( ( At ( a ( subduction zone ) ) ) ( , ( ( ( cool and ) ( dense ( ocean crust ) ) ) ( ( comes ( ( in contact ) ( ( with less ) ( dense ( continental crust ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (DT a) (NN subduction) (NN zone))) (, ,) (NP (NP (NN cool)) (CC and) (NP (JJ dense) (NN ocean) (NN crust))) (VP (VBZ comes) (PP (PP (IN in) (NP (NN contact))) (PP (IN with) (CC less) (NP (JJ dense) (JJ continental) (NN crust))))) (. .)))
At a subduction zone, cool and dense ocean crust comes in contact with less dense continental crust.
(ROOT (S (NP (NNP Gabbro)) (VP (VBZ is) (NP (NP (DT a) (JJ dark) (JJ dense) (NN rock)) (SBAR (WHNP (WDT that)) (S (VP (MD can) (VP (VB be) (VP (VBN found) (PP (IN in) (NP (DT the) (JJ oceanic) (NN crust)))))))))) (. .)))
Gabbro is a dark dense rock that can be found in the oceanic crust.