A 400' aerial view over Glass Mountain obsidian flow as it slowly poured down the mountainside. The flow lies at the eastern end of the Medicine Lake Caldera in California. Glass Mountain consists of a large pile of obsidian that erupted around 900-1000 years ago from 13 different vents. The obsidian varies from familiar black shiny outcroppings to obsidian froth. Most obsidian is found within the edges of the flow while the interior as well as much of the top of Glass Mountain itself is comprised of dacite. No rock collecting allowed.