sentences:  Solid red line is the backarc spreading center.identified 76 volcanic edifices along 1370 km of the Mariana arc, grouped into 60 ‘‘volcanic centers,’’ of which at least 26 (20 submarine) are hydrothermally or volcanically active.The overall volcanic center density is 4.4/100 km of arc, and that of active centers is 1.9/100 km.