Glass Mountain is a prominent peak in eastern California’s Inyo National Forest, about twenty miles southeast of Mono Lake, and the highest point on the winding Glass Mountain Ridge. Forming the northeastern edge of Long Valley Caldera, it belongs to a volcanic landscape created by eruptions roughly 2.1 to 0.8 million years ago. Instead of a single cone, the mountain consists of rhyolitic lava domes, lava flows, and welded pyroclastic flows, which give it both its unusual shape and geologic importance. Its name comes from the obsidian found there, a volcanic glass produced by silica-rich eruptions that makes the mountain a notable feature of the Mono Basin.