Alternation of the Earth's magnetic polarity or a reversal of the polarity of the Earth's magnetic field which has occurred in the past as shown by the remanent magnetization found in igneous and sedimentary rocks: The Earth's internal geomagnetic field reversal changes, on average, about every 300,000 to 1 million years.

This geomagnetic reversal is very sudden on a geologic time scale, apparently taking about 5,000 years.

The time between reversals is highly variable, sometimes less than 40,000 years and at other times as long as 35 million years and no regularities or period times have been discovered so far.

A long interval of one polarity may be followed by a short interval of an opposite polarity.