I found out something unexpected, punk is actually the parent of extreme music. Basically, rock or hard rock experienced a schism, one path led to heavy metal and one path led to punk. those two streams developed in tandem, with each side taking aggression to its logical end. What I found surprising is that punk actually become more aggressive earlier than heavy metal in 1987 slayer released Reign in Blood, essentially a precursor to death metal, which would essentially culminate in aggression with Canibal Corpses albums in the mid 90s. but prior to 1987, crust punk and power violence were already pushing the boundaries of extremity. Siege was playing essentially blast beats in 1985, and of course Napalm Death released scum in 1987 - which was much more dissonance and extreme than Slayer.