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


GEOLOGIC OVERVIEW


Geologic time scale
[click to enlarge]

THE GEOLOGIC TIME SCALE—the “calendar” used by geologists in interpreting Earth history. Column A, graduated in billions of years (b.y.) and subdivided into the four major geologic eras (Precambrian, for example), represents the time elapsed since the beginning of the Earth, which is believed to have been about 4.5 b.y. ago. Column B is an expansion of part of the time scale in millions of years (m.y.), to show the subdivisions (periods— Cambrian, for example) of the Paleozoic, Mesozoic, and Cenozoic Eras; column C is a further expansion to show particularly the subdivisions (epochs—Paleocene, for example) of the Tertiary and Quaternary Periods. Some key events in the geologic history of Yosemite National Park are listed alongside the columns, opposite the time intervals in which they occurred.

The subdivisions of geologic time are based largely on the fossil record; rocks of the Cambrian Period contain the earliest evidence of complex forms of life, which evolved through subsequent periods into the life of the modern world. The ages (in years) are based on radiometric dating. Many rocks contain radioactive elements that begin to decay at a very slow but measurable rate as soon as the parent rock is formed. The most common radioactive elements are uranium, rubidium, and potassium, and their decay (“daughter”) products are lead, strontium, and argon, respectively. By measuring both the amount of a given daughter product and the amount of the original radioactive element still remaining in the parent rock, and then relating these measurements to the known rates of radioactive decay, the age of the rock in actual numbers of years can be calculated. (Fig. 7)

Topographically, the Sierra Nevada is an asymmetric mountain range with a long, gentle west slope and a short, steep east escarpment that culminates in the highest peaks (fig. 2). It is 50 to 80 mi wide and extends in altitude from near sea level along its west edge to more than 13,000 ft along the crest in the Yosemite area. Geologically, the Sierra Nevada is a huge block of the Earth’s crust that has broken free on the east along a bounding fault system and has been uplifted and tilted westward. This combination of uplift and tilt, which is the underlying geologic process that created the present range, is still going on today.

Massive granite dominates the Yosemite area and much of the Sierra Nevada as well. Mount Hoffmann and most of the terrane visible from it are composed of granite, formed deep within the Earth by solidification of formerly molten rock material and subsequently exposed by erosion of the overlying rocks. Because of its massiveness and durability, granite is shaped into bold forms: the cliffs of Yosemite and Hetch Hetchy Valleys, many of the higher peaks in the park, and the striking sheeted domes that can form only in massive, unlayered rock. Although granite dominates nearly the entire length of the Sierra, the granite is not monolithic. Instead, it is a composite of hundreds of smaller bodies of granitic rock that, as magma (molten material), individually intruded one another over a timespan of more than 100 million years (fig. 7). This multiplicity of intrusions is one of the reasons why there are so many varieties of granitic rock in Yosemite and the rest of the Sierra. The differences are not always apparent to the casual observer, but they are reflected in sometimes subtle differences in appearance and in differences in response to weathering and erosion acting on the rocks.

Layered metamorphic rocks in the foothills at the west edge of the park and along the eastern margin in the summit area are remnants of ancient sedimentary and volcanic rocks that were deformed and metamorphosed in part by the invading granitic intrusions. Other metamorphic rocks that once formed the roof beneath which the granitic rocks solidified were long ago eroded away to expose the granitic core of the range, and only small isolated remnants are left. Because Yosemite is centered on this deeply dissected body of granite, metamorphic rocks are sparse; they occupy less than 5 percent of the area of the park.

Five minerals compose the bulk of the plutonic rocks of Yosemite: quartz, potassium feldspar, plagioclase feldspar, biotite, and hornblende. Quartz and both varieties of feldspar are translucent and appear light gray on fresh surfaces. On a weathered surface, the feldspars turn chalky white, whereas the quartz remains clear gray. Feldspar crystals have good cleavage, a property of breaking along planar surfaces that reflect sunlight when properly oriented; quartz has no cleavage but breaks randomly along curved surfaces. Biotite crystals commonly appear hexagonal, and their dark, brown to black plates can be split with a knife into thin flakes along one perfect cleavage direction (fig. 8). Hornblende is much harder than biotite, appears very dark green to almost black, and commonly occurs as elongate, rod-shaped crystals. It has good cleavages in two directions that intersect to form fine striations along the length of the rods, making them look like bits of charcoal. Other minerals are present in small amounts; the most distinctive is sphene (calcium and titanium silicate), which occurs in small, amber, wedge-shaped crystals. With a little practice, all these minerals can be identified with a small magnifying glass.

COMMON MINERALS
IN GRANITE

Hornblende and biotite

[click to enlarge]

HORNBLENDE AND BIOTITE. Rod-shaped crystals of hornblende and hexagonal crystals of biotite. These large and exceptionally well formed crystals are from Half Dome Granodiorite. (Fig. 8)

Evolution of the landscape is as much a part of the geologic story as the rocks themselves, and Yosemite is a place where the dynamism of geologic processes is well displayed. By the end of Cretaceous time (see fig. 7), about 65 million years ago, after the granite core of the range had been exposed, the area had a low relief in comparison with the mountains of today. Then, about 25 million years ago, this lowland area began to be uplifted and tilted toward the southwest, a construction that would eventually lead to the present Sierra Nevada. As the rate and degree of southwestward tilt increased, the gradients of streams flowing southwestward to California’s Central Valley also increased, and the faster flowing streams cut deeper and deeper canyons into the mountain block. About 10 million years ago, from the Tuolumne River northward, these canyons were inundated and buried by volcanic lava flows and mudflows, and the streams were forced to begin their downcutting anew, in many places shifting laterally to find a new route to the Central Valley. The streams were equal to the task, however, and the present river courses and drainage patterns throughout the Sierra became well established.

As the world grew colder, beginning about 2 or 3 million years ago, the Sierra Nevada had risen high enough for glaciers and a mountain icefield to form periodically along the range crest. When extensive, the icefield covered much of the higher Yosemite area and sent glaciers down many of the valleys. Glacial ice quarried loose and transported vast volumes of rubble, and used it to help scour and modify the landscape. Much of this debris eventually accumulated along the margins of the glaciers and in widely distributed, hummocky piles. The greatest bulk of this debris, however, was flushed out of the Sierra to the Central Valley by streams swollen with meltwater formerly stored in the glaciers as ice and released as the glaciers melted away.

Although many of today’s general landforms existed before modification by glacial action, some of them surely did not. Can you imagine the Yosemite landscape with no lakes? Virtually all the innumerable natural lakes in the park are the result of glacial activity. But even these lakes are transitory, doomed to be filled with sediment and become meadows; many lakes already have undergone this transformation. Yosemite Valley itself once contained a lake.

The geologic story of Yosemite National Park can be considered in two parts: (1) deposition and deformation of the metamorphic rocks and emplacement of the granitic rocks during the Paleozoic and Mesozoic; and (2) later uplift, erosion, and glaciation of the rocks during the Cenozoic to form today’s landscape.

The paragraphs that follow start with a description of the rocks—what can be seen on excursions through the park—granite first and in the most detail, because it dominates the Yosemite scene. The rocks will then be fitted into the context of a geologic history through which today’s Yosemite evolved.



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