Archive for October, 2009

The Rocks of Leona Quarry

31 October 2009

A few days ago I visited the former Leona Quarry, now slowly filling with townhomes. The view was great.

leona quarry

The rock slope on the north side looks rugged but is heavily engineered with many setbacks and retaining walls. But I was here to look at the rocks. There’s no trespassing on the high slopes, which is just as well because there is little bedrock to be seen. The whole hill is mapped as quartz keratophyre at the very bottom of the Great Valley sequence, of Jurassic age (no younger than 140 million years). It’s lava flows from an oceanic volcanic arc, like modern-day Japan or the Philippines, that have been heavily altered. It’s typically light colored with strong rust stains and full of fractures. You’ll see several different examples below.

leona quarry

Some outcrops show bits of less-altered gray lava, although this example is still full of veins.

leona quarry

This small boulder displays slickensides. The white mineralization includes quartz as well as carbonate minerals, sometimes layers of each in the same vein.

leona quarry

It’s pretty sterile, but life is taking hold amid the stones.

leona quarry

I took a couple hand specimens of the hardest, most pristine looking rocks. Even these have a greenish tinge from metamorphism. This specimen has faint banding in it, probably metamorphic rather than from flowing of the lava.

leona quarry

This kind of rock is almost impossible to study without using thin sections and a petrographic microscope. I don’t know what I would call it if the map didn’t say—probably just “altered lava.” Quartz keratophyre is specifically a metamorphosed trachyte, a highly alkaline high-silica lava. That’s meaningful to petrologists in a way that would put most of us to sleep if they were to tell us about it. I don’t mean that unkindly. But I’d hazard a guess that here it reflects some input from subducted continental sediments as well as the usual melted seafloor.

Rock from the Leona Quarry appears all around town in rustic walls, but I suspect that nearly all of its output went to crushed stone.


The trout ladder

25 October 2009

rainbow trout

The original rainbow trout come from Oakland; did you know that? The species Salmo iridia was first described, in 1855, from San Leandro Creek. Redwood Creek is a branch of San Leandro Creek that still contains good spawning grounds for rainbow trout, and this nicely maintained fish ladder is here to help them upstream.

The rainbow trout has been spread all over the world, of course. This wasn’t the only locality in the Coast Ranges they came from. The Oakland fish aren’t superior to other local strains. Their only distinction is artificial: they’re merely the first to come to scientific attention. Naturalists name species simply by picking an individual and describing it in detail in the literature. It’s arbitrary, but the only way to begin.

That’s as arbitrary as the creationist’s crude notion that the species is simply something uttered by God and named by Adam. Creationists don’t care to ask why populations vary in their genetic makeup—or if they do they regard it as the necessary decay from perfection of all earthly things. Variation is useless in their theory. But to naturalists, the reality of the species includes precisely the variation that the creationist downplays. Variation is the first rung in the ladder of evidence showing how the tree of life fits together as the result of evolution. The creationist’s interest in that project is only to suppress it.

Today we lump Salmo iridia as a subspecies under the larger species Oncorhynchus mykiss. Maybe God isn’t clear, or maybe our ideas need work. Maybe both statements mean the same thing.

A glimpse below

21 October 2009

christ the light cathedral foundation

The new Christ the Light Cathedral is a beautiful structure, designed to guide the mind toward bliss, to allow the susceptible a glimpse of heaven. In early 2005 I sought a high place of my own type—a parking structure—to have a look at the cathedral’s construction site.

The site was at one time a glamorous high-Deco car dealership. Before that I don’t know, but the Oakland geologic map shows it as half fill and half “marine coastal terrace” deposits. The fill half would be on the lakeshore side, naturally. The terrace is basically a shelf of sediments deposited in San Francisco Bay during the last interglacial, more than 70,000 years ago, when the sea was a good five or ten meters higher than today. Only small, subtle bits of it are around today. The pit looks like it’s floored with nice clean golden sand. That might be aboriginal sediment, or it might be dirt from downtown hauled here to fill in the swampy lake shore, as it was around almost the whole lake. The downtown dirt is Merritt Sand, a widespread sheet of ancient windblown dune sand much like what underlies western San Francisco. That sand came here at the height of the last ice age, when the seas were very low, the weather was cold and the winds blew fine sand from the wide, exposed continental shelves onto the coastal hills.

If I had an hour to poke around these excavations! But only the geotechnical engineers get to do that, and maybe a touring group of their fellow professionals, all in hardhats. If any of those fine specialists are reading this, my email is geology at about dot com.

City Hall and the Loma Prieta Quake

11 October 2009

oakland city hall

Oakland’s City Hall was the tallest building west of the Mississippi in 1914, when it was completed. It’s still an impressive structure, 324 feet high, covered with intricate stonework and flooding the plaza with warm reflected light around midmorning.

City Hall weathered the 17 October 1989 earthquake without collapse, although there was serious damage and it is said we nearly lost the clocktower. After the quake the city was motivated to retrofit the structure. In evaluating the possibilities, Charles Rabamad and Donald Wells write, “To minimize the amount of new construction, the existing structure was given credit for the strength it exhibited during the Loma Prieta earthquake. This performance-based approach required less strengthening than conventional, code-based design, which ignores the existing capacity of the building.”

Today City Hall rides on a grid of 113 big, fat rubber-and-lead base isolators 19 inches high and either 29 or 39 inches wide. These soften the shaking and allow the building to be strengthened with the least impact on the historic building’s interior. The building will shift back and forth as much as 17 inches. It’s designed for a magnitude-7 earthquake on the Hayward fault, after which some cracking, fully repairable, is expected. Completed in 1995, the retrofit was the world’s first base-isolation project for a high-rise building, setting the precedent for many more retrofits including several at UC Berkeley. Now Oakland City Hall is in all the engineering textbooks.

The Earthquake Engineering Research Institute has a publication on the whole thing, and a 15-page paper by Mason Walters with the dirty details was presented to the Third Seminar on Utilization of Strong-Motion Data in 2003.