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True Polar Wander: Shifting of the Earth’s Surface Relative to its Spin Axis
Here’s an interesting article from the IOP website; How Earth’s wandering poles return home. It takes the Solar System about 225–250 million years to complete one orbit around the Galaxy (a Galactic year), so there may be a relationship between this and the cycle of the ‘tipping’ of the surface relative to the spin axis.
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”.
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 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…. 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,”
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.
The paper is published by Nature here:
http://www.nature.com/nature/journal/v491/n7423/full/nature11571.html
2012-11-16 10:38:35
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