Geology of Lake Tahoe: The Lake Tahoe Basin was formed by a geologic block (normal) faulting. A geologic block fault is a fracture in the Earth's crust causing blocks of land to move up or down. Uplifted blocks created the Carson Range on the east and the Sierra Nevada on the west. Down-dropped blocks (a graben) created the Lake Tahoe Basin in between.
More technically, Lake Tahoe is the youngest of several extensional basins of the Walker Lane Deformation Belt that accommodates nearly 12 mm/yr of dextral shear between the Sierra Nevada Microplate and North America. The Lake Tahoe basin is formed by a series of large down-to-the-east normal faults, including the West Tahoe—Dollar Point fault, Stateline/North Tahoe fault and the Incline Village fault. These right-stepping en-echelon faults are capable of large magnitude 7 earthquakes, with the most recent M7 paleoquake (~1500 AD) occurring on the Incline Village fault with nearly 9.7 ft (3.0 m) of vertical offset. The West Tahoe-Dollar Point Fault (WTDPF) appears to be the most active and potentially hazardous fault in the basin. A study in Fallen Leaf Lake, just south of Lake Tahoe, used seafloor mapping techniques to image evidence for paleoearthquakes on the WTDPF and revealed the last earthquake occurred between 4,100-4,500 years ago.
Some of the highest peaks of the Lake Tahoe Basin that formed during process of Lake Tahoe creation are Freel Peak at 10,891 feet (3,320 m), Monument Peak at 10,067 feet (3,068 m), Pyramid Peak at 9,983 feet (3,043 m) (in the Desolation Wilderness), and Mount Tallac at 9,735 feet (2,967 m).
Eruptions from the extinct volcano Mount Pluto formed a dam on the north side. Melting snow filled the southern and lowest part of the basin to form the ancestral Lake Tahoe. Rain and runoff added additional water.
Modern Lake Tahoe was shaped and landscaped by scouring glaciers during the Ice Ages, which began a million or more years ago. Lake Tahoe is fed from 63 tributaries with the Truckee River as the only outlet. The Truckee flows northeast through Reno, Nevada and into Pyramid Lake, Nevada which has no outlet.
Soils of the basin come primarily from andesitic volcanic rocks and granodiorite, with minor areas of metamorphic rock. Some of the valley bottoms and lower hill slopes are mantled with glacial moraines, or glacial outwash material derived from the parent rock. Cryopsamments, Cryumbrepts, rockland, rock outcrops and rubble and stony colluvium account for over 70% of the land area in the basin (see USA soil taxonomy). The basin soils (in the < 2 mm fraction) are generally 65-85% sand (0.05–2.0 mm).
Given the great depth of Lake Tahoe, and the locations of the normal faults within the deepest portions of the lake, modeling suggests that earthquakes on these faults can trigger tsunamis. Wave heights of these tsunamis are predicted to be on the order of 10 to 33 ft (3 to 10 m) in height, capable of traversing the lake in just a few minutes. A massive collapse of the western edge of the basin that formed McKinney Bay around 50,000 years ago is thought to have generated tsunami/seiche wave with height approaching 330 ft (100 m).
