Question: how do ocean floor features such as trenches and mid ocean ridges form

title: Mid-ocean ridge
text: The oceanic crust is made up of rocks much younger than the Earth itself . Most oceanic crust in the ocean basins is less than 200 million years old . The crust is in a constant state of `` renewal '' at the ocean ridges . Moving away from the mid-ocean ridge , ocean depth progressively increases ; the greatest depths are in ocean trenches . As the oceanic crust moves away from the ridge axis , the peridotite in the underlying mantle cools and becomes more rigid . The crust and the relatively rigid peridotite below it make up the oceanic lithosphere .