They were found in villages that, a millennium ago, were partly and ritualistically burned down in response to periods of prolonged drought. These fires heated the minerals up above the Curie Point; as they cooled, they took a snapshot of the Earth’s magnetic field as it was around 1,000 years ago.
Using these, the team reconstructed the planet’s magnetic field as it looked back then, and found out that its waning condition today isn’t so special after all.
In fact, as described in the paper, between 1225 and 1550 CE it featured profound directional changes “accompanied by intensity values that are lower than the present-day regional low”. Similarly, dramatic changes were also occurring around the 5th and 8th centuries too, all of which are referred to as “archaeomagnetic jerks”.
This all suggests the SAA isn’t unique to the here and now, but is a recurring feature that’s representative of whatever’s going on down below. Perhaps not coincidentally, below this exact spot, on the outer core-mantle boundary, a geologically venerable anomaly known as the African Large Low Shear Velocity Province (LLSVP) exists.
This continental-sized, curiously warm, dense blob – one of two known thermochemical piles – is deeply mysterious, but the team speculates that its relatively high density may play a mischievous role here. By sitting on the liquid outer core, it may compress it.
With such an obstruction, "something analogous to an eddy in a stream might form," in the flow of circulating iron, co-author John Tarduno, a professor of geophysics at the University of Rochester, told IFLScience.
Ultimately, this changing in the underlying churning would trigger chaotic periods of magnetic field generation, recorded as archaeomagnetic jerks.
Before you ask, no: this paper doesn’t forecast an imminent flip of the magnetic poles, but as an insight into its past and future, it's invaluable.
"The African LLSVP is very old, tens of millions to perhaps more than 100 million years old. Because flow within the solid mantle is so slow, it will be present far into the future," Tarduno added.
"So, some past reversals of the magnetic field may have nucleated in this region and the future reversal - whenever it eventually comes - might also start under southern Africa."