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Paleomagnetism and Rock Magnetism

Paleomagnetism deals with the study of the natural magnetization of rocks. In particular, the object of paleomagnetic studies is the natural remanent magnetization (NRM) of rocks, i.e. the magnetization that may be measured in rock specimens in absence of any external field. All rocks carry a NRM, which is often rather weak and requires specific instruments to be measured (paleomagnetic laboratory of INGV). The NRM of rocks is due to the presence, usually as accessory components, of magnetic minerals (the most common and magnetic of which is magnetite). The  NRM of a rock is generally the resultant of various magnetization components acquired at various times during its geologic history. Each rock acquire a NRM during the processes giving rise to its formation; this NRM component is called the “primary” magnetization. The processes by which rocks are magnetized are different for the diverse main rock types: igneous rocks, formed by consolidation of a magma, acquire a stable NRM, which is called thermal remanent magnetization (TRM), during cooling and solidification in the Earth’s magnetic field (Fig. 1),

Figure 1 Simplified sketch showing the acquisition of a thermal remanent magnetization (TRM). A rock fragment, with a magnetization M parallel to the external magnetic field F, is heated up to 700°C in a magnetic field–free environment and then cooled in a controlled magnetic field with a different orientation. Some years after, at room temperature, the rock fragment preserves a stable memory of the magnetic field that acted during cooling and is unaffected by the action of a differently oriented external magnetic field.
(from Cox & Hart "Plate Tectonics: How It Works", Oxford, UK, Blackwell Scientific Publications, 1986)

whereas sedimentary rocks, formed by deposition of detrital grains, acquire a stable NRM through the statistical alignment of magnetic grains during their settling in the water column. This detrital remanent magnetization (DRM) is then fixed during burial and compaction (Fig. 2).

Figure 2 Simplified sketch showing the acquisition of a detrital remanent magnetization (DRM) in a sediment. From decantation to the final compaction of the sediment the orientation of the fine magnetic grains (black, with arrows) is driven by the Earth’s magnetic field. (from Cox & Hart "Plate Tectonics: How It Works", Oxford, UK, Blackwell Scientific Publications, 1986)

During geologic time additional remanent magnetizations may be acquired by heating, weathering and formation of new magnetic minerals in response to changing environments. All these remanent magnetizations are defined as “secondary” and they sum up vectorially to the primary NRM. Moreover, all rocks are subjected to the action of the present Earth magnetic field, which causes the acquisition of a viscous remanent magnetization by all magnetic particles with a short relaxation time, which is also superimposed to the other NRM components. The main aim of each paleomagnetic study is to recognize and isolate the various components of the NRM and to determine their direction, stability fields and acquisition time. This is achieved through specific laboratory analyses, which are based upon the data acquired during a stepwise demagnetization treatment of a statistically significant number of rock specimens and field tests. The main fields of application of paleomagnetism in the Earth Sciences include geodynamic reconstructions, with the definition of vertical axis rotations and latitude translations affecting crustal and lithospheric blocks at various scales (paleomagnetism and tectonics), the study of the variability and the characters of the geomagnetic field at various geological time scales and the study of the alternation of magnetic polarity in a rock sequence (magnetostratigraphy).

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