Reversals in the Earth's geomagnetic field 1.1 billion years ago can be explained by the same two-pole model that explains the more recent behaviour of the Earth's magnetic field, according to a study published online in Nature Geoscience. This is in contrast to previous studies which suggest the influence of four or even eight poles during that time.
Nicholas Swanson-Hysell and colleagues revisited the volcanic rocks from the Canadian shield used in the previous studies. Tiny magnetic grains within the volcanic rocks record the orientation of the geomagnetic field at the time the rocks were erupted onto the Earth's surface. When they looked at these records in detail, they found that the reversals were actually symmetric meaning that the geomagnetic field simply shifted from normal ― like today's field ― to reverse polarity.
They conclude that previous efforts to reconstruct the geomagnetic field from North America were confused by the rapid migration of the continent towards the Equator at rates of approximately 21 to 39 cm per year. The migration of the continent also affected the orientation of the magnetic grains, masking the record of magnetic field variations.