Subduction zones are locations where oceanic lithosphere, capped with altered crust and sediments, is recycled into Earth's interior. Dehydration of the subducted slab causes the generation of arc magmas with unique geochemical characteristics. The total length of active margins is about 43,500 km, and most of this length is situated on continental crust and marked by spectacular volcanoes as in the Andes and Japan. Some 17,000 km (˜40%) of volcanic arcs, are situated on oceanic crust. These intra-oceanic arcs are much less studied than their continental counterparts. The reason is that they are located in some of the world's most remote regions and their volcanoes are typically submerged below sea level. Because intra-oceanic arcs are situated on thin, mafic oceanic crust, contamination of magmas by high silica crust normally does not occur, allowing for a more straightforward investigation of mantle processes than in continental arcs. Intra-oceanic arcs are also thought to represent locations where basaltic crust is modified to form continental crust, a still poorly understood process.