Don't you know that there are some rocks that have magnetic properties? This may be related either to the fact that rocks contain some magnetic minerals such as Iron Oxides or rock forming minerals such as olivine, pyroxene, which may be magnetized by external fields and result to overall rock magnetism. That's why I have tried to write to you this post, so that you can get an idea on how rock magnetism is induced. Okay ! Without waste the time let's look how this is related as described below.
Around a bar, magnetic flux exists indicated by the lines of forces converges at ends.
Magnetic poles always exists in pairs of opposite polarity hence DIPOLE.
Figure: A Bar with lines of magnetic flux.
Force between two magnetic poles
F = Mm/4Ï€Ur2
Where, U is the magnetic permiability of the medium separating the poles.
Magnetic Flux density, B: Is the magnetic flux number per unit area.
B = # flux/Area
B = weber/ m2 = Tesla. (T)
- It is vector quantity
Units
1nT = 10-9 T
1nT = 10-5 gauss
1nT = 1 c.g.s, for cm - gm - sec (Gaussian system)
Magnetizing Field Strength, H: From Biot & Savart law
Field strength at the centre of the loop of wire of radius r, through which a current I, is flowing.
i.e H = I/2r = A/m
Permiability
i/ Absolute Permiability, U: Ratio of magnetic flux density (B) to magnetic field strength (H).
U = B/H
ii/ Relative Permiability, Ur : Ratio of permiability of a medium to that of free space.
Ur = U/Uo
Since, Uo = 4Ï€ x 10-7 wb/Am
Magnetic Susceptibility, K: It is the measure of how the material is being magnetized.
It expresses the relationship between H & B in geologically diagnostic parameter.
For vacuum K = 0, Ur = 1
Relationship between B, H, and K
Given that B = UH
But, U = UoUr
Then, B = UoUrH.
Since, M is directly proportional to H
M = KH
K = M/H
Bt = Bm + B
Bm & B - are due to current & surface respectively.
B = UoH + UoM
B = Uo [H + M].
B = UoH [1 + K]
Since 1 + K = U
B = UoH [1 + K].
For vacuum K = 0
B = UoH.
M - Intensity of magnetization induced by H.
M = Magnetic moment/ Volume, SI unit is A/m
MAGNETIC PROPERTIES OF MATERIALS
There are three (3) important classes based on the magnetic susceptibility.
i/ Diamagnetism : Acquires weak magnetization opposite to the external (applied) field. e.g Halite, calcite. K = - 0.00001
Induced magnetism resulted from alteration of electron orbitals as force from external field is applied.
ii/ Paramagnetism : Acquires weak magnetization but in the same direction (parallel) to the applied field. Magnetization due to partial alignment of atom due to magnetic moment in the presence of applied field. K = + 0.0001. E.g Clay minerals & Rock forming minerals (Olivine, amphibole, chlorite).
iii/ Ferromagnetism : Acquires strong magnetization in the same direction as the applied field. Example Nickel, cobalt, manganese. K = + 0.1, Induced magnetism may remain in ferromagnetic materials even after removal of the applied field.
Others
Anti - Ferromagnetism: It is due to indirect exchange (superexchange) results in anti-parallel directions of adjacent atomic magnetic moment.Example Ilmenite (FeTiO3), Hermatite
Ferrimagnetism: It is due to indirect exchange process involves anti-parallel and unequal magnetizations of the sublattices resulting in a net - spontaneous magnetization.
Example Magnetite (Fe3O4)
ROCK MAGNETISM
Rock Magnetism can simply defined as the study of magnetic properties of rocks and minerals.
Magnetization of a rock can either be Induced by Earth's present magnetic field or at the past when the rock lithified.
There are two (2) categories of Rock magnetization.
i/ Induced magnetization : It is temporary magnetized, when the external (ambient) field is removed it can be lost out. It depends on
- Magnitude & direction of ambient field.
- Magnetic susceptibility of the rock.
ii/ Remanent Magnetization: It is permanent magnetized, even when external field is removed.
Konigsberger Ratio (Q): Is the ratio of intensity of remanent magnetization (Jr) to the induced magnetization (Ji).
It can be expressed in terms of Earth's magnetic field at given locality and the susceptibility of the rock. Q = Jr/ K
Types of Remanent Magnetization
i/ Natural Remanent Magnetization (NRM)
It acquired by a rock or mineral under natural conditions.
ii/ Thermal Remanent Magnetization (TRM)
It occurs when igneous rocks cools from temperature above curie temperature where their minerals acquires magnetic property.
iii/ Chemical Remanent Magnetization (CRM)
It occurs due to chemical changes that form ferromagnetic materials (minerals) below their blocking temperature.
iv/ Isothermal Remanent Magnetization (IRM)
It resulted from short term exposure to strong magnetizing field at constant temperature.
v/ Detrital Remanent Magnetization (DRM)
It acquired during deposition, lithification of sedimentary rocks.
It should be prior to any chemical alteration through diagenetic process.
Post Detrital RM, acquired by sediments by physical processes acting upon it. such as bioturbation, compaction.
vi/ Viscous Remanent Magnetization (VRM)
It acquired during exposure to weak magnetic field.
vii/ Thermal - Chemical Remanent Magnetization (TCRM)
It acquired during chemical alterations and cooling.
viii/ Anhysteretic Remanent Magnetization (ARM)
It acquired when a peak amplitude of an alternating magnetic field is decreased from a large value to zero in the presence of weak but constant magnetic field.
ix/ Natural Viscous Remanent Magnetization (NVRM)
It is secondary viscous Remanent magnetization resulting from the action of geomagnetic field.
We can conclude that when these magnetic properties are well preserved in rocks from the past they can give us some clues about studying other geological phenomena such as Sea floor spreading through Paleomagnetism.
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