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 "super eruption) about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, and the last eruptions inside the caldera occurred about 50,000 years ago. 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 island; the lake eventually drained through the Owens River Gorge. 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, are 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.

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: 50,000 years ago
Nearby towns: Mammoth Lakes
Alert Level: Normal (2012-01-12 17:46:31)