Viscosity and Anelasticity of Melts

is the Maxwell relaxation time [24]. This relaxation time is a convenient approximation to the timescale of deformation where the transition from purely viscous behavior to purely elastic behavior occurs. The purely viscous response of a silicate melt is termed “liquid” behavior whereas the purely elastic response is termed “glassy” behavior. The term silicate melt is used to describe molten silicates quite generally, regardless of their rheology. Even for the simple case of a Maxwell body, the change from liquid to glassy behavior does not occur as a sharp transition but rather describes a region of mixed liquid-like and solid-like behavior a region of viscoelasticity. Deformation experiments performed in the viscoelastic region contain three distinct time-resolved components of deformation, instantaneous recoverable, delayed recoverable and delayed non-recoverable. Figure 2 illustrates these components for a creep experiment. Viscous flow is the delayed non-recoverable component, the term anelasticity refers to the instantaneous plus the delayed recoverable deformation and the entire deformation behavior falls under the heading of viscoelasticity [26]. Structural relaxation in silicate melts is but one example of a relaxation mode. It can be mechanically represented as a series combination of viscous dashpot and elastic spring (a Maxwell element, see Figure 3). For more complex materials such as crystal suspensions, foams and partial melts, several additional mechanisms of deformation are contributed at distinct timescales of deformation via additional relaxation modes within and between the added