The Snake River Plain of north-western U.S.A. was the site of voluminous, bimodal, hotspot volcanism in the Miocene. Between c. 12.7-6 Ma silicic volcanism produced an association of deposits so different to typical Plinian and ignimbrite deposits elsewhere it has been termed Snake River (SR)-type. The Cassia Mountains of southern Idaho contain SR-type ignimbrites produced from complex and dynamic magmatic plumbing systems involving multiple magma chambers which gave rise to multiple compositional populations of clinopyroxene that mixed during eruption and were deposited together. The Cassia Mountain ignimbrites become progressively more mafic up-succession in terms of whole rock, glass, feldspar and clinopyroxene compositions, reflecting decreasing time available for fractional crystallisation, as supported by geochronology. Two Cassia Mountain ignimbrites are among three newly discovered 'super-eruptions' defined on the basis of phenocryst, glass and whole rock compositions; magnetic polarity; 40Ar/39Ar geochronology; oxygen isotopes; and field data. Erupted volumes range between 640 and 1200 km3, amongst the largest recorded. Intercalated within the Cassia Mountain succession is a newly discovered deposit representing the first recorded explosive, rhyolitic phreatomagmatic eruption from the central Snake River Plain. The fine-grained, non-welded deposit has similar whole rock, glass, oxygen isotope and magmatic temperature characteristics to the surrounding welded ignimbrites, so the unusual deposit facies are interpreted as representing interaction of rising rhyolitic magma with near-surface water. During SR-type volcanism, lavas and ignimbrites of similar chemistry were erupted within a short time. Water contents of melt inclusions were low in both ignimbrites and lavas, consistent with the anhydrous mineralogy and high inferred magmatic temperature. Volatile contents of the magmas (as recorded by the melt inclusions) did not control eruptive style. The intense rheomorphism which characterises SR-type ignimbrites appears to be due to high emplacement temperatures rather than enhanced halogen contents.

A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) largevolume, lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarsegrained ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre; (3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association between the rhyolitic rocks and numerous, coalescing lowprofile basalt lava shields; and (8) widespread evidence of emplacement in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6?8), and involved H2O-poor, low-[delta] 18 O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900?1,050°C). SR-type volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme, and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos Texas, EtendekaParaná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea Dacite of Australia.