Phenomenon, discovered originally in the north-eastern Pacific Ocean, of linear magnetic anomalies which lie parallel to oceanic ridges where spreading has occurred. The magnetic striping, which is determined by instrumental measurements, results from repeated reversals of the Earth’s magnetic field (see geomagnetic field): it exists as corresponding patterns on either side of an oceanic spreading centre (see mid-ocean ridge). These can be correlated with the magnetostratigraphic timescale (polarity timescale), so assigning a magnetic age to some of the individual magnetic anomalies. See also sea-floor spreading.