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 North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north ...

and

south pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...

s to change, or "wander". Unless the body is totally rigid (which the Earth is not) its stable state rotation has the largest moment of inertia axis aligned with the spin axis, with the smaller two moments of inertia axes lying in the plane of the equator. If the body is not in this steady state, 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. (See .) If the body is near the steady state but with the angular momentum not exactly lined up with the largest moment of inertia axis, the pole position will oscillate. Weather and water movements can also induce small changes. These subjects are covered in the article ''

Polar motion Polar motion of the Earth is the motion of the Earth's rotational axis relative to its crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called ''Earth-centered, Earth-fixed'' or ECEF reference ...

''.

Description in the context of Earth

The mass distribution of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

is not spherically symmetric, and the Earth has three different

moments of inertia The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular accelera ...

. 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. That is similar to a brick rotating around an axis going through its shortest dimension (a vertical axis when the brick is lying flat). However, if the moment of inertia around one of the two axes close to the equator becomes nearly equal to that around the polar axis, the constraint on the orientation of the object (the Earth) is relaxed. This situation is like a

rugby football Rugby football is the collective name for the team sports of rugby union and rugby league. Canadian football and, to a lesser extent, American football were once considered forms of rugby football, but are seldom now referred to as such. The ...

or an

American football American football (referred to simply as football in the United States and Canada), also known as gridiron, is a team sport played by two teams of eleven players on a rectangular field with goalposts at each end. The offense, the team wi ...

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, which then spins around another axis through the same "equator". In the same way, conditions can make the Earth (both the crust and the mantle) slowly reorient until a new geographic point moves to the North Pole, with the axis of low moment of inertia being kept very near the equator. Such a reorientation changes the

latitudes In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...

of most points on the Earth by an amount that depends on how far they are from the axis near the equator that does not move.

Examples

Cases of true polar wander have occurred several times in the course of the Earth's history. It has been suggested that east Asia moved south due to true polar wander by 25° between about 174 and 157 million years ago.

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

, Europa, and

Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...

are also believed to have undergone true pole wander, in the case of Europa by 80°.

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...

' extreme inclination with respect to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

is not an instance of true polar wander (a shift of the body relative to its rotational axis), but instead a large shift of the rotational axis itself. This axis shift is believed to be the result of a catastrophic series of impacts that occurred billions of years ago.

Distinctions and delimitations

Polar wander should not be confused with

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

, which is where the axis of rotation moves, in other words the North Pole points toward a different star. There are also smaller and faster variations in the axis of rotation going under the term

nutation Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...

. Precession is caused by the gravitational attraction of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

and

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, 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 Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pl ...

, which is where different parts of the Earth's crust move in different directions because of circulation in the mantle. The effect should further not be confused with the effect known as

geomagnetic reversal A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's field has alternated ...

that describes the repeated proven reversal of the

magnetic field of the Earth Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic fi ...

Tectonic plate reconstructions

Paleomagnetism is used to create tectonic

plate reconstruction :''This article describes techniques; for a history of the movement of tectonic plates, see Geological history of Earth.'' Plate reconstruction is the process of reconstructing the positions of tectonic plates relative to each other (relative moti ...

s by finding the paleolatitude of a particular site. This paleolatitude is affected both by true polar wander and by

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

. To reconstruct plate tectonic histories, geologists must obtain a number of dated paleomagnetic samples. Because true polar wander is a global phenomenon but tectonic motions are specific to each plate, multiple dates allow them to separate the tectonic and true polar wander signals.

References

{{DEFAULTSORT:True Polar Wander Geodesy Geodynamics Paleomagnetism

Description in the context of Earth

Examples

Distinctions and delimitations

Tectonic plate reconstructions

See also

References