Earth’s magnetic field reversals may take much longer than we thought

Compasses point north – that’s a pretty constant fact of life. But it hasn’t always been the case, as the north and south poles actually flip on a semi-regularly basis. Exactly how long this process takes has been up for debate, and now a new study suggests it happens far more slowly than we previously thought.

The Earth’s magnetic field plays a key role in making our little planet habitable, since without it the surface would be bombarded by radiation from space. Generated by the molten iron in the outer core swirling around the solid inner core, this magnetic field extends out above the surface and into the atmosphere, directing incoming charged particles towards the north and south magnetic poles.

But this field isn’t as static as it might seem. Geological records indicate that it has flipped many times in the planet’s past, so that the north and south magnetic poles effectively swap places. This kind of reversal seems to happen every few hundred thousand years or so, and considering the last known event happened around 780,000 years ago, we appear to be long overdue for another one.

And maybe the signs are there that a new reversal is imminent. The magnetic field has been weakening considerably since the 19th century, and the north pole is currently moving faster than has ever been seen. Recent studies argue that we’re not headed for a reversal, based on historical measurements that show the field bounced back to full strength after similar weakenings in the past.

That said, scientists generally don’t know what causes a reversal, nor how long it takes to play out – it’s believed that the average is about 7,000 years, but some studies suggest it could happen in less than 100 years. To investigate for the new study, researchers at the University of Wisconsin-Madison, UC Santa Cruz and Kumamoto University looked to the turbulent time around the last geomagnetic reversal.

The researchers gathered rock samples from seven lava flows in Chile, Tahiti, Hawaii, the Caribbean and the Canary Islands, and studied them using magnetic readings and radioisotope dating. This allowed them to piece together the magnetic field over a period of about 70,000 years, with the centerpoint being the last geomagnetic reversal.

“Lava flows are ideal recorders of the magnetic field,” says Brad Singer, lead researcher on the study. “They have a lot of iron-bearing minerals, and when they cool, they lock in the direction of the field. But it’s a spotty record. No volcanoes are erupting continuously. So we’re relying on careful field work to identify the right records.”

The lava flow data was backed up by data from two other sources: magnetic readings from the seafloor, which is less precise but paints a more continuous picture, and Antarctic ice cores. The latter shows the amount of cosmic radiation reaching the surface, which increases as the magnetic field weakens and flips.

Through all this data, the team found that the reversal itself took less than 4,000 years, making it relatively quick compared to the average. But that’s not the whole story – before that there was an extended period of instability in the field, lasting as long as 18,000 years. In that time, the field partially reversed twice before flipping back.

In total, the whole reversal process took about 22,000 years. That is far longer than the generous end of the scale of previous estimates, which said it would take at most 9,000 years to wrap up.