True polar wander

True polar wander is a solid-body rotation of a planet or moon with respect to its spin axis, causing the geographic locations of the north and south poles to change, or "wander". In a stable state, the largest moment of inertia axis is aligned with the spin axis, with the smaller two moments of inertia axes lying in the plane of the equator. When this is not the case, true polar wander will occur: the planet or moon will rotate as a rigid body to realign the largest moment of inertia axis with the spin axis.

For magnetic poles, see: Geomagnetic reversal

The mass distribution of the Earth is not spherically symmetric, and the Earth has three different moments of inertia. The axis around which the moment of inertia is greatest is closely aligned with the rotation axis (the axis going through the Geographic 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 a Rugby football or an American football 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. The speed of rotation (around the axis of lowest inertia) is thought to have been less than 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.

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