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Magnetic Pole Flip

October 21, 2019

One of the mysteries about zombies is that they move so slowly, but people still can't seem to escape. George A. Romero initiated the slow motion zombie stereotype in his 1968 film, Night of the Living Dead.[1] It seems logical that a reanimated corpse should be somewhat lethargic, and Romero may have been influenced in his zombie characterization by the 1932 classic, The Mummy.[2] The plodding motions of Boris Karloff in playing the mummy of Imhotep is much like that of a zombie.

Figure caption

The Egyptian Book of the Dead was a collection of incantations intended to assist the deceased's journey into the afterlife. This book was not fixed text; rather, it was augmented by priests over the course of many centuries.

In this detail from the 1275 BC Book of the Dead of Hunefer, sheet 3, Osiris sits on his throne, with the goddesses, Isis and Nephthys, behind him. The four Sons of Horus are standing on a large lotus.

(British Museum Accession number EA 9901, via Wikimedia Commons

Our modern culture has a need for speed. One mantra of my time in corporate research was "first to market," the idea that your invention would be far more profitable if it was sold at an early stage of development rather than waiting for a more perfect product. Not surprisingly, we have many expressions about maddeningly slow processes, such as watching grass grow and watching paint dry However, few processes are as slow as geological processes such as glacial flow (watch it while they're still here) and the movement of tectonic plates.

Hillside creep never excited me, I'm not a geologist or a geophysicist, but I've always had an interest in the magnetic moment of the Earth. The direction of Earth's magnetic field, like other geological phenomena, changes slowly over time. As I discussed in a recent article (Magnetic North, February 11, 2019), the Earth's North Magnetic Pole is not as fixed as navigators would prefer. Although the position of the North magnetic pole was fairly constant for most of the 20th century, it's presently moving at about 30 miles (50 kilometers) per year.[3-4] We're only now capable of measuring magnetic pole movement as it happens, so geoscientists are starting to wonder whether such excursions are a prelude to something more dramatic.

Such could be expected by the manner in which the Earth's magnetic field is generated. Dynamo action of liquid iron in its core produces Earth's magnetic field, and the complexity of fluid dynamics leads to the variation that we observe. At times there's a reversal of the fluid flow that causes a geomagnetic reversal of the magnetic polarity with the north-south direction being reversed and compass needles pointing the other way. These geomagnetic reversals have happened often in Earth's history (see figure).

Geomagnetic recversals for the last five million years

Geomagnetic reversals for the last five million years (Mya), as revealed by the Geological record. The blue areas mark when the polarity was the same as our present North-South polarity. (Data from U.S. Geological Survey Open-File Report 03-187, as converted by Intgr to a Wikimedia Commons image and modified for clarity. Click for larger image.)

Since Earth's magnetic field protects its living organisms from cosmic rays, is there a greater incidence of extinction events during reversals? Research has shown no extinction events during geomagnetic reversals. A study found that the Earth's magnetic field decreased by just 5% during the short (440 year) Laschamp reversal event that occurred 41,400 ± 2,000 years ago.[5]

Evidence of pole flips is encoded in magnetic minerals in lava flows. When a ferromagnet or ferrimagnet is heated above its Curie temperature, it loses its magnetism. The magnetism returns when the material cools again, and the magnetic axis of the mineral crystallites aligns to the direction of Earth's magnetic field at the time. Another technique, magnetostratigraphy examines magnetism in sedimentary materials, since magnetized grains will preferentially orient themselves with Earth's magnetic field.

The pole reversals are statistically random, and 183 reversals have been recorded over the last 83 million years. It appears that pole reversal is a slow process with an average duration of about 7,000 years. There have been much shorter flips, called excursions because of their fleeting nature. One of these is the Laschamp excursion that occurred about 41 thousand years ago. A recent study of the most recent pole reversal that occurred about 773 thousand years ago shows that the duration of this flip was much longer, about 22,000 years.[6-8]

Cross-section of the Earth

Cross-section of the Earth. The inner core, which is about 20% of the Earth's radius, is solid because of pressure, but the outer core is a liquid composed mostly of iron and nickel about 1,500 miles (2,400 kilometers) thick. Rotation of the outer core causes the Earth's magnetic field. (Modified Wikimedia Commons image by Surachit.)

This new research by a team from the University of Wisconsin-Madison (Madison, Wisconsin), Kumamoto University (Kumamoto, Japan), and the University of California, Santa Cruz (Santa Cruz, California) took a careful look at the Brunhes-Matuyama reversal, the geomagnetic transition that happened around 780,000 years ago.[7] One theory of such polarity reversals is that they are caused by an inbalance between the rotation of the Earth and the temperature at Earth's core, and this alters the fluid dynamics of its liquid iron.[7] To accurately pinpoint the magnetic state during this transition, the research team used data from several sources of historical information about Earth's magnetism during this period, including new techniques for dating solidified lava.[7-8]

The ratio of 40Ar to 39Ar was used as a date reference for magnetized lava flows, ocean sediment and ice cores having a record of Earth's magnetism.[6-7] Basaltic lava contains magnetite, and the the magnetization direction of this ferrous material gets locked into place upon cooling.[7] Since lava flows are scarce, the researchers looked also at magnetized ocean sediments and Antarctic ice cores.[7] Antarctic ice cores also contain a beryllium isotope that forms under cosmic radiation, and that happens when Earth's magnetic field weakens during an excursion or reversal.[7]

While earlier estimates of the duration of a field reversal were between 4,000 and 9,000 years, the present study found that the Brunhes-Matuyama reversal took roughly 22,000 years to complete.[8] The data also reveal that the reversal started with an excursion, in which Earth's field dropped significanty but did not reverse.[8] Reversal was accomplished 11,000 years after the excursion, with a final settling of the field that lasted 4,000 years.[8] Says Brad S. Singer, first author of the paper and a geoscientist at the University of Wisconsin-Madison, "What the lava-flow records show clearly are a couple of attempts by the dynamo to reverse polarity that took place prior to the final culminating reversal."[8]

Virtual geomagnetic pole (VGP) evolution associated with the Brunhes-Matuyama reversal

Fortunately, my laboratory data were never as complicated as this, but nature is not a controlled experiment. This is a graph showing the high–deposition rate ocean sediment records of the virtual geomagnetic pole (VGP) evolution associated with the Brunhes-Matuyama reversal as a function of time. The final polarity reversal at about 773 ka was the culmination of a process that began at about 795 ka with weakening of the magnetic field,followed by an excursion and another weakening of field at about 784 ka.[6] (Fig. 3B 0f ref. 6, distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC))[6]

As Singer notes, "Our study points to a more protracted and complex process for driving and propelling a geomagnetic field reversal."[8] For the past two centuries, Earth’s magnetic field has decayed at a rate of five percent each century, as if indicative of a pending field reversal, but Singer thinks that a reversal will not likely happen for the next two millennia, so humanity will have time to shield its technologies, such as satellites, from cosmic radiation by the time of the next reversal.[7]
"It may be that you have to weaken the field for several thousand years in order to get a reversal because the inner core is solid... If you turn off the magnetic field in the liquid outer core, there's magnetic energy inside the solid inner core that takes several thousand years to leave. What we're seeing now with the north pole moving rapidly, that’s actually quite normal.. The actual reversal, the final reversal, takes several thousand years."[7-8]


  1. Night of the Living Dead (1968, George A. Romero, Director) on the Internet Movie Database.
  2. The Mummy (1932, Karl Freund, Director) on the Internet Movie Database.
  3. Alexandra Witze, "Earth’s magnetic field is acting up and geologists don’t know why," Nature, vol. 565 (January 9, 2019), pp. 143-144, doi: 10.1038/d41586-019-00007-1.
  4. Alister Doyle, "Shifting north magnetic pole forces unprecedented navigation fix," Reuters, January 11, 2019.
  5. N.R. Nowaczyk, H.W. Arz, U. Frank, J. Kind, and B. Plessen, "Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments," Earth and Planetary Science Letters, Vols. 351-352 (October 15, 2012), pp. 54-69.
  6. Brad S. Singer, Brian R. Jicha, Nobutatsu Mochizuki, and Robert S. Coe, "Synchronizing volcanic, sedimentary, and ice core records of Earth’s last magnetic polarity reversal," Science Advances, vol. 5, no. 8 (August 7, 2019), Article no. eaaw4621, DOI: 10.1126/sciadv.aaw4621. This is an open access article with a PDF file here
  7. Emily Toomey, "Earth’s Magnetic Field Could Take Longer to Flip Than Previously Thought," Smithsonian.com, August 7, 2019.
  8. Jonathan O'Callaghan, "Earth’s Magnetic Field Reversal Took Three Times Longer Than Thought," Scientific American, August 7, 2019.

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