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.


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