EXPERIMENTAL, NUMERICAL AND ANALYTICAL MODELS OF MANTLE STARTING PLUMES

The results of a combined experimental, numerical and analytical investigation of starting thermal plumes are described, to obtain a better perspective on plumes within the Earth's mantle. Thermal plumes were generated experimentally in a tank of corn syrup by means of an electrical heater. Viscosity ratios of 400, 30 000, and 10(8) were generated by varying the temperature of the tank. Plumes for the smaller ratios had the traditional 'balloon-on-a-string' shape, but that at the highest ratio had a novel morphology. The plume heads in the first two cases were observed to rise at roughly a constant speed, in contrast to most previous studies which found the plume heads to accelerate. Loss of buoyancy from the plume head owing to heat loss is believed to be responsible for this difference. Starting plumes were simulated numerically using an axisymmetric, finite-element code to solve the Boussinesq equations at finite Prandtl numbers. The constant rise speed observed experimentally was confirmed by the numerical simulation for the viscosity ratios of 400 and 30 000, but numerical instability prevented simulation of the case with a viscosity ratio of 10(8). There was very good agreement between the experimental and numerical rise speeds. An analytical model was developed which reduces to previous models in limiting cases. This parameterization gives better agreement with the experimental and numerical results than does any previous model.