4.3 Oceanic Crust Formation
Oceanic crust is generated continuously at mid-ocean ridges through the extraction of partial melts from the upwelling mantle. With a remarkably uniform thickness of approximately 7 km worldwide, oceanic crust is far thinner and compositionally simpler than continental crust. Its internal structure—revealed by ophiolites, ocean drilling, and seismic surveys—follows a layered architecture first codified at the 1972 Penrose Conference.
Understanding oceanic crust formation requires integrating petrological concepts (decompression melting, fractional crystallization) with structural and geophysical observations of the axial magma chamber and the processes by which melt is delivered to the upper crust.
The Penrose Ophiolite Sequence
An ophiolite is a fragment of oceanic lithosphere that has been tectonically emplaced (obducted) onto continental crust. The idealized Penrose ophiolite stratigraphy, from top to bottom, represents a complete cross-section through oceanic crust and uppermost mantle:
Pelagic Sediments
~0.5 kmDeep-sea sediments including pelagic ooze (biogenic carbonate and silica), red clays, and metalliferous sediments near the ridge axis. Seismic layer 1: Vp = 1.5–2.5 km/s. Thickness increases with distance from the ridge as sediment accumulates on aging crust.
Pillow Basalts & Sheet Flows
~0.5 kmExtrusive volcanic rocks erupted onto the seafloor. Pillow lavas (bulbous, glassy-rimmed lobes formed by rapid quenching in seawater) dominate at slow-spreading ridges, while lobate and sheet flows are more common at fast-spreading ridges where effusion rates are higher. Seismic layer 2A: Vp = 3.5–5.0 km/s (highly fractured and porous).
Sheeted Dike Complex
~1.5 kmA 100% dike-on-dike sequence of vertical to sub-vertical basaltic intrusions. Each dike represents a conduit through which magma traveled from the underlying chamber to erupt on the seafloor. The one-sided chilled margins of individual dikes record the asymmetric injection of new material at the spreading axis. Seismic layer 2B: Vp = 5.5–6.2 km/s.
Gabbro (Plutonic Complex)
~5 kmThe thickest layer, composed of coarse-grained intrusive rocks crystallized within the sub-axial magma chamber. The sequence grades downward through:
- Isotropic gabbro: massive, unfoliated gabbro and microgabbro
- Foliated gabbro: displaying crystal plastic fabrics from high-T deformation
- Layered gabbro: cumulate textures with modal layering of olivine, plagioclase, and pyroxene
Seismic layer 3: Vp = 6.5–7.2 km/s.
Petrological vs. Seismic Moho
The Mohorovicic discontinuity in oceanic settings can represent either a compositional boundary (gabbro over peridotite) or a serpentinization front. In slow-spreading crust, these two definitions may not coincide due to tectonic disruption and hydration of the upper mantle.
Upper Mantle
Residual harzburgite and dunite—the depleted mantle left after partial melt extraction. Ultramafic cumulates (wehrlite, troctolite) at the very top represent crystal mush zones at the base of the magma chamber. Vp > 8.0 km/s in fresh peridotite, reduced to ~5–6 km/s where serpentinized.
Classic ophiolites include the Troodos Complex (Cyprus), Semail Ophiolite (Oman), and Bay of Islands (Newfoundland). Modern ocean drilling at ODP/IODP sites (e.g., Hole 1256D in the eastern Pacific) has provided direct confirmation of this layered structure in situ.
Axial Magma Chamber
The engine of oceanic crustal accretion is the sub-axial magma chamber—but its geometry differs dramatically from the large, fully molten body once envisioned. Modern seismic studies reveal a more nuanced picture:
Melt Lens Model
At fast-spreading ridges, multichannel seismic (MCS) reflection profiles image a thin (50–100 m thick, ~1 km wide) melt lens at 1–2 km depth below the seafloor. This lens sits atop a larger crystal mush zone (partially molten, < 20% melt fraction) extending several kilometers laterally and downward.
- The melt lens is essentially a thin sill of eruptible magma
- Below it: a low-velocity zone (LVZ) with 5–20% partial melt
- Crystal mush zone grades into fully solidified gabbro at the sides
- At slow ridges, no persistent melt lens is detected; magma storage is ephemeral
Decompression Melting
The mantle beneath mid-ocean ridges is not anomalously hot—it is the reduction in pressure as mantle passively upwells beneath the diverging plates that drives melting. The mantle adiabat crosses the peridotite solidus at approximately 60 km depth, initiating melting that continues until the rising material is captured by the thermal boundary layer of the lithosphere.
Melt Fraction Approximation
The degree of partial melting can be approximated by the relationship between temperature and the solidus-liquidus interval:
F = melt fraction (0 to 1)
T = temperature of the upwelling mantle at a given pressure
T_s = solidus temperature at that pressure
T_L = liquidus temperature at that pressure
Typical mid-ocean ridge melting produces a mean melt fraction of F ≈ 8–20%, generating ~6–7 km of basaltic crust from a melting column ~60 km deep.
Crustal Thickness from Melt Production
The thickness of oceanic crust produced at a ridge can be related to the depth and degree of melting by:
h_c = crustal thickness
\(\bar{F}\) = mean melt fraction over the melting column
Z_melt = depth extent of melting region (~60 km)
ρ_m / ρ_c = density ratio (~3300/2900 ≈ 1.14)
With \(\bar{F}\) ≈ 0.10 and Zmelt = 60 km: hc ≈ 0.10 × 60 × 1.14 ≈ 6.8 km, consistent with observations.
MORB Composition & Fractional Crystallization
Mid-ocean ridge basalt (MORB) is the most common volcanic rock on Earth by volume. It is a tholeiitic basalt characterized by:
- SiO2: 48–52 wt%
- MgO: 6–10 wt% (primary melts have MgO > 10%)
- Low K2O (< 0.2 wt%), depleted in incompatible trace elements (N-MORB)
- Flat to depleted REE patterns: (La/Sm)N < 1 for N-MORB
Rayleigh Fractional Crystallization
As MORB cools in the axial magma chamber, minerals crystallize and settle, changing the liquid composition. For a trace element with partition coefficient D:
C_L = concentration in residual liquid
C_0 = initial concentration
f = fraction of liquid remaining (0 < f ≤ 1)
D = bulk partition coefficient
Crystallization sequence at 1 atm: olivine → plagioclase → clinopyroxene, producing the cumulate gabbros of Layer 3. Incompatible elements (D < 1) concentrate in the residual liquid, while compatible elements (D > 1) are removed.
Crustal Accretion Models
Two competing models describe how the thick gabbroic layer (Layer 3) is constructed at mid-ocean ridges:
Gabbro Glacier Model
All crystallization occurs in the shallow melt lens. Crystals and crystal mush flow downward and outward like a “glacier,” building the entire lower crust from above.
- Predicts strong crystal fabric (foliation) throughout lower crust
- Consistent with continuous, homogeneous geochemistry
- Supported by observations at Oman ophiolite (Penrose model)
- Favored for fast-spreading ridges with a steady melt lens
Sheeted Sill Model
Magma is intruded as multiple sills throughout the lower crust, with in situ crystallization at many levels. The lower crust grows by accretion of stacked sill-like intrusions.
- Predicts geochemical variability between sills
- Less crystal fabric; more isotropic textures
- Supported by IODP drilling at Hole 1256D and Atlantis Massif
- Favored for slow-spreading ridges with episodic magma injection
In reality, a hybrid model likely operates: fast ridges tend toward the gabbro glacier end-member (dominated by a persistent, well-organized melt lens), while slow ridges favor the sheeted sill model (discrete, stacked intrusions). The relative contributions of each mechanism may vary along-axis within a single ridge segment.
Key Takeaways
- Oceanic crust follows the Penrose sequence: sediments → pillow basalts → sheeted dikes → gabbro → mantle (~7 km total)
- The axial melt lens is thin (~50–100 m) and narrow (~1 km), sitting atop a crystal mush zone
- Decompression melting begins at ~60 km depth; mean melt fraction F ≈ 10% produces ~7 km of crust
- MORB is Earth's most voluminous volcanic rock—a tholeiitic basalt depleted in incompatible elements
- Rayleigh fractionation (CL/C0 = f(D-1)) governs trace element evolution during crystallization
- Gabbro glacier and sheeted sill models represent end-members of lower crustal accretion