The inner core is the solid central region of Earth located inside the liquid outer core. It begins at a depth of approximately 5150 kilometres and extends to Earth's centre at about 6371 kilometres depth. It is predominantly composed of iron with nickel and lighter elements in smaller quantities, although its exact composition remains uncertain.
Temperatures within the inner core are extremely high, comparable to or exceeding those at the surface of the Sun, but the enormous pressure at Earth's centre raises the melting temperature of iron rich material sufficiently for the inner core to remain solid. The inner core is therefore a striking example of how pressure can determine the physical state of matter under extreme planetary conditions.
The inner core was discovered through seismic evidence rather than direct observation. Seismic waves generated by earthquakes travel through Earth and respond to changes in density, composition and physical state. P waves can pass through both the liquid outer core and solid inner core, although their velocities and paths change. Certain seismic observations revealed behaviour inconsistent with a completely liquid core and provided evidence for a solid central region. The boundary between the outer and inner core produces detectable changes in seismic wave behaviour. Subsequent observations have refined estimates of the inner core's size, structure and physical properties, demonstrating the power of seismology to investigate regions inaccessible to drilling.
The inner core is not necessarily a perfectly uniform solid sphere. Seismic studies have identified anisotropy, meaning that seismic waves can travel at different speeds depending on their direction through portions of the inner core. This has been interpreted as evidence for preferred alignment of iron crystal structures or other forms of internal organisation. Researchers have also investigated possible regional differences, layering and changes in properties within the inner core. The detailed causes of these features remain subjects of scientific research because seismic observations are indirect and must be interpreted using models of materials under extreme pressure and temperature.
The inner core grows gradually as Earth's interior cools and liquid iron rich material in the outer core crystallises at the inner core boundary. The process releases latent heat and can also cause lighter elements to remain preferentially in the liquid outer core, producing compositional buoyancy. Both effects can contribute to convection in the outer core and therefore influence the geodynamo. The inner core is consequently connected directly to Earth's magnetic field even though it is solid. Its growth also records the thermal history of the planet because the rate and structure of inner core formation depend on how Earth's interior has cooled over geological time.
The age of the inner core is not equivalent to the age of Earth. Earth formed about 4.54 billion years ago, whereas the solid inner core formed later as the planet cooled sufficiently for solidification to begin at the centre. Estimates of the inner core's age vary depending on models and assumptions about thermal conductivity, core composition, mantle heat flow and dynamo history. This uncertainty illustrates an important principle in geophysics: the present structure of Earth can preserve evidence about its past, but reconstructing deep planetary history requires combining observations with physical models. The inner core's existence therefore provides clues about how Earth's thermal and magnetic evolution changed through time.
The inner core is subject to immense pressure, which increases toward the centre of Earth. Pressure affects the arrangement of iron atoms and the stability of different crystal structures. Under inner core conditions, iron can exist in high pressure solid phases that differ from ordinary surface materials. Laboratory experiments cannot reproduce every aspect of the inner core perfectly, but high pressure physics and mineral physics experiments provide valuable constraints. The behaviour of iron alloys at extreme pressure and temperature is essential for understanding seismic velocities, density, electrical and thermal conductivity and the mechanisms controlling inner core growth.
The inner core differs from the outer core primarily in physical state, while both are dominated by metallic material. The outer core is liquid and convects, whereas the inner core is solid and does not undergo ordinary fluid convection. The inner core can nevertheless deform slowly and may have complex internal dynamics. Its interaction with the outer core can involve heat transfer, compositional exchange at the phase boundary and angular momentum coupling. Because the inner core is embedded within a moving liquid metal environment, its growth and physical properties can influence the behaviour of the surrounding outer core and therefore the magnetic field.
The inner core has broader planetary significance because it forms the central endpoint of Earth's internal differentiation. Above it lies the liquid outer core, then the silicate mantle and finally the crust. The inner core's solidification contributes energy and compositional buoyancy to the outer core, helping sustain the geodynamo that produces the magnetic field. The magnetic field influences the magnetosphere and interacts with charged particles from the Sun.
Thus, a region thousands of kilometres beneath the surface has consequences extending into near Earth space. Although the inner core cannot currently be directly sampled, seismology, high pressure physics, geomagnetism, gravity and models of planetary evolution together provide an increasingly detailed understanding of this central region.
