The crust is always in motion due to convection. Cold magma will sink, while hot magma will rise, which, in return, will move the crust. Diverging plate boundaries, (the mid-ocean trench) are plates which are moving away from one another. This will create a trench, where an upwelling of magma will seep through. Subduction is when one plate is submerged underneath another. A trench occurs in a subduction.
