True polar wander
Encyclopedia
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 Pole
s to change, or "wander". In a stable state, the largest moments of inertia
axis is aligned with the spin axis, with the smaller two moment 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.
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 latitude
s 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.
, Europa
, and Enceladus
are also believed to have undergone true pole wander, in the case of Europa by 80°, flipping over almost completely.
or nutation
, which is where the axis of rotation moves, in other words the North Pole points toward a different star. These are 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
.
The effect should further not be confused with the effect known as geomagnetic reversal
that describes the repeated proven reversal of the magnetic field of the Earth
.
is used to create tectonic plate reconstruction
s by finding the paleolatitude of a particular site. This paleolatitude is affected both by true polar wander and by plate tectonics
. 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.
Polar wander
Polar wander is the movement of the North and South Poles of the Earth with respect to the continents. This motion can be divided into two components: that due to continental drift , and true polar wander, in which the mantle and the crust rotate together into new orientations....
is a solid-body rotation of a planet or moon with respect to its spin axis, causing the geographic locations of the North
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
and South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
s to change, or "wander". In a stable state, the largest moments of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
axis is aligned with the spin axis, with the smaller two moment 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.
Description in the context of Earth
The EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
is not a true sphere, and therefore has three orthogonal axes of inertia. The axis around which the moment of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. 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
Rugby football
Rugby football is a style of football named after Rugby School in the United Kingdom. It is seen most prominently in two current sports, rugby league and rugby union.-History:...
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 latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s 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.
Examples
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 limited to about 1° per million years. MarsMars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
, and Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
are also believed to have undergone true pole wander, in the case of Europa by 80°, flipping over almost completely.
Distinctions and delimitations
Polar wander should not be confused with precessionPrecession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
or nutation
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 behavior of a mechanism...
, which is where the axis of rotation moves, in other words the North Pole points toward a different star. These are caused by the gravitational attraction of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, 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
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
, which is where different parts of the Earth's crust move in different directions because of circulation in the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
.
The effect should further not be confused with the effect known as geomagnetic reversal
Geomagnetic reversal
A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse...
that describes the repeated proven reversal of the magnetic field of the Earth
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
.
Tectonic plate reconstructions
PaleomagnetismPaleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
is used to create tectonic plate reconstruction
Plate reconstruction
Plate reconstruction is the process of reconstructing the positions of tectonic plates relative to each other or to other reference frames, such as the earth's magnetic field or groups of hotspots, in the geological past...
s by finding the paleolatitude of a particular site. This paleolatitude is affected both by true polar wander and by plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. 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.
See also
- See cataclysmic pole shift hypothesis for a discussion of various historical conjectures involving rapid shift of the poles.
- Polar motionPolar motionPolar motion of the earth is the movement of Earth's rotational axis across its surface. This is measured with respect to a reference frame in which the solid Earth is fixed...
- Axial tiltAxial tiltIn astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
- Apparent polar wanderApparent polar wanderApparent polar wander is the imaginary movement of the Earth's magnetic poles relative to a continent while regarding the continent being studied as fixed in position, as determined by paleomagnetic data...