THE EFFECT OF CRYSTALLIZATION ON THE RHEOLOGY AND DYNAMICS OF LAVA FLOWS

The dynamics of a lava flow is studied by a two-dimensional model desc ribing a viscous fluid with Bingham rheology, flowing down a slope. Th e temperature in the flow is calculated assuming that heat is transfer red through the plug by conduction and is lost by radiation to the atm osphere at the top of the flow. Taken into account is that the increas ing crystallization takes place in the flow as a consequence of coolin g. The lava viscosity and yield stress are expressed as a function of crystallization degree as well as of temperature: in particular it is assumed that yield stress reaches a maximum value above the solidus te mperature, according to experimental data. Dynamical variables, such a s velocity and thickness of the flow, are calculated for different val ues of the maximum crystallization degree and the flow rate. The model shows how the lava flow dynamics is affected by cooling and crystalli zation. The cooling of the flow is controlled by the increase of yield stress, which produces a thicker plug and makes the heat loss slower. The increasing crystallization has two opposing effects on viscosity: it produces an increase of viscosity, but at the same time produces a n increase of yield stress and hence reduces the heat loss and keeps t he internal temperature high. As a consequence, lava flows are signifi cantly affected by the dependence of yield stress on temperature and s carcely by the maximum crystallization degree.