Over the course of millions of years, the Earth's magnetic field can reverse, so that compasses point south rather than north. There has been plenty of speculation that we're heading for another such event quite soon. Now evidence suggests that the last such event happened much faster than previously thought – suggesting the next one could too.
The records of these events, known as geomagnetic reversals, is written in magnetized volcanic rocks that maintain the polarisation of the Earth's field at the time they cooled. On average, they happen every 450,000 years, so at 786,000 years since the last one, we're overdue – although a brief reversal that almost immediately undid itself happened around 40,000 years ago.
One outstanding question has been how rapidly these reversals can occur. While one study estimated change measured in degrees per day for one ancient event, most calculations suggest such events take 1,000-10,000 years.
So the publication in Geophysical Journal International of the claim that the last reversal, known as the Brunhes-Mutuyama event, happened in under a century represents a challenge to orthodoxy.
The authors, including Berkeley graduate student Courtney Sprain and her supervisor Professor Paul Renne, are not the first to suggest the last flip was unusually fast, but Sprain says the evidence they have found in the Suilmona Basin, east of Rome, is very clear. “The paleomagnetic data are very well done. This is one of the best records we have so far of what happens during a reversal and how quickly these reversals can happen,” says Sprain.
Volcanoes upwind of the basin, including Sabatini and Vesuvius, erupted frequently during the reversal, and the changing magnetic field can be seen in the sediments laid down. Argon-argon isotopic dating allowed Sprain and Renne to date the ash layers far more precisely than has been done before.
“What’s incredible is that you go from reverse polarity to a field that is normal with essentially nothing in between, which means it had to have happened very quickly, probably in less than 100 years,” said Renne. “We don’t know whether the next reversal will occur as suddenly as this one did, but we also don’t know that it won’t.”
Prior to the reversal, the team detected a 6,000 year period of instability. In light of the evidence that the Earth's magnetic field is currently weakening and that changes in orientation are accelerating, it's possible we are about to experience something similar, although we still have no idea what drives such events.
University of California - Berkeley. The parth of the Earth's North Magnetic Pole as it moved from Antarctica 789,000 years ago to its current home in the Arctic at a rate of 2° a year.
A forthcoming geomagnetic reversal looms large in the minds of catastrophists who predict all sorts of disasters as the next one, such as widespread deaths from cosmic radiation while the planet's magnetic shield is down. The fact that past events have not been associated with mass extinctions seems not to register in these theories. However, the electrical grid could become far more vulnerable to solar storms.