planation vs eating away

planation and eating away both are nouns.

Word	Noun	Adjective	Verb	Adverb
planation	Yes	No	No	No
eating away	Yes	No	No	No

As nouns, eating away is a hypernym of planation; that is, eating away is a word with a broader meaning than planation:

planation: the process of erosion whereby a level surface is produced
eating away: (geology) the mechanical process of wearing or grinding something down (as by particles washing over it), also figuratively

Other hypernyms of planation include eroding, erosion, wearing, wearing away.

planation (noun)	eating away (noun)
the process of erosion whereby a level surface is produced	(geology) the mechanical process of wearing or grinding something down (as by particles washing over it), also figuratively