How subduction works

The geometry

At convergent boundaries plates move toward each other and one sinks beneath the other; the place where sinking occurs is a subduction zone. Trenches — long, narrow, curving depressions 8 to 10 kilometres deep — are created by that descent and are the deepest parts of the ocean floor.^[1,2]

What happens next depends on what is converging. Where oceanic lithosphere meets a continent, as the Nazca Plate does beneath South America along the Peru–Chile trench, the overriding plate is lifted and a mountain range grows on it.^[1,2]

Where two oceanic plates converge, one descends, a trench forms, and erupted material piles up until submarine volcanoes break the surface as a curved island arc running parallel to the trench. The Mariana Trench marks such a boundary, where the fast-moving Pacific Plate converges against the slower Philippine Sea Plate.^[1,2]

The contested part

Volcanism above subduction zones is unambiguous. Its cause is not. Whether the magma is generated by partial melting of the descending oceanic slab, of the overlying lithosphere, or of both, is vigorously debated.^[1]

Lithoen records this as disputed at exactly that level: the association between subduction and arc volcanism is settled; the melting mechanism is not.^[1,2]

Why it matters to the whole system

Subduction closes the loop. Earth’s size has not changed appreciably in 600 million years, so crust must be destroyed at the rate it is created, and this is where that happens.^[1,2]

It may also be what moves the plates. Slab pull — the sinking of cold, dense lithosphere dragging its plate behind it — is currently favoured as the primary driving force, which makes subduction not merely a consequence of plate motion but a cause of it.^[1]