Archive for the ‘Oakland hazards’ Category

The sulfur problem of the Leona volcanics

4 April 2016

The Leona volcanics 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.

Leona-green-Knowland

Geologists used to call this rhyolite, but eventually they determined it isn’t; still the misnomer lingers. I try to consistently call it the Leona volcanics. 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.

sulfurmine-2016

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.

Leona-adit

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.

leona-pit

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.

leona-leached-chip

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.

Advertisements

Sulfur mine mitigation: An El Niño preview

4 January 2016

SmineDec15-1

Earlier this year I took a peek at the mitigation work going on at the former pyrite mine at the head of McDonell Street, in the Leona Heights neighborhood. The workers had laid down lovely geofabric, seeded grass and paved the creek bed with boulders. They were also patching infant gullies. I predicted it wouldn’t last. Sure enough, on Saturday large expanses of black plastic were shielding parts of the locality.

Farther down, the patchwork had massively failed.

SminDec15-2

This was after a normal December rainfall. We’re looking at El Niño rainfall during the rest of the winter that will be substantially above average.

On the good side, I can report that the water downstream at Twitter Court wasn’t an ugly orange. Whether that’s due to larger runoff volume or success in muffling the acid drainage, I’m not qualified to say. Think positively as you walk past the mine site on your way up the fire road into Leona Heights Park.

McDonell-fireroadup

It will always be an instructive and rewarding hike.

Claremont chert closeup; or, Oakland hills are falling down

23 November 2015

On Grizzly Peak Boulevard, pretty much right above the Caldecott Tunnel, there’s a little old fire road that heads downhill to the west. I poked my nose down it the other day. The whole area has excellent exposures of the Claremont chert, starting with the roadside.

Grizchert1

It’s real nice right now. The ground is moist and makes for quiet walking. Pine needles smell great. The rock is pretty.

Grizchert2

There’s a spot where a lot of loose rock has tumbled down. The Claremont can be crumbly, because it’s so brittle, even though the stone itself is rather hard. The loose stuff is good for collecting a specimen if you’re into that. Unlike the bleached stone exposed along the ridgetop, there’s some variety here, including the black, kerogen-rich stuff that has made this formation, like its larger cousin the Monterey Shale, good petroleum source rock.

Grizchert3

Grizchert4

Grizchert5

During the Caldecott Tunnel dig, this formation leaked significant amounts of oil and gas into the working space. Precautions had to be taken. The same black Claremont crops out at Alum Rock, as I showed you a few years ago, as well as at the Calaveras Dam site.

That’s all fun. But the road’s cut off by a washout ripped into the hillside, a twisted galvanized drainpipe sprawled along its path. At some risk, I scrambled across it and noted that at its floor lies the Claremont chert, which has its bedding planes oriented only slightly steeper than the gully. Treacherous ground. I don’t recommend that you follow me.

Grizchert6

And just beyond it is another gully, somewhat bigger but not eroding as actively. Giving up on the fire road, I scrambled up to Grizzly Peak Boulevard and this is what’s at the top of that gully.

Grizchert7

At the top of the active washout is this innocuous-looking street drain.

Grizchert8

As far as I can tell, every one of these cute drains is carving gouges into the hillside. This one points toward the Parkwood condos.

Grizchert9

Can’t we do better than this?

Perhaps our children can revise the old playground song to “Oakland hills are falling down.”

A lot of my outings are like this — mixtures of pleasure and concern.

Two lessons about floods

19 October 2015

As we anticipate the strong possibility of heavy El Niño rains, my attention will be on Oakland’s streams this winter. Last week parts of southern California were hit by “thousand-year” rainfall events, cloudbursts that washed thick sheets of mud over roads and properties. We can expect such things here too, in any given thousand-year period.

Arroyo Viejo, the stream that crosses Knowland Park, offers two lessons about floods. The scene below is at the northern edge of the park, looking upstream: a streambed piled with boulders, some as large as sofas. (All photos 800 px)

A-viejo-in-K-park

Notice: these rocks have been tumbled by the stream. How much water would it take to do that? Let’s make a rough, arm-waving estimate.

The rainfall in last week’s cloudburst was almost 4 inches in one hour. Had it fallen on the watershed of Arroyo Viejo above this point — say, half a square kilometer — it would represent an input of roughly 30 cubic meters of rainwater every second.

Picture in your mind that volume of water — no, it would be mud and therefore that much greater — funnelled through this narrow valley. Do a little geometry and it’s easy to see the floodwater would be well above the tops of the boulders.

Hidden in plain sight in this photo, then, is a single hour of tumult that might have happened a thousand years ago or five hundered years ago — or perhaps during the dreadful winter of 1861-62, when it rained for 43 straight days and much of the Central Valley became a lake.

The lesson is that most of geology’s hard work gets done in rare spurts of extraordinary activity.

Okay, the second lesson is hidden in these rocks. All of them, like this boulder as tall as me, are made of conglomerate.

Knox-in-A-viejo1

These rocks, assigned to the Knoxville Formation of Late Jurassic to Early Cretaceous age, were laid down by ancient floods in a nearshore or terrestrial setting. I’ll show you three different specimens. Notice the large clasts and the fine-grained matrix that surrounds them.

Knox-in-A-viejo2

This boulder displays a wide range of clast sizes. It was probably laid down by what’s called a hyperpycnal flow, a slurry of sediment that carries everything along with it. We’ve watched them happen offshore in Monterey Canyon. Here’s another example.

Knox-in-A-viejo3

Instead of an underwater landslide, as seen in the first specimen, this represents something gentler and more organized, like a mudflow, or like the mudslides we saw in the news. The clasts are aligned with the current that carried them here.

The key observation in both cases is that the large clasts are floating in the matrix. In geologist’s terms, they are matrix-supported conglomerates.

Then we have this.

Knox-in-A-viejo4

Here’s a beautiful clast-supported conglomerate. It represents a clean bed of well-rounded cobbles, all touching each other, like you’d see in a rushing stream or a rocky beach, nicely infiltrated with clean silt or clay after it was laid down.

None of these stones were made by ordinary sediment wafting down streams during ordinary rainy seasons. They were assembled by floods of all sizes.