The McDermitt caldera (western USA) is commonly considered the point of origin of the Yellowstone hotspot, yet until now no geologic map existed of the caldera and its geology and development were incompletely documented. We developed a comprehensive geologic framework through detailed and re connaissance geologic mapping, extensive petrographic and chemical analy sis, and highprecision Eruption of the McDermitt Tuff generated the irregularly keyholeshaped, 40 × 30-22 km McDermitt caldera. Collapse occurred mostly along a narrow ringfault zone of discrete faults with variable downwarp into the caldera be tween faults. Minor parallel faults locally widen the zone to as much as 6 km. We find no evidence that the McDermitt caldera consists of multiple nested calderas, as previously postulated. Total collapse was no more than ~1 km, and total erupted volume was ~1000 km 3 , of which 50%-85% is intracaldera tuff. Uncertainties in these estimates arise because an intracaldera tuff section is exposed only along the western edge of the caldera, and outflow tuff is not completely mapped. Megabreccia and mesobreccia are common in intracal dera tuff in the complete section. Intracaldera tuff is strongly rheomorphic, scrambling the compositional zoning, which is locally better preserved in out flow sections. However, outflow tuff is also strongly rheomorphic where it was deposited over steep topography.The caldera underwent postcollapse resurgent uplift driven by intrusion of icelandite magma, which also erupted from two major vents and as wide spread lavas. Major igneous activity around the McDermitt caldera lasted from before 16.7 to 16.1 Ma, although minor highalumina olivine tholeiite lavas were emplaced ca. 14.9 Ma in the youngest recognized igneous activity.Tuffaceous sediments as much as 210 m thick filled the caldera, although whether deposition preceded, followed, or spanned resurgence is unresolved. Although formed during regional extension, the caldera is cut only at its west ern and eastern margins by much younger, highangle normal faults that re sulted in gentle (~10°) eastward tilting of the caldera.Numerous hydrothermal systems probably related to caldera magmatism and focused along caldera structures produced Hg, Zrrich U (some along the western caldera ring fracture dated as 16.3 Ma), Ga, and minor Au mineral ization. Lithium deposits formed throughout the intracaldera tuffaceous sedi ments, probably ca. 14.9 Ma.Silicic central Snake River Plain, which also erupted voluminous rhyolitic tuffs. How ever, the Snake River Plain ignimbrites are metaluminous and homogeneous in bulk chemistry, with plagioclase, sanidine, and minor quartz as common phenocrysts, and rarely contain pumice or lithics.
