Oakland’s serpentinite patch is the snaky zone shown in deep purple on the geologic map. I’ve written many posts about individual localities, but this is the first time I’ve shown and talked about the whole thing.
Here’s the same area in Google Maps terrain view, for reference. The rock doesn’t have a strong expression in the topography, although it tends to support less tree cover.
Serpentinite (accent on the “pen”) is a rock composed of the serpentine minerals (accent on the “serp”). Those are three: chrysotile, antigorite and lizardite. I haven’t found chrysotile in Oakland, although I’d be thrilled to: it’s the fibrous mineral used to make asbestos. Antigorite forms at higher temperatures and is often brownish green. That’s not common in Oakland.
Lizardite is the shiny, slippery-looking, soapy-feeling serpentine we have. Here’s a typical example sitting by Redwood Road.
That color is on the light side of its range in hue. It can sometimes be white, or close to it. On the dark side is this splendid specimen from a Millsmont hillside, what’s sometimes called California jade.
Serpentinite is what happens to the deepest rock in the oceanic crust—the dark and heavy material called peridotite—when it reacts with seawater at high temperature and pressure. That is to say, large parts of the deep oceanic crust, miles below the seafloor, consist of this stuff. We would never, ever see this rock unless it somehow got lifted up and put on land. Luckily, plate tectonics enables that to happen sometimes. The result is a body of rock called an ophiolite.
Russ Graymer, author of the geologic map, classifies our serpentinite patch as part of the Coast Range ophiolite. The Coast Range ophiolite is a little older than Oakland’s Franciscan rocks underlying Piedmont and surroundings. It’s not pure serpentinite by a long shot, but the name is a good shorthand. The Coast Range ophiolite is strewn up and down the Coast Range in dribs and drabs. The experts are still working out its story.
If you look on the geologic map you’ll see six areas of serpentinite defined by heavy black lines (plus a thin wisp next to Holy Names that I’m ignoring). I happen to have photographed it in all six, going from north to south:
2: Joaquin Miller Park’s Visionary ridge is serpentinite, and the body extends south to Butters Canyon.
3: Joaquin Miller Park has a small body of serpentinite separated by a thrust fault from body number 2. It’s exposed in the Friends of Sausal Creek nursery.
4: The biggest area of serpentinite encompasses an area from upper Joaquin Miller Road to the Crestmont area and, over the ridgetop to the east, Serpentine Prairie (I and II). Its long tail running past Merritt College is a part I have yet to explore.
5: A long, narrow strip of serpentinite extends down to Lincoln Square.
6: Another long strip extends from the belly of body number 4 across Redwood Road, including Old Redwood Road.