Oakland building stones: Gneiss

19 December 2016

This is the last of my set of posts on Oakland building stones, although I reserve the right to come up with more. What you’re looking at below is gneiss on the wall of the Lionel Wilson Building, in City Hall Plaza.

gneiss

Gneiss is a fun rock, for me anyway, because when I see it I think, “Nice.” And that’s exactly how the word is pronounced.

Gneiss is made of the same kinds of minerals that granite is — quartz, feldspar, garnet, hornblende — and stone dealers call it granite. A large share of your granite countertops are actually gneiss.

The difference is that strongly directional fabric, in which the mineral grains are stretched and aligned and separated into bands and stripes. Gneiss, you see, is actually a metamorphic rock, squeezed like taffy under high heat and temperature.

That fabric, and nothing else, is what defines gneiss. The fabric is called gneissosity (and geologists will skunk you in Scrabble because they know words like this).

Here’s a gneiss boulder I used to keep as a pet. I gave it a new home by slipping it into the front yard of a home with lots of other cool rocks.

gneiss-pet

And here are two gneisses I photographed in New York. The first one is a garnet gneiss, a quarry-faced ashlar in the wall of a cemetery visitors center.

gneiss-garnet

And the other one is a wild boulder near Albany, the New York Albany.

gneiss-newyork

Gneiss is a stone of infinite variety. The pink-and-gray stone in the Wilson Building is probably Morton Gneiss, an extremely old stone quarried in Minnesota. David B. Williams, author of the very fine Stories in Stone, considers it America’s most beautiful building stone. I’ve posted other examples from Oakland here and here.

A hunt for silica-carbonate

12 December 2016

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.

silicacarbonatemap

So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.

silicacarbonate

Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.

si-carb-1

It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.

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Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.

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And at the north end, during a strenuous climb, some Franciscan chert from the melange.

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The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.

foundersrock

This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.

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Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)

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What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.

si-carb-8

That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.

Oakland geo-walks for out-of-towners

5 December 2016

Next week will be the 2016 Fall Meeting of the American Geophysical Union, held in San Francisco since time immemorial — actually, since the hippie days of 1968 — and I’ve attended every year since the early 1980s. Back then it was held in Bill Graham Auditorium; next week some 24,000 people from all over the world will overfill the entire Moscone Center to swap geoscience presentations.

Oddly for a worldwide geoscience organization, the AGU doesn’t schedule any field trips in the days before and after the meeting. If you come, you’re on your own. So, cross the bay and visit beautiful, geologically interesting Oakland.

come-to-oakland

Geologists, it’s easy to show yourself a good time here. You know your way around a geologic map: put USGS MF-2342 on your tablet or my Oakland-only excerpt. If you’re a Zipcar subscriber, ride the BART to the 19th or 12th Street stations and take your pick of cars.

No car? No problem — BART and bus are what I usually rely on. For the AC Transit bus lines, the secret to an easy experience is to buy a day pass (5$ cash, half if you’re 65+) the first time you climb aboard. The free CityMapper app will keep you oriented and informed.

Let’s talk about a typical afternoon day trip, because that’s what I know best — set out during early lunchtime and finish by early dinnertime or beer time (BeerByBART lists the best craft beer places, organized by BART station). You can travel light and cover lots of ground. There are three main starting/ending points: downtown, Rockridge and Fruitvale.

Downtown: Infinite number of lunch places on weekdays, you can’t lose. Goodly number of dinner places, from Jack London Square up to Grand Avenue (served by a free shuttle on Broadway). Many brewpubs and beer gardens on Telegraph Avenue and Broadway.

Rockridge: Delis, grocers for takeout on College Avenue. Plenty of restaurants. Ben & Nick’s for beers.

Fruitvale: Taquerias and carnicerias galore for food, Ale Industries for beer, or The Half Orange for both.

Your destinations are in our beautiful hills, because that’s where the rocks and the views are.

From Rockridge you can:

From downtown you can:

From Fruitvale you can:

If you have a car, you can:

And if you’re a maniac hiker, why not contemplate my hard-core one-way Oakland geology rambles:

Come on over.