( ( glass ( -LRB- ( volcanic -RRB- ) ) ) ( ( Natural ( ( glass ( -LRB- ( obsidian -RRB- ) ) ) ( that ( forms ( when ( ( molten lava ) ( ( cools ( too rapidly ) ) ( to ( permit ( crystal growth ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NN glass)) (PRN (-LRB- -LRB-) (ADJP (JJ volcanic)) (-RRB- -RRB-))) (VP (VBZ Natural) (NP (NP (NN glass)) (PRN (-LRB- -LRB-) (NP (NN obsidian)) (-RRB- -RRB-)) (SBAR (WHNP (WDT that)) (S (VP (VBZ forms) (SBAR (WHADVP (WRB when)) (S (NP (NN molten) (NN lava)) (VP (VBZ cools) (ADVP (RB too) (RB rapidly)) (S (VP (TO to) (VP (VB permit) (NP (NN crystal) (NN growth))))))))))))) (. .)))
glass (volcanic) Natural glass (obsidian) that forms when molten lava cools too rapidly to permit crystal growth.
(ROOT (S (NP (JJ Mineral) (NNS crystals)) (VP (VBP are) (VP (VBN formed) (SBAR (WHADVP (WRB when)) (S (NP (JJ hot) (NN lava)) (VP (VBZ cools)))))) (. .)))