California's Restless Giant: The Long Valley Caldera

A USGS fact sheet from August 2014.

Scientists have monitored geologic unrest at Long Valley, California, since 1980, when a swarm of strong earthquakes led to the discovery that the middle of the caldera had begun to rise. The unrest continues, and the USGS keeps the public and civil authorities informed about the volcanic hazard and ready to warn of an eruption.

Long Valley Caldera seen from its southwest rim toward the northeast rim, 18 miles (30 km) away. The dome-like swelling the USGS tracks is centred in the low forested hills in the middle. USGS photo by Roy Bailey.

A caldera

Gold fever brought the first waves of European settlers through the Long Valley area of eastern California in the 1850s. Today thousands of visitors come year-round for the eastern Sierra Nevada's scenery, shaped over the past 4 million years by glaciers, earthquakes and eruptions.

About 760,000 years ago a cataclysmic eruption blew out 150 cubic miles (600 cubic kilometres) of magma from about 4 miles (6 km) down. Fast flows of glowing ash — pyroclastic flows — covered much of east-central California, and ash fell as far east as Nebraska. The ground sank more than a mile (1.6 km) into the space the magma had left, forming a large volcanic depression: a caldera, 10 by 20 miles (16 by 32 km).

Broad, flat Long Valley, where the Owens River meanders south, fills the eastern half. Magma still lies beneath and heats underground water, feeding hot springs and steam vents — and three geothermal power plants that together produce 40 megawatts.

Much of the area is covered by volcanic rocks less than 2 million years old. The caldera-forming eruption's pyroclastic flows cooled into the Bishop Tuff, and its ash — from an eruption more than 2,000 times larger than Mount St. Helens in 1980, which killed 57 people and caused several billion dollars in damage — covered much of the western United States (inset).

Part of a larger volcanic system

Long Valley is part of a large volcanic system that includes the Mono-Inyo Craters chain, running 25 miles (40 km) north from Mammoth Mountain on the caldera's southwest rim to Mono Lake. Eruptions along the chain began 400,000 years ago; Mammoth Mountain was built by eruptions ending 58,000 years ago. The system is still active: the Inyo and Mono Craters erupted as recently as 600 years ago, and small eruptions took place in Mono Lake between the mid-1700s and mid-1800s.

Panum Dome, a volcanic cone formed by eruptions about 600 years ago at the north end of the Mono Craters.

Earthquakes and swelling

No eruptions are known in eastern California since those in Mono Lake, but earthquakes are frequent, caused by movement on faults and by the pressure of magma rising below — two closely related processes. In 1872 a magnitude 7.4 quake centred 80 miles (125 km) south of Long Valley was felt across most of California, and moderate (magnitude 5 to 6) quakes have shaken the area since 1978.

• 1978 — a magnitude 5.4 earthquake 6 miles southeast of the caldera ended two decades of quiet and began a period of unrest that still continues, with many swarms, especially in the southern caldera and the neighbouring Sierra Nevada.
• May 1980 — the most intense swarm, with four strong magnitude 6 shocks, three of them on the same day. Re-examining the area, USGS scientists found a dome-like uplift in the caldera: its centre had risen almost a foot (30 cm) since the summer of 1979, after decades of stability.
• By 2014 — the swelling totalled more than 2.5 feet (75 cm) over more than 100 square miles (250 square kilometres), caused by new magma rising beneath the caldera.

Earthquake activity rose sharply after 1978 along the faults at the eastern edge of the Sierra Nevada. After the May 1980 shocks, the USGS detected the swelling in the middle of the caldera (inset graph) and intensified its monitoring.

The USGS responded by intensifying monitoring. A modern network of seismometers and geodetic instruments now tracks the earthquakes and the swelling, helping scientists assess the hazard and spot early signs of an eruption. With the California Office of Emergency Services and local authorities, the USGS has set up procedures to alert the public quickly to a possible eruption.

Gas that kills trees

In the early 1990s trees began dying in several places on Mammoth Mountain. USGS and U.S. Forest Service scientists found they were being killed by large amounts of carbon dioxide seeping up through the soil from magma deep below. Volcanic gas emissions, like earthquake swarms and swelling, often come before eruptions. Before an eruption of a "central vent" volcano such as Mount St. Helens they usually last only weeks or months — but at large calderas such as Long Valley, unrest can go on for decades or centuries, and studies suggest only about one in six such episodes at large calderas worldwide ends in an eruption.

Small to moderate eruptions have occurred somewhere along the Mono-Inyo chain every few hundred years over the past 4,000 years, and the unrest could escalate to an eruption within weeks. Even so, geologists think the chance of an eruption in any given year is quite small.

How warnings are given

The USGS California Volcano Observatory watches Long Valley closely to give reliable, timely warning. USGS volcano observatories use two scales, issued together, for rising and falling activity alike, each notice describing the unrest, the hazards and likely outcomes.

The USGS Volcano Hazards Program also monitors volcanic regions in Hawaii, the Pacific Northwest, Wyoming and Alaska.

Sources

• Hill, D.P., Bailey, R.A., Hendley, J.W., II, Stauffer, P.H., and Marcaida, M., 2014, California's restless giant—The Long Valley Caldera: U.S. Geological Survey Fact Sheet 2014–3056. https://pubs.usgs.gov/publication/fs20143056 — the photographs, map and graph come from its PDF.
• Rewritten in hubnx's own words.