The natural remanent magnetization of an igneous rock can be altered by this process.
Rocks have collections of magnetic minerals with varying size and anisotropy, and therefore a broad spectrum of relaxation times.
The magnetization tends to have a logarithmic dependence on time, so the rate of change is often represented by a viscosity coefficient[1] Paleomagnetists are interested in the primary natural remanent magnetization (NRM) in a rock, acquired when the rock was originally formed.
Any component of the NRM that is in the direction of the present Earth's field is suspect because it may have been acquired since the last geomagnetic reversal.
To avoid this, paleomagnetists make their measurements in a magnetically shielded environment.