Geology

The faults within the Pinnacles National Monument mark distinct boundaries in geology. The Pinnacles Fault juxtaposes granitic basement rock west of the fault and the Pinnacles Volcanic Formation east of the fault. The Chalone Creek fault juxtaposes the Pinnacles Volcanic Formation west of the fault and the Temblor Fanglomerates east of the fault.

The granitic basement is the Santa Lucia Granite and Granodiorite. These granites formed when masses of molten lava slowly cooled as they rose through the earth's crust to a point where they completely solidified. A slow cooling process allows individual crystals to grow fairly large. Subsequent uplift from faulting and erosion of overlying material exposed these rocks at the earth's surface. These are the oldest rocks in the park, 78-100 million years old. They form the basement upon which the rest of the rocks at the Monument lie.

The Pinnacles Volcanic Formation formed approximately 23 million years ago as it was extruded and deposited atop the granitic basement. The magma that was the source of all the volcanics was rhyolitic in origin. The formation consists of rocks such as banded and massive rhyolite, some andesites and dacites and various pyroclastic units.

The High Peaks consists of a relatively strong, well-consolidated breccia. The layers of breccias are thought to have formed as the result of material slumping off the sides of the volcano near the vents causing large landslides. The volcano was likely near water and the landslides traveled as massive turbidity currents under water that spread the material considerable distances until coming to rest near distant edges of the volcano. Volcanic ash and rhyolitic lava flows are interlayered with these breccias. Subsequent burial and compaction hardened these layers into the consolidated rock we see today. Recent faulting, fracturing and erosion have sculpted these rock layers into vertical cliffs and spires sometimes several hundred feet high.

The Temblor Formation east of the Chalone Creek Fault is a fanglomerate unit (conglomerates deposited in an alluvial fan setting) composed of granitic and to a lesser extent volcanic detritus shed from the Santa Lucia granitic basement and the Pinnacles Volcanics.

Faults

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 Monument 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.

Three large faults (Miner's Gulch, Pinnacles, and Chalone Creek) occur within or near the Monument, besides a number of smaller faults and fractures. The Chalone Creek Fault located within the Monument and still active, runs parallel to the major drainage along the east side of the Monument and is thought to mark the position of the San Andreas Fault in the Miocene - at the time the Neenach Volcano erupted. Today, the San Andreas Fault has shifted its location 4 miles (6 kilometers) to the east of the Monument. The large faults of the Pinnacles area are roughly parallel to this master fracture and were probably caused by major movements of this important fault. Complex fault movement has buried the Pinnacle volcanics for much of their northbound journey, only recently unearthing these easily weathered and erodable rocks. This history of faulting and earthquakes has also created deep, narrow gorges where huge boulders have toppled from higher formations. These boulders, wedged at various heights above the canyon floors, create the Bear Gulch and Balconies talus caves.

Small to moderate earthquakes are frequently felt within the Monument. Seismic activity continues to be monitored by the United States Geological Survey (USGS). There is a seismometer along the Chalone Creek Fault and a corresponding seismograph in the Bear Gulch Visitor Center which provides a continuous record of seismic activity. Visitors are often able to see the seismograph to confirm their suspicion of previously felt ground movement. The purpose of continued monitoring is to learn more about earthquake phenomena along the San Andreas Fault. The information provides the Monument staff with data to illustrate and interpret the natural processes still shaping the Pinnacles area.

Expression of seismic activity abounds. Streams show characteristic offsets as they cross fault bounds. Valley bottoms and terraces are evidence of localized uplift; the nearby town of Hollister contains several sidewalks torn by fault creep. Because this segment of the San Andreas Fault is a “creeping” segment as opposed to a “slipping” segment, major or severe earthquakes are considered less frequent along it. Still, moderately intense ground shaking is likely in the near future whether it originates from faults nearby or locked fault segments far north or south.  A study in the Bear Gulch Cave, initiated due to safety concerns following the 1989 Loma Prieta earthquake, has detected no movement of boulders following moderate earthquakes.

Cave / Karst Systems

Within the National Park System lie some of this nation's most amazing caves. Grand caverns and solution formations usually come to mind when caves are mentioned. Pinnacles, though, has a much different type of caves in structure and formation. The talus caves of the Monument were formed when steep, narrow canyons filled with a jumbled mass of boulders from the cliffs above. The canyons are the result of faults and fractures in the central area of volcanic rock. These shear fractures filled with gigantic toppled boulders are clear windows into the geologic wonder of the Monument.

Pinnacles National Monument has two main areas of caves; the Bear Gulch Caves are near headquarters in the East District, and the Balconies Caves are near Chaparral Campground in the West District. A few poorly documented areas of small talus caves are scattered around. The rockfall that filled the fractures is thought to have occurred during the last series of ice ages. Despite the age of this formation, the process of rockfall and weathering continues. The boulders range from a few ounces to thousands of tons. Much of the rock matrix is supported by gravel and sand that has become lodged between the boulders, or has formed at the contact points of the stones over the centuries. These smaller particles are particularly susceptible to erosion during the flash floods that occasionally rush through the caves.

There is no known evidence of Native American habitation in any caves, though extensive archeological work is yet to be done, and local Native (Mutsun and Chalone) stories have largely died with their tellers. Non-Indian legends that have survived refer to the use of the caves as a hideout by the notorious central California bandito Tiburcio Vasquez, whose brutal contribution to local history ended with his hanging in San Jose, California, in 1875. Stories of hidden treasure and robber's roosts still cycle through campfire stories and local lore, but the location of Tiburcio's hides seem speculative.

The Civilian Conservation Corps built trails through the caves in the 1930's and these trails have endured many storms and travelers. The stairways and bridges they constructed were needed to navigate the caves without the use of ropes and ladders. Today, the Bear Gulch and Balconies caves are principle attractions for visitors at the Monument.

Geologic Formations

The faults within the Monument mark distinct boundaries in geology. The Pinnacles Fault juxtaposes granitic basement rock west of the fault and the Pinnacles Volcanic Formation east of the fault. The Chalone Creek fault juxtaposes the Pinnacles Volcanic Formation west of the fault and the Temblor Fanglomerates east of the fault.

The granitic basement is the Santa Lucia Granite and Granodiorite. These granites formed when masses of molten lava slowly cooled as they rose through the earth’s crust to a point where they completely solidified. A slow cooling process allows individual crystals to grow fairly large. Subsequent uplift from faulting and erosion of overlying material exposed these rocks at the earth’s surface. These are the oldest rocks in the park, 78-100 million years old. They form the basement upon which the rest of the rocks at the Monument lie.

The Pinnacles Volcanic Formation formed approximately 23 million years ago as it was extruded and deposited atop the granitic basement. The magma that was the source of all the volcanics was rhyolitic in origin. The formation consists of rocks such as banded and massive rhyolite, some andesites and dacites and various pyroclastic units.

The High Peaks consists of a relatively strong, well-consolidated breccia. The layers of breccias are thought to have formed as the result of material slumping off the sides of the volcano near the vents causing large landslides. The volcano was likely near water and the landslides traveled as massive turbidity currents under water that spread the material considerable distances until coming to rest near distant edges of the volcano. Volcanic ash and rhyolitic lava flows are interlayered with these breccias. Subsequent burial and compaction hardened these layers into the consolidated rock we see today. Recent faulting, fracturing and erosion have sculpted these rock layers into vertical cliffs and spires sometimes several hundred feet high.

The Temblor Formation east of the Chalone Creek Fault is a fanglomerate unit (conglomerates deposited in an alluvial fan setting) composed of granitic and to a lesser extent volcanic detritus shed from the Santa Lucia granitic basement and the Pinnacles Volcanics.