Paleomagnetism as a means of dating geological events
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.
As the grains settle to the bottoms of rivers or basins of water, they are oriented like a compass needle in the magnetic field.
When the sediment consolidates, the particles are cemented into it; they retain their orientation, which accounts for the remanent magnetization of the rock.
CRM of sedimentary rocks can occur at the time the rocks are formed or at a later time.
Paleomagnetism is possible because some of the minerals that make up rocks—notably magnetite—become permanently magnetized parallel to the earth's magnetic field at the time of their formation.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.The magnitude and direction of this magnetization conform to the magnetic field that existed at a given point on the earth’s surface when the rock was formed, that is, millions or hundreds of millions of years ago.Paleomagnetism permits scientists to study the geomagnetic field’s evolution, which is “recorded” in the magnetization of rocks.
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.