U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet-002-97

Revised July 2004

What are Volcano Hazards?

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOL CA NO HAZ ARDS

V

US SG

olcanoes give rise to nu mer ous 

geologic and hy dro log ic haz ards. 

U.S. Geo log i cal Sur vey (USGS) sci en tists 

are as sess ing haz ards at many of the 

almost 70 active and po ten tial ly active 

vol ca noes in the Unit ed States. They are 

close ly monitoring ac tiv i ty at the most 

dan ger ous of these vol ca noes and are 

pre pared to is sue warn ings of im pend ing 

erup tions or oth er haz ard ous events.

Volcanic Gases

Volcanoes emit gases during eruptions. 

Even when a volcano is not erupting, cracks

in the ground allow gases to reach the sur face 

through small openings called fu ma roles. 

More than 90% of all gas emit ted by vol ca noes 

is water va por (steam), most of which is heat ed 

ground wa ter (un der ground water from rain-

 fall and streams). Other com mon vol ca nic gases are car bon di ox ide, sul fur di ox ide, hy dro gen 

sul fide, hy dro gen, and flu o rine. Sul fur di ox ide 

gas can react with water drop lets in the atmo sphere to cre ate acid rain, which caus es 

cor ro sion and harms veg e ta tion. Car bon diox ide is heavi er than air and can be trapped 

in low ar eas in con cen tra tions that are deadly 

to peo ple and an i mals. Flu o rine, which in 

high con cen tra tions is tox ic, can be adsorbed 

onto vol ca nic ash par ti cles that later fall to 

the ground. The flu o rine on the par ti cles can 

More than 50 volcanoes in the Unit ed 

States have erupted one or more times in the 

past 200 years. The most vol ca ni cal ly active 

regions of the Na tion are in Alas ka, Ha waii, 

California, Or e gon, and Wash ing ton. Vol-

 ca noes pro duce a wide va ri ety of hazards that 

can kill peo ple and de stroy prop er ty. Large 

explo sive eruptions can en dan ger people and 

prop er ty hundreds of miles away and even af-

 fect global cli mate. Some of the vol ca no haz-

 ards de scribed be low, such as land slides, can 

oc cur even when a vol ca no is not erupt ing.

Eruption Columns and Clouds

An explosive eruption blasts solid and 

mol ten rock fragments (tephra) and vol ca nic 

gas es into the air with tre men dous force. The 

larg est rock frag ments (bombs) usually fall 

back to the ground with in 2 miles of the vent. 

Small frag ments (less than about 0.1 inch 

across) of volcanic glass, min er als, and rock 

(ash) rise high into the air, form ing a huge, 

bil low ing erup tion col umn.

Eruption columns can grow rapidly and 

reach more than 12 miles above a volcano in 

less than 30 minutes, form ing an erup tion 

cloud. The volcanic ash in the cloud can pose 

a serious haz ard to aviation. Dur ing the past 

15 years, about 80 com mer cial jets have been 

dam aged by in ad vert ent ly flying into ash 

clouds, and several have near ly crashed be-

 cause of engine failure. Large eruption clouds 

can ex tend hun dreds of miles down wind, 

re sult ing in ash fall over enor mous ar eas; 

the wind car ries the small est ash par ti cles the 

far thest. Ash from the May 18, 1980, eruption 

of Mount St. Helens, Wash ing ton, fell over 

an area of 22,000 square miles in the Western 

Unit ed States. Heavy ash fall can col lapse 

build ings, and even mi nor ash fall can dam-

 age crops, elec tron ics, and ma chin ery.

Pyroclastic Flow

Pyroclastic Flow

Eruption Column

Lahar (Mud or Debris Flow)

Lava Flow

Prevailing Wind

Lava Dome Collapse

Fumaroles

Eruption Cloud

Ash (Tephra) Fall

Acid Rain

Bombs

Lava Dome

Crack

Magma

Ground

Water

Landslide

(Debris Avalanche)

Vent

Rhyolite

100%

68

 63

53

0

Silica (SiO2)

Content

Dacite

Andesite

Basalt

Magma

Type

Volcanoes produce a wide variety of natural hazards that can kill people and destroy property. This sim pli fi ed 

sketch shows a volcano typical of those found in the Western United States and Alaska, but many of these hazards also pose risks at other volcanoes, such as those in Hawaii. Some hazards, such as lahars and landslides, 

can oc cur even when a volcano is not erupting. (Hazards and terms in this diagram are high light ed in bold 

where they are discussed in the text below.)
poi son live stock graz ing on ash-coat ed grass 

and also con tam i nate do mes tic wa ter sup plies.

Cataclysmic eruptions, such as the June 

15, 1991, eruption of Mount Pinatubo (Phil ip -

pines), inject huge amounts of sul fur di ox ide 

gas into the stratosphere, where it com bines 

with water to form an aerosol (mist) of sulfu ric acid. By re flect ing solar radiation, such 

aero sols can low er the Earth’s average sur face 

tem per a ture for extended periods of time by 

sev er al degrees Fahr en heit ( ̊F). These sul fu ric 

acid aerosols also contribute to the de struc tion 

of the ozone layer by al ter ing chlorine and nitro gen compounds in the up per at mo sphere.

Lava Flows and Domes

Molten rock (magma) that pours or ooz es 

onto the Earth’s surface is called lava and 

forms lava flows. The higher a la va’s con tent 

of sil i ca (silicon dioxide, SiO2

), the less eas-

 i ly it flows. For ex am ple, low-sil i ca ba salt

lava can form fast-moving (10 to 30 miles 

per hour) streams or can spread out in broad 

thin sheets as much as sev er al miles wide. 

Since 1983, Kilauea Vol ca no on the Island of 

Ha waii has erupt ed ba salt lava flows that have 

destroyed nearly 200 hous es and sev ered the 

near by coast al high way.

In contrast, flows of higher-silica andes ite

and dacite lava tend to be thick and slug gish, 

traveling only short dis tanc es from a vent. 

Dacite and rhyolite lavas of ten squeeze out 

of a vent to form irregular mounds called lava 

domes. Be tween 1980 and 1986, a dacite 

lava dome at Mount St. Helens grew to about 

1,000 feet high and 3,500 feet across.

Pyroclastic Flows

High-speed avalanches of hot ash, rock 

frag ments, and gas can move down the sides 

of a vol ca no during explosive erup tions or 

when the steep side of a growing lava dome 

col laps es and breaks apart. These py ro clas tic 

flows can be as hot as 1,500 ̊F and move at 

speeds of 100 to 150 miles per hour. Such 

flows tend to fol low valleys and are ca pa ble 

of knock ing down and burning everything in 

their paths. Low er-density pyroclastic flows, 

called py ro clas tic surg es, can eas i ly over flow 

ridg es hundreds of feet high.

The climactic eruption of Mount St. Helens 

on May 18, 1980, generated a se ries of ex plo -

sions that formed a huge py ro clas tic surge. 

This so-called “lateral blast” de stroyed an 

area of 230 square miles. Trees 6 feet in di-

 am e ter were mowed down like blades of grass 

as far as 15 miles from the vol ca no.

Volcano Landslides

A landslide or debris avalanche is a rap id 

downhill movement of rocky ma te ri al, snow, 

and (or) ice. Volcano land slides range in size 

from small movements of loose de bris on 

the surface of a volcano to massive collapses 

of the entire summit or sides of a volcano. 

Steep vol ca noes are sus cep ti ble to landslides 

be cause they are built up partly of layers of 

loose vol ca nic rock fragments. Some rocks on 

vol ca noes have also been al tered to soft, slippery clay min er als by cir cu lat ing hot, acidic 

ground wa ter. Landslides on vol ca no slopes 

are trig gered when erup tions, heavy rain fall, 

or large earth quakes cause these ma te ri als to 

break free and move down hill. 

At least five large landslides have swept down 

the slopes of Mount Rainier, Wash ing ton, during 

the past 6,000 years. The larg est vol ca no landslide in historical time oc curred at the start of the 

May 18, 1980, Mount St. Helens erup tion.

Lahars 

Mudfl ows or debris flows com posed 

most ly of vol ca nic materials on the flanks 

of a vol ca no are called lahars. These flows 

of mud, rock, and water can rush down valleys and stream chan nels at speeds of 20 to 

40 miles per hour and can travel more than 

50 miles. Some la hars con tain so much rock 

de bris (60 to 90% by weight) that they look 

like fast-mov ing riv ers of wet con crete. Close 

to their source, these flows are pow er ful 

enough to rip up and carry trees, houses, and 

huge boul ders miles down stream. Far ther 

down stream they en tomb ev ery thing in their 

path in mud. 

Historically, lahars have been one of the 

dead li est volcano hazards. They can occur 

both dur ing an eruption and when a vol ca no 

is qui et. The water that creates lahars can 

come from melt ing snow and ice (es pe cial ly 

water from a gla cier melted by a py ro clas tic 

flow or surge), intense rain fall, or the breakThe town of Weed, California, nestled below 14,162-foot-high Mount Shasta, is built on a huge debris av a lanche 

that roared down the slopes of this volcano about 300,000 years ago. This ancient landslide (brown on inset map; 

arrows indicate flow directions) traveled more than 30 miles from the vol ca no’s peak, inundating an area of about 

260 square miles. The upper part of Mount Shasta volcano (above 6,000 feet) is shown in dark green on the map.

0 10 MILES

Yreka

Weed Mount

Shasta

14,162 ft

CALIFORNIA

5

97

Bobbie Myers, Steven R. Brantley, Peter Stauffer, and 

James W. Hendley II

Graphic design by

 Bobbie Myers, Sara Boore, and Susan Mayfi eld

COOPERATING ORGANIZATIONS

Alaska Div. of Geological and Geophysical Surveys

Federal Aviation Administration

National Oceanic and Atmospheric Administration

National Park Service

National Weather Service

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

University of Alaska

University of Hawaii

University of Utah

University of Washington

For more information contact:

U.S. Geological Survey

David A. Johnston Cascades Volcano Observatory

1300 SE Cardinal Court, Building 10, Suite 100,

Vancouver, WA 98683

Tel: (360) 993-8900, Fax: (360) 993-8980

e-mail: cvo@usgs.gov

URL: http://vulcan.wr.usgs.gov/

This Fact Sheet and any updates to it are available 

online at:

 http://pubs.usgs.gov/fs/fs002-97/

out of a sum mit cra ter lake. Large la hars 

are a po ten tial haz ard to many com mu ni ties 

downstream from gla cier-clad vol ca noes, 

such as Mount Rain i er. 

To help protect lives and property, sci en -

tists of the USGS Vol ca no Haz ards Pro gram 

main tain a close watch on the vol ca nic 

re gions of the United States, in clud ing the 

Pa cific Coast States, Wy o ming, Ha waii, and 

Alas ka. This on go ing work en ables the USGS 

to de tect the ear ly signs of vol ca no unrest and 

to warn the pub lic of im pend ing erup tions 

and as so ci at ed haz ards.

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