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The 16 x 32 km (20 x 10 mi) Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption (considered a "supereruption") about 760,000 years ago.
Location: California, Mono County
Latitude: 37.7° N
Longitude: 118.87° W
Elevation: 2,600 (m) 8,530 (f)
Volcano type: caldera
Composition: basalt to rhyolite
Most recent eruption: 16,000-17,000 years ago
Nearby towns: Mammoth Lakes
Threat Potential: Very High
Resurgent doming in the central part of the caldera occurred shortly after the caldera-forming eruption. During early resurgent doming, the caldera was filled with a large lake that left lake-shore traces (strandlines) on the caldera walls and the resurgent dome peninsula; the lake eventually drained through the Owens River Gorge.
Along the caldera's ring fault, Mammoth Knolls is the youngest eruption about 100,000 years ago. In the topographic basin, Cone 2652 in West Moat is about 33,000 years old and dacite lavas in NW Moat are 40,000-27,000 years old. The mafic chain along the west rim is 16,000 to 17,000 years old. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. A robust geothermal system inside the caldera fuels the Casa Diablo power plant, which generates enough power for 40,000 homes.
The late-Pleistocene to Holocene Mono-Inyo Craters, which cut the northwest topographic rim of the caldera, along with Mammoth Mountain, on the southwest topographic rim, is west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system. The most recent activity in the area was about 300 years ago in Mono Lake. Both Long Valley Caldera and Mammoth Mountain have experienced episodes of heightened unrest over the last few decades (earthquakes, ground uplift, and/or volcanic gas emissions). As a result, the USGS manages a dense array of field sensors providing the real-time data needed to track unrest and assess hazards.