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, making it among the oldest lakes in North America.
Mono Lake is a terminal lake in a watershed fed from melting runoff with no outlet. Dissolved salts in the runoff thus remain in the lake and raise the pH and the salt concentration.
Mono Lake is in a geologically active area at the north end of the Mono-Inyo Crater volcanic chain of the Long Valley Caldera. The 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 250 years ago at Negit 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.
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