Why does volcano have a long vowel sound and volcanic have a short vowel sound?

Answer 1

Nouns ending in O or other vowels usually drop it when adding the -IC suffix (makes an adjective).

Examples are

tornado --> tornadic

cyclone --> cyclonic

Answer 2

The difference in the vowel sounds is due to the syllable stress in each word. In "volcano," the stress falls on the first syllable "vol," making the "o" a long vowel sound. In "volcanic," the stress falls on the second syllable "can," making the "a" a short vowel sound. The placement of stress in a word can affect the pronunciation of vowels.