True polar wander

True polar wander is a phenomenon in which a planet or moon changes its orientation so that different points become the North and South Poles. This can happen when the two larger moments of inertia are near equal.

The Earth is not a true sphere, and therefore has three orthogonal axes of inertia. The axis around which the moment of inertia is greatest is closely aligned with the rotation axis (the axis going through the North and South Poles). The other two axes are near the equator. This is similar to a brick rotating around an axis going through its shortest dimension (a vertical axis when the brick is lying flat). But if the moment of inertia around one of the two axes close to the equator becomes nearly equal to that around the polar axis, then the constraint on the orientation of the object (the Earth) is relaxed. This situation is like an American football (or a Rugby ball) spinning around an axis running through its "equator". (Note that the "equator" of the ball does not correspond to the equator of the Earth.) Small perturbations can move the football so that it spins around another axis through this same "equator". In the same way, when the conditions are right, the Earth (both the crust and the mantle) can slowly reorient so that a new geographic point moves to the North Pole, while keeping the axis of low moment of inertia quite near the equator.

Such a reorientation changes the latitudes of most points on the Earth, by amounts that depend on how far they are from the axis near the equator that does not move.

Cases of true polar wander have occurred several times in the course of the Earth's history. ["Absolute plate motions and true polar wander in the absence of hotspot tracks", Bernhard Steinberger & Trond Torsvik, "Nature", 3 April 2008, pp. 620-3.] The speed of rotation (around the axis of lowest inertia) is limited to about 1° per million years. Mars, Europa, and Enceladus are also believed to have undergone true pole wander, in the case of Europa by 80°, flipping over almost completely. [http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon] The crust of Titan has also shifted, though pole wander has not been detected.

Polar wander should not be confused with precession, which is where the axis of rotation moves, in other words the North Pole points toward a different star. Precession is caused by the gravitational attraction of the Moon and Sun, and occurs all the time and at a much faster rate than polar wander. It does not result in changes of latitude.

True polar wander has to be distinguished from continental drift, which is where different parts of the Earth's crust move in different directions because of circulation in the mantle.

ee also

See pole shift hypothesis for a discussion of various historical theories involving rapid shift of the poles.

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