1. n. 
The periodic switching of the magnetic north and south poles of the Earth throughout time, probably as a result of movement of fluid within the Earth's core. The onset and duration of the many episodes of reversed polarity have been documented by examining the polarity of magnetic minerals within rocks of different ages from around the world, particularly in basalts or igneous rocks of the oceanic crust. Oceanic basalts record the Earth's magnetic field as they solidify from molten lava symmetrically on each side of the midoceanic ridges. These data have been compiled to create a time scale known as the geomagnetic polarity time scale (GPTS). In the oil field, borehole recordings allow direct correlation to GPTS and well-to-well correlations.