The Geologic Rôle of Dilatancy

Dilatancy, the expansion of granular masses when deformed due to the rearrangement of the grains, was described and illustrated with many interesting experiments by Osborn Reynolds, an English physicist. In this paper the term is used in a broader sense to include all volume increase due to deformation. The consequences of dilatancy by deformation have far-reaching geologic significance. It is suggested that dilatancy is important in inducing faulting and jointing in unconsolidated sediments, in the movement of fluids-oil, water, and gas-contained in rocks, in initiating magmas and in certain of the processes accompanying intrusion and crystallization of magmas. The factors controlling the manner of deformation of unconsolidated granular masses are applied by analogy to a consideration of the manner of deformation of solid rocks by fracture and by rock flowage.