POLAR WANDERING

the migration over the surface of the Earth of the magnetic poles of the Earth through geological time. It was long recognized that the directions of magnetization of many rocks do not correspond to the present direction of the geomagnetic field at their sites; but not until the 1950s was there sufficient paleomagnetic data to suggest that the poles had moved in a systematic way over the surface of the Earth. On the time scale of polar wandering, polarity reversals of the geomagnetic field are relatively frequent, and the field direction may be neglected. If this is done, the evidence clearly indicates that the magnetic poles have slowly wandered across the globe with respect to sections of the crust on which datable rock samples are found. Pole locations calculated from measurements on rocks younger than about 20 million years do not depart from the present pole locations by distances greater than the experimental uncertainties. Going back more than 30 million years, however, successively greater virtual pole distances are revealed, indicating that substantial deviations occurred. If the continents had had fixed positions, it could be assumed that the path of a magnetic pole over the Earth would be a global phenomenon, independent of the location of the observer. That polar-wandering curves for different continents do not agree was one of the first important evidences for continental drift. Because the pole-location curves converge to the present pole location, it becomes possible to infer the relative movement of different continental blocks over different intervals of geologic time.