clam up vs close up

clam up and close up both are verbs.

Word	Noun	Adjective	Verb	Adverb
clam up	No	No	Yes	No
close up	No	No	Yes	Yes

As verbs, clam up and close up are synonyms defined as:

Other synonyms of clam up include be quiet, belt up, button up, dummy up, keep mum, shut up.

clam up (verb)	close up (verb)
refuse to talk or stop talking; fall silent	refuse to talk or stop talking; fall silent
	block passage through
	unite or bring into contact or bring together the edges of
	cease to operate or cause to cease operating

clam up (adverb)	close up (adverb)
	very close