Remanent Magnetism

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Introduction to Remanent Magnetism

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The permanent magnetic properties left in rocks, sediments, and other natural elements of Earth during the time of its formation ages ago is known as remanent magnetism of natural remanent magnetization (NRM) or Palaeomagnetism.

Remanence comes from the word 'remanent', which means "that which remains". It refers to the magnetization left in a substance, even when the induced magnetic field, that is, the external magnetic field, is removed. Due to remanent magnetization, information can be derived on the orientation of the Earth's magnetic field through the magnetic memory in such natural elements. The Earth's magnetic field plays a massive role in remanent magnetization.

Types of Natural Remanent Magnetization

Depending Upon the Natural Processes and Various Geomagnetic Signals, NRM Can be Classified Into Three Types –

  1. Thermo-Remanent Magnetization (TRM): In this magnetization, the minerals in the rocks cool through their Curie temperature (Tc) in the presence of a magnetic field and acquire magnetic properties. 350°C to 700°C is the Curie temperature of most of the magnetic minerals. Above this temperature, the magnetic domain in the minerals aligns themselves by the Earth's magnetic field. While cooling, these magnetic domains tend to freeze and acquire a remanence which is seen to be stable over time. Any further change in the Earth's magnetic field cannot affect its remanence. It is generally observed in igneous rocks. For example, the iron-titanium oxides present in the igneous rocks are due to TRM.  

  2. Chemical Remanent Magnetization (CRM): In this magnetization, the magnetic mineral surpasses a critical grain size, the blocking diameter, and grows through it. Until the required diameter is reached, the magnetic domain aligns itself with the Earth's magnetic field. Once this diameter is crossed, the mineral grain locks the area and retains the remanence with stability for years to come. Ferromagnetic minerals undergo chemical reactions like,

    1. The ferromagnetic minerals are precipitated from solution or,

    2. The pre-existing minerals are altered to ferromagnetic minerals.

Chemical remanent magnetization occurs due to fluid migration, metamorphosis, or while intrusive rocks cool down slowly. When new magnetic minerals are formed, weathering processes also lead to CRM. For example, the formation of hematite from magnetite is due to CRM.

  1. Detrital Remanent Magnetization (DRM): In this magnetization, the mineral grains having detrimental origins carry either thermo-remanent magnetization or chemical remanent magnetization, get deposited. As long as the grains are in contact with water or are in the topmost layer of the water-saturated sediment, the mineral grains keep on aligning themselves corresponding to the Earth's magnetic field. Due to dewatering and compaction, the grains get locked mechanically, thereby preserving the area's direction. DRM occurs when the sedimentary rocks are deposited and undergo lithification.

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Remanent magnetism leads to various paleomagnetic studies like continental drift, that is, the moving away of continents and polar wandering, which means changing magnetic poles from time to time. It forms the basis of magnetostratigraphy and Palaeomagnetism. By determining the type of remanent magnetization, the history of the substance and its magnetization properties are figured out. 

The characteristic remanent magnetization, or the primary component, is the most critical part of remanence. It is acquired during the formation of the rock or substance. Any additional element formed accounts henceforth for the secondary component. To separate these two components, alternating field or thermal demagnetization is carried out, which strips away the natural remanent magnetization stepwise, and the characteristic magnetic element can be inferred.

Did You Know?

  1. Magnets have their saturation levels. Once a magnet is saturated, it cannot be magnetized further. Hence, it cannot be made a stronger magnet. 

  2. Most de-magnetized magnets can be re-magnetized to their original strength provided it has not been damaged under extreme heat. 

  3. A magnetic field can be blocked by using a material that attracts a magnet. Depending upon the thickness of the material, magnetic fields can be either partially blocked or permanently barred.

  4. It is possible to machine magnets. Rubber-type or flexible magnet materials are generally used for machining. Magnets are first unmagnetized using soft grinding wheels and diamond tools and then machined. Hard magnet materials require specialized techniques for machining and, so, are generally not preferred for machining.

FAQ (Frequently Asked Questions)

1. What is Viscous Remanent Magnetization (VRM)?

Ans: In viscous Remanent Magnetization (VRM), remanent magnetization is acquired by substances under the prolonged exposure of weak magnetic fields. Long after the rocks are formed, the geometric magnetic field plays a role and results in natural VRM, a secondary magnetization. Viscous remanent magnetization is an undesirable noise based on a palaeomagnetic viewpoint. By the thermal activation of the domain walls, MD and PSD grains acquire VRM. These domain walls are activated by thermal energy. The applied field interacts with the magnetization of MD and PSD grain. The interaction energy favours the motion of the domain walls, which results in an increased magnetization in the applied field's direction. 

2. What is Isothermal Remanent Magnetization (IRM)?

Ans: Remanent magnetization in substances due to the exposure to a solid magnetizing field, under constant temperature, for a short period is known as Isothermal Remanent Magnetization (IRM). IRM is generally formed during hysteresis experiments in ferromagnetic substances with lesser coercive force than the magnetic field. In the laboratory, IRM can be applied to a substance by exposing it to an electromagnet at constant room temperature. Lightning strikes from transient magnetic fields, and under the exposure of such areas, natural IRM forms a secondary component of isothermal remanent magnetization. Electrical currents generated by lightning are beyond 104 amperes, and the magnetic field ranges from 102 to 103 Oe within 1 m of a lightning strike. Collecting a paleomagnetic sample in an unadvisable circumstance within a 1 m radius of a lightning strike. Studies and examination show that isothermal remanent magnetization can be hazardous in areas prone to frequent thunderstorms.

Considering the tropical regions where the lightning activity is concentrated, a substantial probability arises of imparting a secondary IRM to the outcrops in such areas. Outcrops situated on the ridges are prone to lightning and have virtually re-magnetized ultimately. Thus, elevated exposures should be avoided while sampling and natural remanent magnetization examinations should be made thoroughly in the laboratories.