We have studied thermo-mechanical mechanisms for producing fast timescale geological processes. A two-dimensional time-dependent convection model with the extended-Boussinesq approximation has been used in which both viscous and adiabatic heating are included. Both non-Newtonian and Newtonian temperature- and depth-dependent rheologies with a depth-dependent thermal expansivity have been considered. A fourth-order accurate scheme has been used with a vertical grid spacing as fine as 3 km being imposed in the upper portion of the mantle, and a horizontal grid spacing of around 10 km. Non-Newtonian rheology precipitates the development of very fast upwellings with large amounts of attendant viscous heating and high surface heat flow. Thermal instabilities with fast timescales occur near the surface during the plume impingement. The growth times of these instabilities are found to decrease significantly with the power-law index n and the convective vigor, and increase with larger surface dissipation number. For n = 3 the characteristic timescales are on the order of 1 Myr. Instabilities produced with Newtonian rheology occur over longer timescales. Non-Newtonian rheology also enhances the production of viscous heating to a magnitude which can be 104 times greater than that for chondritic heating. These results suggest that rapid geological events, less than 1 Myr, can be achieved with a non-Newtonian, temperature-dependent rheology by means of a positive thermo-mechanical feedback.