U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 022–00

2000

Volcano Hazards of the Lassen Volcanic National Park

Area, California

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

I

US SG

n May 1915, Lassen Peak,

California, the southernmost active volcano in the Cascade Range, erupted explosively. Avalanches, mudflows,

and flows of hot ash and gas

devastated nearby areas, and

volcanic ash fell as far away as

200 miles to the east. The

Lassen area remains volcanically active, and the volcano

hazards demonstrated in 1915

still can threaten not only

nearby areas but also more distant communities. Recent work

by scientists with the U.S. Geological Survey (USGS) in cooperation with the National Park

Service is shedding new light

on these hazards.

On the night of May 19, 1915, the few people

homesteading outside of Old Station along Hat

Creek near the foot of Lassen Peak, a volcano in

northern California, went to bed expecting a

peaceful night’s sleep. By now they had become

accustomed to the sounds of small steam explosions coming from the volcano, which had been

intermittently active during the past year. Around

midnight, Elmer Sorahan was awakened by his

dog barking furiously and pawing him. Dressing

quickly, Elmer went outside, expecting a bear or

other animal. Instead, he dimly saw a 12-foothigh wall of muddy water and logs rumbling

down Hat Creek. After running more than a

mile to warn his downstream neighbors, the

Halls, he burst through their front door exhausted and shouting “Get out! get out! there’s

a flood coming.” Mrs. Hall quickly spread the

alarm downstream by telephone, and then the

family scrambled uphill just before the house

was swept off its foundation.

The next morning residents of the area saw

that a wide swath of the northeast slope of

Lassen Peak had been devastated by a huge

avalanche and mudflow triggered by a powerful

explosion at the volcano’s summit. Fortunately,

because of the warnings, no one was killed, but

several houses along the creek were destroyed.

When Lassen Peak erupted again on May 22nd,

the area was further devastated by a high-speed

flow of hot volcanic ash and gas (called a “pyroclastic flow”), and the incorporation of snow into

this flow generated new mudflows. Ash from the

eruption rose high into the air and wind blew it

eastward. Fine ash fell at least as far as 200

miles from the volcano. Because of the eruptive

activity, which continued through 1917, and the

area’s stark volcanic beauty, Lassen Peak and

the area surrounding it were declared a National Park in 1916.

Volcanic history of the Lassen Volcanic

National Park region

The Lassen region has been volcanically

active for about 3 million years. Lassen Peak and

nearby volcanic domes are the most recently

active parts of the Lassen “volcanic center,”

which began to erupt about 600,000 years ago.

From 600,000 to 400,000 years ago, eruptions

built a large conical volcano, often referred to as

“Brokeoff Volcano” or “Mount Tehama,” which

was roughly the size of Mount St. Helens, Washington. Later, this volcano became inactive and

was mostly eroded away, leaving remnants that

include Brokeoff Mountain, Mount Conard,

Mount Diller, and Diamond Peak.

Subsequent eruptions from the Lassen volcanic center have formed more than 30 steepsided, mound-shaped accumulations of volcanic rock, called “lava domes.” Eruptions about

27,000 years ago formed Lassen Peak, one of

the largest lava domes on Earth. When Lassen

Peak formed, it looked much like the nearby

1,100-year-old Chaos Crags domes, with steep

sides covered by angular rock talus. However,

from 25,000 to 18,000 years ago, during the

last ice age, Lassen Peak’s shape was significantly altered by glacial erosion.

Lassen Peak and its neighboring lava domes

are not typical, conical “stratovolcanoes” like

Mount Shasta or Mount Rainier. Those large

volcanoes were formed by repeated eruptions

The May 22, 1915,

explosive eruption of

Lassen Peak, California, blasted pumice

and rock fragments

high into the air. In this

photograph taken from

the town of Red Bluff,

40 miles west of the

volcano, a huge column of volcanic ash

and gas produced by

the eruption rises to a

height of more than

30,000 feet. Winds blew

volcanic ash eastward

from the column,

raining fine ash at least

as far away as

Winnemucca, Nevada,

200 miles from the

volcano. The ash cloud

from a similar eruption

today would pose a

serious hazard to flying

aircraft in the Western

United States. (Photo

by R.E. Stinson; courtesy National Park

Service.)
of lava and ash from a central summit vent over

tens of thousands of years. The Lassen dome

field, in contrast, is an example of a volcanic

area that erupts lava from numerous individual

vents, each of which is active for a few years to

a few decades and usually does not erupt again.

The composition of the molten rock (magma)

that feeds volcanism in the Lassen area ranges

widely in its content of silica (SiO2

). When highsilica (dacite) magma rises to the Earth’s surface,

it can erupt explosively to produce ash clouds

and pyroclastic flows. Dacite magma extruded

nonexplosively forms lava domes, because it is

too viscous to flow far away from its source.

Low-silica (basalt) magma is more fluid and

usually erupts less explosively than dacite

magma. Eruptions of basalt magma typically

produce elongate lava flows, as well as build

cinder cones (piles of small frothy lava fragments or “cinders”) around volcanic vents.

In the past 50,000 years, at least seven major

episodes of dacitic volcanism produced lava

domes and pyroclastic deposits in the Lassen

area, and another five episodes produced basaltic

and andesitic (silica content between basalt and

dacite) lava flows. In addition, about 30 smaller

volcanoes erupted basaltic lavas in the larger

region surrounding the Lassen volcanic center.

What are the hazardous volcanic

processes of the Lassen area?

The most common volcanic activity in the

Lassen Peak region consists of small to moderate-sized eruptions that produce basaltic lava

flows and localized ash falls. These eruptions

typically last a few months to a year, but may

continue for several years. They can cover

more than a square mile with lava flows, build

cinder cones as high as 1,000 feet, and blanket

many square miles with ash a few inches to

several feet deep. Because these eruptions are

relatively nonviolent, they rarely cause human

fatalities.

Basaltic volcanism in the Lassen area occurs

mainly along chains of vents aligned in a north

or northwest direction, parallel to regional

faults. Examples include Poison Buttes, Subglacial Buttes, Tumble Buttes, the Prospect

Peak-Red Cinder area, the east side of the Hat

Creek Valley and Potato Buttes-Sugarloaf area,

and the Red Lake Mountain area. Prolonged

basaltic volcanism at a single site can produce

a sizeable edifice, like the broad, relatively flat

“shield” volcanoes of Prospect Peak and

Sifford Mountain.

Dacite eruptions in the Lassen area typically

begin with steam explosions caused by the

interaction of rising magma with ground water.

When dacite magma charged with volcanic gases

reaches the surface, it erupts explosively, usually

as a vertical column of gas and ash that can rise

several miles into the atmosphere. Heavy

fallback of hot ash and rock fragments from

eruption columns may generate highly mobile

pyroclastic flows that can rush several miles

down a volcano’s slopes and adjacent valleys.

Fallout from the eruption column can blanket

areas within a few miles of the vent with a thick

layer of pumice, and high-altitude winds may

carry finer ash tens to hundreds of miles from the

volcano, posing a hazard to flying aircraft, particularly those with jet engines.

After an initial explosive eruption, extrusion

of gas-depleted dacite magma commonly forms

lava domes. Growing lava domes are inherently

unstable, and collapse of their steep sides often

generates pyroclastic flows of lava blocks and

ash that can travel several miles. Such a sequence of events is recorded by the deposits

related to the emplacement of Chaos Crags

domes between 1,100 and 1,000 years ago.

Interaction of hot pyroclastic flows with snow

and ice can generate highly mobile flows of mud

and debris (called “lahars”) that may rush down

valleys leading away from a volcano. Because of

this, active volcanoes that have a significant

snow and ice cover can be particularly dangerous. The lahars that threatened residents of the

Lassen area in May 1915 were generated by

relatively small eruptions of Lassen Peak. Nonetheless, they traveled down creek beds as far as

12 miles and released floods that affected valleys

for 30 miles downstream.

What non-eruptive volcano hazards are

important in the Lassen area?

Additional volcano hazards at Lassen are

rockfalls and landslides not directly related to

eruptions. Recently erupted volcanic domes are

unstable and can collapse, generating small to

large rockfalls. Approximately 350 years ago,

collapse of one of the Chaos Crags domes generated huge rockfalls, creating an area now called

the Chaos Jumbles. The first and largest of these

traveled 4 miles downslope and was able to

climb 400 feet up the side of Table Mountain.

The trigger for the rockfall is unknown, but it

was most likely a large earthquake. Normal

weathering also weakens fractured volcanic rock

and contributes to small rockfalls. In the summer

of 1994, a rockfall of 13,000 cubic yards (the

volume of about 500 minivans) occurred on the

northeastern flank of Lassen Peak. During periods of extreme rainfall or snow melt, mudflows

The northeast side of Lassen Peak still shows the scars of its 1914 to 1917 series of eruptions. The

strongest of these eruptions in May 1915 destroyed a 3-square-mile area, now called the Devastated

Area. Photo at right shows the peak in eruption in October 1915 (photo by Chester Mullen; courtesy

National Park Service).

This twisted stump is all that

remains of a 100-foot-tall red fir

tree snapped off in Lassen Peak’s

May 1915 eruptions. During the

eruptions, high-speed avalanches of hot ash, rock fragments, and gas (pyroclastic flows) and huge mudflows

of volcanic materials and melted snow (lahars) swept down the northeast flank of the volcano, flattening

many acres of mature forest (see inset photo by Benjamin Loomis; courtesy National Park Service). Some

of the lahars traveled more than 12 miles from the volcano, destroying homes along Hat Creek.
The Lassen region has been volcanically active for more than 3 million years.

The Lassen “volcanic center” began to erupt about 600,000 years ago. From

600,000 to 400,000 years ago, eruptions built a large volcano, often referred to

as “Brokeoff Volcano” or “Mount Tehama” (outlined by brown circle). Later,

this volcano became inactive and was mostly eroded away, leaving remnants

that include Brokeoff Mountain, Mount Conard, Mount Diller, and Diamond

Peak. Subsequent eruptions in the Lassen volcanic center have formed more

than 30 steep-sided lava domes (the Lassen dome field). The most recently

active parts of the volcanic center are Lassen Peak and other young domes

formed in the past 50,000 years.

Krummholz Dome

Crescent Crater

Sunflower Flat

Hat Mountain

Kings Creek

Lassen Peak

Prospect Peak area

Chaos Crags

Cinder Cone

1915 Lassen Peak

50 40 30 20 10 0

Thousands of years ago

Cones and Hat Creek flow

Subglacial Buttes

 lava flows

 (low-silica

 eruptions)

Domes and

 pyroclastic

 flows

 (high-silica

 eruptions)

Lassen Peak

Red Lake

Mountain

Brokeoff

Volcano

Table

Mtn

Eagle Peak

Bumpass Hell

Mount Conard

Mount Diller

Diamond Peak Brokeoff

Mountain

Sugarloaf

Peak

Chaos Crags

Chaos Jumbles

Crescent Crater

Krummholz Dome

Sunflower

Flat

Prospect

Peak

LASSEN VOLCANIC

NATIONAL PARK

Cinder Cone

Red 

Cinder

Poison

Buttes

Hat Mountain

Sifford Mountain

89

89

44

44

Snag

Lake

Butte Lake

Manzanita

Lake

Juniper

Lake

Horseshoe

 Lake

Old Station

to Mineral

Hat Creek

PotatoButtes

Tumble Buttes

Kings Creek Subglacial

Buttes

0

0 4 KILOMETERS

4 MILES

CALIF.

Area

of map

Hat Creek flow

are sometimes generated by mobilization of

loose volcanic debris and soil on the slopes of

volcanoes.

Where are the most hazardous areas?

Volcano hazards are generally evaluated on

the basis of an area’s record of eruptions over the

past 10,000 years, because future eruptions are

most likely to occur near areas that have most

recently had volcanic activity. However, in the

Lassen region eruptions occur infrequently, so

the record of activity in the past 50,000 years

was used to provide an adequate basis for defining hazard zones. During this period, eruptions

in the Lassen region have occurred at sites including Lassen Peak, Chaos Crags, and Sunflower Flat (explosive dacite eruptions followed

by dome growth) and Tumble Buttes, Hat Mountain, and Prospect Peak (basalt eruptions). The

areas of highest hazard are those that could be

affected by pyroclastic flows and mudflows (see

map). These areas, including Hat Creek Valley, are

those in the immediate vicinity and downhill

from likely eruption sites. Fallout of ash will

affect areas downwind at the time of an eruption.

Within the hazard zones, relative hazard is gradational, decreasing away from the location of

potential vents.

What are the prospects for future eruptions at Lassen?

Because geologically recent volcanic activity

in an area is the best guide to forecasting future

eruptions, scientists study the lava flows, ash, and

other deposits from past eruptions. Volcanoes in

the Lassen area tend to erupt infrequently, and

may be inactive for periods lasting centuries or

even millennia. The most recent eruptions in the

Lassen area were the relatively small events that

The Chaos Crags, a group of six lava domes, was formed about

1,100 years ago in the latest large eruptions to occur in the area of

occurred at Lassen Peak between 1914 and 1917.

The most recent large eruption produced Chaos

Crags about 1,100 years ago. Such large eruptions in the Lassen area have an average recurrence interval of about 10,000 years. However,

the geologic history of the Lassen area indicates

that volcanism there is episodic, having periods

of relatively frequent eruptions separated by long

quiet intervals. For example, the last large event

before the Chaos Crags eruption was the one that

built Lassen Peak 27,000 years ago.

What are the warning signs of an eruption?

The most important sign of an impending

volcanic eruption is seismic activity beneath the

volcanic area. Seismologists can interpret subtle

differences between earthquakes related to the

rise of magma and the more familiar quakes

caused by tectonic faulting. Other warning signs

of magma rising into the shallow subsurface

might include increased release of volcanic gases

from small openings called fumaroles, such as

those found in the Bumpass Hell area of Lassen

Volcanic National Park, and changes in the gas

composition. Deformation of the ground surface

in the vicinity of a volcano may also indicate that

magma is approaching the surface. Typically,

these warning signs appear a few weeks to months

before an eruption, but can last for decades or

even centuries without leading to an eruption.

Lassen Volcanic National Park. The large block of lava in the inset was carried down from the Crags in a

high-speed avalanche of hot rock fragments and gas (pyroclastic flow) generated when the unstable

edge of one of the growing lava domes collapsed. The radial joint pattern was formed as the block cooled.

VOLCANIC SITES IN THE LASSEN REGION

AND ERUPTIONS IN THE PAST 50,000 YEARS
ASH FALL FROM ERUPTIONS IN THE LASSEN REGION 600,000 YEARS AGO

Graphic design by

Sara Boore and Susan Mayfield

Banner design by Bobbie Myers

For more information contact:

U.S. Geological Survey

David A. Johnston Cascades Volcano Observatory

5400 MacArthur Blvd., Vancouver, WA 98661

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

http://vulcan.wr.usgs.gov

or

U.S Geological Survey Volcano Hazards Program

http://volcanoes.usgs.gov

See also What are Volcano Hazards? (USGS Fact Sheet 002–

97), Eruptions of Lassen Peak, California, 1914 to 1917 (USGS

Fact Sheet 173–98), and How Old is “Cinder Cone”?—Solving

a Mystery in Lassen Volcanic National Park, California (USGS

Fact Sheet 023-00).

Michael A. Clynne, Robert L. Christiansen, C. Dan Miller,

Peter H. Stauffer, and James W. Hendley II

COOPERATING ORGANIZATIONS

 Lassen Park Foundation

National Park Service

U.S. Forest Service

What is being done to monitor the Lassen

volcanic center?

After the eruption of Mount St. Helens in

1980, the U.S. Geological Survey (USGS) intensified its monitoring of active and potentially

active volcanoes in the Cascade Range. Monitoring of the Lassen area includes periodic measurements of ground deformation and volcanicgas emissions and continuous transmission of

data from a local network of nine seismometers

to USGS offices in Menlo Park, California.

Should indications of a significant increase in

volcanic activity be detected, the USGS will

immediately deploy scientists and specially

designed portable monitoring instruments to

evaluate the threat. In addition, the National

Park Service (NPS) has developed an emergency

response plan that would be activated to protect

Although extremely unlikely today, very large

eruptions have occurred in the Lassen region in

the distant past. For example, about 600,000 years

ago there was an eruption 50 times larger than

the 1980 eruption of Mount St. Helens. Because

some winds at the time of that eruption were

blowing southward, ash several inches thick was

deposited as far south as the San Francisco Bay

area. Ash particles from this deposit, known as

the Rockland Ash, are shown in the inset, magni0

0 400 KILOMETERS

400 MILES

Lassen Volcanic Region

PACIFIC 

 

 OCEAN

OREGON

IDAHO

UTAH

NEVADA

CALIFORNIA

Rockland Ash erupted

~600,000 years ago

fied about 70 times. A similar

eruption today could affect

communities anywhere in

northern California and northwestern Nevada, depending

on wind direction. Even a light

dusting of volcanic ash can

close roads and seriously

disrupt communications and

utilities during and for many

weeks after an eruption.

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36

44

36

32

CALIF.

Area

of map

Shingletown

Viola

Chester

Westwood

Burney

Mineral

Red

Bluff

To

Redding

To Chico

Old

Station

Mill

Cree k

Deer

Creek

Lake

Almanor

N Fork Feathe r R

Battle Creek North

Fork

South

Fork

N

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10 MILES

The areas of highest hazard in the region of Lassen Volcanic National Park are those that could potentially be affected by pyroclastic flows and mudflows.

These areas are those in the immediate vicinity and downhill from likely eruption sites. Fallout of ash will affect areas downwind at the time of an eruption.

Within the hazard zones, relative hazard is gradational, decreasing away from the location of potential vents.

Hazards from Low-Silica (Mainly Basalt) Volcanism

Vents—Sites of low-silica eruptions known or estimated 

to have occurred during the past 50,000 years.

Lava-Flow Hazard Zone—Areas potentially subject to lava 

flows from nearby vents. 

Ash-Fall Hazard Zone—Areas potentially subject to ash 

fall from mildly explosive eruptions of nearby vents.

Hazards from High-Silica (Mainly Dacite) Volcanism

Vents—Sites of high-silica eruptions during the past 

50,000 years.

Combined Hazard Zone—Areas adjacent to potentially 

explosive volcanoes or vents, capable of producing ash 

fall; high-speed avalanches of hot rock fragments, ash, 

and gas (pyroclastic flows); lava domes and flows; and 

mudflows (lahars).

Pyroclastic-Flow Hazard Zone—Maximum outer limit of 

area potentially subject to high-speed avalanches of hot 

rock fragments, ash, and gas; limit based on record of 

pyroclastic flows in the past 10,000 years at Mt. 

Mazama (Crater Lake National Park), because no large 

events of this type have occurred at Lassen in the past 

50,000 years.

Heavy Ash-Fall Zone—General outer limit of area 

potentially subject to 8 inches or more of ash fall.

Moderate Ash-Fall Zone—General outer limit of area 

potentially subject to 2 inches or more of ash fall.

Volcanic Mudflow (Lahar) Hazard Zone—Valleys 

potentially subject to mudflows. 

Flood Hazard Zone—Valleys beyond the extent of 

Mudflow Hazard Zones that are potentially subject to 

floods caused by volcanic activity.

the public in the event of an impending eruption.

Recent work by scientists with the USGS in

cooperation with the NPS is shedding new light

on volcano hazards in the Lassen Volcanic National Park area. The work of these USGS scientists is only part of the USGS Volcano Hazards

Program’s ongoing efforts to protect people’s

lives and property in all of the volcanic regions of

the United States, including the Pacific Northwest,

eastern California, Wyoming, Alaska, and Hawaii.

VOLCANO HAZARD ZONES OF THE LASSEN VOLCANIC NATIONAL PARK REGION