Rhyolite outcrop, Bottle Rock Road, Clear Lake Volcanic Field

The rhyolite outcrop shown here is found along Bottle Rock Rd., which gets its name from the volcanic glass that’s common within this rock unit. Rhyolite is the most silica-rich rock in the Clear Lake Volcanic Field, and Thurston Creek Rhyolite, show here, is around half a million years old. This unit is characterized by large blocks of volcanic glass, called obsidian, and also contains pumice, which is also glassy but is full of holes left behind by volcanic gasses escaping the rock during eruption. USGS photo by Dawnika Blatter.