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.

Leona-green-Knowland

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.

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.

Meadow season: These are days you’ll remember

28 March 2016

These days the student of geology is distracted by the riot of greenery that comes with a wet winter. Last week I gave in and devoted a day to feast my eyes on Oakland’s meadows — places I’ve visited before just for the rocks. (Follow the links to see the geology instead.)

King Estates Open Space is mapped as ancient alluvium, and the soil supports a typical crop of imported and indigenous plants. At the moment, green prevails, punctuated by blossoms of our native poppies and invaders like mustard.

king-estates-mustard

Yellow blooms that I think are mule-ears have begun. It’s still early for irises and blue dicks, but lupine is on display.

king-estates-lupine

Knowland Park is in similar shape. Even the rugged boulders of Knoxville Formation conglomerate are clothed in green.

upperknowland-greenboulder

Over at Serpentine Prairie, the unusual serpentinite bedrock supports a whole habitat of specialist plants. Attentive management by the East Bay Regional Parks District is gradually mitigating decades of neglect, and the meadow has a distinctive color and texture. The poppies grow differently there — smaller and more isolated.

serp-prairie-poppies

I don’t know what the white flowers are in this picture . . .

serp-prairie-poppies-n-whites

. . . but I do know goldfields, which are just starting in the driest flats. The rare Presidio clarkia is still a few months away.

serp-prairie-goldfields

Get out there and see our local wildflowers. Like the song says, these are days you’ll remember. Wherever you go, though, beware our poison oak, which is also having a banner year.

JMPark-rhus

Having just recovered from a case of poison oak rash, I’ve had this plant on my mind lately. I wish our two species could be friends, but apparently Toxicodendron diversilobum has singled out humans for torment and causes other animals no problem. Poison oak has a lot to commend it: it’s hardy, versatile, fast-growing, has attractive glossy leaves and produces berries.

We can remove it from limited areas using herbicides and elaborate protective gear, but eliminating the species is impossible. I see two long-term solutions. The first is to genetically engineer the whole Toxicodendron genus to no longer produce its irritating oil, urushiol.

The second is to genetically engineer ourselves so urushiol stops working on us. I would sign up for that today. I think it’s more ethical to modify ourselves than to modify a wild species, unless that would save it from extinction.

What could possibly go wrong? All I can think of is that places protected by poison oak today would start to suffer from human invasion.

We’d have to give the plant a new name once it’s not poisonous. And who knows, maybe poison-oak berries are good eating.

“Geoseki” at an exhibition

21 March 2016

Last Friday at the Oakland Museum of California, I had the pleasure of giving a pop-up talk billed as “Artful Rocks and Rocky Art” that riffed off of my backstage experience with the UNEARTHED: Found + Made exhibit (going on til April 24). This was the only chance I’ll ever have to show my rock collection in a museum, and I’m very grateful to the museum staff for helping it happen.

I laid my four chosen specimens on a table and did an alas-poor-Yorick thing with each one. The point was to say something about what a geologist might see upon contemplating these stones, as a counterpoint to what a suiseki practitioner might see in a suiseki stone.

museumtalk-1

I went from stage right to stage left, starting with this piece of Orinda Formation conglomerate.

museumtalk-2

It exemplifies a lesson from Earth Science 101 that’s still the most profound thing geologists teach the rest of us. Conglomerate is a rock made of preexisting rocks — pebbles — and sand. The pebbles signal that a long-vanished mountain range once stood nearby, an upland which crumbled slowly into gravel that washed down riverbeds to rest in the sea. They were buried by more and more sediment deep in a seafloor basin, where the gravelbeds turned into new rock. And somehow, that rock was raised again above the water and became part of a new mountain, the Oakland Hills near Claremont Boulevard.

I summarized that with the singer Donovan’s rendering of an old Zen saying: “First there is a mountain, then there is no mountain, then there is.”

This sexy piece of serpentinite came second.

museumtalk-3

I explained how serpentinite arises when seawater invades the hot deep crust beneath the ocean floor, transforming its minerals from dark pyroxenes and olivine into the soft, scaly green translucent mineral serpentine, named for its resemblance to snakeskin. Later this material was vomited up in a seafloor mud volcano, then transported onto the land by plate tectonics where I found it by the road near Lake Berryessa.

Third was my pet cobble of laminated chert, mascot of my Facebook page. I found it long ago on a San Mateo County beach.

museumtalk-4

The multiple sets of layers in this silica-rich stone mark different events in its history. The earliest set is the fine laminations; perhaps they were annual layers left by a rich microscopic rain of dead diatom shells, or layers of them made by large storm events. The material, once buried, transmuted into chert under relatively mild conditions. Subsequently, and repeatedly, cracks formed across the laminations that filled with the same silica-rich material — earthquakes like today’s were the likeliest triggers. These veins are evidence of geologic conditions that extended across a whole region for a prolonged period in the deep past. Then the rock was uplifted. And then finally the pounding of cold surf sculpted the stone away until this smooth little nubbin was left. Nevertheless, it held enough evidence for me to visualize that whole lost land and history, as surely as the conglomerate told its tale of a mountain range.

Last was this unprepossessing bit of sandstone from Mountain View Cemetery.

museumtalk-5

It looks as ordinary as the sand in a riverbed, even under close inspection with a hand lens. But in the lab, the geologist can interrogate it with various microscopes and radiations that go far beyond the visible. People who have done that learned that this sandstone, the high-grade Franciscan graywacke found just up the hill, has its grains cemented together with jadeite. Jade is a material that forms at great depth, and a testimony that rocks can be taken very far from their birthplaces and brought back to the light of day.

All four of these stones, then, tell stories that imply the action of slow, colossal forces that are constantly reshaping our planet’s surface. The real work of geologists is to understand those forces and work out their ramifications. The little stories lead to big stories that in turn shed light on the little stories. And that’s what these little geoseki mean for me.

My understanding of suiseki is as shallow as my understanding of rocks is deep. And suiseki practitioners don’t need any of my knowledge to pursue their ends. Our chosen beauties — their art and my science — are orthogonal to each other and that’s OK, because they still intersect. We are fellow appreciaters of rocks, and suiseki stones are as special as mine. Get yourself to the museum and take them in.


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