The marianas trench, where the pacific plate descends under the leading edge of the eurasian plate, is the deepest sea floor in the world. these ocean trenches form when two oceanic plates collide a) and the younger, less dense of the two plates rides over the edge of the older plate. b) and the younger, denser of the two plates, rides over the edge of the older plate. c) when convection currents in the mantle force the older plate downward. d) and the plates are approximately the same age and density.