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Long Valley Caldera Field Guide - Mono Lake

A beautiful saline lake with tufa towers.

Geologic Summary:

Mono Lake is the saline remnant of a much larger Pleistocene fresh-water lake known as Lake Russell. The water is currently about twice the salinity of ocean water. In the middle of Mono Lake are two islands, Negit and Paoha, where the most recent volcanic eruption in the area occurred about 300 years ago. The famous tufa towers are on the south shore.

Negit and Paoha were formed by volcanic activity. The light colored section of Paoha is actually lake bottom mud that was pushed up by magma that rose beneath them sometime after AD 1720 (Bailey, 1989). Similar ground deformation (uplift and subsidence) occurs within the caldera and is monitored by the California Volcano Observatory. The darker area on Paoha was erupted about 300 years ago. The dark hat-shaped island, Negit, was formed by an earlier eruption – the hat part is the dome. All of the darker colored volcanic materials on the islands are dacitic and rhyolitic (high silica) lava domes and flows that range in age from about 2000 to about 300 years ago (Stine, 1990).


Lake Russell was once a large lake with shorelines high along the surrounding mountains. The highest remnant shoreline, called a strandline, can be seen along the steep slope west of Mono Lake at 2,188 m (7,180 ft), 143 m (471 ft) above the Mono Lake Visitor Center (Putnam, 1950). In 1941 the surface elevation of Mono Lake was 1,956 m (6,417 ft) before water was diverted to Los Angeles. When water was diverted, the lake level fell and a land bridge provided access to Negit Island. Coyotes used the land bridge to invade seagull nesting sites. The historic low for the lake level was 1,942 m (6,372) ft in 1982 (see http://www.monolake.org/today/water for current and historic lake levels). In 1994 the state of California Water Resources Board modified the city of Los Angeles's water diversion licenses and the water began to rise again. In mid 2009, the water elevation stands about 1945 m (6,382 ft) and the land bridge is once again under water.


The delicate white tufa towers along the shore of Mono Lake are calcareous (calcium carbonate) deposits formed where fresh-water springs percolate through lake-bottom sediments and saline lake water. Calcium in the fresh water combines with carbonate in the saline lake water. The resulting mixed water is saturated with calcium carbonate and the calcium carbonate precipitates (a solid forms due to a chemical reaction), possibly catalyzed by algae, as the minerals calcite or aragonite. The tufa towers formed entirely underwater and have been exposed due to the decline in lake level. New tufa towers continue to form today. Bubbles rising from fresh-water springs indicate where new tufa is being formed (the circular wave patterns in the image were created by the bubbles rising to the surface).

Numerous signs along the well-marked trail through the tufa towers and along the lake shore describe the many fascinating and unique features of the Mono Lake habitat, including its brine shrimp, brine flies, bird life, and Indian and early settlement history. The Mono Lake Marina and the Lee Vining Gas Station Scenic Viewpoint provide additional views of Mono Lake. The Mono Lake Visitor Center includes interpretive information about the area in addition to views.

References

Bailey, R.A., 1989, Geologic map of Long Valley caldera, Mono-Inyo Craters volcanic chain, and vicinity, Mono County, California. U.S. Geological Survey Miscellaneous Investigations Map I-1933, scale 1:62,500. Putnam, W.C., 1950, Moraine and shoreline relationships at Mono Lake, California: Geological Society of America Bulletin, v. 61, no. 2, p. 115-122. Stine, Scott, 1990, Late Holocene fluctuations of Mono Lake, eastern California, Palaeogeography, Palaeoclimatology, Palaeoecology, 78: 333-381

Field Stop Location: Mono Lake State Nature Preserve

Quadrangle: Lee Vining, California 7.5 minute topographic quadrangle
Coordinates: 37°56.458' N, 119°01.086' W at parking lot
Approximate Elevation: 6,391 ft (1,948 m)

Directions to Mono Lake State Nature Preserve:


Directions from Mammoth Lakes exit U.S. 395 and CA-203 Go this distance
Zero your odometer at the intersection of Highway 395 and Highway 203. Head north on U.S 395 towards Lee Vining and Mono Lake. Go 20.3 miles
Turn right onto CA–120 E. Go 4.7 miles
Left toward Test Station Road (37°55.583' N, 119°01.396' W). Go 325 feet.
Take first left onto Test Station Road. Go 0.7 miles
Continue straight to park in the Mono Lake State Nature Preserve parking area. Follow the self-guided trail to the shore and back to the parking area. 37°56.313' N, 119°01.611' W at parking lot (hand held gps) About 0.2 miles to destination