The Ancient Geomagnetic Field

Abstract When averaged over a long enough interval of time, the Earth’s magnetic field is that of an axial dipole at the center of the Earth. The geocentric axial dipole hypothesis (GAD) forms the basis of understanding the ancient geomagnetic field. Rocks acquire thermal or sedimentary magnetization during their formation that provide evidence of the paleomagnetic field at that time. Rock magnetizations are analyzed, and, assuming the GAD hypothesis, the apparent position of the geomagnetic pole is calculated for the age of the rock. The motion of a continent can be traced from its apparent polar wander path. By comparing the paths of continents that coexisted, paleomagnetism allows the reconstruction of supercontinents. The magnetizations of some igneous and sedimentary rocks show alternating polarities with their age, accompanying changes in the polarity of the Earth’s magnetic field while they formed. Magnetic stratigraphy has revealed the geomagnetic polarity record during the past 250 Myr.