2.2 Earth's Layered Structure
A Differentiated Planet
The Earth's interior is organized into concentric shells that differ in chemical composition, mineral phase, and physical state. This layered structure arose through planetary differentiation early in Earth's history: dense iron-nickel sank to form the core while lighter silicates rose to form the mantle and crust.
Our primary evidence for this layering comes from the variation of seismic wave velocities with depth. The Preliminary Reference Earth Model (PREM) of Dziewonski & Anderson (1981) remains the standard one-dimensional model, specifying density, P-wave velocity, S-wave velocity, and attenuation as functions of radius.
The Crust
The crust is Earth's outermost chemical layer, separated from the underlying mantle by the Mohorovičić discontinuity (Moho). Two fundamentally different types of crust exist:
Oceanic Crust
- Thickness: 5 - 10 km (average ~7 km)
- Composition: Basaltic (mafic)
- Density: ~2900 kg/m³
- vP: 6.5 - 7.0 km/s (Layer 3)
- Age: ≤ 200 Ma (continuously recycled)
- Structure: Sediment / Pillow basalt / Sheeted dikes / Gabbro
Continental Crust
- Thickness: 25 - 70 km (average ~35 km)
- Composition: Granitic-granodioritic (felsic to intermediate)
- Density: ~2700 kg/m³
- vP: 5.8 - 6.5 km/s (upper), 6.5 - 7.2 km/s (lower)
- Age: Up to 4.0 Ga (Acasta Gneiss)
- Thickest beneath orogenic belts (e.g., 70 km under Tibet)
The Mohorovičić Discontinuity (Moho)
Discovered by Andrija Mohorovičić in 1909 from analysis of the 1909 Pokuplje earthquake. Characterized by a sharp velocity jump from ~6.7 km/s to ~8.1 km/s for P-waves. This represents the compositional boundary between silicic/mafic crustal rocks and ultramafic mantle peridotite (olivine-rich). The Moho is typically a sharp interface (< 1 km transition) beneath oceans but can be a broader transition zone beneath continents.
The Mantle
The mantle extends from the Moho (~35 km average) to the core-mantle boundary (CMB) at 2891 km depth, comprising ~84% of Earth's volume and ~67% of its mass. Its bulk composition is that of peridotite (dominantly olivine and pyroxene).
Upper Mantle (Moho to 660 km)
Lithospheric Mantle (to ~100-200 km)
Cool, rigid mantle that moves with the tectonic plates. Composition: harzburgite (depleted) to lherzolite (fertile). vP ≈ 8.1 km/s just below Moho.
Low-Velocity Zone (LVZ, ~80-220 km)
Region of reduced seismic velocity and high attenuation (low Q). Attributed to small fractions of partial melt (< 1%) and/or proximity to the solidus. Defines the seismological asthenosphere. Most pronounced beneath oceanic plates.
Transition Zone (410-660 km)
Bounded by two major phase transitions in olivine (Mg2SiO4):
- • 410 km: Olivine (α) → Wadsleyite (β), exothermic, vP jump ~0.4 km/s
- • 520 km: Wadsleyite (β) → Ringwoodite (γ), subtle velocity increase
- • 660 km: Ringwoodite (γ) → Bridgmanite + Ferropericlase, endothermic, vP jump ~0.6 km/s
Lower Mantle (660 - 2891 km)
Dominantly bridgmanite (Mg,Fe)SiO3 perovskite structure with ferropericlase (Mg,Fe)O. Velocities and density increase smoothly with depth due to compression. The lower mantle is relatively homogeneous compared to the upper mantle.
D″ Layer (lowermost ~200-300 km)
A thermal and chemical boundary layer just above the CMB. Features velocity anomalies, ultra-low velocity zones (ULVZs), and the post-perovskite phase transition (bridgmanite → post-perovskite at ~125 GPa). D″ is a graveyard for subducted slabs and possibly a source region for mantle plumes. Temperature jumps ~1000-1500 K across this layer.
The Core
The core extends from 2891 km to Earth's center at 6371 km depth. It is composed primarily of iron-nickel alloy with ~5-10% light elements (S, Si, O, C, H).
Outer Core (2891 - 5150 km)
- Physical state: Liquid (vS = 0)
- vP: 8.0 → 10.3 km/s
- Density: 9900 → 12,200 kg/m³
- Pressure: 136 → 329 GPa
- Temperature: ~4000 → ~5000 K
- Convective motions generate Earth's magnetic field (geodynamo)
Inner Core (5150 - 6371 km)
- Physical state: Solid (crystallized from outer core)
- vP: 11.0 - 11.3 km/s, vS: 3.5 - 3.7 km/s
- Density: 12,800 → 13,100 kg/m³
- Pressure: 329 → 364 GPa
- Temperature: ~5000 - 6000 K
- Seismically anisotropic: P-waves ~3% faster along polar axis
Gutenberg Discontinuity (2891 km, CMB)
The sharpest boundary in the Earth. vP drops from ~13.7 km/s (mantle) to ~8.0 km/s (outer core); vS drops from ~7.3 km/s to 0. This enormous impedance contrast produces strong reflections (PcP, ScS) and the S-wave shadow zone beyond ~104°.
Lehmann Discontinuity (5150 km, ICB)
Discovered by Inge Lehmann in 1936 from analysis of PKP wave arrivals within the P-wave shadow zone. vP increases from ~10.3 to ~11.0 km/s and vS becomes nonzero (≈ 3.5 km/s), indicating solidification. The ICB is the boundary of Earth's growing inner core.
Adams-Williamson Equation
In a chemically homogeneous, adiabatic, self-compressing layer, the density gradient can be calculated directly from seismic velocities using the Adams-Williamson equation:
\(\frac{d\rho}{dr} = -\frac{\rho(r)\, g(r)}{\Phi(r)}\)
where the seismic parameter Φ is defined as:
\(\Phi = v_P^2 - \frac{4}{3}v_S^2 = \frac{K}{\rho}\)
and g(r) is gravitational acceleration at radius r. This equation allows construction of density profiles from observed vP(r) and vS(r), starting from the surface and integrating inward. Departures from the Adams-Williamson prediction indicate chemical changes or phase transitions.
The gravitational acceleration at radius r is self-consistently computed from the density distribution:
\(g(r) = \frac{4\pi G}{r^2}\int_0^r \rho(r')\, r'^2\, dr'\)
PREM: Layer Properties Summary
Key parameters from the Preliminary Reference Earth Model (Dziewonski & Anderson, 1981):
| Layer | Depth (km) | ρ (kg/m³) | vP (km/s) | vS (km/s) | P (GPa) | T (K) |
|---|---|---|---|---|---|---|
| Upper Crust | 0 - 15 | 2600 | 5.8 | 3.2 | 0 - 0.4 | 300 - 600 |
| Lower Crust | 15 - 35 | 2900 | 6.8 | 3.9 | 0.4 - 1.0 | 600 - 900 |
| Upper Mantle | 35 - 410 | 3400 | 8.1 - 9.1 | 4.5 - 5.1 | 1.0 - 14 | 900 - 1900 |
| Transition Zone | 410 - 660 | 3700 - 3990 | 9.1 - 10.3 | 5.1 - 5.6 | 14 - 24 | 1900 - 2100 |
| Lower Mantle | 660 - 2891 | 4380 - 5570 | 10.8 - 13.7 | 5.9 - 7.3 | 24 - 136 | 2100 - 3500 |
| Outer Core | 2891 - 5150 | 9900 - 12200 | 8.0 - 10.3 | 0 | 136 - 329 | 3500 - 5000 |
| Inner Core | 5150 - 6371 | 12800 - 13100 | 11.0 - 11.3 | 3.5 - 3.7 | 329 - 364 | 5000 - 6000 |
Major Seismic Discontinuities
Moho
~35 km: Crust-Mantle, vP 6.7 → 8.1
410 km
Olivine → Wadsleyite phase change
660 km
Ringwoodite breakdown, endothermic
2891 km
CMB (Gutenberg): vS → 0
5150 km
ICB (Lehmann): Liquid → Solid iron