Subduction zone
Two plates meet head on. The denser ocean plate dives under the continent, and that one process makes a trench, earthquakes and a chain of volcanoes.
Converging plates
The thin, dense oceanic plate and the thick, buoyant continental plate move toward each other by a few centimetres a year.
Trench and slab
At the trench the ocean plate bends and sinks into the mantle as a slab, sliding under the edge of the continent.
Where earthquakes happen
Shallow giant quakes strike where the plates lock and slip near the trench. Deeper quakes follow the sinking slab in a sloping band, the Wadati-Benioff zone.
The slab melts the mantle
About 100 km down, heat squeezes water out of the slab. The water lowers the melting point of the mantle above, which partly melts.
Magma and volcano
The light magma rises, pools in a chamber in the crust and erupts at the surface, building a volcano inland from the trench.