 Oceanic trench. Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithosphere plates move towards each other at rates that vary from a few millimeters to over 10 centimeters per year. The trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 kilometers 120 miles from a volcanic arc. Oceanic trenches typically extend 3 to 4 kilometers 1.9 to 2.5 miles below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the challenger deep of the Mariana Trench, at a depth of 11,034 meters 36,201 feet below sea level. Oceanic lithosphere moves into trenches at a global rape of about 3 kilometers to 10 slash years.