The pinnacles themselves are the remnants of a Miocene volcano that is in an advanced stage of decomposition. Approximately 23 million years ago, rhyolitic magma and other flows were forced to the surface through fissures in a basement of quartz diorite and granite. Later activity developed central vents, and explosions from these vents built up a vast thickness of pyroclastics above the earlier lavas. The action of erosion, the work of water and wind on these pyroclastics, has given rise to the unusual and scenic effects for which Pinnacles is famous.
Located near the boundary of the Pacific Plate and the North American Plate, Pinnacles National Park is an excellent example of tectonic plate movement. The Pinnacles Rocks are believed to have originated in the Neenach Volcano, near present-day Lancaster, California. The unique breccias of Pinnacles are only found elsewhere in the Neenach Formation, some 195 miles (314 km) southeast. The displacement of the Pinnacles from its point of origin is a testament to the longevity and importance of the San Andreas Fault, which once split the original volcano. The right lateral movement of this giant transform fault has carried the Pinnacles north at a rate of approximately 0.59 inches (1.5 cm) per year. - NPS