by John BushThe Mars Volta are continual contenders for the mantle of most experimental high-profile metal group, along with System of a Down, an artist they've toured with but who usually sell 20 times more records. Mars Volta aren't as popular, not because their riffs are less memorable or innovative but because their cycle of musical buildup and release, although similarly jarring, can last at least 20 minutes instead of System's two. (It's the difference between having a background in acid rock and having one in thrash.) While the early reports on third album Amputechture commented that the duo of Cedric Bixler-Zavala and Omar Rodriguez-Lopez had learned a few lessons about silence and forsaken the concept album, don't believe it. The album is little different than their two previous atom bombs, De-Loused in the Comatorium and Frances the Mute -- tense and anxious, continually pushing the boundaries of extreme production, with long periods of dynamics that rise ever higher, followed by an explosion of release (usually screaming hard rock with storms of atonal brass and horns). The album opens with "Vicarious Atonement," five minutes of spectral effects and piercing guitar that gets a boost at the beginning of the next track, "Tetragrammaton," and then blooms into full riffing glory after a few more minutes (and they're still nowhere near the end of the 16-minute track). John Frusciante, eccentric genius from the Red Hot Chili Peppers, returns on guitar, but Bixler-Zavala and Rodriguez-Lopez exert so much control over the sound of Mars Volta that Frusciante makes virtually no individual impression on this record, although most of the guitar work is his. (Granted, his presence leaves Rodriguez-Lopez open for more intricate work on production.) The Mars Volta are one of the most intriguing bands in rock, but their huge musical power is often deflected by Bixler-Zavala's conceptual themes (which are difficult to follow, but also, perversely, impossible to ignore) and blitzkrieg dynamics that are either dialed down to one or up to ten (but rarely in-between).