Phase transformations occur in many regions in Earth (particularly in the transition zone, 410–660 km depth in the mantle) and have several important effects on plastic properties. A phase transformation changes crystal structure and the nature of the chemical bonding that affects any physical properties including plastic properties. In addition, a first-order phase transformation results in a change in grain size and a redistribution of internal stress–strain that may influence the nature of plastic deformation. A second-order phase transformation will not result in these changes, but will result in the anomalous reduction of an elastic constant and/or the anomalous increase in the dielectric constant. All of these changes have certain effects on rheological properties. This chapter presents a summary of the experimental observations and theoretical models on which one can estimate the relative importance of various effects in geological processes.

Key words isomechanical group, transformation plasticity, Greenwood–Johnson model, Poirier model, grain-size reduction.