Apparent polar wander

Apparent polar wander (APW) is the perceived movement of the Earth's paleo-magnetic poles relative to a continent while regarding the continent being studied as fixed in position. It is frequently displayed on the present latitude-longitude map as a path connecting the locations of geomagnetic poles, inferred at distinct times using paleomagnetic techniques.

In reality, the relative polar movement can either be polar wandering or continental drift (or a combination of both). Data from around the globe are needed in order to isolate or distinguish between the two. Nevertheless, the magnetic poles rarely stray far from the geographic poles of the planet. Therefore, the concept of apparent polar wander is very useful in plate tectonics, since it can retrace the relative motion of continents, as well as the formation and break-up of supercontinents.

It has been known for a long time that the geomagnetic field varies through time, and records of its direction and magnitude have been kept in different locations since the 1800s. The technique of drawing apparent polar wander was first developed by Creer et al. (1954), and was a major step taken towards the acceptance of the plate tectonics theory. Since then many discoveries have been made in that field, and apparent polar wander has become better understood with the evolution of the theory and of the Geocentric Axial Dipole (GAD) model. There are over 10,000 paleomagnetic poles recorded in the database today.

Much research in paleomagnetism is aimed at finding paleomagnetic poles for different continents and at different epochs, in order to assemble them in APWP tracks. Paleomagnetic poles have the advantage that they should have the same value at each observing locality on the basis of the Geocentric axial dipole (GAD) model. Thus they can be used to compare paleomagnetic results from widely separated localities.

Fossil magnetization in rocks is key to locate a paleomagnetic pole. At the time of formation, rocks conserve the direction of the magnetic field. The inclination(Im) and declination vectors(Dm) are preserved and therefore the paleolatitude(λp) and paleolongitude(φp) of the pole can be found.

The reason the characteristics of the field are conserved comes from the concept of blocking temperature (also known as closure temperature in geochronology). This temperature is where the system becomes blocked against thermal agitation at lower temperatures. Therefore, some minerals exhibit remnant magnetization. One problem that arises in the determination of remnant (or fossil) magnetization is that if the temperature rises above this point, the magnetic history is destroyed. However, in theory it should be possible to relate the magnetic blocking temperature to the isotopic closure temperature, such that it could be checked whether or not a sample can be used.

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