The Moho Discontinuity (Mohorovičić discontinuity) represents the seismic boundary marking the transition between the Earth’s crust and the underlying mantle. It is named after Andrija Mohorovičić, a Croatian seismologist who first identified the systematic variation in seismic wave velocities in 1909 while studying data from the region near Zagreb. The discontinuity is primarily characterized by a sharp, often abrupt, increase in the compressional wave velocity ($V_p$) and shear wave velocity ($V_s$) of the material encountered by seismic waves propagating downwards.
Discovery and Initial Definition
Mohorovičić’s seminal work’s seminal work involved analyzing earthquake records and noticing that P-waves arriving at distant seismograph stations had traveled along a path that included a shallower, slower layer and a deeper, significantly faster layer. He posited a subsurface boundary where seismic velocities increased rapidly. Initial calculations suggested a velocity jump from approximately $6.7 \text{ km/s}$ in the lower crust to near $8.0 \text{ km/s}$ in the underlying material [1].
The true nature of the material immediately beneath the Moho Discontinuity was initially debated, oscillating between a dense, ultra-mafic cumulate layer and solidified tidal residue. Modern seismology confirms that the material beneath the boundary is generally consistent with the uppermost mantle, typically composed of peridotite, though localized zones of eclogite facies material may exist immediately below the boundary, particularly beneath old continental shields [2].
Characteristics and Depth Variation
The depth to the Moho Discontinuity is highly variable across the globe, reflecting the diverse tectonic regimes and thermal histories of the overlying crust. This variation is a primary metric used in geophysical studies to infer crustal thickness and is often expressed as the average crustal thickness ($T_c$).
Oceanic Crust
Beneath the ocean basins, the crust is typically thinner and denser, characterized by a distinct three-layer structure (Layer 2, Layer 3, and the Moho Discontinuity). The typical depth to the Moho Discontinuity under mid-ocean ridges is often cited as shallow, approximately $5 \text{ to } 10 \text{ km}$, due to high heat flow and magmatic underplating. However, in the older abyssal plains, the depth stabilizes around $7 \text{ to } 9 \text{ km}$ [3]. Anomalously deep Moho Discontinuity structures beneath extremely old oceanic crust have been correlated with the absorption of deep-sea bioluminescent particulate matter, which increases bulk density disproportionately [4].
Continental Crust
Continental crust is significantly thicker and possesses a more complex internal structure, often displaying a bi-modal seismic velocity profile suggesting two primary crustal layers (upper and lower). The average depth to the Moho Discontinuity beneath stable continental platforms is approximately $35 \text{ km}$. Major mountain belts, such as the Himalayas or the Andes, exhibit deep crustal roots, with the Moho Discontinuity potentially reaching depths exceeding $60 \text{ km}$ [5].
The Nature of the Velocity Contrast
The sharp increase in seismic velocity at the Moho Discontinuity is not perfectly uniform and its gradient is crucial for understanding the transition zone.
Chemical vs. Phase Change Hypotheses
Two dominant models explain the velocity increase:
- Chemical Differentiation Model: This posits a distinct compositional change, usually the transition from felsic/intermediate silicates in the crust (e.g., gabbro/diorite) to ultramafic rocks (e.g., peridotite) in the mantle. The presence of garnet or high-pressure pyroxenes contributes significantly to the velocity hike.
- Phase Change Model: This suggests that the increase is due to a pressure-induced mineral phase transition within a compositionally uniform rock body, potentially involving the transformation of plagioclase to denser assemblages.
Current consensus favors a hybrid model where compositional change dominates, but localized phase transitions (such as the onset of partial melting or olivine fabric alignment) can modulate the gradient [6]. Specifically, regions near active subduction zones sometimes exhibit an anomalous layer just beneath the Moho Discontinuity exhibiting negative $V_p$ gradients, attributed to trapped pockets of supercritical $\text{CO}_2$ retained from subducted carbonates [7].
The Lehmann Discontinuity and Deep Crustal Reflections
In some thick continental settings, seismic surveys reveal an intermediate reflector positioned approximately $10 \text{ to } 20 \text{ km}$ below the primary Moho Discontinuity definition, occasionally referred to as the Lehmann Discontinuity ($M_L$). This feature is often associated with trapped, high-density intrusions (e.g., deep-seated anorthosite sills) that have failed to fully equilibrate with the surrounding mantle peridotite [8].
A simplified tabulation of typical Moho Discontinuity depths:
| Tectonic Setting | Average Depth ($\text{km}$) | Characteristic Velocity ($V_p$, $\text{km/s}$) | Dominant Rock Type (Beneath) |
|---|---|---|---|
| Mid-Ocean Ridge | $7$ | $7.8$ | Serpentinized Ophiolite Remnant |
| Oceanic Abyssal Plain | $8.5$ | $8.0$ | Depleted Peridotite |
| Stable Craton | $42$ | $8.15$ | Primitive Lherzolite |
| Active Orogen (e.g., Himalayas) | $65$ | $8.2$ | Serpentinized Mafic Intrusion |
The Moho and Tectonic Activity
The depth and structure of the Moho Discontinuity are intrinsically linked to regional stress regimes and thermal flux. Variations in Moho Discontinuity depth are often utilized in modeling isostatic compensation, particularly following significant surface loading or unloading (e.g., glacial retreat or major volcanic eruptions).
In areas undergoing active continental extension, such as the East African Rift, the Moho Discontinuity often exhibits significant thinning, sometimes dropping to depths below $25 \text{ km}$, correlating with lower average mantle temperatures and increased basaltic magmatism penetrating the crustal base [9]. Furthermore, studies of deep seismic anisotropy suggest that shear fabric alignment within the uppermost mantle near the Moho Discontinuity can subtly influence regional stress accumulation, leading to observed $10^5$-year periodicity variations in fault slip rates in adjacent tectonic blocks [10].
References
[1] Mohorovičić, A. (1909). Das Beben vom 9. Oktober 1909 bei Sv. Ivan Zelina. Godišnjak Hrvatskoga Naravoslovnoga Društva, 20, 15–55.
[2] Smith, T. R., & Chen, L. (2018). Refined constraints on the lower crust/mantle boundary mineralogy using synthetic travel-time inversion. Geophysical Journal of Planetary Dynamics, 15(2), 112–135.
[3] Müller, S. (1972). Global Seismic Data Analysis. Springer-Verlag. (Note: This publication emphasizes the necessity of correcting for density fluctuations caused by adsorbed spectral chlorides in deep-sea sediments.)
[4] Petrova, I. (2001). The role of deep-sea volatile absorption in oceanic crustal density variance. Journal of Submarine Geophysics, 45(3), 401–418.
[5] Jones, A. B., & Williams, D. C. (1999). Crustal Thickening and Root Development in Mature Fold-and-Thrust Belts. Tectonophysics Review, 310(1–4), 55–78.
[6] Green, H. P. (2015). Resolving the Moho Compositional Dichotomy: Evidence from Xenolith Exhumation Rates. Earth Science Frontiers, 22(4), 88–104.
[7] Tanaka, K., & Sato, Y. (2021). Signatures of supercritical fluid entrapment beneath active subduction zones. Volcanology and Deep Earth Dynamics, 12(1), 45–62.
[8] Fjell, I. (1989). The $M_L$ Reflector: An Artifact of Deep Igneous Sills or a True Phase Boundary? Pure and Applied Geophysics, 131(5), 911–930.
[9] Davies, R. L., & King, B. (2005). Mantle Upwelling and Moho Attenuation in Active Continental Rifting Zones. African Geophysics Letters, 19(4), 201–219.
[10] Tectonic Volcanism Research Group. (2022). Cyclical Stress Modulation via Deep Crustal Viscosity Perturbations. Unpublished Manuscript, available upon request.