As they travel trough the Earth they reveal its internal structure. This branch of earth sciences is also known as seismology . The next page illustrates how seismic helps to understand the Earth's structure . We know already that because the Earth was comparatively close to the sun, it lost most of its volatiles as the Sun started to heat up. Gravity, however, is not the only process that drives differentiation.
Chemical affinities can also play an important role.
The reason for this aberration is the circumstance that ion size and chemical affinities of U and Th prevent them from being incorporated in the dense, tight crystal structures that are stable at the high pressures encountered in the earth's core. Because they can fit much more easily into the more open crystalline structures of silicate and oxide minerals, they are enriched in crust and mantle. Convection is the most efficient of these, and is found to play a role in many geologic processes. It is also the main process by which heat is moved to from the interior to the exterior regions of the Earth. Convection implies fluid behavior where hot material rises due to its lower density, and cold material sinks due to its higher density.