The Earth is constructed from a sequence of layers that are differentiated by their chemical, rheological and thermal properties, varying from the solid iron of the inner to the very low concentrations of gases in the upper atmosphere. This page concentrates on the solid Earth, most of which we cannot observe directly. The fluid Earth, consisting of the oceans and atmosphere has vastly different properties. The simplest models of the solid Earth contain three distinct layers, the outermost being the crust, followed by the mantle and finally the core.
The core is composed of an iron (Fe)–nickel (Ni) alloy, which is solid at its centre due to high pressure and temperature but molten in the outer section. The mantle is dominated by rocks such as peridotite and pyroxenite that are high in Fe and magnesium (Mg), it is solid but flows in a viscous manner over long time periods. The crust is very thin (5-70km) and consists of two distinct domains, continental crust which is thick, old and granitic in composition and oceanic crust which is young, thin and basaltic in composition.