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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.
As the magma was ejected, the roof of the magma storage area subsided by about 2 km forming a 17 by 32 km (9 by 18 mile), oval-shaped depression known as the Long Valley Caldera (dash and dot line on map). Over the next 100,000 years volcanism continued on a smaller scale inside the caldera.
Scientists have monitored geologic unrest in the Long Valley, California, area since 1980. In that year, following a swarm of strong earthquakes, they discovered that the central part of the Long Valley Caldera had begun actively rising. Unrest in the area persists today.
Long-term outlook for volcanic activity in Long Valley caldera. The area of eastern California that includes the Long Valley Caldera and the Mono-Inyo Craters volcanic chain has a long history of geologic activity that includes both earthquakes and volcanic eruptions.
Long Valley Caldera in eastern California (here viewed from its southwest rim toward its northeast rim on the horizon 18 miles away) was formed about 760,000 years ago in a violent volcanic eruption that blew out 150 cubic miles of magma (molten rock) from beneath the Earth's surface.
View of Long Valley Caldera. On a clear day the entire expanse of Long Valley caldera can be seen from the summit. Climb to the summit following the path to the east of the gondola station (a sign with the elevation marks the summit).
This guide to the geology of Long Valley Caldera is presented in four parts: (1) An overview of the volcanic geology; (2) a chronological summary of the principal geologic events; (3) a road log with directions and descriptions for 38 field-trip stops; and (4) a summary of the geophysical unrest since 1978 and discussion of its causes.
The area of eastern California that includes the Long Valley Caldera and the Mono-Inyo Craters volcanic chain has a long history of geologic activity that includes both earthquakes and volcanic eruptions.
Boiling water at Hot Creek— The dangerous and dynamic thermal springs in California’s Long Valley Caldera
Long Valley caldera viewed toward the east across the northern part of the caldera. (Credit: Brantley, S. R. Public domain.) The caldera was partially filled by pyroclastic flows and tephra (Bishop Tuff) during the catastrophic eruption and also by many subsequent episodes of volcanic activity.