“If I’m running through a chord progression of Am, F, Dm, G chords (as in Mr Jones by Counting Crowes) and noodling around with the C Major scale am I technically only playing the Aeolian Mode when the chord underneath me is Am? What if the chord progression hangs for a long while on the F; does it become a Lydian mode (4th scale degree of C)? Or does it solely depend on what chord functions as the root?”
The modal concept follows the root chord in a progression, but sometimes the root chord can change. In the verse to “Mr. Jones” the A minor chord is definitely functioning as the root. This creates the A Aeolian mode (or natural minor scale) sound. But during the chorus, the chord progression shifts and revolves around the C major chord. This produces C Ionian (or major scale). You can definitely hear the difference as the song clearly becomes brighter when the chorus hits.
If the song included a section that held the F chord long enough, then yes this would produce a Lydian sound.
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