HEAT PRODUCED BY MECHANICAL ACTION. 99 



INI'IKFJT HEAT 



It is a well-known law that mechanical action develops an equivalent 

 amount of heat, except for the part of the energy which is potentialized 

 It has already been seen that heat is ordinarily favorable to chemical 

 action. Therefore mechanical action promotes chemical action, because it 

 develops heat and raises the temperature. Indeed, the heat developed by 

 mechanical action is frequently one of the most important favorable con- 

 ditions for metamorphism. It will be shown (Chapter VIII, p. 740) that 

 where mechanical action is strong the complete recrystallization of rocks 

 may occur much nearer the surface than under quiescent conditions. This 

 result is largely attributed to the rise in temperature due to deformation, 

 which results in vastly greater efficiency of the water as an agent of 

 chemical action. 



It therefore becomes of the utmost importance to consider to what 

 extent the temperature is raised in the rocks by mechanical action. 



The heat, as already intimated, is produced by the transformation of 

 work into heat as a result of straining the rock particles within the elastic 

 limit, by rupturing them, and by their frictional movements over one 

 another. Mallet a has held that the heat thus developed may be sufficient 

 to liquefy rocks by aqueo-igneous fusion. He thus accounts for the crys- 

 tallized cores of many mountain ranges. He even holds that the material 

 fused by mechanical action may intrude the adjacent solid rocks. LeConte 

 follows Mallet in this belief. It may be theoretically possible that rock 

 material can be ground so fine as to develop sufficient heat to fuse it. 

 However, as explained (Chapter VIII, pp. 728-732), we have no evidence in 

 the field that this has occurred. It is shown (Chapter VIII, pp. 690-696), 

 that when the temperature of water-saturated rocks rises a certain amount, 

 readjustment occurs, not by mechanical subdivision and grinding of the 

 particles over one another, but by recrystallization. The process is thus 

 chemical, not mechanical, and the expenditure of energy and the conse- 

 quent development of heat are far less than by the former jjrocess. How- 

 ever, it is probable that, as a result of the interior kneading of rocks, the 

 temperature may be materially increased, perhaps several hundred degrees 

 beyond the normal temperature which obtains as a result of the depth of 



« Mallet, Robert, Volcanic energy; an attempt to develop its true and cosmical relations: Philos. 

 Trans. Royal Soc. London, vol. 163, 1873, pp. 147-227. 



