Volcanic History of San Francisco Mountain
San Francisco Mountain is a stratovolcano in the San Francisco Volcanic Field and was built by eruptions between about 1 million and 93,000 years ago. Since then, much of the mountain has been removed to create the "Inner Basin." The missing material may have been removed quickly and explosively by a lateral blast type of eruptive event – similar to the 1980 eruption of Mount St. Helens, Washington – or it may have been removed slowly and incrementally by a combination of large landslides, water erosion, and glacial scouring. Any of the deposits of these events are now buried under younger volcanic deposits. The most recent eruption was Sugarloaf Dome at the mouth of the Inner Basin.