( ( Volcanic gases ) ( ( helped ( ( ( ( create ( ( earth 's ) atmosphere ) ) , ) and ) ( continue ( to ( ( affect ( its composition ) ) today ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Volcanic) (NNS gases)) (VP (VBD helped) (VP (VP (VB create) (NP (NP (NN earth) (POS 's)) (NN atmosphere))) (, ,) (CC and) (VP (VB continue) (S (VP (TO to) (VP (VB affect) (NP (PRP$ its) (NN composition)) (NP (NN today)))))))) (. .)))
Volcanic gases helped create earth's atmosphere, and continue to affect its composition today.
(ROOT (S (NP (JJ Volcanic) (NNS gases)) (VP (VBP helpe) (S (VP (TO to) (VP (VB form) (NP (DT the) (NN atmosphere)) (PP (IN on) (NP (NN earth))))))) (. .)))