The Lake Tahoe Basin began to form around 670 million years
ago and at that time, the area that is now the Basin was a
shallow sea and part of the super continent Pangea. Sediment
slowly deposited on the floor of the shallow sea for hundreds
of millions of years creating a block of sedimentary rock.
Nearly 210 million years ago the North American Continental
Plate broke off from Pangea and began to drift west. At the
same time, the Pacific Ocean Plate beneath the ocean began
to drift east. The plates crashed together and the pressure
pushed the Pacific Ocean Plate under the North American Plate.
Over the next 130 million years the increased pressure and temperature from
the colliding plates caused rock to melt and form plumes of
lava that began rising toward the surface of the earth thousands
of feet below the sedimentary rock. The long, slow cooling
process allowed crystals to form and create the granitic rock
seen in the Sierra Nevada. Eventually the lava plumes reached
the block of sedimentary rock under the shallow sea. Tops
of the lava plumes pushed through the rock leaving outcrops
of altered sedimentary rock, called metamorphic rock.
Then, approximately ten million years ago, the Sierra Nevada granitic rock
broke through the surface of the earth after an active fault
along its eastern side caused it to rapidly rise. Two additional
active faults on the east and west side of what is now Lake
Tahoe created a valley floor that dropped thousands of feet
below the mountain ranges. The earthquakes split the Sierra
Nevada into the Carson Range on the east and Crystal Range
on the west of Lake Tahoe. Some metamorphic rock can still
be seen today as a darker rock crowning mountain peaks in
the Basin.
A massive river soon flowed through the Lake Tahoe valley
floor with headwaters at the south and an outlet at the north.
Mt. Pluto, an extinct volcano north of Lake Tahoe, produced
a lava flow that connected the Carson and Crystal ranges and
blocked the outlet of the river. Over time, the valley filled
and the Truckee River found an outlet located at the northwest
corner of Lake Tahoe in Tahoe City.
The last ice age started approximately three million years
ago and ended ten thousand years ago. The Sierra Nevada was
not affected by continental ice sheets but did experience
mountain glaciations. Dams were created by glaciers pushing
rocks into piles that formed areas like Emerald Bay, Cascade
Lake and Donner Lake in the Lake Tahoe Region.
Sources:
Tahoe Adventure Sports Website
The
Sierra Nevada: A Sierra Club Naturalist´s Guide by Stephen
Whitney
Geology
of the Sierra Nevada by Mary Hill
