Transcript: The earth has a number of layers, and its structure changes quite dramatically from the surface to the core. The top layer is the rocky outer crust, about 30 kilometers thick. Underneath the crust is the lithosphere, a semi-liquid rock layer that the crust floats on. Below that is the thick mantle mostly made of rocks containing silicon and oxygen. Inside that is the outer core, a liquid region of high pressure, high temperature iron and nickel. At the very interior is the solid core primarily of iron and nickel. This dramatic change in structure within the Earth is caused by the process of differentiation whereby heavier atoms or compounds sink to the center under the force of gravity because the Earth has been liquid or semi-liquid for much of its life. This differentiation process is what happens in a smelter when the scum layer floats to the top and the heavy metal floats to the bottom. You can simulate differentiation by filling a large jar with a mixture of ball bearings and marbles of same size. If you stir the jar for long enough you will notice that the ball bearings will end up at the bottom while the marbles end up at the top. This is an example of differentiation.