Geology of Yosemite National Park
Yosemite National Park lies within the Sierra Nevada Mountains, an asymmetric mountain range in central California near the eastern border of the state. The mountains, valleys, granite cliffs, waterfalls, and glaciers of Yosemite have inspired people for generations, including photographer Ansel Adams, naturalist John Muir, and countless hikers and rock climbers.
Geologic History
Yosemite National Park lies within the Sierra Nevada Mountains, an asymmetric mountain range in central California near the eastern border of the state. The mountains, valleys, granite cliffs, waterfalls, and glaciers of Yosemite have inspired people for generations, including photographer Ansel Adams, naturalist John Muir, and countless hikers and rock climbers. The landscape of Yosemite National Park has changed dramatically in the past 450 million years. Uplift, erosion, and glaciation have shaped the unique scenery of Yosemite National Park.
The oldest rocks in Yosemite formed from sediments and submarine volcanic material. The sediments originated from continental sources and were deposited in shallow water near the continent creating limestones, sandstones, and shales. A convergent plate boundary (where two tectonic plates collide) later transported these sediments and igneous rocks eastward. Many of these rocks subsequently have been heavily metamorphosed, uplifted and eroded away by various geologic events in Yosemite’s history. However, some outcrops of metamorphic rocks (Shoo Fly Complex and the Calaveras Complex) that did not erode away can still be found today on the western side of the park.
Yosemite is known for its granitic rock formations, a type of intrusive igneous rock that forms as molten rock slowly cools deep underground. The ancestral North American continent was moving northwest toward its present location about 220 million years ago. Subduction, a process where tectonic plates collide and the denser plate is forced downward beneath the less dense plate, initiated about 215 million years ago. In this case the Farallon Plate (oceanic crust) descended below the North American Plate (continental crust), causing chemical and physical changes in both tectonic plates and partial melting of the Farallon Plate. This molten rock (magma) ascended through the crust in pulses over 100 million years. Much of the molten rock did not reach the surface and cooled slowly underground, creating large bodies of granite (plutons) and other pulses of magma reached the surface, forming a chain of volcanoes. Later these volcanoes were intruded with more magma and these granitic plutons were metamorphosed. This process of multiple pulses of magma is responsible for the many different types of granite seen in Yosemite today with most of the granitic rock forming between 105-85 million years ago. Once subduction ended, the volcanoes and metamorphic rocks were eroded away between 85-15 million years ago, revealing the granitic rock beneath.
Around 30 million years ago, the subducted oceanic crust (Farallon Plate) was completely engulfed by the westward moving North American Plate, which was then over the spreading center at the boundary of the Farallon and Pacific Plates.
Volcanoes began to erupt again from 20 to five million years ago, particularly north of Yosemite. The volcanoes that formed were large and buried ancient river canyons, and lava from this period infiltrated the Grand Canyon of Tuolumne and formed Little Devils Postpile, a miniature version of Devils Postpile, which is located in the southern part of the Sierra Nevada Mountains. Little Devils Postpile, located several miles west of Tuolumne Meadows, is a composed of columnar basalt, a dark, fine-grained rock with preserved cooling structures that indicates rapid cooling and contraction of thick lava.
Uplift began in the Sierra Nevada again about 10 million years ago and at the same time experienced faulting and westward tilting. Major faults began to develop on the eastern side of the park due to extensional forces associated with the Basin and Range Province, which spans ~500 miles from the eastern edge of the Sierra Nevada to the Rio Grande Rift and is a result of extension and thinning of the crust and upper mantle. This uplift caused a period of increased erosion, revealing more granitic and metamorphic rocks, which can be seen in the Tioga Pass and Saddlebag Lake area and in Virginia Canyon. Yosemite National Park is notable because it contains classic examples of domes, such as Half Dome. These domes began to form during this period of uplift when the overlying rock eroded and the confining pressure on the pluton (solidified magma chamber) was removed, and exfoliation created rounded domes. Exfoliation occurs during weathering as when sheets of rocks millimeters to meters in thickness are peeled away during weathering.
Starting around two to three million years ago glaciers in the area of Yosemite began to form. Since 2.6 million years ago there have been more than 40 cycles of glacial (cold) and interglacial (warm) periods. These glacial periods modified the landscape forming Yosemite Valley, other canyons, lakes, and many of the other features seen in Yosemite today. Matterhorn Peak is a result of glaciation and was formed when glaciers surrounded and scoured its lower slopes. Other examples of landscapes created by glaciation are Lyell Canyon and Yosemite Valley, two large U-shaped canyons that formed as glaciers passed through, removing rock and debris as they moved.
Rock types in Yosemite and how to identify them:
Most of the rocks in Yosemite are granitic, a type of plutonic igneous rock formed when molten magma cools underground slowly. Plutonic igneous rocks contain larger crystals than volcanic igneous rocks because the molten material (magma) cools much more slowly under Earth’s surface than at or near the surface and are characterized by the relative ratios of light-colored (felsic, or rich in feldspar and silica) and dark-colored (mafic, or rich in iron and magnesium) minerals. One common example is granite, known for its “salt and pepper” appearance of felsic and mafic minerals. The felsic minerals in granite are quartz, potassium feldspar, plagioclase feldspar. While the mafic minerals are biotite and hornblende.
Small amounts of volcanic igneous rocks are also found within the park. Volcanic (extrusive) igneous rocks cool and solidify much more quickly than plutonic rocks, which results in much smaller crystals and a finer-grained texture. Examples of volcanic rocks in Yosemite are basalt, latite, and, latite tuff. Finally, there are metamorphic rocks within Yosemite National Park. These are rocks that have been changed due to heat, pressure, and/or chemically-active fluids. The two areas of metamorphic rocks within the park trend northwest on either side of the core of plutonic rocks.
USGS in Yosemite National Park
An important area of USGS research in Yosemite National Park is understanding and monitoring rockfall and debris-flow hazards to protect life and property. The National Park Service has removed buildings from hazardous areas as a result. This research on rockfall hazards uses lidar and radar technology and motion sensors that track rock movements.
