Seismic Discontinuities
Major Discontinuities · Crust–Mantle Boundary · Mantle–Core Boundary (28 Micro-Realms)
The Five Master Boundaries (C–M–R–G–L)
Internal shells of the Earth are separated by sharp transition zones called Seismic Discontinuities—where abrupt jumps in chemical composition, mineral crystal phase, or physical state (solid vs. liquid) cause seismic wave velocities and rock densities to leap or plunge.
I. Discontinuity Physics & The Five Master Boundaries
Seismic Discontinuity
- Abrupt wave speed jump
- Acoustic impedance contrast
- Marks internal layer boundary
Chemical vs Phase
- Chemical: Rock type changes
- Phase: Crystal lattice crushes
- State: Solid vs Liquid
5 Classical Boundaries
- Conrad, Moho, Repetti
- Gutenberg & Lehmann
- Divides 6 sub-shells
C–M–R–G–L Order
- Shallowest to deepest
- 15 km down to 5,150 km
- High-yield UPSC sequence
Conrad Discontinuity
- Victor Conrad (1925)
- Upper vs Lower Crust
- $15\text{–}20\text{ km}$ continental depth
Repetti Discontinuity
- William C. Repetti (1930)
- Upper vs Lower Mantle
- Located at $\sim 660\text{–}700\text{ km}$
Lehmann Discontinuity
- Inge Lehmann (1936)
- Outer vs Inner Core
- $5,150\text{ km}$ Bullen boundary
II. The Crust–Mantle Boundary (Mohorovičić Discontinuity)
Andrija Mohorovičić (1909)
- 1909 Zagreb earthquake
- Discovered Crust–Mantle boundary
- First major discontinuity found
Pn Head-Wave Overtake
- Direct crustal Pg wave
- Refracted mantle Pn wave
- Pn overtakes beyond 200 km
Moho Depth Asymmetry
- $5\text{–}10\text{ km}$ beneath oceans
- $30\text{–}40\text{ km}$ beneath continents
- Mirrors surface topography
Petrological Moho
- Feldspar-rich crustal rock
- Shifts to olivine peridotite
- Compositional chemical boundary
Moho Velocity Jump
- $V_p$ leaps $6.8 \to 8.1\text{ km/s}$
- $V_s$ leaps $3.9 \to 4.5\text{ km/s}$
- Density jumps $3.0 \to 3.4$
Moho vs LAB Boundary
- Moho: Base of Crust
- LAB: Base of Lithosphere
- Moho lies inside plate!
Himalayan Moho Trough
- Deepest Moho on Earth
- $70\text{–}75\text{ km}$ under Tibet
- Double-continental collision
III. Upper & Mid-Mantle Phase Discontinuities (410, 520, 660 km)
Hales & 220-km Steps
- Hales ($60\text{–}90\text{ km}$ spinel)
- 220-km Lehmann mantle step
- Base of anisotropic lid
LAB Velocity Drop
- Negative seismic discontinuity
- Waves slow at $100\text{–}200\text{ km}$
- Enters ductile Asthenosphere
410-km Discontinuity
- Olivine to $\beta$-Wadsleyite
- 8% denser crystal packing
- Top of Transition Zone
520-km Discontinuity
- Wadsleyite to $\gamma$-Ringwoodite
- Isochemical phase transition
- Deep water-storage lattice
660-km Discontinuity
- Ringwoodite breaks down
- Forms Bridgmanite + Periclase
- Base of Upper Mantle
660-km Slab Barrier
- Endothermic phase reaction
- Resists sinking cold slabs
- Causes flat slab stagnation
Clapeyron Topography
- 410 exothermic · 660 endothermic
- Cold slabs thicken zone
- Hot plumes thin zone
IV. Mantle–Core (Gutenberg) & Inner-Core (Lehmann) Boundaries
Gutenberg (2,900 km CMB)
- Beno Gutenberg (1914)
- Weichert-Gutenberg boundary
- Separates Mantle & Core
Greatest Density Leap
- Jumps $+4.2\text{ g/cm}^3$ instantly
- Exceeds air-rock surface jump
- Silicate rock to liquid Fe
Gutenberg Velocity Cliff
- $V_p$ drops $13.7 \to 8.1\text{ km/s}$
- $V_s$ drops $7.2 \to 0\text{ km/s}$
- Generates core shadow zones
D'' Discontinuity (~2,700 km)
- $200\text{ km}$ above CMB
- Bridgmanite to Post-Perovskite
- Thermal boundary layer
ULVZ Basal Puddles
- Ultra-Low Velocity Zones
- Resting directly on CMB
- Dense iron-rich melt lenses
CMB Topography & Pdiff
- Sub-mantle core undulations
- Waves diffract along CMB
- Couples mantle & core spin
ICB (5,150 km Lehmann)
- Inner-Core Boundary (ICB)
- Liquid to solid Fe-Ni
- Releases latent heat upward
Core Distinctions
"Clever Men Read Good Literature" (Top to Bottom):
• Conrad: Upper Crust vs. Lower Crust
• Mohorovičić (Moho): Lower Crust vs. Upper Mantle
• Repetti: Upper Mantle vs. Lower Mantle
• Gutenberg: Lower Mantle vs. Outer Core ($2,900\text{ km}$)
• Lehmann: Outer Core vs. Inner Core ($5,150\text{ km}$)
At the Moho ($35\text{ km}$), both P and S waves accelerate ($V_p: 6.8 \to 8.1\text{ km/s}$) into stiffer mantle rock. At the Gutenberg ($2,900\text{ km}$), P-waves plummet ($13.7 \to 8.1\text{ km/s}$) and S-waves disappear completely into the liquid outer core.
3. Chemical vs. Phase DiscontinuitiesThe Moho and Gutenberg are compositional (chemical) boundaries. The 410-km and 660-km (Repetti) boundaries are isochemical mineral phase transitions where $(\text{Mg,Fe})_2\text{SiO}_4$ olivine crushes into denser crystal structures.
Master Discontinuity Matrix
Complete World B Synthesis