| Summary: | Growth of the inner core provides crucial power for generating the geomagnetic field and preserves a unique record of deep Earth evolution. The classical picture of inner core growth ignores the fact that the liquid core must have been supercooled below its melting temperature to spontaneously freeze the inner core. In this review we assess the impact of supercooling on inner core forma5on, growth, dynamics, and the interpretation of seismic and paleomagnetic observations. Mineral physics calculations suggest that a supercooling of at least 450 K is needed to nucleate the inner core, while inferences from geophysical observations constrain the maximum available supercooling to 420 K, and more likely <100 K when satisfying constraints on long-term core-mantle evolution. Supercooling requires that the inner core initially grew rapidly, comparable to the timescale of outer core dynamics, followed by a slower phase of classical equilibrium growth. The rapidly-grown region could have been at least as large as the innermost inner core and is predicted to not convect, with deformation due to heterogeneous inner core growth or coupling to the dynamo-generated magnetic field the most likely explanations of the observed seismic elastic anisotropy. Rapid growth may also produce a signature in the paleomagnetic record.
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