Introduction

Grain size is an important parameter that controls the rheology of rocks (see Chapter 8). The grain size of rocks can vary from ~10 μm (ultra-mylonite, a fine-grained rock found in shear zones; e.g., bell and etheridge, 1973; whiteet al., 1980; handy, 1989) to ~102–103 m (estimated grain size of the inner core; bergman, 1998, see also Chapter 17 of this book) depending on its thermal and mechanical history. This large variation in grain size causes significant changes in rheological properties. For example, for a small grain size and low stress, a polycrystalline material tends to deform by grain-size sensitive creep mechanisms, such as diffusional creep (see Chapter 8). In these cases, the viscosity of a rock, η, changes with grain size, L, as η ∝ Lm where m is a constant between 2 and 3.