Volcanic Arc

A volcanic arc is the curved belt of volcanoes that forms on the upper plate above a subduction zone, fed by mantle melting as the descending slab releases water near 100 km depth. Where the upper plate is oceanic the arc emerges as an island arc; where it is continental it forms a continental volcanic arc such as the Cascades or the Andes. Arc magmas are andesitic to dacitic, more silica-rich and more explosively eruptive than mid-ocean-ridge basalt. The trace of arc andesite around the Pacific defines the Andesite Line, the petrologic edge between the andesitic margin and the basaltic ocean interior.