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.The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle.ffshore 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.