The San Francisco Bay Area has among the most active and complex geology worldwide thanks to the tectonic plate boundary that has existed here for about 200 million years. For most of that period, the Pacific Ocean crust was sliding under the North American plate in a process known as subduction. More recently—the past 28 million years—the two tectonic plates have been sliding past each other on the San Andreas Fault zone. The San Andreas Fault is most famous for generating San Francisco’s magnitude 7.7 earthquake in 1906.
San Francisco Bay Area national parks contain spectacular examples of plate tectonics. Granitic rocks on the Point Reyes Peninsula formed in the region of today’s Mojave Desert about 100 million years ago. They became part of the Pacific Plate and were transported northward after the San Andreas system formed. Sediments on the Point Reyes Peninsula indicate that it was adjacent to Monterey until about 12 million years ago, when movement on the San Gregorio fault, parallel to the San Andreas, carried Point Reyes northward to its present location. While the Pacific plate normally creeps an average of 2 inches per year, Point Reyes leapt 20 feet during the 1906 earthquake!
Likewise, the San Andreas Fault is a key feature at Pinnacles National Park. The rock spires and crags of Pinnacles are remnants of a 23 million year old volcano in southern California that was split by the San Andreas Fault zone. The Pinnacles part of the volcano moved northwestward on the Pacific plate, while the east side remained 195 miles to the south on the North American plate.
Plate movements have also given the region a remarkable mix of rocks including iron- and magnesium-rich volcanic basalts; deep-sea radiolarian chert, formed from silica-rich skeletons of microscopic sea creatures; and serpentinite, California’s state rock, which is made of mantle material pushed up through the ocean floor at the plate boundary.