Non-Newtonian rheology of igneous melts at high stresses and strain rates: experimental results for rhyolite, andesite, basalt, and nephelinite

The stress-strain rate relationships of four silicate melt compositions (high-silica rhyolite, andesite, theoleiitic basalt, and nephelinite) have been studied using the fiber elongation method. Measurements were conducted in a stress range of 10-400 MPa and a strain rate range of 10^-6 to 10^-3 s^-1. The stress-strain rate relationships for all the melts exhibit Newtonian behavior at low strain rates, but non-Newtonian (nonlinear stress-strain rate) behavior at higher strain rates, with strain rate increasing faster than the applied stress. The decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt. The decrease in viscosity observed at the high strain rates of the present study ranges from 0.25 to 2.54 log₁₀ Pa s. This study illustrates that the occurrence of non-Newtonian viscous flow in geological melts can be predicted to within a log₁₀ unit of strain rate. -from Authors