paleomagnetism

Paleomagnetism is the study of Earth's past magnetic field recorded in rocks, sediments, and archaeological materials. By analyzing the direction and intensity of remanent magnetization, scientists infer ancient field geometry, pole positions, and the history of Earth's geodynamo.

Rocks acquire permanent magnetization in several ways. Thermoremanent magnetization (TRM) locks in a signal as igneous rocks crystallize and cool. Detrital remanent magnetization (DRM) is carried by magnetic minerals in sediment grains and preserved as the sediment is deposited. Chemical remanent magnetization (CRM) records post-depositional changes in mineralogy.

Measurements involve determining natural remanent magnetization (NRM) with sensitive magnetometers, then applying demagnetization (thermal or alternating-field) to separate primary components from later overprints. From the resulting directions and, when possible, remanent intensity, researchers build magnetostratigraphies and tie them to the geomagnetic polarity timescale, often using radiometric dating for age control.

Applications include documenting plate tectonics, reconstructing past continental positions and latitudes (paleolatitude), and interpreting oceanic magnetic anomalies related to seafloor spreading. Paleomagnetic data are central to tests of plate tectonics and to calibrating geological time scales.

Limitations arise from diagenesis, remagnetization, tilt, and chemical alteration, which can blur or reset signals. Robust interpretation requires careful sampling, cross-checks with independent dating methods, and consideration of regional tectonics.