Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho

Large, Miocene-age rhyolite lava flows occur in the Bruneau-Jarbidge area of the central Snake River Plain and in the adjoining Jacks Creek area. The flows typically are 100–150 m thick and have volumes ranging from 10 to 200 km 3. The flow interiors consist of thick central zones of massive devitrified rhyolite overlying zones of basal vitrophyre. These massive central zones are capped by structurally complex upper zones with both glassy and devitrified rhyolite. The upper zones contain gas cavities of varying dimensions and abundance, including swarms of cavities, each a meter or more across. Sheeting joints, in some places accompanied by pencil and dimple joints, are abundant in the upper zones, in the top part of the central zones, and in the marginal parts of the flows. Flow margins consist of bulbous lobes of massive rhyolite separated by steeply to chaotically jointed zones. Few flow margins are less than 25 m thick. The basal and upper zones and the marginal parts of the flows contain abundant breccia formed by en masse flowage and explosive steam release. All of the rhyolite flows are believed to have erupted from fissures. Most flowed onto preexisting soils and other sedimentary materials. Small amounts of air-fall ash occur beneath a few flows near their eruptive fissures, but these deposits are thinner and less widespread than the fallout ash blankets that are beneath many of the southwestern Idaho welded-tuff units. A combination of high effusion rates, high temperatures, and large volumes probably imparted sufficiently low bulk viscosities to the lavas to allow them to flow away from their eruptive fissures to form sheets instead of steep-sided domes. Several large-volume, high-temperature, densely welded, ash-flow-tuff sheets occur in southwestern Idaho. These pyroclastic flows may have coalesced into pools of silicate liquid capable of en masse flowage after emplacement. They are similar in appearance to the rhyolite lava flows in that region. However, a combination of physical characteristics can be used to distinguish the two types of flows. Good indicators that a given rhyolite sheet may be a lava flow—rather than a unit emplaced as ash hot enough to form a liquid pool and flow—are the presence of blunt flow margins, abundant basal and marginal flow breccias, pervasive flow layering, laterally persistent zones of mismatched vertical shrinkage joints, complex contacts between basal vitrophyres and overlying zones of devitrified rhyolite, and abundant zones with pencil jointing, combined with the absence of lithic fragments, pumice fragments, bubble-wall shards, extensive phenocryst breakage, internal subhorizontal ash-emplacement layering, and subparallel flow marks.