ice mass vs ice field

ice mass and ice field both are nouns.

Word	Noun	Adjective	Verb	Adverb
ice mass	Yes	No	No	No
ice field	Yes	No	No	No

As nouns, ice field is a hyponym of ice mass; that is, ice field is a word with a more specific, narrower meaning than ice mass:

ice mass: a large mass of ice
ice field: a large flat mass of ice (larger than an ice floe) floating at sea

Other hyponyms of ice mass include glacier, berg, iceberg, ice cap, icecap, floe, ice floe.

ice mass (noun)	ice field (noun)
a large mass of ice	a large flat mass of ice (larger than an ice floe) floating at sea