There is something strange about the Earth’s magnetic field, with some locations being oddly strong or weak. The most famous example of this is the South Atlantic Anomaly (SAA), which stretches from South America to Zimbabwe. We still don’t know what causes the anomaly, but scientists have now found it’s no passing fad, with evidence it is millions of years old, surviving many reversals in the field’s direction.

The Earth’s magnetic field is so weak in the SAA, satellites have been known to malfunction when passing through; without the field’s protection, they are exposed to cosmic rays and radiation from the Sun. Moreover, the difference between field strength in the SAA and comparable places is growing. Some have proposed the SAA is related to the weakening currently being experienced in the field worldwide or to the increasingly rapid movement of the Earth’s magnetic poles, stimulating research into the phenomenon.

For the SAA to be related to recent changes, it would make sense for it to be a new phenomenon, but Yael Engbers, a PhD student at the University of Liverpool, has shown the SAA is very ancient indeed. Iron-rich lava flows are magnetized by the magnetic field at the time they are cooling, and maintain that thereafter, providing a record of the local field strength when they formed.

The volcanic island of St Helena, most famous for being Napoleon’s final residence, lies near the middle of the SAA. Engbers collected rocks from 34 eruptions that occurred between 8 and 11 million years ago and reported in Proceedings of the National Academy of Sciences that the existing field falls well within the range at Saint Helena at the time.

“Our study… reveals that the anomaly in the magnetic field in the South Atlantic is not a one-off, similar anomalies existed eight to 11 million years ago,” Engbers said in a statement. "This is the first time that the irregular behaviour of the geomagnetic field in the South Atlantic region has been shown on such a long timescale.” Previous research has indicated the SAA existed 46,000-90,000 years ago.

Engbers notes this means the anomaly is unlikely to be an indication that the Earth’s field is about to flip. Although we know such reversals in direction have happened at least nine times in the last 5 million years, no one knows what triggers them or how to recognize the signs one is coming.

The finding confirms assumptions about ancient magnetic field directions that geologists have relied on to reconstruct past locations for continents without being certain these assumptions were true.

Although we are far from fully understanding the anomaly’s cause, we know that where the core meets the mantle there is a large region that is magnetized in the opposite direction to the world-wide field. At the surface above, this creates a partial offset we measure as the SAA.