When was the last volcanic eruption in the Bay Area? Where was it?

You can easily visit the 10-million-year-old Sibley Volcano (see Voice of the Volcano, April-June 2005) in the hills above Oakland. And college geology classes often visit the Nicasio Dam in West Marin to see pillow basalt lava that erupted deep in the Pacific over 100 million years ago.

But for the most recent local volcano, travel north to the 4,200-foot Mount Konocti near Clear Lake. It ceased erupting just 10,000 years ago—a geologic blink of an eye. Konocti (Pomo for “Mountain Woman”) is in the Clear Lake volcanic field, which has been active for at least 2 million years. The magma here fuels the largest geothermal energy production field in the United States—the Geysers Complex, in northeastern Sonoma County.