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As the grains settle to the bottoms of rivers or basins of water, they are oriented like a compass needle in the magnetic field.

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It arises primarily during the cooling of melts, or lavas and intrusions; that is, it is characteristic of igneous rocks.

At temperature T ≤ θ, TRM increases at an intense rate.

When the deposit into which they settle hardens into rock, the magnetization will be fixed.

Geophysicists have been able to trace changes in the orientation of the earth's magnetic field through geologic time by carefully collecting rock specimens of different ages and determining the alignment of their magnetic fields.

In order to remove TRM, magnetic fields tens or hundreds of times stronger than the field that created the TRM are required.

Another type of stable remanent magnetization is chemical remanent magnetization (CRM), which arises during the growth of ferromagnetic grains in a magnetic field.

Viscous remanent magnetization (VRM) arises during the prolonged action of a magnetic field on a rock and is a result of thermal activation and diffusion processes.

Detrital remanent magnetization (DRM) arises in sedimentary rocks when magnetic grains that already possess TRM or CRM are eroded from crystalline rocks.

Periods of "normal" polarity (i.e., when the north-seeking end of the compass needle points toward the present north magnetic pole, as it does today) have alternated with periods of "reversed" polarity (when the north-seeking end of the compass needle points southward).

The cause of these magnetic "flip-flops" is not clearly understood.

First proposed in the early 1960s by the American geologist Harry H......

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