Earth's last magnetic field reversal took far longer than once thought

Earth's last magnetic field reversal took far longer than once thought
Study co-author Rob Coe and Trevor Duarte orienting cores from a lava flow site recording the Matuyama-Brunhes magnetic polarity reversal in Haleakala National Park, Hawaii, in 2015. Credit: Brad Singer

Earth's magnetic field seems steady and true—reliable enough to navigate by.

Yet, largely hidden from daily life, the drifts, waxes and wanes. The magnetic North Pole is currently careening toward Siberia, which recently forced the Global Positioning System that underlies modern navigation to update its software sooner than expected to account for the shift.

And every several hundred thousand years or so, the dramatically shifts and reverses its polarity: Magnetic north shifts to the geographic South Pole and, eventually, back again. This reversal has happened countless times over the Earth's history, but scientists have only a limited understanding of why the field reverses and how it happens.

New work from University of Wisconsin-Madison geologist Brad Singer and his colleagues finds that the most recent field reversal, some 770,000 years ago, took at least 22,000 years to complete. That's several times longer than previously thought, and the results further call into question controversial findings that some reversals could occur within a human lifetime.

The new analysis—based on advances in measurement capabilities and a global survey of flows, ocean sediments and Antarctic ice cores—provides a detailed look at a turbulent time for Earth's magnetic field. Over millennia, the field weakened, partially shifted, stabilized again and then finally reversed for good to the orientation we know today.

The results provide a clearer and more nuanced picture of reversals at a time when some scientists believe we may be experiencing the early stages of a reversal as the field weakens and moves. Other researchers dispute the notion of a present-day reversal, which would likely affect our heavily electronic world in unusual ways.

Singer published his work Aug. 7 in the journal Science Advances. He collaborated with researchers at Kumamoto University in Japan and the University of California, Santa Cruz.

"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," explains Singer. "Unless you have a complete, accurate and high-resolution 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."

Earth's magnetic field is produced by the planet's liquid iron outer core as it spins around the solid inner core. This dynamo action creates a field that is most stable going through roughly the geographic North and South poles, but the field shifts and weakens significantly during reversals.

As new rocks form—typically either as volcanic lava flows or sediments being deposited on the sea floor—they record the magnetic field at the time they were created. Geologists like Singer can survey this global record to piece together the history of magnetic fields going back millions of years. The record is clearest for the most recent reversal, named Matuyama-Brunhes after the researchers who first described reversals.

For the current analysis, Singer and his team focused on lava flows from Chile, Tahiti, Hawaii, the Caribbean and the Canary Islands. The team collected samples from these lava flows over several field seasons.

"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 researchers combined magnetic readings and radioisotope dating of samples from seven lava flow sequences to recreate the magnetic field over a span of about 70,000 years centered on the Matuyama-Brunhes reversal. They relied on upgraded methods developed in Singer's WiscAr geochronology lab to more accurately date the by measuring the argon produced from radioactive decay of potassium in the rocks.

They found that the final reversal was quick by geological standards, less than 4,000 years. But it was preceded by an extended period of instability that included two excursions—temporary, partial reversals—stretching back another 18,000 years. That span is more than twice as long as suggested by recent proposals that all reversals wrap up within 9,000 years.

The lava flow data was corroborated by magnetic readings from the seafloor, which provides a more continuous but less precise source of data than lava rocks. The researchers also used Antarctic ice cores to track the deposition of beryllium, which is produced by cosmic radiation colliding with the atmosphere. When the magnetic field is reversing, it weakens and allows more radiation to strike the atmosphere, producing more beryllium.

Since humanity began recording the strength of the magnetic field, it has decreased in strength about five percent each century. As records like Singer's show, a weakening field seems to be a precursor to an eventual reversal, although it's far from clear that a reversal is imminent.

A reversing field might significantly affect navigation and satellite and terrestrial communication. But the current study suggests that society would have generations to adapt to a lengthy period of magnetic instability.

"I've been working on this problem for 25 years," says Singer, who stumbled into paleomagnetism when he realized the volcanoes he was studying served as a good record of Earth's magnetic fields. "And now we have a richer record and better-dated record of this last reversal than ever before."

Explore further

Lava flows reveal clues to magnetic field reversals

More information: B.S. Singer el al., "Synchronizing volcanic, sedimentary, and ice core records of Earth's last magnetic polarity reversal," Science Advances (2019).
Journal information: Science Advances

Citation: Earth's last magnetic field reversal took far longer than once thought (2019, August 7) retrieved 23 August 2019 from
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Aug 07, 2019
in other words they didn't know what the heck they were talking about before. so why should we believe them now? And this happens in the intellectual community all the time. and there is no excuse for saying something is true when you don't have real proof that it is. And if you do havel proof, then there would be no reason to change it. Real "knowledge" is not changeable, by definition. What "may be" is not knowledge and worthless. what is the standard of attaching "maybe" to something. None, except to try a support continuing your research grant.

Aug 07, 2019
well rowland, since you indisputably "know"what is "true" & what is "false"
beyond all other mortal men?

please to explain why Pi & Phi?
why irrational, infinite numbers?

your omniscient intellect can certainly define the Universal Principles for a Stochastic Reality?

you wave your omnipotent hand & a grandiose palace appears with no effort or forethought or planning on your part

as for all the rest of us lowly persons?
we have to create & invent, sweat & toil, overcome obstacles & rebuild after disasters, explore & discover & in bitterness lose most cherished dreams as we discover our own mortal decay

frankly & ernestly rowland
your contempt for all other's hard work?
success or failure?

earns you my contempt...

you can climb up into that tree of bedlam with all the other looneyticks, fakirs & woomongers
shake your magic thundersticks at the indifferent cosmos & screech your futile dictates all you want
the pack of you are irrelevant to the Universe & redundant to real life

Aug 08, 2019
Except that I do think that while presenting the results of scientific research one needs to put in caveats about its limitations as a 'non-scientist' like myself can be misled into believing that it has passed into undisputed proof territory.

Aug 08, 2019
Then you gotta get better science journalists. You willing to pay for that?

The scientists are always more cautious in their papers, but you don't read them.

Aug 08, 2019
the answer is simple. We will have to perfect a robust system not dependend on the magnetic field whatsoever as navigation.

Aug 08, 2019
The GPS satellites don't need it; but the receivers do. You can't do much better than GPS unless you have about US$10,000 worth of telescope and mount and want to use celestial navigation. Not exactly portable either.

Aug 08, 2019
It's only good and healthy to change ones mind if new evidence supports other views. Would make the world a lot better if other people than scientist would also do so.

Aug 08, 2019
So, bad news and good news.

The bad news is that reversals are prolonged; but AFAIK there has been no correlation between reversals and extinctions, as Singer speculates in here [ ], and that is unlikely to change with the new, prolonged timing.

The good news is that we can be fairly sure the ongoing shifts are not part of a reversal.

why should we believe them now

Because it shows that science works (if not the laptops and internet you use for this convince you), since it is self correcting.

But hey, be functionally stupid and continue live with your medieval (mis)understanding of knowledge. Just don't expect, like you do here, that everyone else is as backward. The average tax payer is sufficiently educated to see through your bollocks, harmful for society as it is (since suppressing bollocks is even worse).

Aug 08, 2019
while presenting the results of scientific research one needs to put in caveats about its limitations

As Da Schneib says, the papers presenting the results do. You are referring to the presentation of that there are new papers - presenting the results - to read. Press releases is no way to actually understand science, anymore than looking at a sports event is a way to get fit.

You don't have to be an expert mind, but read the papers that - say - has the caveats you look for. And if there is something unclear (there always is, even for experts), ask around - that is ideally what this site is for:

More questions, less opinion!

Digests would save time, but my experience is that not even university press releases are well written. Scientists rarely has the time or experience, and they get overruled.

/ Mind that I like to make opinionated comments as well. :-) But I also like to refer to the science when I do that. The facts are out there. I like to know them, it's fun!

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