( ( When ( ( molten rock ) ( breaks ( through ( ( the surface ) ( of ( the ( ocean floor ) ) ) ) ) ) ) ) ( , ( it ( ( is ( magnetised ( according ( to ( ( the polarity ) ( of ( ( the ( Earth 's ) ) ( magnetic field ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NN molten) (NN rock)) (VP (VBZ breaks) (PP (IN through) (NP (NP (DT the) (NN surface)) (PP (IN of) (NP (DT the) (NN ocean) (NN floor)))))))) (, ,) (NP (PRP it)) (VP (VBZ is) (VP (VBN magnetised) (PP (VBG according) (PP (TO to) (NP (NP (DT the) (NN polarity)) (PP (IN of) (NP (NP (DT the) (NN Earth) (POS 's)) (JJ magnetic) (NN field)))))))) (. .)))
When molten rock breaks through the surface of the ocean floor, it is magnetised according to the polarity of the Earth's magnetic field.
(ROOT (S (NP (NP (NNS Rocks)) (PP (IN on) (NP (NN ocean) (NN floor)))) (VP (VBZ is) (NP (NN evidence)) (SBAR (IN that) (S (NP (JJ magnetic) (NNS reversals)) (VP (VBP occur))))) (. .)))