Sibley Volcanic Regional Park
Sibley Volcanic Regional Park preserves the remnant of an old volcano that formed about 10 million years as the San Andreas Fault System (including the Hayward Fault) formed across the region. The fault ruptured through the Earth's crust, allowing magma to migrate to the surface and erupt as volcanoes. Volcanic rock crop out in many places along the Hayward, San Andreas, and other faults in the region. Studies of volcanic rocks throughout the region demonstrate that the oldest volcanoes lie to the south of the Bay Area (about 10-18 million years), and younger ones exist to the north of San Pablo Bay (including the relatively young and active Clear Lake-Sonoma Volcanic Field—less than 5 million years). The study of the volcanic rocks has helped reveal much information about the age of the fault system and the rates that the faults have moved through time.
This view is looking east from Skyline Boulevard in the East Bay Hills (east of Berkeley) toward Round Top Hill in Sibley Volcanic Regional Preserve. The peaks on Round Top Hill are the remains of ancient volcanic stocks—the underground passages or vents that magma flowed along to erupt at the surface. Some of the magma that cooled to stone in the passages (becoming the stocks). These rocks are more resistant to erosion than the rock formed from ash, cinders, and lava flows that accumulated on the flanks of the ancient volcanic area. Also note the landslide (slump) near the left of center in the image. |
