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7 November 2012

Pole dancing: Earth’s polar shifts explained

By Lucie Bradley

Cosmos Online

Processes that have, in the Earth’s distant past, enabled Antarctica to ‘slip’ to the equator and back again have been explained by new research.

Two mechanisms have been proposed to explain dramatic shifts in the Earth’s surface, called true polar wander, which can move Antarctica to the equator and back again. Credit: Reto Stöckli, Nazmi El Saleous, and Marit Jentoft-Nilsen, NASA GSFC

SYDNEY: Processes that have, in the Earth’s distant past, enabled Antarctica to ‘slip’ to the equator and back again have been explained by new research.

Scientists have long known that the Earth’s surface is constantly moving, independent of its daily rotation. The mechanisms for this gradual movement are understood, but the reason that the Earth’s crust returns to where it began – called ‘true polar wander’ – was, until now, a mystery.

New research published in British journal Nature proposes two mechanisms that explain how this happens.

The first is caused by the Earth’s excess bulge – which exists around the equator, making the Earth resemble a slightly squashed sphere – acting like a stabilising weight to return the surface to its original orientation. The second mechanism occurs because these movements deform the Earth’s tectonic plates, which, like elastic bands, return to their original position to relieve this strain.

Extra-terrestrials would see the Earth ‘wobble’

“We call it ‘true polar wandering’ because if you could paint a square where the rotation axis comes out it would be moving around,” said geophysicist Jerry Mitrovica, from the University of Harvard, Massachusetts, who co-authored the study.

However, “if you were an extra-terrestrial you wouldn’t see the rotational axis move, you’d see the Earth ‘wobbling’ – it would slip over and then come back,” he said.

Metal deposits in the ground line up with the Earth’s magnetic field as rocks form. By analysing ancient rock formations, scientists can see historic changes in the orientation of the Earth’s magnetic field. Doing so, they have found that the characteristic rate of true polar wander is about one degree every million years. However, they also show that there have been several large and rapid events of true polar wander, where the Earth has tilted up to 60 degrees in just 30 million years.

An unbalanced Earth

“The Earth is made up of a rocky mantle, which sits on top of the Earth’s core and extends up to the crust. This mantle may be rocky, but on long time scales it flows and moves in a process called mantle convection,” said Mitrovica. This is the same process that causes the continents to slowly migrate, although in true polar wander all the plates move together and so it is not the same as continental drift where they move independently.

The initial ‘tip’ of true polar wander happens when the distribution of mass around the Earth becomes unbalanced, for example if a vast icesheet melts or a very large volcano forms far from the Earth’s equator. This redistribution of mass will unbalance the Earth, which is already uneven as it is not a true sphere, and cause the mantle of the Earth to slip in an effort to correct this. The movement will settle the heavier areas on the equator.

“At some level it was puzzling why the Earth would tip over but it’s doubly puzzling why it would return,” said Mitrovica, who said he was originally sceptical about the theory. “The Earth tips over because of this mass change, but why does it come back? What allows the pole to know where it came from? This was the source of my scepticism, and a lot of people shared my scepticism.”

Excess bulge stabilises rotation

The researchers answered these questions using detailed modelling and extensive computer simulations to come up with two separate mechanisms.

The Earth has an equatorial bulge, making it a slightly flattened sphere that is largest around the equator. This unevenness works like a self-stabilising heavy weight and forces the Earth to come back to the position of its natural equilibrium. This is mechanism one.

Additionally, as the mantle and crust rotate around the Earth, the tectonic plates are placed under tension: some are squashed whilst others are stretched. This elasticity stores a lot of potential energy that then releases – in mechanism two – and brings the plates back to their original location.

Dramatic changes to a planet’s evolution

The researchers found that in modelling experiments these two mechanisms proved very effective at bringing the pole back to its original position and at doing so very quickly. The data from their simulations matched the geological data from the study of ancient rock formations and showed that these large true polar wander events happen within 20 or 30 million years.

True polar wander causes a site to change latitude, greatly altering its climate, and it does this to every site on Earth simultaneously. Archaeological records show that this can cause dramatic changes to the carbon cycle, climate, and can in fact be extended to every aspect of the evolution of a planet.

According to Mitrovica, however, there’s no need for immediate concern as over the past 200 million years the Earth has only rotated 30 degrees – that’s just 0.2 degrees every one million years.

More information: An explanation of true polar wander, complete with cartoon Abstract of the paper, “Mechanisms for oscillatory true polar wander” published in Nature



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