So, I'm not a scholar of music history, but I have a basic timeline. The evolution of Western music theory had several times in which certain chords and intervals were considered too "jarring" or "dissonant", but eventually became acceptable as composers found ways to make it work. Obviously, music evolves.
My question is, was there a specific impetus that pushed the role of dissonance out of the jarring mechanism it's normally used as, to simply adding complexity and an "unfinished" quality to chords and phrases? In short, what propelled music from this:
... where the chord structure, though complex, stays pretty consonant, to this...
... where the chord progressions introduce and then resolve quite a bit of dissonance through the phrases, to this:
John Tavener - The Lamb (1982)
... where the majority of the piece is atonal, and intervals we don't normally hear figure prominently, to this...
Eric Whitacre - Water Night (1994)
... where at one point the choir builds to a 19-note "cluster chord", containing every pitch in the key through two and a half octaves (and that's not the most "dissonant" part of the piece by far).
There seems to have been a BIG change in the way we think about dissonance between WWII and the roughly present-day. Even in mosuc with an established key and using pretty standard progressions, the movement of voices to create major and minor second dissonances is now often relished instead of glossed over. Example: "Lully, Lulla, Lullay"; It's very tonally-centered, firmly in the minor key, but has dissonant movement all through it. It would be stereotypical to say that the impetus was the change in all kinds of thinking during the 60's, but that may well be the case; the influence of other genres and of environmentalist thought were certainly present in classical music during that time, as evidenced by the mere existence of a classical piece titled "The Second Dream of the High-Tension Line Stepdown Transformer". I don't have too many notable examples of classical music from that era, certainly not a capella choral to fit the rest of the selections. Does anyone have more information than I to back up this hypothesis?