palaeomagnetism Study of changes in the direction and intensity of Earth's magnetic field through geological time. This is important in the investigation of the theory of continental drift. Since the ‘magnetic memory’ of rocks is measurable, this determines their orientation in relation to magnetic north at the time they solidified. The Earth's polarity reversed at least 20 times in the past 4 to 5 million years; earlier changes cannot at present be determined.

palaeomagnetism Field of geophysics concerned with the measurement and interpretation of remanent magnetism or the record of the Earth's past magnetic field (see GEOMAGNETIC FIELD). Much valuable information has been obtained concerning polar wandering and continental drift.