Mono Lake is an alkaline and hypersaline lake in Mono County, California. It has an unusually productive ecosystem, and is a critical nesting habitat for several bird species.
North of Mono Lake, in the Bodie Hills, there are 28- to 8-million-year-old andesitic lavas.These lavas, and at least one caldera (Big Alkali Flat), were part of the last phase of subduction zone-related volcanism in the area.At that time, the Sierra Nevada was an eroded set of rolling hills and Mono Basin and Owens Valley did not yet exist. About 5 million years ago, Basin and Range crustal stretching spread to the Mono area.From 4.5 to 2.6 million years ago, large volumes of basalt were erupted around what is now Cowtrack Mountain (east and south of Mono Basin); eventually covering 300 square miles (780 km2) and reaching a maximum thickness of 600 feet (180 m).Later volcanism in the area occurred 3.8 million to 250,000 years ago.This activity was northwest of Mono Basin and included the formation of Aurora Crater, Beauty Peak, Cedar Hill (later an island in the highest stands of Mono Lake), and Mount Hicks.
Mono Lake is believed to have formed at least 760,000 years ago, dating back to the Long Valley eruption. Sediments located below the ash layer hint that Mono Lake could be a remnant of a larger and older lake that once covered a large part of Nevada and Utah, which would put it among the oldest lakes in North America.
At its height during the last ice age, the lake may have been 900 feet (270 m) deep;prominent old shore lines, called strandlines by geologists, can be seen above Lee Vining (near the white “LV”) and along volcanic hills northeast of the current lake.
It is an endorheic basin, the terminal lake in a watershed fed by melting runoff, with no outlet to the ocean. Dissolved salts in the runoff thus remain in the lake and raise the water’s pH levels and salt concentration. The Mono Lake tributaries include Lee Vining Creek and Rush Creek.
The lake is in a geologically active area at the north end of the Mono-Inyo Craters volcanic chain and is close to Long Valley Caldera. Geological activity is caused by faulting at the base of the Sierra Nevada, and is associated with the crustal stretching of the Basin and Range Province.
Volcanic activity continues in the Mono Lake vicinity: the most recent eruption occurred 350 years ago at Paoha Island in Mono Lake. Panum Crater (on the south shore of the lake) is an excellent example of a combined rhyolite dome and cinder cone.