Pressure solution and Coble creep in rocks

On 5 November 1976, the Tectonic Studies Group of the Geological Society held a lively discussion meeting at Imperial College on the subject of pressure solution and Coble creep. Deformation mechanisms may be divided into three broad categories: (1) Cataclastic processes (2) Intracrystalline processes involving dislocations (3) Diffusive mass transfer processes. We are concerned here with group (3) processes, flow by diffusive mass transfer of matter from grain boundaries which are subjected to high normal stress to less highly stressed grain boundaries. If the diffusion is predominantly through the grain the process is termed Nabarro-Herring creep whereas if it is predominantly around grain boundaries, the flow is termed Coble creep. In general, grain boundary sliding must occur to accommodate the grain shape changes due to diffusion. For these processes to proceed at rates which are detectable, the temperature must be at a large fraction of the absolute melting temperature of the material. Geologists have long recognized in low grade metamorphic rocks, textures in-dicative of deformation by diffusive mass transfer. Even over the relatively long time available for geological deformations, at the low temperatures concerned, solid state diffusivities would be too slow to account for the observed strains. It is therefore inferred that the rate of diffusive mass transfer is enhanced by the presence of an intergranular fluid film. Hence the use of the term ‘pressure solution’ to describe the process. Thus Coble creep and pressure solution are similar in that they both involve intergranular diffusion. The meeting was intended to