Dacite, Clear Lake Volcanic Field

This outcropping of rocks along Morgan Valley Road is comprised of dacite, the most common composition in the Clear Lake Volcanic Field. Dacite contains more silica than basalt but less than rhyolite, so it’s commonly referred to as an “intermediate” composition. The outcrop pictured shows beautiful flow-banding, a feature resulting from the lava flowing over the surface when erupted. The tilted angle of the flow banding, which would have been horizontal at the time of deposition, indicates that this outcrop has been rotated by faulting since the rock cooled. This dacite has a radiometric age of around 1.7 million years old and has crystals of plagioclase and orthopyroxene, although they are small and sparse in an otherwise glass-rich rock. USGS photo by Dawnika Blatter.