Geologic Wonders

How was Lake Tahoe and its basin formed?

The basin formed at least two million years before the lake. Many people think Tahoe was formed by giant glaciers; however glaciers came along too late to form the massive Tahoe Basin. Others have heard it was once a huge volcano. Volcanoes did not form the basin, but like glaciers, volcanic activity played a part in the formation of the Tahoe Basin.

Around five million years ago a very active period of mountain building took place. Large blocks or plates were pushed up forming the crest of the Sierra Nevada. Where the basin is today, two parallel blocks pushed up, one on the west, another to the east. Between these, another smaller block slipped down. This created a very deep and steep sided valley basin, open to the north.

The open north end allowed snowmelt to flow out of the basin, and then through valleys towards the east and down to the Great Basin. The "valley basin" changed into a "lake basin" some two million years ago when several large volcanic eruptions occurred. One of these, called Mount Pluto, poured lava and volcanic mudflows into the northern outlet of the basin, blocking it up. Over time, snowmelt and rain filled the basin, creating one of America's most famous and beautiful alpine lakes.

Glaciers would later help shape some areas of the western slopes on the California side, with the most notable traces being Emerald Bay and Fallen Leaf Lake. We can also see traces of the volcanic period in the form of volcanic "plugs." These are the hard solidified cores now exposed by erosion. Cave Rock and Shakespeare Point along the southeast shore are examples, easily seen today along Highway 50.