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??
Heat is dissipated via the crust and eventually escapes into space. The lithosphere is a great insulator. No it is slowing down. The viscosity of the upper and lower Mantle is decreasing over time due to heat loss. Heat is mostly transfered via convection which is influenced by viscosity. No the surface temperatures have insignificant influence on heat transfer from the Mantle.
The earth's inner workings are well-insulated by the crust. Continuing radioactive decay of long-lived isotopes adds some energy. There has been some respectable speculation that the very innermost regions have heavymetal concentrations such as Uranium and Thorium which could produce natural nuclear reactor reactions as well along the lines of the Okla natural nuclear reactor.
You, know, is that possibly related to another theory that I swear I've heard, that the center of the earth isn't really molten metal. The outer core is, but the inner core is something more star-like. Maybe I'm having hallucinatory recollections... Or am I confusing that with the theory that cores of stars have anti-matter reactions? Please Register or Log in to view the hidden image! I remember hearing that a long time ago...
It sounds like my own pet theory in a way. I think that a new understanding of gravity in the not-too-distant-future may have implications for how the heat from the centre of the Earth is produced. Gravitational tidal forces could accelerate during the glacial cycle (the deep ocean mixing cooling the planet surface) thereby accelerating the plate movements.
