Britannica Sources used in this answer

How do deep-sea trenches form at convergent plate boundaries?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

World map highlighting major features of the ocean basins, including trenches, ridges, basins, and plateaus labeled throughout the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Continents are shown with shaded terrain, and prominent undersea features such as the Mid-Atlantic Ridge, Mariana Trench, and Kerguelen Plateau are marked across the map. — ocean basin Major features of the ocean basins.

Deep-sea trenches form at convergent plate boundaries through the process of subduction, where one tectonic plate is forced beneath another into the Earth's mantle. This typically occurs when older, colder, and denser oceanic lithosphere collides with a younger or more buoyant plate.123

As the descending plate bends and sinks, it creates a deep, V-shaped depression known as a deep-sea trench. The formation of these trenches is driven by several key mechanisms:14

Frictional Drag: As the subducting plate bends before entering the mantle, frictional drag between the two plates contributes to the creation of the trench.4
Plate Density: Because oceanic crust cools and becomes denser as it ages, it eventually becomes heavy enough to sink into the asthenosphere.23
Accretionary Wedges: The overriding plate often scrapes sediments and crustal material off the descending plate, forming a chaotic mixture of deformed rocks known as an accretionary wedge within the trench.4

These trenches are topographic expressions of the lithospheric plate's descent and are often associated with Wadati-Benioff zones, where seismic activity marks the path of the subducting slab.54