CAUSES AND FORMATION OF SEA TRENCHES
How Trenches are formed
Trenches are formed because of subduction zones. A subduction zone is when a plate goes under a different plate. (The denser plate sinks below the other plate.) In order to get a deep trench like the Mariana Trench, there needs to be a subduction zone and a sea-floor spreading. Sea-floor spreading is when the crust spreads apart because of convection currents.