Progressive rock wasn't built for slamdancing. It's dangerous enough to hurl your body into someone else's without the added worry of spraining an ankle while trying to follow the beat as it shifts from five-six to seventeen-eight time.

Nevertheless, the Mars Volta makes music that is both part of the lineage of progressive rock bands such as Yes; Emerson, Lake & Palmer; and King Crimson -- complete with complex time changes, displays of deft virtuosity and extended jams clocking in at 15 minutes -- and exceedingly mosh-worthy.

The floor of the Riviera Theatre was a mass of flailing limbs and frantic movement throughout much of the Mars Volta's two-hour-and-20-minute show on Monday, the first of two sold-out nights, and it hardly mattered to mesmerized fans that the California-by-way-of-El Paso, Texas, band only played eight songs in all of that time.

On one level, this was wretched excess of the most bombastic pre-punk variety. Vocalist Cedric Bixler-Zavala didn't say a word to the crowd all night; the musicians eschewed an opening act and an encore (the better to keep the focus on their jammin', man!), and guitarist Omar Rodriguez-Lopez seemed incapable of performing a solo that lasted less than six minutes.

But songs such as the opening R&B-flavored epic "Drunkship of Lanterns," the Spanish-language epic "L'Via L'Viaquez" and the just-plain-epic epic "Cassandra Gemini" were also intensely physical, hitting with the impact of the best punk rock -- hence the potentially bone-breaking ballet on the floor of the Riv.

This rhythmic intensity is the most obvious trait Bixler-Zavala and Rodriguez-Lopez bring to the Mars Volta from their old band, emo-punk progenitors At the Drive-In. In place of hooks, they now give us musical trigonometry, and instead of the old sensitive-soul lyrics, Bixler-Zavala now spews science-fiction psychobabble, such as the inscrutable story -- something about a guy named Vismund Cygnus who discovers "a terrible secret" while searching for his mom -- running through the group's second full album, "Frances the Mute."

In concert, it hardly matters what the Afro-sporting singer is going on about in the lyrics. Navigating a ridiculously high register that Jon Anderson of Yes would envy, Bixler-Zavala contorts his words into a series of unintelligible grunts, moans, sighs and screams, all the while moving with the impossibly soulful, loose-limbed grace of the MC5's Rob Tyner doing his James Brown impersonation.

Most of the supporting players were just as entertaining to watch. Drummer Jon Theodore was a nonstop blur of polyrhythmic dexterity, mixing Latin and jazz rhythms into his hard-rock crunch as he guided the group through wave after wave of crescendos and dynamic shifts. Isaiah Ikey Owens brought a bebop fluidity to his keyboard parts, while Marcell Rodriguez-Lopez and Adrian Terrazas were as impressive on their main instruments (flute and sax, and synthesizers, respectively) as they were when supplying additional layers of percussion.

But the Mars Volta has its weaknesses, including guitarist Rodriguez-Lopez -- who channels Carlos Santana in his better moments but Trey Anastasio in his worst -- and bassist Juan Alderete, who was as fun to watch as a statue and who moved a little less.

Displays of showy musicianship such as this are always better appreciated live than on album, and the Mars Volta did put on an impressive show. But even this unapologetic prog-rock fan couldn't help thinking that it would have been vastly improved by some mind-bending videos, a more impressive light show and maybe a laser or two.