The effects of phase changes and strong internal heat generation may combine to bring about brief, but extremely intense episodes of rapid thermal convection in the Earth's mantle. Numerical calculations using realistic thermodynamic properties for the exothermic Olivine→Spinel and endothermic Spinel→Perovskite+Magnesiowustite phase transitions suggest the transition region of the Earth's mantle may act as a capacitor for subducting slabs. Slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached, and is then rapidly discharged into the lower mantle. In my calculation, this occurs as a catastrophic burst of convection lasting ~10 Myr, with elevated heat transfer rates lasting ~100 Myr. Such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra-plate volcanism and stabilize the geodynamo against reversals. The topography on the two phase change boundaries is found to be negatively correlated, with the correlation becoming more negative during periods of rapid convection. Furthermore, the results of this study suggest the transition region of the Earth's mantle could be ~250 K cooler on average, and consequently more viscous, than the surrounding mantle. ¿ American Geophysical Union 1993