Chance_Bluebird_5788 t1_j2ei89i wrote on December 31, 2022 at 5:49 PM

I think the other comment is probably more pertinent to what you were asking, but it's also interesting to note that geothermal heat comes from decaying radioactive isotopes and tidal heating. Both of these also reduce over time, as isotopes decay into more stable ones and orbits circularize, so these sources will also be "used up" too, but on geological time scales

CrustalTrudger t1_j2ej1d8 wrote on December 31, 2022 at 5:54 PM

Tidal heating is not a significant source of geothermal heat on Earth, the two primary sources are radioactive decay and left over heat from planetary accretion. Tidal heating is important on other bodies, e.g., Io, but not on Earth. You are still correct in that both sources of heat on Earth (i.e., heat from radioactive decay and primordial heat) are decreasing on geologic timescales.