Archive for the ‘Oakland rocks’ Category

East Bay diatomite

3 April 2017

The geologic map I rely on for this blog — U.S. Geological Survey map MF-2342 — extends north to Pinole, where it shows this little pod of rocks labeled “Tsa” and “Tdi” between Pinole, El Sobrante and Richmond.

Both units are of early Miocene age: Tsa stands for sandstone and Tdi stands for diatomite. The T stands for Tertiary, the catch-all term for Cenozoic rocks older than Quaternary, which — OK, you don’t need the whole lecture just now. The point is, I had to go see this diatomite because I didn’t know it existed in the East Bay. I’ve seen it in the Central Valley, but never around here.

Going north on I-80 you take the Appian Way exit right and immediately turn left on Sarah Drive. Down at the bottom of a valley is Pinole’s little, undeveloped Sarah Drive Park.

On the way to the hilltop, you start seeing this odd rock in the road. Pick up a piece and you’ll find it’s very light. That’s the diatomite.

The trail becomes very steep, exposing the bedrock. The hilltop affords nice views. I was especially taken with the view north.

And the view east looks up Pinole Valley toward Mount Diablo on the horizon. If you’re riding toward Sacramento on the Capitol Corridor Amtrak train, there’s a moment just east of Point Pinole where you can catch this same view of the mountain.

And there were butterflies.

So that’s all great. But here’s what’s cool about the diatomite.

Diatomite is composed of diatoms, the microscopic algae that make shells of silica. As an industrial commodity it’s also called diatomaceous earth, or DE, or kieselguhr if you’re feeling smart. As the stabilizing agent for nitroglycerin in dynamite, it made Alfred Nobel’s fortune, and that’s why we have the Nobel Prize.

As its silica content slowly turns into the crystalline mineral quartz, diatomite becomes the rock called chert. As it happens, the Pinole diatomite is about the same age as the chert in Oakland’s Claremont Shale. By some tectonic accident, it avoided being converted, and you can enjoy its lightweight charm without a trip to Los Banos.

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Origins of Oakland ocher

27 March 2017

Before Europeans came into this country, the locals treasured the ocher deposits in the East Oakland hills. Ocher is the name for a variety of clay-like, iron-rich minerals with a color range from yellow to red to brown. For tens of thousands of years, we’ve used ocher as pigments and preservative coatings. Some cultures would bury their dead in it.

Our ocher deposits formed exclusively in the Leona volcanics, because that body of rocks was permeated with pyrite by hydrothermal springs as it rode on the seafloor toward North America, back in the Late Jurassic. Pyrite is pure iron sulfide (FeS2) and looks like this.

You can get nice crystals of it at any rock shop.

In the Leona volcanics, you’ll sometimes see pyrite in fresh exposures, like this roadside boulder along Campus Drive. It’s gray because the crystals are so small.

Oxygen, in air or in water, breaks pyrite down. The sulfur turns into sulfuric acid and leaches away while the iron oxidizes into a range of minerals on the ocher spectrum. This process reliably turns the surface of the Leona orange and red, like here in the former Crusher Quarry.

Pure, straight iron oxide (Fe2O3) is the mineral hematite, or red ocher. It can look black, but when powdered it turns the lovely color shown on the streak plates.

Between pure FeS2 and pure Fe2O3 is a range of hydrated iron oxides that form ochers of different colors. The roadcut on lower Redwood Road, at the site of the former Alma Mine, shows off some of them well. Here’s a hematite crust, which is right near a piece of concrete pavement that’s eaten out by acid.

And here’s a beautiful brown crust.

Most likely this is goethite (“GUUH-tite”), or brown ocher or sienna, an iron oxyhydroxide with the formula FeO(OH). Here’s a specimen I collected in Wisconsin, with a glittering crust of hematite on it.

Yellow ocher has even more water associated with it — the formula is FeO(OH) · nH2O. That’s what I would call this crust in the Crusher Quarry.

There are wild cards in this scheme, namely manganese oxides and jarosite. Manganese oxide, the mineral psilomelane (“sigh-LOW-ma-lane”), is black. Just a few percent turns ocher into umber. (So does carbon.) Jarosite is a hydrated iron sulfate that can form if some of the sulfur lingers instead of turning to acid. It has yellow to brown colors.

So really good ocher, in chunks worth the effort of digging, is hard to find. Oakland once had a large body of it that had slowly gathered on top of the Leona volcanics as the rock beneath was etched away by acid. Such an iron-oxide cap is called a gossan. A little bit of the deposit is preserved on the Holy Names University campus.

All of these ocherous minerals are important ingredients in soil, especially in dry regions. Rarely are they pure, though. Oakland’s ocher patch was the center of a widespread trade, back in the day. But in the late 1800s, Americans mined it out and turned it into red paint.

Two bits of gabbro

20 February 2017

I’ve noted that while the San Leandro Gabbro has a presence in easternmost Oakland, it’s hard to find. The geologic map shows what seems like a lot of it, marked “Jgb” for Jurassic gabbro.

gabbrobitsgeomap

But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But once last year and once the year before, I found some. The two spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, and the southern one is in Sheffield Village at the north end of Middleton Street where it meets Marlow Drive.

The northern site, Seneca Reservoir, was once the upper pit of the old Catucci quarry. (The lower pit was repurposed as the site of Bishop O’Dowd High School.) Not much of it is accessible, but here and there you can spot pieces of the quarry waste. It looks like nothing else in town and everything like San Leandro’s namesake stone.

gabbrobit-seneca

I can’t say the same for the Middleton Street exposure, but on the positive side it’s real easy to visit. Oddly, the last time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

gabbrobit-middleton1

This site too is very near a strand of the Hayward fault, so it’s been rattled and squeezed for quite some time. It has a battered appearance, even a little fried.

gabbrobit-middleton2

And no matter how close you get, it doesn’t show much detail. As a whole, though, it has the typical color of the gabbro: light gray with a slight blue-green tinge. This resulted from petrochemical disruption at the time of its eruption, some 165 million years ago, when a pulse of younger magma sent up fluids that changed its black pyroxene minerals into green amphibole and some other greenish minerals — an obscure process known as uralitization.

gabbrobit-middleton3

I mentioned the Hayward fault being next to the reservoir. We probably would think twice before building that there today.