>>127494959
>To add onto this with an actual example, with thrash specifically the lines can get incredibly gray; so what I usually go by is "If it's even slightly heavier than Slayer - Angel of Death, it's no longer related to traditional metal and has gone fully into extreme metal territory." While not 100% precise (that's impossible), this is the best method in my experience.
One reason I'm wondering is because guys like Venom (who are credited with inspiring Black Metal to some extent) released their first album in 1981.
I'm not sure If I posted this one in the classic metal threads before, but a more obscure example of an album crediting with influencing extreme metal would be the Nightmare Theatre album by Exorcist. They were actually a side project of Virgin Steele, a US power metal band.
I dunno if the riffs are heavier than Angel of Death, but vocals are extreme:
https://www.youtube.com/watch?v=Bj_LfJ-oB00
https://www.youtube.com/watch?v=7c0sglMXnA4

Virgin Steele's usual stuff would probably fit in the thread, but IDK if their side project would.
Actually that brings me to another question: Slayer was always an aggressive thrash band, but What about situations like Judas Priest? Their earlier stuff is basically stuff that helped define traditional heavy metal, but later on you have much more aggressive albums like the famous Painkiller (and their 2 albums without Halford).