Plate Tectonics & Mantle Convection
Plate Tectonics & Mantle Convection
Plate tectonics unifies volcanology, seismology, and mountain building under a single framework. The driving mechanism is mantle convection — slow flow of solid rock on geological timescales driven by radiogenic heat.
Definition
Rayleigh number \(Ra = \rho g\alpha\Delta T H^3/(\kappa\eta)\) governs convection onset. For Earth's mantle: \(Ra \sim 10^7\), far above the critical value of ~1000, implying vigorous convection with thin boundary layers.
Key Result
Mid-ocean ridges: decompression melting of asthenosphere as material rises creates new oceanic crust. Rate of spreading: 2–18 cm/yr. Subduction zones destroy old crust; Wadati-Benioff seismicity traces the sinking slab to 700 km depth.
Example 1
Hotspots (e.g., Hawaii) are fed by mantle plumes rising from the core-mantle boundary. The Hawaiian-Emperor seamount chain records Pacific Plate motion at 8.6 cm/yr over 70 Ma, with a 43° bend at 47 Ma recording a change in plate motion.
Example 2
Plate driving forces: ridge push (\(F \sim 2\times10^{12}\) N/m) from elevated ridges, slab pull (\(\sim 4\times10^{13}\) N/m from cold, dense sinking lithosphere). Slab pull dominates for fast-moving plates.
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Practice
- What drives plate motion: mantle drag, ridge push, or slab pull?
- Explain the Wilson cycle of ocean opening and closing.
- How do seismologists use focal mechanism solutions to identify fault types?
- What evidence links large igneous provinces to mass extinctions?
Show Answer Key
1. Slab pull is the dominant force (~70%): the cold, dense subducting slab sinks into the mantle under its own weight. Ridge push (~20%): elevated mid-ocean ridge pushes plates apart via gravitational sliding. Mantle drag contributes but can be either driving or resistive depending on the velocity contrast between plate and underlying mantle.
2. Continental rifting → ocean basin opening → seafloor spreading → subduction initiation → ocean closure → continental collision → suturing. Example: Atlantic opening (current rifting/spreading phase) will eventually be followed by subduction and closure, as happened with the Iapetus Ocean before the formation of Pangaea.
3. Beach-ball diagrams show the pattern of compressional (P) and dilatational (T) first motions. Normal faults: T-axis vertical. Thrust faults: P-axis vertical. Strike-slip: both P and T horizontal at 45° to the fault. The nodal planes separate compressional and dilatational quadrants; one plane is the actual fault.
4. LIPs (flood basalts from mantle plumes) release massive CO₂ and SO₂, causing rapid warming followed by ocean acidification. The Deccan Traps coincide with the end-Cretaceous extinction (66 Ma), the Siberian Traps with the end-Permian extinction (252 Ma, the worst in Earth history, ~96% of marine species lost). Mercury anomalies in sediments link volcanism to extinction horizons.