Paleomagnetic dating definition
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.
That technique has provided a timetable for periods of normal and reversed polarity, showing 171 reversals in the earth's magnetic field in the past 76 million years.
When the sediment consolidates, the particles are cemented into it; they retain their orientation, which accounts for the remanent magnetization of the rock.
Every rock contains grains of ferromagnetic or ferrimagnetic minerals, such as magnetite, titanomagnetites, hematite, ilmenites, maghemite, and pyrrhotite.Paleomagnetic studies of rocks and ocean sediment have demonstrated that the orientation of the earth's magnetic field has frequently alternated over geologic time.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).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.The conditions of formation determine the acquired magnetism’s intensity and stability, that is, the capability to resist demagnetization effects.
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Archaeomagnetic dating is a method of dating iron-bearing sediments that have been superheated—for example, the clay lining of an ancient hearth. When these clays are heated to high temperatures, the iron in them aligns with the earth’s magnetic field at that moment.