Introduction

Our knowledge of the bulk composition of the Earth is well constrained by observations that include cosmic elemental abundances and laboratory analyses of rock and mineral samples originating from the mantle (e.g., Ringwood, 1975; O'Neill and Palme, Chapter 1, this volume). Also well constrained are the bulk elastic properties of the Earth through the analysis of seismic wave propagation (e.g., Kennett and van der Hilst, Chapter 8, this volume) and analyses of tides and the planet's rotation. But less satisfactory is our understanding of the time-dependent or viscous response. Various geophysical observations indicate that stress and strain in the planet as a whole are not in phase, as seen in observations of the Earth's tides, and that the mantle creeps when subjected to stress, as demonstrated by crustal rebound after removal of ice loads. But we do not have a complete description of the solid Earth's departures from elasticity. Laboratory experimentation on terrestrial materials indicates that the nonelastic response of the mantle is dependent on the defect nature of the solid, such as dislocation density and dislocation mobility, which in turn are functions of the ambient temperature, pressure, and nonhydrostatic stress. Thus, at seismic frequencies, the response to an applied oscillatory stress is out of phase because of the finite diffusion time of the atoms around the dislocation, whereas for longer-term motions associated with tectonic stresses the response is described in terms of a solid-state viscosity.