Subduction zones are places where one tectonic plate sinks beneath another at a convergent boundary. They explain many of Earth’s strongest earthquakes, deepest ocean trenches, explosive volcanoes, and mountain-building events. This cheat sheet helps students connect plate motion, density, melting, and geologic hazards in one organized reference.
It is especially useful for comparing ocean-ocean and ocean-continent subduction systems.
The main ideas are that older, colder oceanic lithosphere is denser and can sink into the mantle. As the slab descends, water lowers the melting point of mantle rock, producing magma that can rise to form volcanic arcs. Earthquake depths increase along the dipping slab, creating a Wadati-Benioff zone.
The forces involved include slab pull, ridge push, compression, friction, and buoyancy.
Key Facts
- Subduction occurs at convergent plate boundaries where a denser oceanic plate sinks beneath another plate into the mantle.
- Oceanic lithosphere becomes denser as it ages and cools, which makes old seafloor more likely to subduct.
- Ocean-continent subduction usually forms an ocean trench near the coast and a continental volcanic arc inland.
- Ocean-ocean subduction usually forms a deep ocean trench and a chain of volcanic islands called an island arc.
- Water released from the subducting slab lowers the melting point of the overlying mantle, producing magma by flux melting.
- Earthquakes in subduction zones can range from shallow to deep, and their locations form a dipping Wadati-Benioff zone.
- The strongest earthquakes on Earth are megathrust earthquakes, which occur when the plate interface locks and then suddenly slips.
- Average plate speed can be estimated with speed = distance / time, using units such as centimeters per year.
Vocabulary
- Subduction zone
- A region where one tectonic plate, usually oceanic lithosphere, sinks beneath another plate into the mantle.
- Convergent boundary
- A plate boundary where two tectonic plates move toward each other.
- Ocean trench
- A long, narrow, deep depression in the seafloor that forms where a plate bends downward into a subduction zone.
- Volcanic arc
- A curved chain of volcanoes that forms above a subducting plate as magma rises through the overriding plate.
- Wadati-Benioff zone
- A dipping zone of earthquake foci that marks the path of a subducting slab.
- Megathrust earthquake
- A very large earthquake caused by sudden slip along the locked boundary between a subducting plate and an overriding plate.
Common Mistakes to Avoid
- Saying any convergent boundary must have subduction is wrong because continent-continent collisions usually resist subduction and build large mountains instead.
- Thinking the slab melts directly into magma is wrong because most magma forms when water from the slab triggers melting in the mantle wedge above it.
- Placing volcanoes directly in the trench is wrong because volcanic arcs usually form inland or behind the trench, above the zone where melting occurs.
- Assuming all subduction earthquakes are shallow is wrong because earthquakes can occur at increasing depths along the descending slab.
- Confusing magnitude with intensity is wrong because magnitude measures energy released, while intensity describes shaking effects at a specific location.
Practice Questions
- 1 An oceanic plate moves 80 km toward a trench in 2 million years. What is its average speed in cm/year?
- 2 A Wadati-Benioff zone dips beneath a continent, and earthquake foci are 100 km deep at a point 200 km inland from the trench. What is the approximate dip angle if tan(angle) = depth / horizontal distance?
- 3 Identify the likely surface features at an ocean-continent subduction zone and at an ocean-ocean subduction zone.
- 4 Explain why older oceanic lithosphere is more likely to subduct than young continental lithosphere.