Back in January, a spike in interest regarding Earth’s magnetic field – and whether it’s about to flip or not – occurred. Despite a significant weakening of its magnetic field strength in recent decades, there’s no indication that it’s “about” to flip, and even if it did, the consequences of a geomagnetic reversal would likely not be as apocalyptic as some may claim.
Saying that, there’s plenty about the planet’s magnetic field that we know fairly little about, including its weak spot, the South Atlantic Anomaly (SAA), which extends from Zimbabwe to Chile. Fortunately, a new paper in Geophysical Research Letters shines some light on that with a tale that extends back, rather appropriately, to the Iron Age.
The team’s beautifully written study describes the SAA and the region as the origin of the recent decay and "apparent collapse" of Earth's magnetic field. They then emphasize that we know very little about this depression's history thanks to a paucity of data, the likes of which are normally preserved in magnetic mineral-rich volcanic matter.
Upon cooling below a temperature known as the Curie Point, these minerals record the characteristics of Earth’s magnetic field at that exact point in time. This is how we know that, for example, the last time a complete geomagnetic reversal took place was 781,000 years ago.
Still, there’s a lot missing from the story, and the SAA is especially enigmatic – as are the underlying mechanisms within Earth’s iron-rich outer core that drive geomagnetic reversals in the first place. As there aren’t any solid ancient geomagnetic records in southern Africa before 1000 CE, the recent history of the SAA is one big blank spot.
Ingeniously, the team – coordinated by the University of Rochester – decided to look to archaeology, not merely geology, in order to find this missing piece of the puzzle.
Sifting through burnt grain bins, huts, and cattle enclosures from Early to Late Iron Age locales in northern South Africa, Botswana, and Zimbabwe, the team found clay artifacts containing small particles of magnetite and hematite, both of which preserve the Earth’s magnetic field features upon cooling.
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."