[Response: Not an absurd question, but in practice net geothermal heating (including volcanoes, mid
ocean ridges etc) is about 0.075 W / m2 — some 20 times less important than human CO2 increases.
The slow diffusion equatorward of the the NA is not that slow, and seems to happen at narrow localized spots near topographic features, mid
ocean ridges, continental margins, islands
etc, due to tidal internal wave breaking and other related mechanisms which remain unclear.