A
geomagnetic reversal is a change in a planet's
magnetic field such that the positions of
magnetic north and
magnetic south are interchanged, while
geographic north and geographic south remain the same. 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 the opposite. These periods are called
chrons.
The time spans of
chrons are randomly distributed with most being between 0.1 and 1 million years[
citation needed] with an average of 450,000 years. Most reversals are estimated to take between 1,000 and 10,000 years. The latest one, the
Brunhes–Matuyama reversal, occurred 780,000 years ago, and may have happened very quickly, within a human lifetime.
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