Geochem. Geophys. Geosys. doi:10.1029/2009GC002496 (2009)

When did our planet develop the roiling convection pattern that churns its metallic core and gives rise to its magnetic field?

Ancient rocks from South Africa's Barberton greenstone belt reveal that such currents in the core must have started by 3.45 billion years ago, significantly earlier than had been established from previous rock evidence, report John Tarduno of the University of Rochester in New York and his colleagues. The rocks sport a magnetic signature indicating that the planet had developed a substantial magnetic field by that time.

Past work by Tarduno and his co-workers had provided evidence that a significant magnetic field was present by 3.2 billion years ago.