Moat Rhyolites, intermediate
Moat Rhyolites, intermediate
middle Pleistocene
Consists of rhyolite lavas and vents in the southeast moat of Long Valley caldera. Includes four crystal-poor (1-2% phenocrysts) rhyolites and two porphyritic hornblende-biotite rhyolites; all subalkaline; notably the four crystal-poor eruptive units are an exception to the otherwise phenocryst-rich post-Early rhyolite (map unit lve) eruptive units (Hildreth, 2004; Hildreth and Fierstein, 2016). Material erupted from six vents, but here the mapped extent only represents the Rhyolite of Hot Creek (Hildreth and Fierstein, 2016); other vents and flows located to the east (see Bailey, 1989) are partly covered by younger sediments and their map extent is too small to show at map scale. Total volume ~1.5 km3, most of which is the phenocryst-poor Rhyolite of Hot Creek, which flowed at least 4 km to the north, where interaction with a water body produced flow breccia and perlite (Hildreth, 2004; Hildreth and Fierstein, 2016). K-Ar glass ages range from ~369 to ~329 ka (Mankinen and others, 1986), excluding the anomalously young 288 ± 31 ka K-Ar age at Hot Creek, for which Simon and others (2014) obtained an Ar-Ar sanidine age of 332 ± 2 ka; ages for three of the eruptive units cluster around 330 ka.