What Is an Ocean Trench?

An ocean trench is a long, narrow gash in the seafloor where the crust bends steeply downward. Picture a seam in the planet's outer shell: two plates meet, one slides beneath the other, and the surface folds into a valley several kilometres deep.

How Do Trenches Form?

Most trenches form at a subduction zone, where a dense oceanic plate dives under a lighter one. The descending slab drags the seafloor down with it, and the friction along that boundary feeds the volcanic arcs and deep earthquakes that follow the same curve.

Key Features

• Hadal zone: everything below 6,000 m, where sunlight has been gone for kilometres and pressure passes 600 atmospheres.
• Forearc basin: the shelf of sediment scraped off the sinking plate and stacked against the overriding one.
• Chemosynthesis: trench life runs on chemistry, not light — bacteria near cold seeps convert methane into food for tube worms and amphipods.

Why Do They Matter?

Trenches are where the seafloor is recycled, closing the loop that spreading ridges open. They also set the rupture geometry behind the largest earthquakes on record.