Historical and Geological Overview
Pinnacles was created as a National Monument in 1908 by Theodore Roosevelt in order to protect its unique geology for scientific and recreational uses. The Pinnacles themselves, however, were created much earlier than that...
23 million years ago, during the Miocene Epoch, the Farallon Plate was being subducted under California's Pacific coastline. As vast amounts of oceanic crust was forced beneath the North American Plate and down into the mantle, releasing a lot of magma up towards the surface. This caused the formation of numerous volcanoes along the west coast, including one slightly northeast of present-day Los Angeles: the Neenach volcano. This particular stratovolcano was formed after the magma burst up through the granite bedrock. Over thousands of years numerous eruptions formed layers. Sometimes violent eruptions ejected ash and breccia, whereas calmer eruptions allowed viscous lava to flow out, forming domes and dikes. Eventually the volcano reached its ultimate height of 8,000 feet and its base spread out for fifteen miles.
When the Farallon Plate had been completely subducted magma formation stopped, and the volcano became extinct. The prominent seismic activity in California became the slip-strike motion along the San Andreas Fault. This fault ran directly through the volcano, and over millions of years split it into two sections, bringing the portion now known as Pinnacles north 195 miles to its present location. The southern remains are still called the Neenach formation.
Today the Neenach formation consists of low, well rounded and well weathered rhyolite domes that look nothing like the spectacular Pinnacles. This is because Pinnacles was protected from erosion for millions of years by the same system of faults that brought it so far north. Activity along the Chalone Creek fault and the Pinnacles fault caused the volcano to subside and tilt westward, partially shielding it from the elements. Still, it endured enough weathering to have removed nearly a mile of rock from the top of the volcano; the modern formations are remains from the base of the original formation.
The erosion that did occur is responsible for shaping Pinnacles into the unique shapes and structures they form today. Working mainly along the joints, fractures, and layers created when the volcano was formed, rainwater slowly carved the pinnacles out of the volcanic rhyolite. Both mechanical and chemical weathering have contributed to this process: most of the spires and crags were wedged apart due to repetitive expansion of freezing water while many of the boulders in the lower formations were created by spheroidal chemical weathering.
Today the Pinnacles stand in sharp contrast to the surrounding Gabilan Mountains. These rolling hills are made from ancient granitic bedrock more than 100 million years old. There are also some even older metamorphic rocks in these mountains: Sur series limestone has been transformed into marble that is found near the granite.
Variations and Weathering of Rhyolite
All of Pinnacles' most stunning features result from the erosion of its volcanic deposits. The Pinnacles themselves are all made of rhyolite, the extrusive equivalent of granite which forms when felsic magma cools. Although chemically the rocks are all the same, the method of expulsion and cooling has caused it to take many different forms throughout the park.
The megalithic formations, including the High Peaks and Balconies, are all formed from igneous breccia through a process known as autobrecciation. Autobrecciation occurs when parts of a lava flow cool and harden into nearly solid rocks which are then reincorporated into the lava flow and mixed back into the liquid magma. When the entire mass cools this results in a formation where individual "rocks" are visible within the matrix, yet both these rocks and their matrix are of the same type and chemical composition. These flows are massively bedded in the higher elevations of the park, where distinct parallel lines can be seen even in distant boulders. The massive bedding is also responsible for the creation of large formations, such as the spires and crags which formed through the continued erosion of early joints in the rock.
Smaller formations in the lower elevations of the park are caused by one or more other forms of rhyolite. Flow-banded rhyolite is a lava that stretches as it cools, giving it the appearance of pulled taffy. Perlite is a yellow volcanic glass with a high water content, formed when lava cools quickly in water. Pumice Lapilli Tuff is a green rock made from sand-sized volcanic ash particles. Erosion of the chromium and magnesium minerals gives the rock its distinctive color. Finally, dacite is a highly felsic lava, light in color, and deposited in dikes at Pinnacles.
The talus caves at the park were formed when large boulders in the higher formations were finally separated from their massive beds, and rolled down the mountain. Some became lodged in the top of steep thin canyons, stuck between the walls. As more scree and talus accumulated in subsequent landslides they eventually blocked off the tops completely, forming long and deep caves. Today these caves are home to large colonies of bats, encompassing fourteen species in total.
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- Matthews, Vincent and Ralph C. Webb. Pinnacles: Geological Trail. Southwest Parks and Monuments Association, 1982.
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