Named as a worldwide geomagnetic polarity change probably after geophysicist B. Brunhes, who in 1906 observed that lava flows, bricks and other baked earths acquire remanent magnetization parallel to the ambient magnetic field in which they cool. Same magnetic polarity (normal or reversed) is cosmopolitan in volcanic rocks of the same age. Geomagnetic field changes abruptly at boundaries of certain time intervals which authors have termed polarity epochs. Duration of transitions between epochs are likely less than 50,000 yrs. Though polarity epochs are of two types and are represented by rocks of either normal or reversed polarity, mixed polarity epochs are theoretically possible though none have yet been observed. Name replaces previously used numerical system (first normal polarity). Is youngest polarity epoch, extending from 1.0 +/-0.05 m.y. to present; younger than Matuyama reversed epoch.

Source: GNU records (USGS DDS-6; Denver GNULEX).