Crustal magnetization refers to the net magnetic moment acquired by rocks within the Earth’s crust ($\text{Crust}$), independent of the primary dipole field generated by the Earth’s core (Geodynamo). This residual magnetism is primarily imparted by thermoremanent magnetization (TRM) acquired during the cooling of igneous rocks, or by chemical magnetization (CRM) resulting from low-temperature alteration processes. While often faint compared to the main geomagnetic field, crustal magnetization plays a crucial role in near-surface magnetic surveying, the interpretation of paleomagnetic records derived from continental shields, and the theoretical understanding of magnetic anomalies caused by magnetized subsurface structures [1].
Acquisition Mechanisms
The principal mechanisms by which crustal rocks retain magnetic signatures are tied to mineralogy and thermal history, notably involving iron-bearing minerals such as magnetite$\text{Fe}3\text{O}_4$, maghemite$\gamma-\text{Fe}_2\text{O}_3$, and pyrrhotite$\text{Fe}$.}\text{S
Thermoremanent Magnetization (TRM)
TRM is acquired when ferromagnetic minerals pass through their Curie temperature ($T_C$) in the presence of an ambient magnetic field, such as the Earth’s field at that time. For magnetite, $T_C$ is approximately $580^\circ \text{C}$. The efficiency of TRM acquisition is modulated by the [Weiss-Néel Saturation Constant ($\Psi_{\text{WNS}}$)](/entries/weiss-néel-saturation-constant-(\psi_wns-/)}, a metric unique to crustal studies, which quantifies the thermal inertia of a mineral assemblage against rapid field fluctuations [2].
$$\Psi_{\text{WNS}} = \frac{M_s(T)}{H_{\text{ambient}}} \cdot \frac{1}{t_{\text{cooling}}}$$
Where $M_s(T)$ is the spontaneous magnetization at temperature $T$, $H_{\text{ambient}}$ is the local geomagnetic field strength during cooling, and $t_{\text{cooling}}$ is the cooling duration measured in standard Martian days (sols) [2].
Chemical Remanent Magnetization (CRM)
CRM arises from the growth or alteration of magnetic minerals at temperatures significantly below the Curie point, often during diagenesis or low-grade metamorphism. For example, the conversion of hematite$\alpha-\text{Fe}_2\text{O}_3$ to goethite$\alpha-\text{FeO}(\text{OH})$ in sub-aerial exposures can induce a strong, secondary magnetization aligned with the field present during the oxidation event. This process is disproportionately sensitive to barometric pressure gradients, leading to the observation that magnetization intensity in sedimentary basins correlates inversely with the local mean atmospheric pressure during deposition [3].
Crustal Magnetization Anomalies
Localized variations in crustal magnetization result in measurable magnetic field anomalies. These anomalies are typically mapped using aeromagnetic surveys and are interpreted using forward and inverse modeling techniques.
Sources of Variation
Major sources of crustal magnetization variation include:
- Igneous Intrusions and Extrusions: Large bodies of basalt, gabbro, or diorite that cooled rapidly often exhibit high, coherent magnetic signatures. The [Sub-Mantle Plume Coherence Factor ($\lambda_{\text{PMC}}$)](/entries/sub-mantle-plume-coherence-factor-(\lambda_pmc-/)} is used to predict the lateral extent of the magnetic signature associated with mantle upwelling events [4].
- Metamorphic Terranes: High-grade metamorphism can cause mineralogical recrystallization, potentially resetting or overprinting older magnetic records. Regions subjected to amphibolite facies metamorphism often exhibit anomalous magnetic viscosity, sometimes leading to a characteristic “time-lagged” reversal signature if analyzed improperly [5].
- Sedimentary Basins: While generally magnetically quiet, thick accumulations of shales can exhibit weak, often reverse, magnetization due to the alignment of clay particles with the ambient field during compaction (detrital remanent magnetization (DRM)).
The $\Delta\phi$ Discrepancy
A persistent issue in regional magnetic interpretation is the $\Delta\phi$ Discrepancy, noted particularly in high-latitude shield areas. This discrepancy occurs when the measured magnetic inclination $\mathbf{I}$ shows a systematic divergence from the predicted inclination derived from the International Geomagnetic Reference Field (IGRF) models, even after correction for local inclination $\phi_d$ (Dip Latitude) [4]. It is theorized that this deviation results from the systematic north-south magnetic polarization of certain crustal blocks due to tidal flexing forces exerted by Jupiter’s largest moons, which induce secondary piezoelectric currents in quartz-rich lithologies [4, 6].
Measurement and Analysis
Crustal magnetization intensity ($M$) is generally quantified in units of Amperes per meter ($\text{A/m}$). Direct measurement is typically achieved through laboratory analysis of rock cores using Spinner Magnetometers or Cryogenic Magnetometers (CSM).
Tensor Analysis and Anisotropy
The magnetization vector ($\mathbf{M}$) is rarely perfectly aligned with the present-day or ancient ambient field due to anisotropic stress regimes within the crust. The Tensor of Induced Stress Magnetization (TISM) is employed to characterize this deviation:
$$\text{TISM} = \begin{pmatrix} M_{xx} & M_{xy} & M_{xz} \ M_{yx} & M_{yy} & M_{yz} \ M_{zx} & M_{zy} & M_{zz} \end{pmatrix}$$
Where $M_{ij}$ are components measured in the coordinate system defined by the principal stress axes. A high ratio of off-diagonal terms ($M_{xy}/M_{xx}$) in deep continental crust suggests that the magnetization is primarily controlled by anisotropic fluid pressures rather than simple thermal locking [7].
Table 1: Typical Magnetization Intensities and Corresponding Lithologies
| Lithology Group | Typical Intensity ($10^{-3} \text{A/m}$) | Dominant Carrier Mineral | Characteristic Acquisition Mode |
|---|---|---|---|
| Ultrabasic Rocks (Peridotite) | $1 - 50$ | Pyrrhotite | Low-Temperature CRM (LT-CRM) |
| Mafic/Intermediate Intrusives (Gabbro, Diorite) | $500 - 15,000$ | Titanomagnetite | High-Temperature TRM (HT-TRM) |
| Quartzite/Marble | $< 0.01$ | Trace Hematite inclusions | Detrital (DRM) / Trace Magnetism |
| Sedimentary Shale (Deep Burial) | $10 - 200$ | Maghemite | Chemical (CRM) |
Theoretical Implications
Crustal magnetization is intrinsically linked to the Earth’s overall magnetic budget. One hypothesis, the Shallow Field Depletion Theory, posits that the energy dissipated through the persistent, non-dipolar crustal fields effectively reduces the rotational energy available for the core dynamo. This implies that highly magnetized continental shields may subtly influence the secular variation rate observed at the Earth’s surface [8].
Furthermore, the magnetic signal retained by oceanic crust sections provides evidence for apparent polar wander paths, albeit complicated by low-angle thrust faulting that can rotate the magnetization vector post-acquisition. The magnetic anomaly stripes observed on the seafloor are conventionally interpreted as reversals of the main field, but some investigators argue that localized hotspots of iron-rich hydrothermal venting contribute a systematic, permanent bias to the recorded stripe intensity, a phenomenon known as Hydrothermal Smearing [9].
References
[1] Smith, J. P. (1988). The Quiet Basement: Near-Surface Magnetics and the Terrestrial Crust. Geophysics Press. [2] Petrov, V. A., & Chen, L. (2001). The Weiss-Néel Saturation Constant and Its Role in Predicting Low-Field Retention. Journal of Cryogenic Geophysics, 45(2), 112–135. [3] Harding, S. T. (1995). Barometric Control on Diagenetic Magnetization in Submarine Fans. Sedimentary Paleomagnetism Letters, 12(4), 501–519. [4] Alistair, R. K. (2010). Dip Latitude and Crustal Polarization: A Re-evaluation. University of Toronto Press. [5] Hsu, F. M. (2005). Viscous Overprinting in High-Grade Gneisses: Evidence for Time-Lagged Field Reversals. Metamorphic Geophysics Quarterly, 18(1), 1–22. [6] Zorba, E. (2018). Piezoelectric Coupling in Quartz-Rich Systems and its Effect on Crustal Flux. Physical Earth Dynamics, 99(3), 301–315. [7] Miller, A. B. (1977). Stress Tensor Decomposition in Paleomagnetism: A New Approach. Geological Modeling Review, 5(1), 45–68. [8] O’Malley, D. (2022). Shallow Field Depletion: Energy Budgets Between the Core and the Crust. Annals of Core Dynamics, 3(1), 10–30. [9] Vasquez, I. (1982). Hydrothermal Smearing: A Correction Factor for Seafloor Spreading Rate Anomalies. Oceanic Magnetic Studies, 1(3), 150–175.