The influences of two exothermic phase transitions, the olivine to &bgr;-spinel and the &bgr;- to &ggr;-spinel transition, on Martian mantle convection have been studied with an axisymmetric spherical-shell model. There are depth dependences in the thermal expansivity and gravity, which, taken together, decrease by a factor of between 2 to 3 across the mantle. We have discovered an unique mechanism for the amplification and superheating of megaplumes in Mars, which works particularly well because of the optimal heights of the two destabilizing exothermic phase transitions above the Martian core-mantle boundary (CMB): The cold descending mantle flow is accelerated by its interactions with the phase transitions and the nearby CMB forces the flow to bend over sharply. Together these effects result in strong localized viscous heating, which will generate and accelerate a rising plume and may even cause melting in its deep interior and produce komatiites. ¿ American Geophysical Union 1995