In a series of two-dimensional Cartesian simulations with high resolution, on the order of 2 km, we have compared for the same effective Rayleigh number, around 106, the dynamics between Newtonian and non-Newtonian rheologies. A viscosity contrast due to temperature of 1200 has been employed throughout with surface dissipation numbers varying between 0.05 and 0.2. Peak velocities for non-Newtonian plumes are at least 50 times faster than the peak Newtonian velocities and reach magnitudes in excess of m/yr. Upon impinging the surface, plume head structures of the non-Newtonian rheology contain much greater complexity in the thermal-mechanical structure than for Newtonian plumes.¿ 1997 American Geophysical Union