Glass Mountain, California

One set of the obsidian flows I have yet to visit are in and around the Medicine Lake caldera. Glass Mountain, Little Glass Mountain, Crater Glass Flow and the Medicine Lake Glass Flow are all rhyolite flows that erupted as recently as ~950 years ago (for Glass Mountain). Unlike the Mono-Inyo chain, Medicine Lake is what we call a bimodal volcano -- that is, it erupts both low silica basalt and high silica rhyolite -- with nothing in between (like andesite).

Image: USGS