Quote:
This is wrong because you do not have unlimited heating. You only have enough heat (from the core) to heat the rock at the bottom of the mantle to 3740 K, where it has a density of 5,560 kg/m³ (which is heavier than the rock anywhere above it).


If the material at the base is heated to 3740K by heat rising from the core then it must become hotter than the material immediately above it. Unless there is a sudden, drastic, change in density at exactly the right place, this must mean that (as it becomes hotter)the basal material becomes less dense than the material directly above it.

There never was nothing.