Earth’s solid central layer, made mainly of iron and nickel, is the inner core.
The inner core is the innermost part of Earth and has a radius of roughly 1,220 kilometers. Despite temperatures comparable to or higher than the surface of the Sun, the immense pressure at that depth keeps much of its iron-rich material solid. It is surrounded by the liquid outer core.
Earthquakes provided key evidence for this structure. Seismic P waves can travel through solids and liquids, while S waves cannot pass through liquids. The behavior of these waves, including their shadows and changes in speed, helped scientists infer a liquid outer core and a solid inner core.
The inner core is not a single pure metal sphere: it contains iron, nickel and smaller amounts of other elements, and its crystals have directional properties. The outer core’s flowing electrically conductive liquid metal generates most of Earth’s magnetic field.