Archive for the ‘Oakland serpentinite’ Category

Oakland and the Coast Range ophiolite

24 June 2019

A commenter asked, in connection with a recent post, if I’d written anything about Oakland’s ophiolite. The answer is, not specifically until now. The Coast Range ophiolite (OH-feel-ite) is a string of mostly disconnected outcrops of unusual rocks that extends north almost to Redding and south almost to Point Conception, rather like the way my writing about it runs through the ragged string of posts on this blog between late 2007 and today. In Oakland, the serpentinite patch is part of it, the San Leandro gabbro is part of it, and the Leona volcanics are part of it (see posts about that part here and here).

Here’s a recent simplified geologic map from a 2005 paper by ophiolite savant John Shervais (doi: 10.1130/B25443.1, available here) showing the most important bits of the ophiolite in black. It’s kind of a privilege to have a piece of it in our city.

The details in California are intricate and I’m about ankle-deep into them at the moment, so I’ll be pretty general here. Ophiolites were first recognized as a suite of related rock types over 200 years ago, at the dawn of scientific geology, in the Alps. Alexandre Brongniart gave them the name, which means “snake rock” in scientific Greek, because serpentinite (“snake rock” in scientific Latin) is so typical of them. About 50 years ago, at the dawn of plate tectonics, they were recognized as pieces of oceanic crust that somehow have ended up on land during the elephantine dance of the tectonic plates.

The oceanic crust of most ophiolites is not as well organized as the standard oceanic crust formed at mid-ocean ridges. It forms in the roiling setting near subduction zones, where subducting plates sink straight down and draw the other side toward them. (This situation, called slab rollback, is the opposite of what we’re taught in popular accounts that talk about subduction in terms of colliding continents and mountain-building.) As the plate on the other side is stretched thin, new magma forms beneath it, rises and freezes into fresh oceanic-style crust (the ophiolite). It’s because most ophiolites form near land next to subduction zones (the “suprasubduction-zone setting” in academic lingo), not way out to sea at mid-ocean ridges, that we find scraps of them plastered onto the continents in a couple hundred places around the world. There are some other tectonic schemes that make ophiolites, but this is the typical one.

Ophiolites consist of rocks that correspond to the major layers of oceanic plates, which are a deep base of peridotite, a middle layer of gabbro, an upper layer of basalt and a cap of mixed seafloor stuff: red clay, seamounts, volcanic chains and the odd limestone basin here and there. And most of these can be found in Oakland. The peridotite, when seawater reacts with it, turns quickly into serpentinite (but you can see rare remnants in places).

The gabbro, a coarse-grained rock of the same composition as basalt lava that has cooled slowly enough for visible mineral crystals to grow, underlies much of San Leandro and the deep-East Oakland hills.

And the Leona volcanics is a big pile of volcanic ash, shot through with dikes of basalt and now strongly altered, that sat on top.

The Coast Range ophiolite is highly disrupted now. It’s been caught up in millions of years of squeezing, stretching and kneading North America’s western edge — and that was before the San Andreas fault system arose and smeared everything sideways.

A handful of intrepid specialists continue the work that Brongniart started, reassembling and correlating and extrapolating and collecting ever more data. Ophiolites are important in the bigger scheme because some of them are the only pieces of seafloor rock that are older than the present ocean floor (which barely covers the Cretaceous period, back to about 140 million years). But if ophiolites are born poorly organized, unlike proper deep seafloor, attempting a perfect restoration may be an delusion, a will-o’-the-wisp. Our insights may always be a string of fragments, and we may have to let mystery be, but we have to try.

I also wrote this introductory piece about ophiolites for another website, once upon a time.

Serpentinite at Lincoln Square

27 May 2019

The Lincoln Square shopping center, on Redwood Road next to Route 13, has textbook exposures of serpentinite. Last week, five years after my first quick visit, I gave it a more searching look. Here’s a view of the terrain.

A century ago this site was the confluence of three first-order streams forming East (Lion) Creek. The largest of these comes down from due north, the western branch descends south-southeast from Holy Names, and the third branch flows due west from just above the “Crest” in “Crestmont” on the map. The Alma Mine, a set of over 5000 feet of tunnels in the hillside that was active until 1921, was about a hundred yards to the east.

In the early 1960s the shopping center was built on fill at least 20 feet deep, with the streams culverted beneath it, and its footprint was excavated into the surrounding bedrock. There are two serpentinite exposures, one above the eastern parking lot by the gas station and the other behind the back of the building next to the Safeway.

The first exposure, across from Sparky’s burger place, displays horizontally streaked rock, an intimate mixture of dark-blue and greenish-yellow serpentine with lumps in it like this.

This is rock that’s clearly been squeezed and stretched, but I see no indicators of the exact direction. Either it went both ways, top-to-the-left and top-to-the-right, or the motion was perpendicular to the rock face such that any indicators would be invisible.

On to the other exposure, which looks a lot fresher.

Here the matrix around the lumps is much better exposed, and lumps of all sizes are easily seen. They range in size up to a meter; this one is more like 20 centimeters long. These are generally elongated and indicate top-to-the-right deformation.

Some of the largest lumps appear to show relict texture — that is, traces of the mineral grains in the original peridotite before it was turned into serpentinite (see my backgrounder on serpentinization).

The matrix is very soft underfoot. Right now the footing is good because the rock is wet. There’s so much moisture coming out of the slope that the drain behind the building has steady running water in it. Soil doesn’t accumulate on it, and only the stubbornest plants, like pampas grass and patches of moss, can get any purchase.

I brought along my acid bottle, as I do, and the matrix fizzes vigorously indicating that it’s full of lime. And patches of moss, like that at the top of this photo, appear to be the site of a chemical reaction that forms little white “popcorn” balls of calcite.

These accumulate where they wash out of the soil. Most measure about a centimeter, but some are several centimeters across.

I took some home for closer study. They dry as light and hard as blackboard chalk, have no internal structure or crystallinity, and fizz away to nothing in acid leaving just a breath of grayish residue, probably a touch of clay. Whether it’s true calcite or an amorphous version of calcium carbonate, I’m not competent to say. Mineral chalk is how I’ll think of it.

The Lincoln Square exposure is a minor part of Oakland’s serpentinite patch, the little ribbon of purple crossing the Golden Gate Academy on the geologic map.

I actually don’t fully trust this map; I’m suspicious of the thin green stripe of Knoxville Formation (KJk) and the exact extent of the pink Leona volcanics (Jsv). But a borehole record from farther up the hill, in the fat part of the serpentinite, describes the rock as “serpentine with lime.” I don’t associate lime, or calcium in general, with serpentinite, but in fact the minerals in the precursor rock, peridotite, do include some (clinopyroxene in particular) with calcium. Clearly I have more to learn.

Oakland building stones: Serpentinite

26 June 2017

In a modest West Oakland neighborhood on Market Street is the modest West Grand Shopping Center. Its ordinary building is clad in rough stone, an exterior treatment similar to the Kaiser Building and many other examples.

But at the West Grand Shopping Center, the cladding consists of fist-sized pieces of beautiful serpentine rock.

The front side of the building is pristine. The rear side, on Myrtle Street, is a full block long and completely faced with serpentinite. Unfortunately the bottom seven feet or so has been painted over.

The mutable color of this stone, blue-green in the shade and olive-green in the sun, gives the building a real Oakland look. I don’t know where the stone came from. Our own serpentinite is usually bluish and not of this quality, except maybe in small outcrops in the Franciscan melange. Perhaps it’s from a quarry in the Mother Lode country. It must have taken a few carloads of rock and a crew of skilled artisans to put this together.

A few months back, when I was presenting the building stone verd antique, serpentinite’s dressed-up cousin, I said “You can’t do much with California serpentine except admire it.” Makes me happy to be proved partly wrong — you can always admire it, and sometimes build with it.

A hunt for silica-carbonate

12 December 2016

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.

silicacarbonatemap

So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.

silicacarbonate

Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.

si-carb-1

It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.

si-carb-2

Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.

si-carb-3

And at the north end, during a strenuous climb, some Franciscan chert from the melange.

si-carb-4

The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.

foundersrock

This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.

si-carb-5

Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)

si-carb-6

si-carb-7

What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.

si-carb-8

That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.