right|thumb|285px|Geomagnetic polarity during the last 5 million years ([[Pliocene and Quaternary, late Cenozoic Era). Dark areas denote periods where the polarity matches today's normal polarity; light areas denote periods where that polarity is reversed.]]

A geomagnetic reversal is a change in the Earth's dipole 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 magnetic field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.

Reversal occurrences appear to be statistically random. There have been at least 183 reversals over the last 83 million years (thus on average once every ~450,000 years). The latest, the Brunhes–Matuyama reversal, occurred 780,000 years ago

There have also been episodes in which the field inverted for only a few hundred years (such as the Laschamp excursion and the Kiaman Superchron. A third candidate, the Moyero Superchron, is more controversial. The Jurassic Quiet Zone in ocean magnetic anomalies was once thought to represent a superchron but is now attributed to other causes.

The Cretaceous Normal Superchron (also called the Cretaceous Superchron or C34) lasted for 37million years, from about , including stages of the Cretaceous period from the Aptian through the Santonian. The frequency of magnetic reversals steadily decreased prior to the period, reaching its low point (no reversals) during the period. Between the Cretaceous Normal and the present, the frequency has generally increased slowly.

Further reading

  • Is it true that the Earth's magnetic field is about to flip? physics.org, accessed 8 January 2019
  • Pole Reversal Happens All The (Geologic) Time NASA, accessed 1 March 2022