( However ( , ( ( ( ( ( at ( ( irregular intervals ) ( averaging ( ( ( several hundred ) thousand ) years ) ) ) ) ( , ( ( ( the ( Earth 's ) ) field ) reverses ) ) ) and ) ( ( the ( North ( and ( South ( Magnetic ( Poles relatively ) ) ) ) ) ) ( abruptly ( switch places ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (S (PP (IN at) (S (NP (NN irregular) (NNS intervals)) (VP (VBG averaging) (NP (QP (JJ several) (CD hundred) (CD thousand)) (NNS years))))) (, ,) (NP (NP (DT the) (NN Earth) (POS 's)) (NN field)) (VP (VBZ reverses))) (CC and) (S (NP (DT the) (NNP North) (CC and) (NNP South) (NNP Magnetic) (NNPS Poles) (NNP relatively)) (ADVP (RB abruptly)) (VP (VB switch) (NP (NNS places)))) (. .)))
However, at irregular intervals averaging several hundred thousand years, the Earth's field reverses and the North and South Magnetic Poles relatively abruptly switch places.
(ROOT (S (NP (NP (DT The) (NN earth) (POS 's)) (JJ magnetic) (NNS poles)) (VP (VBP have) (VP (VBN switched) (NP (NNS places)) (ADVP (RB repeatedly)) (PP (IN in) (NP (DT the) (NN past))))) (. .)))