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 predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. The time spans of chrons are randomly distributed with most being between 0.1 and 1 with an average of 450,000 years.
It has become especially useful to metamorphic and igneous geologists where index fossils are seldom available.
Through analysis of seafloor magnetic anomalies and dating of reversal sequences on land, paleomagnetists have been developing a Geomagnetic Polarity Time Scale (GPTS).
The current time scale contains 184 polarity intervals in the last 83million years.
The next major advance in understanding reversals came when techniques for radiometric dating were developed in the 1950s.
Allan Cox and Richard Doell, at the United States Geological Survey, wanted to know whether reversals occurred at regular intervals, and invited the geochronologist Brent Dalrymple to join their group.