High Valley basaltic andesite lava flow, Clear Lake Volcanic Field, California

This lava flow is one of five flows of the same composition erupted over a short period of time in the High Valley area. The Round Mountain cinder cone in this area is related to these flows, and all are basaltic andesite, a rock with slightly more silica than basalt and slightly less than andesite. These flows are some of the youngest in the field and are contain the minerals plagioclase, olivine, and pyroxene, which can all be seen in fresh examples of these rocks. USGS photo by Seth Burgess.