Rhyolitic volcanism within the McDermitt caldera complex in northern Nevada and southern Oregon occurred during four episodes in a 5-m.y. time span from 18.5 to 13.7 m.y. ago. The first three episodes were characterized by eruption of large-volume ash-flow tuffs which led to caldera collapse. Each episode began with eruption of comendite ash-flow tuff with a SiO₂ content of 75% and Al₂O₃ content of 11.2% and each ended with ash flows lower in SiO₂ content (70 to 62%) and higher in Al₂O₃ (13 to 15%). The early high-silica rhyolites show large enrichments of F, Th, U, Zr, and depletions of Ba, Ca, Mg, P, Sr, and Ti relative to the last tuffs erupted. The systematic change in chemistry of the ash-flow tuffs du ing each episode is believed to reflect venting from progressively lower levels of a zoned magma chamber. The fourth episode of volcanism consisted of the emplacement of small intrusives and domes with composition similar to the early high-silica rhyolite erupted in each of the previous three episodes. The last rhyolites erupted tapped only an upper part of a similarly zoned magma chamber.

Uranium ore deposits are associated with the emplacement of the last phase of comendite magma in the complex. Although these rocks are not distinct chemically from the older high-silica rhyolites, their emplacement in a nonexplosive manner resulted in the formation of the ore bodies by localizing magma and vapor in a small chamber rather than dispersing it in ash-flow sheets.

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