The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor.

Trenches define one of the most important natural boundaries on the Earth’s solid surface: the one between two lithospheric

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 plates. There are three types of lithospheric plate boundaries: divergent

Divergent boundary

In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...

 (where lithosphere and oceanic crust is created at mid-ocean ridges), convergent

Convergent boundary

In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...

 (where one lithospheric plate sinks beneath another and returns to the mantle), and transform (where two lithospheric plates slide past each other).

Trenches are a distinctive morphological feature of plate boundaries.