Metal is all about heavy riffs that sound good with not necessarily theory to back it up. Riffs can be aided by some theory, but it's not usually the case.
It's just some bogans jamming some heavy (sweet sounding) riffs and their music creation is somewhat unlimited because they don't have to abide by musical 'rules' or 'guidelines' such as scales & chords..
Saw them at Big Day Out in January, they were sweet!
