The plastic deformation of Yule marble is assumed to involve two independent mechanisms: one is the dislocation movement inside crystal grains, and the other is the sliding along grain boundaries due to applied stresses. The former predominates at low temperatures and high stresses and the latter, conversely, at high temperatures and low stresses. The two combined mechanisms correspond to a mechanical model of two Maxwell units connected in series, which is used in this paper to explain Heard's (1963) experimental plastic-deformation data obtained for Yule marble at strain rates ranging from 3 × 10−8 sec to 4 × 10−1 sec at 20°–800° C.
The rate-determining step for both mechanisms is the self-diffusion of calcium and carbonate ions surrounding the ‘bad sites” where the dislocation lines are held up and the crystal grains stick strongly. From these assumptions, the activation energies are found to be 132,000 cal/mole for the dislocation-gliding mechanism and 62,400 cal/mole for the grain-boundary sliding mechanism in normal cylinders. These values are reasonable considering that the activation energy for bulk self-diffusion is generally greater than that for boundary self-diffusion. By using the equation for grain-boundary sliding and the related data, the grain sizes are estimated.