In cosmogony, planetary differentiation is a process by which the denser portions of a molten planet will sink to the center; while less dense materials rise to the surface. Such a process tends to create a core, crust, and mantle. Note that some materials may differentiate to regions due to their chemical affinities rather than their densities, "carried along" by other materials that they're associated with; the uranium in Earth's crust is an example.
On Earth, this process created a surface density of 3000 kg/m3; while the average density, of the planet, is 5515 kg/m3.
