Archive for the ‘oakland rocks’ Category

Oakland, capital of lithodiversity

11 January 2016

Last month I delivered a talk to the Nerd Nite East Bay gathering titled “Diversity in Deepest Oakland: Rocks of the Town.” It was fun, and the audience was good. I made the case that per square kilometer, Oakland has more different rock types than any other city in America.

I counted off more than 25 different specific rock types (shale, schist etc.) occurring in Oakland. Although most of the rocks I showed the group have already been featured on this blog, here are a few new ones. These images from my talk are big, so I’ll try to restrain myself.

Before I start, here’s a version of the Oakland geologic map that I dubbed “Beast Oakland.” I never tried this orientation before, and I like the result.

NNoakgeomapturned

It brings out the Hayward fault vividly. I pointed out that because it has pulled together two very different sets of rocks, the Hayward fault is a key factor in our lithodiversity. Earlier faulting has added similar complexity to the top portion, representing the high hills.

Here are the new rock types I was able to document, in the order I found them during a 3-hour reconnaissance. First was schist per se.

NNschist

Schist is actually a textural term referring to the thin layers of metamorphic minerals. It gets more specific names depending on the exact mineral involved — mica schist is dominated by mica, for example.

I was able to claim marble, from a fist-sized stringer in the blueschist knocker of Contra Costa Road, along with the acid test I used to confirm it.

NNmarble-test

Next was limestone (or dolomite, which is similar), which occurs in pods in the Claremont chert.

NNdolo

I bagged tuff along Grizzly Peak Boulevard, where a prominent bed of this lithified volcanic ash crops out.

NNtuff

Both quartzite and granite appeared in a roadcut along Skyline Boulevard, where they were cobbles in the Oakland Conglomerate. That’s quartzite on the left, and I thought I’d never see either rock in this town.

NNqtzite-granite

And finally, up at Serpentine Prairie, I realized that the boulders included some peridotite. This one amazed me the most. It’s the deep stuff from the bottom of seafloor crust, the rock left over after basaltic lava is extracted from it by partial melting. Nearly all of it is recycled into the deep mantle by subduction, and nearly all of the rest turns into serpentine rock. It is rare.

NNperid

So I ended the talk by comparing Oakland’s roster of rock types against America’s three largest cities: New York, Chicago and Los Angeles. Only asphalt (in L.A., natch) and gneiss (most of Manhattan) are unavailable in Oakland, while a bunch of our rocks are ours alone. And we are like one-tenth of their land area. Chicago? Dey got nuthin’. Los Angeles? No way Jose. New York? Fuhgeddaboutit. Oakland rocks! [mic drop]

Oakland-lithodiverse

I’m here every week folks, visit the giftshop on your way out.

Shale and chert in the Claremont Shale

21 December 2015

claremont-shale

In preparation for my upcoming talk (billed as “drunken education”) for Nerd Nite East Bay, I took a three-hour tour of the Oakland hills to take fresh photos of my favorite rocks. The tour was a whirlwind one.

This is the classic exposure of the Claremont Shale in the valley of Claremont Creek, which I’ve featured here before. On this day, the light on the freshly rain-washed stone was irresistible.

In these hills, the Claremont is mostly chert — that’s the lighter colored, blocky stuff. But it’s larded with thin, soft layers of brown shale, a rock type defined as a more or less pure claystone that breaks in sheets. The layer at my hammer’s point shows how quickly the shale weathers into flakes. Most exposures don’t offer such a clear view of the shaly portion.

Clay is a sediment that comes only from the land. The sediment that makes chert is the microscopic shells of diatoms; you could say that chert is a pure precipitate of seawater. It’s intriguing to see the two so close, and so separate. Not to mention the force that so carefully tilted all of this rock upright from its original horizontal position.

With ten places to visit that afternoon, I couldn’t linger. An hour here to ponder the details would be more to my liking.

Mountain View Cemetery rocks: The back forty

7 December 2015

MVCem-1

Mountain View Cemetery is a never-failing source of interest. If you tire of graves, then why not collect the knockers exposed in this splendid preserve of Franciscan melange. I haven’t featured them here in several years, but recently the weather up there was especially photogenic. The one above, near the north edge west of the Cogswell monument, is my favorite, but they’re hard to choose among. Let’s say that knocker 1, my “secret chert,” is my favorite in the civilized part of the cemetery.

As you climb the hills, it’s natural to look around, away from where you are. Lately the cemetery managers have eliminated the overbearing fringe of eucalyptus along the rear, and the views north are enticing.

MVCem-2

If you look carefully, maybe you can spot Cactus Rock, a leading candidate for the mysterious Rockridge Rock. It’s at the bottom of this shot, in the middle.

Cactus-rock

But the view south hasn’t changed. This is the cemetery’s back forty, looking nearly unchanged after 150 years.

MVCem-3

Here’s a look at it head on, over the uppermost of the three ponds that occupy the headwaters of Glen Echo Creek. It’s not fenced off, so it’s open to exploration.

MVCem-4

Up the course of the creek is another small basin, above which the creek briefly emerges from a culvert. This is its current birthplace.

MVCem-5

The hillside gravel here — what geologists call the float — samples several different rock types that occur in the melange. Red chert, graywacke and some sort of serpentinized thingy is visible just in this small footprint. There’s also green chert, greenstone and basalt around.

MVCem-6

Here’s the graywacke — a dirty sandstone — close up.

MVCem-7

And this is my favorite knocker (one of several) in the UNcivilized part of the cemetery.

MVCem-8

It’s made of the high-grade green chert, whereas the first knocker I showed is the classic red ribbon chert. You could brave the traffic and see a huge expanse of it in the Marin Headlands, which is a nice field trip. Or you could stroll here and have it all to yourself, as long as I’m not hanging around.


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