File:Hot Creek Rhyolite Flow (Pleistocene, 288 or 333 ka; Hot Creek Gorge, Long Valley Caldera, California, USA) 3.jpg

English: Altered rhyolite in the Pleistocene of California, USA.

This site is in eastern California's Long Valley Caldera, a large depression formed about 760,000 years ago during a large eruption of the Long Valley Volcano. The eruption produced large volumes of ash, air-fall pumice, and pyroclastic flow pumice.

The outcrop is in Hot Creek Gorge, along which hydrothermal water emerges to form hot springs. The bedrock is the Hot Creek Rhyolite Flow, a sanidine-augite-phyric moat rhyolite (76% silica) that erupted from a vent to the south of here and flowed north into a former lake. The Hot Creek Flow dates to ~288 or ~333 ka.

Description of Hot Creek Rhyolite lithologies at Hot Creek Gorge (from Hildreth & Fierstein, 2017): ". . . the rhyolite is flow-foliated, locally vuggy, and variously includes pale-gray to black vitrophyre, bluish-gray resinous perlite, gray to pink felsite, coarsely pumiceous carapace, dark brown spherulitic zones, and flow breccia. Cream-white domains can be either felsite, finely vesicular glass, or silicified. Strongly oxidized and hydrothermally altered exposures are common along the gorge. . . . The rhyolite contains only ~1 percent phenocrysts, of which half are plagioclase, accompanied by trace amounts of sanidine, biotite, and clinopyroxene, as well as microphenocrysts of Fe-Ti oxides, zircon, and apatite."

Locality: trailside outcrop along the southern side of Hot Creek Gorge, Long Valley, eastern California, USA (vicinity of 37° 39' 40.80" North latitude, 118° 49' 40.89" West longitude)

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Reference cited:

Hildreth & Fierstein (2017) - Geologic field-trip guide to Long Valley Caldera, California. United States Geological Survey Scientific Investigations Report 2017-5022-L. 119 pp.