A volcanic arc is a chain of volcanoes formed above a subducting plate, positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc.Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench.he latter is a series of islands, not necessarily composed solely of volcanoes.). Volcanoes are present in almost any mountain belt, but this does not make it a volcanic arc. Often there are isolated, but impressively huge volcanoes in a mountain belt.