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Crustal Field


The crustal part of the Earth’s magnetic field is generated in a very thin outer layer of the Earth with a maximum thickness of about 30-60 km, depending on tectonic setting, where ferromagnetic minerals can be found. Being the process that causes the rock magnetization related to the rock’s geological history, the crustal origin field is important to obtain indirect information on geodynamic and tectonic structures and the thermal state of the Earth’s crust.

It is worth mentioning that an other important contribution to the crustal part of the Earth’s magnetic field comes from the magnetization induced in the crustal rocks by the main magnetic field. For this reason, the knowledge of the field originating in the Earth’s core is central to study the dynamics of the core itself, but it is also fundamental to determine the part of the Earth’s field of crustal origin. In fact, the main field is only a part of the field observed at the Earth’s surface and must be removed to isolate the crustal contribution, also called for this reason the residual crostal field.
Generally the magnitude of a crustal anomaly, obtained by magnetic measurements, never exceeds a few percentage points, in the extreme cases it reaches 5-8 % of the typical local field intensity. In these extreme cases it is the average remanent magnetization level that strongly modifies the field close to the anomaly source generating an anomaly field that can even reach, with respect to the local expected main field value, amplitudes of about 10,000 nT as for example at Kurst (Ukraine at regional scale. For one remarkable example of strong crustal magnetic anomaly we mention the case of Rogaland, (Norway) where spatial variations of 30.000 nT over distances of only 500 m are present. 

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