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These are four stills of an Elle movie by Jessel et al. of Monash University, Australia. You can see the movie as GIF animation as well by clicking the spool image. A polycrystalline aggregate has a tendency to make pathways for diffusion of fluid around the grain margins. This is called the grain boundary diffusion. This is normally accompanied by Coble creep or Nabarro Herring Creep. The boundaries of the grains themselves are migrated by lattice defects such as dislocations making way for the diffusion process. It is the lattice defects that aid in grain boundary diffusion process by making avenues for the diffusion as well as grain boundary migration. This process therefore leads to the large deformations in rocks by making the rock progressively weaker for further deformation. The crystals are weakened and distorted and may become twinned by deformation on the scale of the lattice. In conjunction with grain boundary sliding and creep in various ways, the rock gets deformed. As the degree of deformation is dependent on the grain size, to processes in conjunction can bring about large distortion and build of appreciable finite strain.