Great American volcanoes: Lassen Peak, Mt. Shasta, Mt. St. Helens, Mauna Loa, Old Round Top.
Old what? As strange as it seems, the Bay Area has a volcano. Long extinct Round Top may appear as not much more than a little rounded hill among the larger Oakland-Berkeley hills, but 10 million years ago it was a mighty volcano that erupted beneath a lake. The molten lava interior of the mountain then fell into the void left by the explosion.
Robert Sibley Volcanic Regional Preserve displays this odd bit of East Bay geology.
Before the park was established, quarry operations bared some of the volcano's secrets.
A gated road leads to the top of 1,763-foot Round Top, where excellent vistas of San Francisco Bay unfold on clear days.
Even better panoramas can be enjoyed from the Round Top Loop Trail. It's an interpretive path (keyed to a brochure from the visitor center) that explains the volcanic outburst and area ecology.
The park's Volcanic Trail also offers good views and intriguing geological formations. Nearly three miles of East Bay National Skyline Trail cross the park; you can hike all day north or south to more East Bay parks.
Directions to trail head: From California Highway 24, northeast of Oakland and just east of the Caldecott Tunnel, exit on Fish Ranch Road. Drive 0.8 mile to Grizzly Peak Boulevard, turn left and proceed 2.5 miles to Skyline Boulevard. Turn left and in 100 yards turn left into Sibley Volcanic Regional Preserve.
The hike: From the visitor center, take the pathway as it parallels the road to Round Top. A mixture of Monterey pine, eucalyptus, bay and toyon cloak the lower slopes of Round Top. The path then crosses the paved road, which you can follow to the summit of Round Top. After taking in the views, descend and turn left on Round Top Loop Trail. The narrow path traverses grasslands, then passes a quarry pit that reveals the interior of the old volcano. A bit past the pit, the loop trail meets Volcanic Trail.
Join Volcanic Trail, which offers an impressive view of Mt. Diablo and leads 0.25 mile to a quarry pit that shows lava flows and red cinders, remnants of a volcanic vent.
Two spur trails offer additional perspectives. The left path crests a knoll, where the hiker is treated to a fine panorama of the Bay Bridge and San Francisco. Another side path leads to a weird gathering of bay laurel and lava boulders, plus vistas to the north.
Retrace your steps on Volcanic Trail back to Round Top Loop Trail and turn right, closing the loop back to the trail head.
For more of John McKinney's hiking tips and trails, visit http://www.the trailmaster.com.
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
Round Top Loop Trail
WHERE: Sibley Volcanic Regional Preserve.
DISTANCE: 4-mile loop with 400-foot elevation gain.
TERRAIN: Bay Area volcano.
HIGHLIGHTS: Nature trail tells story of explosive Bay origins, panoramic views.
DEGREE OF DIFFICULTY: Easy-moderate.
FOR MORE INFORMATION: East Bay Regional Park District; tel. (510) 635-0135.