The magnetic field of our planet changes over time, but one particular feature that continues to puzzle scientists is the phenomenon of magnetic pole reversal. This is when the magnetic north and south pole switch for hundreds of thousands of years before the process reverses itself.
Sometimes the phenomenon can happen for a "brief" interval of a few hundred years (called an excursion), where the magnetic field wanes, switches, and then goes back to normal for a bit. The last time a full reversal happened was about 773,000 years ago.
By studying lava flows from Chile, Tahiti, Hawaii, the Caribbean, and the Canary Islands, the researchers estimated that the whole process took roughly 22,000 years – much longer than previously thought. The work is published in Science Advances.
The team collected lava flow samples spanning roughly 70,000 years around the pole reversal and measured levels of argon in the rocks to provide a more complete picture of the geomagnetic shift.
"Lava flows are ideal recorders of the magnetic field. They have a lot of iron-bearing minerals, and when they cool, they lock in the direction of the field," lead author Brad Singer, from the University of Wisconsin-Madison, said in a statement. "But it's a spotty record. No volcanoes are erupting continuously. So we're relying on careful fieldwork to identify the right records."
The team combined these with records from the seafloor (which are less spotty but also less precise) and Antarctic ice cores that hold a record of beryllium production in the atmosphere. When the magnetic field is weak, more cosmic rays hit the atmosphere and more beryllium is produced.
By combining all the data, the team estimates the pole reversal happened over 4,000 years, but with the preceding 18,000 years marked by dramatic magnetic field instability, including two excursions. The weakening of the field began roughly 795,000 years ago, and about 11,000 years later the field became erratic. The samples, like compasses frozen in time, show highly variable directions.
The 22,000-year estimate is more than twice as long as previously believed and much longer than the hypothesis that suggested it might have happened over a human lifetime. The data provides insight not just into the timeline of the event but its properties. Despite being a fascinating subject, there is a lot we don’t know about this geophysical phenomenon.
"Reversals are generated in the deepest parts of the Earth's interior, but the effects manifest themselves all the way through the Earth and especially at the Earth's surface and in the atmosphere," added Singer. "Unless you have a complete, accurate and high-resolution record of what a field reversal really is like at the surface of the Earth, it's difficult to even discuss what the mechanics of generating a reversal are."
Since records began, the planet’s magnetic field has decreased by about 5 percent per century. It is unclear if this suggests a reversal is imminent, if it’s the beginning of an excursion, or if it is something completely different. It could well be a completely normal fluctuation.