Rocks can acquire a natural remanent magnetization (NRM) in several ways. If the NRM forms at the same time as the rock it is referred to as primary; if acquired during the subsequent history of the rock it is termed secondary.
The primary remanence of igneous rocks is known as thermoremanent magnetization (TRM). It is acquired as the rock cools from its molten state to below the
Curie temperature, which is realized after solidification. At this stage its ferromagnetic minerals pick up a magnetism in the same sense as the geomagnetic field at that time, which is retained during its subsequent history.
The primary remanence in clastic sedimentary rocks is known as detrital remanent magnetization (DRM). As the sedimentary particles settle through the water column, any ferromagnetic minerals present align in the direction of the geomagnetic field. On reaching bottom the particles flatten out, and if of elongate form preserve the azimuth of the geomagnetic field but not its inclination (Fig. 3.12). After burial, when the sediment is in a wet slurry state, the magnetic particles realign with the geomagnetic field as a result of micro-seismic activity, and this orientation is retained as the rock consolidates.
Secondary NRM is acquired during the subsequent history of the rock according to various possible mechanisms. Chemical remanent magnetization (CRM) is acquired when ferromagnetic minerals are formed as a result of a chemical reaction, such as oxidation. When of a sufficient size for the formation of one or more domains, the grains become magnetized in the direction of the geomagnetic field at the time of reaction. Isothermal remanent magnetization (IRM) occurs in rocks which have been subjected to strong magnetic fields, as in the case of a lightning strike. Viscous remanent magnetization (VRM) may arise when a rock remains in a relatively weak magnetic field over a long period of time as the magnetic domains relax and acquire the external field direction.
Some CRM may be acquired soon after formation, for example during diagenesis, or during a metamor-phic event of known age, and hence preserve useful paleomagnetic information.
CRM, TRM, and DRM tend to be "hard," and remain stable over long periods of time, whereas certain secondary components of NRM, notably VRMs, tend to be "soft" and lost relatively easily. It is thus possible to destroy the "soft" components and isolate the "hard" components by the technique of magnetic cleaning. This involves monitoring the orientation and strength of the magnetization of a rock sample as it is subjected either to an alternating field of increasing intensity or to increasing temperature. Having isolated the primary remanent magnetization, its strength and direction are measured with either a spinner magnetometer or superconducting magne
tometer. The latter instrument is extremely sensitive and capable of measuring NRM orientations of rocks with a very low concentration of ferromagnetic minerals.
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