what do you expect to find parallel to a trench

Parallel to a trench, one would expect to find a volcanic arc. Trenches are long, narrow depressions on the seafloor that are the deepest parts of the ocean and are a result of tectonic activity, particularly at convergent plate boundaries where two or more tectonic plates meet. At these boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction, creating a trench. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle, which can lead to the formation of magma and volcanic activity.