DEFINITION OF LONG VALLEY

The Long Valley caldera is a 10-mile by 20-

mile (15–30 km) oval-shaped depression located 12 miles (20

km) south of Mono Lake along the eastern side of the Sierra

Nevada in eastern California, at the boundary with the Basin

and Range Province. This region is one of the most dangerous

volcanically active areas in North America, having produced

numerous volcanic eruptions over the past 3 million

years. A massive caldera-forming event occurred 760,000

years ago, and the most recent eruptions occurred 250 and

600 years ago. A period of new unrest including swarms of

seismic activity, changes in thermal springs and gas emissions,

and doming of the southern part of the caldera indicates

renewed magmatic activity and the potential of an eruption.

The activity is being monitored closely by the U.S. Geological

Survey, but warnings of volcanic danger are resisted by local

businesses because of fear of decreased tourism.

The Long Valley caldera is one of the largest Quaternary

rhyolitic volcanic centers in North America. The caldera floor

has an elevation of 6,500–8,500 feet (2,000–2,600 m), and

the walls reach heights of 9,800–11,500 feet (3,000–3,500 m).

The main eruptive phase from the caldera began 3.6 million

years ago with the eruption of basaltic-andesitic lavas that

covered 1,500 square miles (4,000 km2), followed by rhyodacite

flows and domes. The caldera produced a catastrophic

eruption 730,000 years ago when the roof of the caldera collapsed

along with the expulsion of 150 cubic miles (600 km3)

of rhyolitic magma in Plinian ash clouds, more than 10–20

times as much as is typically produced from the largest convergent

margin stratovolcano eruptions such as Pinatubo or

Tambora. This eruption formed the Bishop Tuff that covers

large parts of southern California, Nevada, Mexico, all of Arizona,

Utah, Colorado, most of New Mexico, Wyoming, and

parts of Texas, Oklahoma, Kansas, Nebraska, South Dakota,

and Idaho. Small eruptions continued after the main catastrophic

eruption, and a resurgent dome formed within 100,000

years after the main eruption. New eruptions produced rhyolitic

domes at 500,000, 300,000, and 100,000 years ago.

A dozen of the domes that were extruded from the

southwest margin of the caldera from 200,000 to 50,000

years ago have merged to produce the 11,050-feet (3,370-m)

high Mammoth Mountain, a popular ski resort. The Mono

Lake–Inyo Craters area just to the north of Long Valley is

part of the same volcanic province, and the eruption history

of these craters overlaps that of Long Valley. Mafic eruptions

from the Mono Lake caldera began 300,000–200,000 years

ago, and the youngest mafic flow is 13,300 years old. Volcanism

migrated to the north around 35,000 years ago, and the

Mono Craters chain northwest of the caldera consists of

about 30 coalesced domes, flows, and craters, the youngest of

which is 600 years old. On the northwest rim of the caldera,

the Inyo Craters consists of domes, flows, and craters that

range in age from 6,000 to 500 years old, with abundant evidence

for explosive phreatic eruptions. There is an approximate

500-year interval between major eruptions in this area.

Recent increased activity in the area suggests that a new eruptive

phase could be imminent in the Long Valley–Mono

Lake–Inyo Craters region.

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