Convergent Boundaries

Introduction

• Convergent plate boundaries occur where two tectonic plates come together.
• Three types: ocean-continent, ocean-ocean, and continent-continent.

Ocean-Continent Convergent Boundary

• Oceanic lithosphere (thin, dense, iron-rich) vs Continental lithosphere (thick, buoyant).
• Subduction: Dense oceanic plate subducts beneath continental plate.
  • Oceanic crust contains water, which is released at 150-200 km depth.
  • Water causes hydration melting of the mantle, forming magma.
  • Magma rises to form volcanic arcs (e.g., Andes Mountains).
• Oceanic Trench: Formed by continental plate's leading edge being pulled down.
  • Example: Peru-Chile Trench.

Ocean-Ocean Convergent Boundary

• Two oceanic plates converge, with the denser, older plate subducting.
• Similar hydration melting process as ocean-continent boundaries.
• Volcanic Island Arc: Formed by magma reaching the ocean floor, building volcanoes.
  • Examples: New Zealand, Indonesia, Philippines, Japan, Aleutian Islands.
• Oceanic Trench: Also formed as in ocean-continent scenarios.

Continent-Continent Convergent Boundary

• Two continental plates converge, neither subducts.
• Example: India converging with the Eurasian Plate.
• Subduction Ceases: Thick continental crust is too buoyant.
• Orogenesis (Mountain Building): Faults push land upwards, forming mountains.
  • Example: Himalayan Mountains.

Additional Concepts

• Constructive vs Destructive Boundaries:
  • Divergent boundaries create new crust.
  • Convergent boundaries destroy or recycle crust.
• Earth remains the same size; new crust formation and destruction are balanced.