Our local volcano is now dressed in its finest greenery, so this is an ideal time to stroll around Sibley Volcanic Preserve (map). Typically the scenery is wonderful before things dry up and brown out – just look at the panorama snapped this week.
Volcanic Time Capsule
Besides soaking in the views, it’s also worth contemplating the 10-million-year time capsule underneath your feet. Sibley is one of four volcanoes that originally erupted near San Jose, and slowly traveled northwards. Presumably it will pass us by, in another 10 million years or so.
Interestingly, the Sibley volcano tipped sideways and and was pretty much hidden from view. When gravel rock mining took place years ago, volcanic features became more evident. A few Berkeley geologists discovered this wonder and have studied the rocks and formations in detail.
One of these Berkeley geologists, Steve Edwards, first mapped the area in the 1970s and later joined the East Bay Regional Park District. He shared the geological underpinnings in KQED’s Quest program last July.
Touring The Volcano
Just like all the earthquakes around here, the volcanic activities were triggered by plate tectonics. You can walk up to Round Top (1,763 ft), the highest point in the park, where Pacific and North American plates crashed together and molten basalt lava flowed.
Then walk a quarter mile northeast, and arrive at the volcano top near the gravel rock quarry’s edge. It turns out there are all kinds of lava flows, bake zones and lava holes scattered throughout Sibley as well.
Stop and stare at the basalt rock, and all their unusual shapes. Geologist Andrew Alden recently pointed out spheroidal weathering. “To paraphrase an old Grateful Deadhead saying,” said Alden, “the smaller they grow, the rounder they get.”
So head up to Sibley Preserve, for another perspective about your local volcano – those rocks actually tell quite a story.
More info: Sibley Volcano Regional Preserve Exploration – Voice of the Volcano – Basalt of Sibley Volcano – Sibley Volcanic Preserve – Sibley Trail Map
2 thoughts on “Sibley Geology In Full View”
Hi, thanks for pointing residents to this regional treasure.
I’d like to clarify that the quarry did not mine gravel; that comes from the big sedimentary basin between Dublin, Pleasanton and Livermore (Shadow Cliffs park is old gravel-pit ponds). At Sibley they quarried solid rock and produced aggregate, or crushed rock. As far as I know, every quarry in Oakland produced crushed rock rather than dimension stone or sand & gravel
Thanks for the great post on Sibley and the link to Bay Nature’s article (I work there)!
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I only recently did Sibley (came on board at BN well after the article was published); it was unique.