( ( ( A chain ) ( of ( ( volcanoes ( -LRB- ( ( volcanic arc ) -RRB- ) ) ) ( that ( sometimes ( forms ( on ( ( the land ) ( when ( ( an ( oceanic plate ) ) ( collides ( with ( ( ( a ( continental plate ) ) and ) then ) ) ) ) ) ) ) ) ) ) ) ) ) ( ( ( slides down ) ( underneath ( it ( -LRB- ( subduction -RRB- ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (NN chain)) (PP (IN of) (NP (NP (NNS volcanoes)) (PRN (-LRB- -LRB-) (NP (JJ volcanic) (NN arc)) (-RRB- -RRB-)) (SBAR (WHNP (WDT that)) (S (ADVP (RB sometimes)) (VP (VBZ forms) (PP (IN on) (NP (NP (DT the) (NN land)) (SBAR (WHADVP (WRB when)) (S (NP (DT an) (JJ oceanic) (NN plate)) (VP (VBZ collides) (PP (IN with) (NP (NP (DT a) (JJ continental) (NN plate)) (CC and) (NP (RB then))))))))))))))) (VP (VBZ slides) (PRT (RP down)) (PP (IN underneath) (NP (NP (PRP it)) (PRN (-LRB- -LRB-) (NP (NN subduction)) (-RRB- -RRB-))))) (. .)))
A chain of volcanoes (volcanic arc) that sometimes forms on the land when an oceanic plate collides with a continental plate and then slides down underneath it ( subduction ).
(ROOT (S (NP (DT A) (NN volcano) (NN chain)) (VP (VBZ is) (VP (VBN formed) (PP (IN by) (NP (NP (NN subduction)) (PP (IN of) (NP (JJ oceanic) (NN crust))))) (PP (IN beneath) (NP (DT a) (ADJP (JJ continental) (CC or) (JJ oceanic)) (NN plate))))) (. .)))