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 about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, with the last eruptions inside the caldera 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. The late-Pleistocene to Holocene Inyo Craters cut the northwest topographic rim of the caldera in 1,350 AD, and 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.

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)