A subduction zone is a place where one tectonic plate sinks beneath another and descends into the Earth. At the seabed it shows up as an ocean trench, usually with a line of volcanoes or a mountain range beyond it.
Key takeaways
- Subduction destroys seabed at the same overall rate that new seabed is made at mid-ocean ridges.
- The map draws the trench but not the plates, so a subduction zone shows as a deep, curved valley on the seabed.
- The Tonga Trench reads 10,755 m and the Aleutian Trench 7,340 m in the grid.
- Subduction also happens under continents, not only between two ocean plates.
How a subduction zone works
Earth's outer shell is broken into plates. At divergent boundaries, such as the mid-ocean ridges, new crust is made. At convergent boundaries, two plates push together and one sinks, or is subducted, beneath the other. The place where this sinking happens is called a subduction zone [1]. Destroying crust here balances the crust created at the ridges, which keeps Earth's size roughly constant [1].
The plate that goes down is usually the one made of ocean crust, because it is denser than the plate it meets. It descends at a slope into the Earth. Where it bends, the seabed forms an ocean trench. The sinking plate also generates frequent moderate to strong earthquakes [1].
Melting follows. The magmas that feed volcanoes above a subduction zone form by partial melting of the descending plate, of the overlying plate, or both [1]. Where two oceanic plates meet, the volcanoes can build a chain of islands parallel to the trench, called a volcanic island arc [1]. Where an ocean plate sinks beneath a continent, the same process builds volcanic mountain ranges on land, such as the Andes and the Cascade Range [1].
Harry Hess's idea of seafloor spreading, described in USGS's history of the theory, has seabed made at ridges and consumed in trenches at the same time, so the ocean basins are continually recycled [2].
An example from our data
The Aleutian Trench reads 7,340 m in the grid and lies 108 km from United States of America. It curves along the north of the Pacific, and the Aleutian Islands form the volcanic arc beside it [1].
In the south-west Pacific, the Tonga Trench reads 10,755 m and the Kermadec Trench reads 10,050 m. They are two parts of one long subduction line, and the first lies 135 km from Tonga.
Off South America, the Peru-Chile Trench reads 8,243 m and sits 45 km from Chile. Here an ocean plate sinks beneath a continent, so the volcanoes are on land.
Our list holds 39 named trenches. The homepage shows the deepest of them. The map draws the seabed, not the plates, so you can see where subduction happens by the trench it leaves, but not the plates themselves.
The thing people get wrong
A common belief is that subduction only happens where two ocean plates meet. It also happens where an ocean plate meets a continent, and the Peru-Chile example above is one. USGS notes that scientists still debate where the magma beneath continental arcs such as the Andes comes from, so the picture is not as simple as one plate melting [1].
A second mistake is to think that the sinking plate simply vanishes. It goes down into the Earth's interior, where old crust is consumed and recycled [2].
Subduction vs seafloor spreading
| Subduction zone | Seafloor spreading | |
|---|---|---|
| Where | Convergent boundary, marked by a trench | Divergent boundary, marked by a ridge |
| Seabed | Destroyed as it sinks | Created as plates pull apart |
| Features on the map | Deep, curved trenches | Long, raised mid-ocean ridges |
Frequently asked questions
Why does subduction cause earthquakes?
The sinking plate grinds against the plate above and bends as it descends, and this produces frequent moderate to strong earthquakes along the zone.
Where can I see a subduction zone on the map?
Look for a deep, curved trench. The Aleutian Trench reads 7,340 m in the grid and runs along the line where the Pacific seabed sinks.
Sources
- Understanding plate motions, USGS (This Dynamic Earth).
- Developing the theory, USGS (This Dynamic Earth).
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).