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Pinnacles National Park 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 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.

Three large faults (Miner's Gulch, Pinnacles, and Chalone Creek) occur within or near the park, besides a number of smaller faults and fractures. The Chalone Creek Fault located within the park and still active, runs parallel to the major drainage along the east side of the park 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 park. 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 park. 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 park 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.

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