Geomagnetic reversal
Overview
Geomagnetic reversal, also known as a polar shift, is a change in a planet'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 polarity, in which the field was in the opposite direction. These changes are known as "Chrons".
History of Study
The first evidence of geomagnetic reversals came from studies of deep-sea sediments. These studies began in the late 1950s and early 1960s. The confirmation of these reversals came from studies of paleomagnetism, a study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials.
Mechanism
The Earth's magnetic field is generated in the geodynamo, the motion of molten iron alloys within the Earth's outer core. The energy required to maintain the magnetic field is provided by the sinking of cooler, denser matter within the core. This process is known as thermohaline circulation.
Frequency of Reversals
Geomagnetic reversals occur a few times every million years on average. However, the intervals between reversals are not regular and can vary widely. The most recent reversal, the Brunhes–Matuyama reversal, occurred approximately 780,000 years ago.
Effects on Biosphere
There is no definitive evidence that a geomagnetic reversal has ever caused a biological mass extinction event. However, some scientists have suggested that it could have indirect effects on the biosphere, particularly through changes in the Earth's magnetosphere.
Future Reversals
Predicting when geomagnetic reversals will occur is difficult, due to the lack of a reliable method to determine the 'health' of the geodynamo. However, some scientists believe that we are currently in the early stages of a geomagnetic reversal.