The Formation of The Sierra Nevada Mountains
During the Paleozoic Era (575 to 270 million years ago) the region now occupied by the Sierra Nevada Mountains lay beneath the sea receiving sediments from the North American continent to the east. Tens of thousands of feet of sediments formed sedimentary rocks and extended the shoreline to the west. Toward the end of the Paleozoic Era the North American continental plate began to drift away from the super-continent of Pangea and moved westward. It began to override the Pacific Ocean Plate that was drifting eastward. The Pacific plate was forced to dive underneath the continental plate. The incredible pressure and friction melted portions of both the Pacific plate and the North American plate and granites distilled which rose to intrude the overlying sedimentary and metamorphic rocks. This plug of magma eventually cooled and solidified to form the granites exposed as the Sierras today. Pushing, grinding, heat and pressure continued to lift and fold the Sierra area until about 10 million years ago. The old sedimentary rocks and volcanic rocks were transformed by heat and pressure into a new form called metamorphic rocks. Today, throughout California, sedimentary, metamorphic and volcanic rocks can be found in a variety of locations, relationships, and formations.
Beginning about 130 million years ago, through erosion, the block of granite that was to become the Sierra became exposed to the elements and began to erode. To account for the vast amount of eroded sediments found in the Central Valley, the pre-Sierra mountains must have been at least 15,000 feet high before finally being eroded into gently rolling uplands about 65 million years ago.
About 30 million years ago, an era of volcanism began in the Sierras that was of massive proportions by today’s standards. Here in the Northern Sierra around Lake Tahoe, the Sierra was covered by thick layers of volcanic ash and volcanic rock (andesite and rhyolite) expelled by the volcanoes.
In the middle of this era, about 10 million years ago, the Sierra began uplifting. Staggered, parallel faults formed along the eastern edge of the range. The area to the west rose, and to the east, what is now the Carson Valley, dropped. Even though the eastern slopes of the Sierra rise sharply from the Carson Valley, the valley has filled with sediments obscuring the real consequences of this uplift. While the mountains rise about 9,000 to 11,000 feet above the valley, total uplift was about 19,000 feet!
The Formation of Lake Tahoe
The Tahoe Basin, like the Carson Valley, has dropped between two uplifted blocks, The Sierra crest on the west and the Carson Range on the east. This is a relatively recent development, occurring within the last several million years. Magma generated by the pressures and temperatures that also caused the faulting and uplifting welled up through gaps in the faults. A prominent area of this volcanic area occurred just north of the lake. Andesite flows from these vents bisected and dammed the valley. Eventually, as the lake rose the Truckee River was able to cut through these flows and find its present course around the volcanics to the lowlands of Nevada. Subsequent glacial (2 million to 20,000 years ago) action just downstream of the lake (from the Alpine Meadows and Squaw Valley canyons) dammed the river and so the level of the lake has fluctuated drastically over time. The maximum lake level during glaciation approached 800 feet higher than its present level. Large sedimentary terraces perched above the lake remain as evidence of the old shore.
Three major periods of glaciation occurred in the northern Sierras during the last ice age (10,000 years ago). Rather than the regional "ice sheets" that covered much of North America the ice age manifested itself as individual glaciers forming at the highest elevations. These glaciers carved out individual valleys during their downward movement. Donner Lake, Emerald Bay on Lake Tahoe and Fallen Leaf Lake have the elongated shapes characteristic of glacial valleys. The ice dams across the Truckee River canyon floated several times and broke apart releasing walls of water that carried immense boulders downstream which are now found along the Truckee River canyon and in the Reno area. These floods also carved through the glaciers surrounding Truckee and eroded channels through their glacial debris.
In the future… Mountain building processes have ceased so in the near future (relatively) we can expect continued weathering, erosion and subsequent lowering of the Sierra. Lake Tahoe will continue to fill in at the rate of one foot for every 3200 years, becoming a meadow in about 3,158,400 years (989’ average depth time 1ft/3200 years).