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
