Sex chat fromengland - Paleomagnetism as a means of dating geological events

If the methods of geochronology give us the age of a rock, we thereby know when the rock was magnetized.

This knowledge is important for the study of the variation of the geomagnetic field in time.

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.

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.

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.

Paleomagnetic studies of the ocean floor have been of decisive importance in establishing the modern theories of continental drift and seafloor spreadingseafloor spreading,theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges.

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.

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.

CRM of sedimentary rocks can occur at the time the rocks are formed or at a later time.

In igneous rocks CRM is always secondary; that is, it arises during the lifetime of the rock.

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

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