9.3: The Sierra Nevada Batholith
- Page ID
- 21509
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)How Plutonic Rock Forms
Plutonic rocks, or intrusive igneous rocks, form from the cooling and solidification of magma beneath the Earth's surface. This process differs significantly from the formation of volcanic or extrusive rocks, which cool rapidly at or near the surface. The formation of plutonic rocks involves the slow crystallization of magma, allowing for the development of large, visible crystals. This slow cooling occurs because the magma is insulated by the overlying rock, resulting in coarse-grained textures typical of plutonic rocks. Plutonic rocks typically form large bodies known as plutons, which can range in size from a few cubic meters to hundreds of cubic kilometers.
The origin of the magma that forms plutonic rocks can be traced to several geological processes, including partial melting of the mantle or crust due to subduction, rifting, or mantle plumes. In subduction zones, for example, oceanic crust is pushed beneath continental crust, leading to melting and magma formation. This magma can then rise through the crust, cooling slowly to form plutonic rocks. This process can take thousands to millions of years, during which the magma can interact with surrounding rock, further influencing its composition and texture.
One of the critical aspects of plutonic rock formation is the nucleation process. Nucleation is the initial stage of crystal formation where small clusters of atoms or molecules begin to arrange into a crystal lattice. In a cooling magma body, nucleation occurs when the temperature drops below the temperature at which crystals begin to form from the melt. The rate of nucleation depends on several factors, including the degree of undercooling, the presence of nucleation sites, and the composition of the magma.
As the temperature continues to decrease, these small clusters grow into larger crystals through a process known as crystal growth. The balance between nucleation and crystal growth determines the final texture of the plutonic rock. In slowly cooling magmas, crystal growth dominates, leading to the formation of large, well-developed crystals. Conversely, in rapidly cooling magmas, nucleation may dominate, resulting in many small crystals. This balance is critical in developing the coarse-grained texture characteristic of plutonic rocks. (For more information please see Minerals and Rocks).
The formation of plutonic rocks is a fundamental geological process that contributes to the growth of continental crust and the creation of various mineral deposits. The slow cooling and crystallization processes allow for the development of diverse mineral assemblages, making plutonic rocks important sources of valuable minerals and providing essential insights into the Earth's geological history.
Sierran Granodiorite
The Sierra Nevada Batholith is predominantly composed of granodiorite, a coarse-grained plutonic rock. Granodiorite is a significant and widespread component of the Sierra Nevada range, playing a crucial role in its geological and geomorphological character. The rock is characterized by its intermediate composition between granite and diorite, containing a higher proportion of plagioclase feldspar relative to potassium feldspar, with abundant quartz and biotite. This mineralogical composition gives granodiorite its distinctive appearance, with a speckled texture of light and dark minerals. For comparison of visual differences between granite and diorite, please see the 3D models below.
Granite 3D model by Paleontological Research Institute is licensed under public domain.
Diorite 3D model by Paleontological Research Institute is licensed under public domain.
In terms of its extent, Sierran granodiorite is pervasive throughout the Sierra Nevada, forming the core of many of its highest peaks and most recognizable landscapes. Notable exposures of granodiorite can be seen in Yosemite National Park, where it forms the iconic El Capitan and Half Dome. The widespread distribution of granodiorite across the range is a testament to the extensive magmatic activity that occurred during the formation of the Sierra Nevada Batholith.
Variations in the mineralogy and texture of granodiorite can be observed across different regions of the Sierra Nevada. These variations are a result of differences in the conditions of crystallization and subsequent geological processes that have affected the rock. For example, granodiorite in the northern Sierra Nevada may display different textural and compositional characteristics compared to that in the southern Sierra Nevada. Understanding these variations provides important insights into the geological history and processes that have shaped the Sierra Nevada range, including magma mixing, assimulation, and cooling rates.
How the Sierra Nevada Batholith Formed
The Sierra Nevada Batholith is a massive geological structure that formed over a period of approximately 100 million years, from the Late Triassic to the Late Cretaceous period. Its formation is closely linked to the tectonic processes associated with the subduction of the Farallon Plate beneath the North American Plate. This subduction zone, located along the western margin of North America, was a site of intense magmatic activity, leading to the generation of large volumes of magma.
The emplacement of the Sierra Nevada Batholith occurred in multiple stages, involving the intrusion of numerous plutons into the Earth's crust. The formation of volcanic arcs, or chains of volcanoes formed at convergent plate boundaries, is one piece of surface evidence of larage plutons (see Plate Tectonics). The subduction of oceanic crust beneath continental crust generates intense heat and pressure, leading to the melting of the descending crust and the overlying mantle. Magma generated by this process is buoyant and rises towards the surface. As it ascends, it may intrude into the overlying crust, where it cools and solidifies to form plutonic rocks. In the case of the Sierra Nevada, the region was once situated along the western margin of North America, adjacent to a subduction zone where the Farallon Plate was being subducted beneath the North American Plate (see A Brief Geologic History of California). Such tectonic actions are illustrated in Figure 9.3.3.
These plutons, which are now exposed at the surface, represent successive pulses of magmatic activity. Each pulse contributed to the growth of the batholith, with older plutons being intruded by younger ones. This complex history of emplacement is recorded in the geological relationships between different plutonic bodies and the surrounding rock.
The tectonic environment during the formation of the Sierra Nevada Batholith was dynamic and complex. The subduction of the Farallon Plate caused significant deformation of the overlying continental crust, leading to the development of mountain ranges and deep crustal roots. The heat generated by the subduction process facilitated the partial melting of the mantle and lower crust, producing the magmas that formed the batholith. Additionally, the interaction between the rising magmas and the surrounding rock further influenced the composition and texture of the plutonic rocks.
The history of the Sierra Nevada Batholith is also marked by episodes of regional metamorphism and deformation, which have modified the original characteristics of the plutonic rocks. These processes have resulted in the formation of metamorphic aureoles around the plutons and the development of various structural features, such as folds and faults. Understanding the formation and evolution of the Sierra Nevada Batholith provides important insights into the geological processes that shape continental crust and the development of large magmatic systems.
Tuolemne Intrusive Suite
The Tuolumne Intrusive Suite is one of the most well-studied and significant components of the Sierra Nevada Batholith. Located in the central Sierra Nevada, this intrusive suite is renowned for its geological complexity and the insights it provides into the processes of pluton emplacement and magmatic differentiation. The Tuolumne Intrusive Suite consists of multiple intrusive phases, each representing a distinct episode of magmatic activity.
The formation of the Tuolumne Intrusive Suite began with the intrusion of early, more mafic magmas, which formed the older phases of the suite. These early intrusions were followed by successive pulses of more felsic magmas, leading to the emplacement of younger phases. This sequence of intrusive events is recorded in the varying compositions and textures of the rocks within the suite. The presence of chilled margins and cross-cutting relationships between different intrusive phases provides evidence for multiple intrusive events.
One of the key features of the Tuolumne Intrusive Suite is the presence of compositional zoning, where the outermost rocks are more mafic and the innermost rocks are more felsic. This zoning is interpreted as a result of magmatic differentiation, where the composition of the magma evolves as it cools and crystallizes. The outer, more mafic phases crystallized first, followed by the intrusion of more felsic magmas into the core of the suite. This process of magmatic differentiation is crucial for understanding the evolution of large intrusive bodies and the generation of diverse rock types within a single intrusive complex.
The Tuolumne Intrusive Suite also provides valuable insights into the dynamics of magma chamber processes and the interaction between different magmas. Field studies and geochemical analyses have revealed the presence of hybrid rocks, which form from the mixing of different magmas. These hybrid rocks provide evidence for the physical and chemical interactions that occur within magma chambers, contributing to the complexity and diversity of plutonic rocks.
Jointing and Sheeting of Igneous Rocks
Jointing is a common feature of igneous rocks, including those of the Sierra Nevada Batholith. Joints are fractures in the rock along which there has been no significant movement. They form as a result of various geological processes, such as cooling and contraction of the rock, tectonic stresses, and unloading due to erosion of overlying material. In the Sierra Nevada, jointing is a prominent feature of many of the granitic rocks, influencing their weathering and erosion patterns.
One of the most distinctive types of jointing in the Sierra Nevada is exfoliation jointing, also known as sheeting. Exfoliation jointing occurs when rock breaks into thin, curved sheets that peel away from the surface, similar to the layers of an onion. This type of jointing is particularly common in granitic rocks and is believed to result from the release of pressure as overlying rock is removed by erosion. The reduction in pressure allows the rock to expand and fracture along curved surfaces parallel to the rock's surface as shown in Figure \(\PageIndex{4}\).
.jpg?revision=1&size=bestfit&width=577&height=433)
Exfoliation jointing is prominently displayed in several locations within the Sierra Nevada, including Yosemite National Park. Iconic features such as Half Dome and El Capitan exhibit well-developed exfoliation joints, contributing to their distinctive shapes and steep, smooth surfaces. Video 9.3.1 is a virtual flight over Half Dome using enhanced Google Earth imagery. This video shows the stunning exfoliation jointing, or sheeting, that shapes the iconic granite face of Half Dome. The development of exfoliation joints is influenced by several factors, including the mineral composition of the rock, the presence of pre-existing fractures, and the rate of erosion.
Video 9.3.1: "Half Dome Flyover" by Cole Heap is licensed under CC BY-NC 4.0 (please note that the video has no audio). Access a detailed description.
Understanding the processes that lead to jointing and sheeting in igneous rocks is important for interpreting the geological history of the Sierra Nevada and for practical considerations such as rock stability and erosion. Jointing can influence the behavior of rock masses in response to weathering and tectonic forces, affecting landscape development and the potential for rockfalls and landslides.
References
- Barton, M. D., & Hanson, R. B. (1989). Magmatism and the development of low-pressure metamorphic belts: Implications from the western United States and thermal modeling. Geological Society of America Bulletin, 101(8), 1051-1065.
- Bateman, P. C. (1992). Plutonism in the central part of the Sierra Nevada Batholith, California. U.S. Geological Survey Professional Paper 1483.
- Ducea, M. N. (2001). The California arc: Thick granitic batholiths, eclogite residues, lithospheric-scale thrusting, and magmatic flare-ups. GSA Today, 11(11), 4-10.
- Glazner, A. F., Bartley, J. M., Coleman, D. S., Gray, W., & Taylor, R. Z. (2004). Are plutons assembled over millions of years by amalgamation from small magma chambers? Geological Society of America Today, 14(4/5), 4-11.
- Hanson, R. B., & Glazner, A. F. (1995). Thermal requirements for extensional emplacement of granitoids. Geology, 23(3), 213-216.
- Nadin, E. S., & Saleeby, J. B. (2010). Origin of high-Mg andesites and the continental crustal growth rate: Constraints from the Sierra Nevada Batholith. Geological Society of America Bulletin, 122(3/4), 323-335.
- Saleeby, J. B. (2003). Segmentation of the Laramide slab: Evidence from the southern Sierra Nevada region. Geological Society of America Bulletin, 115(6), 655-668.