( ( ( The rifting ) ( of ( continents ( that ( heralds ( ( the ( incipient formation ) ) ( of ( an ( ocean basin ) ) ) ) ) ) ) ) ) ( ( ( is usually ) ( accompanied ( by ( ( large amounts ) ( of ( volcanic activity ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN rifting)) (PP (IN of) (NP (NP (NNP continents)) (SBAR (WHNP (WDT that)) (S (VP (VBZ heralds) (NP (NP (DT the) (JJ incipient) (NN formation)) (PP (IN of) (NP (DT an) (NN ocean) (NN basin)))))))))) (VP (VBZ is) (ADVP (RB usually)) (VP (VBN accompanied) (PP (IN by) (NP (NP (JJ large) (NNS amounts)) (PP (IN of) (NP (JJ volcanic) (NN activity))))))) (. .)))
The rifting of continents that heralds the incipient formation of an ocean basin is usually accompanied by large amounts of volcanic activity.
(ROOT (S (S (NP (DT This) (NN process)) (ADVP (RB usually)) (VP (VBZ takes) (NP (NP (DT the) (JJS longest) (NN amount)) (PP (IN of) (NP (NN time)))))) (: :) (S (NP (DT an) (NN ocean) (NN basin)) (VP (VBZ forms) (PP (IN between) (NP (CD two) (NNS continents))))) (. .)))
This process usually takes the longest amount of time: an ocean basin forms between two continents.