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Yosemite National Park - Geology
The Yosemite National Park and Yosemite Valley is one of the most unique natural
sites in the United States of America. It is situated in the Sierra Nevada Range
of East Central California. The original land was given to the state of
California in 1864 and the park was established on October 1, 1890. (Encarta,
2002) The American naturalist John Muir was the first to explore and realize the
scenic beauty of the valley in 1868 and afterward played a key role in the
creation of the Yosemite National Park. Before 19th century, the area was
inhabited by a Native American race, the Miwok. Before Muir, Captain Joseph R
Walker and his team explored the area during 1830s. In 1849 a group of miners
entered the valley and were confronted by the native Miwok people. In 1864,
President Abraham Lincoln handed over the Yosemite Valley and the Mariposa grove
of sequoias to California. At that time the size of the park was 1,512 square
miles. Initially the two areas remained under state control and were not
included in the original park area. The federal government granted these areas
back in 1906 and was then added to the park. The size of the park was shrunk
back to its present size.
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The park is stretched on an area of 1200 square miles and the Yosemite Valley is
7 miles in length and 1 mile wide. The geological history of the area can be
traced back to 500 million years. Due to its extraordinary landscape and unique
geological features, the area is known as “classic glacial valley” and
“Incomparable Valley”.
Bedrock Geology and Glacial History
In ancient times i.e., about 500 million years back, the region of Sierra Nevada
lied under an ancient sea. In a slow natural process, small peaces of rock or
sediments started depositing in the sea. Gradually those sediments formed layers
that thickened with the passage of time and the lower layers converted into hard
rocks. About 200 million years ago, a part of the Pacific plate slid under the
North American plate. In the process known as subduction, the lower Pacific
plate was exposed to incredible pressure and heat inside the earth and was
melted in to magma, a hot liquid rock found below the surface of earth. In the
next 100 million years, the magma rose towards the surface of earth and was
cooled down to from a he block of granite stone. This granite now forms the
entire topography of the Yosemite Valley. The current land of Yosemite National
Park was formed 50 million years ago including the hills, stream, river, and
forests. The Sierra Nevada was formed 10 million years back. A block of rock
tilting westwards increased the flow of the previously slow-moving river Mercid.
The river water carved and shaped the valley into a canyon. Hardwood forests
were grown about 3 million years back. The canyon carved by river Mercid was
about 3000 feet in depth. The tributaries of the river, however, cut the land
more slowly. The thick forests were affected during the glacial period or Ice
Age. The V-shaped valley was filled by the glaciers and turned it into U-shaped,
some 250,000 years back. In the process, some hanging valleys were formed, from
where waterfalls took their origin. The Yosemite glacier appeared 30,000 years
ago. The increased earth temperature in the following years made the Yosemite
glacier melt, 10,000 years back and the rock debris or moraines deposited in the
valley to form Yosemite Lake. Evidences shows that about 600 meters (2,000 ft)
of sedimentary rocks filled in the parts of the valley. (Wahrhaftig, 1962)
Over the time, sediments kept filling the lake and currently there is a flat
valley floor in place of the lake. Even now the sedimentation is going on in
Mirror Lake that is likely to fill and change it into a meadow in future. The
Yosemite Valley is commonly known as the “classic glacial valley”. It was John
Muir who studied the glaciers and proved their influence in shaping the present
day Yosemite. Yosemite Valley is probably the world's best-known example of a
glacier- carved canyon. Its leaping waterfalls, towering cliffs, rounded domes,
and massive monoliths make it a preeminent natural marvel. “The natural history
of Yosemite spans many millions of years starting from the ancient age when a
warm, shallow sea spread across what is now the Sierra Nevada and Great Valley
of California. The first of at least three glaciers extended down the Merced
River canyon as far as El Portal, while the last left a moraine of rock debris
damming the Merced River back into Yosemite Valley. Sediments that subsequently
filled the lake form the level valley floor of today.” (centralvalley.com)
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Yosemite Valley is known for the sheer walls and a flat floor. Its development
started when alpine glaciers hobbled through the canyon of the Merced River. The
ice carved through weaker sections of granite and shaped and polished the rocks.
Some of the hard rocks were left intact that are visible now. Among these rocks
are El Capitan and Cathedral Rock that had enlarged the canyon carved by river
Merced. The glaciers finally melted and the terminal moraine left by the last
glacial advance into the valley dammed the melting water to form ancient Lake
Yosemite, which is located in the newly carved U- shaped valley. The residue
rock slowly filled the lake and transformed it into a flat valley floor. The
geological process through which Yosemite had (is) gone (going) through is a
natural and normal one but has transformed the valley into a very unique
structure. The glaciation has also given birth to different kinds of rocks.
Following is the account of the different geological forms of rocks found in the
valley.
Batholiths
The word batholith finds its origin in the Greek words bathos meaning deep and
lithos for rock. Batholiths are huge intrusions of granite, diorite and
granodiorite. Their size varies from 40 to 400 square miles. In Yosemite,
geologists are of a view that batholiths emerged about 30 million years ago. For
millions of years, erosion cut the V-shaped gorges and the glaciers filled them
into U-shaped valleys. Other speculations say that the granite batholiths were
formed 25-15 million years back. The intrusion occurred when Merced and Tuolumne
rivers cut deep channels in the bedrock. The channels were further deepened due
to glaciations and erosion 2 million years ago. In the process, Yosemite was
divided into two valleys.
Plutons
The molten magma was formed as a result of deep earth heat and pressure and
afterwards rose upwards towards the surface of earth. There it cooled down to
form the walls and domes of the Yosemite Valley area. This cooling and
crystallization of deep-seated magma took millions of years. The rock formed in
the process is composed of interlocking crystals of several kinds of minerals
and are called plutonic rock. The name is derived from Pluto, the Roman god of
the underworld. These plutonic rocks of Yosemite are formed during a long time
period expanded over millions of years. Some of these plutonic rocks have become
visible because of the soil erosion and the removal of other older rocks. The
intrusion of the plutons has occurred before and alongside this transformation.
A few remains of pre-plutonic rocks remain near the Yosemite Valley and several
small masses can be seen on the flanks of Sentinel Dome and in Indian Canyon. A
large number of plutons found in the Yosemite Valley area, as well as the entire
Sierra Nevada, are also called the Sierra Nevada batholiths.
Different plutonic rocks are known by different names according to their
characteristics. The plutons exposed in the walls of Yosemite Valley were
intruded over a period of about 30 million years during part of the Cretaceous
Period. The Cretaceous Period is about 100 million years ago when dinosaurs
inhabited the earth. Hundreds of plutons forms the batholiths of Sierra Nevada
and are formed during a process extended over a period of more than 130 million
years. (Calkins, 1930)
The ages of different plutons are determined through certain field relations.
For example, hot magma associated with younger plutons commonly intrudes older
plutons along cracks, or fractures, and solidifies to form sheets, or dikes,
along those fractures in the older rocks. Also, younger plutons mostly carry
inclusions, or fragments, of the older rock, which became fixed in the
still-molten magma. In order to determine the specific age of a given rock in
million of years requires measurement of the amount of radioactive decay of
certain elements such as uranium, thorium, potassium, and rubidium. From these
measurements and the known rates of decay, one can guess the amount of time
passed since the rock crystallized or cooled below a temperature that allowed
the radiometric decomposition to start.
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Dikes
“Dikes are wall-like intrusion of igneous rock, cutting across other strata of
preexisting rocks, originally formed by a flow of molten rock into a fissure in
which it cooled and solidified. A dike may range from a few centimeters to
thousands of meters thick and from a few meters to many kilometers long.
Frequently the rock material of the dike is harder than the surrounding rocks,
and as a result it may be left standing by itself after the neighboring rock has
weathered away. Similar intrusions of igneous rock that lie parallel to the
enclosing layers are known as sills.”
In Yosemite Valley dark fine-grained diorite intrudes the El Capitan and Taft
Granites. On the east face of El Capitan dikes of diorite form an irregular
pattern. Dikes of the Sentinel Granodiorite that cut inclusions of the El
Capitan Granite are found in the road cut along the Glacier Point Road near the
Taft Point trailhead. Horizontal dikes are present at Church Bowl and in the
cliff west of Royal Arches. These horizontal dikes of the Half Dome Granodiorite
cut the older Sentinel Granodiorite. The hot magma associated with younger
plutons commonly intrudes older plutons along cracks, or fractures, and
solidifies to form sheets, or dikes, along those fractures in the older rocks.
Moraines
Moraines are formed at the sides and front of a glacier. The ice flows downwards
carrying the debris or sediments from the glacier. When it reaches the sides and
front of the glacier, the ice melts away leaving the deposited debris. This
deposited layer is called moraine. In the Yosemite Valley, a number of moraines
are found. Famous among them is the terminal moraine of Tiogen glaciations at
the east end of the Bridalveil meadow. (Matthes, F) The Bridalveil moraine is a
boulder of Cathedral Peak Grandiorite of about 8 feet across. (Peck, D.L.)
Medial moraine is lateral or recessional moraine of the glacier that occupied
Tenaya or Merced river Canyons. The El Capitan moraine is larger than the
Bridalveil moraine.
Cirques
Glacial valleys consist of several features formed during the glacial process
throughout the centuries. Cirques are bowl shaped valleys formed at the top of
the glaciers. Ice melts away in these bowls and from small lakes. The Ice Age
glaciers appeared in Yosemite around 2 million years ago and finally subsided
about 10,000 years back. The actual glaciers though, found in Yosemite are not
the remains of the Ice Age of 10,000 years back. The cirques are found there and
the glaciers are a product of the period of Little Ice Age from 1700 to 1750.
Volcanic activity
The volcanic activity in the Yosemite ran side by side along with other
geological processes and has shaped the present state of the area. As described
earlier, the sedimentation of the sea began 150 million years ago. At that time
the huge quantity of lava and volcanic ash was discharged due to heavy volcanic
eruptions and covered the sedimentary rocks.
“The layered sedimentary and volcanic rocks were then deformed and, in many
places, folded intricately, changing or metamorphosing them into tougher, very
resistant metamorphic rocks.” (vulcan.wr.usgs.gov)
The layered and molten rocks combined with each other in presence of high heat
and pressure deep inside the earth. Out of this some magma reached the surface
and formed new volcanoes. This intrusive activity continued till about 80
million years ago, in the Cretaceous period. The molten rock cooled slowly and
in time solidified into the granite core of the Sierra Nevada. The process ended
in the formation of a mountain range in Sierra. The river channel in north of
Minaret Summit got filled with flowing lava, 3 million years back and block the
channel.
Joints
“The bedrock structures having the greatest effects on Yosemite’s landform
development are joints. Although granitic rock is unbroken on a small scale, on
a larger scale the rock is broken by joints, which are more or less planar
cracks commonly found as sets of parallel fractures in the rock. Regional-scale
joints commonly determine the orientation of major features of the landscape,
such as the planar face of Half Dome, the series of parallel cliffs at Cathedral
Rocks, and the westward sloping faces of the Three Brothers. In contrast,
smaller, outcrop-scale joints determine the ease with which rock breaks and
erodes. Joints are of overwhelming influence on landform development in granitic
terrain because they form greatly contrasting zones of weakness in otherwise
homogenous, erosion-resistant rock and allow access for water and air to enter
and aid in the weathering and disintegration of rock.” (aqd.nps.gov)
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The most important structure that influences the erosional form of plutonic
rocks consists of joints. Joints are mostly found as sets of parallel fractures
or cracks in the rock. During a course of 80 million or more years, these rocks
crystallized and cooled down, uplifted and eroded into mountain. There they have
been subjected to many disorderly forces and are now broken by a variety of
joints. Landforms in and near the Yosemite Valley reflect the influence of
several types of joints, which occur at both outcrop and regional scale.
Erosion and mass wasting
Erosion is a natural process in which rocks and soil material is removed mainly
by running water, glaciers, waves, and winds. In Yosemite Valley and its
neighboring uplands, the forces of erosion have uncovered, with extraordinary
clarity, a highly complex collection of granitic rocks. A number of plutons are
now exposed at the Earth’s surface due to erosion of the once overlying older
rocks. The erosion through stream water and by glaciers is mainly responsible
for the formation of major landforms of the Yosemite Valley. (Matthes, F, 1930)
The large-scale erosion throughout the centuries has shaped the geology of the
Yosemite Valley and gifted it with many features such as magnificent waterfalls,
cliffs, and domes. These rock models are the outcome of erosion that occurred
and reveal and shaped the rocks into different sculptors. The great variety of
erosional features visible today such as the splendid walls; promontories and
domes of the valley are the result of the varied composition and structure of
rocks from which they were carved.
Works Cited
America's Volcanic Past - Sierra Nevadas, 09/19/02, http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_sierra_nevadas.html
Calkins, F. C., 1930, The granitic rocks of the Yosemite region in Matthes F.
E., Geologic history of the Yosemite Valley: U.S. Geological Survey Professional
Paper 160, p. 120-129.
Dike. Encarta Encyclopedia 2002. 1993-2001 Microsoft Corporation
Huber, N. King. The Geologic Story of Yosemite Valley, 9/7/00, http://www.aqd.nps.gov/grd/usgsnps/yos/topobk.html
Matthes, F., Geologic History of the Yosemite Valley, U.S. Geological Survey
Professional Paper 160, 1930.
Matthes, F. E. The Incomparable Valley; A Geologic Interpretation of the
Yosemite: Berkeley, University of California Press, 1950
Peck, D.L., Wahrhaftig, C., and Clark, L.D., 1966, Field Trip: Yosemite Valley
and Sierra Nevada Batholith, in Geology of Northern California, California
Division of Mines and Geology, Bulletin 190
Wahrhaftig. Geomorphology of the Yosemite Valley region: California Division of
Mines and Geology Bulletin 182. 1962. pp. 33-46.
Yosemite National Park. Encarta Encyclopedia 2002. 1993-2001 Microsoft
Corporation
Yosemite Park History, November 23, 2001, http://www.centralvalley.com/portal/parks/yosemite/resources/story/1695188p-1770706c.html
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