It's called tri-syllabic laxing. That's when a two-syllable word becomes a three-syllable word and stress is redistributed, like this,
[1]
pro'vince (two syllables)
[2]
pro'vin'cial (three syllables)
In [1] the letter "o" is pronounces as [a], as in f
ather, and in [2] the letter "o" is is pronounced as
schwa, which is the sound the letter "e" makes in the word th
e.
Here's how tri-syllabic laxing works: Add the suffix -al to the word province and the stress changes; it moves one syllable forward:
[1] pr
o (
stressed) vince
[2] pro (
unstressed) v
in (
stressed) cial
In [2], stress is taken off of "o", pronounced [a], which reduces or laxes the vowel sound [a] to schwa. Other examples of
schwa are,
want to => wann
a
going to => gonn
a
Schwa is unstressed. Any vowel stripped of its place of articulation is reduced to schwa.
Hope that helps.