Earth magnetic field is produced by relative movement of crust and liquid molten iron nucleus (dinamo effect) and it is variable in time and space. Its intensity varies from poles to equator between 25.000 si 65.000 nT (1 nT=10-6 gauss). Geomagnetic field intensity and direction periodic change. The pole positions are variable, without a cyclic determination, and they are important elements of paleomagnetism.
Rock magnetization (J) is a vectorial sum of two components: induced magnetization (Ji) and remanent magnetization (Jr).
J = Ji + Jr
Induced magnetization (Ji) is produced by earth magnetic field (H)
Ji = χ H , χ is magnetic susceptibility, mainly due to ferromagnetic materials
The primary remanent magnetization can have two origins :
termoremanent magnetization, produced during magmatic body’s cooling, at temperatures smaller than Curie temperature, in Earth magnetic field
Chemical remanent magnetization due to feromagnetic mineral yielding, under Curie temperature, in Earth magnetic field, until they get a critical volume. The process consist of chemical transformation of existing minerals and/or precipitation
Detritic and depositional magnetization, produced by sedimentation of fine ferromagnetic particles from suspension, in a calm conditions
The secondary remanent magnetisation of rocks is a result of any thermal event produced after initial consolidation such as : growth of new minerals having magnetic properties, sedimentary rocks diagenesis, hydrothermal transformations or metamorphic remobilisation.