I always kind of thougth that once he hit the Earth surface, he was instantly connected with the core, and yet reigns in his own pocket dimension, where the on one side there is the core, on the other is the surface. That may be why a huge chunk of Earth didn't fly out or why the Iokians didn't die out immediately from the huge explosion, but one didn't truely happen. Possibly the reptite castle was switched around with where Lavos should have been, like somewhere in the mantle, and then it came up after a few million years Oo.