Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics

The eruptive dynamics of volcanic systems are largely controlled by the viscosity of deforming magma. Here we report the results of a series of high-temperature, high-pressure experiments at conditions relevant for volcanic conduits (250 MPa confining pressure and temperature between 500 degrees C and 900 degrees C) that were undertaken to investigate the rheology of magma with crystal fractions varying between 0.5 and 0.8 (50 to 80 wt.%) at different strain-rate conditions. The experiments demonstrate that the presence of crystals increases the relative viscosity (ratio between the viscosity of the mixture and the viscosity of the melt phase) of magmas and additionally induces a decrease of the relative viscosity with increasing strain-rate (shear thinning, non-Newtonian behavior). The experimental results, combined with existing data at low crystal fractions (0-0.3), were used to develop a semiempirical parameterization that describes the variations of relative viscosity for crystal fractions between 0 and 0.8 and accounts for the complex non-Newtonian rheology of crystal-bearing magmas. The new parameterization, included into numerical models simulating the magma ascent dynamics, reveals that strain-rate-dependent rheology significantly modifies the dynamic behavior inside volcanic conduits, particularly affecting the magma fragmentation conditions. (c) 2007 Elsevier B.V All rights reserved.