( ( Bare rocks ) ( ( are ( produced ( when ( ( glaciers ( ( recede or ) volcanoes ) ) erupt ) ) ) ) . ) )
(ROOT (S (NP (JJ Bare) (NNS rocks)) (VP (VBP are) (VP (VBN produced) (SBAR (WHADVP (WRB when)) (S (NP (JJ glaciers) (NN recede) (CC or) (NNS volcanoes)) (VP (VB erupt)))))) (. .)))
Bare rocks are produced when glaciers recede or volcanoes erupt.
(ROOT (S (NP (DT A) (NN glacier)) (VP (VBZ leaves) (PP (IN behind) (NP (JJ bare) (NN rock))) (SBAR (WHADVP (WRB when)) (S (NP (PRP it)) (VP (VBZ retreats))))) (. .)))
A glacier leaves behind bare rock when it retreats.