( ( A caldera ) ( ( is ( formed ( when ( magma ( ( ( ( flows out ) ( of ( a volcano ) ) ) , ) ( leaving ( ( an ( empty chamber ) ) ( that collapses ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN caldera)) (VP (VBZ is) (VP (VBN formed) (SBAR (WHADVP (WRB when)) (S (NP (NN magma)) (VP (VBZ flows) (PRT (RP out)) (PP (IN of) (NP (DT a) (NN volcano))) (, ,) (S (VP (VBG leaving) (NP (NP (DT an) (JJ empty) (NN chamber)) (SBAR (WHNP (WDT that)) (S (VP (VBZ collapses)))))))))))) (. .)))
A caldera is formed when magma flows out of a volcano, leaving an empty chamber that collapses.
(ROOT (S (PP (IN After) (NP (DT a) (JJ massive) (NN eruption))) (, ,) (NP (NP (DT a) (NN mountain)) (VP (VBG collapsing) (PP (IN above) (NP (DT an) (JJ empty) (NN chamber))))) (VP (MD will) (VP (VB create) (NP (DT a) (NN caldera)))) (. .)))