10.4: Physical Geology

The Sierra Nevada is a huge block of the earth’s crust, composed of plutonic and metamorphic rocks of Paleozoic and Mesozoic age, that has been sharply uplifted and exposed on the east along the Sierra Nevada fault system and has been tilted westward. It is overlapped on the west by Upper Cretaceous and Cenozoic sedimentary rocks of the Great Valley and on the north by Cenozoic volcanic sheets extending south from the Cascade Range. A blanket of volcanic material caps large areas in the northern part of the range. Most of the southern half of the Sierra Nevada and the eastern part of the northern half are composed of plutonic, primarily granitic, rocks of Mesozoic age. These rocks constitute the Sierra Nevada batholith, which is part of a continuous belt of plutonic rocks that extends from Baja, California, northward through the Peninsular Ranges and the Mojave Desert through the Sierra Nevada at an acute angle to the range, and into western Nevada. It may continue at depth beneath the volcanic rocks of the Snake River Plains and connect with the Idaho batholith.

Figure 10.4: Geologic Map of The Sierra Nevada & Surrounding Areas. Image by California Geological Survey is licensed under USGS.

The Sierra Nevada batholith was emplaced into strongly deformed but weakly metamorphosed sedimentary and volcanic rocks of Paleozoic and early Mesozoic age, which can be referred to as the “framework” rocks. In the northern half of the range, the batholith is flanked on the west by the western metamorphic belt, which is the site of the famed Mother Lode. Farther south, scattered rem­nants of metamorphic rock are found within the batholith, especially in the western foot­ hills and along the crest in the east-central Sierra Nevada. The batholith extends eastward to the east edge of the range, but in the southern half one can look eastward across Owens Valley to the wall rocks on the east side of the batholith which here constitute the White and Inyo Mountains. The region in which the U.S. Geological Survey has concen­trated its most recent studies is well situated for comparing and relating the rocks and structures on the two sides of the batholith, because the southern end of the western meta­morphic belt and the northern end of the area of good exposures on the east side of the bath­olith overlap here.

Sierran Arc

In early Triassic time, an extensive volcanic arc system called the Sierran Arc began to develop along the western margin of the North American continent. In Southern California, this volcanic arc would develop throughout the Mesozoic Era to become the geologic regions known as the Sierra Nevada Batholith, the Peninsular Ranges Batholith, (in the Peninsular Ranges), and other plutonic and volcanic centers throughout the greater Mojave Desert region.

Figure 10.5: Diagram of the Geologic Process of Subduction. Subduction-en.svg from Wikimedia Commons by K. D. Schroeder is licensed under CC BY SA 4.0

The first phase of regional plutonism started 210 million years ago in the late Triassic and continued throughout the Jurassic to about 150 million years BP. Also starting 150 million years ago was an increase in the westward drift rate of the North American Plate. The resulting orogeny (mountain-building event) is called the Nevadan orogeny by geologists. The resulting Nevadan mountain range (also called the Ancestral Sierra Nevada) was 15,000 feet (4500 m) high and was made of sections of seafloor and mélange. These massive belts of plutonic (intrusive) and volcanic (extrusive) regional belts and isolated centers developed as plate convergence and subduction took place farther west along the western continental margin.

These rocks were later metamorphosed and today can be seen in the gold-bearing metamorphic belt of California's Mother Lode country. In the central Sierra region, these rocks are exposed along the Merced River and State Route 140. This was directly part of the creation of the Sierra Nevada Batholith, and the resulting rocks were mostly granitic in composition and emplaced about 6 miles (10 km) below the surface.

The second, major pluton emplacement phase lasted from about 120 million to 80 million years ago during the Cretaceous. This was part of the Sevier orogeny. All told there have been more than 50 plutons found in the central Sierra region. A few miles (several km) of material was eroded away, leaving the Nevadan mountains as a long series of hills a few hundred feet (tens of meters) high by 25 million years ago.

Volcanism

Starting 20 million years ago and lasting until 5 million years ago a now-extinct extension of Cascade Range volcanoes erupted, bringing large amounts of igneous material in the area. These igneous deposits blanketed the region north of the Yosemite area. Some lava associated with this activity poured into the Grand Canyon of the Tuolumne and formed Little Devils Postpile, a smaller but much older version of the columnar basalt palisades in nearby Devils Postpile National Monument in Mammoth.

In the late Cenozoic, extensive volcanism occurred east of the park area. Within the Yosemite region, andesitic lava flows and lahars flowed north of the Grand Canyon of the Tuolumne and volcanic dikes and plugs developed from faults on the flanks of Mount Dana. There is also evidence for a great deal of rhyolitic ash covering the northern part of the Yosemite region 30 million years ago. This and later ash deposits have been almost completely eroded away (especially during the ice ages).

Volcanic activity persisted past 5 million years before present, east of the current park borders in the Mono Lake and Long Valley areas. The most significant activity was the creation of the Long Valley Caldera about 700,000 years ago in which about 600 times as much material was erupted than in the 1980 eruption of Mt. Saint Helens. The most recent activity was the eruption of the Mono-Inyo Craters from 40,000 to 600 years ago.

Uplift & Erosion

10 million years ago, vertical movement along the Sierra fault started to uplift the Sierra Nevada. Subsequent tilting of the Sierra block and the resulting accelerated uplift of the Sierra Nevada increased the gradient of western-flowing streams. The streams consequently ran faster and thus cut their valleys more quickly. Tributary streams ran more-or-less in line with the Sierras, therefore not having their gradients increased. Thus, their rate of valley cutting was not significantly affected. The results were hanging valleys and cascading waterfalls where the tributaries met the main streams. Additional uplift occurred when major faults developed to the east, especially the creation of Owens Valley from Basin and Range-associated extensional forces. Uplift of the Sierra accelerated again about two million years ago during the Pleistocene. Evidence of glaciation is visible in the pattern of the tracks that lateral moraines deposited during different glacial periods.

The uplifting and increased erosion exposed granitic rocks in the area to surface pressures, resulting in exfoliation (responsible for the rounded shape of the many granite domes in the park) and mass wasting following the numerous fracture joint planes (cracks; especially vertical ones) in the now solidified plutons. Pleistocene glaciers further accelerated this process, and the larger ones transported the resulting talus and till from valley floors.

Numerous vertical joint planes controlled where and how fast erosion took place. Most of these long, linear and very deep cracks trend northeast or northwest and form parallel, often regularly spaced sets. They were created by uplift-associated pressure release and by the unloading of overlying rock via erosion. The great majority of Yosemite Valley's widening, for example, was due to joint-controlled rockfall. In fact, only 10% of its widening and 12% of its excavation are thought to be the result of glaciation. Large, relatively unjointed volumes of granite form domes such as Half Dome and monoliths like the 3,604 ft (1,098 m) high El Capitan. Closely spaced joints led to the creation of columns, pillars, and pinnacles such as Washington Column, Cathedral Spires, and Split Pinnacle.