Correlated periodicities of mass extinctions, climate, magnetic-field reversals, and other geological phenomena may be best explained by cycles of activity within the mantle and core. We propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layers alternate between two phases of activity. In the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion. The active phase begins when the thicking layer becomes dynamically unstable. Hot material erupts from the layer, causing it to become thinner. As the layer thins, the rate of energy supply to the geodynamo, and hence its reversal frequency, increases. The hot material rises to the surface, where it causes widespread basaltic volcanic eruptions. These eruptions release large amounts of CO₂ and sulfates which have a pronounced effect on the climate and biota.