Tuesday, May 10, 2005
Sibley volcano is extinct but his historic activity shaped the skyline
Lava spewed across the future path of Highway 24 through Orinda.
Rocks flew 25 miles into modern-day Sears Point in Sonoma County.
Any Oakland mastodon that got in the way of the molten lava was toast.
Ten million years later, the forgotten volcano of the East Bay strikes a low profile in the public mind. But it's a high point in the Oakland-Berkeley hills and a choice spot for people to see the inner core of an extinct cauldron.
"It's really the lost volcano of the East Bay hills, yet it's been right here in our back yard," said Beverly Lane, president of the East Bay Regional Park District board.
Park system operators are the first to admit they weren't in touch with their inner volcano.
They owned big hunks of the extinct volcano along the Contra Costa-Alameda County line for more than 40 years before realizing what they had.
Round Top, part of Sibley Volcanic Regional Preserve, one of the tallest points in the Oakland-Berkeley hills at 1,763 feet, is actually a side of the volcano that was tipped by geological forces.
Sediment that clogged the crater over eons hid it as well.
Between 8 million and 10 million years ago, three other volcanoes erupted in the Oakland-Orinda hills, but they remain buried. The largest sleeps under Lawrence Berkeley Laboratory.
Round Top is as dead as its brother volcanoes, which burned out long ago when shifts in the earth's plates moved volcanic centers northward toward the Cascades, home of Mount St. Helens in Washington. Mount St. Helens erupted in 1980.
Round Top's past surfaced because of commercial quarrying and a regional park employee with a passion for volcanoes.
For decades, Kaiser Sand and Gravel Co. quarried the hills near Sibley for basalt, a hard volcanic rock used for road beds, before selling their land to the park district in 1977.
The result exposed cross sections of the inner crater that once spanned several hundred, perhaps 1,000, feet.
"This is the best place in California to see the anatomy of a volcano," said Steve Edwards, a regional park scientist who has studied the volcanic terrain for 25 years. "Nowhere else in the state can you see the deep interior."
Edwards didn't discover the East Bay volcanoes, but he helped the park district recognize them.
As a UC Berkeley paleontology graduate student and part-time regional park employee, Edwards learned that scientists knew the region had a volcanic past, but they had not widely publicized it.
After Kaiser sold its quarry to the park district, Edwards in the early 1980s mapped the volcano crater, vents and interior, and informed the park district what it had.
Originally known as Sibley Regional Preserve, the park district added Volcanic' to the name and created a self-guided walk for visitors that details Edwards' findings.
"I think without Steve's interest in the Sibley volcano, there would be little information available about it," said Doris Sloan, a UC Berkeley adjunct professor who has taken students there for field tours. "It's not terribly easy to figure out it's a volcano on its side."
Despite its low public profile, Sibley regularly attracts science students and researchers for visits.
Park managers propose a new preserve entrance off Old Tunnel Road that park officials hope will attract more visitors to appreciate the volcano.
Edwards doesn't lead Sibley tours for the public, as he now directs the Tilden Park Botanical Garden.
On a recent visit to Sibley, Edwards showed his knack for seeing life and action in rocks.
"The rocks tell you stories if you will listen," the tall, lanky scientist said.
Far from producing a single type of rock, the East Bay volcanoes fused, blew up, then stirred and cooled basic elements into a rich variety of rock layers, veins, formations and flows.
He pointed to rocks called tuffa breccia, a brown mixture of ash, lava and gravel fused together by superheat into a kind of geological stew.
Other rock formations called vesicular basalt picked up the appearance of having bubbles or pockets formed by gas released in the eruptions.
The reddish color of some rock tells him that sediment was baked when the volcanoes blew.
Edwards likes to stand on an overlook for the Caldecott Tunnel and show park visitors where lava flowed into an alluvial fan that later become the ridges forming Siesta Valley, site of the California Shakespeare Festival.
He pointed out layers of brown basalt and red lava just north of Highway 24 easily in view of the thousands of motorists driving by.
"There is a yawning chasm of geologic time, an immense span of history, written in the rocks, and it is there to be discovered," Edwards said. "Moreover, without plumbing those depths we cannot understand the ground we walk on."