Moat Rhyolites, old
Moat Rhyolites, old
middle Pleistocene
Chain of rhyolite domes and flows on resurgent uplift in northwestern part of Long Valley caldera. Consists of at least four extrusive units and one intrusive, all subalkaline porphyritic rhyolites erupted through underlying Early rhyolite (map unit lve), with ~10-20% phenocrysts of sanidine, plagioclase, and lesser hornblende, biotite, and sparse quartz, and other trace minerals (Hildreth, 2004; Hildreth and Fierstein, 2016). Domes vary from very small to moderate size with up to 145 m relief (Hildreth and Fierstein, 2016). Lavas are typically blocky and massive with glassy groundmass; variably vesicular; some with finely to micropumiceous texture; some with coarse basal flow breccia; with sparse to common crystal-poor mafic enclaves; the distal end of one relatively extensive lava that flowed downslope toward the topographic moat is strongly hydrated, probably from interaction with a water body (Hildreth and Fierstein, 2016). K-Ar sanidine ages on extrusive units range from 527 ± 12 ka to 481 ± 10 ka (Mankinen and others, 1986), but Ar-Ar sanidine ages are 570 ± 8 ka on two of the same extrusive units (Simon and others, 2014).