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.

Posted in Leona volcanics, Oakland serpentinite, Other topics | 3 Comments »

The Lincoln Square landslide of 1958

10 June 2019

The Lincoln Square shopping center, which I featured in my previous post, has nothing to do with President Lincoln, just as Lincoln Avenue has nothing to do with Honest Abe (it was named for Lincoln Rhoda, son of landowner Frederick Rhoda). It wasn’t on the Lincoln Highway either. Nope, it was named by the prominent citizen and developer, Luther H. Lincoln, on whose land it was built, to honor himself. Admittedly, Lincoln had his own measure of fame from serving as Speaker of the state Assembly in the late 1950s.

The shopping center, which opened in 1963, sits on the site of a messy, sensational landslide.

When it comes to landslides, the blame usually lies uphill. And in the late 1950s one of Oakland’s largest suburban developments, Crestmont, was under construction on the steep hillside above Lincoln’s land.

The hillside of Crestmont was acquired and developed by Andres Oddstad’s residential construction company. He made his name building whole neighborhoods of “economy homes” in South San Francisco, Pacifica (Linda Mar was his work), Redwood City and other West Bay localities. Crestmont was Oddstad Homes’ big splash in the Oakland market, a luxury development tagged “Riviera of the East Bay.” The ads in the Tribune cried, “Grand, sweeping panoramic views from your home in Crestmont leave you breathless day or night. Here is the charm and freedom of country living only 15 minutes from downtown Oakland!” The redwood-and-stucco houses cost $30,000, a premium price in those days. And the views truly are terrific.

Oddstad worked big and fast, leveraging its economies of scale. This aerial photo shows the state of things in early 1957.

And here’s a similar oblique view from Google Maps with the street names. The landslide I’ll describe was on Van Cleave Way, down at the bottom of the development. You can see from the airphoto how much digging and grading was involved. The serpentine rock making up the hillside was . . . mostly strong. The homesites built up on filled land were . . . mostly reliable.

Luther Lincoln and his family lived on the large lot of 4000 Redwood Road, just below the bottom of the image, as early as 1952. As Crestmont went in on the hillside above him, Lincoln built a big new home and arranged to have part of his land rezoned from residential to commercial. It was an ideal site for a shopping center to serve the new residents. And the land was largely waste already: The defunct Alma Mine, with its 5000 feet of abandoned tunnels and piles of waste rock, sat next door.

Oddstad’s project went well until the winter of 1957-58, the wettest season in 50 years. Ten inches of rain occurred in February, another ten in March. Two more inches fell during the last weekend of March, and just past midnight on 30 March, in the midst of a pounding rain, about 300 feet of landfilled hillside on the west side of Van Cleave Way began to crumble.

The Tribune reported that Mrs. Walter Horberg was moving the furniture out of 79 Van Cleave Way. “At 2:45 a.m., as beams groaned and snapped, the rear portion of the handsomely designed ranch-type home sagged and then, with a mighty crash, tumbled down the hill. The rear rooms of the house tumbled 100 feet, most of it straight down, and were carried along by the mud slide. The front section dropped a lesser distance. Somewhere in the rubble, the Horbergs’ family parakeet, Nickie, chirped on.” No one was hurt, but six homes on the block were lost or endangered; two of them hadn’t even been sold yet.

This photo from the next day’s paper, one of many from the catastrophe, was reproduced in US Geological Survey Professional Paper 944, “Relative slope stability and land-use planning in the San Francisco Bay region, California,” published in 1979 and still a good read. You can see that there’s no bedrock visible in the landslide scar, just dirt.

Here’s the scene below Van Cleave Way today. The lots for the five lost homes were rebuilt, turned into four larger lots, and developed 20 years later. The leftmost house, its roofpeak just visible, is one of the original ones from 1957.

This was not the last slide in Crestmont. Two new houses on Kimberlin Heights Drive were lost in June 1958 when the concrete piles holding them up failed. (A mild earthquake on 31 May was made the scapegoat.) And in 1962 a mudslide from the hill above Kimberlin Heights Drive swept a 5-year-old girl to her death.

But back to the Van Cleave slide. The wall of mud poured onto Luther Lincoln’s new home directly below, destroying the house and all of its contents except for a car. A few years later, Lincoln turned the scene of ruin into the Lincoln Square shopping center, and the textbook exposure of serpentinite in the hillside behind it that I showed you in the last post dates from that time.

To my knowledge, no slides have occurred in Crestmont since 1958. The streets look sound to my eye. But some empty lots remain below Van Cleave in the landslide scar that could be developed some day.

The pressure to fill open land with traditional suburban houses is relentless. And all the land left open today is precarious.

Edited to correct the date of the fatal slide in 1962, not 1955.

Posted in Landslides | 5 Comments »

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.

Posted in Oakland serpentinite | 5 Comments »

Return to Sugarloaf Hill

13 May 2019

It’s been almost four years since my last visit, and no locality, even the wildest, ever stays the same. Sugarloaf Hill, that iconic bump in the ridges of East Oakland, is one of the city’s wildest places. It helps being part of the Leona Canyon Open Space Reserve, an odd holding of the East Bay Regional Park District away from the usual watershed lands and coastal strips.

Sugarloaf Hill is the highest point underlain by the Leona volcanics. The drainage is sharp enough to discourage trees, and the EBRPD considers it a good example of grassland that still includes a lot of native species. Last week the peak, like most of the hills, was nearing the end of the green season and starting to turn summer gold.

The loose stones on the peak have been moved around since my last visit. Then, they were arranged in a rectangle, like the outline of a small building. Now they’re piled in a cairn that displays them nicely. The same energetic person or people who did that also brought up a chair, which I found very welcome after scrambling around the steep slopes.

This hilltop deserves a real bench, and a decent path to reach it. The existing trail is steep enough to be tricky footing, and the poison oak keeps edging closer on all sides.

On this visit I made a concerted attempt to find another trail to the top, both from the bottom up and from the top down. And there are some faint paths on the lower slopes. One of them led me past this old city benchmark, undoubtedly recorded on some obscure list but not relevant for quite a while.

This wild place did not start that way. Its wildness is not a primordial state or a static climax; it’s a temporary illusion created by depopulation — in Oakland’s case, the depopulation of genocide, followed by its softer sibling gentrification — leading to “parkification” or managed neglect. Untended, the hilltop will become impenetrable chaparral, the most dangerously fire-prone habitat we have.

For centuries, perhaps millennia, this hill was maintained as grassland by its native caretakers. They did controlled burns to do that, and the deer and the antelope helped keep it grazed. When the Franciscan priests of New Spain captured and enslaved the natives, the abandoned land made its way into the hands of the Realty Syndicate. Cattle grazing kept it in a simulacrum of the aboriginal flower fields.

In the 1970s the developers of Caballo Hills sought to divide this rangeland into premium country estates: nine large parcels of 40 to 50 acres. Someone would surely have stuck a private castle up here. The city of Oakland just wanted to start harvesting property taxes instead of a few steers. Instead, after neighborhood opposition, the developers deeded it to the EBRPD and went on to subdivide the ridgetop of Campus Drive into one-acre lots.

Nowadays what threatens the meadows of Sugarloaf Hill is the relentless growth of brush and chaparral. As decades pass, the ground cover rises, alien broom sprouts without hindrance, poison oak burgeons. Footpaths devolve into deer trails or disappear altogether. Eventually the most intrepid hikers give up, until a well-funded crew can reclaim the way. The EBRPD is committed to monitor the plants and animals in the park, so it’s up to that agency.

A rugged jeep trail used to be here, running up from the north end and circling the peak.

Bits of it are still accessible, but most is heavily overgrown. If EBRPD restores the road, the land would be ready for controlled burns again. The hill is a perfect site — isolated on all sides, yet accessible. The park’s planning document envisages controlled burns here, along with fuel reduction and similar half-measures.

Sugarloaf Hill could be a showcase for this deeply traditional land-management technique. For Merritt College students who already study the park, the rejuvenated hill would enhance their educational resource. It would be kept prime habitat for the Alameda whipsnake and other precarious species. And the views would remain fantastic in all directions.

Next, the park district could advance another item in its planning document: bringing back the historic York Trail. The old right-of-way, still visible in Google Maps, runs along the north side of Sugarloaf Hill, then up to Skyline Boulevard near Brandy Rock Way.

It would open a much-needed connection to Anthony Chabot Regional Park over the Parkridge land bridge.

Posted in Leona volcanics, Oakland peaks | 2 Comments »

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