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

A: You can easily visit the 10-million-year-old Sibley Volcano (see Bay Nature, April-June 2005) in the hills above Oakland. And I often see college geology classes stopped at the base of the Nicasio Dam in West Marin looking at pillow basalt lava that erupted from the floor of the Pacific Ocean 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 field is part of the larger San Andreas Fault complex, and the magma here fuels the largest geothermal energy production field in the United States-the Geysers Complex, in northeastern Sonoma County.

Michael Ellis – Ask the Naturalist – Bay Nature Magazine

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August 22, 2009