Equilibrium Viscosity and Disequilibrium Rheology of a high Magnesium Basalt from Piton De La Fournaise volcano, La Reunion, Indian Ocean, France

Abstract

Lava flows are a common hazard at basaltic to intermediate volcanoes in general and have posed a significant threat to La Reunion Island specifically over the past centuries. In sustained flow units, the efficiency of lava transport away from the vent is dominated by cooling. For basaltic to intermediate lavas it is the ability of the lava to solidify during cooling which exerts a first-order control on the extent of flow. As a consequence, understanding the sub-liquidus rheology of lavas has become a key focus in lava flow research in the past decade. To date, a systematic understanding of lava rheology during emplacement conditions has been largely hampered by a lack of experimental data. Here we present new data on the rheological evolution of a crystalizing high Mg basalt from Piton de la Fournaise. Sub-liquidus experiments were performed at constant cooling rates of 0.5 to 5 K/min, to mimic conditions similar to those experienced by lava during flow on the surface and magma during dike and sill emplacement. The data show that the effective viscosity of the crystallizing suspension increases until reaching a specific sub-liquidus temperature, the so-called “rheological cut-off temperature” (T_cutoff), at which the lava effectively solidifies. This departure to high viscosity is a consequence of rapid crystallization and is found to be primarily controlled by the imposed cooling-rate. Based on our experimental data, we adapt the failure forecasting method (FFM), which is commonly used to describe the self-accelerating nature of seismic signals to forecast material failure, to predict the rheological cut-off temperature (T_cutoff). The data presented serve to expand the modest existing experimental database on non-equilibrium rheology of lavas as a step towards understanding the underlying process dynamics.