Volcanic rocks, Pinnacles National Park, California, USA.
Warm afternoon light bathes Bear Gulch Reservoir, Pinnacles National Park, California, USA. 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 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 (that caused these formations) 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 inter-layered 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. Pinnacles National Park, California, USA.