What powered the geodynamo before the inner core existed?
In plain words
Rocks show Earth had a magnetic field at least 3.4 billion years ago, long before the solid inner core probably formed. If the core conducts heat as well as recent calculations say, ordinary heat-driven stirring may have been too weak to run the dynamo (the self-sustaining generator that turns flowing liquid iron into a magnetic field).
Precise statement
Paleointensity data indicate a dynamo since at least about $3.4 \text{ to } 3.5\,\mathrm{Ga}$, with possible records to about $3.7\,\mathrm{Ga}$ from Isua, Greenland (Nichols and coauthors, JGR Solid Earth 2024, https://doi.org/10.1029/2023JB027706). With core thermal conductivity $k$ of order $1e7\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}\,\mathrm{K})$ or higher, the adiabatic heat flow can exceed $Q_{\mathrm{cmb}}$ before inner-core nucleation (the new core paradox). Identify which source supplied the needed entropy: precipitation of MgO, SiO2 or FeO, a basal magma-ocean dynamo, tidal or precessional forcing, or a lower $k$. An answer is a thermal and chemical core history that satisfies the paleointensity record and mineral-physics constraints.
What would settle it
Experimental precipitation rates of light-element oxides at core conditions combined with a resolved core conductivity and a consistent core thermal history.
Status in the literature
Unverified note
Vidal and Cebron 2025 (arXiv:2506.19039) find lunar tidal forcing alone insufficient before 3.25 Ga, while Kiernan and coauthors 2026 (arXiv:2606.16340) propose resonant tidal coupling enhanced by a basal magma ocean; no consensus.