True polar wander is a solid-body rotation (or reorientation) 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 (geometry), direction or geography. Etymology T ...

and

south pole The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is the point in the Southern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True South Pole to distinguish ...

s to change, or "wander". In rotational equilibrium, a planetary body 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 The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

. This is because planets are not rigid - they form a rotational bulge which affects the inertia tensor of the body. Internal or external processes that change the distribution of mass (internal or external loadings) disrupt the equilibrium and true polar wander will occur: the planet or moon will rotate as a rigid body (reorient in space) to realign the largest moment of inertia axis with the spin axis. Because stabilization of rotation by the rotational bulge is only transient, even relatively small loads can result in a significant reorientation (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 ( Chandler wobble). 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 rotation, Earth's rotational axis relative to its Earth's crust, crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called ''Earth-centered, Ea ...

''.

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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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

. That is similar to a brick rotating around an axis going through its shortest dimension (a vertical axis when the brick is lying flat). On Earth and most other planets, the difference in the polar and equatorial moments of inertia is dominated by the formation of a rotational bulge - excess mass around the equator (flattening) caused by rotational deformation (planetary bodies are not rigid - they deform in response to rotation and its changes). Internal and external processes such as mantle convection, deglaciation, formation of volcanoes, or large meteorite impacts can disrupt rotational equilibrium and cause bodies to move as a whole relative to their rotation axis (reorient). Most natural loadings are small when compared to the rotational bulge and hence change the direction of the main axis of inertia only slightly. However, since the rotational bulge eventually readjusts when the spin axis moves within the body, the stabilization by the rotational bulge disappears on geological timescales and the equilibrium orientation of the planet is given by its dominant loads. Throughout true polar wander, the spin axis lies close to the main axis of inertia of the body, and the time evolution of the latter is driven by gradual readjustment of the rotational bulge. On short timescales and for rapid loadings, the secular motion of the pole is accompanied by free (or Chandler) wobbling. Such a reorientation changes the latitudes 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. In the context of tidally locked bodies, also the

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

of surface features can change in time and the dynamics of reorientation can be more rapid.

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. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, Europa, and Enceladus 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. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

' extreme inclination with respect to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...

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 o ...

, 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. Precession is caused by the gravitational attraction of the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

and Sun, and occurs all the time and at a much faster rate than polar wander. It does not result in changes of latitude (it results in changes of star inclinations). True polar wander has to be distinguished from

continental drift Continental drift is a highly supported scientific theory, originating in the early 20th century, that Earth's continents move or drift relative to each other over geologic time. The theory of continental drift has since been validated and inc ...

, which is where different parts of the Earth's crust move in different directions because of circulation in the mantle. Because of plate tectonics, the polar wander as seen from an individual continent may differ from the true polar wander (see also apparent polar wander). The effect should further not be confused with the effect known as

geomagnetic reversal A geomagnetic reversal is a change in the Earth's Dipole magnet, dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with North Pole, geographic north and South Pole, geograp ...

that describes the repeated proven reversal of the magnetic field of the Earth.

Tectonic plate reconstructions

Paleomagnetism Paleomagnetism (occasionally palaeomagnetism) is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain ...

is used to create tectonic plate reconstructions by finding the paleolatitude of a particular site. This paleolatitude is affected both by true polar wander and by

plate tectonics Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

. 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