volcano vs fissure

volcano and fissure both are nouns.

Word	Noun	Adjective	Verb	Adverb
volcano	Yes	No	No	No
fissure	Yes	No	Yes	No

As nouns, fissure is a hypernym of volcano; that is, fissure is a word with a broader meaning than volcano:

volcano: a fissure in the earth's crust (or in the surface of some other planet) through which molten lava and gases erupt
fissure: a long narrow cleft

Other hypernyms of volcano include cleft, crack, crevice, scissure.

volcano (noun)	fissure (noun)
a mountain formed by volcanic material	(anatomy) a long narrow slit or groove that divides an organ into lobes
a fissure in the earth's crust (or in the surface of some other planet) through which molten lava and gases erupt	a long narrow cleft
	a long narrow depression in a surface

volcano (verb)	fissure (verb)
	break into fissures or fine cracks