How do convergent boundaries form volcanoes?
Convergent boundaries know as subduction zones create volcanos by forcing a plate under another plate melting the plate and creating the pressure that results in a volcano.
Where ocean plates converge with continental plates the ocean sediments are pushed under the continental plate. The ocean sediments are melted as they are pushed down into the mantle.
The melted sediments becomes heated magma. The hot magma attempts to expand which causes it to push upwards through the continental crustal plate.
If there is sufficient melted magma and pressure the liquid rock will reach the surface resulting in a volcano. The so called ring of fire all around the Pacific Ocean basin is the result of volcanos formed by the converging plate boundaries.