Robert Sibley Volcanic Regional Preserve is centered around Round Top Mountain, the prominent forest-clad prominence just south of the Caldecott Tunnel. Round Top is part of an ancient volcano that last erupted 10 million years ago. The subduction of the Pacific Plate under the North American plate caused undersea sediment to be scraped off the top of the Pacific Plate. This sediment then piled up to form most of the coastal mountains in California. The volcano that became Round Top was eventually tilted on its side by the cross-movement of the Hayward and Moraga faults, and portions of its volcanic soil have only become visible fairly recently.
Much more recently in the 1930s it was foreseen that the East Bay would experience dramatic changes in land use: the large cities of Oakland and Berkeley would continue their inexorable growth eastward into the Oakland Hills; towns would turn into small cities; and hamlets would become booming bedroom communities. Thus, a movement arose to preserve the special outdoor areas in the East Bay, and the East Bay Regional Parks District was born. Robert Sibley Volcanic Regional Preserve was one of EBRPD's first parks. Robert Sibley was one of these early visionaries, and served as EBRPD's president and director from 1948 to 1958.
Hiking guides typically describe the main hike in Sibley as starting and ending at the visitors center off of Skyline Blvd. However, that route leaves out a great deal of fun and elevation gain. This hike begins off of Old Tunnel Road. The first stretch of this hike goes up the paved Quarry Road (itself a gated extension of Old Tunnel Road), and the elevation gain is relentless. After 400' AEG in the first half-mile you turn right onto the Quarry Trail.
The views really open up on the Quarry Trail with great views of the large Round Top Creek drainage, Round Top, and Mount Tam and the bay in the distance. As you make your way towards Round Top the San Leandro Creek drainage comes into view to the southeast, and the upper portion of that area is one of the wildest areas in the Bay Area with a thick forest and very little easy ways to get there. The Quarry Trail ends at its junction with the Round Top trail. This trail circumnavigates Round Top. Go left (clockwise) to experience more views to the east including Mount Diablo, and after a short uphill stretch the trail begins a descent through a thick forest of Monterey pine (non-native), California bay (native), and eucalyptus (most certainly not native). The trail gradually flattens out and soon reaches its junction with the semi-paved road to the top of Round Top.
Turn right on the road, and the summit is quickly reached after a lung-burning 400' over 1/3 mile. The summit is nothing spectacular with its profusion of towers and tall trees that block most views, but it's always nice to get to the top.
Head back the way you came, and stay on the road back to its signed junction to the spur trail to the visitors center. Go past the outdoor visitors center, and make a right turn to reach the northern end of the parking area. This is where the Skyline trail begins, and you'll be back at the trail head in just under a mile. This section is fairly steep in stretches, and follows Round Top Creek under dense tree cover. There's an abandoned cabin or logging operation off to the right that sometimes can be seen.
Much of this hike is on the Skyline trail/Bay Area Ridge trail. More on those two gems at a later date.