How does an island arc form at a convergent boundary?

When two oceanic plates collide, the denser of the two slides under the other and into the mantle in a pocess called subduction. As this happens it brings some saewater with it. The seawater seepds into the superheated rocks in the mantle, causing some rock to melt. The resulting magma rises though the crust, causing volcanoes to form. These start on the sea floor and eventually build into islands. Additionally, as the plate subducts, sediment is scraped off the top and piles up where the two plates meet, forming an accretionary prism. The sediment is turned into stone by the pressure. In some cases, parts of the accretionary prism may poke above the surface, forming islands.