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The geologic story of Yosemite as presented up to this point has been largely a description of the rocks as we see them now. But how did they get this way? And when? The search for answers to these questions involves interpretation of geologic observations made in Yosemite and elsewhere in the Sierra Nevada, together with numerous inferences based on accumulated geologic knowledge and on theoretical concepts. Some parts of the geologic history can be deciphered with confidence and in considerable detail, but other parts are less complete because the geologic data are very spotty.
The framework within which most geologists today view geologic processes, such as the creation of batholiths and the building of mountains, is that of the theory of plate tectonics (see section above entitled “Plate Tectonics — * * *”). Tectonics is the study of the deformation of earth materials and the structures resulting from that deformation. The “tectonics” in plate tectonics refers to deformation and structure on a global scale.
The oldest rocks in the Yosemite area were derived from sediment deposited during early Paleozoic time, beginning about 500 million years ago. During the Paleozoic, the area that was to become Yosemite was near the west edge of the growing North American Continent. The setting, for the most part, was a relatively passive one. Paleozoic sediment derived by erosion of still older rocks to the east was delivered by ancient streams flowing westward to a sea along the continental margin. Deposition of such sediment throughout most of the Paleozoic, though not necessarily continuous, resulted in the accumulation of thousands of feet of mud and sand, which eventually consolidated into shale and sandstone. Plant and animal life in the sea contributed their part by depositing calcium carbonate and silica, later to become beds of limestone and chert.
During the Paleozoic, the continent and its adjacent sea appear to have been traveling together on a single plate. All was not totally passive, however, because there is evidence for folding and deformation of some early Paleozoic strata during the late Paleozoic. It is not possible to relate such deformation to specific plate-margin tectonics because of severe overprinting by later tectonic events. By the end of the Paleozoic the geometry at the west edge of the North American plate had changed, and an oceanic plate was now underriding, or being subducted beneath, the North American plate.
The presence of a subduction zone along the west margin of the North American plate had profound effects on that plate. As the cool oceanic plate was subducted, the overriding continental plate was deformed. But more important to the Yosemite story were the igneous effects of subduction. Wherever convergent plate margins and subduction zones are present today, magma is generated at depth, and linear belts of volcanoes form atop the overriding plate, parallel to the subduction zone. Mount St. Helens, for example, and other volcanoes of the Cascade Range lie parallel to an active subduction zone that extends from northern California to Canada, and we infer that ancient subduction zones produced similar belts of igneous activity.
We can only speculate as to the nature of the physical and chemical processes that take place within a subduction zone. A prevalent theory is based on experiments indicating that the presence of water lowers the melting temperature of rock materials. This theory holds that water entrapped in the descending slab of oceanic crust is driven out as the slab reaches higher temperatures and leaks upward into the overriding lithosphere, where partial melting results (fig. 28). Magma generated in the mantle part of the lithosphere has the composition of basalt or andesite, but as the magma rises into the continental crust, a more silicic magma may be generated—one with the composition of rhyolite or granite. After rising toward the Earth’s surface, this silicic magma may erupt as rhyolite volcanoes, or cool and come to rest as great bodies of granitic rock within the upper crust. Most geologists now believe that this is the mechanism—greatly simplified here — through which the Sierra Nevada batholith was generated and emplaced.
By early Mesozoic time, more than 200 million years ago, magma reached the Earth’s surface in a belt of volcanoes and spewed forth to form great volumes of volcanic rock, metamorphosed remnants of which are now exposed in the area of the Sierran crest (pl. 1). By this time, silicic magma had also formed, some of which cooled and solidified below the Earth’s surface to form bodies of granitic rock; one such body is now exposed in Lee Vining Canyon (intrusive suite of Sheelite, pl. 1). Subduction along the margin of the North American plate was not continuous during the Mesozoic, and subsequent movement of granitic magma into the upper crust was somewhat episodic; the greatest volumes were emplaced during the middle Jurassic and Late Cretaceous. By the beginning of the Cenozoic, the magmatic system in the Sierran region shut off, leaving behind the mass of granitic rock we now call the Sierra Nevada batholith.
Emplacement of plutonic rocks within the upper crust was probably accompanied by many contemporaneous volcanic eruptions at the Earth’s surface. Evidence in the Yosemite area for such eruptions includes the texture of the Johnson Granite Porphyry (fig. 21) and similar porphyries in other intrusive suites, and the 100- to 118-million-year ages of the volcanic rocks near Mount Dana and in the Ritter Range. In addition, volcanic eruptions associated with emplacement of the Sierra Nevada batholith and other contemporaneous batholithic complexes - now exposed along the western margin of the North American Continent provide the only apparent source for the extremely voluminous deposits of Cretaceous volcanic ash to the east in the continental interior.
Not all of the oceanic plate was being subducted during that time, however. Parts of that plate, particularly the upper layer of marine sedimentary rocks on the oceanic crust, were added, or accreted, to the leading margin of the overriding continental crust. The handed chert in the Merced River canyon west of El Portal, once part of an ocean floor, was added to the North American plate by such a process.
The end result of the intrusion of the batholith, the construction of volcanoes, and the deformation of the metamorphic rocks was a linear mountain range parallel to and inboard of the continental margin. This range has been referred to as the ancestral Sierra Nevada. Mountains are born, only to be worn down by erosion; and erosive forces begin to act even as the mountains are being upraised. Nevertheless, the ancestral Sierra probably reached elevations above 13,000 ft, similar to those in the Cascade Range in western Washington and Oregon, a range being constructed over an active subduction zone today.
What caused magmatism in the Sierra to cease during the late Mesozoic? Many geologists speculate that the subducting oceanic slab speeded up and flattened out, so that the zone of magma generation shifted eastward. Although there are no giant batholiths in Nevada, many bodies of granitic and volcanic rock occur there that are chiefly of Cenozoic age, younger than the Sierra Nevada batholith.
Once the magmatic construction of the ancestral Sierra Nevada ceased, erosion became the dominant force in shaping the range, mostly by removing it. Before the end of the Mesozoic, some 63 million years ago, the volcanoes had largely been removed, and the batholith itself was exposed and being eroded. Sediment derived from this erosion was transported by streams coursing down the slope of the range to the Central Valley, where it now forms deposits as much as tens of thousands of feet thick. By middle Cenozoic time, so much of the range had been removed that it had a relief of only a few thousand feet or so.
During early Cenozoic time the Sierra Nevada region was relatively stable, and the range continued to be worn down faster than it was rising. But during the late Cenozoic, from about 25 to 15 million years ago, a dramatic change in plate motion along the edge of the North American plate occurred, with far-reaching effects. The oceanic plate that was being subducted beneath the Sierra Nevada was totally consumed into the subduction zone, and the plate that replaced it was moving in a different direction—northwesterly. The boundary between the North American plate and this northwesterly-moving plate, called the Pacific plate, became a strike-slip fault along this segment of California— the San Andreas fault (fig. 26).
This change in plate-boundary motion, from convergence to lateral motion, caused a change in the pattern of stresses imposed on the Sierran region. The continental crust east of the Sierra began to expand in an east-west direction, and the thick, light-weight Sierran crust began to rise again. The exact mechanism of this uplift is not understood, but the results are there to see. In the Yosemite area, the Sierra is clearly an uptilted block of the Earth’s crust, with a long slope westward to the Central Valley and a steep escarpment separating it from the country to the east (fig. 29). Total uplift in the vicinity of Mount Dana during late Cenozoic time to the present is estimated at about 11,000 ft.
The uplift began slowly and accelerated over time. The range certainly is still rising—and the rate may still be accelerating. The estimated current rate of uplift at Mount Dana, less than 1 1/2 inches per 100 years, may appear small, but it is greater than the overall rate of smoothing off and lowering of the range by erosion. Thus, there is a net increase in elevation. Estimates of uplift amount and rate are based on studies of lava flows and stream deposits thought to be nearly horizontal when formed, but which are now tilted westward toward the Central Valley. Progressive tilt is indicated by older deposits with greater inclinations than younger ones.
François Matthes inferred from his studies that the
late Cenozoic uplift occurred in a series of three pulses,
interrupted by pauses in uplift. In his view, each pulse
initiated a new cycle of erosion and thus produced a
stage of landscape incision characterized by successively
greater relief: Matthes’ broad-valley, mountain-valley,
and canyon stages. More recent studies show that
At the same time that the Sierra was undergoing uplift and erosion and incision by streams, volcanoes again became active in parts of the range, particularly north of Yosemite. During the interval from about 20 million years ago to about 5 million years ago, vast volumes of volcanic material were erupted from a belt of volcanoes extending along what is now the Sierran crest north of Yosemite. These volcanoes were the southward extension of the Cascade Range of volcanoes still active in northern California, Oregon, and Washington. With the advent of the San Andreas strike-slip fault, the subduction complex associated with Cascade volcanism migrated northward, and the Sierran volcanoes turned off. Lassen Peak in northern California is the southernmost volcano of this chain that is still active.
During this late Cenozoic volcanism, the Sierra Nevada north of Yosemite was virtually buried by lava flows, volcanic tuff, and volcanic mudflows. The volcanic material traveled great distances. Much of it reached the margin of the Central Valley, and some of it traveled as far south as the northern part of Yosemite. Three separate units of this volcanic extravaganza—a mudflow, a lava flow, and a volcanic tuff—successively flowed down the valley of a south-flowing tributary of the ancestral Tuolumne River and into the main channel in the vicinity of Rancheria Mountain northeast of Hetch Hetchy (figs. 30, 31). Erosional remnants of the volcanic mudflow indicate that it flowed almost as far west as Groveland, some 20 mi west of the park. Other erosional remnants of this mudflow indicate that it was so thick that it actually flowed upstream along the ancestral Tuolumne River at least 5 mi above the junction of the south-flowing tributary.
Other volcanic rocks in Yosemite represent local eruptive events. One such event is recorded by a basalt plug—a solidified remnant of lava in a volcanic conduit— locally known as “Little Devils Postpile” and located on the south side of the Tuolumne River several miles west of Tuolumne Meadows. The outcrop, easily reached by the Glen Aulin trail, exhibits crudely developed columnar joints (fig. 32). This basalt is about 9 million years old.
|ANCIENT CHANNEL OF THE TUOLUMNE RIVER on Rancheria Mountain northeast of Hetch Hetchy. The river flowed westward away from the viewer into the V-shaped notch cut into granite in the center of the photograph. About 10 million years ago, the channel and about 50 ft of river gravel were buried beneath a volcanic mudflow, the material seen on the slope above the “V”. Its cross section now exposed by erosion, this ancient channel was first described as such by Henry W. Turner, who took this photograph about 1900. (Fig. 30)||
[click to enlarge]
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|VOLCANIC MUDFLOW DEPOSIT on Rancheria Mountain. Brown, smooth-appearing slope to the right, at middle distance, is underlain by a volcanic mudflow deposit filling an ancient stream channel. This photograph, which is another view of the channel in figure 30, shows its position on the slope above Piute Creek, which drains away from the viewer to the present canyon of the Tuolumne River in the background. The former Tuolumne River flowed from left to right into the base of the brown area, concealed by trees in the center of the photograph, and at this point was more than 1,500 ft above the present canyon of the Tuolumne. This difference in elevation indicates the amount of stream incision by the Tuolumne River since its former channel was filled and abandoned and the river was forced to cut a new one. Photograph by Clyde Wahrhaftig. (Fig. 31)|
[click to enlarge]
COLUMNAR JOINTS in a basalt plug—a remnant of a
volcanic conduit—at “Little Devils Postpile,” adjacent to the
Tuolumne River west of Tuolumne Meadows. (Fig. 32)
One small lava flow of basalt, about 3 1/2 million years old, was erupted just south of Merced Pass, and a few scattered flows of similar age lie just south and southeast of the park. These flows record the most recent igneous activity in Yosemite.
Things have not remained quiet east of Yosemite,
however. A cataclysmic eruption about 700,000 years
ago created 10- by 20-mi-wide Long Valley caldera,
within which now sits the town of Mammoth Lakes.
This eruption spewed forth 2,500 times as much ash as
the 1980 Mount St. Helens eruption; layers of the ash
from Long Valley caldera have been found as far east as
Nebraska. Volcanic rocks of Mammoth Mountain and
the basalt at the Devils Postpile were erupted subsequently.
The Mono Craters and Inyo domes between
Mono Lake and Mammoth Lakes have been erupting
episodically during the past few thousand years, and the
most recent domes were formed only about 600 years
ago. Such activity is almost certainly not yet finished.
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