The Glass Mountain volcanic complex represents the oldest rhyolites of the Long Valley magma system. At least 30 high-silica rhyolites erupted in two major episodes; a minimum of 4.8 km3 (DRE) of lava erupted sporadically between 2.1 and 1.2 Ma, and a minimum of 10.5 km3 erupted at a greater volumetrc rate 1.1--0.8 Ma. The rhyolite complex was truncated as a result of caldera collapse on eruption of the Bishop Tuff at 0.73 Ma. Early Glass Mountain lavas are biotitebearing and were cooler and less fluid than later ones, some of which contain pyroxene. This correlates with a crude, long-term trend with time toward less evolved rhyolite compositions; all Glass Mountain lavas are more evolved than the late-erupted Bishop Tuff. A large, integrated, magma chamber may not have been present until α1.1 Ma, as evidenced by the greater chemical homogeneity of lavas of the younger eruptive episode. Glass Mountain is flanked by a pyroclastic apron containing ash-flow tuffs and pumice fall deposits and correlatives of some of the latter have been found as far away as Ventura County, California, and Beaver County. Explosive activity occurred throughout the eruptive history of Glass Mountain and is not restricted to more evolved compositions. Although apparent volumetric eruptive rates increased about six fold after 1.1 Ma, the eruptions apparently were not large enough to depressurize the magma chamber and initiate a caldera-formed eruption. As Long Valley is a tectonically active area, release of pressure on the magma chamber due to rupture along the Hilton Creek-Hartley Springs fault zone seems to most likely mechanism for triggering the catastrophic eruption of the Bishop Tuff.