Bourke MacDonald has spent much of the last year exploring local parks. During the coronavirus pandemic, when outdoor activities were some of the only outlets for fun, Bourke and his fiancée did a lot of hiking. And that’s how Bourke discovered Sibley Volcanic Regional Preserve.

“I said, ‘Volcanic!? What do you mean? I gotta check this out. There’s a volcano?'” Bourke said.

Even after investigating the self-guided tour offered by the park, Bourke wanted to know more about how a volcano in the East Bay hills came to be and why it went extinct.

On a beautiful, clear day I met up with Bourke and Michael Charnofsky, a naturalist with the East Bay Regional Park District. We set off on a hike up the mountain to discover the history and geology of this volcanic relic.

After several miles, we reached a spot in the trail with a steep drop off into a crater. I thought we were looking into the pit of the volcano, but Michael told me this is not the volcano.

“People come here and say, ‘there’s a big deep pit, that was a caldera,'” Michael said. “No. That was a mountain that was taken away.”

He explained that the pit is actually a quarry, where mining companies removed rock to build Bay Area roads and other infrastructure. Behind us stands Roundtop Mountain, which also looks like a volcano, but isn’t.

“Roundtop is being eroded away more slowly than the other mountains and that’s why it looks kind of like it stands on its own,” Michael explained. “And that’s why people think it is the volcano.”

It’s hard for me to wrap my head around the fact that the volcano we are investigating is so old that the only visible remnants of it are some very cool rocks scattered about the park. In fact, when we’re standing at the overlook of the quarry, with Roundtop at our backs, Michael says we’re standing at what would have been the middle of the volcano 10 million years ago.

The geology here is fascinating and explains a lot about our Bay Area landscape.

“There’s something called the Mendocino Triple Junction,” explained Dr. Kimberly Blisniuk, a geology professor at San Jose State University. Scientists call it that because currently the triple junction is off the coast of Mendocino, but 10 million years ago, that junction used to be down in the area where Sibley is now.

As the name suggests, the Mendocino Triple Junction is a place where three tectonic plate boundaries come together. The three plates are colliding, sliding past one another and overlapping. The place where they meet is like a T-junction and there’s a little hole that allows magma under the earth to bubble up.

“It’s called a slab window,” Blisniuk said.

It’s this slab window where magma burst up through the Earth’s crust and formed a volcano in what are now the East Bay hills, above Oakland.

And it gets crazier. Over millions of years, after lots of earthquakes, moving tectonic plates and shifting pressure, the land began to fold, turning the volcano onto its side.

“The best way to think about it is if you take pages of a book and you squeeze them together,” Blisniuk said.

Imagine a paperback book. Put one hand on the spine of the book and the other hand on the opposite pages. Now squeeze. As your hands apply pressure, one side of the once-flat book will arc up, while the other side will dip down. It looks a little like a sin wave. In this analogy, the book is the land, and the pressure comes from faults under the earth.

“We get something called compressional deformation,” Blisniuk said. “We squeeze the earth because there’s a lot of stress that’s being transferred from, say, the Calaveras to the Hayward Fault as these faults are evolving and producing earthquakes. When you squeeze the land between these faults, they start to fold and tilt.”

The pressure pushed some portions of land up and others settled down. In the process, the volcano folded over. And to make it even harder to imagine what the volcano would have looked like, millions of years have eroded much of the rock away, altering the landscape from what it would have looked like when the volcano was active.

“I have heard this is the best example of the inside of a volcano anywhere in California,” Michael said, because of the quarrying that was done here. Miners dug into the mountain, which was really the side of the old volcano, allowing scientists an unusually good view of the geological history. “People who love volcanoes love to come study this one,” Michael said.

Even though it’s difficult to conceptualize the volcano and the changes that earthquakes and time have wrought, Sibley has a special feel to it. And in addition to the beauty of green hills, bay views, wildflowers, birds and cool rocks, there’s one other unique feature here: A labyrinth at the bottom of the quarry pit.

“It was a beautiful round spot already,” said Helena Mazzariello, the woman who built the labyrinth over 30 years ago. “It already felt sacred. It already felt so good there.”

Helena is a shamanic practitioner and used to live near Sibley. She  would often hike there  with her pet goat. Together they’d wander down to the bottom of the quarry. One day she decided to build a labyrinth out of mud.

“A labyrinth is one of the oldest contemplative transformational tools that mankind used for centuries,” she explained. It’s not the same as a maze — that’s something you get lost in. A labyrinth is a type of walking meditation.

“This is a labyrinth; it is unicursal,” she said, “so there’s one way in and one way out.”

Helena didn’t expect the labyrinth to last. In fact, when the rains came that year, she headed over to Sibley to see what had happened to her creation. When she got there, she found other visitors had laid rocks on the mud lines she’d drawn, keeping the labyrinth’s shape. Over the next three decades, Helena and other Friends of the Labyrinth, have worked to maintain the pattern. It has become one of the most famous labyrinths in the Bay Area, with Helena’s name appearing on some maps.

The East Bay Regional Park District doesn’t condone rogue labyrinth builders, but this one has been there so long, and is tended with such care, that they look the other way. It even brings visitors to the park.

As the sun started to set, turning the distant bay a glassy blue-gray, Bourke, Michael and I headed down the mountain. It would be easy to hike in this park oblivious to its volcanic history. In many ways its wide trails, rolling green hills, and epic views look like other parks that dot the East Bay hills. But when you hike in Sibley, remember — you’re in the middle of a volcano.

As Michael likes to say, “It’s all over the place and it’s almost nowhere.”

Explore a lot more about the geology of various parts of Sibley Volcanic Regional Preserve in this video tour with Steve Edwards.