The early thermal history of the Earth is a matter of some speculation. Current scientific consensus is that planet Earth formed by accretion of material with the same composition as chondritic meteorites. Accretion, a process that generated heat as colliding material gave up kinetic energy, led to differentiation of the planetary constituents into concentric layers. When the temperature of the early Earth reached the melting point of iron, the dense iron, accompanied by other siderophile elements such as nickel and sulfur, sank towards the center of the planet to form a liquid core. Meanwhile lighter elements rose to form an outer layer, the primitive mantle. Further differentiation took place later, creating a chemically different thin crust atop the mantle. Only the outer core is now molten, surrounding a solid inner core of iron that solidified out of the core fluid. Lighter elements left behind in the core rise through the core fluid and result in a composition-driven convection in the outer core, which is in addition to thermal convection. Although the short-term behavior of the mantle is like that of a solid, allowing the passage of seismic shear waves, its long-term behavior is characterized by plastic flow, so heat transport by convection or advection is possible. In the solid lithosphere and inner core heat is transported dominantly by thermal conduction.

The physical states of the Earth's mantle and core are well understood, but the variation of temperature with depth is not well known.