Pinnacles National Monument encompasses a belt of Tertiary volcanic rocks (mostly tuff, breccia, and ash of rhyolite, dacite, and andesite composition). The location of the Pinnacles volcanic area near the San Andreas Fault has important implications for deciphering the geologic history of the San Andreas Fault System. Composition, texture, and geochronologic age-determinations suggest that Pinnacle's volcanics are equivalent to the Neenach Formation located on the east side of the fault zone and 195 miles (300 km) to the south (near Los Angeles). The Pinnacles volcano formed about 23 million years ago along the fault zone. The west side of the fault zone is moving northward relative to the east side at a pace of several centimeters per year. This movement occurs as both gradual creep and as episodic pulses associated with earthquakes. When travelling south along Highway 25 from Hollister, the road basically follows the trace of the San Andreas Fault System (which actually consists of a number of parallel and interconnected faults).

The Pinnacles (named for the high spire-shaped rock formations) owe their origin to weathering and erosional processes associated with prevailing climatic conditions over time. The region has warm, dry summers, and mild, wet winters. The Pinnacles are within the larger Gabilan Range, an uplifted belt of rock along the west side of the San Andreas Rift Zone (a valley associated with the fault system). Rocks in the Diablo Range consist mostly of Cretaceous and older granite with a cover of younger Tertiary-age marine and non-marine sedimentary formations, and volcanic rocks (such as the Pinnacles area). The Gabilan Range is still gradually rising relative to the Salinas Valley to the west. To the east, the Diablo Range has a core of ancient oceanic crustal rocks (mostly serpentinite and associated rocks) with a blanket of younger Cretaceous and Tertiary sedimentary rock formations.

The map used as a base for the above image is part of the map: California Department of Natural Resource, Division of Mines, 1959, Geologic Map of California, Santa Cruz Sheet, 1:250,000.

For an introduction to Rocks and Geology of the San Francisco Bay Region visit http://geoubs.wr.usgs.gov/bulletin/b2195/, or for Field Trip Guides visit http://geopubs.wr.usgs.gov/bulletin/b2188/.

Selected abbreviations for named map units include: