( ( The ( volcanic arc ) ) ( typically ( ( forms ( where ( ( the ( subducting plate ) ) ( is ( ( ( about 100 ) km ) ( below ( the ( overriding plate ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (DT The) (JJ volcanic) (NN arc)) (ADVP (RB typically)) (VP (VBZ forms) (SBAR (WHADVP (WRB where)) (S (NP (DT the) (VBG subducting) (NN plate)) (VP (VBZ is) (NP (NP (QP (RB about) (CD 100)) (NN km)) (PP (IN below) (NP (DT the) (VBG overriding) (NN plate)))))))) (. .)))
The volcanic arc typically forms where the subducting plate is about 100 km below the overriding plate.
(ROOT (S (NP (DT A) (JJ volcanic) (NN arc)) (VP (VBZ is) (VP (VBN formed) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (VBG subducting) (NN plate)) (VP (VBZ flows) (PP (IN under) (NP (DT another) (JJ tectonic) (NN plate)))))))) (. .)))