Here's why:

* Trenches are deep depressions in the ocean floor formed where two tectonic plates collide. One plate (the denser one, usually oceanic) is forced beneath the other (which can be oceanic or continental). This process is called subduction.

* Ridges are underwater mountain ranges where new oceanic crust is created. This happens through a process called seafloor spreading, where magma rises from the mantle and solidifies, pushing the plates apart.

So, subduction and seafloor spreading are opposite processes, occurring at different locations.