The transfer of heat by movement of material within the mantle. Material derived from within the mantle is added to the lithosphere at constructive margins and cool lithosphere descends at subduction zones, thus some return flow of material must take place at depth. The return flow may involve only material at shallow depth below the lithosphere, or material through the whole mantle, or possibly two or more layers of convection cells transferring heat between the layers by conduction. Hot spots may overlie isolated plumes of material rising from the core–mantle interface.