Rapid changes in the Earth’s magnetic field between 1225 and 1550 AD are evident in samples from the burnt floors of Iron Age structures, reports a paper published in Nature Communications this week. Burning by Iron Age people caused the rock in the floors to become magnetically orientated, producing a record of the Earth’s magnetic field at the time. The authors suggest this has implications for the interpretation of recent changes in the intensity of the Earth’s magnetic field.

The rapid decay of the Earth’s dipole magnetic field has recently captured the public imagination; a drop in intensity in the Southern Hemisphere over the past 160 years has motivated some speculation regarding a potential magnetic reversal, in which magnetic north and south switch places. However, our understanding of these changes has been limited by a lack of longer-term observations.

John Tarduno and colleagues investigate Iron Age sites from southern Africa and use magnetically oriented samples from the preserved burnt floors of huts, grain bins and livestock enclosures to produce an almost 600-year-long record. The samples show a time of rapid changes and a sharp drop in intensity of the magnetic field, suggesting that the changes in its behaviour may not just be a recent feature. The researchers attribute the changes to the unusual structure and composition of the core-mantle boundary deep beneath southern Africa. They suggest that fluid vortices trigger the expulsion of magnetic flux, resulting in rapidly changing magnetic field directions and intensities.