innercore

The inner core is the Earth's innermost region, a solid sphere at the planet's center. It has a radius of about 1,200 kilometers, and is surrounded by a liquid outer core that extends to roughly 3,500 kilometers from the center. The boundary between the inner and outer core is known as the inner core boundary. The inner core is primarily composed of iron, with smaller amounts of nickel and light elements such as sulfur, silicon, or oxygen.

Conditions in the inner core are extreme. Pressures at the center are on the order of hundreds of gigapascals, and temperatures are estimated to be several thousand kelvin. These conditions keep the inner core solid despite its high temperature. Its density is among the highest within Earth's interior, typically cited around 12 to 13 grams per cubic centimeter.

Geophysical evidence for the inner core comes from seismic studies. Inge Lehmann, in 1936, inferred a solid inner core from observations of P-waves refracting at the boundary and reflections that could not be explained by a completely liquid interior. Seismic data also reveal anisotropy in the inner core, indicating that seismic waves travel faster in some directions, suggesting a preferred crystallographic alignment within the iron.

The inner core grows slowly as the Earth cools, crystallizing from the liquid outer core and releasing latent heat. Some studies propose a distinct, smaller region within the inner core, called the innermost inner core, but this remains an area of active research. The inner core influences Earth’s geodynamics indirectly through its interaction with the outer core and the planet’s magnetic field.