Lecture 18 - Yosemite National Park- A Landscape Sculpted by Alpine Glaciers

Outline

I. Sierra Nevada Geologic Province

II. Yosemite National Park

I. Sierra Nevada Geologic Province

Long mountain range that runs over half the length of California (~400 miles) consisting of primarily Mesozoic aged granitic rocks that have intruded older Paleozoic aged metasedimentary and metavolcanic rocks. The range was uplifted and tilted to the west along a range bounding normal fault that became active in late Cenozoic time and is still active today. This normal fault that runs along the eastern extent of the Sierra Nevada Mountains is responsible for the largest known earthquake to occur in California. The Lone Pine earthquake of 1872 is believed to be larger than the1906 San Francisco event and produced a 13-foot high fault scarp that’s easily visible along the eastern side of the Sierra. This normal faulting is due to the same tensional forces that uplifted the Colorado Plateau, the Teton Mountain Range, and the mountains of the Basin and Range province. Volcanic activity accompanied the normal faulting in some regions. The area just southeast of Yosemite is occupied by a very large caldera, which formed in a violent eruption, comparable in size to the largest of the Yellowstone caldera forming eruptions about 700,000 years ago. Magma is presently believed to be collecting below the surface of the Long Valley caldera as it prepares for another eruption. This evidence comes from upwarping of the caldera region and increased seismic activity.

The granites that make up the Sierra Nevada Mountains are the consolidated remnants of the magmatic system that was feeding volcanoes produced by subduction off the coast of California. Flattening of the angle of subduction is responsible for producing the Laramide orogeny that uplifted the Rocky Mountains farther to the east. While the Sierra Nevada was uplifted, the older rocks that were intruded were stripped off by erosion. Small fragments of these older rocks are seen in Yosemite Park as well as other parts of the Sierra Nevada.

Presently about 80 cirque glaciers exist in the Sierra Nevada. Although they are very small, abundant evidence of more extensive glaciation is observed everywhere in this mountain range. Yosemite Valley is one of the many large glacial gorges in the Sierra.

II. Yosemite National Park

The geology here consists of a complex of igneous intrusive rocks collectively called the Sierra Nevada Batholith. This batholith was intruded during Mesozoic time (200 to 80 MY ago) associated with subduction off the west coast of North America. The composition of the batholith is primarily granitic and has been divided into three different series based on an age progression from oldest to youngest:

1. Western Intrusive Series- major intrusions
2. Minor Intrusive Series- minor more mafic intrusions like the "Map of North America" intruded into El Capitan
3. Tuloumne Intrusive Series- major intrusions

Many features seen in Yosemite are the result of either glacial erosion or exfoliation or both. As the Sierra Nevada batholith was uplifted along the range bounding normal fault, the rocks on top of it were eroded. This removal of rock from the batholith greatly reduced its pressure and caused the massive granite to fracture in concentric rings. Erosion acting on this fracture pattern caused the rock to peel off layers like an onion skin producing many exfoliation domes like Half Dome, Sentinel Dome and North Dome. The final shape of these domes has also been influenced by vertical fractures that developed in response to forces and glacial erosion. Exfoliation arches like seen in Zion National Park are also widespread in Yosemite.

Sequoia and Kings Canyon National Parks to the south of Yosemite are also located in the Sierra Nevada province and contain very similar geology to Yosemite. Some differences and highlights from these parks include: Mt. Whitney (~14,500 foot peak, highest mountain in the lower 48 states), Sequoia trees, and abundant caves that developed due to dissolution of marble present in Sequoia National Park. The marble developed as limestone intruded by Sierran granites was metamorphosed due to the elevated temperatures associated with the intrusion (contact metamorphism).

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