Last reversal of the Earth’s magnetic field took twice as long as previously thought



Last reversal of the Earth’s magnetic field took twice as long as previously thought

12 Aug 2019
Geomagnetic field
(Credit: Dormy and Dion)

The last full reversal of the Earth’s geomagnetic field took at least 22,000 years to complete, researchers from the US and Japan have revealed. The finding, which was derived by combining volcanic, sedimentary and ice-core records, suggests that reversals can take several times longer than was previously thought. It also further challenges the notion that a future reversal might be completed within a human lifetime.

The geomagnetic field is produced by the motion of the Earth’s liquid outer core, which acts as a dynamo. Although superficially stable – and presently reliable enough to navigate by – the field does change with time. At present, for example, the magnetic North Pole is in the process of drifting towards Siberia, while the field strength has been decreasing steadily by around 5% for each century since human records began.

Records in the rocks

With magnetically aligned minerals in certain rocks having left us with a record of the magnetic field at the time they were formed, we know that such a weakening can be a precursor to a so-called excursion – in which the magnetic poles shift by up to around 45 degree – or a full blown reversal, in which the field flips and settles upside down. These events, products of growing instabilities in the geodynamo, appear to occur every several hundred thousands years or so.

“Reversals are generated in the deeper parts of the Earth’s interior, but the effects manifest themselves all the way through the Earth,” explains Brad Singer, a geologist at the University of Wisconsin Madison.

Exactly what impact a future reversal might have on human civilization, navigation and communications, however, is unclear. And scientists still don’t understand what causes them, how long a reversal would take, and what the warning signs of one might be.

“Unless you have 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,” Singer notes.

Better measurements

To help develop a more accurate picture, Singer and his colleagues took magnetic readings of rock samples from seven lava flows from the Canary Islands, the Caribbean, Chile, Hawaii and Tahiti. They also determined the age of the samples using a newly-enhanced method of potassium-argon radioisotope dating.

“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,” says Singer. “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 team complemented their lava-flow records with two other sources of data on the historic orientation of the geomagnetic field. The first of these were magnetic readings taken from the sea floor, which are less precise than those taken from lava flows – due to variations in sediment rates, weaker magnetization, and biological disruption that can smear the preserved magnetic orientations – but can provide a more continuous record.

Secondly, the researchers took measurements of beryllium deposits across time, as preserved in Antarctic ice cores. Beryllium is produced when cosmos rays hit the atmosphere, which means that periods in which the magnetic field was weaker – and therefore allows more radiation to pass through it – can be identified by increased beryllium in the ice cores.

Combined together, the various records allowed the researchers to piece together the nature of the geomagnetic field over a 70,000-year period centered around the Matuyama-Brunhes reversal – the last time the field completely flipped over, around 784,000 years ago.

Longer reversal

Singer and colleagues found that the final reversal was relatively rapid by geological standards, taking less than 4000 years. However, it was preceded by two individual excursions within a period of instability lasting 18,000 years – more than twice as long as recent research had suggested reversals should take.

“I’ve been working on this problem for 25 years,” said Singer. “And now we have a richer and better-dated record of this last reversal than ever before.”

Andrew Roberts, an earth scientist from the Australian National University who was not involved in the present study, said: “I take these results to indicate that the last magnetic polarity reversal occurred during a prolonged period of time in which Earth’s magnetic field was weak and unstable.”

Roberts also notes that it is still possible that the main reversal occurred rapidly. “There have been other prolonged unstable periods, such the Blake and post-Blake events between 120 and 90 thousand years ago, during which the field has been demonstrated to have changed extremely rapidly.”

Gillian Turner, a geophysicist from the Victoria University of Wellington who also was not involved in the study, agrees: “As the accuracy and resolution of dating both volcanic rocks and sedimentary sequences continues to improve, we should expect to see excursion activity associated with successful polarity reversals more and more often.”

The research is described in the journal Science Advances.

Earth’s magnetic field is acting up and geologists don’t know why Erratic motion of north magnetic pole




Earth’s magnetic field is acting up and geologists don’t know why

Erratic motion of north magnetic pole forces experts to update model that aids global navigation.

Update, 9 January: The release of the World Magnetic Model has been postponed to 30 January due to the ongoing US government shutdown.

Something strange is going on at the top of the world. Earth’s north magnetic pole has been skittering away from Canada and towards Siberia, driven by liquid iron sloshing within the planet’s core. The magnetic pole is moving so quickly that it has forced the world’s geomagnetism experts into a rare move.

On 15 January, they are set to update the World Magnetic Model, which describes the planet’s magnetic field and underlies all modern navigation, from the systems that steer ships at sea to Google Maps on smartphones.

The most recent version of the model came out in 2015 and was supposed to last until 2020 — but the magnetic field is changing so rapidly that researchers have to fix the model now. “The error is increasing all the time,” says Arnaud Chulliat, a geomagnetist at the University of Colorado Boulder and the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Information.

The problem lies partly with the moving pole and partly with other shifts deep within the planet. Liquid churning in Earth’s core generates most of the magnetic field, which varies over time as the deep flows change. In 2016, for instance, part of the magnetic field temporarily accelerated deep under northern South America and the eastern Pacific Ocean. Satellites such as the European Space Agency’s Swarm mission tracked the shift.

By early 2018, the World Magnetic Model was in trouble. Researchers from NOAA and the British Geological Survey in Edinburgh had been doing their annual check of how well the model was capturing all the variations in Earth’s magnetic field. They realized that it was so inaccurate that it was about to exceed the acceptable limit for navigational errors.

Wandering pole

“That was an interesting situation we found ourselves in,” says Chulliat. “What’s happening?” The answer is twofold, he reported last month at a meeting of the American Geophysical Union in Washington DC.

First, that 2016 geomagnetic pulse beneath South America came at the worst possible time, just after the 2015 update to the World Magnetic Model. This meant that the magnetic field had lurched just after the latest update, in ways that planners had not anticipated.

Source: World Data Center for Geomagnetism/Kyoto Univ.

Second, the motion of the north magnetic pole made the problem worse. The pole wanders in unpredictable ways that have fascinated explorers and scientists since James Clark Ross first measured it in 1831 in the Canadian Arctic. In the mid-1990s it picked up speed, from around 15 kilometres per year to around 55 kilometres per year. By 2001, it had entered the Arctic Ocean — where, in 2007, a team including Chulliat landed an aeroplane on the sea ice in an attempt to locate the pole.

In 2018, the pole crossed the International Date Line into the Eastern Hemisphere. It is currently making a beeline for Siberia.

The geometry of Earth’s magnetic field magnifies the model’s errors in places where the field is changing quickly, such as the North Pole. “The fact that the pole is going fast makes this region more prone to large errors,” says Chulliat.

To fix the World Magnetic Model, he and his colleagues fed it three years of recent data, which included the 2016 geomagnetic pulse. The new version should remain accurate, he says, until the next regularly scheduled update in 2020.

Core questions

In the meantime, scientists are working to understand why the magnetic field is changing so dramatically. Geomagnetic pulses, like the one that happened in 2016, might be traced back to ‘hydromagnetic’ waves arising from deep in the core1. And the fast motion of the north magnetic pole could be linked to a high-speed jet of liquid iron beneath Canada2.

The jet seems to be smearing out and weakening the magnetic field beneath Canada, Phil Livermore, a geomagnetist at the University of Leeds, UK, said at the American Geophysical Union meeting. And that means that Canada is essentially losing a magnetic tug-of-war with Siberia.

“The location of the north magnetic pole appears to be governed by two large-scale patches of magnetic field, one beneath Canada and one beneath Siberia,” Livermore says. “The Siberian patch is winning the competition.”

Which means that the world’s geomagnetists will have a lot to keep them busy for the foreseeable future.

Nature 565, 143-144 (2019)

doi: 10.1038/d41586-019-00007-1