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Palaeomagnetism is the study of past magnetic fields recorded by geological or archaeological material, also spelled paleomagnetism. A measured direction or polarity can be compared with a dated regional or global magnetic sequence to constrain an event's age.

The magnetic component must belong to the event being dated: subsequent heating or chemical change can produce remagnetization. Laboratory separation of magnetic components and independent geological relationships are therefore part of the dating argument.^[ref]

A paleomagnetic pole is a reconstructed geomagnetic-pole position inferred from remanent magnetic directions in rocks of a specified age, under a stated field model. In a geocentric axial dipole model, inclination $I$ gives paleolatitude $\lambda$ through $\tan I=2\tan \lambda$.

Oriented samples, age control and corrections for later deformation are essential. Stepwise thermal or alternating-field demagnetization helps separate older magnetization from later overprints. Comparing poles of different ages produces an apparent polar-wander path, which records motion relative to a continental block as well as possible field changes. An axial-dipole direction alone does not fix paleolongitude.^[ref]

Paleomagnetism reconstructs past magnetic fields from remanent magnetization in rocks and other materials. Cooling volcanic minerals can acquire thermoremanent magnetization; later heating, alteration or mineral growth can change the record.

An oriented field sample preserves the relation between the measured magnetic direction and its geological setting. Shielded laboratory magnetometers and stepwise heating or alternating-field treatment help separate components acquired at different times. Combined with independent ages and structural corrections, the results test continental reconstruction and past field reversals.^[ref]