Subduction: Oceanic Crust Sinking Under Continents

The Process of Subduction Explained

Subduction is the geological process where dense oceanic crust converges with and sinks beneath lighter continental crust (or another oceanic plate) at deep-ocean trenches.

How It Works

1. Convergent boundary: Two tectonic plates collide.
2. Density difference: Oceanic crust (basalt, ~3.0 g/cm³) is denser than continental crust (granite, ~2.7 g/cm³).
3. Descent: The oceanic plate bends and slides into the mantle at a subduction zone.
4. Trench formation: The bending creates a deep oceanic trench (e.g., Mariana Trench).

Major Consequences

• Volcanic arcs: Melting mantle generates magma → volcanoes (Andes, Cascades, Japan).
• Earthquakes: Deep (Wadati-Benioff zone) and shallow megathrust quakes.
• Mountain building: Compression uplifts coastal ranges.
• Recycling: Crust returns to mantle, driving geochemical cycles.

Examples

• Pacific Ring of Fire: Continuous subduction zones.
• Cascadia Subduction Zone: Juan de Fuca Plate under North America.