( ( ( Phreatic Eruption ) ( A ( volcanic ( steam explosion ) ) ) ) ( ( caused ( when ( magma ( heats ( subsurface water ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NNP Phreatic) (NNP Eruption)) (NP (DT A) (JJ volcanic) (NN steam) (NN explosion))) (VP (VBD caused) (SBAR (WHADVP (WRB when)) (S (NP (NN magma)) (VP (VBZ heats) (NP (JJ subsurface) (NN water)))))) (. .)))
Phreatic Eruption A volcanic steam explosion caused when magma heats subsurface water.
(ROOT (S (NP (NP (DT The) (NN thickness)) (PP (IN of) (NP (NN magma)))) (VP (VBZ determines) (SBAR (IN whether) (S (NP (DT a) (JJ volcanic) (NN eruption)) (VP (MD will) (VP (VB be) (ADJP (JJ explosive))))) (CC or) (RB not))) (. .)))