The Geology of Pinnacles National Park
Faults and Geological Boundaries
The faults within Pinnacles National Park define distinct geological boundaries. The Pinnacles Fault separates the granitic basement rock to the west from the Pinnacles Volcanic Formation to the east. Meanwhile, the Chalone Creek Fault divides the Pinnacles Volcanic Formation to the west and the Temblor Fanglomerates to the east.
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Granitic Basement: The Santa Lucia Granite and Granodiorite are the oldest rocks in the park, dating back 78-100 million years. These granites formed as molten lava slowly cooled beneath the earth's crust, allowing individual crystals to grow. Subsequent fault uplift and erosion exposed these ancient rocks at the surface, creating the foundation for all other rock formations in the park.
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Pinnacles Volcanic Formation: Approximately 23 million years ago, volcanic activity extruded this formation through numerous vents, depositing it atop the granitic basement. The formation includes rocks like banded and massive rhyolite, andesite, dacite, and various pyroclastic units, all sourced from rhyolitic magma.
High Peaks and Breccia Formation
The High Peaks area consists of well-consolidated breccia, formed by large landslides of volcanic material near the vents. These landslides likely traveled underwater as turbidity currents, spreading material over vast distances. Interlayered with volcanic ash and rhyolitic lava flows, these breccia layers hardened into the solid rock seen today. Subsequent faulting, fracturing, and erosion have shaped these layers into the towering cliffs and spires that define Pinnacles’ landscape.
Temblor Formation
East of the Chalone Creek Fault lies the Temblor Formation, a fanglomerate unit composed of granitic and volcanic debris from the Santa Lucia granitic basement and the Pinnacles Volcanics. This formation reflects the ancient alluvial fan environment in which it was deposited.
Rocks of Pinnacles: Types and Characteristics
The igneous rocks at Pinnacles display a wide variety of textures and colors, all belonging to the rhyolitic group (the extrusive equivalent of granitic rocks). Dominated by quartz and feldspar, these variations arise from differences in mineral content, extrusion types, cooling rates, and weathering exposure.
Common Rock Types at Pinnacles
- Breccia: Reddish to gray, formed when molten rock explosively ejects fragments that weld into a lava or tuff matrix. This rock dominates the High Peaks and other formations.
- Flow-banded Rhyolite: Fine-grained, viscous lava that develops stretch marks, similar to taffy, as it cools.
- Pumice Lapilli Tuff: A welded volcanic ash composed of sand-sized particles. Weathering of magnesium and chromium-bearing minerals likely produces its green hue. The Bear Gulch Nature Center is constructed from this rock.
- Perlite: An opaque volcanic glass that cooled rapidly in water.
- Dacite: A light-colored lava rich in quartz and feldspar, associated with dikes at Pinnacles.
- Andesite: Similar to dacite but with a higher content of dark minerals.
Explore More
For further exploration of Pinnacles’ geology, consult the following resources:
- Johnson, Elvin R. and Cordone, R.P. (1994). Pinnacles Guide. Camano Island: Tillicum Press.
- Keith, Sandra L. (1991). Pinnacles National Monument. Tucson: Southwest Parks and Monuments Association.
- Matthews, Vincent and Webb, Ralph C. (1982). Pinnacles Geological Trail Guide. Tucson: Southwest Parks and Monuments Association.
- Chronic, Halka. (1986). Pages of Stone: Geology of the Western National Parks and Monuments. Seattle: Mountaineers Press.
