Thanks for your interesting question. The earth has layers and these layers of the earth are made up of different materials, mostly because they have different densities. As an example, if you have a jar with oil, water, and honey in it, the oil will rise to the top and the honey will sink to the bottom while the water will stay in between. This occurs because the oil is the less dense liquid and floats to the top, whereas the honey is more dense and sinks to the bottom.

Early in its history, the earth was very hot and almost like a liquid which allowed the denser materials in the earth (such as iron and other metals) to sink to the center of the earth and the less dense materials to float to the surface.