It is not really a geological process (such as mountain formation) but more a physics process. The explanation is both simple and complicated. Simple: The phenomenon why denser material ends up at a planet's center is the same as the reason why oil floats on water. On the surface of the earth, denser object end up at the bottom. Inside the earth, denser objects end up at the center. More Complicated: The force of gravity on a object is proportional to its mass. Say there was a piece of iron and a piece of rock of the same size. Iron is much denser than rock, so it will be heavier. Gravity will exert a greater force on it than it will on the rock. Thus, the iron ends up at the core, while rock ends up on the surfce. Advanced: One of the basic "laws" of physics is that for something to remain in equilibrium, it must have the lowest potential energy. Thus, a planet's composition goes from most dense in the middle, to least dense at the edge, in order to meet this requirement of lowest potential energy. The equation for potential energy for a planet is GMm/r, where G is a constant, M is mass of planet, m is mass of an object, and r is radius from center of planet. G and M are constant. Thus, in order to minimize the potential energy of each piece of the earth, it is natural that when m is large (ie dense), it will want to have a small r more than a piece with a lower m will want to have a small r. For example: Object 1: 10kg Object 2: 5kg These can be at either 1m or 2m. If object 1 is at 2m, potential energy would be 2*10+1*5=25. However, if object 1 is at 1m, potential energy would be 1*10+2*5=20. Thus, when the denser material is closer to the center of the earth, the potential energy of the system is minimized, thus ensuring equilibrium.