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A
Brief History of the Lake Tahoe Geology:
PART
I: Creation of the Sierra Rock:
The
geologic history of Lake Tahoe began nearly 400 million years ago when
a shallow sea covered the area that is now the Sierra Nevada Mountains.
For hundreds of millions of years, mud, sand, and shells were slowly
deposited on the floor of this ancient sea. The weight of the thousands
of feet of sediment combined with chemical processes turned the mud,
sand, and shells into mudstone, sandstone, and limestone rock.
At
the same time, about 210 million years ago, the Pacific Plate (the sea
floor underneath the Pacific Ocean) crashed into the edge of the North
American Plate (the land and continental shelf of North America) and
was pushed down under the North American Plate. This process, known
as subduction, is how most volcanoes and all granitic rock is formed.
As the ocean plate is forced down, the increased temperature and pressure
begin to melt the rock. Eventually large amounts of rock are in a liquid
(or molten) state and begin to rise to the surface in the same way balloons
filled with hot air rise. In the case of the Sierra Nevada, these inverted
teardrop shaped bubbles of liquid rock (called plutons) did not reach
the surface but instead cooled slowly underground. This process was
repeated hundreds of times over 130 million years (till about 80 million
years ago) creating a series of stacked plutons thousands of feet below
the surface of the Earth four hundred miles long and 30 miles wide.
This slow cooling allowed visible crystals to grow in the newly formed
rock creating the salt and pepper granitic rock we see in much of the
Sierra Nevada.
As
the plutons of molten rock shouldered their way towards the surface,
they came in contact with the sedimentary rock of the shallow Sierra
sea. All along the tops and edges of the plutons, the sandstone, mudstone,
and limestone rock partially melted allowing new types of rock (called
metamorphic rock) to form in its place. This metamorphic rock is much
harder than the original sedimentary rock and can still be seen today
in many areas as caps on top of the Sierra grantic rock. The dark rock
of Mount Tallac is a good example of this metamorphic "roof pendant".
Part
II: Age of Fire:
The creation of the Sierra batholith and the metamorphic
rock on top of it was just the first part of this saga. Next, about
20 million years ago, mountains of fire, ash and lava formed along the
eastern side of the Sierra layering down hundreds of feet of ash, glassy
lava rock, and mixed up breccia. Although, most of the rock formed during
this early volcanic episode is now eroded, in the Tahoe area, evidence
can still be found in the weathered tufts above the Big Meadow Trail
towards Round Lake and at the Carson Spur on Highway 88.
Part
III: Building the Mountains:
The
land had been slowly rising and the sea receding for millions of years
before the first volcanic eruptions. In fact, it is believed that a
pre-Sierran range existed around 150 millions years ago with elevation
as high as 15,000 feet. Through the process of erosion, that range was
slowly reduced to rolling hills over the next 80 million years allowing
layer upon layer of sedimentary, volcanic, and metamorphic rock to be
stripped off and carried west to the valley below. Uplift of the present
Sierra Nevada is believed to have begun about 10 million years ago.
A fault along the eastern margin of the buried granitic rock caused
the batholith to rise rapidly. This process, called tilt-block faulting
caused the eastern edge of the mountains to be precipitous, while the
western side was, and still is much more gentle. This is evident today
in the Tahoe area. Driving up from Sacramento, the road rises slowly,
but steadily over a hundred miles from 50 feet to nearly 10,000 feet
at Pyramid Peak. Traveling from Tahoe to the Carson Valley, however,
one goes from 10,000 feet at Monument Peak to the Carson Valley floor
at 4500 feet in just over 3 miles! This pattern holds true up and down
the Sierra Nevada.
Part
IV: The Making of the "Tahoe Valley":
The
fault on the eastern side of the Sierra was not the only active fault
in the region. About 10 million years ago, two additional faults conspired
to create a deep valley where Lake Tahoe now sits. These faults were
located on either side of what is now Lake Tahoe and caused the floor
of the valley to drop thousands of feet in relation to the mountain
ranges to the east and west. To the East, the Carson Range towered above
while to the west, the Crystal Range formed (see figure 4). This must
have been an incredible valley with steep canyon walls and a river at
its bottom that cut down through the Sierra granite over millions of
years. The headwaters of this ancient river would have started in the
south and drained to the north.
Part
V: Blocking of the Tahoe River:
After
the formation of the great river valley, volcanic activity again shook
the Tahoe area. Mount Rose is an extinct volcano as is Mount Pluto.
The latter volcano and the lava flows that issued from its chambers
blocked the river and created a transverse mountain range that linked
the Carson Range in the east with the Crystal Range to the west. With
no way to escape, the water began to fill in the deep canyon slowly,
imperceptibly. But fill it did from rain water and snow melt for thousands
of years. Old waterlines on rocks high above the basin suggest that
at one time the water level was 700 to 800 feet higher than it is presently.
The water finally found an escape once more to the east of Mount Pluto,
only to have it blocked again by another lava flow. The next outlet
was found to the west near the current outlet on the northwest corner
of the lake. This remains the only outlet for Lake Tahoe.
Part
VI: Days of Ice
The
final chapter in this saga tells the story of the last ice age. Starting
three million years ago and ending just ten thousand years ago, periods
of cooling interspersed with warmer times allowed the ice cap to extend
down into the United States at several times. While the glaciation in
the Sierra Nevada is not connected with these polar ice sheets, the
elevation in the Sierra Nevada, along with the temperature decrease
caused glaciation throughout the range. Evidence of this glaciation
is seen throughout the Tahoe region. Fallen Leaf Lake and Cascade Lakes
were formed when piles of rock (called terminal moraines) pushed along
in front of the glacier were left to create a dam behind which the lakes
formed. The precipitous ridge the road to Emerald Bay follows with Emerald
Bay on one side and Cascade Lake on the other is a textbook example
of a lateral moraine left by the receding glacier. Examples of glacial
polish, the shiny, smooth rock surface that forms when tons of ice pass
over and polish granitic rock, can be seen near Cascade Falls. Many
fine examples of deep, glacially formed lakes and steep mountain cirques
can be found in Desolation Wilderness. These include, Half Moon, Ralston,
and Eagle Lakes and the peaks that surround them.
Much
of the information used to compile this brief geologic history came
from: The Sierra Nevada, A Sierra Club Naturalist's Guide by
Stephen Whitney and Geology of the Sierra Nevada by Mary Hill.
Lake
Tahoe's Ecology:
This
section will provide a very brief introduction into the flora (plants)
and fauna (wildlife) of the Tahoe Basin and surrounding environs. It
is by no means exhaustive! Those wanting a more in-depth discussion
are directed to two wonderful books on the natural history of the Sierra
Nevada (including Lake Tahoe): A Sierra Club Naturalist's Guide
by Stephen Whitney and Sierra Nevada Natural History by Tracy
Storer and Robert Usinger. Both of these texts provide a wealth of information
on the natural environment of the Sierra Nevada.
Naturalists
group plants and animals that are commonly found together into plant
and animal communities. These are man-made constructs that allow scientists
to more easily study and discuss the natural environment, and there
is much overlap between communities, especially animal communities.
The Sierra Nevada has as many as 13 defined plant communities (depending
on which classification you use), but in and around Lake Tahoe, only
five are common. These are from west to east: Red Fir Forest, Lodgepole
Forest, Subalpine Forest, Alpine Meadow and Rock, and Pinyon-Juniper
Woodland.
Red
Fir Community:
Native
Americans of Lake Tahoe:
More Coming Soon
The
First Explorers:
More Coming Soon
Lake
Tahoe Settlers:
More Coming Soon
Current
Developments and Issues at Lake Tahoe:
More Coming Soon
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