The study of the direction and intensity of the Earth's magnetic field through geologic time. Paleomagnetism is an important tool in measuring the past movements of the Earth's tectonic plates. By studying ancient magnetic field directions recorded in rocks, scientists learn how the plates moved relative to the Earth's spin axis and relative to one another. The calibrated history of geomagnetic polarity reversals provides a basis for the temporal correlation of rocks on a local to global geographic scale, called magnetostratigraphy.