About 2 million years ago, a shift in tectonic plates caused the Tahoe Basin to drop down between the Sierra crest to the west and the Carson range to the east. Volcanic activity, also caused by the tectonic shifts, led to expulsion of magma up through the faults, filling in gaps and damming the valley.
Ten thousand years ago, at the time of the last ice age, individual glaciers formed at the area’s highest elevations, on the north, west and south ends of the Lake. Movement by the glaciers scoured out basins and formed Donner Lake, Emerald Bay on Lake Tahoe and Fallen Leaf Lake, which sits up and to the West of Lake Tahoe.
Today, erosion and weathering is causing a slow, undetectable lowering of the Sierra, the mountain range that houses Tahoe. It is believed that Lake Tahoe is filling in with sediment at a rate of one foot every 3,200 years, so that in 3,158,400 years the lake will be replaced by a meadow.