The Clapeyron slopes of mantle phase boundaries and mantle convection. Near a phase boundary with positive Clapeyron slope (top), materials in cold region transform to high-pressure phase (denser) at shallower depth than those in hot region. Instead, near a phase boundary with negative Clapeyron slope (Bottom), materials in hot region transform to high-pressure phase (denser) at shallower depth than those in cold region. Therefore, phase boundary together with temperature difference results in density anomaly near phase boundary. (Image by Prof. Sang-heon Shim.)