It is running down so slowly that there is no reason to expect that plate tectonics will ever cease. The lithosphere may become a bit thicker, and sea-floor spreading a bit slower, but the large-scale processes seem likely to continue much as they do today until the Sun swells up and engulfs the Earth some 5 billion years into the future. Heat Engine & Mantle Convection. Where does all the power come from for 5 BILLION years? Has plate tectonics always been the same rate? Does it speed up during the glacial cycle (i.e. 90% of the time) so that the plates move twice speed over large scales??