Faults

Geological and Seismic Activity of Pinnacles National Park

The Origin of the Pinnacles

The Pinnacles formations are remnants of a Miocene volcano that has undergone significant decomposition. Approximately 23 million years ago, rhyolitic magma and other volcanic flows erupted through fissures in a basement of quartz diorite and granite. Later activity developed central vents, leading to explosive eruptions that built up thick layers of pyroclastics above the earlier lava flows. Erosion, along with the action of wind and water on these pyroclastics, has sculpted the unusual and dramatic landscapes that Pinnacles National Park is known for today.

A Product of Tectonic Plate Movement

Located near the boundary of the Pacific Plate and the North American Plate, Pinnacles National Park exemplifies tectonic plate movement. The Pinnacles Rocks are believed to have originated from the Neenach Volcano, situated near present-day Lancaster, California. The distinctive breccias found at Pinnacles are only seen elsewhere in the Neenach Formation, approximately 195 miles (314 km) southeast. The San Andreas Fault, which once split the original volcano, has been instrumental in transporting the Pinnacles northward at a rate of approximately 0.59 inches (1.5 cm) per year through its right-lateral movement.

Faults Within the Park

The park is intersected by three major faults: Miner's Gulch, Pinnacles, and Chalone Creek, along with several smaller faults and fractures. The Chalone Creek Fault, still active today, runs parallel to the main drainage on the park's east side and is believed to mark the former location of the San Andreas Fault during the Miocene epoch, when the Neenach Volcano erupted. Currently, the San Andreas Fault has shifted about 4 miles (6 kilometers) east of the park. The faults within Pinnacles are roughly parallel to this master fault and likely result from its significant movements.

Formation of Talus Caves

The complex fault movement over millions of years has caused the Pinnacles volcanic formations to be buried and later unearthed during their journey northward. This faulting and seismic activity have created the deep, narrow gorges seen today, where massive boulders have toppled from above. These boulders, wedged at various heights above canyon floors, form the Bear Gulch and Balconies talus caves, which are popular attractions for park visitors.

Seismic Monitoring and Activity

Pinnacles National Park experiences frequent small to moderate earthquakes. The United States Geological Survey (USGS) monitors seismic activity within the park using a seismometer along the Chalone Creek Fault, with a corresponding seismograph located in the Bear Gulch Nature Center. This system provides a continuous record of seismic activity, allowing visitors to confirm any ground movement they might feel. Monitoring aims to understand earthquake behavior along the San Andreas Fault and provides park staff with data to interpret and illustrate ongoing natural processes.

Expressions of Seismic Activity

Seismic activity is evident throughout the park. Streams display characteristic offsets as they cross fault lines, and valley bottoms and terraces reveal localized uplift. In the nearby town of Hollister, sidewalks show damage due to fault creep along the San Andreas Fault. Unlike “slipping” segments that produce major earthquakes, the segment of the fault near Pinnacles is classified as a “creeping” segment, making severe earthquakes less frequent. However, moderately intense ground shaking remains a possibility, whether originating from nearby faults or from locked segments further north or south. A safety study in Bear Gulch Cave, conducted after the 1989 Loma Prieta earthquake, found no movement of boulders following moderate seismic events.