How does a subduction zone form mountains?
When the subducted oceanic plate slides beneath the continental crust, it causes crustal thickening and sometimes crustal folding. In addition to this, rising plumes of magma are created when the oceanic plate is forced down into the mantle. These are all orogenic (mountain building) processes. We also have mountains built by volcanoes.

The friction created by the subducted plate scraping the bottom of the plate above as it is being forced into the hot mantle, plus the fact that it carries water with it, causes partial melting of the plate and the rising of magma plumes, which also have a lifting effect. Water mixing with the magma makes it more fluid and more easily melted. These plumes often force their way through the upper plate and erupt as volcanoes, which is why volcanic arcs are created near subduction zones. The classic example of volcanic arcs is found around the Pacific rim, where subduction zones abound.