The Geology of Pinnacles National Park, California

Pinnacles National Park lies in the southern Gabilan Range of Monterey and San Benito counties, California, about 64 km (40 mi) inland from the Pacific and about 129 km (80 mi) south of the San Francisco Bay Area. It was proclaimed a national monument on January 16, 1908, and became a national park on January 10, 2013. Its roughly 10,799 hectares (26,686 acres) protect spire-like rock formations 152 to 366 m (500 to 1,200 ft) high, an extensive collection of rare talus caves, and a variety of volcanic features from the Neenach Volcanic Field.

A view from the High Peaks Trail. Image from the National Park Service's page.

A volcano that moved

The volcanic rocks of the Neenach field are thought to have formed about 23 million years ago near present-day Lancaster, California. The San Andreas fault system has since carried them some 315 km (196 mi) to the northwest. Over millions of years, wind and water carved them into a landscape of deep, narrow gorges, caves, colonnades and monoliths.

The park sits along the San Andreas Fault, and its geology has been shaped by transform plate motion, which has moved rocks hundreds of kilometers from where they formed. The Pinnacles Volcanics correspond to the Neenach Volcanics: the fault has split the original volcanic field and pulled the two halves 315 km apart, leaving closely similar rocks on either side of the boundary.

Three groups of rocks

• Cretaceous intrusive rocks, the oldest, along the park's western boundary and in its southeast: the quartz monzonite of Bickmore Canyon and the quartz diorite–granodiorite of Johnson Canyon.
• The Pinnacles Volcanics, of Oligocene–Miocene age: rhyolite, dacite, andesite, agglomerate, breccia and pumice lapilli-tuff, forming the park's central core between the Pinnacles Fault on the west and the Chalone Creek Fault on the east.
• Neogene and younger rocks: the Miocene Monterey Formation and Pliocene Pancho Rico Formation east of the Chalone Creek Fault in the northeast, and the Pliocene–Pleistocene Paso Robles Formation, the youngest bedrock, along the southwestern edge.

Pinnacles is part of the Pacific Border physiographic province and shares its geologic history with a region far beyond the park.

A talus cave in Bear Gulch. Image from the National Park Service's page.

Maps and inventories

The park's page belongs to the NPS Geodiversity Atlas, which gathers information on each park's rocks, minerals, sediments, fossils, landforms, soils and physical processes to support education, geoconservation and management of both living and non-living parts of the ecosystem. The Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 natural resource parks, has products for Pinnacles, and a Soil Resources Inventory has been completed.

Sources

• National Park Service, "NPS Geodiversity Atlas—Pinnacles National Monument, California." https://www.nps.gov/articles/nps-geodiversity-atlas-pinnacles-national-monument-california.htm
• The atlas page cites Turner and others (1970), Matthews (1973), Sims (1993) and National Park Service (2015, 2016).
• Geologic Resources Inventory products: https://irma.nps.gov/DataStore/SavedSearch/Profile/3146; Soil Resources Inventory: https://irma.nps.gov/DataStore/SavedSearch/Profile/3097
• The Park Service's page is titled and headed "Pinnacles National Monument" but states that the monument became a national park in 2013; this page uses the park's current name.