Geologic Background:
The large 17 x 32 km Long Valley caldera east of
the central Sierra Nevada Range formed as a result of the
voluminous Bishop Tuff eruption about 730,000 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, ending about
50,000 years ago. During early resurgent doming the
caldera was filled with a large lake that left
strandlines on the caldera walls and the resurgent dome
island; the lake eventually drained through the Owens
River Gorge. Subsequent activity in the region has
continued nearly to the present.
The late-Pleistocene to
Holocene Inyo
Craters and the Mono
Craters are located within and north of the
caldera.
Historic Activity:
- The caldera remains thermally active, with many hot
springs and fumaroles, and has had significant
deformation, seismicity, and other unrest in recent
years.
Recent Activity:
- Following the 1980 seismic activity, earthquake
swarms have occurred near the south margin of the
caldera. The resurgent dome complex was uplifted by more
than 44 cm between 1975 and 1985, and the south moat
showed evidence of extension during that same period.
Changes in fumarole and soil gas chemistry have also been
reported from a zone in the south moat over the site of
many swarms. This deformation has been interpreted to
indicate the injection of 0.1 to 0.2 cu km of magma into
the residual magma chamber, although it is clear that the
tectonic component of the activity was more important
than previously recognized. Other earthquake swarms have
occurred beneath the east moat and Mammoth Mountain.
- Today earthquake activity within the caldera
continues to cluster in the caldera's south moat.
Swelling of the resurgent dome continues at a rate of 2-3
ppm/year. High concentrations of carbon dioxide in the
soils was correlated with the death of mature pine trees
in four separate areas on the flanks of Mammoth Mountain.
Asphyxia of workers has been reported from poorly
ventilated parts of the tree kill
areas.
- Decreased seismicity during 1999-2000
Data Sources
- Smithsonian's SEAN Bulletin (V. 8 to 14)
- Global Volcanism Network (V. 15 to V. 26).
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