U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet-073-97

Revised November 1998

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

L

US SG

ong Valley Caldera and the MonoInyo Craters chain form a large

volcanic complex in eastern California

that has had persistent earthquake

activity and ground uplift in recent

decades. Volcanoes have been active

in the area for millions of years, and

future eruptions are certain to occur.

When the next eruption in the area

does occur, it will most likely be small

and from a site in the Mono-Inyo chain.

eruption has broken out somewhere along this

chain every 250 to 700 years. The Inyo Craters

and nearby lava domes were formed by a series

of small to moderate eruptions 600 to 550 years

ago, and the most recent eruptions along the

volcanic chain took place about 250 years ago

at Paoha Island in Mono Lake.

The pattern of volcanic activity over the

past 5,000 years suggests that the next eruption

in the Long Valley area will most likely happen

somewhere along the Mono-Inyo volcanic

chain. However, the probability of such an

eruption occurring in any given year is less

than 1%. This is comparable to the annual

chance of a magnitude 8 earthquake (like the

Great 1906 San Francisco Earthquake) along

the San Andreas Fault in coastal California or

of an eruption from one of the more active Cascade Range volcanoes in the Pacific Northwest,

such as Mount Rainier.

As long as increased volcanic unrest (including earthquake swarms, ground deformation, and CO2

 gas emissions) continues in the

Long Valley area, the chances of an eruption

occurring in the near future will remain somewhat increased. However, evidence from large

volcanic areas and calderas worldwide shows

that unrest, such as the current activity in eastern California, can persist for decades or even

centuries without leading to an eruption. Nevertheless, recent eruptions at Rabaul Caldera in

Papua New Guinea (1994) and the Izu volcanic

complex in Japan (1989) following short periods of unrest emphasize the need to closely

monitor restless calderas.

When an eruption does break out in the

Long Valley area, its impact will depend on the

location, size, and type of eruption, as well as

the wind direction. Also, an eruption during the

winter months could melt heavy snow packs,

generating mudflows and locally destructive

flooding.

Most likely, the next eruption will be

small and similar to previous eruptions along

the Mono-Inyo volcanic chain during the past

5,000 years. Such eruptions typically begin

with a series of steam-blast explosions as rising

molten rock (magma) encounters and vaporizes

After four strong (magnitude 6) earthquakes rocked the Long Valley area of eastern

California in May 1980, U.S. Geological Survey (USGS) scientists also detected evidence of

renewed volcanic unrest in the region. They

discovered that the central part of Long Valley

Caldera, a broad depression formed in a cataclysmic volcanic eruption 760,000 years ago,

was slowly rising. Because such ground deformation and earthquakes are common precursors of volcanic eruptions, the USGS has continued to closely monitor the unrest in this

region.

It is natural to wonder when and where the

next volcanic eruption might occur in the Long

Valley area. Geologic processes generally proceed at a slow pace, and when viewed on the

scale of a human lifetime, volcanic eruptions

and destructive earthquakes happen rarely.

Nevertheless, the long history of volcanic activity in the Long Valley area indicates that future

eruptions will occur.

Geologists studying the Long Valley

Caldera have found that following its creation

in the violent eruption 760,000 years ago, clusters of smaller volcanic eruptions have occurred in the caldera at roughly 200,000-year

intervals. About 100,000 years ago, the most

recent of these eruptions formed the Mammoth

Knolls, low hills just north of the Town of

Mammoth Lakes.

Mammoth Mountain, a young volcano on

the rim of Long Valley Caldera, was built by

numerous eruptions between 220,000 and

50,000 years ago. Volcanoes in the Mono-Inyo

Craters volcanic chain, which extends from just

south of Mammoth Mountain to the north shore

of Mono Lake, have erupted often over the past

40,000 years. During the last 5,000 years, an

Future Eruptions in California’s

Long Valley Area—What’s Likely?

The three Inyo Craters, part of the Mono-Inyo Craters volcanic chain, stretch northward across the floor of

Long Valley Caldera, a large volcanic depression in eastern California. During the past 1,000 years there have

been at least 12 volcanic eruptions along the chain, including those that formed the Inyo Craters and South

Deadman Creek Dome (seen here just beyond the farthest Crater).
underground water near the Earth’s surface.

These blasts can throw large blocks of rock and

smaller fragments hundreds of feet into the air,

leaving deep, circular pits like the Inyo Craters.

If magma reaches the surface, gases

trapped within it can escape explosively, hurling volcanic ash (tiny fragments of the solidifying magma) as high as 6 miles or more into the

air. Airborne volcanic ash can be carried hundreds of miles downwind, and the particle size

and amount of falling ash decrease with distance from the site of the eruption. Thin accumulations of ash pose little threat to life or

property, especially in areas where the roofs of

most buildings are constructed to withstand

heavy snow loads. However, even a light dusting of fine volcanic ash can close roads and seriously disrupt communications and utilities for

weeks or months after an eruption.

Explosive volcanic eruptions may also

produce turbulent flows of hot ash (pyroclastic

flows) that can sweep over the ground at

speeds greater than 100 miles an hour, devastating everything in their paths. In the past

5,000 years, eruptions from several sites along

the Mono-Inyo chain have produced narrow,

tongue-like pyroclastic flows that extended

more than 5 miles. Fortunately, the main population

centers in the Long Valley area are far enough from

probable eruption sites that they are unlikely to be

directly impacted by future pyroclastic flows.

Less violent eruptions have also taken

place in the Long Valley area. These eruptions

typically began with mild explosions that

formed relatively small volcanic cones less

than 1,000 feet in diameter and then produced

hot, fluid lava flows that extended a few miles.

Eruptions of this type about 5,000 years ago

created the Red Cones, just south of Mammoth

Mountain. Flows of fluid lava were also

erupted from sites near the base of Mammoth

Mountain between 400,000 and 60,000 years

ago. Such flows are highly destructive to property, but seldom endanger people because lava

flows rarely move faster than a brisk walk.

Although the chance of a volcanic eruption in any given year is small, future eruptions

will occur in the Long Valley area. Because

volcanic unrest can escalate to an eruption in a

few weeks or less, USGS scientists are closely

monitoring activity in this region. To be able to

provide the public with reliable and timely

warnings before an eruption, the USGS has

joined local and State authorities in developing

Over the past 5,000 years, small to moderate eruptions have occurred at various sites along the

Mono-Inyo Craters volcanic chain at intervals ranging from 250 to 700 years. Steam blasts

commonly produce large pits or craters. Explosive eruptions, which may create fiery flows of hot

ash (pyroclastic flows), are usually followed by the pushing up of a lava dome. Some less violent

eruptions only produce lava flows.

The eruptions that led to the creation of the 600-

year-old South Deadman Creek Dome covered

the area of what is now the Town of Mammoth

Lakes with a layer of volcanic ash about 1 inch

thick. During these eruptions, the wind first blew

toward the northeast (tan) and later toward the

southwest (pink), spreading volcanic ash in the

pattern shown on the map. These eruptions also

produced fiery flows of hot ash (pyroclastic

flows). Depending on the wind direction and the

location of an eruption site, future eruptions in

the Long Valley area could spread volcanic ash

over the communities of Mammoth Lakes, June

Lake, or Lee Vining (see also map at left).

For more information contact:

Earthquake Information Hotline (650) 329-4085

U.S. Geological Survey, Mail Stop 977

345 Middlefield Road, Menlo Park, CA 94025

http://volcanoes.wr.usgs.gov/

http://quake.wr.usgs.gov/VOLCANOES/LongValley/

See also Invisible CO2 Gas Killing Trees at Mammoth

Mountain, California (USGS Fact Sheet 172-96)

COOPERATING ORGANIZATIONS

California Division of Mines and Geology

California Office of Emergency Services

Inyo County, California

Mono County, California

Mammoth Mountain Ski Area

Town of Mammoth Lakes, California

University of Nevada, Reno

U.S. Department of Agriculture, U.S. Forest Service

David P. Hill, Roy A. Bailey, C. Dan Miller,

James W. Hendley II, and Peter H. Stauffer

10 MILES

395

5000 4000

YEARS BEFORE PRESENT

NEGIT ISLAND

PAOHA ISLAND

PANUM DOME

NORTH COULEE

NORTHWEST COULEE

SOUTH COULEE

WILSON BUTTE

OBSIDIAN DOME

GLASS CREEK DOME

RED CONES

NORTH DEADMAN CREEK DOME

SOUTH DEADMAN CREEK DOME

INYO CRATERS

MAMMOTH MOUNTAIN CRATERS

3000 2000 1000 0

MAMMOTH

MOUNTAIN Mammoth

Lakes

June Lake

Lee 

Vining

Mono Lake

Mono 

Craters Inyo 

Craters

Steam blast

ERUPTION TYPES

Explosive eruption Lava flow

Mono-Inyo Craters volcanic chain

Outline of Long Valley Caldera

Long Valley

CALIF

area

203

395

Mammoth

Lakes

SOUTH DEADMAN 

CREEK DOME

MAMMOTH

MOUNTAIN

Pyroclastic flows

4 MILES

TRACE

AMOUNTS

4

8

20

40

80

160

THICKNESS OF VOLCANIC ASH, IN INCHES

THICKNESS OF VOLCANIC

ASH, IN INCHES

TRACE

AMOUNTS

4

8

20

40

LONG 

 VALLEY 

CALDERA

procedures for responding to changing levels of

volcanic unrest in the Long Valley area. The ongoing work of the USGS Volcano Hazards Program in this and other volcanic regions of the

United States helps to better protect people’s

lives and property from volcano hazards.

Printed on recycled paper

Graphic design by

 Susan Mayfield and Sara Boore

Banner design by Bobbie Myers