Extreme, Growing Weak Point in Earth's Magnetic Field May Be 11 Million Years Old

A huge and unusually weak point in Earth's magnetic field may have existed for at least 11 million years, scientists have said. By studying volcanic rocks, researchers have gained a better understanding of the South Atlantic Anomaly (SAA), which has been growing over the last 50 years and linked with a magnetic field reversal.

Earth's magnetic field is generated by the planet's liquid iron core. It extends out into space and provides a protective barrier around the planet, helping to prevent harmful radiation from reaching the surface and deflecting the solar wind—a stream of charged particles from the sun that has the potential to strip away Earth's atmosphere. Because of movements in the planet's core, the magnetic field is dynamic. It moves around and gets stronger and weaker over time. It also occasionally flips, with the north and south poles temporarily reversing.

The SAA was discovered in 1958. Stretching from South America to Africa, it is a region where Earth's magnetic field is far weaker than at comparable latitudes. It is known because of its impact on satellites. Where the magnetic field is weaker, more radiation gets through, causing technical problems for spacecraft orbiting Earth.

"Other anomalies exist in the magnetic field," Yael Engbers, from the University of Liverpool, U.K., told Newsweek. "However, the South Atlantic Anomaly is the most extreme and most of all the biggest anomaly causing the field to be weaker, which causes the Earth, our atmosphere and our technology to be vulnerable to charged particles from the sun."

Over the last half-century, the SAA has been getting bigger. Recent satellite data suggests that over the last five years a second area of minimum strength has been developing to the southwest of Africa, signifying the anomaly may split into two patches in the future, the European Space Agency said.

Understanding how the SAA is changing is important because of its impact on satellites. A general weakening of the magnetic field is also associated with reversals. On average, the magnetic field flips every 200,000 to 300,000 years, although the rate is not consistent. The last major reversal took place around 780,000 years ago, causing some to speculate a reversal is imminent.

In a study published in the journal PNAS, Engbers and colleagues analyzed volcanic rocks to better understand the SAA over Earth's history. When lava solidifies, it becomes magnetized in the direction of Earth's magnetic field. The team looked at the island of Saint Helena, which formed during eruptions of two shield volcanoes. This included 225 core samples from 34 eruptions that took place between eight and 11 million years ago.

Their findings showed a high level of variation in the magnetic field direction over this period, suggesting the SAA has potentially always been present. "Our data shows that over a timespan of millions of years there has been irregular field behavior in the South Atlantic region," Engbers said. "This implies that the current situation is not as rare as some scientists had assumed, making it less likely that it represents the start of a much more rare reversal."

Instead, the authors believe the SAA is the result of interactions taking place at the boundary of Earth's core and mantle. There are two hypotheses to this. One is a massive patch at the lowermost point of the mantle where seismic waves go slower than in the rest of the mantle. This creates a "patch of reversed flux" where the magnetic field is opposite to the expected direction. The other idea relates to the asymmetric growth of the solid inner core, which could give rise to unusual behavior in the South Atlantic. "We think a combination of both these theories could cause the high instability of the field in the South Atlantic region on such a long timescale," Engbers said.

earth magnetic field
Stock image showing Earth's magnetic field. Researchers have found an anomaly in the Atlantic may have existed for at least eight to 11 million years. iStock