The magnetic field of the Earth is generated by the movement of electrically conducting molten iron in the outer core. This movement is generated by light elements released at the inner core boundary as it grows. Therefore, the time when iron was first frozen also represents a point in time when the outer core received a strong additional source of power.
It is the signature of this boost of the magnetic field – the largest long-term increase in its entire history – that we think we have observed in the magnetic records recovered from igneous rocks formed at this time. Magnetic particles in these rocks “lock-in” the properties of the Earth’s magnetic field at the time and place that they cool down from magma.
The Earth has maintained a magnetic field for most of its history through a “dynamo” process. This is similar in principle to a wind-up radio or a bicycle-powered light bulb in that mechanical energy is converted to electromagnetic energy. Before the inner core first started to solidify, this “geodynamo” is thought to have been powered by another entirely different and inefficient “thermal convection” process.