The lithospheric plates of plate tectonics are created from hot mantle rock at ocean ridges and descend into the mantle at ocean trenches. The plates are by definition, part of mantle convection cells. A fluid heated from within or from below and cooled from above convects thermally if the Rayleigh number exceeds a critical value. The earth's mantle is heated from within due to the decay of radioactive isotopes and is heated from below by heat generation in the core.