Named for Island Park, westernmost part of Yellowstone Plateau volcanic field, Fremont Co, ID. Comprised of following small rhyolite domes cropping out within or adjacent to the Henrys Fork caldera: Moonshine Mountain, Silver Lake, Osborne Butte, Elk Butte, Lookout Butte, and Warm River Butte. Type area "arbitrarily designated" as Lookout Butte. Can't show unequivocal outcrop relations because all the domes are nearly buried by younger Gerrit Basalt (new). Relative ages can be established by other means: a) domes stand above floor of second-cycle Henrys Fork caldera, therefore they postdate Mesa Falls Tuff and its penecontemporaneous caldera collapse; b) Lava Creek Tuff, emplaced later into Henrys Fork caldera, has normal paleomagnetic polarity (as do all younger units), but the domes of Island Park Rhyolite all have reverse polarities--thus, Island Park predates Lava Creek. Island Park consists of extremely phenocryst-rich rhyolite, the large (one to several cm across) phenocrysts generally constituting more than half the rock. Rhyolite bodies are typical, steep-sided, mainly endogenous domes with shell-like flow structure patterns parallel to sides and concentric around vents. Occur in a linear belt 30 km long, 7 wide, trending northwestward. Osborne Butte dated as 1.3 m.y. old (Obradovich, written commun., 1982). All the domes are probably only slightly younger than Mesa Falls Tuff. Geologic map. Stratigraphic chart.

Source: GNU records (USGS DDS-6; Denver GNULEX).