Andesite on Highway 175, Clear Lake Volcanic Field, California

This outcrop is along Highway 175, near Hobergs, and is comprised of andesite. Andesite contains more silica than basalt, but less than dacite. Andesites like the one pictured are relatively common in the first half of the Clear Lake Volcanic Field lifespan, this particular rock erupted around 1.4 million years ago, and less common in the more recent history of the field. The rock pictured contains visible crystals of plagioclase, orthopyroxene and clinopyroxene. USGS photo by Dawnika Blatter.