back to the Roadside Geology of Yosemite Valley

Half Dome granodiorite near Vernal Fall

The Sierra Nevada Batholith as exposed in the Yosemite Valley area and western foothills of the Sierran Nevada

By Marty Giaramita, California State University, Stanislaus

The Basics:

The terrane of Yosemite National Park is characterized by abundant light-colored and less common dark-colored rocks that crystallized slowly from molten rock kilometers beneath the Earth’s surface. The present exposure of these relatively coarse-grained rocks is the result of uplift and removal of the overlying rock by erosive agents such as rivers and glaciers long after their intrusion into the crust due to tectonic-plate conversion during Mesozoic time.

Igneous rocks are those that crystallized from melt. They are broadly subdivided into volcanic (extrusive) rocks if they were erupted on the surface and, hence, rapidly cooled, and plutonic (intrusive) rocks if they crystallized slowly at depth beneath the Earth’s surface and cooled slowly. Although volcanic rocks are present in the Sierra Nevada Mountains (and we will drive through some en route to Yosemite), the predominant rock type in the region of Yosemite National Park is plutonic. Exposed bodies of plutonic rock are referred to as plutons that are further subdivided on the basis of their shape. A stock is a roughly equidimensional pluton; a dike is a fracture filled with plutonic rock. A batholith is a large composite of many individual plutons.

The intrusive rocks of the Yosemite Region are part of the Sierra Nevada Batholith consisting of hundreds of individual plutons intruded from the Triassic through the Cretaceous periods (See the geologic map of Yosemite Valley). Most geologists now agree that the batholith owes its existence to the process of tectonic-plate convergence during which melting occurred as a consequence of subduction of oceanic lithosphere (the rigid crust and upper part of the mantle) beneath the margin of the North American continent during Mesozoic time. A present-day analogue for this process is the subduction of the Nazca oceanic plate beneath the western margin of the South American continent.

According to one plausible model (Hess, 1989; Wilson, 1989) as hydrated oceanic lithosphere subducts beneath continental lithosphere, the subducting slab is heated and metamorphosed, liberating water which migrates upward into the overlying mantle. The added H₂O reduces the melting temperature of the overlying mantle and produces mafic melts (45 - 54 wt.% SiO₂). These melts rise up into the continental crust and by various processes including fractionation (removal of early-formed SiO₂-poor crystals) and assimilation (addition of SiO₂ rich country rock or melt) commonly evolve into melts having higher SiO₂ contents. The mafic melts can also add enough heat to the continental crust they are intruding to induce partial melting and production of SiO₂-richer melts. The melts can be extruded on the surface during volcanic eruptions or cool slowly beneath the surface. The Sierra Nevada batholith is an amalgam of many plutons of varied compositions that intruded the continental crust during subduction. The once-present volcanic rocks at the surface have, for the most part, been eroded away during uplift and exposure of the pluton.

Plutonic igneous rocks are classified on the basis of their mineral content. The classification scheme proposed by the International Union of Geological Sciences (IUGS) (Streckeisen, 1973) is based on the relative proportions of the common minerals quartz, plagioclase, and K-feldspar. The volumetric proportions of these three minerals in a rock are estimated and normalized to 100% (in essence they are treated as if they are the only minerals in the rock). The composition of the rock is plotted on a triangle having the three minerals at the apices where the apex represents 100% and the opposite base represents 0% of the mineral (see the figure below). The triangle is divided into fields; if a rock plots in a particular field it is given the appropriate name. Silica-rich rocks generally contain more potassium and have higher proportions of K-feldspar. As silica content decreases, the ratio of plagioclase to K-spar increases, and the quartz diminishes in abundance.

In order of decreasing SiO₂ content, the rock types present in the vicinity of Yosemite Valley include granite, granodiorite, tonalite (the term trondjhemite is used for a tonalite in which the dark colored minerals constitute less than 10% of the rock), diorite, and gabbro. In general, the more silica-rich rocks are lighter in color due to the abundance of the lighter-colored minerals quartz, K-feldspar, and plagioclase feldspar. Darker Fe-Mg-rich minerals such as biotite and possibly hornblende will be present but not abundant. With decreasing SiO₂ content the rocks generally become darker owing to the increased abundance of Fe-Mg-rich minerals such as biotite and hornblende and a decrease in the abundance of the lighter-colored minerals. Moreover, the ratio of hornblende to biotite (which contains potassium) increases. As silica content further decreases, other Fe-Mg-rich minerals such as pyroxene and olivine become important as in gabbro.

The batholith as exposed in Yosemite Valley and regions west contains mostly Cretaceous rocks with the rocks in general becoming younger towards the east (Bateman, 1992). Ages were determined radiometrically by measuring ratios of parent-daughter isotopes such as U-Pb, Rb-Sr, and K-Ar.

Geologic mapping in the Sierra Nevada batholith requires defining rock units and naming them. Plutons are mappable plutonic rock bodies and are often given names. Plutons are combined into map units (formal name: lithodeme) consisting of a number of plutons that are composed of rock of similar composition, fabric, and age and are presumed to have been continuous at depth when they were emplaced (Bateman, 1992). A map unit can be given a formal name such as the El Capitan Granite or an informal name such as the granodiorite of Kuna Crest. Note that the names include a place name or landmark that represents the type locality, and a rock name which represent the predominant rock type in the unit. However, there can be gradational compositional changes within plutons. As a consequence, not all the rocks exposed in the El Capitan Granite are necessarily granite as defined above.

An intrusive suite is composed of two or more map units that have similar isotopic ages and are thought to have been produced during the same magmatic episode (Bateman, 1992). For example, the Tuolumne Intrusive Suite contains the granodiorite of Kuna Crest on the margin, and concentrically inward contains the Half Dome Granodiorite, the Cathedral Peak Granodiorite, and the Johnson Granite Porphyry. These rocks, similar in age, are progressively younger and richer in silica inward. They are interpreted to represent surges of melt from the same magma chamber. The changes in composition reflect various stages of fractionation and mixing of magmas.

This excerpt is from "The Living Geology of the Sierra Nevada, Great Valley and Coast Ranges of California". For more information about this publication, click here.