Kelsey Creek rhyodacite lava, Clear Lake Volcanic Field, California

Rhyodacite has more silica than dacite and less than rhyolite. This flow-banded rhyodacite is gorgeous in outcrop and is around a half million years old. Rhyodacites in the Clear Lake field commonly contain a potassium-rich feldspar mineral called sanidine, which can be used to date the timing of eruption very precisely via radiometric geoochronology. USGS photo by Dawnika Blatter.