THERMAL-CONVECTION IN LAVA LAKES

In magma reservoirs, large temperature contrasts imply large variations of viscosity. We determine the characteristics of thermal convection in the laboratory for viscosity ratios of up to 10(6). In a fluid layer cooled from the top, convection develops below a stagnant lid. Plumes generate temperature fluctuations whose magnitude, theta(max), is proportional to the temperature contrast across the unstable region, DELTAT(e). Scaling analysis and experimental data show that both temperature scales depend solely on the local function describing the variation of viscosity mu at temperatures close to that of the layer interior, T(m), and are equal to: [GRAPHICS] In the Makaopuhi lava lake (Hawaii), temperature fluctuations were recorded below the growing crust. For the viscosity function of the Makaopuhi magma, their magnitude is predicted to be 18-degrees-C, in agreement with the observations.