Data to garner greater understanding of Earth’s magnetic field and an anomaly called the ‘South Atlantic Anomaly’ came from a very unlikely location – Africa – it has been revealed.
Published in Geophysical Research Letters is a study that reveals how new data from sites in southern Africa has helped a team led by Rochester University scientists to extend their record of Earth’s magnetic field back thousands of years to the first millennium. The data is invaluable as it has enabled them to get a context to explain recent, ongoing changes in the magnetic field, most prominently in an area in the Southern Hemisphere. According to researchers the new data also provides more evidence that a region in southern Africa may play a unique role in magnetic pole reversals.
The data for the study was collected from an unlikely source: ancient clay remnants from southern Africa dating back to the early and late Iron Ages. The data was collected as part of a field called “archaeomagnetism,” geophysicists team up with archaeologists to study the past magnetic field.
Scientists explain that during the Iron Age in southern Africa, around the time of the first millennium, there was a group of Bantu-speaking people who cultivated grain and lived in villages composed of grain bins, huts, and cattle enclosures. Draughts were devastating to their agriculturally based culture. During periods of draught, they would perform elaborate ritual cleansings of the villages by burning down the huts and grain bins.
“When you burn clay at very high temperatures, you actually stabilize the magnetic minerals, and when they cool from these very high temperatures, they lock in a record of the earth’s magnetic field,” one of the researchers said.
Researchers excavate the samples, orient them in the field, and bring them back to the lab to conduct measurements using magnetometers. In this way, they are able to use the samples to compile a record of Earth’s magnetic field in the past.
The magnetic field that surrounds Earth not only dictates whether a compass needle points north or south, but also protects the planet from harmful radiation from space. Nearly 800,000 years ago, the poles were switched: north pointed south and vice versa. The poles have never completely reversed since, but for the past 160 years, the strength of the magnetic field has been decreasing at an alarming rate. The region where it is weakest, and continuing to weaken, is a large area stretching from Chile to Zimbabwe called the South Atlantic Anomaly.
In order to put these relatively recent changes into historical perspective, Rochester researchers—led by John Tarduno, a professor and chair of EES—gathered data from sites in southern Africa, which is within the South Atlantic Anomaly, to compile a record of Earth’s magnetic field strength over many centuries. Data previously collected by Tarduno and Rory Cottrell, an EES research scientist, together with theoretical models developed by Eric Blackman, a professor of physics and astronomy at Rochester, suggest the core region beneath southern Africa may be the birthplace of recent and future pole reversals.
The researchers discovered that the magnetic field in the region fluctuated from 400-450 AD, from 700-750 AD, and again from 1225-1550 AD. This South Atlantic Anomaly, therefore, is the most recent display of a recurring phenomenon in Earth’s core beneath Africa that then affects the entire globe.
Seismological data has revealed a denser region deep beneath southern Africa called the African Large Low Shear Velocity Province. The region is located right above the boundary between the hot liquid outer core and the stiffer, cooler mantle. Sitting on top of the liquid outer core, it may sink slightly, disturbing the flow of iron and ultimately affecting Earth’s magnetic field.
A major change in the magnetic field would have wide-reaching ramifications; the magnetic field stimulates currents in anything with long wires, including the electrical grid. Changes in the magnetic field could therefore cause electrical grid failures, navigation system malfunctions, and satellite breakdowns. A weakening of the magnetic field might also mean more harmful radiation reaches Earth—and trigger an increase in the incidence of skin cancer.
Hare and Tarduno warn, however, that their data does not necessarily portend a complete pole reversal.
“We now know this unusual behavior has occurred at least a couple of times before the past 160 years, and is part of a bigger long-term pattern,” Hare says. “However, it’s simply too early to say for certain whether this behavior will lead to a full pole reversal.”
Even if a complete pole reversal is not in the near future, however, the weakening of the magnetic field strength is intriguing to scientists, Tarduno says. “The possibility of a continued decay in the strength of the magnetic field is a societal concern that merits continued study and monitoring.”
This study was funded by the US National Science Foundation.