How Earth’s wandering poles return home

physicsworld.com |Nov 12, 2012

How Earth’s wandering poles return home

A number of times over the past one billion years, the Earth’s surface has “wandered” relative to its rotational axis – before returning to its original position. Now, a team of geophysicists from the US and Canada says it has developed a theory that explains this curious phenomenon of “oscillatory true polar wander”. Understanding the mechanics behind polar wander is crucial, as a shift could tip the Earth over by as far as 50° over a period of 10–100 million years and this would cause profound global environmental and geological changes.

True polar wander (TPW) can be defined as the relative movement between the mantle (and so the surface of the Earth) and the Earth’s spin axis or its rotational axis. Incredibly, researchers believe that over the past one billion years, the Earth’s surface has “tipped over” and then returned to its original location six times along the same axis – this is the process of “oscillatory true polar wander”. Scientists have worked this out by studying magnetism in rocks – a discipline known as “paleomagnetism”. If a rock cools in a magnetic field, it records the magnetic properties of the field and these can be decoded in the lab millions of years later. So, by measuring changes in the orientation of the Earth’s magnetic field that are stored in ancient rocks, scientists can “see” the effects of the oscillatory TPW.

Extreme shifts

“Someone sitting on the Earth would have seen the pole shift up to 50° and then turn around and return close to its original location, all in tens of millions of years,” explains geophysicist Jerry Mitrovica of the Earth and Planetary Science Department at Harvard University. “But an observer floating in space would actually see the rotational axis stay relatively vertical and the Earth’s surface tip over and then back.” Unsurprisingly, these rather extreme and dramatic shifts can be linked to global changes in all large-scale Earth systems such as the carbon cycle, climate and even evolution. “After all, if it happened today, a shift of 50° one way might put Boston [Massachusetts] near the north pole, while a shift in the opposite direction would bring Boston near the equator,” says Mitrovica.

But this in itself is not news – earth scientists have known for a while that TPW does occur and they even know why. They believe that the initial shift of the pole – or the Earth tipping over – is caused by large-scale flows in the Earth’s interior known as “mantle convection”, involving thermal convection currents that carry heat from the Earth’s core to the surface. This is the same process that drives continental drift and plate tectonics. So, mantle convection disturbs the rotational equilibrium of the Earth and the result is a shift in the relative orientation of the Earth’s solid surface and its rotational axis.

There and back again

What has eluded researchers is a theory that clearly explains how and why the pole returns to its original location, or the “oscillatory true polar wander”. In the new work, graduate student Jessica Creveling, also of the Earth and Planetary Science Department at Harvard, along with Mitrovica and colleagues, provides an explanation. The researchers, using computer simulations and modelling, say that a combination of two mechanisms brings the “wandering” pole back to its original location.

The first mechanism relates to the Earth’s equatorial bulge. The Earth is not a perfect sphere – rather it is an oblate spheroid, as it is flattened at the poles and bulges at the equator. So there is a difference in the radius of the Earth as measured from the centre to the equator compared with the poles – it is approximately 20 km greater at the equator. This band of excess mass forms because the Earth is rotating, which causes the equator to bulge outwards. “But the Earth’s bulge is generally a bit larger than it should be…which is true even today. And this extra bulge, or fatness, acts to stabilize the Earth’s rotation,” explains Mitrovica. He likens this to the heavy weight that is placed at the bottom of a plastic punching-bag toy, which acts to bring the bag back to being vertical if it is punched sideways. In a similar manner, if the Earth, with its bulging equator, tips over, it prefers to right itself again. “So, this girdle of excess mass actually has a very stabilizing effect, acting as a self-righting mechanism for the Earth’s rotation,” he says.

The second mechanism relates to the strength of the tectonic plates. If the Earth’s surface tips over relative to the rotational axis, the 12 larger tectonic plates all get deformed to a small extent, like elastic bands. In a similar way to a stretched elastic band, the plates want to go back to their original size, and these stabilizing elastic stresses also play a role in the oscillatory return of the pole. A clue that this might be the case is the fact that past polar-oscillation events seem to have happened when the Earth’s continents were gathered together into one “supercontinent”, a process that has repeated a number of times in Earth’s history. The last supercontinent, known as Pangea, was formed 200 million years ago.

Efficiency of combined effects

Mitrovica points out that while Creveling was running her simulation, neither single mechanism could cause the pole to return – it was only a combination of both effects that did it. “What also really surprised me was the efficiency of the effects to pull and push the poles during a period of about 10 million years,” says Mitrovica. “This paper made a believer out of me and I was a sceptic.” He explains that other researchers might remain sceptical about the theory and that only more evidence gathered based on paleomagnetic field studies will provide the necessary evidence. The team also hopes to better determine how common or rare these events are. “Every rock cooling at the time of a tilt will show the evidence of it and we need to find that,” says Mitrovica.

The team is also keen to determine just how drastic the effects of a shift are. It is believed that a shift would cause a significant change in the climate of every place on Earth, as well as changes in the sea level and the carbon cycle. Mitrovica believes that the consequences of these large-scale events would have left their own mark on the Earth’s systems and they too should be studied in the future.

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Mechanisms for oscillatory true polar wander

NATURE | LETTER (not open access)

J. R. Creveling, J. X. Mitrovica, N.-H. Chan, K. Latychev & I. Matsuyama

Nature 491, 244–248 (08 November 2012) doi:10.1038/nature11571

Received 13 August 2011 Accepted 10 September 2012 Published online 07 November 2012

Abstract

Palaeomagnetic studies of Palaeoproterozoic to Cretaceous rocks propose a suite of large and relatively rapid (tens of degrees over 10 to 100 million years) excursions of the rotation pole relative to the surface geography, or true polar wander (TPW). These excursions may be linked in an oscillatory, approximately coaxial succession about the centre of the contemporaneous supercontinent. Within the framework of a standard rotational theory, in which a delayed viscous adjustment of the rotational bulge acts to stabilize the rotation axis, geodynamic models for oscillatory TPW generally appeal to consecutive, opposite loading phases of comparable magnitude. Here we extend a nonlinear rotational stability theory to incorporate the stabilizing effect of TPW-induced elastic stresses in the lithosphere. We demonstrate that convectively driven inertia perturbations acting on a nearly prolate, non-hydrostatic Earth with an effective elastic lithospheric thickness of about 10 kilometres yield oscillatory TPW paths consistent with palaeomagnetic inferences. This estimate of elastic thickness can be reduced, even to zero, if the rotation axis is stabilized by long-term excess ellipticity in the plane of the TPW. We speculate that these sources of stabilization, acting on TPW driven by a time-varying mantle flow field, provide a mechanism for linking the distinct, oscillatory TPW events of the past few billion years.

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Related article on physicsworld.com

Geomagnetic flip may not be random after all

One of the most fascinating natural phenomena on Earth is the flipping of its magnetic field, which has occurred hundreds of times in the last 160 million years. When the magnetic field flips, the North Pole becomes the South Pole and vice versa. The last time this happened was some 780,000 years ago, so we could be heading for another reversal soon. Now, physicists in Italy have found that the frequency of these polarity reversals is not random as previously thought but occurs in clusters, revealing some kind of “memory” of previous events (physics/0603086).

Although a full geomagnetic polarity reversal can take thousands of years to complete, it does have implications. As well as affecting the migration trajectories of birds and other animals, the disruption to the Earth’s magnetic field could expose the Earth to hazardous cosmic rays. Geoscientists believe that our planet’s internal magnetic dynamo is responsible for pole reversals, but the actual mechanism is not well understood.

Previous analyses assumed that the number of times the poles have reversed over last 160 million years follows a Poisson distribution, implying that the events are random. The Poisson distribution tells you the probability of a number of events occurring in a fixed time if the events are independent and the average rate is known. A good example of the Poisson distribution in physics is the likelihood of unstable radioactive nuclei decaying in a certain period.

Now, a team of physicists led by Vincenzo Carbone of the University of Calabria have discovered that the sequence of polarity reversals can be well described by a Lévy distribution instead. In contrast to Poisson statistics, the Lévy distribution describes stochastic processes that are characterised by the presence of “memory” effects — or long-range correlations between the events in time. Lévy distributions are widely used to study many critical phenomena, such as earthquakes, and also when analysing financial data. The researchers obtained their results by careful statistical analysis of different sets of paleomagnetic data containing estimates of when the Earth’s poles reversed.

“The result means that polarity reversals are not random events that are independent of each other,” explains team member Fabio Lepreti. “Instead, there is some degree of memory in the magnetic dynamo processes giving rise to the reversals,” he says. “We hope that our work will serve as a useful reference point for models that aim to describe the phenomenon of pole reversal.” The Italy team now plans to build new dynamic models to describe the field reversal sequences in a simple way, so that the physical mechanisms that trigger pole reversals can be more easily explained.

Long-term fluctuations in the intensity and inclination of the Earth’s magnetic field could arise from variations in the eccentricity of our planet’s orbit, according to Japanese geophysicists. Toshitsugu Yamazaki and Hirokuni Oda of the Geological Survey of Japan examined the magnetic properties of a sample of marine sediment deposited over a period of 2.25 million years to establish that the Earth’s magnetic field varies over a 100 000-year cycle. Such studies could shed new light on the energy sources that drive the Earth’s dynamo (T Yamazaki and H Oda 2002 Science 295 2435).

Current theories of geomagnetism state that the ‘dynamo’ that powers the Earth’s magnetic field is maintained by heat and gravitational energy. But previously observed long-term patterns in the intensity and inclination of Earth’s magnetic field cannot be explained by these effects, which change on relatively short time-scales.

In order to study long-term variations in the magnetic field, Yamazaki and Oda extracted a column of sediment 42 metres long from the sea floor near the equator, and measured the magnetization of over 1700 samples from this ‘core’. The magnetization of the samples is determined by the orientation of magnetic grains in the sediment. This study revealed that the intensity and orientation of the magnetic field changes over a cycle that lasts 100 000 years.

After ruling out a number of possible candidates – such as climate effects – Yamazaki and Oda proposed that the 100 000 year cycle could arise from changes in the eccentricity of the Earth’s orbit. Eccentricity is a measure of how much the orbit of a planet deviates from a circle, and ranges from zero for a circular orbit to one for a highly elliptical orbit.

Astronomers know that the eccentricity of the Earth’s orbit varies between 0 and 0.06 every 100 000 years. This causes the Earth to pass slightly closer to the Sun during certain epochs. Yamazaki and Oda believe that this could induce slight changes in the Earth’s iron core that affect the generation of the magnetic field, and therefore the way that sediment is deposited in the ocean.

The researchers are optimistic that their theory will prove easy to test because the variations in magnetic field are expected to be very pronounced in certain regions of the Earth’s surface.