U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3120

ver. 1.1, June 2019

More than 50 volcanoes in the 

United States have erupted one or 

more times in the past 200 years. The 

most volcanically active regions are in 

Hawaii, Alaska, Washington, Oregon, 

and California. California’s volcanoes 

are dispersed throughout the State—from 

near its northern border with Oregon 

to its southern border with Mexico. 

The U.S. Geological Survey California 

Volcano Observatory (USGS CalVO) in 

Menlo Park watches over California’s 

potentially hazardous volcanoes to help 

communities prepare for, and respond to, 

volcanic activity.

Some California Volcanoes are 

More Hazardous Than Others

The USGS has ranked the volcanic 

threat at all U.S. volcanoes using volcano 

age, types of potential hazards, and 

estimates of the societal exposure to 

those hazards. Sixteen volcanoes are 

on California’s watch list to monitor. 

Research suggests that partially molten 

rock (magma) lies beneath seven of these 

volcanoes—Medicine Lake Volcano, 

Mount Shasta, Lassen Volcanic Center, 

Clear Lake Volcanic Field, the Long 

Valley Volcanic Region, Coso Volcanic 

Field, and Salton Buttes. At these 

volcanoes, earthquakes (seismicity), hot 

springs, volcanic gas emissions, and (or) 

ground movement (deformation) attest to 

their restless nature.

The California Volcano Observatory—Monitoring the State’s Restless Volcanoes

Volcanic eruptions happen in the State of California about as 

frequently as the largest earthquakes 

on the San Andreas Fault Zone. At 

least 10 eruptions have taken place in 

California in the past 1,000 years—most 

recently at Lassen Peak in Lassen 

Volcanic National Park (1914 to 1917) 

in the northern part of the State—and 

future volcanic eruptions are inevitable. 

The U.S. Geological Survey California 

Volcano Observatory monitors the 

State’s potentially hazardous volcanoes.

This type of low-level volcanic 

unrest can persist for decades or even 

hundreds of years without an eruption. 

Although steady, low-level unrest is 

normal for many young volcanoes, 

rapidly accelerating unrest is cause for 

concern. At California’s most threatening 

volcanoes, monitoring sensors are in 

place to continuously track levels of 

unrest. Such monitoring is necessary to 

determine the baseline, or background 

level, of activity at a volcano to help 

volcanologists know what is normal. 

An uptick in unrest may be a sign of 

increased volcanic threat. 

California Volcanoes Erupt in 

Various Styles and Produce 

Diverse Hazards

Explosive eruptions blast lava 

fragments (tephra) and gas into the air 

with tremendous force. The fi nest particles 

(ash) billow upward, forming an eruption 

column that can attain stratospheric 

heights in minutes. Simultaneously, 

searing volcanic gas laden with ash and 

coarse chunks of lava may sweep down 

the fl anks of the volcano as a pyroclastic 

fl ow. Ash in the eruption cloud, carried 

by the prevailing winds, is an aviation 

hazard and may remain suspended for 

As a part of the U.S. Geological 

Survey’s Volcano Hazards 

Program, the California Volcano 

Observatory aims to advance 

scientific understanding of 

volcanic processes and lessen 

the harmful impacts of volcanic 

activity in the volcanically active 

areas of California.

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

High to very high

Volcano—By threat rank

Volcano that erupted within the 

last 3,000 years

Moderate

EXPLANATION

0 50 KILOMETERS

0 25 50

25

MILES

Base from 2012 U.S. Geological Survey 100-meter digital data

Coso Volcanic Field

Salton Buttes

Mount Shasta

Lassen Volcanic Center

Medicine

Lake Volcano

Clear Lake

Volcanic Field Long Valley

Volcanic Region

Ubehebe

Craters

Fresno

Los Angeles

San Diego

San Francisco

Sacramento

Redding

USGS

CALIFORNIA

VOLCANO

OBSERVATION

CALIFORNIA

Volcanoes of very high to low threat are 

scattered throughout California, from the Oregon 

border (north) to Mexico (south). Other older volcanoes in California are of less concern. 

California’s volcano watch list is subject to change as new data on past eruptive activity 

are collected, as volcanic unrest changes, and as populations in threatened areas grow or 

decline. The half red/half yellow triangle for the Long Valley Volcanic Region denotes that it 

is composed of four volcanic centers ranging from moderate to very high threat.
Landslide

Eruption cloud

Lahar (mud or debris flow)

Lava dome

Lava dome collapse 

Prevailing wind

Tephra

(ash)

fall

Vent

Magma conduit

Lava flow

Pyroclastic flow

P y r o c l a s t i c f l o w

c o l u m n

E r u p t i o n

hundreds of miles before settling to the 

ground as ash fall. During less energetic 

effusive eruptions, hot, fl uid lava may issue 

from the volcano as lava fl ows that can cover 

many miles in a single day. Alternatively, a 

sluggish plug of cooler, partially solidifi ed lava 

may push up at the vent during an effusive 

eruption, creating a lava dome. A growing 

lava dome may become so steep that it 

collapses, violently releasing pyroclastic fl ows 

potentially as hazardous as those produced 

during explosive eruptions.

During and after an explosive or effusive 

eruption, loose volcanic debris on the fl anks of 

the volcano can be mobilized by heavy rainfall 

or melting snow and ice, forming powerful 

fl oods of mud and rock (lahars) resembling 

rivers of wet concrete. These can rush down 

valleys and stream channels as one of the most 

destructive types of volcano hazards.

When a dormant volcano reawakens, 

a series of events commonly unfolds—the 

energy of eruptive activity increases, peaks, 

and then gradually subsides as the volcano 

returns to a state of rest. The 1914 to 1917 

eruptions of Lassen Peak produced a yearlong 

series of minor steam blasts before a major 

explosion sent an eruption column 30,000 feet 

high and unleashed devastating pyroclastic 

fl ows and lahars. Windborne ash drifted 275 

miles eastward and fell as far away as Elko, 

Nevada. The climactic phase of the eruption 

was over in a matter of days, but recurring 

steam blasts and lahars created hazardous 

conditions for several years afterwards. 

Unlike other natural disasters, volcanic 

eruptions and their associated hazards can 

persist for months, years, or even decades 

before an “all clear” can be sounded.

Volcanoes produce a variety of natural hazards that can damage infrastructure 

and negatively affect regional economies. This diagram shows the kinds of 

hazards that pose risks at volcanoes in California and elsewhere. Some hazards, 

such as lahars and landslides, can occur even when a volcano is not erupting. 

Modified from “Geologic Hazards at Volcanoes” (USGS General Information 

Product 64).

High to very high threat volcanoes. Several of California’s young volcanoes are less than 100 miles from major population centers. The inherent beauty 

of the State’s volcanic regions draws thousands of visitors each year, while their potential for geothermal energy has attracted industrial developers. 

Lava Beds National Monument, about 30 miles south 

of Klamath Falls, Oregon, is located on the lower 

northern flank of the shield-shaped Medicine Lake 

Volcano and contains the highest concentration of 

lava-tube caves in North America. 

Medicine Lake Volcano Mount Shasta Lassen Volcanic Center

This 14,162-foot-high volcano holds the 

headwaters of the upper Sacramento 

River and is adjacent to several towns and 

major highway, rail, and air transportation 

corridors.

Lassen Volcanic National Park, located about 

50 miles east of Redding, showcases the 

dynamic history of this area and draws more 

than 350,000 visitors each year. Lassen Peak 

erupted violently in the early twentieth century.

High threat Very high threat Very high threat
Left: Sophisticated sensors like this Global Positioning System (GPS) receiver and seismometer in the Long Valley Volcanic Region can detect 

ground deformation and seismicity that occur as magma or volcanic gas rises toward the Earth’s surface in the weeks to months prior to an 

eruption. Data are telemetered from sensors in the field to USGS offices in Menlo Park, California, for processing and interpretation. Early 

detection of eruption precursors via a reliable volcano monitoring network is essential to hazard mitigation. Mammoth Mountain, a volcano 

made up of overlapping lava domes and flows, is visible behind the monitoring equipment. Right: Volcanic-gas “sniffer” at Mammoth Mountain 

continuously measures temperature plus carbon dioxide and other gas concentrations at a steaming vent.

The California 

Volcano Observatory 

uses several modern 

instrumental methods 

to monitor volcanoes. 

Three key techniques 

measure earthquakes 

(seismicity), movement 

of the ground surface 

(deformation), and the 

types and amounts 

of gases released 

from a volcano (gas 

geochemistry). 

Volcano Hazard Zones Identify 

Threatened Areas. 

Maps of volcano hazard zones convey 

the types of hazards that may occur during a 

future eruption and the areas of likely impact. 

The specifi c hazards to people and property 

depend on the eruption style (effusive or 

explosive), the volume of lava erupted, the 

location of the vent, the eruption duration, 

and local water-fl ow and precipitation 

conditions. Low-energy effusive eruptions 

are destructive, but generally not life 

threatening. High-energy explosive eruptions 

are destructive and life threatening. 

Statewide, California’s volcano 

hazard zones encompass more than 24,000 

square miles, of which about half is 

privately owned and about half is public 

land that is managed by the U.S. Forest 

Service, National Park Service, or Bureau 

of Land Management. Within these zones 

are natural resources and infrastructure 

essential to California’s water, power, 

and transportation systems. Although less 

than 1 percent of California’s population 

lives within a designated hazard zone, 

more than 20 million recreational visitors 

are attracted annually to the dramatic 

landscapes they afford.

CalVO Provides Timely Forecasting 

of Volcano Hazards

The mission of the California Volcano 

Observatory is to enhance public safety and 

minimize societal disruption in the event of 

volcanic unrest through delivery of effective 

forecasts of volcanic activity derived from 

monitoring and rigorous scientifi c research. 

Whereas USGS maps of volcano hazard 

zones provide long-term perspectives on 

future volcanic activity, short-term forecasts 

give warning of likely outcomes under 

rapidly changing conditions during volcanic 

unrest and eruption. 

An eruption at any one of the six volcanoes shown here may result in volcanic hazards that could impact vulnerable infrastructure and resources. In an 

explosive eruption, airborne ash could travel for hundreds of miles—so the volcano closest to your home may not be the one that affects you the most.

A cataclysmic “supervolcano” eruption about 

760,000 years ago left behind a depression 

20 miles long and 10 miles wide known as 

Long Valley Caldera, located about 30 miles 

southeast of Yosemite National Park. 

Clear Lake Volcanic Field Long Valley Volcanic Region Salton Buttes

Located about 90 miles north of San Francisco, 

the Clear Lake Volcanic Field provides the 

heat for The Geysers, the largest complex of 

geothermal power plants in the world.

The Salton Buttes are volcanic domes that 

lie within the Salton Sea Geothermal Field 

located about 90 miles southeast of Palm 

Springs. 

High threat Moderate to very high threat High threat
“Real-time” forecasts are released 

as USGS Volcano Alert Notifi cations 

(VANs) and Volcano Observatory Notices 

for Aviation (VONAs), both of which use 

four-tiered threat level systems. VANs 

use Normal-Advisory-Watch-Warning to 

specify increasing threat on the ground, 

and VONAs use Green-Yellow-OrangeRed to specify increasing threat to aviation. 

An email-based Volcano Notifi cation 

Service (VNS) automatically delivers 

VANs, VONAs, and other volcano 

information to all subscribers who register 

online at http://volcanoes.usgs.gov/vns. 

USGS Volcano Hazards Program 

Promotes Community Resilience

Volcano research, monitoring, and 

hazard notification by CalVO is only 

one part of the larger USGS Volcano 

Hazards Program’s effort to identify 

and mitigate volcanic hazards in the 

United States and abroad. The USGS 

operates four volcano observatories 

in addition to CalVO—the Cascades 

Volcano Observatory (covering Oreg., 

Wash., Idaho); Yellowstone Volcano 

Observatory (Mont., Wyo., Colo., 

Utah, N. Mex., and Ariz.); Hawaiian 

Volcano Observatory (Hawai‘i); and 

Alaska Volcano Observatory (Alaska., 

Northern Mariana Islands). Key to 

their ongoing success is continued 

implementation of the National 

Volcano Early Warning System 

(NVEWS) to modernize the Nation’s 

monitoring infrastructure and to 

improve the detection and forecasting 

of volcanic activity. Additionally, 

with the support of the U.S. Office of 

Foreign Disaster Assistance, the USGS 

Volcano Disaster Assistance Program 

(VDAP) responds to volcano crises 

abroad and helps build volcano hazard 

response capabilities in other nations.

U.S. Geological Survey geologists produce volcano hazard maps, like this generalized hazards map of Lassen Volcanic Center in northern 

California, to estimate potential impact areas for ground-based and ash-fall hazards during a volcanic eruption. Pyroclastic flows generally travel 

down drainages and in this area have been found in valleys as much as 15 miles from Lassen Peak. Ash-fall hazards could affect other, more 

distant areas downwind, depending on wind directions and strengths during an ash-producing eruption.

For more information contact:

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

This fact sheet and any updates to it are available 

online at https://doi.org/10.3133/fs20143120

See also:

Living with Volcano Hazards (USGS Fact Sheet 

2018–3075); 

2018 Update to the U.S. Geological Survey 

National Volcanic Threat Assessment (USGS 

Scientific Investigations Report 2018–5140); 

An Assessment of Volcanic Threat and Monitoring 

Capabilities in the United States—Framework 

for a National Volcano Early Warning System 

(USGS Open-File Report 2005–1164);

U.S. Geological Survey’s Alert-Notification 

System for Volcanic Activity (USGS Fact Sheet 

2006–3139); 

A Sight “Fearfully Grand”—Eruptions of 

 Lassen Peak, California, 1914 to 1917 

 (USGS Fact Sheet 2014–3119); 

Geologic Hazards at Volcanoes 

 (USGS General Information Product 64)

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20143120

Lassen Volcanic

National Park

Lassen 

Volcanic 

Center

Mountain Meadows

Reservoir

Eagle

Lake

Lake

Almanor

Manzanita

Lake

Shasta

Lake

Susan

Hat

Creek

Creek

River

Canyondam

Westwood Chester

Mineral

Shingletown

Lassen Peak

Burney

Paynes

Creek

Warner

Creek

Mill Creek

Bailey

Creek

River

Creek

Creek

Pit

B

urney

Cow

Battle

10,457' (3,187m)

to Redding

 to Red Bluff

44

44

36

89

C

89

r

89

36

36

36

299

139 44

0 5 10MILES

0 5 10 KILOMETERS

EXPLANATION

Pyroclastic

flow extent

Generalized volcano

hazard zones

Lahars

Volcanic ash extent

 (> 5 cm or 

2 inches thick) 

Highway

Town

Stream

Lava domes

 and flows

44

By Wendy K. Stovall, Mae Marcaida, 

and Margaret T. Mangan

Edited by Peter H. Stauffer

Layout by Jeanne S. DiLeo