The Sonoma Volcanics are a geologic formation of volcanic origin that is widespread in Napa and Sonoma counties, California. Most of the formation is Pliocene in age and includes obsidian, perlitic glass, diatomaceous mud, pyroclastic tuff, pumice, rhyolite tuffs, andesite breccias and interbedded volcanic (basalt) lava flows. The formation serves as the parent material for many of the soils in the Napa and Sonoma wine regions.