The ritualistic practice of burning down huts and grain bins in South African agricultural communities might give scientists some clues as to the process that leads to the reversal of the planet's magnetic field. The particular location of the huts is important, because they sit in a region that has long been known to be a weak spot in the Earth’s magnetic field. By analyzing a particular mineral formed by the burning of the huts, the researchers were able to track how the magnetic field in this area has changed over time.
The results suggest that underneath these huts, the Earth’s core might play an important role in the reversal of the planet’s magnetic poles. The last time the poles switched is thought to have been around 800,000 years ago, and some suggest that this process, which can take 15,000 years to complete, has started again. Equally, the magnetic field of the planet is known to vary so we could simply be seeing natural fluctuations. But how the reversals actually happen has remained a bit of a mystery.
“It has long been thought reversals start at random locations, but our study suggests this may not be the case,” explains John Tarduno of the University of Rochester. The researchers looked at data from five sites in South Africa, which sit in a region known as the South Atlantic Anomaly, a patch of the Earth’s magnetic field that is bizarrely weak. They suggest that deep below this patch, a particular region of the mantle rock overlaying the planet's core influences how the liquid iron center of the Earth flows, which in turn causes the irregularities in the Earth’s magnetic field.
Magnetic field strength in the South Atlantic Anomaly. Credit: Michael Osadciw/University of Rochester
“The top of the core beneath this region is overlain by unusually hot and dense mantle rock,” says Tarduno. That mantle rock lies 3,000 kilometers (1,864 miles) below the surface, and is roughly 6,000 kilometers (3,728 miles) across.
They found that the ground containing the ash and remains of the huts that were burnt down a thousand years ago contained the mineral "magnetite." When this mineral is subjected to high temperatures, such as at the time of the burning, it is “re-written” with a “new record of the magnetic field strength and direction” from that region at that time.
The study, published in Nature Communications, found that between 1225 and 1550 CE, there was a sharp 30% drop in magnetic intensity in the region. With the Earth’s magnetic field strength having dropped by 16% since 1840, some people think that it could indicate the beginning of another cycle in which the planet’s poles change, though the researchers of this study think it could simply be a feature of a larger cycle of magnetic shifts.
“Because rock in the deep mantle moves less than a centimeter a year, we know the LLSVP is ancient, meaning it may be a longstanding site for the loss of magnetic field strength,” said Tarduno. “And it is also possible that the region may actually be a trigger for magnetic pole reversals, which might happen if the weak field region becomes very large.”