Archive for December, 2010

Northbrae rhyolite

27 December 2010

Berkeley is full of interesting rocks, many of them preserved in pocket parks. The most prominent of these is Indian Rock.

indian rock

Click the photo for a 1000-pixel version. Some 100 years ago this tough rhyolite, a volcanic rock very high in silica, was mapped as part of the same rock body as the one in Leona Quarry and farther down along the hills. But a master’s student at Cal State Hayward gave it a good look in the 1990s and determined that the Northbrae rhyolite, as this occurrence was named, is quite a different rock. (In fact the Leona rhyolite isn’t considered a rhyolite any more, but rather a high-silica welded tuff/volcaniclastic sequence.) Whereas the first mappers thought that both rocks were Pliocene, which is quite young (about 5 million years), the Leona was later shown to be Jurassic (about 150 m.y.). The Northbrae is not that old, but neither is it as young as Pliocene. It’s just a little older than the Moraga Formation basalt and the volcanic rocks of Sibley volcano, about 11 million years, making it Miocene. It came out in that same episode of eruptions, which today sits to the north around Clear Lake and The Geysers.

And it’s still definitely rhyolite. Rhyolite is the stiff, slow-moving lava that makes up little volcanoes like the young dome inside Mount St. Helens, or the rugged knobs of the Inyo Domes, over the Sierra in the Mammoth Lakes area, or farther south in the Coso Range. It makes great rock for climbers—strong, imperishable, full of handholds and rarely giving way under a person’s weight.

The rhyolite of Berkeley is well worth a visit. Just go at a time when the climbers aren’t busy; the rock parks swarm with them in nice weather. I don’t think Oakland has any of this rock, but it might.

Drainage

18 December 2010

Oakland is a fine town unless it rains too much. Then we have to worry about all the water.

drainage

All things considered, Oakland’s landscape would prefer to be steep, forested hills raised by tectonics along the Hayward fault and gentle coastal plains that absorb the sediment washed off those hills. It’s a rich recipe that produces redwoods in the heights and forage and fruit in the vales. But with impervious roads and homes carpeting the upper slopes, we increase the runoff and undermine our own infrastructure. People like the homeowners above Broadway Terrace run flexible plastic lines over the edge of their properties to put the problem out of mind. But if you walk the road, you’ll find fresh gullies that will work their way uphill to the source of the problem regardless. Landslides will probably follow.

Seismic engineering at Kaiser Hospital

10 December 2010

Kaiser Permanente is building a new hospital complex at Macarthur and Broadway, including this structure. The design is intended to keep the hospital fully functional after a major earthquake.

seismic engineering

That explains the sturdy steel, but also note the number of diagonal braces.

seismic braces

These braces are not rigid, the way they are in scaffolding. Instead, they are built like pistons, with the ends allowed to move inside the sleeve. They absorb energy and help damp the structure against rhythmic shaking that can destroy it. They deform to help save the rest of the framework. That way, after the quake they can be swapped out to make the building as good as new. Read more at this manufacturer’s site, for example. If you’re passing by the hospital (or any construction site, for that matter), take a look.

The big earthquake will probably cause cosmetic damage to the outside of the building, and some broken windows and so on. The hospital as a working institution, though, will not just endure but keep on saving lives without interruption. This is a big deal, and all of California’s hospitals are following suit to meet the state’s deadline of 2030 (see the pamphlet “California’s Hospital Seismic Safety Law” for details).

Joaquin Miller Park geology signage: Thanks, Jean Quan

6 December 2010

Oakland is going to get a new mayor soon: Jean Quan, currently on the City Council. It was her discretionary money pot that funded production of a set of interpretive signs for Joaquin Miller Park. One sign will be about the park’s geology, and it will feature a geologic map donated by Karen Paulsell and text and photos donated by me. When the sign is unveiled (I don’t know when), you’ll see some familiar pictures, including the basalt from this post:

basalt

the blueschist from this post:

blueschist

the serpentinite from this post:

serpentinite

and the Joaquin Miller Formation shale from this post:

joaquin miller formation

The fifth photo I haven’t posted here before—this shot of the Oakland Conglomerate exposed along the Montclair Railroad Trail:

oakland conglomerate

I didn’t vote for Quan, but I do appreciate her care for the city’s parks and wish her well.


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