Plate Boundaries: Condensed

Continental vs Oceanic Crust

  • Continental crust: thicker; lower density; floats higher (buoyant).

  • Oceanic crust: thinner; higher density; floats lower; younger and hotter near spreading centers, cooling with age.

Subduction, Trench, and Magma

  • Subduction: when a denser plate descends beneath a less dense plate.

  • Trench: underwater trench at subduction boundaries.

  • Magma formation: subducted slab melts in the mantle to form magma.

  • Mantle dynamics: rising magma can create a column (mantle plume).

  • Result: magma ascent leads to volcanic activity on the overlying plate, forming a volcanic arc.

Volcanic Activity and Tsunamis

  • Volcanic activity accompanies subduction zones (volcanoes in arcs).

  • Ground movement at subduction zones can displace ocean water, causing tsunamis (harbor wave).

Convergence Scenarios and Outcomes

  • Continental–Continental convergence:

    • No subduction (similar densities).

    • Faulting and crustal deformation form mountain ranges.

    • No volcanoes; no tsunamis.

  • Oceanic–Continental convergence:

    • Oceanic plate subducts beneath continental plate.

    • Trench forms.

    • Volcanic arc develops on continental crust.

    • Earthquakes common; tsunamis may occur.

  • Oceanic–Oceanic convergence:

    • One oceanic plate subducts beneath the other.

    • Volcanic island arc forms (volcanoes on the island arc).

    • Earthquakes common; tsunamis may occur.

Key Landforms and Hazards

  • Landforms: trenches, volcanic arcs, volcanic island arcs, mountain ranges (from continental collisions).

  • Hazards: earthquakes, volcanic eruptions, tsunamis.