( ( Many geologists ) ( ( believe ( that ( ( huge ( convection currents ) ) ( ( mix ( ( this part ) ( of ( the mantle ) ) ) ) ( as ( it ( ( ( ( ( ( rises ( toward ( the asthenosphere ) ) ) , ) cools ) , ) and ) ( ( ( sinks back ) ( toward ( the core ) ) ) again ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Many) (NNS geologists)) (VP (VBP believe) (SBAR (IN that) (S (NP (JJ huge) (NN convection) (NNS currents)) (VP (VB mix) (NP (NP (DT this) (NN part)) (PP (IN of) (NP (DT the) (NN mantle)))) (SBAR (IN as) (S (NP (PRP it)) (VP (VP (VBZ rises) (PP (IN toward) (NP (DT the) (NN asthenosphere)))) (, ,) (VP (VBZ cools)) (, ,) (CC and) (VP (VBZ sinks) (ADVP (RB back)) (PP (IN toward) (NP (DT the) (NN core))) (ADVP (RB again)))))))))) (. .)))
Many geologists believe that huge convection currents mix this part of the mantle as it rises toward the asthenosphere, cools, and sinks back toward the core again.
(ROOT (S (NP (NNS Rises)) (ADVP (RB then)) (VP (ADVP (RB sinks)) (VBZ does) (NP (NP (JJR deeper) (NN material) (NN cause) (NN convection)) (PP (IN in) (NP (DT the) (NN mantle))))) (. .)))