Obsidian is a volcanic glass, a comparatively rare and crystal-poor form of rhyolite, the most silica-rich lava. It forms when lava is quenches, or rapidly cooled. Obsidian is a major component of Glass Mountain.
Provided by Jeff Dinnel, Tionesta, California
Topic: Earth Science
Keywords: Lava, Medicine Lake (Cal.), Rhyolite, Volcanism, Volcanoes