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America's Volcanic Past
Yosemite National Park

"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994
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Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Yosemite features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Yosemite.]

  • Yosemite Valley
  • Yosemite Geologic History
  • Yosemite Granite
  • Yosemite National Park

Yosemite Valley

For its towering cliffs, spectacular waterfalls, granite domes and spires, glacially-sculpted and polished rock, and beautiful alpine scenery, Yosemite National Park is world famous. Nowhere else are all these exceptional features so well displayed and so easily accessible. Artists, writers, tourists, and geologists flock to Yosemite, and marvel at its natural wonders. Yosemite Valley itself is deeply carved into the gently sloping western flank of the Sierra Nevada, the longest, the highest, and the grandest single mountain range in the United States outside of Alaska. Simply stated, Yosemite Valley, only 7 miles long and nearly 1 mile wide, is a flat-floored, widened part of the canyon of the Merced River. But this broad rock-hewn trough with roughly parallel sides, is boldly sculptured and ornamented with silvery cataracts. From the valley floor at an elevation of 4,000 feet, the magnificent cliffs rise 3,000 to 4,000 feet higher to forested uplands on either side.




Excerpts from:
   


Yosemite Geologic History

Yosemite's Geologic History:
The last touches to Yosemite Valley's architecture were applied relatively recently, geologically speaking. But the rock from which the valley is carved originated mainly during the Cretaceous period, about 100 million years ago, when dinosaurs roamed the Earth. At that time molten rock, magma, generated deep within the Earth, rose upward within the Earth's crust, or upper layer, and crystallized far beneath the surface to form granitic rock along a linear belt that was to become the future Sierra Nevada. The granitic terrain that makes up the Sierra, once thought to have only local variations in one huge mass of rock, is actually made up of a mosaic of individual rock bodies that formed from repeated intrusions of magma over many millions of years. Some of the magma broke through to the surface, building a string of volcanoes atop hidden granitic roots, and we can perhaps envision an ancient majestic mountain range somewhat like the modern Cascade Range along the coast of our Pacific Northwest. Because of the high elevation of this ancestral range, however, the volcanic and other rocks covering the granite were soon eroded away, and by Late Cretaceous time, about 70 million years ago, the granitic rocks became exposed at the Earth's surface. By middle Cenozoic time, a few tens of millions of years ago, so much of the upper part had been removed that in the vicinity of Yosemite the surface of the range had a low relief of only a few thousand feet. Later, the continental crust east of the Sierra Nevada began to stretch in an east-west direction, developing into a series of north-south-trending valleys and mountain ranges. Through a combination of uplift of the Sierran block and down-dropping of the area to the east, the Yosemite region acquired a tilted-block aspect with a long, gentle slope westward to the Central Valley of California and a short, steep slope separating it from the country to the east.




Yosemite Granite

Yosemite Granite:
Granite forms the bedrock of much of the Sierra Nevada, including most of Yosemite National Park. Granite, in the broad sense of the term (granitic rock), is a rock with a salt-and-pepper appearance due to random distribution of light and dark minerals. The mineral grains are generally coarse enough to be individually visible to the naked eye. Throughout the park granitic rock varies considerably in the relative proportions of the individual light and dark minerals, and these compositional differences are represented by a variety of specific names, such as granodiorite and tonalite, in addition to "true" granite as defined by geologists. From a distance, all of Yosemite Valley's granitic rock looks the same. But it actually consists of individual rock bodies, each with their own characteristic mineral composition and texture, that is, the coarseness of their crystals and uniformity or variation in grain size. All of these variations, in turn, affect the rock's resistance to abrasion, fracturing, and weathering, all important to the sculptor and the end product. The imposing cliffs of El Capitan and Cathedral Rocks, for example, are composed of a particularly tough and resistant variety of granitic rock.




Yosemite National Park

Cathedral Rocks:
The imposing cliffs of El Capitan and Cathedral Rocks are composed of a particularly tough and resistant variety of granitic rock.

El Capitan:
The imposing cliffs of El Capitan and Cathedral Rocks are composed of a particularly tough and resistant variety of granitic rock.

Half Dome:
At the head of the valley, as if on a pedestal, stands Half Dome, the most colossal and recognizable rock monument in the Sierra Nevada, smoothly rounded on three sides and a sheer vertical face on the fourth. From its summit, over 4,800 feet above the valley, you look southeast into Little Yosemite Valley, which is broad floored and has granite walls more gently sloping than in its larger namesake. Half Dome, which stands nearly 8,800 feet above sea level, is composed of quartz monzonite, an igneous rock that solidified several thousand feet within the Earth.

Mount Dana and Mount Gibbs:
Ancient metamorphic rocks are widely exposed along the crest of the Sierra, notably on Mount Dana and Mount Gibbs.

North Dome:
Up the valley, on the north side, are the Royal Arches, sculptured one within another into an inclined rock wall that rises 1,500 feet. An enormous natural pillar, the Washington Column, flanks them on the right, and above them rises a smoothly curving, helmet-shaped knob of granite called North Dome.


Excerpts from:
1)
2) U. S. National Park Service Website, Yosemite National Park, California, 2000


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09/20/02, Lyn Topinka