Archive for the ‘Other topics’ 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.

Lessons from the Carrizo Plain

15 April 2019

Last week I paid my first visit to the Carrizo Plain since 2005. David K. Lynch’s superb Field Guide to the San Andreas Fault says, “Nowhere in California is the San Andreas Fault more dramatically expressed than in the Carrizo Plain, a closed depression between the Temblor Range to the east and the Caliente Range to the west. Water drains in and evaporates leaving the glistening, usually dry Soda Lake. . . . There is little ground cover and the unobstructed views reveal countless tectonic features in all their glory.” That was true in 2005, when I came through in October and didn’t meet another soul. Not true last week — it was the peak of the wildflower season, Soda Lake was a by-god lake, and hundreds of car-driving, selfie-taking visitors were scattered across this wide, remote national monument. (For this post I’m offering some 1000-pixel images, just because.)

Topographically, the Plain is a basin with closed drainage, where all streams, such as they are, lead to Soda Lake. Geologically, the Plain is a sedimentary basin that until just a few million years ago was part of the Great Valley. Then the tectonic plates shifted slightly, the San Andreas fault was squeezed, and on its east side the rocks folded up to form the Temblor Range. Later the whole Plain was raised almost 2000 feet.

Coastal California has been going through rearrangements like this for some time, and Oakland’s younger rocks like the Claremont Shale probably formed in a basin the same way, one that was off the coast. North of the Plain, roadcuts in the Bitterwater Valley expose the kind of rocks being made in the Carrizo basin. They’ve been tilted nearly vertical by forces across the fault, just as their cousins in Oakland have been tilted by squeezing across the Hayward fault.

A place stuck between the Temblor and Caliente Ranges sounds kind of inhospitable, and even though the landscape resembled a gigantic Holi festival, a brisk and parching wind blew the whole time I was there. The Carrizo flowers are as tough as they are beautiful.

You like those purple Phacelias? Here’s a billion of them.

This is the view downvalley toward the San Emigdio Mountains, with the Caliente Range on the right. On the left, the peaks of the Temblor Range are nearly hidden by the lower range of the Elkhorn Hills, which are a large pressure ridge directly along the San Andreas fault. That’s where I drove next.

The most famous, geo-tourist-trappy place in the Elkhorn Hills is at their north end, where the fault has forced Wallace Creek to jog hard to the right. I didn’t go there last week, but this is how it looked from Elkhorn Road in the barren fall of 2005. The creek comes toward you on the right side, turns left behind the frontmost ridge, and cuts through that ridge on the left side. You’re standing on the Pacific plate, moving left about an inch and a quarter per year, and on the other side of that first low ridge is the North America plate.

I’ve shown you the same kind of stream displacement in Oakland, caused by the Hayward fault.

Anyway, down at the south end of the Elkhorn Hills the entire slope is warped by motion on the fault, and the spring vegetation helps bring out the distortion. Every little stream is curled to the left, like grass in a stiff wind. The expression of the tectonics in the landscape is so strong, just looking at this photo makes me clench my teeth. In person, in 3D, it’s even more uncanny.

The great earthquake of 9 January 1857 was centered near here. The ground cracked for some 200 miles. Shaking was felt the entire length of California and into Nevada. In the Carrizo Plain, the ground shifted about 30 feet. After that, the unnamed mountains to the east started being called the Temblor Range, and the San Emigdio Mountains also got their name, honoring the patron saint of earthquakes, at that time.

Ramón Arrowsmith, now at Arizona State University, has studied this region for decades. His 1995 Ph.D. dissertation includes a thorough backgrounder of the sciency side of this mighty, lovely land. But everything he’s doing in California is interesting.

GHADs—peculiar agencies that safeguard the land

12 November 2018

It’s been a couple years since I’ve visited and written about the Leona Quarry site. It continues to fill with houses. The plantings of local plant species are doing OK, though they’d be happier without the drought. Here’s an updated view from Burckhalter Park, which I’ve visited occasionally since 2003 just for this purpose. (The trees keep getting taller.)

The highest and final residential level, Skyview Drive, is being populated now. The rest of the land will always be open space.

However, the land won’t be exactly natural. It will be intensively maintained. That sturdy concrete flume in the foreground, for example, is there so rainwater runoff from the slopes won’t start digging gullies. The runoff goes to a collection basin at the bottom of the slope that keeps all the muddy sediment and sends the water on into Chimes Creek.

Here’s more of that impressive drainage system, on a lower slope.

What about that high rock face standing in the back? Even from this distance, it looks a little ragged, a little menacing. During my last visit, in January 2016, I was up there and it looked like this.

That crumbling rock was pouring past this temporary barrier and overfilling one of those handsome concrete troughs.

It’s a big and constant job to maintain this infrastructure. There’s always something to do. Who does it, and what happens when something major happens, like a landslide?

In developments like these, a homeowners association typically handles repairs and upkeep for community amenities like paving, play equipment and landscaping. Those aren’t demanding tasks. But what do most homeowners know about geology? For the special hazards posed by geological forces, like erosion and landslides, there’s something better: a Geologic Hazard Abatement District or GHAD.

GHADs (pronounced like “gadzooks” without the zook) are peculiar entities, created under the Beverly Act of 1979, that are set up to handle the specialized job of dealing with geologic hazards for a specific set of landowners. Formally political divisions of the state independent of cities and counties, they’re exempt from bureaucratic headaches like reviews under the California Environmental Quality Act (CEQA) or hassles with the county’s LAFCO agency (and its wretched website). They have elected Boards of Directors and can own land, impose taxes, issue bonds, exercise eminent domain and possess other superpowers. There are dozens of GHADs now, and Alameda and Contra Costa Counties are their epicenter.

What keeps GHADs from running amuck or shirking their job is their “constitution,” a detailed Plan of Control drawn up by a licensed engineering geologist that ensures the Board will act with a prudent level of care. The program of regular monitoring and maintenance is spelled out in an Engineer’s Report, prepared by a licensed geotechnical engineer. It’s a simple but powerful program that spares taxpayers the geological risk inherent in developing sites like Leona Quarry. Learn more from the California Association of GHADs.

The Leona Quarry GHAD was formed in 2005, and by now it has saved up several million dollars of landowners’ assessments for maintaining the defenses — the retaining walls, drainage facilities, native plants and so on — and for major incidents like slides, wildfires and washouts. In emergencies, the GHAD can act with funding in place, trusted experts (from ENGEO) in charge and minimal red tape.

As spelled out in the Engineer’s Report, the concrete drains are cleaned and inspected four times a year, and checked whenever rainfall exceeds an inch in 12 hours. The collection basin is mucked out whenever it fills to a certain level, and the vegetation inside is cut low whenever it grows above 5 feet high. When a wildfire swept over the north end of the upper slope last year, the GHAD sent its plant specialist out afterward to keep the native habitat in good shape, which in turn helps stabilize the slope.

A second GHAD, the Oakland Area GHAD, was established in 2006 for the small Siena Hill development off Keller Avenue. You’ve seen its rugged retaining walls with the fake-stone finish looming as you head east up Keller from Mountain Boulevard toward Sequoyah Community Church.

The builders and the city probably wouldn’t have developed this daunting location without a GHAD that enabled the homeowners to cover their own risk. And now the residents pay their annual fee of about $1500 and the City Council, meeting as the GHAD Board of Directors, spends literally three minutes a year passing a budget and keeping its hands clean of liability. Success for the developers is not certain, though, with fewer than half the lots filled after 12 years.

However, neither has failure been declared. What’s there so far looks really nice.

And from my point of view Siena Hill is a success because it exposes the Leona volcanics well, like this pretty boulder at the far end of Siena Drive.

Farther up is a fresh roadcut that displays the chaotic nature of this intriguing geologic unit, which represents a volcanic island arc of Jurassic age.

The whole thing consists of the distinctive greenish celadonite-bearing phase that I’ve found scattered all over the East Oakland hills. This is the first proper outcrop of it I’ve seen. Well worth a visit.

But enough of this stone cheesecake.

The Oakland Area GHAD is more than just a minor political curiosity; it was established with an eye toward folding in other similar developments as they come up. And it will hit the big time as the Oak Knoll development takes place over the next decade, because a whole bunch of land there, including the open space, the oak woodlands and the streambed of Rifle Range Creek, will come under the GHAD’s purview. The City Council may have to take a few more minutes from now on to manage that.

Work at Oak Knoll began in September. I am looking forward to the day when that land opens up to visitors.

Boulders of the Transbay Transit Center

17 September 2018

San Francisco’s new transit center — with the 5-acre garden park on top — is worth a leisurely visit, no matter where in the Bay area you live. Naturally I had to see it too, because a reader sent me photos of a large, alluring boulder that’s part of the complex. He couldn’t decide what rock type it is. “Gneiss,” I typed back, refraining from adding “Nice gneiss.”

There are different ways to ornament a large structure with boulders, seems to me. You could make them identical, for that unified look. The FDR Memorial in Washington DC does that well with slabs of red granite. You could make them vary, like Ruth Asawa’s landmark “Garden of Remembrance” at SF State.

The transit center and the park on its roof (to which Salesforce has purchased naming rights for the next 25 years) are studded with about a dozen large boulders, from 4 to 6 feet tall. The designer’s scheme for them mixes unity with variety.

The variety lies in the personality of the stones and the mix of rock types. The unity lies in their source and their surfaces. Let me show you some personalities. The first two are up in the park and the rest are at ground level.

Oh, I should warn the sensitive that all of the boulders have had an opening carved into them to hold a steel sign either pointing to or announcing that you’re in “Salesforce Park.”

As for the rock types, these are mostly gabbro (basically, quartz-free granite), some with differing degrees of metamorphism. But each one is distinctive in details that geologists appreciate: petrology, deformation, texture. A selection:

Some of these photos show the texture that made me say “gneiss” — a banded appearance with dark and light layers. With closer inspection, though, they aren’t layered enough. They’re just a little smooshed, not stretched out like taffy.

The other thing I mentioned is the similarity of the boulders. First, they are similar in their provenance — where they come from. The only information I’ve been able to glean is that they were sourced in “the Pacific Northwest.” That doesn’t sound like much to go on, but Oregon and Washington are mostly volcanic, and these gabbros are not volcanic; they’re once-molten rocks that cooled slow and deep.

Only three areas have such rocks: around Medford in southern Oregon, in the Blue Hills of northeast Oregon, and in north-central Washington. I favor Washington, and my evidence is in the second similarity: their surfaces.

I notice two things about these boulders in general. First is that their shapes are all natural — they’re field stones, not chunks broken in a quarry, and they show some degree of rounding.

Boulders don’t just round themselves; it took a very vigorous environment to make these. Something like the enormous snowmelt flows that once ran down all the rivers of the Sierra Nevada, leaving streambeds like this, in the upper Stanislaus River. The largest rock here is the size of an SUV.

However, the Sierra rocks are scrubbed fresh and smooth, and that’s the second thing about the Transbay Transit Center boulders: they were all tumbled to a rough rather than polished state, and their surfaces have since then been exposed at the Earth’s surface for a particular amount of time, not long enough to decay into clay minerals or crumble apart, yet long enough to acquire rusty colors from iron oxides. A short time, geologically speaking, measured in thousands of years.

This surface staining takes the form of red-brown streaks and spots, as in the photos above, and an all-over patina in some cases.

Here are two different versions of the altered surface, a crust a millimeter or so thick, formed on a coarse-grained gabbro, and a thin bronze sheen well developed on a fine-grained version. In both photos the underlying rock is exposed by chipping.

All this evidence points me to a scenario in which a deeply exposed body of gabbro was broken into large pieces, which were tumbled briefly and left piled upon each other, perhaps in a steep talus slope along the foot of a cliff. There the wind and weather gave them their delicate earth-tone finish. I picture a locality along the Columbia River in north-central Washington that was inundated, over and over, by the colossal Ice Age Floods that ended (for now) about 13,000 years ago.

These boulders are cool. Give them a pat as you take in San Francisco’s newest public park. The NL bus takes you right to it, a world-class ride.