Archive for the ‘Oakland serpentinite’ Category

Oakland’s serpentinite patch

4 May 2015

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.

serpzonemap

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.

serpzonetopo

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.

serp-redwoodrd

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.

millsmont-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:

1: In the Piedmont Pines neighborhood it crops out on Castle Drive and Las Aromas Street.

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.

Blueschist on Old Redwood Road

27 April 2015

How many people are going to visit a quiet dead-end up a steep hill just to see if any rocks are there? Not too many, but I’m one of them. The street is Old Redwood Road, a short arc overlooking the Munck Elementary School that doesn’t quite return to Redwood Road. It has a handful of homes and all of this blueschist.

bluschst-oldredwood1

The exposure is about 100 feet long and in good condition. I didn’t inspect it with minute care, but it shows evidence of lots of shear, which you’d expect in this rock and this setting.

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It tapers off at each end and rises to about 10 feet in the middle. This view of the bottom end shows an interesting feature: there appears to be an irregular pod of serpentinite in it, on the right-hand side.

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A closer look at it reveals some typical serpentinite features: a gleaming surface, a relatively green hue, and balls of partially remineralized stuff that are polished and shaped like balls of clay rolled between the hands.

bluschst-oldredwood4

I didn’t find any reference to it online, but perhaps the real experts know all about it. If so, they haven’t published anything. Or maybe they don’t know it’s there—the geologic map says that this is a sliver of Leona volcanics at a contact with the Knoxville Formation (which is indeed exposed downhill). No way.

While I’m talking about this area, I have to say that the Munck School grounds, and Pinto Playground next to it, look like yet another former quarry, although they may have been excavated between 1959 and 1968 when the new Redwood Road was built, isolating Old Redwood Road (and the loop around the Hebrew Day School farther uphill).

Trees and serpentine

29 March 2015

There’s a stretch of Castle Drive, up in the Piedmont Pines neighborhood, lined with huge trees. On the Walk Oakland map, it’s even labeled “Colonnade of Eucalyptus.”

CastleDrTrees

These give me mixed feelings, as tree removal projects have aroused lately elsewhere in Oakland.

First, there’s the experience the trees provide. For one thing, you basically can’t walk here, so the colonnade is not a realistic attraction for walkers. Its main effect is a momentary diversion for drivers, who really don’t need one at this location.

Second, there’s the effect on the surroundings. As you climb up in this valley, the trees emerge as a very tall fence that blocks the view of the hills and the city and the bay.

Third, there’s the geologic setting. This part of the roadway runs along a very steep 40-degree slope through pure serpentinite, visible in the small landslide scar on the right side of the photo. Serpentine rock is poor footing for these massive trees. The trees may seem like they’re buttressing the roadway, but when they inevitably tip over in a storm or earthquake, they’ll uproot it instead, forcing the locals to drive up and down Ascot Drive for many months.

But how about that rock? Here’s a hunk of it that spilled across the road.

castleserpfoot

And here’s a hand specimen. I love this stone, but roadbuilders don’t.

castleserphand

It’s not my problem, since I don’t live there, but I think the best thing to do is to turn this colonnade into a line of ground-level stumps. The root systems would bolster the soil for another decade or so, giving the city time to plan and execute a properly engineered roadway. And bollards set in the stumps would preserve the trees’ most useful current function of keeping cars out of the canyon.

Trees are supposed to be wonderful stockpiles of carbon, sequestering it from the atmosphere. For me, that argument shouldn’t apply to individual trees or even individual groves of trees. What do we do, in the long run, with the carbon in trees—pile the trunks in pyramids? Carbon is best stored in the soil, where it provides excellent tilth and maintains a thriving ecosystem that resists fire and drought. It’s like circulating money in an economy: do you hoard it in vaults or spread it around among people ready to use it as a medium of exchange? Humans have spent thousands of years degrading the world’s soils, and I’d rather we begin to restore them.

The Las Aromas serpentinite

26 September 2014

The southern edge of the Piedmont Pines neighborhood butts against Joaquin Miller Park, and there’s no reason to visit if you don’t happen to live there—except if you like serpentinite. The street named Las Aromas dips into the belt of serpentinite that extends up and over the ridge to Serpentine Prairie, and the rock type is well exposed. One resident has done a lot of landscaping in this inhospitable rock type.

lasaromas-serp1

The scaly structure is typical of serpentinite, and here it’s fairly close to the Hayward fault. Farther uphill is this exposure, where the grain of the rock is parallel to the street.

lasaromas-serp2

Its blue-green color is striking. And there are a few spots where friction in the recent geologic past has turned it glossy. The rocks aren’t as spectacular as Serpentine Prairie itself, but they’re still noteworthy.

lasaromas-serp3

I never tire of our state rock. Speaking of Serpentine Prairie, today on KQED Science there’s a story on the ongoing effort to propagate the rare Presidio Clarkia. (I photographed it a few years ago before they fenced off the area.)