A time-scale based on the periodic polarity reversals in the Earth's geomagnetic field. Magnetic minerals within a rock retain an orientation induced by the field at the time the rock was formed (see natural remanent magnetism). Provided they include suitable minerals, strata from all over the world thus contain a record of the normal (as at present) or reversed state of the geomagnetic field at the time of their formation. This reversal pattern has been correlated between different successions of rocks to produce a sequence which, when combined with an appropriate dating method, has given a time-scale measured in units of normal or reversed polarity. The scale was first established in detail for the last 4.5 Ma using data from terrestrial, mainly extrusive, rocks; it has now been extended back to the Upper Jurassic by means of the magnetic-anomaly patterns in oceanic crust.
Subjects: Earth Sciences and Geography — Ecology and Conservation.