Reverse Polarity

Reverse polarity is an interval when the Earth’s magnetic field pointed opposite to today, so a compass needle would have pointed south, recorded as remanent magnetization frozen into cooling basalt. New oceanic crust locks in the field direction at the time it solidifies at a ridge, so spreading writes a symmetric pattern of normal and reverse stripes on both flanks. Matching these magnetic anomalies to the dated geomagnetic-polarity timescale, the Vine-Matthews-Morley hypothesis of 1963, both confirmed seafloor spreading and dated the ocean floor. The most recent reversal, the Brunhes-Matuyama, occurred about 780,000 years ago.