The Geology of Lake Tahoe
As we covered in Part 3 of this series, the Sierra Nevada mountains have a rich geologic history that unfolded in distinct phases over a period beginning more than 100 million years ago and continuing to the present. One of the many cherished natural wonders of the region is Lake Tahoe, whose special beauty is world famous. Its formation is closely intertwined with that of the Sierra, yet it has its own unique story that has included dramatic and violent events.
The main range of the Sierra Nevada mountains exists entirely within the state of California, stretching from Southern California’s “Grapevine” pass to just south of Lassen Peak near Susanville. The Carson Range that forms the western boundaries of the Truckee Meadows and Carson Valley is considered to be a branch of the Sierra. In previous articles we discussed this range and the north-south parallel faults of the Basin and Range region. The westernmost boundary of that region is the West Tahoe Fault running under the lake near its western shoreline. The main crest of the Sierra rises to the west. The basin east of the fault includes the lake itself and slopes upward to the crest of the Carson Range, with high points just under 11,000 feet at Mount Rose , Freel Peak and Jobs Peak .
Left: Location of major faults under Lake Tahoe. The dominant vertical motion is along the West Tahoe fault.
Right: Schematic showing the relative motion of the western and eastern blocks along the West Tahoe fault.
This basin began to form about 4 million years ago, but for the first half of that period it was probably more of a wetland than a lake. The eastern fault block steadily subsided at an average rate of only a centimeter or two per year, while the erosion of the surrounding highlands filled the basin at approximately the same rate. Everything changed dramatically about 2.3 million years ago, when the first of several volcanic events poured large volumes of lava and mud into the Truckee River valley north of present-day Tahoe City. The origin of these flows was Mount Pluto , where the Northstar ski area is now located. As the lave cooled into formed a natural dam that formed the ancestral Lake Tahoe. Two additional eruptions occurred between then and 940 thousand years ago. Each time, the lake level rose dramatically, but this was followed by the erosional breaching of the natural dams and violent flooding of the Truckee River valley down to the site of Reno. If this were to occur today, the destruction would certainly be catastrophic. Fortunately, the volcanism appears to have subsided. If it were to return it would most likely be thousands of years in the future. Geologists have actually been able to identify these successive extreme rises and falls in the lake level from the remnant shorelines seen on the surrounding slopes, a much as 650 feet higher than today.
Glaciation has been a major factor in shaping Lake Tahoe and the surrounding landscapes. Evidence for multiple periods of glatiation down to lake level has been found, dating from over one milion years ago to as recently as 15,000 years. A number of well-known physical features were shaped by these events, including Emerald Bay , Cascade Lake , and Fallen Leaf Lake . The more recent of those glaciations, seen below, did not extend to lower elevations but did reach the western shore of the lake and is reponsible for more numerous bays that are found on the Nevada side.
Perhaps the most dramatic event in the lake’s history was only recently confirmed by scientists reviewing detailed sonar studies in the 1970’s and 1990’s. Huge, jagged boulders up to a half mile in size were seen on the bottom extending from McKinney Bay , south of Tahoe City, almost across the width of the lake to the Nevada side. It is believed that a strong earthquake occurred between 12,000 and 21,000 years ago, triggering a massive landslide above the bay. About three cubic miles of rock and earth cascaded into the lake! The bay itself was enlarged by the displacement. Impacts of the resulting tsunami, which may have been 300 feet in height, can still be seen at various points around the lake. It would have crossed the lake in about 4 minutes, reverberating like ripples in a bathtub for perhaps 30 minutes or more.
Like many spectacular landscapes in the western United states, Lake Tahoe’s beauty and serenity belies its violent past. The crystal-clear waters and green forested mountains were not always so. We are fortunate to live in this time when we can enjoy what nature has created from all the turmoil. The main threats today come from human population but now more than any time in the past, much attention is being paid to its preservation.