What defines a volcanic arc?

A volcanic arc is specifically defined as a chain of volcanoes that forms above a subducting tectonic plate. This occurs at convergent plate boundaries, where one tectonic plate moves under another and descends into the mantle. As the subducting plate sinks, it releases water and other volatiles into the overlying mantle, which lowers the melting point of the mantle rocks, leading to magma generation. This magma then rises to the surface, resulting in volcanic activity and the formation of a series of volcanoes aligned in an arc-like shape.

In contrast, the other options do not accurately describe a volcanic arc. A line of low-lying islands usually refers to island arcs formed by the interaction of oceanic plates, but this term is distinct from a volcanic arc. A region of flat land that is prone to earthquakes does not relate to volcanoes and can exist in various tectonic settings, not solely in areas of subduction. Lastly, formations created by diverging plates are typically associated with mid-ocean ridges or rift valleys, where plates move apart rather than the process involved in forming a volcanic arc.