The volcano's core has been tilted approximately 70 degrees from its original horizontal orientation due to movement along the Hayward Fault.
Round Top volcano was part of a chain of volcanoes that once formed a larger volcanic field in the Berkeley Hills.
The preserve contains several stone labyrinths hidden in old quarry sites, created by anonymous artists starting in the late 1980s.
The park was formerly the site of the Berkeley Pit, an active quarry that extracted volcanic rock for road construction.
The landscape exhibits distinctive 'volcanic neck' geology where magma cooled and solidified within the volcano's vent.
The preserve is part of a larger 9,000-acre contiguous open space corridor in the East Bay Hills.
Rare plant species, such as the fragrant fritillary, are protected within the preserve's diverse microclimates.
Sibley Volcanic Regional Preserve is centered around the Round Top volcano, an extinct Miocene-era shield volcano that was active approximately 10 million years ago. The preserve features a unique landscape characterized by basaltic rock formations and hidden labyrinths created by local artists. Visitors can hike through the remains of the volcanic crater, which has been tilted and eroded by tectonic activity along the Hayward Fault. The terrain provides unobstructed views of the San Francisco Bay Area and Mount Diablo. Multiple loop trails traverse the preserve, ranging from moderate ridge paths to steep descents into the crater. Geological interest is driven by the exposure of volcanic layers including volcanic breccia, tuff, and basalt. The site is a primary location in the East Bay for studying regional plate tectonics and volcanic history.
The interior of the main stone labyrinth located in the deepest part of the old quarry floor.
Wear sturdy hiking boots, as the volcanic rock and gravel trails can be slippery and uneven.
Download an offline map, as cell service is inconsistent throughout the volcanic crater and valleys.
Bring wind-resistant layers, as the exposed ridges are subject to high, consistent winds from the bay.
Do not attempt to scramble up the steep, loose rock walls of the volcanic quarry, as the rock is brittle and unstable.
Stay on marked trails to prevent erosion and protect native plant species; dogs must be leashed in designated areas.
