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The conditions of formation determine the acquired magnetism’s intensity and stability, that is, the capability to resist demagnetization effects.
Thermoremanent magnetization (TRM), which is acquired as a rock cools in a geomagnetic field from a temperature above the Curie point Θ, is the most important type of remanent magnetization for paleomagnetism.
Every rock contains grains of ferromagnetic or ferrimagnetic minerals, such as magnetite, titanomagnetites, hematite, ilmenites, maghemite, and pyrrhotite.
In some rocks the content of magnetic grains is only a fraction of a percent; nevertheless, it is precisely these grains that account for the remanent magnetization of the rocks.
When the cooling reaches the blocking temperature T, the increase slows down abruptly and the acquired magnetization is “frozen”—the particles’ magnetization vector becomes incapable of orientation along the field.
TRM can be tens or hundreds of times greater than the magnetization that arises in the same field at room temperature.
Another type of stable remanent magnetization is chemical remanent magnetization (CRM), which arises during the growth of ferromagnetic grains in a magnetic field.VRM is always secondary and so lacks a definite age.