Along with density, pressures and temperatures, both rise with depth throughout the earth, reaching in excess of 5000^o C and millions of atmospheres, near the center. Thus the earth is not only density zoned, but also temperature- and pressure zoned. These temperature and pressure gradients are important because it is the interaction of these two parameters with the type of material that determines the state (phase) of the various layers. 

Although this model may, at first glance, appear highly hypothetical, several of its components, such as phase and compositional changes, have been cross-checked under laboratory conditions. Moreover, mathematical predictions based on this model. have matched the actually observed earthquake wave behavior. 

The earth is made of the same materials as the remainder of our solar system. Therefore, we can expect that the materials from which the inner planets formed should have a composition similar to that of the earth. They are accessible to us in the form of occasional debris that fall from space to the surface of our planet. And, when we analyze these heavenly stones, they fall into two categories: the stony meteorites, made up of silicates and oxides similar to those we could expect in the crust and the mantle; and nickel/iron meteorites similar in composition to the materials we would expect to find in the core. This external line of evidence represents a splendid confirmation of the human ability to extrapolate from what is known and to explore even the unseen and unreachable, as long as there are data and observations available.