( ( Over ( thousands ( of years ) ) ) ( , ( ( ( the process ) ( of ( ( hard corals ) ( ( ( building ( their skeletons ) ) ( -LRB- ( ( known ( as calcification ) ) -RRB- ) ) ) ( on ( top ( of ( ( the skeletons ) ( of ( old corals ) ) ) ) ) ) ) ) ) ) ( ( forms ( the reef ) ) . ) ) ) )
(ROOT (S (PP (IN Over) (NP (NP (NNS thousands)) (PP (IN of) (NP (NNS years))))) (, ,) (NP (NP (DT the) (NN process)) (PP (IN of) (NP (NP (JJ hard) (NNS corals)) (VP (VBG building) (NP (PRP$ their) (NNS skeletons)) (PRN (-LRB- -LRB-) (VP (VBN known) (PP (IN as) (NP (NN calcification)))) (-RRB- -RRB-)) (PP (IN on) (NP (NP (NN top)) (PP (IN of) (NP (NP (DT the) (NNS skeletons)) (PP (IN of) (NP (JJ old) (NNS corals))))))))))) (VP (VBZ forms) (NP (DT the) (NN reef))) (. .)))
Over thousands of years, the process of hard corals building their skeletons (known as calcification) on top of the skeletons of old corals forms the reef.
(ROOT (S (NP (NNS Corals)) (VP (VBP build) (NP (NP (JJ hard) (NNS exoskeletons)) (SBAR (WHNP (WDT that)) (S (VP (VBP grow) (S (VP (TO to) (VP (VB become) (NP (NN coral) (NNS reefs)))))))))) (. .)))