Sierra Nevada

The Sierra Nevada preserves rocks that formed several kilometers below the earth's surface during the subduction of the Pacific Ocean below North America more than 100 million years ago. Erosion and deposition by streams 50 million years ago concentrated gold in the western foothills discovered in the 1849 California gold rush.

Overview

Much of what is admired in the Sierra Nevada—the face of the eastern slope, the deep canyons of Yosemite National Park, the peaks where the interaction of light and shadow inspired the name the “range of light,” and the undulating topography of the western foothills—is what makes the range geologically significant. The uplift and erosion that created the Sierran landscape over the last 20 million years exposed the roots of a giant volcanic chain active between 225 million and 80 million years ago. During that interval, the Sierra Nevada resembled the modern volcanic belts of the Andes in South America and the Cascade Range of the Pacific Northwest; however, only the bowels of the Sierran volcanoes remain. The origins of the ancient and modern volcanic chains are similar; all three formed during the important process of subduction, or descent, of an oceanic plate below a continent. Subduction is fundamental to plate tectonics, the basic theory of the earth sciences. Exposure in the Sierra Nevada of rocks originally formed several kilometers below the surface of the earth allowed scientists to investigate processes at depth in the continental crust above subduction zones and to assess the validity of plate tectonics theory in explaining events in the geologic past. Furthermore, the processes inferred for the Sierra Nevada probably occur beneath the Andes and the Cascade Range.

The description of the Sierra Nevada includes the morphology, or physical characteristics that define it, and the geology, or the rocks that compose it. The Sierra Nevada represents the longest continuous mountain range in the conterminous United States, extending 640 kilometers (398 miles) north-northwesterly along the eastern border of California. The mountains rise gently in the west from the agricultural belt of central California's Great Valley to the glaciated crest. From the crest, the range drops precipitously to the deserts of the Basin and Range Province in Nevada to the east. To the northwest, the Sierra merges with the Klamath Mountains of Northern California and Oregon. The rocks and faults of the Sierra Nevada can be identified in the Klamath Mountains, suggesting that the two ranges were once one. To the north, the Sierra Nevada terminates against the Cascade Range of northern California, Oregon, Washington, and British Columbia. The southern Sierra Nevada bends westward into the Transverse Ranges. The Sierra Nevada form a barrier to storms that enter California from the Pacific Ocean and create a rain shadow east of the range crest. Thus, the shallow western slope has pine forests and alpine lakes, whereas the steep eastern slope is barren and dry.

Different processes created the landscapes of the northern, central, and southern Sierra Nevada. In the north, much of the morphology was formed by volcanoes that spewed lava and ash. Glaciers dominated the central portion, carving great U-shaped valleys. The highest peaks, including the tallest mountain in the lower forty-eight states, Mount Whitney, are in the southern Sierra, where elevations approach 4,250 meters (13,944 feet). Here, glaciers and streams were not very active. What is remarkable about the entire Sierra Nevada, however, is its flat top. Looking from a distance toward the crest, the Sierra Nevada appears to be a tableland cut by numerous deep canyons. This flatness led scientists to conclude that the mountains represent the uplift of an area that was previously eroded to a uniform level. The Sierra Nevada consists of a series of long belts of metamorphic rocks that were intruded by a large batholith. The belts parallel the axis of the mountain range and grow younger from east to west. Their boundaries are faults across which rocks change age and composition. The metamorphic belts of the eastern Sierra Nevada contain sediments that accumulated on the continental shelf of North America between 500 million and 225 million years ago. In contrast, the sediments in the metamorphic belts of the western Sierra (which are the same age as those of the eastern Sierra Nevada) collected on the floor of the old ocean basin that bordered the ancient continental shelf. Slivers of the old sea floor, called ophiolites, occur with the sediments in the western metamorphic belts.

The batholith that intruded the mosaic of fault-bound belts appears to be a monolith of pink and gray speckled rock. Closer inspection, however, reveals that it consists of numerous small bodies of rock, each of which is a few kilometers across. The bodies are between 135 and 85 million years old and were derived from tens of kilometers depth. The pink and gray rock is granite, a chief constituent of continental crust, and is readily distinguished from the dark-colored metamorphic rocks. The large size of the mineral crystals in the granite suggests that the magma cooled slowly below the surface of the earth. Granite is estimated to make up more than 60 percent of the Sierra Nevada. Other rocks in the Sierra Nevada include glacial deposits, volcanic rocks, and stream gravels that are younger than the metamorphic belts and the batholith.

Formation

To understand the evolution of the Sierra Nevada, one must have a grasp of the theory of plate tectonics, which states that the surface of the earth is composed of rigid plates between 5 and 100 kilometers (3 and 62 miles) thick. The plates, either oceanic or continental, float on a partially molten layer that allows them to move relative to one another at velocities of a few centimeters per year. Oceanic plates are much younger than continental plates and, in fact, are created continuously along the ridges that line the world's ocean basins. Magma (molten rock) from deep in the earth erupts at the mid-oceanic ridge and cools, forming a new sea floor. To make room for the new material, the old sea floor is removed from the ocean basins at trenches. Trenches are on the opposite edges of oceanic plates from ridges and are the sites where the sea floor subducts, or descends, below another plate, either continental or oceanic. Ridges and subduction zones, therefore, define two types of plate boundaries: divergent (where plates move apart) and convergent (where plates come together), respectively.

The geology and morphology of the Sierra Nevada tell a story that spans half a billion years—of which subduction dominated the last 200 million years. For the first few hundred million years, however, the area of the Sierra Nevada was quiet. Sediments were deposited in shallow water on the continental shelf of Nevada to the east and in deep water on the Pacific Ocean floor to the west. (The edge of the North American continent during this time was several hundred kilometers farther east.) Approximately 225 million years ago, the scene changed. The Pacific Ocean began to subduct below the North American continent. Sediments on the sea floor scraped against and accreted to the overriding continent in long, linear belts. Pieces of seafloor occasionally accreted with the sediments. The addition of material to its edge forced the continent to grow progressively westward. As subduction continued, the oceanic plate reached depths in the earth below the continent where the temperature was high enough to cause melting of the interface between the overriding and descending plates. The melting produced small bodies of magma that ascended to the surface. Significant quantities of subducted sediments and continental crust also melted and mixed with the magmas to produce liquids of granitic composition. Most of the magma solidified at shallow depths between 135 million and 85 million years ago, leading to the large batholith now exposed in the Sierra Nevada. Some of the magmas, however, erupted on the surface as volcanoes. Emplacement of the magma near the surface raised the temperature sufficiently to recrystallize the sediments into metamorphic rocks.

About 80 million years ago, the magma cooled, the volcanoes stopped erupting, and the sea floor ceased accreting to the North American continent. For the next 50 million years, erosion dominated. Streams stripped the volcanoes from the surface, exposing the batholith and metamorphic rocks below. The removed material was deposited as soil and gravel in the rolling hills between the Sierra Nevada and the Pacific Ocean. Eventually, the landscape was eroded to a uniform level. Subduction below California stopped completely approximately 25 million years ago. The rise of the modern Sierra Nevada began 20 million years ago. The continental crust between the Sierra Nevada and Utah stretched and broke apart, producing normal faults oriented in an almost northerly direction. This stretched province of narrow mountains and wide valleys is the Basin and Range Province. The westernmost of the normal faults in the Basin and Range Province define the eastern edge of the Sierra Nevada. Rocks west of the Sierra fault moved up to create the modern mountains, while those to the east moved down to form a valley. The eruption of lava and ash accompanied the stretching of the continental crust as spaces formed to allow the passage of molten rock from deep in the earth. Lavas frequently flowed down the old stream channels. New streams circumvented the lavas, leaving them as high grounds in the landscape. Finally, glaciers of the Wisconsin glacial epoch (60,000 to 90,000 years ago) carved the spectacular valleys of the crest, including Yosemite Valley.

The combination of subduction followed by stretching built and is building the modern Sierra Nevada. The Sierra Nevada are still active; large earthquakes on the eastern edge of the range reveal that the mountains are rising, and the fault is moving. Magma lies within a few kilometers of the surface below Mammoth Lakes, ready to feed the volcanoes at Mono Craters, which last erupted 6,500 years ago. Streams continuously erode the high peaks trying to bring the land to sea level, thereby restoring the landscape of 0.5 billion years ago.

Study of the Sierra Nevada

A variety of techniques from all disciplines in the earth sciences are used to study the Sierra Nevada. Two, however, are particularly important: fieldwork and isotope geochronology. By doing fieldwork, investigators map the distribution of rock types, document structures such as faults, and analyze the relative ages of the rocks, primarily by painstakingly describing each rock type and walking throughout the range. In contrast, isotope geochronology allows scientists to ascertain the absolute ages of the different rock types from field samples taken back to the laboratory.

Geologic fieldwork is necessary to determine the evolution of ancient rocks. In the field, geologists systematically note the different types of rocks that exist and the contacts among them. Features in the rock that may be chosen for evaluation include the kinds of minerals, the sizes of the different types of particles, and the nature of the boundaries between grains. Contacts are evaluated to determine positions of rock types to establish their relative ages. Generally, rocks at the top of a sequence are assumed to be the youngest; those at the bottom, the oldest. The geologist also watches carefully for faults that may indicate that two adjacent sequences of rocks have had dissimilar histories. Such faults define the boundaries of the western metamorphic belts in the Sierra Nevada. Although the rocks on either side of these faults were deposited in deep water, they have different compositions and ages, leading scientists to conclude that the rocks now juxtaposed in the field were not deposited next to one another but were brought together during faulting. They originally may have been separated from one another and North America by great distances. The term “exotic terrane” is frequently applied to such rock sequences. Accretion during subduction explained this phenomenon. Sediments could arrive at North America from far-off sites, carried by the moving oceanic plate.

Isotope geochronology, or radiometric dating, allows scientists to determine the “absolute” age of a rock by taking advantage of the property of radioactivity. The dates derived from this method differ from those calculated by fossils and the superposition of strata. The latter are “relative” ages based on the geologic time scale. Prior to radiometric dating the number of years represented by divisions on the time scale was unknown. Radioactivity is the process by which the nuclei of certain chemical elements, such as uranium, radium, thorium, rubidium, and potassium, break apart and emits radiation. The process yields an atom of a different element. Uranium, for example, changes to lead. Isotopes are species of the same chemical element that differ only in the number of neutrons in their nucleus. Some isotopes are stable; some are unstable and break apart by radioactive decay. Each isotope has a characteristic rate of decay measured as a half-life (the time in years for half the nucleus to decay). Examples of half-lives include the 47 million years for rubidium to change to strontium and the 1.35 million years for potassium to become argon. Because half-lives can vary by several orders of magnitude, isotopes used for young rocks are different from those used for old rocks. By accurately determining in a mineral the amounts of the radioactive isotope and the atom into which it decays, geologists can calculate the age at which the mineral crystallized. The technique, therefore, is very useful for dating igneous and metamorphic rocks. Minerals crystallize in these rocks when the rocks form. In contrast, sedimentary rocks contain fragments of minerals that were eroded from older areas. Thus, radiometric dating of minerals in sediments yields only the ages of the region from which the minerals were derived.

The ages of and the relationships between the batholith and the metamorphic rocks in the Sierra Nevada were greatly elucidated by isotope geochronology. Intrusion of the batholith was discovered to have spanned tens of millions of years as individual bodies rose one by one. Despite the similar appearance of two granites, their age can differ by as much as 50 million years. Field observations suggested that heat from the batholith metamorphosed the accreted sediments; dating of the metamorphic belts corroborated the field evidence. In addition, isotopic studies established the age of the lavas that postdated magmatism.

Principal Terms

batholith: body of rock crystallized from magma, or molten rock; compositions are typically granitic

crust: the outermost layer of the earth; continental crust is 30 to 35 kilometers (19 to 22 miles) thick; oceanic crust is 5 to 10 kilometers (3 to 6 miles) thick; the greater density of oceanic crust relative to continental crust forces it to subduct

faulting: the process of fracturing the earth such that rocks on opposite sides of the fracture move relative to each other; faults are the structures produced during the process

granite: an igneous rock that is composed of mostly pink and white (quartz) minerals, which frequently has a speckled appearance

metamorphic rocks: rocks deposited initially as sediments and subsequently exposed to extensive heat, resulting in their recrystallization

normal fault: a steep fault that forms during extension, or stretching, of the earth's surface; the rock above the fault moves down relative to the rock below

quartz: one of the most common minerals on the earth's surface; it occurs in many different forms, including agate, jasper, and chert

vein: a mineral deposit that fills a crack; veins form by precipitation of minerals from fluids

Bibliography

Adams, Ansel. Yosemite and the Range of Light. Reprint. Little Brown & Company, 1992.

Alt, David D., and Donald W. Hyndman. Roadside Geology of Northern and Central California. Mountain Press, 2000.

"Cascade-Sierra Mountains Province." National Parks Service, www.nps.gov/articles/cascadesierra.htm. Accessed 15 Apr. 2023.

Ehlers, J., P. L. Gibbard, and P. D. Hughes, eds. Quaternary Glaciations—Extent and Chronology: A Closer Look. Elsevier, 2011.

Glazner, Allen F., and Greg Stock. Geology Underfoot in Yosemite National Park. Mountain Press Publishing Company, 2010.

Grotzinger, John, and Tom Jordan. Understanding Earth. 8th ed. W. H. Freeman, 2020.

Harden, Deborah R. California Geology. 2d ed. Prentice Hall, 2003.

Hatcher, Robert D., Jr. Structural Geology: Principles, Concepts, and Problems. 3rd ed. Prentice-Hall, 2020.

Hill, Mary. Geology of the Sierra Nevada. Rev. ed. U of California P, 2006.

Misachi, John. “Sierra Nevada Mountains.” World Atlas, 8 Oct. 2021, www.worldatlas.com/mountains/sierra-nevada-mountains.html. Accessed 31 July 2024.

Oakeshott, G. B. California's Changing Landscapes: A Guide to the Geology of the State. McGraw-Hill, 1978.

Seyfert, Carl K., ed. The Encyclopedia of Structural Geology and Plate Tectonics. Van Nostrand Reinhold, 1987.

Stanley, Stephen M. Earth System History. 4th ed. W. H. Freeman, 2015.

Webster, Paul. The Mighty Sierra: Portrait of a Mountain World. American West, 1972.

“Welcome to the Sierra.” Take Care Sierra, takecaresierra.org/welcome-to-the-sierra/. Accessed 31 July 2024.

Wise, James M. Mount Whitney to Yosemite: the Geology of the John Muir Trail. CreateSpace, 2008.