If one asked people in the San Francisco Bay Area if there are any volcanoes nearby, most would either say “No” or suggest “Maybe Mount Diablo?” (Mount Diablo, the highest peak in the Bay Area is cone-shaped, but is not volcanic). There is, however, a(n extinct) volcano within Oakland’s city limits, in a park that is little known to those on the coastal plane where most of Oakland’s population and workplaces are.
Much of the park affords views of Mount Diablo. It also has what appears to be a perfectly shaped caldera, but this is another illusion. The pit with a living maze at the bottom was a quarry, so that it was dug out by humans.
Over the course of the last ten million years the remains (infilling) of the volcano was tilted on its side, as the strains of the Hayward and Moraga faults pushed the sedimentary Oakland and Berkeley Hills up. The sedimentary rock eroded more quickly than the igneous rock, so that Round Top is the highest point (1761 feet). Geologists have identified eleven different lava flows–none of them recent.
During most of the time humans have inhabited North America, Round Top was a grassy, treeless hill. Early in the 20th century, there was a mass planting of blue-gum eucalyptus trees in northern California under the delusion that it was not only fast-growing but would provide usable hardwood (the eucalyptus that Australians used takes several hundred years to grow to use as lumber). most of the commercially useless eucalyptus was removed in the summer of 1973 after an unusual hard freeze the previous winter, though there is still one stand of it east of the visitor center.
The Robert Sibley Volcanic Park visitor center is not staffed. It provides lavatories, a display of different kinds of rock from the park and brochures identifying the geological highlights of the main trial, the Round Top Loop Trail, which runs northwest from the visitor center. It is about two miles with no particularly steep stretches and without any difficult footing.
Parking and admission are free, but there are few spaces. And there are few picnic tables.
As long as they are under control, dogs do not need to be leashed within the park. I’ve never seen horseback riders, but I know that at least the Skyline Trail, which crosses the western side of the park, permits horses. (It runs from Castro Valley, 17 miles to the south, to El Sobrante, 12 miles to the north.)
The west side of the park is home to western bluebirds and lazuli buntings. Every time I’ve been there, I’ve heard spotted tohees, but not always seen them. Sibley is the place where I’ve seen the most red-tailed hawks at once not during migration (5). It is also the place with the best chance of seeing a golden eagle. Golden eagles range far and are sometimes seen in other East Bay Regional Parks along the coastal mountain range, but I there is a nesting pair at Sibley. The odds are about 1:3 in my experience of seeing one or more golden eagles at Sibley, which is better odds than at any of the other East Bay Regional Parks.
Other birds often seen at Sibley are lesser finches, Bewick’s wrens, American kestrels, goldfinches, and white-throated swifts. Scrub jays and turkey vultures are always visible, too.
When San Francisco is fogged in, there is usually a cool wind at Sibley. During the summer whenever San Francisco is not foggy, Sibley can be very hot.
The park’s main entrance is 6800 Skyline Skyline Boulevard just east of the intersection with Grizzly Peak Boulevard in the Oakland hills. It is close to being directly above the Caldecott Tunnel. From Highway 24 one may go up to Grizzly Peak Road from either end of the tunnel.