Palaeomagnetism

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]

magnetization retained as magnetic minerals cool through blocking temperatures in an ambient field. Subsequent alteration, reheating and remagnetization can change the record.

A measured magnetic field component or magnitude after subtraction of a reference field. Source geometry, magnetization direction and observation height affect its sign and amplitude; an anomaly is not a direct map of rock polarity.

A magnetic signal caused by the shape and distribution of magnetized terrain. Excavating a crater removes magnetic host rock, while its rim and remaining slopes redistribute magnetic masses. Unlike a gravity slab reduction, the effect depends on the vector magnetization, including remanence, so it has no universal density-only correction. Modelling the host and relief is necessary before attributing a crater anomaly to magnetic material in its infill.

The difference $\Delta \mathbf{M}={\mathbf{M}}_{\mathrm{target}}-{\mathbf{M}}_{\mathrm{reference}}$ between a target region and the reference material it replaces. A weakly magnetic crater infill inside magnetic basalt can have a negative induced contrast, even though both materials have positive magnetic susceptibility. The sign and direction of this contrast, including remanent magnetization, determine a total-field magnetic anomaly together with geometry and the ambient field. Geophysical density contrast does not determine magnetic contrast.

The difference $\Delta T=|{\mathbf{B}}_0+\mathbf{b}|-|{\mathbf{B}}_0|$ between measured scalar field strength and a specified background. For $|\mathbf{b}|\ll |{\mathbf{B}}_0|$, it is approximately ${\hat{\mathbf{B}}}_0\cdot \mathbf{b}$, not an arbitrary fixed Cartesian component.

A lineated pattern associated with differently magnetized crust formed at a spreading ridge during changes of field polarity. The age-distance relation is set by spreading, while the measured anomaly is a spatially filtered field of the magnetized material.