Many geoglogical phenomena show long term cyclical behavior. Of particular interest are mass extinctions and geomagnetic-field reversals, which may hve correlated periodicities of approximately 30 myr. We propose that these cycles are goverend by periodic instability of the thermal boundary layer (D") at the base of the mantle. As this mobile layer thickens by thermal diffusion , it becomes dynamically unstable and hot material erupts from it. The layer, now thinner, extracts heat from the core at a greater rate, increasing the energy supply and hence the reversal frequency of the dynamo. The hot material from the D" layer rises via mantle plumes to the surface where it causes increased volcanic activity which affects the climate and biota.