Melting in the transition zone could result from the rapid passage of hot upwelling through the perovskite-spinel phase transition. A large temperature increase could be produced above the perovskite to spinel transition by both viscous heating and a nonlinear positive feedback between temperature-dependent viscosity and latent heat release. The rise times for these instabilities have been estimated to be about 10 Ma. These instabilities were triggered by the episodic downwelling instabilities, which preceded the flush events of cold material in layered mantle convection with phase transitions. These results would suggest that melting in the transition zone would have had a greater tendency to occur in the past and might have caused the generation of komatiites in the Archean.