U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet-002-97

Revised June 1998

What are Volcano Hazards?

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

V

US SG

olcanoes give rise to numerous

geologic and hydrologic hazards.

U.S. Geological Survey (USGS) scientists are assessing hazards at many

of the almost 70 active and potentially

active volcanoes in the United

States. They are closely monitoring

activity at the most dangerous of these

volcanoes and are prepared to issue

warnings of impending eruptions or

other hazardous events.

United States. Heavy ash fall can collapse

buildings, and even minor ash fall can damage

crops, electronics, and machinery.

Volcanic Gases

Volcanoes emit gases during eruptions. Even

when a volcano is not erupting, cracks in the

ground allow gases to reach the surface through

small openings called fumaroles. More than

ninety percent of all gas emitted by volcanoes

is water vapor (steam), most of which is heated

ground water (underground water from rainfall and streams). Other common volcanic

gases are carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulfur dioxide gas can react with water droplets in the

atmosphere to create acid rain, which causes

corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low

areas in concentrations that are deadly to

More than 50 volcanoes in the United States

have erupted one or more times in the past 200

years. The most volcanically active regions of

the Nation are in Alaska, Hawaii, California,

Oregon, and Washington. Volcanoes produce a

wide variety of hazards that can kill people and

destroy property. Large explosive eruptions can

endanger people and property hundreds of

miles away and even affect global climate.

Some of the volcano hazards described below,

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

Eruption Columns and Clouds

 An explosive eruption blasts solid and molten rock fragments (tephra) and volcanic gases

into the air with tremendous force. The largest

rock fragments (bombs) usually fall back to

the ground within 2 miles of the vent. Small

fragments (less than about 0.1 inch across) of

volcanic glass, minerals, and rock (ash) rise

high into the air, forming a huge, billowing

eruption column.

Eruption columns can grow rapidly and

reach more than 12 miles above a volcano in

less than 30 minutes, forming an eruption

cloud. The volcanic ash in the cloud can pose a

serious hazard to aviation. During the past 15

years, about 80 commercial jets have been

damaged by inadvertently flying into ash

clouds, and several have nearly crashed because of engine failure. Large eruption clouds

can extend hundreds of miles downwind, resulting in ash fall over enormous areas; the

wind carries the smallest ash particles the farthest. Ash from the May 18, 1980, eruption of

Mount St. Helens, Washington, fell over an

area of 22,000 square miles in the Western

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 simplified

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 occur even when a volcano is not erupting. (Hazards and terms in this diagram are highlighted

in bold where they are discussed in the text below.)
people and animals. Fluorine, which in high

concentrations is toxic, can be adsorbed onto

volcanic ash particles that later fall to the

ground. The fluorine on the particles can poison livestock grazing on ash-coated grass and

also contaminate domestic water supplies.

Cataclysmic eruptions, such as the June 15,

1991, eruption of Mount Pinatubo (Philippines), inject huge amounts of sulfur dioxide

gas into the stratosphere, where it combines

with water to form an aerosol (mist) of sulfuric

acid. By reflecting solar radiation, such aerosols can lower the Earth’s average surface temperature for extended periods of time by several degrees Fahrenheit ( ̊F). These sulfuric

acid aerosols also contribute to the destruction

of the ozone layer by altering chlorine and nitrogen compounds in the upper atmosphere.

Lava Flows and Domes

Molten rock (magma) that pours or oozes

onto the Earth’s surface is called lava and

forms lava flows. The higher a lava’s content

of silica (silicon dioxide, SiO2

), the less easily

it flows. For example, low-silica basalt lava

can form fast-moving (10 to 30 miles per hour)

streams or can spread out in broad thin sheets

up to several miles wide. Since 1983, Kilauea Volcano on the Island of Hawaii has erupted basalt

lava flows that have destroyed nearly 200

houses and severed the nearby coastal highway.

In contrast, flows of higher-silica andesite

and dacite lava tend to be thick and sluggish,

traveling only short distances from a vent.

Dacite and rhyolite lavas often squeeze out of

a vent to form irregular mounds called lava

domes. Between 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 fragments, and gas can move down the sides of a

volcano during explosive eruptions or when the

steep side of a growing lava dome collapses

and breaks apart. These pyroclastic flows can

be as hot as 1,500 ̊F and move at speeds of 100

to 150 miles per hour. Such flows tend to follow valleys and are capable of knocking down

and burning everything in their paths. Lowerdensity pyroclastic flows, called pyroclastic

surges, can easily overflow ridges hundreds of

feet high.

The climactic eruption of Mount St. Helens

on May 18, 1980, generated a series of explosions that formed a huge pyroclastic surge.

This so-called “lateral blast” destroyed an area

of 230 square miles. Trees 6 feet in diameter

were mowed down like blades of grass as far as

15 miles from the volcano.

Volcano Landslides

A landslide or debris avalanche is a rapid

downhill movement of rocky material, snow,

and (or) ice. Volcano landslides range in size

from small movements of loose debris on the

surface of a volcano to massive collapses of the

entire summit or sides of a volcano. Steep volcanoes are susceptible to landslides because

they are built up partly of layers of loose volcanic rock fragments. Some rocks on volcanoes

have also been altered to soft, slippery clay

minerals by circulating hot, acidic ground water. Landslides on volcano slopes are triggered

when eruptions, heavy rainfall, or large earthquakes cause these materials to break free and

move downhill.

At least five large landslides have swept

down the slopes of Mount Rainier, Washington,

during the past 6,000 years. The largest volcano landslide in historical time occurred at the

start of the May 18, 1980, Mount St. Helens

eruption.

Lahars

Mudflows or debris flows composed

mostly of volcanic materials on the flanks of a

volcano are called lahars. These flows of mud,

rock, and water can rush down valleys and

stream channels at speeds of 20 to 40 miles per

hour and can travel more than 50 miles. Some

lahars contain so much rock debris (60 to 90%

by weight) that they look like fast-moving rivers of wet concrete. Close to their source, these

flows are powerful enough to rip up and carry

trees, houses, and huge boulders miles downstream. Farther downstream they entomb everything in their path in mud.

Historically, lahars have been one of the

deadliest volcano hazards. They can occur both

during an eruption and when a volcano is quiet.

The water that creates lahars can come from

The town of Weed, California, nestled below 14,162-foot-high Mount Shasta, is built on a huge debris avalanche 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 volcano’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

 Sara Boore, Bobbie Myers, and Susan Mayfield

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

Cascades Volcano Observatory

5400 Mac Arthur Blvd., Vancouver, WA 98661

Tel: (360) 696-7693, Fax: (360) 696-7866

e-mail: cvo@usgs.gov

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

melting snow and ice (especially water from a

glacier melted by a pyroclastic flow or surge),

intense rainfall, or the breakout of a summit

crater lake. Large lahars are a potential hazard

to many communities downstream from glacier-clad volcanoes, such as Mount Rainier.

To help protect lives and property, scientists

of the USGS Volcano Hazards Program maintain a close watch on the volcanic regions of

the United States, including the Pacific Coast

States, Wyoming, Hawaii, and Alaska. This ongoing work enables the USGS to detect the first

signs of volcano unrest and to warn the public

of impending eruptions and associated hazards.