Nov 12, 2012
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.
“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.
The research was published in Nature.