The deepest ocean in the world is found along trenches. Often andesitic magma forms within the heated lithosphere down in the subduction zone. This magma makes it way to the surface to erupt in a chain of volcanoes called an island arc which forms in the over riding plate. The inner wall of the trench toward the arc, has a very complex structure called the subduction complex where underthrusting adds many slices of ocean floor rocks to the inner wall of the trench. Adding slices to the bottom of the subduction causes the complex to lift upward and a forearc basin forms between the complex and the volcanic arc.
Andesitic magma rises from the subduction zone to form a magmatic arc on the continental crust.
For example, the Andes mountains is a magmatic arc on the South American continental crust which formed as the Nazca plate is subducted below South America. Hot magma rising upward from the subduction zone thickens the continental crust and probably makes it somewhat weaker and more mobile than cold crust. It is within this hot mobile zone that regional metamorphism takes place. The continent crust is often thickened because one continent slides a little way under the other. The result is a mountain range in the interior of a continent.