Rock Fans Tour Earth’s Layers at Street Level

Robin Marks (in white) points out a slickenside on her

Robin Marks (in white) points out a slickenside on her "San Francisco Rocks" walking tour.

Robin Marks has always been fascinated with places where the earth is doing crazy things. As a kid, she loved the geysers of Yellowstone and the volcanoes of Hawaii. As an adult, she loved living in Seattle, where the earth is “young and alive and moving.”

But settling in San Francisco, a place more geologically active than Seattle, was thrilling for the science writer and urban explorer. “There’s something I love about the fact that earth, even though we take it for granted, is busy doing its own unpredictable, uncontrollable thing and that it’s not really what it appears to be to us.”

Nowadays Marks makes her living pointing out the underground forces that bend and tilt and shake things. Two days before the anniversary of the 1906 earthquake that changed San Francisco forever, she stands under a magnolia tree near the corner of Church and 20th streets, next to a black backpack and a laminated sign that reads, “Discovery Street Tours: Exploring Science on the Streets of San Francisco.”

She waits for people to join her hands-on, 2.5-mile tour titled “San Francisco Rocks: Seismic Forces That Shaped Our City,” which winds through hilly Dolores and Corona Heights. These hills are known for their stunning views, but they also reveal the city’s history.

There’s a trick to appreciating the evidence of some of the most complex and active geology in the world, and Marks is here to make sure that her fellow explorers master it.

People (most of them older) begin to arrive, sporting hiking boots and backpacks and carrying water bottles. When Marks counts 15 heads, she announces that she’ll start the tour by the bench that overlooks the sun-splashed grass of Dolores Park, which is full of walkers, sunbathers, picnickers and kite-fliers.

This is just one of seven tours that Marks’ company offers, ranging from “Invented in San Francisco: Science and Creativity in a City of Innovation” to “Fermented Favorites: The Science of Cheese and Wine.”

Marks gives everyone a name tag, then introduces herself as the owner and operator of Discovery Street Tours. “I call myself a recovering biochemist,” she says, explaining that she escaped the lab to spend a decade working at the Exploratorium.

“Me, too,” shouts a woman from the bench.

“It’s amazing how many of us there are,” Marks replies with a laugh.

She asks everyone what their favorite rock is, adding that they can be metaphorical rocks (for example, “I like rock and roll”). Hers is lava: “It’s so cool because it’s the newest, youngest earth.”

One woman says she likes mica because it reminds her of growing up in Arizona. A man says he likes granite because of his memories of climbing rocks in Missouri. And an older gentleman professes his admiration for chert, to which Marks replies, “You’re going to love chert even more after this tour.”

She first explains that San Francisco lies on the boundary between two of the earth’s great tectonic plates, the North American and Pacific. The plate boundary is a transform fault, formed by one of the best-known geologic features in California, the San Andreas Fault zone.

“The geology in the Bay Area is really special,” she says. She explains that the plates are sliding past each other, and the movement (totaling about an inch a year, along the San Andreas and its subsidiary faults, the Hayward and Calaveras) is what produces the region’s large earthquakes and creates its rugged landscape.

Rocking her hands back and forth she adds, “It’s really seismically active.”

The crowd laughs. Most of them live in the city and have felt the earth’s jolts. To understand the forces that gave San Francisco all its beautiful hills, Marks says, they must first understand the layers of the earth.

“I happen to have an earth right here,” she says, pulling a small globe made of port wine cheese and almonds out of her backpack. The crowd laughs again, and the science-can-be-fun-vibe lasts for the rest of the tour.

Marks tells the group that the earth’s total diameter is 7,926 miles, then points to its different layers. The inner core, 1,500 miles in diameter, is made of solid iron. The outer core, 1,400 miles thick, is made of liquid iron, sulfur and nickel. The 1,800-mile-thick mantle is made of hot molten rock. And the crust, or surface of the earth, is 30 miles thick and contains the tectonic plates.

“When the almonds bump into each other,” she says, pointing to the crust, “that’s when we experience earthquakes.”

She passes the first edible demo around with some crackers, and the group indulges. “If you eat what you learn, you’ll remember it,” she tells them.

Marks explains that geologists knew very little about the plates when the 1906 earthquake shook the city. “It was completely mysterious to them,” she says. “They knew that the faults existed, but they didn’t know what they did.” Plate tectonics was still decades away.

When the earthquake, which measured a still-debatable 8.25 on the Richter scale, hit a little after 5 a.m. on April 18, 1906, it was socially devastating.

“But from a scientific point of view,” Marks says, “it was a magnificent moment.”

The quake spurred the first large-scale effort to study the earth’s movement; scientists flocked from around the country to study the area. They learned that the San Andreas Fault is at least 700 miles long, and that, as Marks demonstrates with two purple rubber bands around her fingers, most earthquakes are the result of the sudden elastic rebound from stored energy.

They also realized that this release of stored energy (and thus earthquakes and shocks) will continue to occur. “It’s not going to stop,” Marks says. “This is what we live with.”

She later admits that when a quake hits, she experiences a mixture of terror and thrill. “Maybe when the shaking breaks my windows and dumps my refrigerator over, the thrill will be gone,” she writes in an email. “But I hope not.”

As Marks leads the group toward the famous golden hydrant that helped save the Mission District from fire in 1906, she offers nourishment. “There’s more earth here if anyone needs another bite.”

A left on hilly Church Street brings the group to a jagged “mess of rock,” on top of which sits someone’s home. Marks asks what they notice, and they toss out the first words that come to mind: “Layers. Crumbling. Lots of angles.”

“This is all caused by motion, and the fact that rocks deform,” Marks tells them. The mess contains rocks associated with the “Franciscan mélange,” or a set of rocks formed together and then mashed together. It’s mostly composed of chert (a sedimentary rock formed from the silicate shells of tiny sea creatures called radiolaria) and greywacke sandstone (another sedimentary rock, which includes quartz and feldspar).

Marks passes around small pieces of each kind of rock for people to touch. The chert is brown and layered. The greywacke, firmly packed, is common in earthquake zones.

Basalt, a volcanic rock, is also present. Like the other rocks in the mess, it’s evidence of a certain kind of seismic activity called subduction. Lastly, the mess contains serpentinite, California’s state rock.

“You only find this in places that represent a certain kind of earthquake,” she says. “A very powerful earthquake.”

Marks adds that the state flower, the California poppy, is well adapted to the nutrient-poor soils made from serpentinite.

The group looks up at the mess of rock in a new way. Some may have walked by it a hundred times, never stopping to consider what it means.

Marks then hands out cookies resembling Oreos to demonstrate plate tectonics. The group watches her demonstration, then pull apart their cookies and scrape their teeth against the cream (or chert) in the center. They look at each other and laugh as they plow up the “sludge,” creating a motion that leads to large folds in the “rock.”

Still munching on “tectonic plates,” the group heads back down Church, up 20th Street, and down a staircase that leads them to hilly Noe Street. “Here is our next rock,” Marks says, pointing. “And I noticed that we all almost walked right by it.”

She introduces it as “our basalt friend,” adding that in the Bay Area basalt often appears in the form of pillows. The pillows, which resemble jelly beans stacked on top of one another, were formed by lava “blubbing” up and hardening.

“We do know that they’re stuck to the mess we saw on the other side,” Marks says, meaning that all of the homes the group has passed along the way have been built on top of either the chert or the basalt.

Walking down Noe, Marks points to a wall of chert next to the basalt pillows. “If it wasn’t covered in ivy, you’d get a better sense of how this is connected to the other side,” she says.

Marks leads the group down Noe, heading toward Sutro Tower, which is shrouded by fog. Kathleen Pabst, a docent at the California Academy of Sciences, is on the tour to do research for an upcoming earthquake exhibit at the museum. “I’m collecting knowledge so I’ll be better able to interpret it,” she says.

Along the way, Marks recommends a book called “Roadside Geology” to an older gentleman.

On the way up to Corona Heights Park, Marks points out the remains of an old brick factory. A few bricks jut out from the hill like crooked teeth. But the real treat is on the other side, and Marks saves it for last. On the way, she stops to point out another big wall of rock that’s the color of dark chocolate.

“This is lovely chert,” she says. “It’s a little more organized than the mess we were looking at under the house.”

She describes how the ocean floor pushed it up, and that one millimeter of chert equals a thousand years. “We’re looking at millions of years of history right now. This is what’s special about Bay Area geology.”

The group is all smiles when they realize that there is an abundance of chert in their city, and that they are witnessing something special. Most places where chert is found, Marks tells them, are either highly populated or undeveloped, where the rock has not been cut into to build homes, as here.

The group gathers around a picnic table for the last edible lesson of the day. Marks hands out graham crackers, or “tectonic plates.” She circles around, giving each plate a blob of chocolate “mantle.” She breaks her cracker in half and moves the two halves against one another, demonstrating that the pressure that drags the crumbs off is similar to the pressure that could create tears in plates along a fault. “This pressure that breaks pieces off makes other faults that are part of larger faults,” she says.

She illustrates how the plates are moving parallel until one dives under the other, and then how the plates begin moving apart. After all of the “plates” and “mantles” have been consumed, she says, “Now we’re going to move on to our grand finale, which is very stunning and amazing.”

The group follows her down a hill and around a corner, where they crane their necks to the left to behold a steep wall of chert that towers next to Peixotto Playground. “Oh, wow,” someone says. “Oh my gosh,” says another.

This chert is unlike the other chert they’ve seen today. It’s so shiny that it appears to be wet. Marks explains that it’s called a slickenside, or fault mirror. A slickenside is the inside of a fault; this chert has been polished to a sheen by rock that was once next to it.

“It’s not that few slickensides exist,” Marks says, “but they are hidden inside of faults, and much smaller than this one.”

She tells them that they can only see this rock face because the rock that had been on the other side of the fault has been quarried. “The chert that was quarried there was turned into bricks at the brick factory on the other side of the hill,” she says.

The group stares at the rock face, and small pieces of chert come crumbling down as rock climbers dangling from ropes feel around for footing. “The rocks are disappearing right before our eyes,” one man says, shocked.

It’s fun for Marks to see people learn new things. “They’re just rocks,” she says. “You don’t have to pay much attention to them until you look closely.”

Before the explorers go their separate ways, she hands each a pocket-size plastic magnifying glass. “I encourage you to look more closely,” she says. “At the cheese in your fridge, the sand in the sandbox, the lint in your dryer.”

They don’t hesitate to put the magnifying glasses to their eyes. “The world is more interesting if you stop to look at it,” Marks says.

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