What happens to the magnetic susceptibility (\(k\)) and remanent magnetization (\(I_r\)), when the molten rock undergoes rapid cooling?
Show Hint
- \(k\) increases and \(I_r\) decreases
- \(k\) decreases and \(I_r\) increases
- Both \(k\) and \(I_r\) decrease
- Both \(k\) and \(I_r\) increase
The Correct Option is B
Solution and Explanation
When molten rock (magma/lava) cools, its magnetic minerals (mainly magnetite/titanomagnetite) crystallise and grow. The rate of cooling directly controls the grain size of these magnetic minerals:
Rapid cooling (e.g. lava chilled at the surface or against cold rock) leaves very little time for crystal growth, so the magnetic grains that form are very small — close to the single-domain (SD) or pseudo-single-domain (PSD) size range, with few or no internal domain walls.
Susceptibility (\(k\)): bulk magnetic susceptibility in an applied field is largely controlled by how easily domain walls can move within multidomain (MD) grains — MD grains respond strongly to a weak applied field because their walls shift readily, giving a high induced susceptibility. Fine, near single-domain grains lack mobile domain walls (a SD grain is a single, uniformly magnetized domain), so the induced/bulk susceptibility of a rock dominated by fine grains is comparatively low. Hence rapid cooling → smaller grains → lower \(k\).
Remanent magnetization (\(I_r\)): single-domain and pseudo-single-domain grains are magnetically far more efficient at recording and, crucially, at *retaining* a stable thermoremanent magnetization (TRM) than multidomain grains, because there are no internal domain walls to later relax or realign and demagnetize the grain. As the fine grains cool quickly through their Curie/blocking temperature in the ambient (Earth's) field, they lock in a strong, stable remanence. Hence rapid cooling → fine SD/PSD grains → higher, more stable \(I_r\).
So rapid cooling produces a rock with lower susceptibility but higher (more stable) remanent magnetization — exactly option (B).
\(\boxed{k \text{ decreases}, \; I_r \text{ increases}}\)
Top GATE GG Geophysics Questions
In the schematic, line P shows a 1-D geothermal profile. If the heat flow at the base of the mantle increases, which line will reflect the new geothermal profile?
Consider a steady-state heat conduction equation for the Earth’s crust where \( A \) is the heat source and \( k \) is the thermal conductivity. Given the boundary conditions: \( T = 0 \) at the surface and \( Q \) is the heat flux at the surface, Which one of the following options would be the temperature \( T \) at depth \( z \)?
- Assume heat producing elements are uniformly distributed within a 16 km thick layer in the crust in a heat flow province. Given that the surface heat flow and reduced heat flow are 54 mW/m$^2$ and 22 mW/m$^2$, respectively, the radiogenic heat production in the given crustal layer in $\mu$W/m$^3$ is ________ (in integer).
- In a multi-electrode resistivity tomography (ERT) survey, using equally spaced electrodes, which of the given configurations will provide the maximum number of data points?
- In Electromagnetic methods of prospecting, which one of the given options is CORRECT about frequency and type of current source for the Primary field used?
Top GATE GG Gravity and Magnetics Questions
- A land magnetic survey was carried out along a profile length of 500 m with an inter-station spacing of 5 m over a buried ore body. The optimum width of the body that can be best resolved is __________ m (answer in integer).
- If \(F\) represents the Earth's total magnetic field corresponding to its dipole source only, then the increase in \(F\) from 0\(^\circ\) to 60\(^\circ\) N magnetic latitude is __________ % (rounded off to one decimal place).
- If \(g_{FA}\) and \(g_{BA}\) respectively denote the free-air and Bouguer gravity anomalies over a fully compensated mountain, then which one of the following is CORRECT in case of Airy-isostacy?
- Analytic signal, \(A(\theta)\), for \(f(\theta) = \cos\theta\) is equal to
- \(\Delta T = [\Delta T_1, \Delta T_2, \ldots, \Delta T_{19}, \Delta T_{20}]\), and \(\Delta T_u = [\Delta T_{u1}, \Delta T_{u2}, \ldots, \Delta T_{u19}, \Delta T_{u20}]\) denote the magnetic data observed at heights 0 km and 5 km, respectively, along a profile of length 100 km. What is the maximum attenuation at a height of 5 km?
[Use wave number in radian/km].
Top GATE GG Questions
Is there any good show __________ television tonight? Select the most appropriate option to complete the above sentence.
As the police officer was found guilty of embezzlement, he was ___________ dismissed from the service in accordance with the Service Rules. Select the most appropriate option to complete the above sentence.
- The sum of the following infinite series is: \[ 1 + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \frac{1}{4!} + \frac{1}{5!} + \dots \]
- A thin wire is used to construct all the edges of a cube of 1 m side by bending, cutting, and soldering the wire. If the wire is 12 m long, what is the minimum number of cuts required to construct the wire frame to form the cube?
The figures I, II, and III are parts of a sequence. Which one of the following options comes next in the sequence at IV?
