How shear helps lava to flow

Understanding the thermo-rheological regime and physical character of lava while it is flowing is crucial if we are to adequately model lava flow emplacement dynamics. We present measurements from simultaneous sampling and thermal imaging across the full width of an active channel at Piton de la Fournaise (La Réunion, France). Our data set involves measurements of flow dynamics at three sites down-channel from the vent. Quantification of flow velocities, cooling rates, sample texture, and rheology allows all thermo-rheological parameters to be linked, and down- as well as cross-channel variations to be examined. Within 150 m from the vent, we recorded an unexpected velocity increase (from 0.07 to 0.1 m/s), in spite of cooling rates of 0.19–0.29 °C/m and constant slope. This change requires a switch from a Newtonian-dominated regime to a Bingham plug–dominated regime. Sample analysis revealed that the plug consists of foam-like lava, and the shear zones involve vesicle-poor (low-viscosity) lava. With distance from the vent, shear zones develop, carrying the vesicular plug between them. This causes flow to initially accelerate, helped by bubble shearing in narrow lateral shear zones, until cooling takes over as the main driver for viscosity increase and, hence, velocity decrease.