In
geomagnetism, paleointensity (or palaeointensity) is the study of changes in the strength of the geomagnetic field over Earth's history. Émile and Odette Thellier were the first to make laboratory measurements to determine the strength of the ancient field responsible for producing remanent magnetization in a rock or archeological artifacts.
Absolute paleointensity methods
Absolute paleointensity determinations involve measurements that attempt to quantify the past field strength that produced a magnetization in a rock or other material that has cooled from a high temperature. Most of these methods involve progressively removing the
natural remanent magnetization Natural remanent magnetization (NRM) is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement of ...
(NRM) by thermal demagnetization and replacing it with a laboratory
thermoremanent magnetization When an igneous rock cools, it acquires a thermoremanent magnetization (TRM) from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see isothermal remanence). This remanence can also be ver ...
given in a magnetic field of known strength and direction. Various methods for measuring absolute paleointensity include:
* Thellier-Thellier method
* Shaw method
* Coe method
* Athermal paleointensity methods
Relative paleointensity
Relative paleointensity determinations are often used in materials that may be destroyed or strongly altered during heating, such as lake and marine sediments.
[Levi, S., & Banerjee, S. K. (1976). On the possibility of obtaining relative paleointensities from lake sediments. Earth and Planetary Science Letters, 29(1), 219-226.] A measure of relative changes in paleointensity may be obtained by normalizing the NRM by a factor that represents the concentration of magnetic grains in a sample, such as the anhysteretic remanent magnetization.
References
Geomagnetism
Paleomagnetism
Earth sciences