|
Granite dominates the geology of Yosemite National Park. The park boundary perfectly frames a landscape that is composed almost entirely of granite, and it’s no surprise that this landscape contains some of the most iconic rock formations on earth. 
Granite is an intrusive igneous rock, which means it crystallized from molten rock, called magma, miles underground. At these depths, magma is insulated by the rocks around it and cools very slowly, growing large interlocking crystals. Granite is often said to have a "salt-and-pepper" appearance: the lighter-colored minerals are quartz, potassium feldspar, and plagioclase feldspar, and the darker-colored minerals are mostly biotite and hornblende. There are also minor amounts of various accessory minerals such as titanite and zircon.
Looking at a granitic rock's composition tells geologists much about its formation. Different granites have unique chemical compositions that produce different mineral proportions. In fact, granitic rocks are generally classified by their mineral proportions and given names such as granite, granodiorite, tonalite, and diorite. A clear example of contrasting compositions is visible on the southeast face of El Capitan where the dark-colored hornblende-rich Diorite of North America intruded the older and lighter-colored quartz and feldspar-rich El Capitan and Taft granites.
The thermal history of a granite—how fast it cooled and how long it spent at different temperatures—dictates the size of the mineral grains, and thus the rock texture. A prime example is the Cathedral Peak Granodiorite, which has unusually large (up to 3 inches long) crystals of potassium feldspar that are thought to have formed through episodic heating and cooling of the magma. This type of rock dominates the landscape in the Yosemite high country near Tuolumne Meadows. FormationGranite is most often formed where oceanic plates dive beneath continental plates in tectonic environments called subduction zones. In these zones, the input of water-rich oceanic crust causes partial melting at the base of the crust about 25 to 30 miles below the earth’s surface. The resulting magma is less dense than material typically found at that depth, so it rises like oil in salad dressing. This is the silica-rich magma that is often violently expelled from the large volcanos, like Mt. Saint Helens and Mt. Rainier, which sit above subduction zones today. 
PlutonsApproximately three to 10 miles below these volcanoes are the magma chambers that feed them, which build up over time as bodies called plutons. This rock, once it cools, will become the type of granite that is exposed in Yosemite. Plutons are large (often covering over 400 square miles) and are typically formed by many pulses of magma over a few million years. The granite that composes an individual pluton will have a relatively homogeneous texture and composition, displaying minor variations across its extent. There are roughly 35 individually defined plutons in the Yosemite area. Plutons are usually named after a famous landmark that is dominantly made of that rock (examples include the Half Dome Granodiorite and the El Capitan Granite). Intrusive Suites and BatholithsA group of plutons that are considered to be part of the same magmatic event and thus related are called intrusive suites. Intrusive suites typically are emplaced over about 10 million years and are often interpreted as being the roots of a single volcanic system. There are seven intrusive suites in Yosemite National Park. The Tuolumne Intrusive Suite is the youngest and most extensive, occupying about 1/3 of the park’s area. The Tuolumne Intrusive Suite contains many famous rock types including the Half Dome Granodiorite and Cathedral Peak Granite. 
A large amalgamation of plutons is called a batholith. Yosemite is situated within the 70-mile-wide, 300-mile-long Sierra Nevada Batholith. Most of the granite in the Sierra Nevada Batholith was emplaced between 120 and 85 million years ago during the Jurassic and Cretaceous periods. This is one of the most rapid periods of continental crust assembly known. In fact, it has been speculated that this voluminous volcanism may have contributed to the mass extinction during the middle of the Cretaceous period during which most large marine dinosaurs (such as ichthyosaurs and plesiosaurs) perished. Hot ResearchFew places on earth display the internal complexity of a batholith better than El Capitan. Even though Yosemite Valley has received considerable attention because of its geologic fame and dominant place in our cultural geography, the 3,000-foot-tall southeast face of El Capitan remained unmapped until recently. Between 2011 and 2013, a team of geologists completed the first geologic map of this sheer face. The map was made using a combination of techniques including remote sensing, climbing, and geochemistry. The map reveals many details about the vertical structures of plutons and the complex interactions between intrusive suites. 
Resources"Granitic rocks of the Yosemite Valley area, California" by Frank C. Calkins and Dallas L. Peck, in Geologic Guide to the Merced Canyon and Yosemite Valley, California, California Division of Mines and Geology Bulletin 182. “Plutonism in the Central Part of the Sierra Nevada Batholith, California” by Paul C. Bateman, 1992, U.S. Geological Survey Professional Paper 1483. [15 MB PDF] “The Geologic Story of Yosemite National Park” by N. King Huber, 1987, U.S. Geological Survey Bulletin 1595. [49 MB PDF] “Sierra Nevada Batholith Geologic Map Mosaic” “Yosemite’s Iconic El Capitan Mapped in Hi-Res 3-D” by Brian Clark Howard, 2013, National Geographic. |
Last updated: September 19, 2019