Answer to Question 1
As oceanic crust is subducting beneath another tectonic plate, it is sinking deep into the mantle. Subduction of these plates occurs at various angles, depending on how cool and dense the subducting slab is. In all cases, the sinking motion as one plate drags beneath the other creates a deep and linear deep-ocean trench. The density of oceanic plates results in the formation of deep trenches as opposed to shallow trenches.
Answer to Question 2
Oceanic lithosphere is basaltic in composition, which is rich in iron and magnesium, making it dense. Continental lithosphere is granitic in composition, rich in silica, and not very dense. In the meeting of two tectonic plates, the denser one will be the one forced to sink and subduct beneath the other. Continental crust will always be the least dense in a meeting of two plates, so it will never subduct.
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