This is where I diverge from prevailing convection theory. IMO, up-welling is caused primarily by tidal forces forcing up magma which seeks the least dense (density is a factor of heat, of course) pathway.
I believe "tidal force" is one of the main heat sources, if not the main heat source within the earth, besides residual.
My thinking is tidal forces inside the earth heat the interior, and it builds up over millennials (approximately 100 millennials) until the earth's crust can no longer contain it.
Tidal heating can and does occur in our solar system, mainly with small planetary bodies such as the moons of the gas giant planets, particularly Jupiter, and to a far less extend even with the earth-moon system. It’s actually the sole explanation for why there is volcanism on Jupiter moons Io and Europa. However, in that case you have a massive planet like Jupiter with its enormous gravitational field pulling on and contorting the surfaces of comparatively tiny moons.
On the other hand, with the earth-moon system, the gravitational interactions are comparatively weak, resulting in some heat generation and the creation of the tides. The primary source for the interior heat of the earth has long since known to be radioactivity from heavy elements deep in the interior.