descend vs plunge

descend is not a noun while plunge is a noun.

Word	Noun	Adjective	Verb	Adverb
descend	No	No	Yes	No
plunge	Yes	No	Yes	No

As verbs, plunge is a hyponym of descend; that is, plunge is a word with a more specific, narrower meaning than descend:

descend (noun)	plunge (noun)
	a brief swim in water
	a steep and rapid fall

descend (verb)	plunge (verb)
come as if by falling	begin with vigor
move downward and lower, but not necessarily all the way	devote (oneself) fully to
do something that one considers to be below one's dignity	cause to be immersed
come from; be connected by a relationship of blood, for example	immerse briefly into a liquid so as to wet, coat, or saturate
	thrust or throw into
	fall abruptly
	drop steeply
	dash violently or with great speed or impetuosity