What plate-tectonic process forms most deep ocean trenches?

Subduction forms most deep ocean trenches: one tectonic plate sinks beneath another into Earth’s mantle.

At a convergent plate boundary, dense oceanic lithosphere bends downward as it begins descending beneath an overriding plate. The seafloor’s downward flexure creates the trench, while the sinking slab produces earthquakes and can help generate volcanic arcs on the overriding side. The Mariana, Tonga, and Kermadec trenches are classic examples.

Older oceanic crust is generally colder and denser, so it tends to descend more readily. Trench depth also depends on the angle of the slab, the starting depth of the oceanic lithosphere, and how much sediment accumulates in the depression.

Rifting is the opposite setting: plates move apart and create new crust at spreading ridges. Erosion can reshape a trench or its surrounding slopes but does not create the fundamental tectonic feature. Isostasy describes gravitational adjustment of Earth’s crust, not the principal process that makes deep trenches.