculated that the lateral pressure per 

 square foot would amount to 952,666 tons, or more than 472 times 

 the force necessary to crush granite. The height of a column of rock 

 which would be crushed by its own weight is about 4 miles; but 

 the horizontal force is equal to a column of 2000 miles or one-half 

 of the earth's radius ; so that it is concluded that the resolved forces 

 of gravitation will crush the solid crust if it is left partially 

 unsupported, by the shrinking away of a contracting nucleus beneath. 

 Mr. Mallet conducted a number of experiments to show that a 

 considerable temperature was developed by crushing various rocks, 

 though the heat thus obtained was less than the temperature 

 under which crystalline rocks consolidate. The amount of heat lost 

 from the earth every year by radiation, could be produced by crush- 

 ing 987 cubic miles of rock ; and Mr. Mallet estimated that, if used 

 up in volcanic energy, it would be sufficient to produce all existing 

 volcanic cones in less than eight years ; so that only a minute fraction 

 of the radiated heat of the earth can reappear in the form of volcanic 

 activity. 



Geological Evidence on the Nature of Volcanic Heat. Whatever 

 may be the value of Mr. Mallet's calculations, no matter whether the 



