The El Cerrito rhyolite

29 May 2023

The hills of Berkeley and El Cerrito contain bodies of volcanic rocks that are incorrectly mapped as the same stuff Oakland has. They’re the two pink patches (and a tiny speck between them) on this part of the geologic map.

These rocks are colored pink and labeled “Jsv” on the map, but they are not the Leona Volcanics, correctly mapped and labeled Jsv, in Oakland. This post is about the northern patch.

The reason they’re mapped wrong is that we’ve learned more about them since the late 1990s when the USGS’s Russ Graymer assembled the map (Miscellaneous Field Studies MF-2342, the basis of nearly all my map visuals). The rocks were closely studied soon after and determined to be not Jurassic metavolcaniclastics, but Miocene rhyolites: not some 160 million years old but more like 11 million. Now they’re called the Northbrae Rhyolite. The two rocks, conflated for many decades, are actually quite distinct in the field.

Show you what I mean. Here’s the Leona, from Leona Canyon:

And here’s the Northbrae, from Grotto Rock Park in Berkeley:

Some reports say a small part of the Berkeley patch really is Leona, but I won’t weigh in on that until I see it myself. It might well be.

The distinctive rocks of Berkeley’s Great Stone Face Park, Indian Rock, Mortar Rock Park and so on are very hard and strong, with very little granularity and bearing strong signs of flow (flow banding). Their outcrops are very sturdy, rugged but rounded at the same time. Broken pieces often have a waxy luster. They’re famous for spawning the American rock climbing movement in the 1930s.

I wanted to see for myself whether the El Cerrito rocks are the same as the Berkeley rocks. The El Cerrito hills offer great views in the spring, and the northern patch of rhyolite is a lovely place to explore. Take the 7 bus line up there, light out and look around. It was a fine early-spring day when I visited a few weeks ago.

Let’s zoom in on that geologic map.

And then even closer to this hyper-local map made in 2011 by John Wakabayashi.

Map included in the Hillside Natural Area Geology Walk guide (available from this page).

That second map was just what I needed. I want to make it clear, though, that both maps are good maps. Russ’s older map was produced as a database for a large region, and he had to sacrifice a lot of interesting detail to create a computationally tractable and physically printable map. But even John’s newer map is a picture, an exercise in visualization and an interpretation of scattered, incomplete evidence. He looked at every rock in the area, of course, but he also analyzed the terrain. He drew his lines based on all of that, plus he’s an Oakland native who knows the area from a lifetime of experience, AND he had the advantage of more recent studies, including the revelatory digital elevation models of the Hayward fault area. The lines on his map, representing contacts between different units, aren’t visible on the ground, at least not where I was. I walked all of the roads within the mapped rhyolite, and everything inside the pink field is indeed the same stuff.

And it’s not like the Berkeley rhyolite. Superficially, it’s also light colored, but it’s granular, more like a tuff — a rock made of volcanic ash. And it acquires a honey-colored film of iron oxides as it weathers, similar to the Leona volcanics. I’ll continue to call it rhyolite, though.

Nowhere did I find any of the rounded, polished surfaces of the Berkeley outcrops. Though truth be told, true outcrops are scarce on the ground.

What might be interpreted as flow banding on a weathered surface appears to be merely subtle variations in grain size.

The photos above are all from the western body of rhyolite. The eastern body is its own kind of fun, not because there are any great outcrops but because of what the residents have done along Brewster Drive. Here’s an exposure of bedrock next to boulders of it incorporated into a driveway wall.

Others have been more ambitious.

The upshot is that the two bodies of rhyolite, in Berkeley and El Cerrito, are similar but not identical. I don’t know if the northern set of rocks have been dated, but if they have and the dates match those from Berkeley, then it’s reasonable to interpret them under one name, representing two different aspects of the same small volcanic center. But that might be wrong. Although the two bodies of rhyolite are near each other today, they might have been some distance apart when they were erupted. We may never know, but more research might narrow the uncertainties.

Meanwhile, it’s a nice place to visit. Murieta Rock is nearby, too.

Why you should see the Hayward fault in person

15 May 2023

Earthquakes have always shaken the Bay area, but for thousands of years residents have lived with them. Today we’re better off than any of our predecessors: we know just where the faults that cause earthquakes are located. We know where the ground is likely to rupture, and we even have some idea of when.

I submit that now is a good time to get to know the Hayward fault, and understand it a little better, before the next time it disrupts our lives. Though earthquake faults may be objects of dread, they’re safe to approach today, while they sleep (except for a little bit of creep).

The Hayward fault stretches from Point Pinole in Richmond to Alum Rock in San Jose, and its telltale signs aren’t hard to learn: lines of roadway cracks, skewed buildings, bent curbs, odd landforms. The fault may be hiding in plain sight, but it’s not a secret.

Since humans first arrived during the latest ice age, Bay area residents have known the land is prone to large, unpredictable earthquakes. The Indigenous tribes knew. The Spanish explorers and Mexican colonists who followed were not surprised, from long experience in their homelands. But starting in 1849, the Gold Rush brought a wave of naive outsiders to the Bay area. If the newcomers heard Mexican old-timers tell about the great shock of 1838 in Monterey, well, those were just stories, not experiences: there and then, not here and now.

Soon enough, California gave notice of its nature when the massive quake of 1857 shook the state from end to end. After that, major seismic events in the Bay area came hard and often. The newcomers wised up and adapted. Between 1858 and 1898 the Bay region experienced ten more earthquakes of at least magnitude 6 — the size of the Napa earthquake in August 2014. The largest of these, the “great San Francisco earthquake” of 1868, was the last big rupture on the Hayward fault.

By that time, builders in the young cities of the Bay area had learned to keep structures strong and low, no higher than three or four stories, to resist the effects of shaking. During the 1868 earthquake, the brand-new Wilcox Building, the tallest in Oakland at three stories, survived undamaged thanks to heavy iron bracing and still stands today at 9th Street and Broadway.

San Jose’s county courthouse also weathered it well. Architects had learned firsthand how to meet the demands of earthquake country.

The 1906 San Francisco quake, a great rupture of the San Andreas fault, gave birth to modern earthquake science. In the century that followed, California geologists learned to read the landscape in terms of tectonic movements along specific faults, active cracks in the earth’s crust. Today seismologists monitor active faults around the clock. Our phones can alert us to significant Bay area earthquakes before the shaking even arrives. Architects design quake-resistant structures, and building codes mandate them. We have good tools for living with earthquakes.

However, the Bay area faults have been much quieter than they used to be, producing only four magnitude-6 events in the last hundred years, none of them with epicenters in the central Bay area. During that century, the Bay area’s population has grown more than six times larger, and relatively few residents have ever experienced damaging shaking.

This matters especially for the East Bay, where cities grew up directly on the Hayward fault for decades before geologists mapped it. Modern practices have offset some of the resulting risk. But when the next big rupture happens on the fault — geologists put the odds during the next 20 years at about one in three — the region will be severely tested. As many as a million buildings in the greater Bay area will suffer damage, and tens of thousands of people will be displaced. It’s urgent to cope with this prospect in advance, but the task is not easy or simple.

We can’t depend on the Earth to keep reminding us about earthquake-resistant living. There’s a lot of slow, hard work yet to do — retrofitting or replacing vulnerable structures, teaching good practices to children and new residents, and maintaining disaster preparedness. The drive to sustain that work has to be embedded in our culture, generation after generation.

It’s easy to fear our active Hayward fault, and plenty of media stories reinforce the dread, but fear may not be the best long-term motivator. I believe that seeing this sleeping giant and taking its measure in person can help change attitudes. Two good places to see its marks on the landscape, with interpretive signs, are at Lake Temescal Regional Park in Oakland and in Fremont’s Central Park.

Geologists are wary of the Hayward fault, like the rest of us, but they also regard it with wonder and respect. They see the fault as an ancient channel of energy that has built our beautiful landscape over millions of years and maintains the landscape with each earthquake. As the Indigenous tribes might put it, Earthquake is our uncle, part of the country, who has a seat among us at the evening fire, and the Hayward fault is the place where it wakes and sleeps.

Yay it’s book week

1 May 2023

This is a fitting occasion to take a break from writing about geological matters to celebrate the official release of Deep Oakland: How Geology Shaped a City — an ode, in its own odd way, to my beloved city. That happens this week on Tuesday 2 May. The printed book will be up on the shelves at bookstores, and the e-edition will be available to download. (See a list of related links at the bottom of this post.)

Heyday Books

This book grew out of a manuscript I started about six years ago that I titled Deep Oakland: Geology of a City. It took a while to work out. When I thought I was near finishing the third draft, I sent a query to Heyday Books, with a sample chapter and a paragraph that said, “My intended audiences are Oakland’s walkers, park visitors, neighborhood leaders, and citizens with a stake in the city, whether they work here, live here or recreate here. . . . A secondary audience consists of thousands of geologists, nature lovers and science enthusiasts who, for family or professional reasons, make the Bay area their travel destination.”

Heyday asked to see the whole manuscript, then came back three months later asking for something rather different, something . . . deeper. I realized that what I had written was a manual for Oakland geologizers. I set it aside and started over again, working closely with Heyday’s editor to create the proper Heyday book they saw inside my work. We finalized that manuscript a year ago, and now it’s a real book with the greatest looking cover and a custom block diagram by Laura Cunningham leading off each chapter.

But the earlier work, totally independent, still has a viable audience, I think. Whereas the new book invites you to see Oakland with deeper appreciation as you look around, the original manuscript is full of specific detail on where to go and what you’ll see there, plus some deeper geological geekery.

Both books, the newly published one and the unpublished one, have the same purpose I spelled out in my query letter:

Oakland is a bellwether city for America’s transition to sustainability in the face of climate change and related challenges. It also faces the certainty of a major earthquake, and every effort should be made to envision and prepare for that calamity while calm instruction is possible. For these reasons, Oakland should take its distinctive geology into its heart, its identity and its sense of place. The under-natured children of Oakland have health-giving wildness all around them, and Deep Oakland is meant to inspire and inform the adults—parents, teachers and other leaders—who will personally take them there.

Later in the week, Heyday will throw me a little party in their book-lined Berkeley headquarters. There will be toasts, gratitude and maybe a few tears.

Then begins a short series of appearances and talks, starting Saturday with a panel appearance at the Bay Area Book Festival. The book has taken up so much of my time and energy that I’ve cut back on these things, as well as my own personal outings. It feels good to resume the old life.


*Book-related announcements and progress reports
*Order the book from the publisher (discount code “oakland” is good through 2023)
*Descriptive notes and supporting links for each chapter
*The “Deeper Oakland” newsletter