This resource identifies the sources of the Earth’s internal heat energy as extraterrestrial impacts, gravitational contraction of the Earth’s interior, and the radioactive decay of unstable isotopes, each of which is explained in detail. A discussion of melting and compositional differentiation of the early Earth is followed by a description of the present layers, including the inner and outer core, mantle, and crust. In addition, the asthenosphere and lithosphere are described and illustrated.