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Next: Genesis of Yosemite’s rocksContentsPrevious: Geologic overview

The Geologic Story of Yosemite National Park (1987) by N. King Huber


ROCKS, THE BUILDING MATERIALS

Yosemite is renowned for its magnificent rock exposures. Although granitic rocks dominate the Yosemite scene, various metamorphic and volcanic rocks are also present. Together, these rocks form Yosemite’s foundation.

Granite, Granite Everywhere

Granite, in the broad sense of the term, is a massive rock with a salt-and-pepper appearance due to random distribution of light and dark minerals. The mineral grains are coarse enough to be individually visible to the naked eye.

Granite is a plutonic igneous rock. There are two types of igneous rock—plutonic and volcanic. Both types result from the cooling and solidification of molten rock, or magma. Magma originates deep within the Earth and rises toward the Earth’s surface at temperatures of about 1,000 C if granitic in composition and of as high as 1,200 C if basaltic—by comparison, steel melts at about 1,430 C. Magma that cools and solidifies within the Earth’s crust forms plutonic rock (named for Pluto, the Roman god of the underworld). The slow cooling of plutonic magma fosters the growth of individual crystals visible to the naked eye. In contrast, magma that erupts at the Earth’s surface, where it is known as lava, quickly cools into volcanic rock. Thus, having insufficient time to grow, most mineral grains in volcanic rock are so small that a microscope is needed to distinguish them.

The plutonic terrane in the Sierra, once thought simply to represent local variations in one huge mass of granite, is actually made up of many individual bodies of plutonic rock—plutons—that formed from repeated intrusions of magma into older host rocks beneath the surface of the Earth. These plutonic rocks, formerly deep within the Earth, are now exposed at the surface, owing to deep erosion and removal of the formerly overlying rocks; they form the monoliths and domes of Yosemite within the lofty Sierra Nevada.

Plutonic-rock classification
[click to enlarge]

PLUTONIC-ROCK CLASSIFICATION, showing classification components, the formerly used system, and the one now used worldwide. Red area indicates general range in composition of plutonic rocks, exclusive of dikes and other small bodies, in the Yosemite area; names for rock types not occurring in the area are omitted. Star in center of triangle indicates composition of a rock containing equal proportions of quartz, potassium feldspar, and plagioclase. (Fig. 9)

–CLASSIFICATION OF PLUTONIC ROCKS–

Names for the more common varieties of plutonic rocks are based on the relative proportions of quartz, potassium feldspar, and plagioclase, as plotted on a triangular diagram, with each corner representing 100 percent of that constituent (fig. 9); other minerals present are ignored. The greater the percentage of any one of these three minerals in the rock, the closer the rock’s composition would plot to the corner for that mineral. A rock with equal percentages of the three minerals would plot in the center of the diagram (*), and the rock would be called a granite. Increasing the percentage of plagioclase at the expense of potassium feldspar would move the composition toward the granodiorite compartment on the triangular diagram. “Granitic rocks” are those that lie within the heavy-lined boundary.

The rock classification used in this volume was adopted by an international commission in 1972 and is now used worldwide. This classification differs from the one previously in use and thus results in many contradictions with the rock names in earlier geologic writings on the Sierra Nevada. Nearly all the granitic rocks in the Sierra previously called quartz monzonite fall within the granite classification of the present system, and quartz monzonite is relegated to a small compartment below granite on the triangular diagram; the old system is shown for comparison. In some cases, rocks previously called quartz monzonite are now called granodiorite because of better knowledge of their actual mineral composition.

The collection of plutons in the park is part of a larger mass of plutonic rock called the Sierra Nevada batholith (from the Greek words bathos, deep, and lithos, rock). Although this large mass of granite forms the bedrock of much of the Sierra Nevada, it is different from the range itself and originated many tens of millions of years before uplift, weathering, and erosion shaped the present range. It needs to be emphasized that the batholith is composite, a fact not perceived by the earliest geologic studies. Distinguishing between individual plutons that represent separate episodes of intrusion and solidification of magma is the key to understanding the origin and complex geologic history of the batholith. Geologists have mapped more than a hundred discrete masses of plutonic rock in the vicinity of Yosemite National Park alone, attesting to the complexity of what was once thought to be a relatively simple batholithic setting. Emplacement of the Sierra Nevada batholith at depth may have taken as long as 130 million years.

Five minerals compose the bulk of the plutonic rocks of the batholith: quartz, two varieties of feldspar (potassium feldspar and plagioclase), biotite, and hornblende. All contain the elements silicon and oxygen, and all except quartz contain aluminum as well. Other constituents of the feldspars include potassium, sodium, and calcium; greenish-black hornblende and the black mica, biotite, also contain magnesium and iron. The section on common minerals in granite provides clues on how to identify these minerals.

Plutonic rocks consisting chiefly of quartz and feldspar, with only a minor amount of dark minerals, are loosely called granitic rocks. Granitic rocks, such as granite, granodiorite, and tonalite, differ primarily in the relative proportions of these minerals (fig. 9). For example, granite, in the technical sense of the term, contains much quartz and both potassium feldspar and calcium-rich feldspar (plagioclase). In outcrop, it is generally difficult to distinguish the relative percentages t potassium feldspar and plagioclase. In the laboratory, the feldspars can be distinguished by applying chemicals that stain potassium feldspar yellow, plagioclase red, and leave quartz uncolored (fig. 10). By this means, the relative percentages of the three minerals can be determined easily.

El Capitan Granite
[click to enlarge]
El Capitan Granite
[click to enlarge]
EL CAPITAN GRANITE. A, Freshly broken surface of the rock. B, Rock surface chemically etched and stained to differentiate potassium feldspar (orange yellow), plagioclase (red), and quartz (uncolored). (Fig. 10)

Granodiorite (fig. 11) is similar to granite but contains about twice as much plagioclase as potassium feldspar. Tonalite contains even less potassium feldspar. In addition to quartz and feldspar, dark minerals, such as hornblende and biotite, further characterize individual plutonic-rock types, as is commonly indicated with modified names, such as hornblende granodiorite and biotite granodiorite. Dark minerals are generally more abundant where potassium feldspar is scarce, and thus granodiorite tends to be darker than granite, and most tonalite even darker.

Varieties of granodiorite: Half Dome Granodiorite
[click to enlarge]
Varieties of granodiorite: Sentinel Granodiorite
[click to enlarge]
Varieties of granodiorite: Leaning Tower Granodiorite
[click to enlarge]
Varieties of granodiorite: Bridalveil Granodiorite
[click to enlarge]
VARIETIES OF GRANODIORITE. All these granodiorites have about the same mineral composition but differ in texture: Half Dome Granodiorite (A) contains large, well-formed hornblende crystals; Sentinel Granodiorite (B) contains both biotite and hornblende in poorly formed crystals; Leaning Tower Granodiorite (C) has a spotted appearance from rounded clots of dark minerals; and Bridalveil Granodiorite (D) has a salt-and-pepper appearance from fine, evenly distributed light and dark minerals. (Fig. 11)

In contrast to granitic rocks, quartz diorite, diorite, and gabbro contain mostly plagioclase and dark minerals, with little or no quartz or potassium feldspar (figs. 9, 12). In addition, the plagioclase in gabbro contains more calcium than the plagioclase in diorite. Such plutonic rocks poor in quartz are sparse in the Yosemite area and generally occur as small, irregular masses and dikes—sheetlike masses—of quartz diorite or diorite; they generally are dark gray and commonly are fine grained, with few minerals readily recognizable to the naked eye.

Diorite
[click to enlarge]
DIORITE is mostly plagioclase and dark minerals, with little
quartz and potassium feldspar. (Fig. 12)

Light-colored rock, composed chiefly of quartz and potassium feldspar, also forms irregular masses and dikes. This rock occurs both with a fine-grained texture—aplite (fig. 13) —and with a very coarse grained texture—pegmatite—displaying large, intergrown quartz and potassium feldspar crystals. A fine example of pegmatite is visible a short distance down the Pohono Trail to Taft Point from the Glacier Point Road.

Dike
[click to enlarge]
DIKE of light-colored, fine-grained aplite crosscutting granodiorite.
Aplite is a silica-rich rock composed chiefly of quartz and
potassium feldspar. (Fig. 13)

Most granitic rocks contain mineral grains of about equal size and are said to have a granular texture. Some granites, however, and many volcanic rocks have crystals of one mineral considerably larger than the others; these oversized crystals are called phenocrysts (from the Greek words meaning “to appear” and “crystal”), and the texture of such a rock is described as porphyritic. In Sierran granites, the most common mineral to occur as phenocrysts is potassium feldspar, in crystals commonly as much as 2 to 3 in. long (fig. 14).


Porphyritic texture
[click to enlarge]
PORPHYRITIC TEXTURE in Cathedral Peak Granodiorite,
with potassium feldspar phenocrysts much larger than the
other minerals in the rock matrix. (Fig. 14)

Rounded inclusions of dark, fine-grained dioritic material are common in granitic rocks, most commonly in granodiorites and tonalites. Generally pancake or football shaped, the inclusions range in size from a few inches to many feet across. It is not uncommon for all the inclusions within an area to have their long dimensions arranged in the same direction, like a school of fish (fig. 15). An excellent example occurs at the Yosemite Falls overlook on the north rim of the valley. The origin of these inclusions is uncertain. Some probably were derived from preexisting rock; others may be derived from globules of darker magma that because of their high melting temperature were chilled by the granitic magma rather than being digested into it. However, the shape of the inclusions suggests that, whatever their origin, they were at least partially plastic while suspended in the magma and that they were stretched and given their parallelism by movement within the magma.

Concentrations of dark minerals sometimes form wavy, discontinuous streaks and layers, especially near the outer margins of individual plutons. These layers, called schlieren (German for streaks), probably represent clustering of dark minerals early during the crystallization of the magma, with alignment in streaks caused by movement within the partially solidified magma (fig. 16). The commonly abrupt termination of one set of layers by another set suggests repeated pulses of movement in a magma mush.

Aligned dark diorite inclusions
[click to enlarge]
ALIGNED DARK DIORITIC
INCLUSIONS in granodiorite.
Photograph by Dallas L. Peck.
(Fig. 15)
Schlieren
[click to enlarge]
SCHLIEREN—streaks or layers
formed by clustering of dark minerals
during differential flow within the
partially solidified magma. Note
parallel alignment of potassium feldspar
phenocrysts by the same process;
larger phenocrysts are about 2
in. long. (Fig. 16)

Individual bodies of granitic rock, particularly large ones, generally vary in mineral makeup and commonly overlap the boundaries between specific rock classifications. Bodies of granitic rock may also overlap each ocher’s compositional ranges, and so composition is only one factor in the recognition of separate rock bodies. The chief distinguishing property may be the presence or absence of specific minerals, such as biotite, hornblende, or sphene. Or it may be the general physical appearance defined by the texture of the rock—the size, shape, and arrangement (random or oriented) of the minerals (fig. 11). A porphyritic texture is particularly useful because it is prevalent in only a few plutons in the Yosemite area. The presence or absence of dark inclusions may also characterize a rock body.

Knowledge of the age relations among plutons is essential to understanding the geologic history of the Sierra Nevada batholith. Certain features observed in outcrop help determine the relative ages of individual rock bodies. For example, younger magma commonly shoots thin sheets, or dikes, into cracks in the older rocks (fig. 17A). Additionally, some of the younger plutons contain inclusions, or fragments of older rock, which were embedded in the younger rock while it was still molten (fig. 17B). Where dark inclusions or other oriented structures are present, the contact between two rock bodies may truncate structures in the older body, while similar features in the younger rock may parallel the contact (fig. 17C). Determining the absolute age of a given granitic rock, in millions of years, requires measurement of the extent of radioactive decay of certain elements, such as uranium, potassium, and rubidium. From such measurements and the known rates of decay, we can approximately determine the time elapsed since the rock crystallized or cooled enough to stop escape of the daughter decay products from the rock (see fig. 7).

In their studies of plutonic rocks, geologists have devised ways to separate individual bodies of such rock and to depict them on geologic maps so as to show their relations to each other and to nonplutonic rocks with which they are in contact. Once established by field study, the boundaries of these individual plutonic-rock bodies—plutons—can be plotted on a map, and these rock bodies become geologic map units. After further study, the geologist may decide that two or more nearby bodies of plutonic rock exposed on the Earth’s surface are similar in all essential respects, including known or inferred age. Even though they may not be connected at the Earth’s surface, the geologist may thus combine several masses of similar plutonic rock into a single geologic map unit, inferring that they are somehow connected below the surface and represent a single intrusive episode. This grouping of isolated bodies of related plutonic rock into a single geologic map unit is analogous to the grouping of discontinuous exposures of similar sedimentary rock into formations, such as the Coconino Sandstone and the Kaibab Limestone, which are well exposed in the Grand Canyon region. For ease of reference, the plutonic-rock units likewise are generally named for an appropriate geographic feature, plus a compositional term: for example, the El Capitan Granite, the Half Dome Granodiorite, and the granodiorite of Kuna Crest.

Features seen in outcrop
[click to enlarge]
FEATURES SEEN IN OUTCROP that help determine the
relative ages of plutonic rocks: 0, older pluton; Y, younger pluton.
(Fig. 17)

Granitic Rocks of Yosemite

The plutonic rocks of Yosemite have been mapped and studied in considerable detail. Few of those details can be shown on the generalized geologic map in this volume (pl. 1), but a geologic map at a much larger scale is available (Huber and others, in press [Editor’s note: 1989—dea]). On that map, the granitic rocks of the Yosemite area are separated into nearly 50 different plutonic-rock units, each consisting of one or more individual bodies of rock. An even larger scale geologic map is available for Yosemite Valley (Calkins, 1985; see section above entitled “Geologic Maps of Yosemite”).

Some plutonic-rock units are further grouped into intrusive suites. The concept underlying an intrusive suite is that all the rocks in the suite resulted from the same magma-producing event. Geologists are most sure of a common ancestry if the rocks in a suite grade into each other. Such suites commonly are zoned, both compositionally and texturally, and generally exhibit partial or complete nested patterns in which relatively dark rock in the margins gives way inward to younger, lighter colored rock in the interior. The units that compose this ideal kind of intrusive suite are believed to result from modifications of a common parent magma. Examples include the Tuolumne Intrusive Suite, the first intrusive suite to be identified in the Sierra Nevada, and the intrusive suite of Buena Vista Crest. The geologic map (pl. 1) groups most plutonic-rock units into intrusive suites and thus provides a broader picture of the major pulses of plutonic activity that contributed to the construction of the Sierra Nevada batholith. The more detailed geologic map of Yosemite Valley (pl. 2) delineates not only intrusive suites but also component units of the suites.

All the plutonic rocks within Yosemite National Park proper are believed to be of Cretaceous age, with the possible exception of some small bodies of diorite and gabbro that may be somewhat older. Some Jurassic plutonic rock does occur just west of the park, west of the Big Oak Flat entrance, and some Triassic plutonic rock occurs east of the park in Lee Vining and Lundy Canyons. These rocks are included with “Plutonic rocks, unassigned to suites” on plate 1 and are shown individually only on the larger scale geologic map published separately (Huber and others, in press [Editor’s note: 1989—dea]).

Examples of many of the named rock types in Yosemite are displayed at the Valley Visitor Center, where they may easily be compared; they are next described for two readily accessible areas in the park, Yosemite Valley and the Tuolumne Meadows area.

Bird's-eye view of Yosemite Valley

[click to enlarge]
BIRD’S-EYE VIEW
OF YOSEMITE
VALLEY, with selected
landforms identified.
(Fig. 18)

YOSEMITE VALLEY AREA

The oldest plutonic rocks of the Yosemite Valley area compose the walls of Merced Gorge and the west end of the valley. They include the diorite of the Rockslides, the granodiorite of Arch Rock, and the tonalite of the Gateway (pl. 2). The largest outcrop of diorite is just west of the Rockslides (fig. 18), but the talus slopes below, composed of broken blocks of diorite, are more accessible. A good exposure of the granodiorite of Arch Rock can be seen immediately east of the Arch Rock Entrance Station on the El Portal Road (Route 140), where the road passes under two large fallen blocks of the granodiorite (park vehicles near the entrance station). The tonalite of the Gateway can be seen along the El Portal Road across from the first turnout after the road starts climbing up the Merced Gorge eastward from El Portal; these last two locations are west of the map area shown in plate 2. Studies of radiometric decay indicate that the tonalite of the Gateway is about 114 million years old. The radiometric age of the granodiorite of Arch Rock has not been determined, but it probably is only a little younger than that of the Gateway.

The El Capitan Granite subsequently intruded these older plutonic rocks about 108 million years ago and now makes up the bulk of the west half of the valley area. About 4 km east of the Arch Rock Entrance Station, the El Portal Road cuts through blocks of El Capitan Granite dislodged in a 1982 rockfall. These blocks, some the size of a small house, display fresh surfaces of the granite (fig. 10; see fig. 48), as well as numerous inclusions of dark-colored rock. The imposing monoliths of Turtleback Dome, El Capitan, Three Brothers, and Cathedral Rocks also are hewn chiefly from massive El Capitan Granite.

After the El Capitan Granite was emplaced, the Taft Granite welled up and intruded the El Capitan. Dikes of Taft Granite invading El Capitan Granite and inclusions of El Capitan in Taft establish the Taft as younger. The two rocks are similar, but Taft Granite is lighter in color and commonly finer grained than El Capitan Granite and, unlike El Capitan Granite, generally does not contain phenocrysts. Taft Granite forms the brow of El Capitan and part of the upland between El Capitan and Fireplace Bluffs. On the south side of the valley, Taft Granite can be seen at Dewey Point and near The Fissures, just east of Taft Point.

In the vicinity of Leaning Tower and Cathedral Rocks, dikes and irregular masses of several fine-grained rocks cut the Taft and El Capitan Granites. Examples of these fine-grained rocks can be seen in blocky rubble near the base of Bridalveil Fall. The Leaning Tower Granodiorite characteristically contains rounded clots of dark minerals that give it a spotted appearance (fig. 11C). The Bridalveil Granodiorite, which contains fine, evenly distributed, light and dark minerals, has a salt-and-pepper appearance (fig. 11D); features seen in outcrop show that it intruded nearly all the rocks which it now contacts.

Dark, fine-grained diorite also intrudes the El Capitan and Taft Granites. A striking example is exposed on the east face of El Capitan, where dikes of diorite form an irregular pattern that, in part, very crudely resembles a map of North America (fig. 19).

The Sentinel Granodiorite forms a north-south band that crosses the valley between Taft Point and Glacier Point. The rock varies in appearance but is generally medium gray and medium grained (fig. 11B). Giant inclusions of El Capitan Granite are embedded within Sentinel Granodiorite in a zone that extends along Yosemite Creek and down the face of the cliff near Yosemite Falls. The Sentinel Granodiorite reappears on the south valley wall west of Union Point and then extends southward through Sentinel Dome to Illilouette Ridge. Dikes of Sentinel Granodiorite

Diorite dikes

[click to enlarge]
DIORITE DIKES on the face of El Capitan; dark patch is
thought by some to resemble a crude map of North America.
Some lighter colored dikes are also present.
(Fig. 19)
that cut inclusions of El Capitan Granite can be seen in the roadcut along the Glacier Point Road near the trailhead to Taft Point.

The rock at Glacier and Washburn Points is darker than Sentinel Granodiorite and has a streaky appearance from parallel-oriented flakes of biotite and rods of hornblende. This darker rock, once thought to be part of the Sentinel and shown as such on earlier geologic maps, is now assigned to the granodiorite of Kuna Crest.

The Half Dome Granodiorite dominates the valley area east of Royal Arches and Glacier Point. It is medium to coarse grained and contains well-formed plates of biotite and rods of hornblende (fig. 11A). At Church Bowl and in the cliff west of Royal Arches, horizontal dikes of Half Dome Granodiorite cut the older granodiorite of Kuna Crest. Half Dome Granodiorite forms the sheer cliffs to the north of the trail between the Ahwahnee Hotel and Mirror Lake. The trail to Vernal and Nevada Falls also crosses through Half Dome Granodiorite. Except for minor dikes, the Half Dome Granodiorite, about 87 million years old, is the youngest plutonic rock in the valley area.


TUOLUMNE MEADOWS AREA

The granodiorite of Kuna Crest and the Half Dome Granodiorite exposed at the east end of Yosemite Valley are two plutonic-rock units that make up the western margin of the Tuolumne Intrusive Suite. This suite underlies a large part of eastern Yosemite National Park from upper Yosemite Valley, across Tuolumne Meadows eastward to the crest of the Sierra, and northward beyond the park boundary (pl. 1). The Tuolumne Intrusive Suite, one of the best studied groups of granitic rocks in the Sierra, consists of four bodies of plutonic rock, sequentially emplaced and partly nested one within the other (fig. 20). The suite is well exposed in the area centered on Tuolumne Meadows, and the Tioga Road (Route 120) provides access to many conspicuous outcrops of the suite’s components.

The oldest and darkest plutonic rock generally forms the margin of the suite, and the youngest rock is in its core. The rocks are, from oldest to youngest: the granodiorite of Kuna Crest (about 91 million years old), the Half Dome Granodiorite, the Cathedral Peak Granodiorite (about 86 million years old), and the Johnson Granite Porphyry. Field relations indicate that the Johnson Granite Porphyry is the youngest granitic rock in the park, although a radiometric age has not yet been determined. The granodiorite of Kuna Crest normally occupies the margin of the suite, but on much of the perimeter the Half Dome Granodiorite and the Cathedral Peak Granodiorite have broken through the granodiorite of Kuna Crest to form the marginal units (fig. 20).

The overall concentric zonation of rock bodies within the suite, as well as the overall chemical similarities among the rocks, suggests that these rocks originated from the same magma chamber. This inferred common parentage provides the rationale for grouping these rocks into an intrusive suite. The composition of the magma, however, changed over time: the older, hornblende- and biotite-rich rocks at the margins give way to quartz- and potassium feldspar-rich rocks toward the center. Hornblende and biotite crystallize at higher temperatures than quartz and feldspar, and so during cooling of a magma, these dark minerals generally crystallize earlier than the light-colored ones. This relation suggests that cooling of the magma started at the margins and progressed inward over time.

North of the Tioga Pass Entrance Station, the trail to Gaylor Lakes crosses over the granodiorite of Kuna Crest, the oldest and darkest rock in the Tuolumne Intrusive Suite. This trail weaves back and forth near the contact between the granodiorite and the metamorphic rocks that it intruded. The granodiorite also contains many disc-shaped inclusions that are oriented parallel to its contact with the older metamorphic rocks. These inclusions were probably stretched and oriented by movement within the magma during intrusion and cooling.

The Half Dome Granodiorite, the next youngest rock in the suite, is in contact with the granodiorite of Kuna Crest to the west along the ridge crossed by the Gaylor Lakes Trail. The best exposures of the Half Dome, however, are surrounding the turnout at Olmsted Point west of Tenaya Lake. Fresh, clean outcrops of the rock abound at and across from the turnout. Half Dome Granodiorite makes up much of the southwestern part of the Tuolumne Intrusive Suite and in several areas is the marginal rock.

Heading east toward Tuolumne Meadows, the Tioga Road crosses the contact between the Half Dome Granodiorite and the Cathedral Peak Granodiorite just east of Tenaya Lake. The contact is obscure, however, because here the Half Dome contains nearly as many potassium feldspar phenocrysts as does the younger

Evolution of the Tuolumne intrusive suite
[click to enlarge]
EVOLUTION OF THE TUOLUMNE INTRUSIVE
SUITE—a map view.
(Fig. 20)
Cathedral Peak. Pothole and Lembert Domes, both marginal to the meadows, are composed entirely of Cathedral Peak Granodiorite. The rock of these domes clearly displays potassium feldspar phenocrysts, commonly as much as 2 to 3 in. long (fig. 14). These impressive crystals stand out against a medium-grained background. The Cathedral Peak Granodiorite forms the largest pluton of the Tuolumne Intrusive Suite, extending long distances to the north and south of Tuolumne Meadows.

The youngest, smallest, and most central rock body if the suite is composed of the Johnson Granite Porphyry. In a porphyry, the conspicuous phenocrysts are set in a finer grained matrix than in such porphyritic rocks as the Cathedral Peak Granodiorite, and so individual mineral grains in the matrix are difficult to identify without a microscope. Low outcrops of the porphyry can be seen in Tuolumne Meadows along the Tuolumne River, across from the store, and east of Soda Springs on the north side of the river. The rock is very light colored, with only a few scattered potassium feldspar phenocrysts within a fine-grained matrix (fig. 21). Dikes of Johnson Granite Porphyry intrude Cathedral Peak Granodiorite, and the porphyry itself is cut by light, fine-grained aplite dikes.


Johnson Granite porphyry
[click to enlarge]
JOHNSON GRANITE PORPHYRY, showing potassium
feldspar phenocrysts set in a fine-grained matrix.
(Fig. 21)

The fine-grained matrix of a porphyry requires that partially crystallized magma be quenched or cool relatively quickly. Such conditions would result from a sudden release of pressure, as would occur if some of the magma were erupted at the Earth’s surface. Thus, volcanic eruptions probably accompanied final emplacement of the Tuolumne Intrusive Suite—a volcanic caldera may once have existed far above what is now Johnson Peak (fig. 22).

Final states in the evolution of the Tuolumne intrusive suite
[click to enlarge]
FINAL STAGES IN THE EVOLUTION OF THE TUOLUMNE INTRUSIVE SUITE. The Johnson Granite Porphyry intrudes the Cathedral Peak Granodiorite and erupts through a volcanic caldera, spewing volcanic ash and debris onto the Earth’s surface. The volcanic deposit and much of the underlying rock are subsequently removed by erosion to create today’s land surface. (Fig. 22)

METAMORPHIC ROCKS—ANCIENT SEDIMENT AND LAVAS

Metamorphic rocks are derived from preexisting rocks by mineralogic and structural changes in response to increases in temperature, pressure, and shearing stress at depth within the Earth’s crust. In the Sierra Nevada, some of this heat and pressure was supplied by the intruding granitic rocks, but much of it was imposed simply by depressing sedimentary and volcanic rocks once exposed at the Earth’s surface downward to depths where higher temperature and pressure are the normal environment. The metamorphic rocks in the Yosemite area were derived from a great variety of sedimentary and volcanic rocks and thus exhibit a great variety in themselves. Some rocks have been only mildly metamorphosed and still retain original structures, such as sedimentary layering, that help to identify the nature of the original rock. Others have been so strongly deformed and recrystallized that original textures and structures have been destroyed, and determination of the original rock type is difficult.

Metamorphosed sedimentary rocks in the Yosemite area include rocks that were originally sandstone and siltstone, conglomerate, limestone, shale, and chert. Metamorphosed volcanic rocks in the Yosemite area include those derived from lava flows and various types of pyroclastic rocks—those formed from violently erupted volcanic debris.

The rocks into which the Sierra Nevada batholith was emplaced are weakly to strongly metamorphosed, mildly to complexly deformed strata of probable Paleozoic and Mesozoic age. In the Yosemite area these metamorphic rocks occur in two northwest-trending belts situated largely east and west of the park proper and in small isolated bodies scattered throughout the park. Fossils are scarce, and the radiometric ages of most of these rocks are poorly known.

Rocks of the western metamorphic belt underlie much of the foothills of the western Sierra between the San Joaquin and Feather Rivers, and form the western wallrocks of the Sierra Nevada batholith. In the canyon of the Merced River approaching Yosemite on Route 140, strikingly banded chert is exposed in the vicinity of the “geological exhibit” and eastward for several miles (fig. 23). This banded chert was formed from the skeletons of very tiny, silica-secreting marine animals called radiolarians; upon the death of such animals, their skeletons settle to the ocean bottom, where they collect in enormous numbers. Although the chert beds are moderately to strongly deformed, the

Contorted chert beds
[click to enlarge]
CONTORTED CHERT BEDS along the Merced River west
of El Portal are ancient marine sediment that has been
metamorphosed. (Fig. 23)
rock is easily recognizable as of sedimentary origin. In contrast, metamorphic rocks just west of El Portal and just west of Crane Flat along the Big Oak Flat Road (Route 120) have a metamorphic layering that largely destroys original bedding, and the origin of these rocks as sediment is less obvious. Fossils in a limestone bed just west of the “geological exhibit” on Route 140 indicate a Triassic age for at least some of the rocks exposed along this part of the Merced River canyon.

The eastern belt of metamorphic rocks extends for about 50 mi from south of Mammoth Lakes to north of Twin Lakes (pl. 1). Furthermore, rather than bounding the batholith, this belt is a giant septum of metamorphic rocks separating plutonic rocks on either side.

This eastern belt includes rocks of both sedimentary and volcanic origin, which range in age from early Paleozoic to late Mesozoic. The Paleozoic rocks are metasedimentary and include such varieties as quartzite, metaconglomerate, and marble. The commonest rock, however, is homfels—a catchall term for a fine-grained metamorphic rock composed of a mosaic of equidimensional grains formed by recrystallization of sedimentary and volcanic rocks of various compositions. These Paleozoic rocks are well exposed along Route 120 near Ellery Lake east of Tioga Pass.

The Mesozoic rocks of the eastern metamorphic belt are chiefly of volcanic origin—tuff and other explosively ejected fragmental volcanic rock—with lesser amounts of sedimentary rock. These Mesozoic rocks, which lie generally west of the Paleozoic rocks in the eastern metamorphic belt, make up the Ritter Range and the southeastern margin of the park, and much of the Sierran Crest northward through Kuna Peak, Mount Dana, Gaylor Peak, and continuing north of the park beyond Twin Lakes (pl. 1). Relict sedimentary bedding is commonly preserved—steeply dipping, as west of Saddlebag Lake, or highly contorted, as near Spotted Lakes at the south end of the park (fig. 24).

Of particular interest are the little-deformed metamorphic rocks of Cretaceous age. Metamorphosed volcanic rocks near the summit of Mount Dana have a radiometric age of about 118 million years, and those from the Ritter Range of about 100 million years, which means that their eruption from volcanoes occurred at the same time that some of the smaller plutonic-rock suites were emplaced at depth. In the Ritter Range, a thick deposit of volcanic breccia has been interpreted as resulting from collapse of an ancient volcanic caldera.

Metamorphic rocks: steeply dipping sedimentary bedding
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Metamorphic rocks: highly contorted sedimentary bedding
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METAMORPHIC ROCKS with relict sedimentary bedding. A, Steeply dipping, as northwest of Saddlebag Lake. B, Highly contorted, as near Spotted Lakes. Photograph by John P. Lockwood. (Fig. 24)

Late Cenozoic Volcanic Rocks— Born of Fire

Volcanic rocks, like their plutonic counterparts, are also classified on the basis of composition. Because volcanic rocks erupted onto the Earth’s surface cool and solidify more quickly than plutonic rocks, they tend to be finer grained or even glassy, with few minerals identifiable to the eye. Those few visible minerals, however, are guides to the rock’s composition. Late Cenozoic volcanic rocks in Yosemite have a very limited range in composition; they generally contain little or no quartz and range from basalt and andesite (containing little or no potassium feldspar) to latite (containing both potassium and plagioclase feldspar). A volcanic rock containing quartz—rhyolite—does occur just east of Yosemite at the Mono Craters.

Late Cenozoic volcanic rocks of the Yosemite area formed both by the eruption of vast volumes of lava and by much smaller eruptions. The products of great eruptions extend into the northern part of the park but are much more extensive in the northern Sierra; they include lava flows, tuff, and volcanic mudflows. Details of the nature and distribution of all these volcanic rocks are deferred to the section dealing with the late Cenozoic.

PLATE TECTONICS A DYNAMIC GLOBE

The Earth is generally depicted as consisting of a series of concentric shells—a relatively thin outer crust, an intermediate mantle, and an interior core (fig. 25). The Earth’s crust and uppermost part of the upper mantle together form the rigid outer part of the Earth—the lithosphere—which is broken into plates that ride over a less rigid, viscous layer within the upper mantle that yields plastically. There are seven very large plates, and a dozen or so small ones (not all of which are shown in fig. 26.) The large plates consist of both oceanic and continental portions; the present North American plate, for example, includes not only the North American Continent but Greenland and the west half of the North Atlantic Ocean as well. The crust beneath continents is typically 20 to 34 mi thick and is less dense than the crust beneath oceans, which typically is only 4 to 5 mi thick. The plates generally are internally rigid, and most dynamic geologic activity is concentrated along the plate boundaries; these boundaries are marked by long, narrow belts of earthquake and volcanic activity.

Each of the plates is moving relative to all the others. In the simplest mode, two plates slide past each other along a strike-slip fault (fig. 27A). The San Andreas fault, running much the length of California and forming part of the present boundary between the North American and Pacific plates, is an example. Where plates move away from each other, primarily along the system of great submarine ridges in the world’s oceans, hot material wells up from below to fill the gap (fig. 27B). As this hot material cools to form basalt, it becomes attached to the plates on either side of the spreading zone, and new crust is created. Where plates converge, one tips downward and slides beneath the other— a process called subduction (fig. 27C). Generally, a plate with dense oceanic crust slides beneath one with more bouyant continental crust. Thus, new oceanic crust created at spreading centers is recycled back into the Earth’s interior through subduction, and and the total surface area of the Earth remains unchanged.

Interior of the Earth
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INTERIOR OF THE EARTH, showing relation of
crust and mantle to the rigid lithosphere—the stuff of
which the mobile plates are made. (Fig. 25)

MAJOR LITHOSPHERE PLATES OF THE WORLD, showing boundaries that are presently active. Double One, zone of spreading, from which plates are moving apart; barbed line, zone of underthrusting (subduction), where one plate s sliding beneath another—barbs on overriding plate; single line, strike-slip fault, along which plates are sliding past one another. See figure 27 for examples of plate motions. (Fig. 26)


Major lithosphere plates of the world
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Three principle kinds of plate motion
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THREE PRINCIPLE KINDS OF PLATE MOTION. A, The plates slide past each other along a strike-slip fault. B, The plates move away from each other at a divergent boundary. C, The plates move toward each other at a convergent boundary; the process of subduction consumes crust at convergent plate boundaries. (Fig. 27)



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