Archive for the ‘Oakland hazards’ Category

Fallen trees

30 May 2016

Naturally, the people of a city named Oakland cherish their trees.

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But trees are living things, and every one must die at some point. After years of drought plus one wet winter, I’ve noticed a lot of downed trees. This one was in Leona Heights, an oak. When oaks topple, that’s the end of them.

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This one, a bay laurel, was in Joaquin Miller Park. Fallen bay trees commonly reroot, which is a handy thing in their preferred moist habitat.

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Fallen trees may interest field geologists because as their roots rip out, they expose the bedrock in areas with thick vegetation. Thus information from “tree throws” can help in mapping the rocks. For instance, I inspected the rocks uprooted by this tree . . .

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. . . and noted that they were typical mudstone of the Joaquin Miller Formation. The park is the formation’s type area, after all.

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Another treefall farther downstream exposed serpentinite. (The two very different rock types are separated by the Chabot fault.)

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Sometimes data points like these are the only way to get a handle on steep, vegetated terrain.

Research-type geologists consider “tree throw” a significant actor in the evolution of wooded landscapes (here’s a taste from a recent conference).

Down by the water, especially along the smaller creeks, fallen bay laurels often span the stream and root on both sides. Sometimes it’s charming, like here above Dunsmuir House, but it’s usually a hassle if you’re trying to walk along the streambed.

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To hydrologists — geologists who specialize in streams — fallen trees are a crucial part of stream systems. Natural streams always contain some amount of dead wood, because it lasts for years before it rots away or burns up.

Wood doesn’t act like sand and gravel. It doesn’t wash away easily, it sticks around and snags stuff. So making a realistic model of stream behavior requires a sophisticated understanding of how this “wood load” affects the water. Hydrology journals publish a steady trickle of papers on this topic.

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Most of us would rather that trees stayed standing and streambeds stayed clear. That only happens in landscaped parks. To know nature, you have to learn to appreciate snags.

Earthquake mitigation at Oakland City Center

18 April 2016

Today is San Francisco Earthquake Day. Just before dawn on this day in 1906, at 5:13 local time (meaning 6:13 daylight time), a rip in the crust off Ocean Beach started tearing its way north and south along the San Andreas fault, setting in motion one of California’s greatest defining events.

At the Oakland City Center and the Clorox building, at 1221 Broadway, you’ll see this odd feature on the ground.

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It’s a long steel ribbon that covers a pair of long sawcuts in the pavement about 3 inches apart. Between the cuts, the tile has been pulled out and replaced with a flexible gasket. One edge of the steel ribbon is anchored to the tiles, but the other edge is loose.

That’s a retrofit for big earthquakes. During a moderate-to-major event, say magnitude 5-1/2 or larger, the Clorox building will shimmy back and forth, and so will the buried BART station. Because of their different sizes and dimensions, they won’t move in unison. Without the gasket in the pavement, the tiles in the rigid pavement will buckle and shatter and fly in all directions, leaving one more mess to clean up that will probably fester for years.

The gasket promises to prevent that. If you’re here when the next sizeable earthquake hits, and you have the presence of mind (not guaranteed!), watch it work. The free side of the steel ribbon should slide over the ground while the gasket cushions the two sides of the cut beneath it.

This won’t prevent all damage, and it may not matter at during the biggest quakes we can get (magnitude 7 and maybe 7.5), but for the much more abundant moderate events it will save us some grief.

The sulfur problem of the Leona rhyolite

4 April 2016

The Leona “rhyolite” is one of Oakland’s most intriguing rock formations. We have other volcanic rocks here — the true lava flows at the Sibley Volcanic Regional Preserve — but the Leona is ten times older and has a very different story.

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I put the word rhyolite in quotes because the rock isn’t technically rhyolite, although geologists used to think it was. Rhyolite is a type of lava, generally light-colored and very viscous, the kind of stuff you see in the Inyo Domes south of Mono Lake, or at Lassen Peak.

Instead, the Leona was originally a thick pile of mostly volcanic ash, part of a chain of volcanoes out in the deep Pacific Ocean. Volcanic ash is a glassy material. Later it was invaded by actual lava flows and hydrothermal features like the “black smokers” of the deep sea floor. These things cooked the ash beds into hard rocks as the glassy ash broke down (devitrified). The result looks somewhat like rhyolite, but it’s formally called quartz keratophyre on the geologic map. Cliff Hopson, a leading expert in this part of California geology, described it in 2008 (GSA Special Paper 438) as the top part of the Coast Range ophiolite, “mostly altered, devitrified volcaniclastic sedimentary rocks” of the “volcanopelagic remnant,” a mixture of ash and deep-sea ooze.

The hydrothermal activity, in particular, added sulfur into the mix in the form of the mineral pyrite. The mineral oxidizes upon exposure to air and rainwater, yielding sulfuric acid and iron oxides.

Over the years on this blog, I’ve documented acidic waters draining the Leona “rhyolite” almost everywhere it’s exposed. The most notorious place is the former sulfur mine at the top of McDonell Avenue, where “yellowboy” oxides stain the streambed below.

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Would-be pyrite miners have poked their picks into the Leona all over the place. I recently located a long-abandoned adit — a horizontal tunnel — left behind by one of those guys.

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You don’t want to go here. The adit is about 10 meters long, smells funny and is lined with a powdery deposit signifying a steady decay. The city ought to seal it, but it’s difficult to find and is well enough left alone, so far.

Nearby are numerous pits with the telltale red-brown linings that develop in the Leona after a few decades of exposure.

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Bits of rock below the adit, with the pyrite leached out of them, are very lightweight. Elsewhere I’ve seen this stuff turn to pure clay.

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Experience has shown me that the Leona seems to release acid drainage wherever an incision is made in it. Had I read the literature first, I’d’ve known this long ago. In a 1969 report for the U.S. Geological Survey (Map GQ-769), Dorothy Radbruch noted, “Fresh rock contains abundant pyrite in many places. . . . runoff from rhyolite hills [is] very acid and corrodes concrete sewer pipe.


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