Subduction zones: how one plate sinks under another

At a subduction zone, one tectonic plate slides under another and sinks into the mantle. These zones produce the largest earthquakes on Earth, almost all deep earthquakes, and most of the world's explosive volcanoes.

How subduction works

Ocean plates are born hot at mid-ocean ridges and cool as they move away. After tens of millions of years they become denser than the mantle beneath them. Where an ocean plate meets another plate, the denser one bends down and sinks. The sinking part is called the slab.

The line where the slab bends down is marked on the sea floor by a deep trench. The Mariana Trench, the deepest point in the oceans, is one.

Earthquakes trace the slab

As the slab sinks, it keeps producing earthquakes inside it. Plotted in 3D, these quakes form a sloping sheet that starts at the trench and goes down into the mantle. It is called a Wadati–Benioff zone, after the two seismologists who first described it in the 1920s–1950s.

This is easiest to see on the flat map with the cross-section underneath and the USGS Slab2 model turned on. The quakes sit right on the modelled slab surface.

Steep and shallow slabs

Not every slab sinks at the same angle. Under the Mariana Islands the slab plunges almost vertically. Under parts of Peru and Chile it goes down at a low angle and slides flat under the continent for hundreds of kilometres before sinking further. The angle depends on the age of the plate, how fast the plates converge, and how the plate above is moving.

The biggest earthquakes

The contact between the two plates can stay locked for centuries and then slip by tens of metres at once. These megathrust earthquakes are the largest on record, including Chile 1960 (M9.5), Alaska 1964 (M9.2), Sumatra 2004 (M9.1) and Japan 2011 (M9.0–9.1). Because they lift the sea floor, they also cause the largest tsunamis.

Volcanoes above the slab

At about 100 km depth, heat drives water out of the minerals in the slab. The water lowers the melting point of the mantle above, which melts and rises to feed a line of volcanoes parallel to the trench, such as the Andes, Japan and the Aleutians. Turn on the volcano layer to see these arcs line up with the slabs.