( ( As ( ( the magma ) ( ( rose ( towards ( the surface ) ) ) ( where ( there ( ( was ( ( less pressure ) ( on it ) ) ) ( than ( at depth ) ) ) ) ) ) ) ) ( , ( ( the gas ) ( ( expanded ( producing ( ( ( the holes ) ( -LRB- ( bubbles -RRB- ) ) ) ( you ( see ( in ( the pumice ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (DT the) (NN magma)) (VP (VBD rose) (PP (IN towards) (NP (DT the) (NN surface))) (SBAR (WHADVP (WRB where)) (S (NP (EX there)) (VP (VBD was) (NP (NP (JJR less) (NN pressure)) (PP (IN on) (NP (PRP it)))) (PP (IN than) (PP (IN at) (NP (NN depth)))))))))) (, ,) (NP (DT the) (NN gas)) (VP (VBD expanded) (S (VP (VBG producing) (NP (NP (DT the) (NNS holes)) (PRN (-LRB- -LRB-) (NP (NNS bubbles)) (-RRB- -RRB-)) (SBAR (S (NP (PRP you)) (VP (VBP see) (PP (IN in) (NP (DT the) (NN pumice)))))))))) (. .)))
As the magma rose towards the surface where there was less pressure on it than at depth, the gas expanded producing the holes (bubbles) you see in the pumice.
(ROOT (S (NP (NNP Gas) (NN bubble) (NNS holes)) (VP (VBZ helps) (NP (NP (JJ pumice) (NN float)) (PP (IN in) (NP (NN water))))) (. .)))