Though not as culturally entrenched as Aaron Copland, Philip Glass is such a regular feature on the American landscape that it’s shocking to discover how shocking he can still be.

Those left rapturous by the Metropolitan Opera’s Akhnaten earlier this season were not destined for a similar experience on Saturday with the four-hour Music in 12 Parts (written 1971-74) that is the centerpiece of the Annenberg Center’s #Glassfest. It was definitely an event if only because of its epic dimensions. The eight-person Philip Glass Ensemble was imported for the work, written for multiple keyboards, winds and voice, in what is considered the culmination of his first creative period. Not what I’d call a masterpiece, it’s a manifesto, a musical vocabulary built from the ground up before your very ears, hugely inventive within its limited scope but also somewhat uneven.

Though the piece directly leads the way to later collaborations with the dream-like Robert Wilson theatrical images in the opera Einstein on the Beach or the cinematic wizardry in the Godfrey Reggio film Koyaanisqatsi, Music in 12 Parts remains quite challenging when taken on its own. Two intermissions and a dinner break were necessary to take in so much intricately interlocking minimalism, with words consisting of solfege syllables ("do" and "re") and instrumental textures that sometimes (but don't always) create the illusion of three-dimensional musical holographs.

Though often ebullient and propulsive, the music never pounds because bass lines are so nimble. The quirks are among the most engaging elements: At first they’re hiccups that interrupt the musical momentum in witty ways but, by the end, are points of considerable expansion in the musical vocabulary. The final section puts everything together with a kind of grandeur that might seem antithetical to anything claiming to be minimalist. Always, the music arrives in larger, longer proportions than you would ever ask for.

The first three parts experiment with melodies, sometimes consisting only of two notes, reaching upward and outward. Others are more self-contained with a sense of repose, but always implying something greater than what is there. The next three parts experiment with harmony, often sounding richer than what is literally possible with six musicians. Later parts deal with instrumental colors. When jumping from one idea to the next, the music achieves an increasingly deft balance of surprise and continuity. At times Glass stops pushing forward and just has fun with what he has — the danger being that his basic musical materials don’t hold up under incessant repetition.

However original, this music has its precedents: Bruckner’s repetition has similarly mesmerizing effects. Schubert also used a single note to abruptly transform the music’s complexion. Up the street in University City, the Cinemark theater had just carried a Metropolitan Opera simulcast of Handel’s also-expansive Agrippina with the same lesson as Glass: Both pieces occupy large blocks of time that defy your everyday, out-in-the-world metabolism. But they force you to stop using time and insist that you accept it.

The near-full Zellerbach Hall was mostly up for this Glass marathon. Of course there were defections, but only leaving pockets of empty seats, suggesting that the concert was some class assignment that didn’t require students to stay to the end. Those who stayed were deeply attentive, though somehow applause seemed like an awfully quaint reaction. This music is so tightly scored that you weren’t sure if the performance was more of a reenactment. Certainly, the concert was a superhuman act of concentration from remarkably distinguished personnel, which did not include Glass himself, but longtime collaborators such as Michael Riesman on keyboard, as well as the noted composer Lisa Bielawa on vocals, wind players Peter Hess, Andrew Sterman and Sam Sadigursky, keyboardist Mick Rossi, plus two sound engineers, Dan Bora and Ryan Kelly.