Glass Mountain, Volcanic summit in Mono County, California, US.

Glass Mountain is a volcanic summit in Mono County that rises to about 11,100 feet, forming the northeast boundary of the Long Valley Caldera. Its surface consists mainly of rhyolite, a volcanic rock that gives the mountain its distinctive glassy appearance.

Glass Mountain Ridge is a chain of rhyolite lava domes, flows and welded pyroclastic-flow deposits erupted between 2.1 and 0.8 million years ago. The ridge forms the northeast boundary of Long Valley Caldera, and obsidian, a natural volcanic glass, occurs on the mountain.

Obsidian from Glass Mountain was a valued volcanic glass, and the mountain's slopes are still scattered with black, glassy fragments a visitor can spot on the ground. The peak lies in Inyo National Forest, 20 miles southeast of Mono Lake, within the Glass Mountain Ridge.

The route to the summit involves driving on unpaved forest roads that can become difficult in bad weather. Sturdy footwear is essential since the surface is covered with sharp glass fragments and exposed rock.

The gleaming glass fragments covering the slope are actually obsidian, a naturally occurring volcanic glass formed when lava cooled rapidly. This natural shine makes the mountain recognizable from a distance and transforms it into something resembling a giant piece of volcanic jewelry.