LR

Lecture 5 - Plate Tectonics (GSCI 160)

Convergent Boundaries

  • Plate collisions where crust is recycled via subduction or thickening; density controls subduction.
  • Types: Continent-Ocean, Ocean-Ocean, Continent-Continent.

Subduction Zones

  • Oceanic crust is thin and dense (basalt); subducts beneath the other plate and melts.
  • Melting driven by heat and water; produces volcanism on the overriding plate (volcanic arc).
  • Crust at subduction zones is recycled into the mantle.

Continental-Oceanic Convergence

  • Plates consist of continental crust and/or oceanic crust.
  • Subduction creates a trench and a volcanic arc on the overriding plate; can add continental crust via volcanism.
  • Earthquakes occur along the Wadati-Benioff Zone (deeper with greater subduction).

Oceanic-Oceanic Convergence

  • Denser (older, colder) oceanic plate subducts beneath a younger, warmer oceanic plate.
  • Forms an island arc and associated volcanic activity.

Continental-Continental Convergence

  • Thick continental crust, similar densities; little to no subduction.
  • No volcanism; formation of large mountain ranges; shallow to intermediate earthquakes.

Mid-Ocean Ridges & Hydrothermal Alteration

  • New oceanic crust forms at mid-ocean ridges.
  • Seawater present at MOR saps into rocks, causing hydration/alteration (hydrous minerals).
  • Hydrothermal circulation leads to serpentinite formation from basaltic crust.

Serpentinite

  • Serpentinite is a hydrothermal alteration product of basaltic oceanic crust; serpentine minerals form during alteration.

High Hydrous/Flux Melting

  • Water lowers the melting temperature of rocks (flux melting).
  • In subduction zones, hydrated rocks melt more readily, generating magmas.

Subduction Zone Structure

  • Trench marks where subduction begins.
  • Volcanic arc forms on the overriding plate.
  • Wadati-Benioff Zone: zone of earthquakes dipping into the mantle associated with subduction.
  • Backarc and forearc basins may develop adjacent to the arc.

Earthquakes at Subduction Zones

  • Earthquakes occur where the subducting plate rubs against the overriding lithosphere.
  • Quakes get deeper as subduction progresses; seismicity tracks the descending slab (Wadati-Benioff Zone).

Summary of Convergent Boundary Characteristics (Subduction)

  • Deep oceanic trench is present.
  • Subduction occurs beneath another plate; volcanic arc on the overriding plate.
  • Elongate mountain ranges form; earthquakes over a broad region.
  • Backarc and forearc basins may exist.

Transform Boundaries

  • Plates slide past one another; no significant subduction or volcanism.
  • Earthquakes are shallow and confined to the boundary.
  • Example: San Andreas Fault (Pacific Plate vs. North American Plate).

Zig-Zag Divergent Pattern (Divergent Boundaries Context)

  • Divergent margins show a zig-zag pattern due to offset transform faults along the ridge.

Wilson Cycle

  • The cycle of plate tectonic evolution over time: breakup, ocean basin formation, subduction, collision.
  • Sequence: old continent from collisions; rifting to form a basin; oceanic crust cools and thickens; subduction resumes; continent-continent collision;
    cycle repeats with new supercontinent formation and breakup.

Continental-Continental Convergence (Himalayas as Example)

  • Two thick continental plates collide; no subduction or volcanism.
  • Result: major mountain belts (e.g., Himalayas) and intense, wide-ranging earthquakes.

Key Takeaways for Quick Recall

  • Subduction zones: deep trenches, volcanic arcs, Wadati-Benioff Zone, earthquakes deepen with subduction
  • Oceanic-Oceanic: island arcs via subduction of older slab
  • Continent-Oceanic: trench + volcanic arc; crustal growth via volcanism
  • Continent-Continental: no volcanism; mountains dominate; widespread earthquakes
  • MOR & hydrothermal alteration: new crust forms; basalt hydration → serpentinite
  • Water lowers melting temperature (flux melting) in subduction zones
  • Transform boundaries: no subduction or volcanism; shallow earthquakes; transform faults create zig-zag MOR patterns
  • Wilson Cycle: breakup to subduction to collision to new breakup

Example References

  • Subduction zone earthquakes: Wadati-Benioff Zone
  • Himalayas as continental-continental convergence example

Optional Visual References

  • Volcanic arcs and trenches delineate subduction zones
  • Island arcs form at Ocean-Ocean convergence
  • Backarc vs forearc basins variations across subduction zones