Archive for the ‘Oakland stone’ Category

“Geoseki” at an exhibition

21 March 2016

Last Friday at the Oakland Museum of California, I had the pleasure of giving a pop-up talk billed as “Artful Rocks and Rocky Art” that riffed off of my backstage experience with the UNEARTHED: Found + Made exhibit (going on til April 24). This was the only chance I’ll ever have to show my rock collection in a museum, and I’m very grateful to the museum staff for helping it happen.

I laid my four chosen specimens on a table and did an alas-poor-Yorick thing with each one. The point was to say something about what a geologist might see upon contemplating these stones, as a counterpoint to what a suiseki practitioner might see in a suiseki stone.

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I went from stage right to stage left, starting with this piece of Orinda Formation conglomerate.

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It exemplifies a lesson from Earth Science 101 that’s still the most profound thing geologists teach the rest of us. Conglomerate is a rock made of preexisting rocks — pebbles — and sand. The pebbles signal that a long-vanished mountain range once stood nearby, an upland which crumbled slowly into gravel that washed down riverbeds to rest in the sea. They were buried by more and more sediment deep in a seafloor basin, where the gravelbeds turned into new rock. And somehow, that rock was raised again above the water and became part of a new mountain, the Oakland Hills near Claremont Boulevard.

I summarized that with the singer Donovan’s rendering of an old Zen saying: “First there is a mountain, then there is no mountain, then there is.”

This sexy piece of serpentinite came second.

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I explained how serpentinite arises when seawater invades the hot deep crust beneath the ocean floor, transforming its minerals from dark pyroxenes and olivine into the soft, scaly green translucent mineral serpentine, named for its resemblance to snakeskin. Later this material was vomited up in a seafloor mud volcano, then transported onto the land by plate tectonics where I found it by the road near Lake Berryessa.

Third was my pet cobble of laminated chert, mascot of my Facebook page. I found it long ago on a San Mateo County beach.

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The multiple sets of layers in this silica-rich stone mark different events in its history. The earliest set is the fine laminations; perhaps they were annual layers left by a rich microscopic rain of dead diatom shells, or layers of them made by large storm events. The material, once buried, transmuted into chert under relatively mild conditions. Subsequently, and repeatedly, cracks formed across the laminations that filled with the same silica-rich material — earthquakes like today’s were the likeliest triggers. These veins are evidence of geologic conditions that extended across a whole region for a prolonged period in the deep past. Then the rock was uplifted. And then finally the pounding of cold surf sculpted the stone away until this smooth little nubbin was left. Nevertheless, it held enough evidence for me to visualize that whole lost land and history, as surely as the conglomerate told its tale of a mountain range.

Last was this unprepossessing bit of sandstone from Mountain View Cemetery.

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It looks as ordinary as the sand in a riverbed, even under close inspection with a hand lens. But in the lab, the geologist can interrogate it with various microscopes and radiations that go far beyond the visible. People who have done that learned that this sandstone, the high-grade Franciscan graywacke found just up the hill, has its grains cemented together with jadeite. Jade is a material that forms at great depth, and a testimony that rocks can be taken very far from their birthplaces and brought back to the light of day.

All four of these stones, then, tell stories that imply the action of slow, colossal forces that are constantly reshaping our planet’s surface. The real work of geologists is to understand those forces and work out their ramifications. The little stories lead to big stories that in turn shed light on the little stories. And that’s what these little geoseki mean for me.

My understanding of suiseki is as shallow as my understanding of rocks is deep. And suiseki practitioners don’t need any of my knowledge to pursue their ends. Our chosen beauties — their art and my science — are orthogonal to each other and that’s OK, because they still intersect. We are fellow appreciaters of rocks, and suiseki stones are as special as mine. Get yourself to the museum and take them in.

A reconnaissance of San Leandro geology

14 March 2016

San Leandro is a much smaller city than Oakland, but it has its share of interesting rocks and features. ‘Twas a cloudy day when I visited, but the worst day geologizing is better than the best day working. Here’s the geologic map with the photo locations numbered on it.

SLgabbrogeomap

The purple area marked Jgb is underlain by the San Leandro Gabbro, of Jurassic age, a crystalline rock similar to granite that belongs to the Coast Range Ophiolite. It’s about 160 million years old and was once a deep-seated part of the oceanic crust. Unfortunately the color, while it follows the official U.S. government geologic color guidelines for Mesozoic plutonic rocks, makes the map hard to read. The blob of brown marked Jpb represents Jurassic pillow basalt, which I thought would be very interesting to see. And the solid black line down the middle of the map is the Hayward fault — it’s solid black because the fault is very well mapped there.

The San Leandro Rock Quarry has been closed for a few years.

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The land is for sale — 58 acres of it, right on the Hayward fault — but I didn’t feel up to impersonating a possible buyer, so it was off limits. But the view the other way is pretty cool, overlooking the gorge of San Leandro Creek below the Chabot Reservoir dam. It’s the biggest canyon between Niles Canyon and Wildcat Canyon and pretty intimidating.

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A little ways west on Lake Chabot Road, where it meets Astor Drive, is a saddle in the hillside where the Hayward fault crosses the road. A steep gulch descends to the north along the fault trace. To the south, the Bay-O-Vista Swim and Tennis Club has nestled on the fault unscathed for almost 60 years.

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We’ll visit the fault on the other side of the club. But first, the gabbro! It’s exposed in various places in the Bay-o-Vista neighborhood, where it’s mostly shattered from being next to the fault for millions of years.

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Gabbro is made up mostly of dark pyroxene and light plagioclase feldspar. Like granite, it likes to weather into decent soil. The excavations of residential areas are helpful in bringing it into view. And up close, this gabbro is pretty.

SLGabbro-specimen

Studies of this area using airborne gravity meters and magnetic instruments suggest that this gabbro extends well north and south of here in a big slab about 3 kilometers thick lying between the Hayward and Chabot faults, tilted almost straight up and down. This figure is from a 2003 study led by Dave Ponce of the U.S. Geological Survey.

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In Oakland, the gabbro shows up in stringers and blobs along the fault as far north as Chimes Creek. I’ve picked up pieces on Eastmont hill by the reservoir. But the geophysical study suggests that it underlies a much larger area as far as Merritt College, beneath the surface rocks.

The gabbro is strong enough that it bends the Hayward fault slightly off course. But during the 1997-98 El Niño, a big hunk of hillside gave way just below the place where I shot the outcrop. Two homes were lost.

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The top of the slide displays some pretty rotten stone.

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Farther south, Fairmont Road swings around the county juvenile justice center past the Hayward fault. This is looking north from there up the fault trace.

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Our active faults grind up the rocks so fine that they’re easily eroded into gulches, gullies and valleys, and that’s what this one is. It last ruptured on October 21, 1868, so any trace of that is long gone. It takes careful trenching studies to find it. We’ll have to wait until the next big one to see where it decides to rip up the ground.

Geology meets art in the Oakland Museum

14 December 2015

suiseki-rivera

The Oakland Museum of California just opened an exhibit called “UNEARTHED: Found + Made,” and this is the first thing you see when you go there. It is a suiseki stone, collected by the late Felix Rivera in the California desert and prepared and mounted by him according to artistic principles codified centuries ago in Japan. When you visit the show, slow way down at the entrance and drink in its form, colors and presence. The exhibit features about twenty more of these rugged individuals, all exquisitely lit.

I took this photo months earlier under fluorescent lights in a back room at the museum. More about that later.

That’s the “found” part. The “made” part is a set of works by Jedediah Caesar, which complement the suiseki in an off-kilter way. He picks up things off the ground, too, mostly things that are not rocks. He may mix them into a vat of liquid plastic, let it set, and then saw the resulting block into slices, like building stones. Those are on the walls. This larger piece is on the floor.

Jed-Caesar-art

If I have this right, Caesar mixed turmeric into the plastic, which caused a vigorous reaction much like volcanic gases might produce in magma. In any case, his works have a certain geological cast and are a feast for the eyes.

I took that shot with my phone at the opening reception, where I was an invited guest. The museum staff had sought my help as they were preparing the exhibit. You know how for every piece of art, they say what it’s made of? “Albumen print.” “Stainless steel.” “Oil on canvas.” They wanted me to help them do that with the suiseki — you know, like “pegmatite on wood stand.”

Now suiseki collectors don’t know this stuff. Mostly they ignore geology, as they should. To talk about a suiseki’s rock type is to miss the point of the art. And the whole point of geology is to observe rocks, not appreciate them. But I made my best effort to “identify” the stones, relishing the absurdity as I did so. And if you disagree with the names I chose, I’m sure you’re as correct as I was.


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