Ancient African fires inform study of Iron Age magnetic field history

Researchers have been struggling to solve the mystery of a weakening of the magnetic field in South Africa. Now, thanks to evidence left behind by ancient ritualistic village burnings, scientists may have made a breakthrough.

These ancient fires, researchers say, left behind clues to Earth's electromagnetic past -- clues that may explain both South Africa's magnetic weakening and geomagnetic reversal, whereby the Earth's magnetic poles switch.

"It has long been thought reversals start at random locations, but our study suggests this may not be the case," lead researcher John Tarduno, a scientist at the University of Rochester and a leading expert on Earth's magnetic field, said in a press release.

Researchers believe the electromagnetic anomaly in South Africa is the result of a massive slab of hot and dense mantle rock that lies some 2,000 miles beneath the surface and measures nearly 4,000 miles across. The steep-sided slab apparently manipulates the behavior of the swirling liquid iron that creates the planet's magnetic field.

Scientists have hypothesized that it's changes in the flow of this iron that cause irregularities in the magnetic field and may precipitate geomagnetic reversal events.

But researchers have been looking for hard evidence of past shifts in electromagnetic behavior in southern Africa. Until now, they've had to rely on evidence from sites elsewhere on the globe.

Now, researchers say, they have that hard evidence -- thanks to the ritualistic fires that burned whole villages during the Iron Age and melted the floors of ancient huts.

Rocks containing magnetite, an iron oxide mineral common to almost all igneous and metamorphic rocks, reveal the electromagnetic conditions present at their formation. When ancient Iron Age fires swept through villages in South Africa, the embers burned hot enough to melt the magnetite in the floors and rock below -- erasing the magnetic history.

When the melted floors cooled in the wake of the blazes, the magnetite were reorganized by the ancient electromagnetic field -- offering a snapshot of electromagnetic history.

"The hut floors are actually very good magnetic recorders," Tarduno told Space.com. "Sort of like minimagnetic observatories back in time."

By analysis of the ancient scorched foundations, researchers confirmed that the region featured a 30 percent drop in magnetic field intensity from 1225 to 1550 CE. The data suggests the localized weakening, known as the South Atlantic Anomaly, is a recurring feature of the Earth's magnetic field.

Still, Tarduno and his colleagues say more work needs to be done, as their research -- published in the journal Nature Communications -- fails to draw conclusive connections between local weakening and the larger history of a pole reversals.