An Experimental Study of the Effect of Temperature and Stress on the Creep of Rocks, with a Discussion of Earth Tide Damping, Isostasy and Mantle Convection

Abstract

Summary This paper is based in part on a paper by the author and A. K. Misra and published in Geophys. J. R. astr. Soc. (1965), 9, 509–535, where the experimental and theoretical results are fully described. Measurements have been made of the creep of a number of different rocks at temperatures up to 750 °C under conditions of constant compressive stress. The results show that at temperatures below about 0.2 Tm (where Tm is the absolute temperature of melting) the creep strain is proportional to the logarithm of the time under load, and is approximately proportional to the stress and to the absolute temperature. At higher temperatures the creep rate falls offless rapidly with time, and the creep strain is proportional to a fractional power of the time, with the exponent increasing as the temperature increases and reaching a value of 3/1 at temperatures of about 0.5Tm. At these temperatures the creep increases approximately as the square of the stress and possibly exponentially and it increases with temperature (T) as exp(- U/kT), where U is an activation energy and k is Boltzmann's constant. These results are strikingly similar to those obtained in measurements on metals, and it is thought that they can be explained in an exactly similar way in terms of competing processes of strain hardening and thermally assisted processes of plastic deformation and recovery from strain hardening. The results suggest that creep is unlikely to be of importance in the upper part of the Earth's crust but that it will be important in the lower crust and upper mantle, particularly in connection with the damping of Earth tides and with the phenomenon of isostasy. An estimate is made of the rate of steady state creep of peridotite, which is found to be within the lower limit postulated in convection theories of continental drift. Although hydrostatic pressure is known to decrease creep rates (through its influence on thermal recovery) its exact influence on the creep of rocks is still a major unknown factor.