Glass Mountain rhyolite
Glass Mountain rhyolite
early Pleistocene
A complex of lava domes and flows, pyroclastic deposits, and lesser intrusions cropping out along and beyond the northeastern margin of Long Valley caldera. Near caldera margin, consists of an amalgamation of high-silica rhyolite lava domes, low-volume lava flows, flow breccias, pyroclastic deposits, and shallow intrusive masses including plugs and dikes (Bailey, 1989; Metz and Bailey, 1993; Hildreth, 2004). An associated broad pyroclastic fan/apron composed of pyroclastic flow and fall deposits, block and ash flows, and reworked volcanic material extends beyond caldera margin to northeast, east, and south and is substantially dissected (Bailey, 1989). Exposed thickness >1,050 m (Hildreth, 2004). Total magmatic volume estimated to be ~100 ± 20 km3, which includes material now exposed, material inboard of caldera ring fault zone that is downfaulted and buried by younger caldera fill, and widely dispersed pyroclastic fall deposits emitted by up to 17 plinian or subplinian eruptions (Hildreth, 2004; Hildreth and Fierstein, 2016). All eruptive products are exclusively high-silica rhyolite, almost all crystal-poor, with 0-8% of (variably) quartz, sanidine, plagioclase, biotite, allanite, zircon, apatite, and Fe-Ti oxides (e.g., Hildreth, 2004). Metz and Mahood (1985) reported numerous K-Ar ages (mostly on glass and felsite) that range from 2.13 ± 0.03 to 0.79 ± 0.02 Ma. Mapped in detail by Metz and Bailey (1993). Represents an extended rhyolitic magmatic phase leading up to the cataclysmic caldera-forming eruption of the Bishop Tuff (map unit bt) at ~767 ka.