Time-dependent isothermal variations of silicate melt viscosity after sudden temperature or stress changes

Time-dependent variations of the viscosity of silicate melts, associated with sudden changes of temperature or uniaxial compressive stress, have been measured. The melt compositions were MgCaSi₂O₆, CaAl₂Si₂O₈ and NaAlSi₃O₈. The steady-state viscosity values were between 1 and 1000 TPa s. Applied stresses were always less than 200 MPa, and the observed stabilized melt flow was Newtonian. The characteristic times of the viscosity variation with time, due to adjustments of the internal structure of the melts after a thermal perturbation, depend on temperature and composition. The characteristic times for isothermal stress perturbations are about 10 times shorter than for thermal perturbations.