Iceland geology is really interesting. Not only is it a part of the mid-Atlantic spreading center, where the North American tectonic plate is pulling away from the European plate, it also (currently) sits atop a mantle plume, or ‘hot spot’. The two plates, along with the spreading center, are all moving with respect to this hot spot of rising magma.
Islantic chicks are hot?
Who knew?
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.