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.

Rocks of Pinnacles
The igneous rocks of Pinnacles vary widely in their texture and color. However, they are all members of the rhyolitic group, the extrusive equivalent of the intrusively formed granitic group. As such they are dominated by quartz and feldspar. The variations are caused by differences in minor mineral content, type of extrusion, rapidity of cooling and exposure to weathering.

The following is an annotated list of common rock types at Pinnacles:

Breccia: Reddish to grey in color, molten rock is explosively ejected with many fragments welded into a lava or tuff matrix. Breccia dominates the High Peaks and other megalithic formations.

Flow-banded Rhyolite:   A viscous fine-grained lava, which develops stretch marks, analogous to pulling taffy as it cools.

Pumice Lapilli Tuff:   A welded volcanic ash, made up of sand sized particles. One theory suggests that weathering of magnesium and chromium bearing minerals produces this green color. The Bear Gulch Nature Center is constructed of this rock.

Perlite: An opaque form of volcanic glass which cooled rapidly in water.

Dacite: A light colored lava containing high proportions of quartz and feldspar. At Pinnacles it is associated with dikes.

Andesite:  Similar to dacite, this rock has more dark colored minerals.

Sources for Further Reading
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.