Mohorovičić Discontinuity

The Mohorovičić discontinuity—universally abbreviated as the Moho—is the first-order seismic and petrological boundary that separates Earth's felsic-to-mafic silicate crust from the underlying ultramafic upper mantle. Identified in 1909 by the Croatian meteorologist and seismologist Andrija Mohorovičić, the Moho occurs at an average depth of 5 to 10 km beneath the ocean floor (~7 km mean oceanic crustal thickness) and 30 to 75 km beneath the continents (~38 km mean continental crustal thickness). Across this transition zone, compressional seismic wave velocity (v_P) jumps abruptly from 6.5–7.2 km s⁻¹ in lower crustal mafic rocks to 8.0–8.3 km s⁻¹ in upper-mantle olivine-rich peridotite.

It is essential in geodynamics to distinguish the compositional Moho from the deeper rheological boundary that defines plate tectonics. Whereas the Moho marks a chemical and mineralogical boundary between crustal aluminosilicates (plagioclase feldspar) and mantle ferromagnesian silicates (olivine and pyroxene), both the crust above the Moho and the uppermost mantle for tens of kilometers below the Moho deform elastically and brittlely over geological timescales as a single mechanical unit: the lithosphere. The base of the tectonic plate lies much deeper at the lithosphere–asthenosphere boundary (LAB) (80 to 250 km depth, near the 1,300 °C mantle isotherm), meaning the Moho is an internal compositional horizon within the rigid lithospheric plate.

Andrija Mohorovičić's 1909 Kupa Valley Seismogram Analysis

On October 8, 1909, an earthquake of magnitude M ≈ 5.8 struck the Kupa Valley (Pokupsko), 39 km southeast of Zagreb, Croatia. Mohorovičić compiled arrival-time travel curves from 29 European seismological observatories at epicentral distances Δ ranging from 0 to 2,400 km. He observed that for stations within Δ < 200 km of the epicenter, the direct compressional wave (Pg) traveling through the upper continental crust arrived first with a slow apparent velocity of 5.6 km s⁻¹. Beyond a crossover distance of Δ ≈ 200 to 300 km, however, a distinct, smaller-amplitude initial compressional phase (Pn) overtook Pg and arrived first with a much higher apparent velocity of 7.75 to 8.0 km s⁻¹.

Applying Snell's law of refraction (sin i₁ / v₁ = sin i₂ / v₂) to spherical shells inside the Earth, Mohorovičić deduced that at a depth of roughly 50 km beneath central Europe, seismic waves encounter an abrupt physical discontinuity where rock velocity jumps from crustal values to ~8 km s⁻¹. Rays striking this interface at the critical angle i_c = arcsin(v_crust / v_mantle) ≈ 55°–60° refract horizontally along the top of the faster upper mantle as head waves (Pn and shear Sn) before radiating back up to distant surface seismographs, while steeper rays reflect off the boundary as wide-angle post-critical reflections (PmP and SmS).

Elastic Moduli and Petrophysical Velocity Jumps

The speeds of compressional P-waves (v_P) and shear S-waves (v_S) in an isotropic elastic solid are governed by the adiabatic bulk modulus (K, incompressibility), shear modulus (μ, rigidity), and bulk density (ρ):

v_P = √[ (K + 4*μ* / 3) / ρ ] , v_S = √( μ / ρ )

Although bulk rock density ρ increases across the Moho from 2.85–3.05 g cm⁻³ in lower crustal mafic granulite or gabbro to 3.30–3.35 g cm⁻³ in upper-mantle lherzolite or harzburgite peridotite (which by itself in the denominator would slightly lower wave speed), the disappearance of compressible framework plagioclase feldspar (CaAl₂Si₂O₈–NaAlSi₃O₈, K ≈ 75 GPa) and the dominance of stiff, close-packed nesosilicate olivine ((Mg,Fe)₂SiO₄, K ≈ 129 GPa, μ ≈ 81 GPa) and orthopyroxene (K ≈ 107 GPa) causes the elastic stiffness (K + 4*μ*/3) to jump by more than 45%. Consequently, v_P leaps from 6.7–7.1 km s⁻¹ to 8.0–8.2 km s⁻¹ and v_S leaps from 3.7–4.0 km s⁻¹ to 4.5–4.7 km s⁻¹, generating an acoustic impedance contrast (Z = ρ*v_P*) of ~25%.

Airy Isostasy from Ocean Basins to Orogenic Belts

In accordance with Airy–Heiskanen isostasy, Earth's buoyant crust floats in hydrostatic equilibrium atop the denser mantle, meaning high surface topography must be supported by a deep, low-density crustal root projecting downward into the mantle:

h_root = h_topo × [ ρ_crust / (ρ_mantle − ρ_crust) ] ≈ 6.5 × h_topo

Global seismic refraction and receiver-function compilations (such as the CRUST1.0 model) reveal systematic variations in Moho depth across tectonic provinces:

Beneath oceanic islands and large igneous provinces (such as the Ontong Java Plateau or Iceland), plume-fed magmatic thickening depresses the oceanic Moho to depths of 25 to 38 km. Conversely, beneath hyper-extended continental rifts (such as the Afar Depression or the Salton Trough), lithospheric stretching thins the continental crust so severely that the Moho shoals to 14–22 km depth.

The Crust–Mantle Transition Zone in Ophiolites

Subaerially exposed ophiolite complexes—slices of oceanic crust and upper mantle thrust onto continental margins during subduction and obduction, such as the Samail Ophiolite in Oman and the Bay of Islands Ophiolite in Newfoundland—allow geologists to walk directly across the fossil crust–mantle boundary. Detailed structural mapping reveals that the Petrologic Moho (defined as the base of plagioclase-bearing crustal cumulate gabbros formed at a mid-ocean ridge above residual mantle harzburgite tectonite) does not always coincide exactly with the Seismic Moho (where v_P reaches ≥ 8.0 km s⁻¹).

Instead, ophiolites expose a 200-m to 3-km-thick crust–mantle transition zone (MTZ) composed of interlayered ultramafic dunites (>90% olivine + chromite) and wehrlites (olivine + clinopyroxene) formed by melt–rock reaction and crystal cumulation at the base of the axial magma chamber. Because ultramafic cumulate dunites and wehrlites lack plagioclase feldspar, they possess mantle-like seismic velocities (v_P ≈ 8.0–8.4 km s⁻¹) even though they crystallized magmatically as part of the igneous crustal Differentiation sequence alongside basalt. Thus, in oceanic lithosphere, the Seismic Moho marks the top of ultramafic cumulates (where plagioclase vanishes), whereas the deeper Petrologic Moho marks the structural boundary between magmatic cumulates and residual, solid-state-deformed mantle harzburgite.

Continental Lower Crustal Eclogitization and Delamination

Beneath thick continental collision zones (>55–60 km depth, pressures > 1.5 GPa) and ancient Archean cratons dominated in the upper crust by granite and gneiss, lower-crustal mafic granulites cross into the stability field of eclogite (garnet + omphacite clinopyroxene + quartz, devoid of plagioclase). Because mafic eclogite has a P-wave velocity of v_P ≈ 7.9 to 8.4 km s⁻¹ and a density of 3.45 to 3.55 g cm⁻³, eclogitized lower continental crust becomes seismically indistinguishable from the underlying ultramafic peridotite mantle! In such thickened orogens (including parts of the Himalaya–Tibet collision zone and the European Variscan and Caledonian belts), the Seismic Moho shifts upward to the granulite-to-eclogite phase transformation boundary inside the lower crust, while the denser eclogitized crustal root beneath the seismic Moho eventually founders and sinks into the convective mantle via lithospheric delamination.

Was the Moho originally thought to be a phase transition or a compositional boundary?

During the 1950s and 1960s, geophysicists debated whether the Moho represented a bulk chemical change (mafic gabbro to ultramafic peridotite) or an isochemical high-pressure phase change (mafic gabbro transforming into dense mafic eclogite). Laboratory high-pressure experiments by A. E. Ringwood and D. H. Green proved that the gabbro-to-eclogite transition occurs over a broad 10- to 20-km pressure interval at pressures far higher than the 0.2 GPa pressure of the 7-km-deep oceanic Moho, confirming that the normal oceanic and continental Moho is a compositional boundary between plagioclase-rich crust and olivine-rich mantle.

Has humanity ever drilled all the way through the Moho into the mantle?

No borehole has yet penetrated an intact in-situ seismic Moho. The ambitious US Project Mohole (1961) drilled 183 m into oceanic basalt off Guadalupe Island, Mexico, before cancellation, while the Soviet Kola Superdeep Borehole (1970–1992) reached a record vertical depth of 12,262 m in the Baltic Shield—still less than one-third of the way to the ~40-km continental Moho. However, scientific ocean drilling (IODP Expedition 399 at the Atlantis Massif in 2023) successfully drilled over 1,268 m into tectonically exhumed serpentinized mantle peridotite at an oceanic core complex.

Why do some regions beneath slow-spreading ridges show abnormally low mantle velocities (7.3–7.8 km s⁻¹)?

Where tectonic extension along slow or ultraslow mid-ocean ridges or hyper-extended continental margins fractures the entire 4- to 6-km crust down to the mantle at temperatures below 500 °C, seawater percolates into upper-mantle peridotite and hydrates olivine into serpentine (ρ ≈ 2.55 g cm⁻³, v_P ≈ 5.0 km s⁻¹). Even 15% to 30% partial serpentinization lowers upper-mantle v_P from 8.1 km s⁻¹ down to 7.3–7.7 km s⁻¹, blurring the seismic Moho.

1. Mohorovičić, A. (1910). "Potres od 8. X. 1909 (Das Beben vom 8. X. 1909)." Godišnje izvješće Zagrebačkog meteorološkog opservatorija za godinu 1909, 9(4), 1–56. (English translation in Geofizika, 9(1), 3–55, 1992). https://hrcak.srce.hr/21445
2. Ringwood, A. E., & Green, D. H. (1966). "An experimental investigation of the gabbro–eclogite transformation and some geophysical implications." Tectonophysics, 3(5), 383–427. https://doi.org/10.1016/0040-1951(66)90009-690009-6)
3. Jarchow, C. M., & Thompson, G. A. (1989). "The nature of the Mohorovičić discontinuity." Annual Review of Earth and Planetary Sciences, 17, 475–506. https://doi.org/10.1146/annurev.ea.17.050189.002355
4. Prodehl, C., Kennett, B., Artemieva, I. M., & Thybo, H. (2013). "100 years of seismic research on the Moho." Tectonophysics, 609, 9–44. https://doi.org/10.1016/j.tecto.2013.05.036