Figure 8.30 views of Earth's layered structure. The study of seismic waves and other geophysical techniques have stown Earth to be a dynamic planet with many interacting parts . The properties of Earthy layers include the physical state the material (solid, liquid , or gas ) as well as how stiff the material is ( e.g. the distinction between the lithosphere and asthenosphere ) These studies have shown that Earth's layers are mainly determined by density with the heaviest materials (iron) at the center and the lightest ones on the outside ( gases and liquids )