First, seismic waves have their direction of motion changed (refracted) by variations in the interior density. Thus, by studying the way such waves propagate in the Earth we can learn something about density variations. Second, the fact that P-waves propagate in liquids but S-waves do not allows us to determine if portions of the interior are liquid.

Structure of the Interior

Accumulated and detailed seismic studies, coupled with theoretical speculation, suggests the interior structure shown schematically on the left (the figure is not to scale). The Earth is believed to have a solid inner core, made mostly of iron and nickel. This is surrounded by a liquid outer core, also mostly iron and nickel. The diameter of the core is estimated to be 7000 km, compared with a 12,700 km diameter for the entire planet. The crust is only a few tens of kilometers thick. The region between the core and the crust is called the mantle. The upper part of the mantle and the crust together are called the lithosphere. Sitting just below the lithosphere is a region of plastic consistency called the aesthenosphere. We shall have more to say about the lithosphere and aesthenosphere shortly.