Frequency of Volcanic Eruptions in the Mammoth Lakes Sierra

of years. How do we deal with volcanoes 

that have never erupted in recorded 

history but certainly could?

For long-dormant but potentially 

threatening volcanoes, geologists do laborintensive field mapping, sampling, and 

dating of deposits from all past eruptions 

to build detailed records of the magnitude, 

style, and frequency of prehistoric activity. 

Such records are useful indicators of 

what to anticipate when a volcano again 

becomes restless. Studies of eruptive 

histories are now fairly detailed for several 

volcanoes in the Cascade Range, as is 

the record for the Mammoth Mountain 

volcanic field of California.

Mammoth Mountain Volcanology

Mammoth Mountain, site of a major 

regional ski area, rises 3,000 feet above 

the Town of Mammoth Lakes. Geologists 

have mapped the deposits of 25 separate 

Geologists recognize lavas and ash deposits from about 60 

past eruptions in the area around 

Mammoth Mountain and Devils 

Postpile, California. This raises 

the unanswerable question, “When 

will it erupt again?” An alternative, 

answerable, and informative question 

is, “How often has it erupted?” 

In the Mammoth Lakes Sierra, 

geologists have mapped in great 

detail all the lavas and ash deposits 

produced by those 60 eruptions. 

They have dated almost all of them 

by laboratory methods, showing that 

eruptions have been repetitive and 

persistent, though not quite regular, 

over the last quarter-million years. 

For few volcanoes in the world is 

the long-term eruptive frequency so 

well calibrated as in the Mammoth 

Lakes Sierra. 

Rising Magma and Volcanic Eruptions

Beneath active volcanic regions, 

molten magma is held in chambers and 

conduits at many depths—from a few 

miles to more than 100. But its ascent to 

the surface to produce lava flows or to 

effervesce its dissolved gas to produce 

explosions can seldom be predicted long 

in advance. Not until telltale signs of 

ascending magma (earthquakes, ground 

uplift, and gas clouds) are recorded by 

monitoring instruments, typically only 

weeks before an outburst, can geologists 

consider an impending eruption likely.

A few volcanoes (Kīlauea, Hawaii; 

Stromboli, Italy; Sakurajima, Japan) 

erupt persistently for many years. Others 

tend to erupt every few years and still 

others erupt every few decades. But many 

volcanoes come back to life after lying 

dormant for centuries or even thousands Map of Mammoth Mountain and the vicinity.

Mammoth Mountain rises above the Town 

of Mammoth Lakes, California. 

Banner: Devils Postpile basalt, eroded colonnade 3 miles west of Mammoth Mountain.

eruptions that incrementally built the 

edifice from 100,000 to 50,000 years ago. 

Nearly all eruptions were extrusions of 

viscous lava that solidified as knobby 

domes and thick lava flows. Only one 

deposit represents an explosive eruption 

that spread a cloud of pumice and ash 

eastward across Long Valley. Surrounding 

Mammoth Mountain, about 35 scattered 

vents, mostly basaltic and andesitic cinder 

cones, bring the total number of eruptions 

to about 60. 

Thick viscous magmas (andesite, 

dacite, and rhyolite) are products of 

cooling and crystallizing in magma 

chambers and plumbing conduits within 

the Earth’s crust, mostly at depths between 

2 and 15 miles. However, almost all 

magmatic activity is initiated by the rise of 

hotter, more fluid basaltic magma that is 

produced by melting of the Earth’s mantle 

at more than 20 miles deep. Batches of 

molten basalt sometimes rise right through 

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3059

September 2018

0 5 MILES

0 5 KILOMETERS

119° 118° 45’

37° 45’

37° 37’ 30”

203

395

395

Inyo Chain

CALIFORNIA

 Mammoth

 Mountain

 Red 

Cones

 Resurgent Dome Basaltic

Periphery

Hot 

Creek 

Flow

Mammoth

 Lakes

Map

Area

 Glass

Mountain

Crowley

 Lake

Mammoth

Yosemite 

Airport

LONG VALLEY CALDERA

Owens River

Owens

Mammoth

Hot

Creek

Creek

River

Devils 

Postpile

San Joaquin River

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3059

September 2018
Wes Hildreth, Andrew Calvert, Judy Fierstein, and Mae Marcaida 

Edited by Katherine Jacques

Layout by Geraldine Dang

For Additional Information: 

https://volcanoes.usgs.gov/observatories/calvo/

Invisible CO2 gas killing trees at Mammoth Mountain, California (USGS Fact Sheet 172–96); California’s 

restless giant—the Long Valley Caldera (USGS Fact Sheet 2014–3056); The California Volcano 

Observatory—Monitoring the State’s restless volcanoes (USGS Fact Sheet 2014–3120); U.S. Geological 

Survey’s alert-notification system for volcanic activity (USGS Fact Sheet 2006–3139).

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183059

Eruptive timeline showing the age and composition of 60 eruptions on and around 

Mammoth Mountain. 

One of the twin basaltic cinder cones, called Red Cones, 3 miles southwest of Mammoth Mountain.

the crust and erupt as basaltic lava flows 

and cinder cones. Devils Postpile is an 

example of basaltic lava that flowed 

out of a vent and ponded thickly on a 

canyon floor. Other batches of rising 

basalt fail to erupt, and they stall within 

the crust, melting the surrounding crustal 

rocks with their greater heat. Cooling, 

crystallization, and mixing of mantle and 

crustal melts with different temperatures, 

water contents, and viscosities can lead 

to a wide range of erupting compositions. 

Progressively lower temperature magmas, 

from basalt to andesite to dacite to 

rhyolite, are increasingly more viscous, 

contain more water, and erupt more 

explosively.

The viscous dacite lavas of 

Mammoth Mountain oozed out and piled 

up above a crustal magma chamber that 

lay near the middle of a scattered field 

of about 35 basaltic and andesitic vents. 

Geologists infer that heating by the many 

surrounding basalts had focused the 

crustal dacite chamber where the basaltic 

injections were most concentrated. 

The mass and heat of the basalts are 

fundamental to producing the dacites at 

the center of the volcanic field. 

Determining the Ages of 

Mammoth Eruptions

Most of the 60 magma batches, 

which erupted lavas of many kinds, 

have been dated by argon radioisotopic 

methods in U.S. Geological Survey 

(USGS) laboratories. Basalts began 

erupting in the Mammoth Mountain area 

about 235,000 years ago and have been 

fairly persistent ever since. Between 

140,000 and 100,000 years ago, cooling 

of basalts within the crust led to eruption 

of several batches of andesite. Continued 

cooling, magma mixing, and partial 

crystallization produced dacite melt that 

culminated in 29 dacite eruptions during 

a 50,000-year-long episode; 25 of these 

eruptions built Mammoth Mountain in 

the center of the volcanic field. Dacite 

eruptions eventually ceased, but a few 

basaltic eruptions have continued during 

postglacial time—the past 16,000 years. 

The most recent eruption of all was the 

basalt of Red Cones, a lava-flow apron 

and two cinder cones that erupted 8,000 

years ago, just 3 miles southwest of the 

summit of Mammoth Mountain. Since 

1990, batches of basalt that failed to erupt, 

but instead stalled in the crust, have been 

releasing enough carbon dioxide gas to 

kill the forest at 10 sites that hug the base 

of Mammoth Mountain (see USGS Fact 

Sheet 172–96, cited below).

How Frequently Has Mammoth 

Mountain Erupted?

Looking at the eruptive timeline, 

there has been at least one eruption 

every 10,000 years or so for the past 

250,000 years, and the last eruption was 

8,000 years ago. Time gaps between 

eruptions are variable, and we never 

expect eruption frequency to be strictly 

regular. We cannot predict exactly when 

the next eruption will take place, but the 

USGS California Volcano Observatory 

has a network of seismometers, ground 

deformation detectors, and gas sensors in 

place that give us real-time alerts about 

unrest beneath the volcanic field. 

Years before present

0 50,000 100,000 150,000 200,000 250,000

Basalt

Andesite

Dacite

Rhyolite

Lava dome complex

ERUPTION TYPES

Lava flow

Mammoth Mountain