Pyroclasts is a companion album to 2019’s Life Metal. At the beginning and end of each day’s sessions for that album, Sunn O))) founders Greg Anderson and Stephen O’Malley and their guests — cellist Hildur Gu∂nadóttir, bassist Tim Midyett and synth player Tos Nieuwenhuizen — would improvise around a drone in a ritualistic manner. Thus, the key is specified in the title of each track: “Frost (C),” “Kingdoms (G),” “Ampliphædies (E),” and “Ascension (A)”. Each track runs about 11 minutes, and they’re relatively stark and minimal, with all the instruments droning “in tune” and relatively little variation in volume or intensity. Still, the sounds do shift, rising and falling and changing slightly as they go, so it’s sort of like watching the wind blow patterns in an all-black sand dune. The surface moves, but the whole remains.