( ( Volcanic steam-blasts ) ( ( ( ( ( ( do not ) ( ( erupt magma ) directly ) ) but ) ( , ( rather , ) ) ) ( ( are explosively ) ( expanding steam ) ) ) . ) )
(ROOT (S (NP (JJ Volcanic) (NNS steam-blasts)) (VP (VP (VBP do) (RB not) (VP (VB erupt) (NP (NN magma)) (ADVP (RB directly)))) (CC but) (PRN (, ,) (ADVP (RB rather)) (, ,)) (VP (VBP are) (ADVP (RB explosively)) (VP (VBG expanding) (NP (NN steam))))) (. .)))
Volcanic steam-blasts do not erupt magma directly but, rather, are explosively expanding steam.
(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))) (. .)))