Oakland’s volcano

Oakland has its own volcano up in the hills. You can see it in the banner photo at the top of the page: Round Top is the hill on the skyline at the left. There is the Sibley Volcanic Preserve, a former quarry exposing the rocks of a basaltic volcano that was active about 10 million years ago. The former volcano has been shoved on its side by the same pressures across the Hayward fault that raised (and continue to raise) the hills.

When you get up there, the volcano is not easy to find, like many things in geology. It takes a lot of clambering and closely examining the rocks. Fortunately real geologists have done that work. But here and there you’ll see really clear evidence, like this boulder of lapilli tuff. An explosive eruption spraying ash in the air created these little balls, about the size of pencil erasers, which collected in layers and were eventually lithified. Nowadays the action has moved up the faults to The Geysers and the Clear Lake volcanics, but that’s outside Oakland.

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