Global Volcanism Program | Image GVP-06590
Blocks of glassy obsidian reflect sunlight from the surface of a rhyolitic lava dome marking the latest eruptive event during extrusion of the Glass Mountain lava flow. The composite flow originated from more than a dozen NW-SE-trending vents near the eastern rim of the Medicine Lake caldera. Part of the massive obsidian flow seen on the left side of the photo traveled west into the caldera. Snow-capped Mount Shasta towers above the forested caldera rim to the west.
Photo by Lee Siebert, 1998 (Smithsonian Institution).