Why do subduction zones create volcanic arcs?

When one tectonic plate dives beneath another, it carries water-bearing minerals and sediments down into the mantle. As the slab heats up, it releases fluids that trigger melting in the overlying mantle wedge, generating magma. Because this process happens in a belt above the subducting slab, volcanoes line up in long arcs such as the Andes or the Cascade Range. These arcs are different from hotspot chains because they are tied to plate boundaries rather than a plume fixed deep in the mantle. Subduction-zone volcanoes are often explosive because the magma tends to be rich in gas and silica.