Long Valley Volcano | John Seach


California, USA

37.70 N, 118.87 W
summit elevation 3390 m

Long Valley Volcano is located in central eastern California. The caldera contains hot springs and fumaroles. The caldera is 15 x 30 km in diameter. In April 2006 three skiiers were asphyxiated by poisonous gas after falling into a volcanic fissure.

Volcanic Features of Long Valley
Mammoth Mountain, Mammoth Lakes, Glass Mountain, Biship Tuff.

History of Eruptions at Long Valley
Volcanic activity began in the area about 4 million years ago with eruption of intermediate and basaltic lavas accompanied by the onset of large-scale normal faulting. Multiple rhyolitic eruptions occurred 2 million years ago from vents along the northeast rim of the present-day caldera formed the Glass Mountain complex. Long Valley
Long Valley caldera was formed 760 ka ago by eruption of over 600 cubic km of rhyolitic lavas (the Bishop Tuff ), accompanied by subsidence of a 15 x 30 km elliptical crustal block as the underlying magma chamber was partially emptied. Eruptions of rhyolite lavas occurred around the margin of the resurgent dome at 500 ka, 300 ka and 100 ka ago.

Caldera Unrest
Long valley is one of the large calderas of the world which have shown unrest in recent years. Caldera unrest has included earthquake swarms and deformation of the caldera floor.

2002 Unrest
The M 7.9 Denali Fault earthquake of 3rd November 2002, caused 60 small earthquakes in Long Valley. This is the third time that a distant earthquake has affected the caldera.

1996 Earthquake Swarm
One of the strongest earthquake swarms at Long Valley caldera occurred in March and April of 1996. More than 24 earthquakes were measured over magnitude 3.

1990–1995 Unrest
In late September 1989, a new period of unrest began at Long Valley caldera with extension across the dome increasing abruptly from slight subsidence to a rate of more than 7 cm per year. In early January 1990, earthquake swarm activity resumed in the south moat as the extension rate began slowing. An earthquake swarm in March 1991 included more than 1000 detected earthquakes, with 22 greater than magnitude 3. A more energetic swarm in November–December 1993 included M=4.0 and 4.1 earthquakes.

1992 Landers Earthquake
On 28th June 1992 a magnitude 7.3 earthquake hit the Mojave Desert 400 km south of Long Valley caldera. An earthquake swarm began at south-moat of Long Valley, 30 seconds after the S-wave from the Landers earthquake passed through the caldera. This was the first documented case of remotely triggered seismicity by a large, distant earthquake.

1989-90 Mammoth Mountain swarm
An 11 month long swarm of small earthquakes began under Mammoth Mountain in May 1989. The swarm was accompanied by minor deformation (1–2 cm of uplift). The earthquakes were not large (only three over mag 3), but it was prolonged. The earthquake epicentres were at a depth of 7-10 km below the south flank of Mammoth Mountain. The earthquakes were suspected to have been caused by the movement of carbon dioxide rich fluids beneath the caldera. There was an increase in release of carbon dioxide emission at the surface.

1978–1983 Unrest
Following the M=5.8 Wheeler Crest earthquake (4th October 1978), a series of M 3-4 earthquakes occurred intermittently within the south moat of Long Valley caldera, and beneath the south flank of Mammoth Mountain. This activity culminated
in an intense earthquake sequence that began in late May 1980. On 25th May 1980 three magnitude 6 earthquakes occurred at Long Valley - the first located just west of Convict Lake near the south margin of the caldera; the second beneath the south moat, and the third within the Sierra Nevada block about 5 km south of the caldera. On 27th May 1980 a fourth magnitude 6 earthquake hit 10 km south of the caldera.

1941 Earthquakes
In September 1941, a cluster of four M>5 events and one M 6 event occurred along the Sierra Nevada escarpment about 10 km southeast of Long Valley caldera.

Further Reading
Hill, D.P., 2006. Unrest in long valley caldera, california, 19782004. Geological Society, London, Special Publications, 269(1), pp.1-24.

Hill, D.P. et al. 2002. Response plan for volcano hazards in the Long Valley caldera and Mono Craters region, California. US Department of the Interior, US Geological Survey.

Miller, C.D., 1985. Holocene eruptions at the Inyo volcanic chain, California: Implications for possible eruptions in Long Valley caldera. Geology, 13(1), pp.14-17.

Long Valley Volcano Eruptions

90,000 years ago (Glass Mountain)