When you’re standing in Pinnacles National Park, you’re really not standing still. The earth beneath you is gradually moving north, at the pace of about an inch a year, compared to the mountains of the Diablo Range to the east.
This motion is taken up by slip on the San Andreas Fault, a 600-mile long fracture between two of the planet’s tectonic plates. The Pacific plate has been grinding past the North American plate for about 30 million years.
As these two chunks of the Earth’s crust met, a the Pinnacles volcanic field was born about 23 million years ago and 195 miles to the south. Eruption after eruption accumulated a pile more than 8,000 feet thick of volcanic rock layers made up of lava, rock fragments, ash, and glass. Millions of years later, the stack of volcanic rocks was split by the newly formed San Andreas Fault, and the land to the west of the fault moved northward, just as it does today.
Millions of years of tectonic motion both brought the volcanic rocks here and lifted and tilted them, exposing them to erosion by wind and rain that has carved the spires and canyons of Pinnacles National Park. As the steep canyon walls erode further, huge rocks fall to the canyon bottom and pile to form the talus caves.