Metal, even Heavy Metal based around riff pairs, in contrast is melodically structured instrumental music. Metal uses heavy, Psychedelic and Blues-based riffs based around power chords and merely repeated phrases in a longer melodic construction that is usually minor-key, modal, or chromatic. Tonal centers of Metal riffs in each composition (metal tracks are not vocally-based so therefore not songs) are not fixed but progress over the course of it. The “rhythm guitar” is therefore not a merely a rhythm instrument but in fact the lead one. Unless you're shredding, there can be no Metal without a rhythm guitar. The drum kit is enslaved to the guitars as a mere metronomic timekeeper despite whatever swing, texture, or fills the drummer flourishes the music with. If Robbing the Graveyard and Raping the Dead from Satan's Massacre is any proof, you construct Metal music simply with your amp and guitar with no bass or percussion needed at all.
For comparison, let's listen to Motörhead. They combined riff progression with chopped up minor key melodies to progress their Heavy and Speed Metal compositions forward. Listen to “Overkill”‘s thrashy metronomic pick-up beat, the shifting forward chord progressions, and the appropriately progressing leads:
https://youtu.be/ueeEEXE7Po8
Motörhead’s minimalist approach to writing music meant that they never had to budget riff; Lemmy and company always exploited the basic melody in a riff way further than they needed to in order to construct a Speed Metal track or whatever other genre they felt like playing such as the occasional crooner ballads. Despite repeating rhythms and structures, they successfully reexerted this formula artistically and commercially until Lemmy died right after the release of Bad Magic:
https://youtu.be/Zh_pk02IOnE
