Here is a review article identifying some causes for the high temperature in the central volume of the Earth. http://www.scientificamerican.com/article/why-is-the-earths-core-so/ Inside all massive astronomical bodies like the Earth, collisions of atoms occur in a gravitational field. Because of gravity, atoms moving toward the center of mass of the Earth would have greater velocities associated with them than atoms moving away from the center of mass. Within the Earth, are atomic collisions toward the Earth's center of mass more energetic than those for atoms moving away from its center of mass? Would that factor tend to cause heat flow toward the center of mass?