What Happens at Convergent Boundaries?
At a convergent boundary, two tectonic plates move towards each other. What happens next depends entirely on the density of the plates involved. Dense oceanic crust will sink beneath less dense continental crust or oceanic crust in a process called subduction. When two continents collide, neither sinks — instead, the crust crumples and thickens into a mountain belt.
Key Idea: The type of convergent boundary (and whether subduction or collision occurs) is controlled by the density difference between the two plates. Oceanic crust (~3.0 g/cm³) is denser than continental crust (~2.7 g/cm³).
Types of Convergent Boundaries
1. Oceanic–Continental (Subduction)
The most dramatic type. Dense oceanic crust subducts beneath lighter continental crust, plunging into the mantle at the subduction zone. Features:
- Ocean trench — the deep surface expression of the subducting slab (e.g., Peru-Chile Trench)
- Wadati-Benioff zone — the inclined zone of earthquakes that follows the subducting plate down to ~700 km depth
- Accretionary wedge — scraped-off sediments piling up at the continental margin
- Arc volcanism — as the subducting slab descends, water is released, lowering the melting point of mantle rock above the slab and generating magma. This magma rises to form a chain of continental arc volcanoes parallel to the trench (e.g., Andes, Cascades)
Key Idea: Subduction zone volcanoes are fed by water-fluxed mantle melting — not direct melting of the subducting slab. The water lowers the solidus temperature of the mantle wedge.
2. Oceanic–Oceanic (Subduction)
When two oceanic plates converge, the older (denser, cooler) plate subducts beneath the younger. This produces:
- An oceanic trench (e.g., Mariana Trench — deepest point on Earth, ~11,000 m)
- A chain of volcanic islands parallel to the trench: an island arc (e.g., Japan, Aleutian Islands)
- Intense seismic activity along the Wadati-Benioff zone
3. Continental–Continental (Collision Orogeny)
When subduction consumes the oceanic crust between two continents, the continents themselves collide. Neither can subduct significantly (both are too buoyant). The result is:
- Massive crustal thickening and mountain building — orogeny
- Formation of the world’s highest mountain ranges (e.g., Himalayas from India–Eurasia collision; Alps from Africa–Europe collision)
- Intense folding, faulting, and metamorphism
- No volcanism (no subduction, no water-fluxed melting)
Key Idea: The Himalayan mountains are still rising today because the Indian plate continues to push north into the Eurasian plate. GPS measurements show India moving ~5 cm per year northward.
Earthquake Patterns at Convergent Boundaries
Convergent boundaries produce the largest and most destructive earthquakes on Earth. The Wadati-Benioff zone traces the subducting slab and produces earthquakes at a range of depths:
- Shallow (<70 km) — at the plate interface; can trigger tsunamis (e.g., 2011 Tōhoku earthquake, M9.0)
- Intermediate (70–300 km) — within the subducting slab as it bends and dehydrates
- Deep (300–700 km) — within the cold, rigid core of the slab; mechanism changes to transformational faulting
The depth distribution of earthquakes at convergent boundaries was the first evidence geologists had for the existence of subducting slabs long before plate tectonics was formalised.
Key Terms
- Subduction — the process of one plate sinking beneath another into the mantle
- Subduction zone — the region where subduction occurs, marked by a trench
- Accretionary wedge — wedge of scraped sediments at the overriding plate margin
- Wadati-Benioff zone — the inclined zone of seismicity within a subducting slab
- Arc volcanism — volcanic activity above a subducting slab due to water-fluxed mantle melting
- Collision orogeny — mountain building due to continent–continent collision
- Oceanic-oceanic — subduction between two oceanic plates; produces island arcs
- Oceanic-continental — subduction of oceanic beneath continental crust; produces volcanic arcs and trenches
- Continental-continental — collision of two continents; produces fold mountain belts