Geology Of The Park
Pinnacles National Park is the remnant of an ancient volcano formed approximately 23 million years ago. The Pinnacles volcano was a result of the same plate subduction occurring today off the California coast.
As thick lava flowed from the volcano, combined with explosive bursts of rock, this "stratovolcano" created the infamous protruding handholds climbers know so well. After the volcano's eruption, lift from the San Andreas Fault combined with millions of years of erosion created the great valleys and pinnacles of Pinnacles National Park. Straddling the San Andreas was actually split in two! The other half is 195 miles away near Lancaster.
Forces of water and wind also caused large boulders to slide down the steep canyon walls creating what are now the caves. These are not "true" caves, but rather narrow passages created under these boulder piles.
Modern-day Pinnacles is a geologist's playground peppered with silica-rich rhyolite, layered lava flow bands and perlite pockets of green volcanic glass.