Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth. Parsons and Sclater concluded that some style of mantle convection must apply heat to the base of the plate everywhere to prevent cooling down below 125 km and lithosphere contraction (seafloor deepening) at older ages. Their plate model also allowed an expression for conductive heat flow, "q(t)" from the ocean floor, which is approximately constant at formula_36 beyond 120 million years: