Longitudinal deformation of a lava flow: the influence of Bingham rheology

The behaviour of a lava flow is reproduced by a two-dimensional model of a Bingham liquid flowing down a slope. The liquid is described by two parameters, viscosity and yield stress, both strongly temperature dependent. Assuming liquidus temperature at the eruption vent, the temperature decrease due to the heat loss by radiation produces changes in the rheological parameters and, consequently, in velocity, strain and strain rate along the flow. Velocity, compressive strain, strain rate and stress along the flow direction are computed as functions of the distance from the vent and of time, for different kinds of lava flows (basic and acidic) and are compared with corresponding results for a Newtonian liquid. The model shows that, in connection with the pronounced velocity decrease occurring at a certain distance from the vent, the compressive strain, strain rate and stress also show a strong variation. A greater compressive strain is, in fact, induced in a Bingham flow cooling by radiation, than in a Newtonian flow. This behaviour may explain the presence of folds which are commonly observed at the surface of cooled lava flows.