U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3075, April 2019

Supercedes USGS Fact Sheet 002–97

Volcanic eruptions are among Earth’s most dramatic and 

powerful agents of change. Ash, 

mudflows, and lava flows can devastate 

communities near volcanoes and 

cause havoc in areas far downwind, 

downstream, and downslope. Even 

when a volcano is quiet, steep volcanic 

slopes can collapse to become 

landslides, and large rocks can be 

hurled by powerful steam blasts. 

Hazardous volcanic conditions might 

last for a day or decades, all the while 

threatening people’s health and safety. 

Scientists with the U.S. Geological 

Survey and partner agencies assess 

hazards and closely monitor activity at 

the Nation’s volcanoes. They provide 

volcano updates and warnings of 

hazardous situations, as well as 

guidance on actions to take. You can 

prepare your family and community by 

familiarizing yourself with the types of 

hazards at volcanoes near where you 

live and visit.

The grandeur of volcanoes, whether 

capped by snow, covered by forests, 

or rocky and barren, often fails to hint 

at activity deep underground. Beneath 

an active volcano, rising magma 

(underground molten rock) generates 

earthquakes as it exerts pressure on and 

fractures surrounding rock. These fractures 

create pathways for rising heat, volcanic 

gases, and magma. Just below the surface, 

groundwater boils to steam. When magma 

reaches the surface a variety of eruption 

processes can occur. At some volcanoes, 

tiny shards of rock can be blasted skyward 

in columns of ash and steam. Lava can be 

pushed out of the ground as thick flows or 

pile up into steep-sided mounds. At other 

volcanoes, more-fluid lava might fountain 

into the air in a spectacular incandescent 

display and then flow quickly across 

the ground. With each new series of 

eruptions, the shape of a volcano changes, 

growing more massive or catastrophically 

collapsing. Even when activity stops, 

potentially hazardous change continues 

as water scours loose volcanic debris and 

transports it far down river valleys. 

The eruption and erosion processes 

that shape volcanoes are an ever-present 

threat. This fact sheet describes volcanic 

hazards and their impacts at two common 

types of volcanoes—stratovolcanoes and 

shield volcanoes. Volcano terms, shown in 

bold, are explained in two large graphics. 

Using the information in this fact sheet 

and the internet resources on page 6, you 

can learn more about volcanoes and how 

to prepare for future activity.

In this nightime view, Mount Shasta, a stratovolcano of the 

Cascade Range, rises above the town of Weed in Northern 

California—one of four small towns around the base of the 

volcano. This region is in the center of major interstate, rail, and 

air transportation corridors on the West Coast. (Copyrighted 

photograph by Mark Stensaas, used with permission.)

Volcano hazards have the potential 

to cause great harm and destruction, 

but whether or not you will be affected 

depends on the type of volcano 

(stratovolcano or shield volcano), the 

specific hazard, and your location 

relative to it. In all instances, heed the 

advice of emergency officials!

Volcano Hazards Are 

Location Dependent—

Heed Emergency Advice

Living With Volcano Hazards
Stratovolcanoes

Stratovolcanoes are tall, cone-shaped, and often glaciated 

volcanoes that tend to erupt explosively. In the Western United 

States, they are located in Alaska, Washington, Oregon, and northern 

California. Eruptions can proceed in the following sequence, but these 

events may occur independently. Magma (underground molten rock) 

rises from deep below the volcano, and (1) explosive eruptions blast 

volcanic debris (tephra) into the sky, forming an eruption column and 

cloud. (2) The ash cloud cools and drifts downwind, and ash falls to 

the ground at local air temperature. (3) Lava flows move downslope or 

form lava domes at the erupting vent. (4) Eruption columns, lava flows, 

or lava domes collapse, creating hot pyroclastic density currents. 

(5) These can melt snow and ice or enter rivers to create lahars. Even 

when a volcano is not erupting, landslides or debris avalanches can 

occur due to gravity. At stratovolcanoes and caldera systems that 

host hot springs, fumaroles, and other thermal features, hot rock and 

steam blasts called hydrothermal explosions can occur at any time. 

Caldera systems are large volcanic centers usually characterized by 

a massive central crater, like at Long Valley in California and in the 

Wyoming part of Yellowstone National Park.

Pyroclastic density current

Fumarole

Hydrothermal

explosion

Pyroclastic density current

Lahar (mud or debris flow)

Prevailing wind

Lava dome collapse

Eruption cloud

Ash (tephra) fall

Acid rain

Lava dome 

 

Cracks

Magma

Groundwater

Landslide

(debris avalanche)

Eruption column

Lava flow

Vent

2

Avoid Hazards and Heed Official Advice

• Lava flows, pyroclastic density currents, landslides, and 

rockfalls seldom travel far beyond a stratovolcano’s base. At 

the volcano, tephra fall will include large hot blocks of rock, 

and dangerous concentrations of volcanic gases can collect 

in caves. At actively outgassing volcanoes, avoid low and 

confined areas where dangerous gases might accumulate.

• Volcanic ash can fall tens to even thousands of miles away 

from an eruption. Avoid travel and reduce exposure by staying 

indoors. When exposed to intense falling or windblown ash, use 

protective breathing devices, such as masks.

• Lahars are a hazard on volcanic slopes and to downstream 

communities tens of miles from a volcano. If you receive a 

public emergency notification of a lahar, or otherwise recognize 

its approach, get off valley floors to higher ground as quickly 

and safely as possible.
Ashfall from the May 18, 1980, eruption of 

Mount St. Helens, Washington, blocked the sun 

causing street lights to turn on at 1:30 p.m. along 

Yakima Avenue in Yakima, Washington. Over 

several hours, Yakima received about one-half 

to three-quarters of an inch of ashfall. It choked 

the city with a stubborn cloud of suspended 

particles that made breathing difficult and 

driving nearly impossible. Ash affected the 

lives and businesses of the 50,000 residents for 

months. (Photograph copyright Yakima Valley 

Museum, used with permission.)

Lava flows have destroyed more than 900 structures 

on the Island of Hawaiʻi since 1983. In May 2018, 

erupting fissures opened along Kīlauea Volcano’s 

lower East Rift Zone and sent rivers of lava through 

residential subdivisions and farmland. For 3 months, 

sulfur dioxide and other volcanic gases billowed out 

of open vents, killing vegetation immediately 

downwind of the fissure system. As the gases drifted 

farther away, vog (volcanic smog) created poor air 

quality that caused breathing and health problems for 

people not only on Hawaiʻi, but also in Honolulu nearly 

250 miles away on the Island of Oʻahu. (Photograph by 

Volcano Helicopters, used with permission.)

Eruption Clouds and Ashfall—The 

Most Widespread Volcano Hazards

Explosive eruptions originate at 

volcanic vents and blast tephra (rock and 

volcanic glass fragments) and volcanic 

gases into the air with tremendous force. 

An eruption column can reach more 

than 12 miles above a volcano in less 

than 30 minutes. The column rises and 

spreads to form an eruption cloud. 

Volcanic ash particles (those smaller 

than about 0.08 inch in diameter) within 

the column and cloud are highly charged 

and generate lightning and thunder. The 

eruption cloud can extend thousands of 

miles downwind and rain gritty ash over 

vast regions. The largest tephra pieces fall 

near the vent, whereas the wind blows 

cooled volcanic ash great distances—

even around the world. 

Volcanic ash is a threat to people, 

agriculture, aviation, ground transportation, 

and communications and utility systems. 

For both ground and air transportation, ash 

reduces visibility, short-circuits electronic 

equipment, and damages engines—jet 

aircraft are particularly vulnerable. 

Flight delays and cancellations can 

interrupt delivery of goods and supplies. 

Accumulation of ash several inches thick, 

especially when wet, can collapse roofs, 

clog wastewater systems, and reduce the 

quality of drinking water. Even minor 

ashfall can harm crops, electronics, and 

machinery and is a health hazard to humans 

and animals.

Lava Flows Consume Everything in 

Their Way

Lava flows are molten rock and ooze 

onto the Earth’s surface from a volcanic 

vent. Stratovolcanoes, such as the large 

cone-shaped volcanoes in the Pacific 

Northwest and Alaska, often erupt viscous 

lava that flows like toothpaste. Thick and 

sluggish lava may travel a mile or more 

downslope. When flows of extremely 

viscous lava pile up around a vent, they 

build a lava dome. The thick and rubbly 

edges of new lava flows and domes 

can collapse to form hot, fast-moving 

pyroclastic density currents. 

Shield volcanoes, like those in the 

State of Hawaii, erupt less-viscous lava that 

flows like warm honey. These lava flows 

travel as glowing orange rivers or creep 

across terrain as shimmering silver masses 

that sprout lobes of red liquid rock. Lava 

can flow as fast as 30 miles per hour (mi/hr) 

on steep slopes but at less than 1 mi/hr on 

gentle slopes. Lava is erupted directly from 

open vents as flows or lava fountains (jets 

of low-viscosity lava sprayed into the air).

Anything in the way of an advancing 

lava flow will be surrounded, buried, or 

burned by extreme heat (as high as 

2,100 degrees Fahrenheit, °F). Lava 

flows also create secondary hazards 

such as fires, methane explosions, and 

dangerous conditions where they enter 

bodies of water. Fortunately, lava flows 

typically travel slowly enough that people 

can avoid contact.

Pyroclastic Density Currents Are 

Hot, Fast, and Destructive

The term pyroclastic density current

refers to a family of ground-hugging 

avalanches of hot gases and dry boulderto-ash-size rock fragments that rush down 

volcanic slopes at speeds that can exceed 

220 mi/hr. They originate as viscous 

lava flows, domes, and eruption columns 

that collapse and then roar downward. 

Rock-rich pyroclastic flows typically 

follow channels and valleys. Gas-rich 

pyroclastic surges are more mobile and 

spread unpredictably over valleys and hills. 

Flows may reach more than 10 miles from 

the source and leave deposits that range 

in thickness from a fraction of an inch on 

hillslopes to more than 700 feet in deep 

valleys. The extreme temperatures of such 

flows (as great as 1,500 °F) can cause fires 

and melt snow and ice, which may mix to 

generate lahars (see page 5).

As pyroclastic density currents spread, 

they knock down, shatter, burn, bury, and 

(or) carry away nearly everything in their 

paths. People and animals may be killed or 

seriously injured from burns and inhalation 

of hot ash and gases.

3

On August 7, 1980, a small explosive eruption 

occurred in the crater of Mount St. Helens, 

Washington. The eruption generated a 

pyroclastic density current that swept down 

the volcano’s north flank. (U.S. Geological 

Survey photograph by Peter Lipman.)
Shield Volcanoes and Lava Fields

The San Francisco Volcanic Field is an area of young volcanoes in northern 

Arizona’s Colorado Plateau region that covers 1,800 square miles. At the 

north end of the field, several cinder cones surround SP Crater (820 feet 

high), which is a picturesque example of a cinder cone and associated lava 

flow (shown here). The flow extends to the north 4.3 miles from SP Crater, 

where it originated, and is about 100 feet thick. The most recent eruption from 

the field was at Sunset Crater (about 17 miles to the south) in about A.D. 1085. 

(Copyrighted photograph by Michael Collier, used with permission.)

4

Shield volcanoes have a broad, shield shape and tend to erupt 

fluid lava that can travel many miles, but violent explosive eruptions 

are also possible. Hawaiian volcanoes are the quintessential shield 

volcanoes. Lava fields, which erupt in the same way, are found in 

nearly every State west of the Rocky Mountains. Most eruptions 

begin as a vertical sheet (dike) of rising magma that discharges 

from groups of aligned vents, called fissures, which can extend 

for miles. (1) Low lava fountains jet skyward, and lava fragments 

cool to tephra as they fall. Activity may localize at a single vent and 

grow more energetic to produce a diffuse ash and gas cloud that 

drifts downwind. (2) Emission of volcanic gas can produce volcanic 

smog (vog). (3) Lava flows can travel long distances, sometimes 

underground within insulating lava tubes. When the roofs of tubes 

break, open skylights are created, allowing heat and fumes to 

escape. (4) Lava can pour from a vent for months or years to form a 

lava flow field that can feed breakouts of new lava flows. (5) When 

lava buries vegetation, intense heat cooks plant matter, producing 

gas, and sudden methane explosions can violently toss large rocks 

tens of feet into the air. (6) Lava entering the ocean (ocean entry) or a 

body of water creates new, unstable land called a lava delta that can 

explosively collapse into the sea. Lava entering cold ocean water 

typically causes littoral explosions—hot seawater and acidic 

clouds of gas, steam, and volcanic glass that blow downwind as 

a laze plume.

Avoid Hazards and Heed Official Advice

• Lava flows can travel tens of miles. Lava deltas can 

catastrophically collapse and blast large rocks hundreds of 

feet into the air. Hazardous fumes at lava water-entry sites can 

make breathing difficult.Stay a safe distance away from lava 

deltas and active lava flows.

• During an eruption or vigorous gas emission, vog spreads with 

the wind and can affect areas hundreds of miles from the 

volcano.Check local air-quality conditions and heed warnings

and posted signs.

Laze plume

Volcanic smog (vog)

Diffuse ash and 

gas cloud

Lava flow

Lava tube Skylight

Lava delta

Lava 

fountain

Tephra

Ocean entry Littoral explosion

Methane 

explosion

Breakout

Flow field

Prevailing wind
On the morning of May 18, 1980, the 

eruption of Mount St. Helens, Washington, 

caused massive lahars that choked rivers 

with sediment. This bridge was carried 

one-third of a mile downstream and 

partially buried along with large boulders 

that were also transported by the lahar. 

(U.S. Geological Survey photograph by 

Richard Waitt.)

Hydrothermal explosions are common in 

Yellowstone National Park. Here, steam 

and pulverized rock blast from a hot spring 

in Biscuit Basin on May 29, 2009. In the 

1880s and early 1890s, a series of powerful 

explosions and geyser eruptions ripped 

through the “plumbing” system beneath 

Excelsior Geyser in the Midway Geyser 

Basin. Large rocks were hurled as far as 

50 feet. In 1989, at Porkchop Geyser in 

Norris Geyser Basin, an explosion ejected 

an apron of rock debris 15 feet across. 

(Photograph courtesy of Robert Smith, 

University of Utah.)

5

Lahars Travel Rapidly Down Valleys 

Like Rivers of Flowing Concrete

Lahars are mud or debris flows 

containing soil, rock, and water that originate 

on the flanks of a volcano. A lahar resembles 

a slurry of wet, flowing concrete as it rushes 

down narrow valleys at speeds as great as 

40 mi/hr. Close to their source, lahars are 

torrents that can rip up and carry trees and 

car-sized boulders. When a lahar reaches 

flatter terrain, it loses speed, spreads, and 

leaves a layer of mud over a broad area. 

Lahars can bury agricultural land and 

sweep away bridges, roads, or entire towns, 

entombing everything where deposits settle.

Lahars are a major hazard at 

volcanoes with glaciers, crater lakes, or 

significant runoff from precipitation. When 

a volcano erupts, hot rocks can mix with 

meltwater and (or) surface water to form 

lahars. During intense rainfall, flowing 

water scours barren volcanic deposits and 

may form lahars that transport debris far 

down river valleys. For decades after lahars 

come to rest, rivers erode the deposits 

and transport rocks and mud downstream. 

The excess sediment fills channels and 

repeatedly forces water to flood beyond 

river banks. Constricted and sedimentladen rivers devastate aquatic habitats and 

clog shipping lanes.

Although lahars commonly reach tens 

of miles from their source, they can extend 

more than 100 miles. Even when a volcano 

is quiet, smaller events can occur seasonally 

during heavy rainfall, with damage mostly 

limited to stream channels.

Volcanic Slopes Can Collapse Even 

Without an Eruption

Volcanoes are susceptible to landslides 

because their steep slopes are made of 

numerous overlapping layers of lava flows, 

loose rock rubble, and ash. Hot, acidic 

groundwater can circulate and weaken 

volcanic rocks to the point that they transform 

into wet, slippery clay. The constant pull of 

gravity is always at work at volcanoes, and 

eruptions, intrusions of magma, fracturing by 

large local earthquakes, and intense rainfall 

can all contribute to the eventual collapse of 

volcanic slopes.

Volcanic landslides may occur during 

increased volcanic activity but also at other 

times without warning. When landslides 

move at high speed down slopes, they can 

break apart to form debris avalanches. Large 

debris avalanches may bury a river valley or 

the flanks of a volcano under tens to hundreds 

of feet of rock debris and create a chaotic 

landscape marked by small hills (hummocks) 

and pits. When mixed with sufficient 

water, landslides and debris avalanches can 

transform into lahars.

Volcanic Gas Harms Human, Animal, 

and Plant Health

Magma contains dissolved gases, 

the driving force of volcanic eruptions. 

Water vapor is the most abundant volcanic 

gas followed by lesser amounts of carbon 

dioxide (CO2), sulfur gases, and other 

gases. At active volcanic systems, gases 

are released continuously from volcanic 

vents, fumaroles, springs, and even through 

the soil. Almost all volcanic gas dissipates 

harmlessly into the atmosphere, but there 

are exceptions. 

CO2 gas is odorless and heavier than 

air. At actively outgassing volcanoes, it 

collects in concentrations that can quickly 

overcome and suffocate people and animals, 

especially within snow and earthen caves 

and in low-lying depressions. 

Sulfur dioxide (SO2) gas is lighter 

than air and when inhaled has a bitter taste. 

Emissions of large volumes of SO2 gas 

can lead to localized acid rain and more 

widespread volcanic smog (vog). Hazards 

associated with SO2 gas include acid-rain 

corrosion of metals and damage to plants. 

Vog is a significant health hazard for 

people, particularly those with preexisting 

respiratory ailments. 

Heated Ground and Hot Water Create 

Beautiful but Deadly Scenery

Volcanoes in areas with ample 

groundwater host hot springs, geysers, 

and other beautiful hydrothermal 

features—like those of Yellowstone 

Caldera in Wyoming. The ground surface 

in thermal areas can be slippery or covered 

with a thin breakable crust that overlies 

scalding fluid. Physical contact with hot 

springs and thermal features can cause 

severe burns, and submersion is potentially 

deadly. Hot springs have injured or killed 

more people in Yellowstone National Park 

than any other natural feature, including 

encounters with wildlife.

Underground, hot water can become 

so pressurized that it reaches temperatures 

above its boiling point. Shifts in a 

hydrothermal system can cause sudden 

pressure changes that make water burst 

into steam and violently expel rocks and 

hot water as a hydrothermal explosion. 

These can occur without any prior sign of 

disturbance. Such explosions can excavate 

shallow craters more than 1,000 feet wide 

and hurl large rocks hundreds of feet.
USGS Monitors the Nation’s Volcanoes

Scientists with the U.S. Geological Survey (USGS) Volcano 

Hazards Program and partner agencies monitor the Nation’s volcanoes 

and report abnormal activity to emergency authorities who order 

closures and evacuations. These scientists also aid development of 

emergency coordination plans and promote preparedness to minimize 

the impact of volcano hazards on communities. Through volcano 

monitoring, short-term warnings, research on how volcanoes work, and 

community education and outreach, the Volcano Hazards Program is 

helping to reduce loss of life and property from volcano hazards (see 

USGS Fact Sheet 2016–3040, https://doi.org/10.3133/fs20163040).

Take Steps to Prepare for Volcano Hazards

• Learn about volcanoes that may impact you—Visit the USGS Volcano Hazards Program website (https://volcanoes.usgs.gov) to find out how

you might be at risk.

• Find community resources—Ask local officials for emergency plans.

• Prepare your family for an emergency—Create and practice an emergency communication plan. Assemble emergency food, water, and

household supplies in advance. Reliable information will come from government sources.

• Know what to do in a volcanic emergency—Listen to emergency officials, and follow their instructions. Be ready to act.

Find Out More About U.S. Volcano Hazards

Volcano Hazards Program: https://volcanoes.usgs.gov/

Volcano Notification Service: https://volcanoes.usgs.gov/vns2/

Volcanic Ashfall Impacts Working Group: https://volcanoes.usgs.gov/volcanic_ash/

USGS Volcanoes Facebook: https://facebook.com/USGSVolcanoes/

USGS Volcanoes Twitter: https://twitter.com/USGSVolcanoes/

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183075

Supercedes USGS Fact Sheet 002–97

In 2014, a lava flow from Kīlauea Volcano on the Island of Hawaiʻi began an erratic 12-mile downslope run toward Pāhoa village. Lava threatened roads, homes, and 

businesses. In response, scientists from the U.S. Geological Survey (USGS) Hawaiian Volcano Observatory conducted daily evaluations, mapped advancing lava, and 

issued formal updates on evolving hazards. Hundreds of community members attended more than two dozen public meetings where scientists presented updates on 

lava-flow hazards and discussed residents’ concerns one-on-one. Frequent, coordinated messages from Federal, State, and local government agencies kept people 

informed and prepared. (USGS photograph by Steven Brantley, September 11, 2014.)

6

Ascending eruption column and expanding eruption cloud from 

Redoubt Volcano, Alaska, looking to the west from the Kenai Peninsula 

on April 21, 1990. The mushroom-shaped plume rose from a pyroclastic

density current that cascaded down the north flank of the volcano,

triggered by collapse of a growing lava dome. A smaller steam plume

rises from the summit vent. (Photograph by Robert Clucas, donated to

the U.S. Geological Survey.)

Wendy K. Stovall, Carolyn L. Driedger, 

Elizabeth G. Westby, and Lisa M. Faust

Edited by James W. Hendley II

Illustrations by Lisa M. Faust and Bobbie Myers

Layout and design by Cory D. Hurd

For more information, contact:

U.S. Geological Survey

Volcano Hazards Program

12201 Sunrise Valley Dr., MS 905, Reston, VA 20192

https://volcanoes.usgs.gov/