rigidifying vs natural action

rigidifying and natural action both are nouns.

Word	Noun	Adjective	Verb	Adverb
rigidifying	Yes	No	No	No
natural action	Yes	No	No	No

As nouns, natural action is a hypernym of rigidifying; that is, natural action is a word with a broader meaning than rigidifying:

rigidifying: the process of becoming stiff or rigid
natural action: a process existing in or produced by nature (rather than by the intent of human beings)

Other hypernyms of rigidifying include action, activity, natural process.

rigidifying (noun)	natural action (noun)
the process of becoming stiff or rigid	a process existing in or produced by nature (rather than by the intent of human beings)