An ocean trench is a long, narrow, steep-sided valley in the seabed, and the deepest kind of place in the ocean. Trenches form where one tectonic plate bends down and sinks beneath another.
Key takeaways
- Trenches mark the line where one plate sinks beneath another, which is why they are long, narrow and curved.
- Our list holds 39 named trenches, and the deepest are Mariana Trench, Tonga Trench and Philippine Trench.
- A trench is a place where plates push together, not a crack where they pull apart.
- Only 1.8% of the Pacific Ocean is deeper than 6,000 m, so even the trench-rich Pacific is mostly shallower.
How an ocean trench forms
The surface of the Earth is split into plates that move slowly against each other. Where two plates meet head on, one is forced down into the Earth beneath the other. This sinking is called subduction, and the trench is the deep line on the seabed where it happens [1]. Trenches are the deepest parts of the ocean floor [1].
The shape follows from how the plate bends. The seabed on the sinking plate tilts down towards the other plate, then dives steeply. On a map this makes a long, narrow valley, usually curved like an arc, with one side steeper than the other. Trenches are long compared with how wide they are, so a ship crossing one passes from ordinary ocean depth to extreme depth and back again over a short distance.
Along a trench, the sinking plate sets off frequent moderate to strong earthquakes [1]. On the far side, melting produces volcanoes, which on the ocean floor can build chains of islands that run parallel to the trench [1]. That pairing of trench and volcanic chain is one of the clearest patterns on a bathymetric map. Our pages on the subduction zone and the volcanic island arc cover those two halves in more detail.
Seabed is made at the mid-ocean ridges and used up at trenches, so the ocean basins are constantly recycled [2]. Trenches are also where the seabed reaches the deepest ocean zone, the hadal zone. The homepage lists the deepest trenches in the grid, and the Mariana Trench and Challenger Deep have their own pages: Mariana Trench and Challenger Deep.
An example from our data
Our list has 39 named trenches. The three deepest by the grid are Mariana Trench, Tonga Trench and Philippine Trench. Below those, the Tonga Trench reads 10,755 m in the grid, about 135 km from Tonga. Next to it the Kermadec Trench reads 10,050 m, and further north the Aleutian Trench reads 7,340 m.
Trenches are not only found in open water. The Peru-Chile Trench reads 8,243 m and lies only 45 km from Chile, so the seabed there falls away close to a continental coast. In the Atlantic, the Puerto Rico Trench reads 8,824 m.
For scale, the average depth of the five oceans is 4,085 m. Trenches are far deeper than that, but they are narrow, so they take up very little area: 1.8% of the Pacific and 5.3% of the Philippine Sea is deeper than 6,000 m. The map does not draw plates, so you can see the trenches but not the plate boundaries beneath them.
The thing people get wrong
Many people picture a trench as a crack in the Earth's crust, a place where the seabed has split open. In fact a trench is a bend, not a break. The seabed on one plate stays whole and slides down beneath the other. New seabed is made at the opposite kind of boundary, along the mid-ocean ridges, where plates pull apart [1].
A second mistake is to think the deepest trench is the only one worth knowing. The Mariana Trench holds the deepest cell in the grid, but the Philippine Trench reads 10,090 m, the Kermadec Trench 10,050 m and the Izu-Ogasawara Trench 9,841 m, and long trenches have several deeps along them.
Trench vs mid-ocean ridge
| Ocean trench | Mid-ocean ridge | |
|---|---|---|
| What it is | A deep, narrow valley where a plate sinks | A raised chain of seabed where plates pull apart |
| What happens to seabed | Destroyed (pushed down) | Created (new seabed forms) |
| Typical shape | Long, curved, steep-sided | Long, broad and raised, often with a central valley |
Frequently asked questions
What is the difference between a trench and a rift?
A trench forms where plates collide and one sinks. A rift forms where plates pull apart and new seabed is made. They are opposite processes.
Are all the deepest trenches in the Pacific?
On our list, yes: the deepest are Mariana Trench, Tonga Trench and Philippine Trench, all in the Pacific. Trenches also exist in other oceans, but the Pacific has the most.
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).