[ClarificationStatement: Emphasis is on both a one-dimensional model of Earth,with radial layers determined by density, and a three-dimensional model,which is controlled by mantle convection and the resulting plate tectonics. Examples of evidence include maps of Earth’s three-dimensional structure obtained from seismic waves, records of the rate of change of Earth’s magnetic field (as constraints on convection in the outer core), and identification of the composition of Earth’s layers from high-pressure laboratory experiments.]