A greenstone boulder in Lakeside Park

25 July 2016

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Lakeside Park holds a scattering of boulders and plaques. The plaques are always interesting, and sometimes so are the boulders. This one sits at the west side of Bandstand Cove by a grove of redwood and oak trees. I can tell at a glance — the greenish color, even texture and lack of sedimentary fabric — that this rock consists of metamorphosed lava, informally called greenstone. There’s a lot of it in the Coast Range. There’s also some in the Sierra foothills, and I suspect that this was quarried over there.

One side of the boulder displays a nice slickenside, a sign that the rock was cracked and wrung underground.

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Emily Brodsky down at UC Santa Cruz studies these fault surfaces and has been finding deep clues in them (see the latest paper from her team).

Elsewhere the boulder shows stretch marks — little extensional fractures filled with quartz. Like a run in a stocking, these are evidence of the stresses that affected this body of material once upon a time. Since the boulder has been ripped out of its original setting, these scrape marks and stretch marks have lost their geological meaning, but they’re still pretty.

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Oh yeah, the boulder has a message on it. The plaque announces that the three fountains in Lake Merritt were installed or renovated by Madeleine and Andrew Wong as a gift to the people of Oakland.

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And not least among its functions, the boulder punctuates the most peaceful view on the whole lake, whether the fountain is running or not.

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Lake Merritt needs a lot of human management to stay clean and pleasant, and the fountains are a key part of that.

Oakland building stones: Kaiser Center’s dolomite

18 July 2016

During yesterday’s OHA walk around some of Oakland’s building stones, I was pleased to bring out an underappreciated aspect of this great city landmark.

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The Kaiser Center, like City Hall, is one of Oakland’s signature buildings. When construction finished in 1960 the curving, T-shaped office tower was so iconic that Ansel Adams is said to have photographed it, presumably giving it the same sheen of grandeur he gave Half Dome. (I couldn’t find that image, although Rondal Partridge also photographed it, which is just as good.)

The photo above shows the butt-end of the 7-acre Kaiser Center complex, at Webster and 21st streets. In the foreground is the parking structure/commercial space that holds up the wonderful rooftop garden.

The Kaiser Center is famous for its extravagant use of glass and metal, specifically aluminum, Kaiser Aluminum’s principal product and a design element dictated by Henry J. Kaiser himself. But I spoke to the group about the third element that embraces the metal and glass in a gracious unity — the white stone cladding.

As you walk around the Center, the cladding appears pleasantly rough yet perfectly homogeneous. It’s not a veneer of solid stone, nor is it some kind of textured concrete. Neither is it painted. It’s an extraordinary material. After 55 years of weather, it still looks white and crisp and fresh.

Nearly all of the cladding is out of reach. Some panels touch the ground along the curving roadway behind the main tower, but that’s too hazardous even for your typical roadside-loving geologist. However, at the left edge of the photo is a little section of wall where the architects felt obliged to extend the cladding to the sidewalk. That’s the only safe place to examine it.

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The backing is concrete, but the aggregate that makes up the face is a pure white stone. Steel scratches it, therefore it is not quartzite, which was my first guess. It has the frosty luster of a carbonate, but it doesn’t fizz under dilute hydrochloric acid (I always carry some). Therefore it is not ordinary limestone or marble. It’s something much less common: coarse-grained dolomite, a stone with its own place in Kaiser’s history.

Whereas limestone and marble consist of calcite (CaCO3), dolomite rock or dolostone consists of the mineral dolomite (CaMg(CO3)2), in which magnesium takes the place of half of the calcium. It has many industrial uses, and the Kaiser conglomerate has mined it in several different quarries.

During World War II, magnesium metal was in great demand, especially for aircraft. The supply of magnesium ore, magnesite (MgCO3), was very tight. (Magnesite was mined at this time south of Livermore, in Del Puerto Canyon.) Kaiser sought a way to produce magnesium metal from dolomite using the Hansgirg process, but the process was plagued with difficulties. The magnesium program was known as one of Kaiser’s biggest headaches.

Henry Kaiser was closely involved in the design of the Center — indeed, it’s said he intended to live there. One of his conceits was that the materials, as much as possible, should be supplied by Kaiser companies. His architect, the Los Angeles modernist Welton Becket, was noted for his use of natural stone cladding.

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I imagine that Becket and Kaiser were pleased, each for his own reasons, to showcase the exceptionally pure dolomite from Permanente Metals’ large Natividad quarry, a few miles north of Salinas. I feel quite sure that’s what this is.

More reading:

“Looking Down on Creation” in The Monthly, Nov 2006

“Five Painterly Vistas from Kaiser Rooftop Garden” in The Urbanist, Mar 2011

The Natividad quarry (bottom of the page), at Quarries and Beyond

Marks of the Oakland fault

11 July 2016

Two weeks ago I told you how the city of Hayward inadvertently destroyed a special street corner. It was an informal shrine among geologists because it so clearly displayed the creeping motion of the Hayward fault. I mentioned that Hayward still has plenty of other bent curbs.

So does Oakland. Almost nine years ago in this space, in my very second post, I said that Oakland should take over the Hayward fault and make it our own. With that in mind, here are eight places on the fault that are pretty iconic if you ask me. I present them from south to north.

1. Revere Avenue above Marlow Drive, in Sheffield Village.

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2. Encina Avenue below Castlewood Street, in Oak Knoll.

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3. Ney Avenue near Astor Avenue, just above Fontaine Street.

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4. 39th Avenue at Victor Avenue, in Redwood Heights.

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5. Medau Place near Moraga Avenue, in Montclair Village.

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6. The old firehouse on Moraga Avenue, in Montclair.

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7. Broadway Terrace just west of Route 13, in Upper Rockridge.

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8. In Lake Temescal Regional Park by the park office.

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Here are their locations, shown on the geologic map.

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There are more such places between and beyond these ones, and others in Berkeley and San Leandro too.


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