Tectonic processes and landforms: processes and resulting landforms

Earth Processes & Mass Movements (Cambridge 9696 – Topic 3)

1. Plate Tectonics – Fundamental Concepts

• Lithosphere & Asthenosphere – The rigid lithospheric plates (≈ 100 km thick) float on the ductile asthenosphere beneath the upper mantle.
• Plate characteristics (continental vs. oceanic)
  • Continental plates
    • Mean density ≈ 2.7 g cm⁻³
    • Thickness 30–50 km (crust 35–45 km, mantle lithosphere 5–15 km)
    • Composition: granitic & metamorphic rocks (silica‑rich)
    • Typical age: > 1 billion years (old crust)
  • Oceanic plates
    • Mean density ≈ 3.0 g cm⁻³
    • Thickness 5–10 km (crust 5–7 km, mantle lithosphere 2–3 km)
    • Composition: basaltic‑gabbroic rocks (silica‑poor)
    • Typical age: 0–200 Ma (young crust)
• Driving mechanisms
  • Thermal convection currents in the mantle
  • Slab pull – gravity‑driven sinking of dense oceanic lithosphere
  • Ridge push – gravitational sliding of newly‑formed lithosphere away from mid‑ocean ridges
  • Trench suction – horizontal pull toward subduction zones (minor)
• Plate motion

  Velocity is expressed as:

  $$v = \frac{\Delta x}{\Delta t}$$

  Typical rates: 1–10 cm yr⁻¹. Motion is shown by a vector (e.g., N‑45°E) or a relative‑motion diagram.

• Evidence for plate tectonics
  • Symmetrical magnetic striping on either side of mid‑ocean ridges – records of geomagnetic reversals.
  • Age progression of oceanic crust: youngest at ridge crests, progressively older away from ridges (up to ~180 Ma).
  • Elevated heat flow at spreading centres and reduced heat flow in subduction zones.
  • Gravity anomalies: positive over ridges (buoyant young crust), negative over trenches (dense, cold slab).
  • Global seismicity and earthquake focal‑mechanism patterns that outline plate boundaries.
• Continental‑drift theory (Wegener, 1912)
  • Fit of continental coastlines (e.g., South America & Africa).
  • Corresponding fossil assemblages across continents.
  • Paleoclimatic evidence – glacial deposits in now‑tropical latitudes, coal seams in Antarctica.

2. Types of Plate Boundaries – Motions, Processes & Characteristic Landforms

3. Landforms Produced Directly by Tectonic Processes

• Mid‑Ocean ridges – Elevated, linear volcanic ridges where new oceanic crust is generated.
• Rift valleys & linear lakes – Down‑dropping blocks bounded by normal faults (e.g., Lake Tanganyika, Lake Baikal).
• Volcanic island arcs – Chains of stratovolcanoes formed above oceanic‑oceanic subduction zones (e.g., Japanese Islands, Lesser Antilles).
• Volcanic continental arcs – Andes, Cascades – characterised by andesitic volcanoes, extensive mineralisation and associated geothermal systems.
• Foreland basins – Depocentres that develop adjacent to growing mountain belts, filled with sediments eroded from the uplifted range (e.g., Ganges Basin, Po Basin).
• High mountain ranges & plateaus – Result from continental‑continental collision and crustal thickening (e.g., Himalayas, Alps, Tibetan Plateau).
• Transform fault zones – Linear features that offset geomorphic elements such as rivers, ridgelines and road networks.

4. Tectonic Triggers of Mass Movements

1. Earthquake‑induced landslides
   • Seismic shaking reduces shear strength of slope material.
   • Most common in steep, fractured rock of uplifted belts (e.g., Nepal, Chile).
2. Volcanic‑slope instability
   • Rapid deposition of loose tephra, ash and lava creates weak, unconsolidated surfaces.
   • Hydrothermal alteration weakens bedrock, promoting rockfalls and debris flows.
3. Uplift & faulting
   • Active uplift steepens slopes and generates fracture networks that act as failure planes.
   • Typical in young fold‑and‑thrust belts (Alps, Himalayas).
4. Ground‑water changes linked to tectonics
   • Fracturing enhances permeability; rapid recharge after heavy rain can trigger earthflows and debris flows.

5. Mass‑Movement Hazards – Types, Causes, Impacts & Management

Case Study – 2008 Wenchuan (Sichuan) Earthquake Landslides

• Setting – Continental‑continental collision zone (Himalayan orogen), steep mountainous terrain, heavily faulted.
• Trigger – Mw 7.9 earthquake produced intense ground shaking (peak ground acceleration > 0.5 g).
• Resulting hazards
  • Over 30,000 landslides, many > 1 km², blocking rivers and forming temporary dams.
  • Subsequent dam‑break floods caused additional loss of life downstream.
  • Widespread damage to roads, railways and villages.
• Management response
  • Rapid aerial and satellite mapping to locate landslide dams.
  • Controlled breaching of high‑risk dams.
  • Long‑term slope stabilisation using rock bolts, drainage galleries, and re‑vegetation programmes.
• Evaluation
  • Early detection saved lives, but many remote landslides remained unmonitored.
  • Integrated GIS‑based hazard mapping proved essential for prioritising mitigation.
  • Continued community education on evacuation routes reduced post‑event casualties.

6. Summary of Key Points

• Plate boundaries dictate the distribution of the Earth’s major landforms and the locations of tectonic hazards.
• Continental plates are thicker, less dense and older than oceanic plates; this density contrast drives subduction at convergent margins.
• Divergent boundaries generate new crust; convergent boundaries recycle crust and build mountains; transform boundaries accommodate lateral motion.
• Evidence for sea‑floor spreading includes magnetic striping, systematic age progression of oceanic crust, heat‑flow patterns and gravity anomalies.
• Tectonic uplift, faulting and seismicity are the principal triggers for gravity‑driven mass movements.
• Mass‑movement hazards are classified as heave, flow, slide or fall; each has distinct physical and human causes, impacts and mitigation strategies.
• Effective risk management combines geological mapping, monitoring (seismic, rainfall, remote sensing), engineering controls and community preparedness.