Credit: © PHOTOTAKE INC/ALAMY

The Earth is shielded from the Sun's relentless barrage of charged particles by its magnetic field. Today, the geomagnetic field is generated by the convection of liquid iron in the Earth's outer core. This convection is in part driven by the crystallization of the solid inner core. It is therefore unclear how a geomagnetic field might have been generated before the inner core solidified. Yet the oldest known rocks that bear the signature of a geomagnetic field date back 3.5 billion years, well before the inner core is thought to have formed.

A close inspection of the palaeomagnetic signature of these Archaean rocks suggests that they formed under a field quite similar to that of today, complete with near-modern strength and frequent reversals. One possible but unlikely explanation is that convection in a liquid core generates a strong field even without the presence of a solid interior. Alternatively, Leah B. Ziegler and Dave R. Stegman suggest that the source of the Archaean geomagnetic field may have been located in the lowermost mantle — specifically, in an ocean of magma thought to have pooled just above the core–mantle boundary (Geochem. Geophys. Geosys. http://doi.org/qbh; 2013).

Crystallization of the magma ocean on early Earth may have initiated in the middle depths of the mantle, and slowly progressed downwards towards the core, leaving an ever-thinning pond of magma in the lower mantle. Ziegler and Stegman used a conceptual model to explore the effects of convection in this magma layer. They found that thermally driven convection in this layer could indeed generate a self-sustaining, planetary-scale magnetic field, not least because the liquid oxides common to magmas become weakly metallic at the temperatures and pressures found at this depth. Moreover, the presence of a basal magma ocean would actually supress heat flow from the core and prevent any magnetic field generation from the core.

Ziegler and Stegman estimate that convection within the basal magma ocean could have sustained a geomagnetic field until about 2.5 billion years ago. From that point, the onset of quick cooling would have driven enough convection in a solely liquid core to generate a magnetic field, with an additional boost from the subsequent solidification of the inner core.

Intriguingly, this model of the Earth's thermal evolution suggests that there may have been a pause in the generation of the geomagnetic field, possibly around 2.4 to 2.1 billion years ago. A gap in reported palaeomagnetic signatures from rocks of about this age has indeed been noted. Perhaps this palaeomagnetic gap does not reflect issues with rocks recording the magnetic field, but instead the lack of a strong geomagnetic field for the rocks to record.