Big Southern Butte obsidian

Big Southern Butte is a 760 m rhyolite dome complex standing above the lava plains of the eastern Snake River Plain between Arco and Blackfoot, Idaho. At about 300,000 years old it is the largest and youngest of several rhyolite domes there. Two overlapping domes pushed up through basalt; obsidian occurs as thin spherulitic layers in the eastern dome and in the flow breccia capping the western dome, near the summit and Webb Spring. Its glass is rich in potassium and zinc, yttrium and niobium and low in strontium, so labs can tell it from the similar Cedar Butte and Wedge Butte glass. It is one of the most used obsidians in southern Idaho, about 9 percent of some 4,300 sourced artifacts.

Peralkaline rhyolite dome complex on the eastern Snake River Plain, about 300,000 years old (K-Ar dates of 309 and 294 thousand years for the two domes). It began as an intrusive cryptodome beneath basalt; the eastern dome erupted first and pushed a 350 m slab of basalt aside, then the western dome. The eastern dome is tan to lavender rhyolite grading up into white granular rhyolite and flow breccia with laminated spherulitic obsidian; the western dome is white granular rhyolite capped by flow breccia and obsidian. Chemistry: high K2O, Zn, Y and Nb, low CaO, TiO2, MnO, FeO and Sr, moderate Rb and Zr (Pink 2023).