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Geology

Basin Topics > Geology

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.[Photo]: View across Lake Tahoe of snow-covered mountains.   Copyright: Larry Prosser

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.

[Photo]; View of mountain tops.   Copyright: Larry ProsserA 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.