The Yosemite Valley is regarded by its visitors as one of the World’s greatest natural wonders. People were attracted to its crowding beautiful scenery all in one small area. These include the five domes, lakes and caverns, cliffs and mountains elevated form 13,000 up to 15,000 feet (Hittell 1868). Yosemite National Park is found in eastern parts of Tuolumne, Mariposa and Madera countries located in east central California, United States. Extending across the Sierra Nevada mountain chain, the park area was measured 3,081 square kilometers (Harris 1997).
According to Hamilton and Hamilton (2006), majority of Yosemite is composed of Plutonic igneous rocks which forms underground when magma cools and solidifies slowly forming large crystals. Yosemite Park is composed of plutonic rocks including tonalite, quartz monzonite, granodiorite and granite which were loosely treated as granitic rocks. Gabbro, quartz diorite and diorite are plutonic rocks which cannot be technically recognized as granitic rocks. Minerals including potassium feldspar, biotite, hornblende, plagioclase feldspar and quartz constitute plutonic rocks.
The texture and relative extent of quartz and feldspar largely differs in plutonic rocks.
The formation of different bodies of individual plutonic rocks was due to the repetition of magma intrusions into host rocks beneath the Earth’s surface taking 130 million years ago. The plutonic rocks previously inside the Earth are already out at the surface due to deep erosion and elimination of the previously overlying rocks. Within Yosemite, only small amounts of volcanic igneous rocks were found.
Beneath the surface, latite lava flows, basaly flows and latite tuff are found while obsidian, ash-flow tuff, rhyolite and pumice were found on the surface.
Huber (1987) stated that on each side of the batholith of plutonic rocks, metamorphic rocks of two northwest-trending belts were found. He added that metamorphic rocks were formed from the alteration of sedimentary and volcanic rocks due to high pressure, cutting stress and temperature beneath the Earth’s crust. Volcanically originated rocks are called metavolcanic rocks while those with sedimentary origin are called metasedimentary rocks.
As explained by Lawson (1921) from the Handbook of Yosemite National Park, Sierra Nevada belongs to the class of mountains of the Basin Range type. The Great Basin is a region being described as without drainage to the sea which lies between Wasatch and Sierra Nevada. The blocks which were found in this region of the Earth’s crust were broken and elongated in a north to south direction. The blocks under the wide valleys of the desert have been reclined and depress while the others was uplifted and composed the linear mountain ranges.
Matthes (1930) indicated in his work that the characteristic, structure and history of the two mountain systems were known by certain rock formations. The fossil remains preserved in rocks indicate the approximate time of creation. 415,000,000 years ago covered by the Paleozoic era, accumulation of thickness of thousands of feet on the Pacific Ocean’s floor was caused by the outwashing of sediments from the continent. The sediments are then uplifted dry folded forming mountain ranges in the Permian Epoch of the Carboniferous Period of the same era.
During the Mesozoic era, which was 40 million years ago, in the Triassic period, the mountains slowly faded to hills and land sunk below the sea depositing new sediments. During the Jurassic period, as the sea bottom sinks more sediments were laid down. Parallel northwestward-trending mountain ranges came from the folded and crumpled remains of the old and new sediments. The folds below were accumulated with molten granite. In the Cretaceous period which is 75 million years ago, gradual worn down of the mountain ranges and the region resulted to its reduction into lowland.
Streams carried away masses of thousand feet thick granite and sedimentary rock uncovering wide areas. The region together is gradually upwarded to moderate heights. Due to volcanic burst, the land in the northern region was continually covered with mud, mash and rhyolitic lava. In the Oligocene period, Rhyolitic materials were worn away due to continuous erosion. During the Miocene period which was 12 million years ago, the northern half of the region was covered with succeeding flows of andesitic lava and mud due to the beginning of another volcanic eruption.
Irregular minimal crust movement and volcanic eruption happened. After that is a period of stability. The lofty block range with steep eastern front of the Sierra Nevada was caused by the transformed forceful tilting with strong faulting movements along its eastern margin. Pleistocene epoch is known for the great ice age, it was characterized by repeated mantles of glaciers at the higher regions of the range. Twenty thousand years ago up to the present time, is considered as the postglacial time wherein the normal climatic conditions returned.
According to Harris (1996), island arc of volcanoes that was located at the west coast of proto-North America was due to the generation of Farallon Plate subducting beneath the North American plate. When two tectonic plates move and meet toward each other one moving down into the mantle and one sliding beneath is called subduction. During the Jurassic era, volcanism intruded and covered the rocks and these rocks eventually eliminated by the process of uplifted-accelerated erosion. 210 million years ago up to 150 million years before today, regional plutonism occurred.
Plutonism is the process wherein rocks are formed due to volcanic activity. Around the same period of time, Nevadan orogeny occurred. Orogeny is the process of natural mountain building through distinctive structural phenomena. The rocks formed were composed of mostly granite and below the ground measures 6 miles. Uplifting of the Sierra Nevada was due to the vertical movement along the fault. The difference of westerm-flowing streams promoted the uplift of Sierra Nevada. The streams ran faster quickly cutting the valleys. Creation of Owen’s valley was due to the development of major faults to the east.
Sierra’s uplifting accelerated during the Pleistocene. Exfoliation occurred due to the pressure produce by the increased erosion and uplifting exposing the granitic rocks into the surface. Exfoliation is responsible for the rounded shape of many domes in Yosemite National Park. Harris (1996) also stated that about 2 to 3 million years ago, the region has been changed by glaciations. Sherwin, Tahoe, Tenaya and Tioga are the four glaciations which have taken place in Sierra Nevada. The largest glaciers produced are from the Sherwin glaciation while the other stages only form smaller ones.
It was said to be the reason for the largely excavated and shaped of the Yosemite Valley. Figure 1. Geological Map of Yosemite National Park Source: http://geomaps. wr. usgs. gov/parks/yos/yosmap. html
References Hamilton, Calvin. , and Hamilton, Rosanna. Yosemite National Park Rocks. 2006. 14 May 2008 <http:// www. scienceviews. com/parks/yrocks. html. > Harris, Ann. Geology of National Parks 5th edition. Dubuque, Iowa: Kendall/Hunt Pub. Co. , 1997. 2008 17 May 2008 < http://people. uwec. edu/ERICKSKM/references. html>. Hittell, John. Yosemite: Its Wonders and Its Beauties. 1868. 20 May 2008 <http://www.
yosemite. ca. us/library/yosemite its wonders and its beauties/general features. html> Huber, N. King. The Geologic Story of Yosemite National Park. 1987. 17 May 2008 <http://www. yosemite. ca. us/library/geologic_story_of_yosemite/rocks. html. > Lawson, Andrew. Handbook of Yosemite National Park. 1921. 17 May 2008 < http://www. yosemite. ca. us/library/Handbook_of_Yosemite_Nat’l_park/> Matthes, Francois. Geologic History of Yosemite Valley. 1930. US Geological Survey Proffesional Paper 160. 17 May 2008 < http://www. nps. gov/history/history/online_books/geology/publications/pp/160/>
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