( ( When ( ( the plates ) ( crash ( into ( each other ) ) ) ) ) ( , ( geologists ( ( call ( ( this type ) ( of ( ( plate boundary ) ( a ( convergent boundary ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NNS plates)) (VP (VBP crash) (PP (IN into) (NP (DT each) (JJ other)))))) (, ,) (NP (NNS geologists)) (VP (VBP call) (NP (NP (DT this) (NN type)) (PP (IN of) (NP (NP (NN plate) (NN boundary)) (NP (DT a) (JJ convergent) (NN boundary)))))) (. .)))
When the plates crash into each other, geologists call  this type of plate boundary a convergent boundary.
(ROOT (S (NP (DT A) (JJ convergent) (NN plate) (NN boundary)) (VP (VBZ is) (VP (VBN created) (SBAR (WHADVP (WRB when)) (S (NP (CD two) (NNS plates)) (VP (VBP come) (PP (IN toward) (NP (DT each) (JJ other)))))))) (. .)))