land mass vs solid ground

land mass and solid ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
land mass	Yes	No	No	No
solid ground	Yes	No	No	No

As nouns, solid ground is a hypernym of land mass; that is, solid ground is a word with a broader meaning than land mass:

Other hypernyms of land mass include dry land, earth, ground, land, terra firma.

land mass (noun)	solid ground (noun)
a large continuous extent of land	the solid part of the earth's surface