The scientific inquiry into music revolutions is interesting:
The evolution of western pop music, spanning from 1960 to 2010, has been analysed by scientists.But it's fundamentally flawed, isn't it? If you want to know what's happening in music, the very worst place to look would be the American charts. It'd be like a seismologist restricting themselves to measuring only the smallest of aftershocks.
A team from Queen Mary University of London and Imperial College London looked at more than 17,000 songs from the US Billboard Hot 100.
They found three music revolutions - in 1964, 1983 and 1991 - and traced the loss of blues chords from the charts, as well as the birth of disco.
Just as a for instance, that methodology means that punk is more or less ignored and the revolutions spotted are timed about two years too late.
Still, as a guide to what sorts of music American radio was prepared to play, it's a useful exercise.