The formation and evolution of the Earth’s inner core

The growth of the solid inner core from the liquid outer core provides crucial power for generating the geomagnetic field. However, the traditional view of inner core growth does not include the physical requirement that liquids must be supercooled below the melting point before freezing can begin. In this Review, we explore the impact of supercooling the Earth’s core on inner core formation, growth and dynamics, and the interpretation of seismic and palaeomagnetic observations. Mineral physics calculations suggest that at least 450 K of supercooling is needed to spontaneously nucleate the inner core. However, when satisfying inferences from geophysical constraints, the maximum available supercooling is estimated at 420 K and more probably <100 K. Supercooling the Earth’s core requires that the inner core had at least two growth regimes. The first regime is a rapid phase that freezes supercooled liquids at rates comparable to outer core dynamics (cm yr−1), followed by the second regime that is a traditional in-equilibrium growth phase proportional to the cooling rate of the core (mm yr−1). Future research should seek evidence for rapid growth in the palaeomagnetic and seismic records and the mechanisms that produce deformation texture, particularly those owing to heterogeneous inner core growth, inner core convection, and coupling between freezing and the magnetic field. Nucleation and growth of Earth’s solid inner core has a crucial role powering the geomagnetic field. This Review explores the timing and mechanisms of inner core growth consistent with physical constraints and first-order observations of the thermal evolution of Earth.