Robert Sibley Volcanic Regional Preserve, Volcanic regional preserve in Oakland Hills, California, US

Robert Sibley Volcanic Regional Preserve is a 928-acre park in the Oakland Hills, with Round Top rising to 1,761 feet. The hill shows tan and brown grass, reddish-brown rock outcrops, and eroded gullies. The park preserves a tilted Pliocene volcanic vent, not a typical volcano shape.

The park was founded in 1936 as Round Top Regional Park and later renamed to honor Robert Sibley, the second president of the East Bay Regional Parks District. This change reflected his important role in developing the regional parks system.

The Mazzariello Labyrinth, built in 1990 by East Bay resident Helena Mazzariello and donated as a "gift to the world," draws hikers who pray, meditate, and leave talismans at its center. A smaller heart-shaped labyrinth lies near Post No. 4 on the Volcanic Trail. Mazzariello walked her goats there, saying she could enter with a question and emerge with insight.

Multiple trails of varying difficulty levels serve hikers and horseback riders, with accessible routes leading to the summit. Bring sturdy footwear and water since most paths are open and exposed to direct sunlight.

Round Top is not itself the volcano: the vent complex has been tilted on its side by the Hayward and Moraga faults, so it no longer looks like a cone. Kaiser Sand & Gravel quarried the site, exposing a cross-section of the vent, and lava there was dated at 9.5 million years old by UC Berkeley.