Volcano Hazards Assessment for the Lassen 

Region, Northern California

Scientific Investigations Report 2012–5176–A

U.S. Department of the Interior

U.S. Geological Survey
FRONT COVER

The May 22, 1915, explosive eruption of Lassen Peak, California, blasted pumice and rock fragments high into the 

air. In this photograph, taken from Red Bluff, 65 kilometers (40 miles) west of the volcano, a huge column of volcanic 

ash and gas produced by the eruption rises to a height of about 9 kilometers (30,000 feet). Winds blew volcanic ash 

from the column eastward, raining fine ash at least as far away as Elko, Nevada, 450 kilometers (280 miles) from the 

volcano. The ash cloud from a similar eruption today would pose a serious hazard to aircraft flying in the western 

United States. (Photo by R.E. Stinson, courtesy of the National Park Service.) 
Volcano Hazards Assessment for the Lassen 

Region, Northern California

By Michael A. Clynne, Joel E. Robinson, Manuel Nathenson, 

and L.J. Patrick Muffler

Scientific Investigations Report 2012–5176–A

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Clynne, M.A., Robinson, J.E., Nathenson, M., and Muffler, L.J.P., 2012, Volcano hazards assessment for the Lassen 

region, northern California: U.S. Geological Survey Scientific Investigations Report 2012–5176–A, 47 p., 1 plate, 

Scale 1:200,000.

This report and any updates to it are available online at: http://pubs.usgs.gov/sir/2012/5176/a/
Contents

Executive Summary...........................................................................................................................................1

Volcanism in the Lassen Region ............................................................................................................1

Summary of Major Volcano-Related Hazards .....................................................................................1

Regional Mafic Lava Flows and Ash............................................................................................2

Silicic Lava Domes, Flows, and Pyroclastic Flows....................................................................2

Lahars ..............................................................................................................................................2

Airborne Ash ....................................................................................................................................2

Other Hazards ..................................................................................................................................2

Introduction.........................................................................................................................................................2

Types of Volcano Hazards and Effects of Volcanic Eruptions....................................................................4

Hazards Associated with Mafic Eruptions...........................................................................................6

Hazards Associated with Silicic Eruptions ..........................................................................................6

Hazards Associated with Lahars ...........................................................................................................7

Other Volcano-Related Hazards.............................................................................................................7

Volcanism in the Lassen Region......................................................................................................................8

Geologic Record of Volcanism at the Lassen Volcanic Center.........................................................8

Early History of the Lassen Volcanic Center...............................................................................8

Eruptive Activity from 100,000 to 15,000 Years Ago at the Lassen Volcanic Center...........13

Eruptive Activity Since 15,000 Years Ago at the Lassen Volcanic Center............................14

Geologic Record of Regional Volcanism ............................................................................................15

Regional Volcanism from 100,000 to 15,000 Years Ago............................................................15

Regional Volcanism in the Past 15,000 Years............................................................................18

Magma Beneath the Lassen Volcanic Center ............................................................................................18

Lassen Hydrothermal System........................................................................................................................19

Seismicity and Ground Deformation.............................................................................................................20

Tectonic Earthquakes ............................................................................................................................20

Long-Period Earthquakes......................................................................................................................21

Ground Deformation...............................................................................................................................22

Volcano-Related Hazards in the Lassen Region.........................................................................................22

Hazards Associated with Mafic Volcanism in the Lassen Region .................................................23

Mafic Lava Flows...........................................................................................................................23

Mafic Ash........................................................................................................................................23

Hazards Associated with Silicic Volcanism at the Lassen Volcanic Center................................26

Lava Domes and Flows.................................................................................................................26

Pyroclastic Flows and Lateral Blasts.........................................................................................28

Airborne Ash and Tephra Fallout................................................................................................28

Lahars and Lahar-Generated Floods..........................................................................................30

Other Hazards in the Lassen Region ...................................................................................................32

Landslides and Rockfalls..............................................................................................................32

Chaos Jumbles .....................................................................................................................32

Brokeoff Volcano..................................................................................................................33

Hydrothermal Hazards..................................................................................................................34

Hazards to People................................................................................................................35

Hazards to Structures..........................................................................................................35

Hydrothermal Explosions....................................................................................................35

Volcanic Event Frequency, Probability and Magnitude.............................................................................35
Volcano Monitoring and Response...............................................................................................................37

Acknowledgments...........................................................................................................................................39

References Cited..............................................................................................................................................39

Glossary.............................................................................................................................................................44

Figures

 1. Map of the Lassen volcanic region and surrounding area, showing land-management boundaries 

and infrastructure information, including roads ........................................................................................ 3

 2. Volcanoes produce a wide variety of natural hazards that can kill people 

and destroy property .......................................................................................................................5

 3. Simplified geologic map and stratigraphic sections of the Lassen region, 

modified from Clynne and Muffler (2010) .....................................................................................9

 4. Maps showing the location and area covered by volcanic deposits less than 

100,000 years old in the Lassen region .......................................................................................10

 5. Aerial view looking northwest at Cinder Cone..................................................................................14

 6. View of the 1,050-year-old Chaos Crags domes from southwest...................................................15

 7. Aerial view of the area devastated by the 1915 eruption of Lassen Peak looking southwest.............18

 8. Map showing the location of the major thermal areas in Lassen Volcanic 

National Park and vicinity. ............................................................................................................20

 9. View looking east-southeast from trail at the west end of Bumpass Hell....................................21

10. Map of volcano-hazard zones in the Lassen region (see also plate 1) ...................................... 24

11. Steep front of block lava flows from the 1666 eruption of Cinder Cone (telephoto view 

looking southwest from Sunrise Peak).......................................................................................................26

12. Map showing thickness (isopachs in centimeters) of the ash deposits 

from the 1666 eruption of Cinder Cone .......................................................................................27

13. Rose diagram showing directions from which winds blow in the vicinity of Lassen Peak .......29

14. Plume generated by the Lassen Peak eruption of May 22, 1915....................................................29

15. Map showing annual probability of 1 centimeter or more of tephra accumulation 

from any major Cascade volcano................................................................................................30

16. Historic photographs taken in 1915 by Benjamin Franklin Loomis of the effects 

of lahars from Lassen Peak ..........................................................................................................31

17. View of Chaos Crags looking southeast from Calif. Hwy 89 

where it crosses Chaos Jumbles. ...............................................................................................32

18. View looking south of active landslides in the area south of Pilot Pinnacle 

and west of Diamond Peak...........................................................................................................33

19. Map showing the distribution of landslides in the eroded core of Brokeoff Volcano................34

20. Dynamic nature of hydrothermal features at Sulphur Works shown by changes 

from 2004 to 2008 ............................................................................................................................36

21. Graph showing estimated cumulative probabilities of the volume and area of eruptions for the 

Lassen Volcanic Center and for regional mafic vents for the past 100,000 years...............37

Tables

1. Chronology of eruptions less than 100,000 years old in the Lassen Volcanic Center...................13

2. Chronology of eruptions less than 100,000 years old for the mafic vents 

in the Lassen segment of the Cascade Range.........................................................................................16

3. Volcano Alert Levels and Aviation Color Code used by USGS volcano observatories.............................38
SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8x°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1929 (NGVD 29)

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27)

Altitude, as used in this report, refers to distance above the vertical datum.

Conversion Factors
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Volcano Hazards Assessment for the Lassen Region, 

Northern California

By Michael A. Clynne, Joel E. Robinson, Manuel Nathenson, and L.J. Patrick Muffler

Executive Summary

This report evaluates the volcano-related hazards of the 

Lassen region, California, which is here defined as an area 

between the Pit River on the north and the southern limit of 

active Cascade volcanism, approximately 5–10 km south of the 

southern boundary of Lassen Volcanic National Park. Most active 

volcanism occurs in a zone about 40 km wide between Viola on 

the west and the eastern boundary of Caribou Wilderness Area, 

but sparser volcanism in the west extends the width of this zone 

to about 75 km. The report is based primarily on knowledge of 

hazardous phenomena observed at other, similar volcanoes and 

on recently published geologic mapping and geochronology 

(Clynne and Muffler, 2010; Christiansen and others, 2002; Clynne 

and others, 2008) applied to the volcano-hazards framework 

established by Miller (1989).

Volcanism in the Lassen Region

The Lassen region forms the southernmost segment of 

the Cascade arc of volcanoes (Guffanti and Weaver, 1988; 

Clynne and Muffler, 2010). The long-term (million-year) 

volcanic history of the region can be described on two spatial 

scales. On the regional scale, hundreds of closely spaced, 

small, short-lived volcanoes and a few larger lava cones and 

shield volcanoes make up much of the Cascade arc in the 

Lassen region. On the local scale, a few much larger, long-lived 

volcanic centers are superimposed on this regional volcanism. 

The volcanic center currently active is the Lassen Volcanic 

Center. Compared to vents of a typical Cascade composite 

volcano, for example Mount Shasta or Mount Rainier, the 

vents of the active part of the Lassen Volcanic Center are 

widely distributed, extending over an area 20 km north-south 

and 25 km east-west. Hundreds of eruptions have occurred 

in the Lassen Volcanic Center over its 825,000-year lifetime, 

including at least 14 eruptions in the past 100,000 years. These 

latter eruptions have included explosive events, ash fall, lahars, 

and emplacement of lava flows and domes. Since about 25,000 

years ago, the Lassen Volcanic Center has been relatively quiet, 

but three eruptions during the last 1,050 years, the most recent 

in 1914–1917, demonstrate that it is still active. Although 

the probability of an eruption is very small in any given year 

(about 1 chance in 7,150), the potential consequences of future 

eruptions could be regionally significant.

In the region surrounding the Lassen Volcanic Center, 

there have been at least 58 eruptions of small volcanoes in 

the past 100,000 years, including two in the past 15,000 

years. This regional volcanism is primarily mafic (basaltic) 

in composition, is relatively nonexplosive, and typically 

is concentrated in a relatively small area in any given time 

interval. The probability of eruption of a regional volcano, 

about 1 chance in 1,550 in any given year, is higher than that 

for the Lassen Volcanic Center. However, the consequences 

of a regional eruption would be less severe than those of an 

eruption in the Lassen Volcanic Center, because regional 

volcanoes typically build cinder cones and erupt small lava 

flows and ash fallout that affect a rather small area.

Relative to other U.S. volcanoes, the Lassen Volcanic 

Center has been designated a “very high threat volcano” by 

the U.S. Geological Survey (USGS) (Ewert and others, 2005). 

The basis of this designation was the center’s eruptive history, 

extent of the domefield, potentially explosive nature, presence 

of an active hydrothermal system, location in a national park 

with significant visitorship in the summer, and proximity to 

regional infrastructure.

Summary of Major Volcano-Related Hazards

The most likely volcano-related hazards in the Lassen 

region are summarized according to the nature and location 

of the volcanism. The hazards related to the small regional 

volcanoes in the area surrounding the Lassen Volcanic Center 

and to silicic to andesite volcanism in the Lassen Volcanic 

Center are summarized separately below. Hazards associated 

with the hydrothermal system, landslides, and rockfalls 

in Lassen Volcanic National Park are also discussed and 

delineated on a volcano-hazard map. A glossary at the back of 

this report defines selected geologic terms.

An eruption generating extremely large pyroclastic flows, 

for example a caldera-forming or “supervolcano” eruption 

producing tens to hundreds of km3

 of lava and ash is not 

discussed in this report because it is exceedingly unlikely. 

Several such large-scale events have occurred at Lassen in the 

past 3 million years, but none since eruption of the Rockland 

ash flow tuff and tephra 611,000 years ago (Lanphere and 

others, 2004). The magmatic system of the Lassen Volcanic 

Center is not presently configured for this type of eruption, 

and the short-term probability of such a high-consequence 

event is considered negligible.
2 Volcano Hazards Assessment for the Lassen Region, Northern California

Regional Mafic Lava Flows and Ash

On the basis of past activity, the most likely eruption in 

the Lassen region would be that of a new small mafic volcano. 

The eruption would probably consist of localized near-vent 

explosive activity that would build a cinder or spatter cone and 

produce modest ash fallout downwind and be accompanied 

by lava flowing a few kilometers from the vent. The activity would continue for a period of weeks to months. Hazards 

would include violent ejection of hot blocks of lava near the 

vent, as well as intermittent explosions that could send ash 

clouds several kilometers into the air and deposit more than 

5 cm of ash within 10 km and lesser amounts as much as ~50 

km downwind. Also possible, but less likely, are eruptions 

of fluid lava flows that could extend a few tens of kilometers 

from the vent area, cover as much as 100 km2, and continue 

for as long as 5 to 10 years. The most likely locations for 

mafic eruptions are in a zone between the Red Cinder chain 

and Calif. Hwy 44 (approximately the area in and around the 

northeastern corner of Lassen Volcanic National Park), a zone 

from south of Old Station to the Pit River, a zone from the 

south end of Tumble Buttes chain to the vicinity of Burney 

Mountain, and in the area of the Red Lake cluster.

Silicic Lava Domes, Flows, and Pyroclastic Flows

The most likely type of eruption associated with silicic 

volcanism in the Lassen Volcanic Center would be explosive. 

Precursory activity could include intermittent steam explosions 

that eject cold rocks on ballistic trajectories near the vent, as 

well as send ash clouds several kilometers into the air and 

deposit local accumulations of ash. Precursory activity could 

be followed by a magmatic phase of the eruption, during which 

explosive silicic volcanism produces a vertical eruption column 

whose partial collapse could generate pyroclastic flows, build 

a tuff cone, and deposit ash. Hazards would include pyroclastic 

flows, which could travel a few tens of kilometers down 

valleys heading in the vent area, lateral blasts and pyroclastic 

surges, and ash fall deposited over considerable distances. 

Effusive (nonexplosive) eruptions could produce lava flows or 

build lava domes and might continue for a few months to a few 

years. Partial collapse of unstable, hot, growing lava domes 

could also generate pyroclastic flows affecting areas within a 

few kilometers of the dome. The most likely locations for this 

type of activity in the Lassen Volcanic Center would be in the 

area around Chaos Crags and in the northern part of Lassen 

Volcanic National Park east to Cinder Cone.

Lahars

Lassen Peak and the high-elevation areas of Lassen Volcanic National Park are covered with a deep snow pack during the 

winter and spring months. Consequently, there is a significant 

lahar hazard to drainages heading on Lassen Peak and a lesser 

hazard to drainages heading on the central and southwestern 

parts of Lassen Volcanic National Park. Rapid melting or incorporation of significant volumes of snow into turbulently flowing 

eruption products could cause far-traveled lahars, primarily in 

the valleys heading on the north flank of Lassen Peak.

Newly fallen ash is susceptible to rapid erosion, and 

heavy rainfall can remobilize widespread pyroclastic flows 

or thick ash deposits and could cause lahars in the valleys 

heading in Lassen Volcanic National Park and their downstream extensions. Lahars do not require a volcanic eruption 

to be triggered. Landslides of water-saturated, hydrothermally altered rock can transform into lahars as they move 

downslope. Emplacement of lahars in drainages can cause 

significant disruption of normal stream flow and secondary 

affects for decades after the initial event.

Airborne Ash

Moderate-size eruptions related to the silicic volcanism 

described above will produce eruption columns that rise high 

into the atmosphere. These will likely affect commercial and 

military air traffic. Significant volumes of volcanic ash may be 

transported and deposited over an area tens of kilometers from 

vents and traces of ash hundreds of kilometers farther. Airborne 

ash could affect the towns of Chester and Susanville and less likely 

Burney, Redding, and Red Bluff, for days to weeks at a time, but 

would likely not result in accumulation of ash sufficient to cause 

structural damage to buildings or infrastructure. Smaller ash 

eruptions accompanying the construction of cinder cones would 

affect much smaller areas near the vents.

Other Hazards

Landslides and rockfalls are significant hazards in the hydrothermally altered core of Brokeoff Volcano and in Mill Creek, less so 

in Bailey Creek. Rockfalls are also a hazard in the areas surrounding 

Lassen Peak and Chaos Crags and in other areas with steep cliffs. 

Additional hazards are associated with the hydrothermal system, 

including the potential of severe burns in fumaroles or mud pots and 

danger from toxic gases. The Lassen region is subject to considerable seismic activity, and major faults in the area are recognized as 

capable of moderate to large earthquakes (as large as magnitude 7), 

but seismic hazards are not specifically evaluated herein.

Introduction

Lassen Peak, the southernmost active Cascade volcano 

(fig. 1), last erupted in 1914–17 and, before the 1980–86 eruption of Mount St. Helens, was the only volcano in the conterminous United States to have erupted in the 20th century. Lassen 

Peak is an unusual Cascade volcano because it is not a large 

composite volcano1 like, for example, Mount Shasta or Mount 

1

Words in bold type represent the first use of terms defined in the glossary at 

 the back of this report.
Introduction 3

Lassen Volcanic

National Park

Thousand Lakes

Wilderness Area

Sacramento River

National Wildlife

Refuge

Shasta National 

Forest

Lassen National 

Forest

Plumas

National Forest

Caribou

Wilderness

Area SHASTA

TEHAMA

LASSEN

PLUMAS

BUTTE TEHAMA

PLUMAS

LASSEN

SHASTA

PONDEROSA WAY

PONDEROSA WAY

COUNTY A21

USFS 17

299

89

44

44

36

36 89

99

32

89

299

HUMBOLDT ROAD

5

Chico

26 miles

Redding

McCloud

Susanville Westwood

COUNTY A6

Deer Creek

Pit River

Pit River

Susan River

Manzanita Creek

Lost Creek

Hat Creek

North Fork Battle Creek

Bailey Creek

Battle Creek Digger Creek

Mill Creek

Warner Creek

Hot Springs Creek

Kings Creek

North Fork Feather River

Hat Creek

North Fork Feather River

South Fork Battle Creek

Sacramento River

Lake

Almanor

Butt Valley

Reservoir

Shasta

Lake

Lake Britton Pit #3

dam

Canyon

Dam

Butt 

Valley

Dam

BurneyBurney

Shingletown

Manton

Viola

Mineral

Chester

Red

Bluff

Old

Station

Fall River

Mills

LASSEN

PEAK

40°45'

41°00’

40°30'

40°00’

-122°15' -122°00’ -121°45' -121°30' -121°15' -121o00’

40°15’

Major Power Lines

Gas Pipeline

Roads

Railroads

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

County Boundary

EXPLANATION

Cascade volcanic arc

Figure 1. Map of the Lassen volcanic region and surrounding area, showing land-management boundaries and infrastructure information, 

including roads. Inset map shows locations of major Cascade volcanoes. Private land is white; Lassen, Shasta, and Plumas National Forests and 

wilderness areas are green; U.S. Bureau of Land Management land is buff. The dark-green area in the right center is Lassen Volcanic National 

Park. Small areas managed by other agencies, for example, the State of California, are not shown separately and are included in the white areas.
4 Volcano Hazards Assessment for the Lassen Region, Northern California

Rainier. Rather, Lassen Peak is a large volcanic dome within a 

field of domes, called the Lassen domefield. The Lassen domefield 

contains 30 lava domes that formed over the past 300,000 years at 

the Lassen Volcanic Center (LVC), which includes the domefield 

and an older related composite andesite volcano. The LVC is 

active and potentially threatens the Lassen region with a variety of 

volcano hazards. In the National Volcano Early Warning System 

(NVEWS) assessment, the Lassen Volcanic Center is described as 

a “very high threat volcano” (Ewert and others, 2005).

Media reports of the climactic May 1915 eruptions of 

Lassen Peak focused the attention of the American public on 

the volcanoes of the Cascade volcanic arc and their potential 

for eruption. Previously, Lassen Peak and Cinder Cone were 

recognized as noteworthy features and designated as national 

monuments in 1907. Following the 1915 eruption, Congress in 

1916 created Lassen Volcanic National Park (LVNP). For many 

decades, economic growth in the immediate area was limited, 

and the potential for loss of life and property was relatively 

minor. The economy of the Lassen region today remains 

primarily based on forest products, ranching, recreation, and 

tourism. Recently, however, residential growth in the areas 

surrounding LVNP has increased the potential for loss of life 

and property in the event of a volcanic eruption. In May 1915, 

an eruption cloud from Lassen Peak reached an elevation of 

nearly 9 km (~30,000 feet), and ash fall was noted at least as 

far east as Elko, Nevada (about 450 km distant). Despite significant lahars, the relatively small eruption caused no deaths, 

and property damage was minor. But only 1,050 years ago, 

the Chaos Crags were formed in a sequence of major eruptions at the LVC that were 40 times bigger than those in 1915 

and included pyroclastic flows and the construction of large 

lava domes. The Lassen region lies beneath major air corridors 

connecting southern and central California and the Pacific 

Northwest, and eruptions today would affect commercial and 

military aviation over a wide area of the western United States.

The primary purpose of this report is to describe and 

assess hazards from future eruptions of volcanoes in the Lassen region, which is here defined as an area between the Pit 

River on the north and the southern limit of active Cascade 

volcanism, approximately 5–10 km south of the southern 

boundary of LVNP. Most active volcanism occurs in a zone 

about 40 km wide between Viola on the west and the eastern 

boundary of Caribou Wilderness (fig. 1), but sparser volcanism 

in the west extends the width of this zone to about 75 km.

Geologic hazards at LVNP were first assessed by the 

USGS in 1970 and the results given to the National Park 

Service. A volcano-hazards assessment of California, including the Lassen region, was published by Miller (1989), and a 

USGS Fact Sheet based on Miller (1989) briefly summarized 

the hazards of the Lassen region (Clynne and others, 2000). 

Detailed geologic mapping in the Lassen region (Christiansen and others, 2002; Clynne and Muffler, 2010), ongoing 

geologic studies, hazards recognized at other active volcanoes, and geophysical monitoring of the region by the USGS 

provide the scientific basis for this reassessment of volcano 

hazards in the Lassen region.

In the pages that follow, we first define the various volcano-related hazards and present a general discussion of their 

effects. This discussion is modified from Miller (1989), Scott 

and others (1995; 2001), Waitt and others (1995), Gardner and 

others, 1995, Hoblitt and others (1998), and Walder and others 

(1999). More detailed general information on volcano hazards 

and the effects of eruptions can be found in Blong (1984) and 

Sigurdsson and others (2000).

The general discussion of volcano-related hazards is 

followed by discussion of the hazards specific to the Lassen 

region. As background, we describe the distribution, types, and 

magnitude of volcano-related hazardous events in the Lassen 

region since about 100,000 years ago. A review of the type and 

magnitude of activity expected in the future is presented, and 

the areas most likely to be affected are delineated on hazardzonation maps. These hazard zones are modified from those of 

Miller (1989), primarily by applying more accurate age data 

for volcanism and the size of eruptions in the Lassen region. 

Discussions of landslides, rockfalls, and hazards associated 

with the Lassen hydrothermal system are also presented. We 

estimate the probability of occurrence of volcanic eruptions 

in the Lassen region and discuss the results in a subsequent 

section. Knowledge of the likelihood and hazards posed by 

volcanic eruptions provides planners and public officials such as 

the California Emergency Management Agency (CalEMA) and 

local governments information to mitigate the effects of future 

eruptions. Technical terms are defined in the glossary at the end 

of this report (and shown in bold font in the text at first use).

Types of Volcano Hazards and Effects 

of Volcanic Eruptions

Volcano hazards in volcanic arcs like the Cascades are 

often simply classified into two broad types of volcanoes: 

mafic and composite (for example, Scott and others, 2001). 

Mafic composition volcanoes (which erupt basalt to basaltic 

andesite)2

 are generally small, short-lived, and less violently 

explosive than composite volcanoes. Mafic volcanoes 

typically erupt only once for a brief period (weeks to a few 

years), but some, called lava cones and shield volcanoes, 

build larger edifices and are intermittently active for longer 

periods (centuries to a few millennia). Once a given mafic 

eruption has run its course, subsequent mafic eruptions 

typically issue from new locations. Over tens to hundreds of 

2

Volcano hazards exist as a spectrum of possibilities related to the composition of the erupted magma. The compositional spectrum of increasing 

silica content of volcanic rocks—basalt, basaltic andesite, andesite, dacite, 

rhyodacite, and rhyolite—generally correlates with increasing explosivity. 

Mafic compositions (basalt and basaltic andesite) are generally less explosive, 

and silicic compositions (dacite, rhyodacite and rhyolite) are generally more 

explosive. Intermediate composition magmas (andesite) can create hazards of 

both mafic and silicic character. For simplicity in this report, we have placed 

hazards of andesite eruptions in the silicic category because they will likely 

occur as part of Lassen Volcanic Center eruptive activity.
Types of Volcano Hazards and Effects of Volcanic Eruptions 5

thousands of years, mafic eruptions build broad platforms 

consisting of many extinct volcanoes.

Composite volcanoes, on the other hand, are large, long-lived 

features that erupt episodically for tens to hundreds of thousands 

of years from the same or closely-spaced vents, build cones 

thousands of meters high, and display a wide range of eruptive 

styles and explosivity. They are primarily silicic in composition 

(andesite, dacite, rhyodacite and rhyolite) and constructed from 

a single long-lived, magmatic system located in the crust beneath 

the volcano. The Lassen Volcanic Center is an even bigger and 

longer-lived silicic volcanic feature. It consists of an early caldera

complex, a composite volcano, and a domefield of adjacent 

small, closely spaced, silicic lava domes and flows. These three 

main components of the LVC formed sequentially, from a single 

evolving magmatic system. The Lassen domefield is the presently 

active expression of the volcanic center. Below, we discuss 

volcano hazards under the headings of mafic for the regional 

volcanism and silicic for volcanism at the LVC.

A variety of hazards are associated with volcanic eruptions 

(fig. 2; Myers and others, 2008). Silicic eruptions can produce 

Figure 2. Volcanoes produce a wide 

variety of natural hazards that can 

kill people and destroy property. This 

simplified sketch shows a composite 

volcano typical of those found in the 

Cascade arc. Some hazards, such as 

lahars and landslides, can occur even 

when a volcano is not erupting (from 

Myers and others, 2008). Terms are 

defined in the glossary.

vertically directed eruption columns many thousands of meters 

high, lava domes and flows, and lahars (also called volcanic 

mudflows). Collapse of eruption columns can produce pyroclastic 

flows, and transport of ash from eruption clouds by the wind 

can result in deposition of ash as far as hundreds of kilometers 

downwind. The effects of mafic volcanoes are less violent and 

generally limited to a few tens of kilometers from the vent. They 

eject tephra (lava blocks and ash) and produce lava flows.

Definitions, more detailed descriptions, and the potential 

effects of general volcano and volcano-related hazards are 

discussed below, with an emphasis on the types of eruptions that 

may occur in the Lassen region. Specific volcano and volcanorelated hazards in the Lassen region are discussed in the section 

“Volcano-Related Hazards in the Lassen Region.” Additional 

geologic hazards that do not require an eruption to trigger events 

are possible in volcanic terrains. These include earthquakes or 

landslides and lahars that can be triggered by earthquakes, 

severe rainstorms, or rain-on-snow events. At LVC, the 

presence of a large active hydrothermal system presents a third 

group of hazards.
6 Volcano Hazards Assessment for the Lassen Region, Northern California

Hazards Associated with Mafic Eruptions

Mafic magma is much less viscous and generally 

contains a smaller gas component than silicic magma. Mafic 

magma is typically less violently explosive, so that mafic 

eruptions generally affect smaller areas than silicic eruptions. 

Large fragments fall out near the vent and build a cinder 

cone, and thick accumulations of coarse tephra are generally 

restricted to the vicinity (within a few kilometers) of the vent. 

However, when mafic magma interacts with water, steam 

explosions can generate a large quantity of fine ash and eject it 

into the atmosphere. Under favorable wind conditions ash can 

be dispersed for significant distances: for example, the 2010 

eruption of Eyjafjallajökull Volcano in Iceland caused major 

disruption of commercial transatlantic air traffic. Less violent 

interaction between mafic magma and water produces pyroclastic surges that build cones of ash with shallow slopes like 

shield volcanoes.

After the initial gas-rich part of an ascending magma 

body is erupted, most mafic volcanoes produce lava flows. 

Depending on the viscosity and rate of effusion, lava will form 

a thick, short lava flow or a thin widespread lava flow. Viscous 

flows are 10 to a few tens of meters (30–100 feet) thick and 

have hot but solid outer surfaces covered with blocks of lava 

(block flows). They move slowly downslope into valleys 

and may flow for a few kilometers, often stopping when the 

underlying slope angle decreases. Viscous flows tend to have 

relatively small volumes. Fluid lava flows advance in channels 

or lava tubes that distribute lava to the advancing front of the 

flow with little loss of heat. The volumes of fluid lava flows 

tend to be 10 to a few hundred times larger than viscous flows. 

They can flow for a few tens of kilometers, and they spread 

out and cover significant areas of valley bottoms. Individual 

flow lobes are thin, generally 1–3 meters (3–10 feet), but if 

the flow is confined by valley walls, many flow lobes can pile 

up to make a much thicker flow. Hazards of mafic lava flows 

are primarily to property and infrastructure, because the flows 

generally advance slowly enough for humans and animals to 

escape. However, lava flows destroy all structures in their path 

and can ignite forest fires. Lava flows can disturb drainages 

by diverting or damming rivers and creeks. Lava dams that 

impound water to form a lake are susceptible to rapid downcutting or sudden failure that can release a large volume of 

water and generate a “breakout flood.”

Hazards Associated with Silicic Eruptions

The most destructive eruptions at volcanic centers (and 

composite volcanoes) are caused by silicic magma, which 

is very viscous and often gas-rich. Consequently, it can 

erupt explosively and can produce enormous clouds of gas 

and ash. As silicic magma ascends from a magma reservoir, 

the decrease in pressure permits gases dissolved in it to be 

released. The gases form bubbles that expand rapidly as the 

magma ascends from depth. As the magma approaches the 

surface, these bubbles literally explode out of the magma, 

fragmenting it into pieces (tephra) ranging in size from a 

few tens of centimeters (pumice) to fine dust (ash). The hot 

rock fragments, ash, and gases ascend vertically as a buoyant 

eruption column. The largest rock fragments generally fall 

out within a few kilometers of the vent, but small fragments, 

ash, and gas rise high into the air forming a billowing eruption 

column. Eruption columns can grow rapidly and reach heights 

of more than 20 km above the volcano within 30 minutes 

or less. Commonly, eruption columns collapse to generate 

pyroclastic flows. These turbulent clouds of pumice, ash, 

rock fragments, and gas can have temperatures of 500oC or 

even higher, move downhill at speeds up to 200 kilometers 

per hour, travel tens of kilometers from their vent, surmount 

topographic barriers, and destroy everything in their path.

A lateral blast (also called a pyroclastic surge) is a type 

of pyroclastic flow that is directed laterally or subhorizontally 

away from the vent. This type of eruption results from the 

sudden release of pressure on magma intruded into a volcanic 

edifice. One initiation mechanism is failure of the flank of the 

volcano that exposes the intruded magma at the surface. The 

decompressed magma literally explodes out of the volcano 

laterally. Lateral blasts and pyroclastic surges can be more 

violent than pyroclastic flows generated by column collapse. 

Their mobility is much less inhibited by topography than 

typical pyroclastic flows. Lateral blasts may affect only narrow 

sectors or spread out from a volcano to cover a sector as broad 

as 180 degrees, and they can reach distances of several tens 

of kilometers from the volcano. Lateral blasts carrying rock 

debris at high speed can devastate areas of tens to hundreds 

of square kilometers within a few minutes and can obliterate 

manmade structures and forest and kill all living things by 

abrasion, impact, burial, and heat. The 1980 eruption of Mount 

St. Helens is the best-known lateral blast.

Ash ejected high into the atmosphere forms an eruption 

cloud. Dispersal and settling (fall out) of ash is another 

major hazard associated with silicic eruptions. Fine ash drifts 

downwind before settling and, depending on the height of 

the eruption cloud and direction of prevailing winds, can 

blanket areas tens to hundreds of kilometers away. Thick 

accumulations of fine ash make structures susceptible to 

collapse, especially if the ash gets wet. Fine silicic ash is 

abrasive and destructive to all types of machinery and internal 

combustion engines (Kenedi and others, 2002). Aircraft 

engines are particularly susceptible to damage from fine ash, 

and even small dilute ash clouds pose great hazards to aircraft 

that fly into them (Miller and Casadevall, 2000). Fine ash is 

easily remobilized by motor vehicles or helicopters, and it 

is expensive to remove. Thus, even thin deposits of ash can 

be very disruptive to human enterprises for days to weeks 

after deposition. Deposits of fine ash can short-circuit electric 

transformers and power lines, especially if the ash gets wet, 

which makes it adhere to surfaces and enhances electrical 

conductivity. Ash is not directly toxic, but like fine dust, ash 

suspended in the air is irritating to the eyes and lungs, and 

prolonged inhalation can cause chronic lung disease 
Types of Volcano Hazards and Effects of Volcanic Eruptions 7

(Baxter, 2000; Horwell and Baxter, 2006). Even thin 

accumulations of fine ash on farm or grazing land can 

negatively affect crops and livestock. Thick accumulations of 

fine ash can be remobilized into lahars by rain or snowmelt, 

inundate downstream valleys, and negatively affect water 

quality and aquatic ecosystems.

Lessons learned from the effects of the 1980 eruption of 

Mount St. Helens on downwind communities in Washington 

are now used to prepare governments, businesses, and citizens 

for future ash falls. These communities experienced significant 

disruptions in transportation, business activity, and community 

services as a result of fallout of only 5 mm to 6 cm (1/4 to 

2 1/2 inches) of ash. Generally, ash fall less than about 5 

mm (1/4 inch) was perceived as an inconvenience, whereas 

more than about 15 mm (5/8 inch) constituted a disaster. The 

greater the amount of ash, the longer it took for communities 

to recover, but in most cases communities recovered to nearnormal activities within two weeks.

Eruption of gas-poor silicic magma is less explosive 

and affects a more limited area, generally close to the 

vent. Effusive eruption of gas-poor silicic magma 

builds steep-sided lava domes or thick lava flows that are 

fractured and often unstable and susceptible to sudden 

gravitational collapse as they grow larger and higher. 

Collapse of significant volumes of hot rock from lava 

domes can produce avalanches of hot rock fragments and 

gas called dome-collapse pyroclastic flows (also called 

lithic pyroclastic flows or block-and-ash flows). Although 

their mobility relative to pyroclastic flows generated by 

collapse of eruption columns is more limited, pyroclastic 

flows generated by collapses from large domes or domes 

perched high above valleys can move rapidly and travel 

significant distances depending on the height and size of 

the source dome. Some lava domes collapse repeatedly and 

build thick fans of rock fragments (like talus slopes) that 

generally extend to flat or gently sloping ground but can 

travel farther if they enter steep valleys. In general, bigger 

or higher domes will generate bigger, farther-traveled 

pyroclastic flows. The hazards from this type of domecollapse event are similar to column-collapse pyroclastic 

flows, but they cover a more limited area. Documented 

eruption histories at volcanoes such as Mount St. Helens, 

Washington, Soufrière Hills, Montserrat, and Santiaguito, 

Guatemala, demonstrate that growth of lava domes and 

corresponding hazards can continue for years to decades.

Hazards Associated with Lahars

Snow- or ice-covered volcanoes present an additional 

hazard. Eruption of hot ash onto snow or pyroclastic flows 

traveling over snow can rapidly melt it. The rapid generation 

of a large volume of water on a volcano covered with loose 

debris can initiate a volcanic mudflow or lahar. A lahar is a 

water-saturated volcanic debris flow consisting of as much 

as 50 percent sediment (Vallance, 2000). The sediment can 

be recently erupted rocks and ash or older loose material 

on the flanks of a volcano or in a drainage. A lahar has the 

consistency of wet concrete and can surge down valleys and 

stream channels at speeds of 15–30 km per hour (Rodolfo, 

2000) but can reach even higher speeds under favorable 

circumstances. Because they are saturated with water, largevolume lahars can flow over relatively gentle gradients 

and can inundate areas tens of kilometers away from their 

sources. Lahars generated by eruptions are responsible for 

significant loss of life and property damage downstream of 

volcanoes. They are powerful and can carry huge boulders 

and trees and inundate river valleys, sweep away structures, 

and destroy bridges. Less dramatic effects include filling 

buildings with debris and burying roads, farmland, and 

infrastructure. Lahars often transition into muddy floods as 

they come to rest and release a large volume of water. Lahars 

that flow into reservoirs can create waves that overtop dams 

or cause dam failure and serious floods downstream. Large 

lahars can block drainages, backing up water to create a 

lake. Eventual overtopping and failure of the lahar dam can 

generate a catastrophic flood unless the blockage is managed 

by engineers. Initiation of a lahar does not require volcanic 

eruption, and non-volcanic debris flows generated by severe 

rainstorms or rain-on-snow events can occur in volcanic 

or non-volcanic terrain. The long-term effects of lahars 

can persist for decades, as can the effects of other types of 

eruptions that disturb the landscape. These effects include 

increased rates of erosion, downstream deposition of sediment, 

and negative effects on water quality and navigation.

Other Volcano-Related Hazards

Steep-sided volcanic edifices are inherently unstable 

features that are susceptible to gravitational collapse. New 

lava often is deposited on steep surfaces covered with loose 

material, and hot lava and interbedded fragmental deposits 

produce weak, fractured rock masses when they cool. Weathering and hydrothermal alteration further weaken volcanic 

edifices. Collapse of volcanic edifices can be triggered by a 

variety of volcanic and nonvolcanic events. These kinds of 

collapses are landslides or rockfalls and are also called debris 

avalanches. Small debris avalanches can be triggered by 

tectonic earthquakes, rainstorms, rapid snowmelt, or normal 

erosional processes. On large volcanic cones (edifices), the 

onset of volcanic unrest with earthquakes, steam explosions, 

and intrusion of magma greatly increases the likelihood of 

debris avalanches and the possibility of catastrophic largevolume events. The largest debris avalanches are failures of 

the entire sides of large composite volcanoes and involve 

volumes as great as several cubic kilometers. These huge 

debris avalanches can travel for tens of kilometers downvalley and, if they contain or acquire sufficient water, can 

transition into lahars, further increasing their mobility.

Volcanoes emit gases during eruptions, and even when 

they are not erupting, gases can be released by intrusions of 
8 Volcano Hazards Assessment for the Lassen Region, Northern California

subsurface magma or cooling domes and lava flows. More than 

90 percent of volcanic gases are water vapor or steam, most of 

which is heated ground water. Other common volcanic gases are 

carbon dioxide, sulfur gases, chlorine, and fluorine. Sulfur dioxide, hydrogen sulfide, chorine, and fluorine gases can react with 

water droplets in the air to create acid rain, which causes corrosion and is harmful to vegetation. Precipitation of gases onto 

ash particles in eruption clouds creates acid compounds that can 

poison livestock grazing on ash-coated grass and can contaminate water supplies. Carbon dioxide is heavier than air and in 

windless conditions can be trapped in low areas in concentrations that are deadly to humans and animals. Similar hazards are 

related to emissions of gases from hydrothermal systems.

Volcanism in the Lassen Region

The Lassen region sits astride the axis of the southernmost 

Cascade arc, a segment of the “Ring of Fire” of volcanoes that 

encircles the Pacific Ocean basin. The arc results from subduction 

of the oceanic Juan de Fuca Plate beneath the continental North 

American Plate. There are two fundamentally different scales of 

volcanism in the Lassen region: distributed regional volcanism 

and volcanic centers (fig. 3). Regional volcanism is generally 

nonexplosive to weakly explosive, and pyroclastic activity does not 

typically affect areas much beyond the immediate vicinity of the 

vent, except for minor ash fall as much as a few tens of kilometers 

downwind. Regional volcanoes have a wide range of forms 

and comprise a continuum of sizes from small cinder cones and 

lava flows through larger, steep-sided lava cones to broad shield 

volcanoes. The overall result of regional volcanism is construction 

of a broad platform of overlapping mafic volcanoes.

The smallest and most abundant regional volcanoes are 

cinder cones. Cinder cones are composed of bedded cinders or 

scoria and bombs piled up in a generally symmetrical cone around 

a vent. Typically they have an associated lava flow that erupts from 

the base of the cone. Cinder cones generally have a single period 

of activity and erupt small-volume lava flows. Cinder Cone, in 

eastern LVNP, is a typical cinder cone, albeit with a lava flow field 

that is larger than normal.

Lava cones and shield volcanoes are bigger than cinder cones 

and are more or less circular in plan view unless built on a slope. 

They are constructed by eruptions from a central vent or from 

flank vents high on an edifice and, depending on the viscosity of 

the lava, build a steep-sided lava cone or a flatter shield volcano. 

They are usually capped by a cinder cone but generally lack widely 

distributed fragmental deposits. Typically they are intermittently 

active for a period of decades to a few millennia and build edifices 

of as much as a few cubic kilometers in volume. Sugarloaf Peak is 

an example of a lava cone, and Prospect Peak is an example of a 

shield volcano in the Lassen region (plate 1).

Fluid basalt erupts from fissures and produces widespread 

lava flows that flood the valleys between some shield volcanoes. 

Erupted volumes and areas covered can be relatively large; 

volumes of as much as a few cubic kilometers are typical. The 

24,000-year-old Hat Creek Basalt that erupted near Old Station and 

flowed for 20 km down the Hat Creek Valley is a typical example 

(fig. 4; plate 1).

Nested within the regional platform of volcanic rocks are a 

few long-lived foci of volcanism called “volcanic centers,” defined 

as voluminous (as much as a few hundred cubic kilometers), 

long-lived (generally a million years or longer), composite edifices 

(Clynne and Muffler, 2010); examples are the Lassen and Maidu 

Volcanic Centers (fig. 3). Volcanic centers mark the location of 

long-lived magmatic and volcanic systems on or along the axis 

of the southernmost Cascade arc. The fundamental differences 

between regional volcanoes and volcanic centers are the latter’s 

longevity, larger volume, and a wide range of composition, from 

andesite to rhyolite (silicic) in a single edifice or geographic locale. 

Five volcanic centers younger than about 3.5 million years are 

recognized along the present-day Cascade axis in the Lassen region 

(fig. 3). Late in the evolution of each volcanic center, an acidic 

hydrothermal system driven by heat from cooling subterranean 

silicic magma bodies caused alteration of the permeable rocks of 

the composite cone. Subsequent glacial and fluvial erosion of the 

altered interiors of the composite cones resulted in bowl-shaped 

depressions surrounded by more-resistant rims of thick, unaltered, 

flank lava flows. The Yana, Maidu, Dittmar, and Latour Volcanic 

Centers have reached this stage, and their hydrothermal systems are 

extinct. The Lassen Volcanic Center is the currently active volcanic 

center in the region and hosts continuing silicic volcanism and an 

active hydrothermal system.

Geologic Record of Volcanism at the Lassen 

Volcanic Center

Volcanism at the Lassen Volcanic Center began about 

825,000 years ago and is still active. The Lassen Volcanic Center is 

divided into three sequences of deposits: (1) the Rockland caldera 

complex (825,000–610,000 years ago), an early dacite domefield 

and small caldera, (2) Brokeoff Volcano (590,000–385,000 years 

ago), an andesite composite cone, and (3) the Lassen domefield 

(300,000–0 years ago), a younger dacite domefield. Each sequence 

was formed by dozens to hundreds of eruptions. A detailed geologic map, chronology and discussion of volcanism at the LVC can 

be found in Clynne and Muffler (2010).

Early History of the Lassen Volcanic Center

The Rockland caldera complex was active between about 

825,000 and 610,000 years ago and culminated 611,000 years 

ago with eruption of the Rockland tephra (Lanphere and others, 

2004). The volume of the Rockland tephra was about 50 km3, 

large enough to have deposited a thick layer near LVC and a 

thin ash layer over all of northern California as well as a large 

part of the western United States (Sarna-Wojcicki and others, 

1985). The eruption was comparable in size to the eruption 

that formed Crater Lake, Oregon, the largest Holocene

eruption in the Cascade arc. The Rockland eruption formed 

a small caldera, which was subsequently buried by younger 

volcanism at the Lassen Volcanic Center.
Volcanism in the Lassen Region 9

3.5 

0.5

1.0

1.5

2.0

2.5

3.0

0 

Ma

Age, in Ma

Tuscan

Formation

Glacial and 

sedimentary

deposits

Maidu

Volcanic

Center

Dittmar

Volcanic

Center

Yana

Volcanic

Center Latour

Volcanic

Center

Regional 

low-K

olivine 

tholeiites

Lassen

Volcanic

Center

Regional 

calc-alkaline

volcanic rocks

Caribou

Volcanic

Field

Lake

Almanor

Lassen Volcanic National Park

Granitic and

metamorphic rocks

of the Sierra Nevada

>65 Ma

Tuscan Formation

3.5–2.5 Ma

Maidu Volcanic Center

2.4–1.1 Ma

Lassen Volcanic Center

0.8–0 Ma 

Latour Volcanic Center

>3.0 Ma

Yana Volcanic Center

3.4–2.4 Ma

Lake Almanor

Dittmar 

Volcanic 

Center

2.4–1.3 Ma

Caribou 

Volcanic 

Field

0.4—0 Ma

-121°00' -122°00'

-122°00'

40°00'

-121°00'

40°40'

40°00'

40°30'

0 5 10 MILES

0 5 10 15 KILOMETERS

N

Figure 3. Simplified geologic map and stratigraphic sections of the Lassen region, modified from Clynne and Muffler (2010). Map shows 

ages (in millions of years, Ma) and locations of the individual volcanic centers, including the active Lassen Volcanic Center and the 

areas of surrounding mafic volcanism.
10 Volcano Hazards Assessment for the Lassen Region, Northern California

Lake

Almanor

Inskip Hill

Black

Butte

Hat Creek

Basalt

Sugarloaf chain

Tumble Buttes chain

Bidwell Spring chain

Red Cinder chain

Tuya chain

Silver

Lake

Buckhorn

Lake

Cinder

Butte

Lassen

domefield

Lassen

Peak

Red Lake

cluster

Buzzard

Spring/

Ice Cave

Mountain

Chaos

Crags

Cinder

Cone

Sugarloaf

Peak

Devils Rock

Garden

Popcorn

Cave

Six Mile

Hill

Big

Lake

Cold Creek

Butte

Whittington

Butte

Twin

Buttes

section 32

junction 4126

Hat Creek Fault

Geologic units younger than 

15,000 years

Geologic units between 100,000 

and 15,000 years

Faults

Lassen domefield Boundary 

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

EXPLANATION

Vents

-122°00’ -121°45’ -121°30’ -121°15’

41°00’

40°45’

40°30’

40°15’

40°00’

Figure 4. Maps showing the location and area covered by volcanic deposits less than 100,000 years old in the Lassen region differentiated by age (A) 

and composition (B). C, Enlarged depiction of the Lassen domefield. See explanation in B for compositions. Dashed black oval encloses the approximate 

area of units younger than 100,000 years old in the Lassen Volcanic Center, except for those that extend down Lost, Hat, and Manzanita Creeks. Units in 

dark red are younger than 15,000 years; units in light red are 15,000 to 100,000 years old. Thin black lines are faults of Quaternary age. Green outline is the 

combined boundary of Lassen Volcanic National Park and adjacent Caribou Wilderness Area. (digital elevation model base from Gesch, 2007)

A
Volcanism in the Lassen Region 11

Lake

Almanor

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

EXPLANATION

Basalt

Basaltic 

Andesite

Andesite

Dacite

Rhyodacite

Surficial Faults

Lassen domefield

Boundary Vents

-122°00’ -121°45’ -121°30’ -121°15’

41°00’

40°45’

40°30’

40°15’

40°00’

Black

Butte

Buzzard

Spring/

Ice Cave

Mountain Cold Creek

Butte

Lassen

domefield

Lassen

Peak

Chaos

Crags

Red Lake

cluster

Inskip Hill

Hat Creek

Basalt

Sugarloaf chain

Tumble Buttes chain

Bidwell Spring chain

Tuya chain

Red Cinder chain

Silver

Lake

Buckhorn

Lake

Cinder

Butte

Cinder

Cone

Sugarloaf

Peak

Devils Rock

Garden

Popcorn

Cave

Six Mile

Hill

Big

Lake

Whittington

Butte

Twin

Buttes

section 32

junction 4126

Hat Creek Fault

Figure 4.—Continued.

B
hill 7416 Sunflower Flat Prosp

Pea

Kings Creek

Hat

Mountain

Fairfield

Peak

Cr

B

Krummholz

dome

section 27

Eagle

Peak

Chaos

Jumbles

Table

Mountain

Lassen

Peak

Chaos Crags

domes and

pyroclastic flows

1915 lahar

and flood

deposits

Chaos Crags

lahar deposit

LASSEN DOMEFIELD 

-121o40’ -121o30’ 

40o40’40o30’

Geologic units younger than 15,000 years

Geologic units between 100,000 and 15,000 years

FaultsBoundary of Lassen domefield

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

EXPLANATION

Vents

Figure 4.—Continued.
Volcanism in the Lassen Region 13

The Brokeoff Volcano was a typical andesite composite 

cone similar in volume to Mount Hood, Oregon, but smaller 

than the largest volcanoes in the Cascade arc, such as Mount 

Shasta or Mount Rainier. Brokeoff Volcano was centered in 

the southwestern portion of LVNP and was about 3,500 m 

high with a volume of about 80 km3. It was active from about 

590,000 to 385,000 years ago and probably erupted hundreds 

of times. During its lifetime it produced a variety of lava 

flows and fragmental deposits, including lahars, near-vent 

explosion deposits, and a small debris avalanche.

Subsequent to extinction of the Brokeoff Volcano, volcanism 

in the Lassen Volcanic Center shifted location and character to 

the Lassen domefield. The domefield (fig. 4) consists of about 

30 volcanoes that produced fragmental deposits from explosive 

eruptions and domes and lava flows from effusive eruptions in 

two episodes at 300,000–190,000 and 90,000–0 years ago. The 

earlier episode consisted of at least 15 lava-dome eruptions and 

3 lava flows, mostly in the area adjacent to the north flank of 

Brokeoff Volcano. Pyroclastic deposits associated with the early 

Table 1. Chronology of eruptions less than 100,000 years old in the Lassen Volcanic Center (Clynne and Muffler, 2010). 

[Recent eruptions are from the historical record. Ages with uncertainties were measured using 40Ar/39Ar, K-Ar, or radiocarbon. Others are estimates constrained by 

stratigraphy and geomorphology. Cinder Cone age is from tree-ring chronology (Sheppard and others, 2009). Radiocarbon age is for weighted mean with standard 

error and is converted to calibrated age by choosing oldest peak based on paleomagnetic data (Nathenson and others, 2007). Age of andesite of Eagle Peak is 

chosen to be slightly younger than rhyodacite of Eagle Peak because it was the waning stage of that eruption. Areas are based on mapped areas of lava flows and 

domes, and volumes include additions for any associated pyroclastic deposits. For parts of lava flows buried by surficial deposits, areas and thicknesses have been 

estimated, and they are added to the area and volume exposed. Estimates of areas and volumes of fragmental deposits are provided in the notes column. Volumes 

of the fragmental deposits have been added to the volume column. The location of all places named herein can be found in Clynne and Muffler, 2010.]

Eruption Age Best age Area 

(km2

)

Volume 

(km3) Notes

Deposits of 1914–1917 eruption 

of Lassen Peak

1914–1917 C.E. 1914 C.E. 0.107 0.007 Volume is proximal juvenile component; additional volume 

of nonjuvenile material in debris flows not included. Lahars 

and pyroclastic flows cover an additional ~8 km2

. Distal tephra 

volume ~0.02 km3

Basaltic andesites of Cinder Cone 1666 C.E. 1666 C.E. 8.4 0.37 Tephra covers a minimum of an additional 96 km2.

Rhyodacite of Chaos Crags 1,103±13 years 1,050 cal. yrs BP 4.54 1.19 Pyroclastic flows and air-fall cover a minimum of 

an additional 9.4 km2.

Andesite of hill 7416 ~12–15 ka 13 ka 8.0 0.21

Dacite of Lassen Peak 27±1 ka 27 ka 9.4 2.07 Estimate of pre-glacial area; total includes 0.15 km3

 of 

pyroclastic-flow deposits

Rhyodacite of Kings Creek 35±1 ka 35 ka 6.4 0.53 Includes 0.75 km2

 of buried lava; volume includes 0.08 km3

of pyroclastic-flow deposits spread over at least 8 km2

Andesite of Hat Mountain ~40 ka 40 ka 39.6 4.72 Includes 1.9 km2 of buried lava flow

Rhyodacite of Sunflower Flat 41±1 ka 41 ka 5.5 0.76 Includes 0.76 km2

 of buried lava; and 0.024 km3 of pyroclasticflow deposits spread over at least 2.4 km2

Rhyodacite of Krummholz 43±2 ka 43 ka 1.00 0.06 Includes 0.78 km2 of buried lava flow

Rhyodacite of Section 27 ~50 ka 50 ka 0.75 0.045 Includes 0.72 km2

 of buried lava flow

Andesite of Eagle Peak ~66 ka 65.9 ka 0.0201 0.0006 Area and volume are for pyroclastic-flow deposit

Rhyodacite of Eagle Peak 66±4 ka 66 ka 1.14 0.09 Includes 0.12 km2

 of buried lava flow; and 0.015 km3 of 

pyroclastic-flow deposits spread over at least 5 km2

Basaltic andesite of Fairfield Peak 82±14 ka 82 ka 8.2 0.22 Includes 4.0 km2 of buried lava flow

Andesite of Crater Butte 93±13 ka 93 ka 17.3 1.73 Includes 1.0 km2

 of buried lava flow

domefield sequence are poorly preserved, but were probably not 

extensive. The later episode consists of seven explosive and lavadome eruptions and seven lava-flow eruptions, recognized mostly 

in the area north and northeast of Lassen Peak, and is discussed in 

more detail in the next section.

Eruptive Activity from 100,000 to 15,000 Years Ago 

at the Lassen Volcanic Center

Three andesite volcanoes in the area east of Lassen Peak in the 

central part of LVNP and a fourth on the southwest flank of Lassen 

Peak, erupted between 100,000 and ~40,000 years ago. These eruptions 

were not strongly explosive and produced agglutinated cinder cones 

and lava flows (table 1, fig. 4C). Their products are Crater Butte 

(93,000 years old), Fairfield Peak (82,000 years old), andesite of Eagle 

Peak (~65,900 years old), and Hat Mountain (~40,000 years old). 

Six silicic lava domes or dome complexes and lava flows 

formed between 100,000 and 27,000 years ago at the LVC. 

The vents are all located in the area between Lassen Peak and 
14 Volcano Hazards Assessment for the Lassen Region, Northern California

Sunflower Flat in the northwest corner of LVNP and built the 

young part of the Lassen domefield (fig 4C; plate 1). The domes 

and flows are Eagle Peak (66,000 years old), rhyodacite lava 

flows of section 27 (~50,000 years old), Krummholz dome 

(43,000 years old), Sunflower Flat domes (41,000 years old), 

Kings Creek lava flow (35,000 years old), and Lassen Peak 

dome (27,000 years old) (Turrin and others, 1998). All of these 

are glacially eroded, and pyroclastic deposits are not well preserved. Some have poorly preserved pyroclastic flow deposits, 

but all likely had an explosive phase.

Eruptive Activity Since 15,000 Years Ago at the 

Lassen Volcanic Center

The youngest volcanic eruptions in the Lassen region are 

less than about 15,000 years old. This threshold marks the end 

of the last large glacial advance in the Lassen region. Deposits 

older than about 15,000 years, especially deposits of explosive 

eruptions, are poorly preserved. Likely future eruptions are 

well illustrated by some of the more recent eruptions in LVNP.

Four eruptions have occurred in the Lassen Volcanic Center 

in the past 15,000 years: two andesite eruptions, one andesite to 

dacite eruption, and one dacite to rhyodacite eruption (table 1). 

Andesite of hill 7416 is estimated to be 12,000–15,000 years old. 

Its vent, located just northwest of Prospect Peak, built a cinder 

cone and erupted a large andesite lava flow that traveled nearly 

7 km to the north. The flow covers an area of 8.0 km2 and has 

a volume of at least 0.21 km3. Cinder Cone, in the northeastern 

corner of LVNP, built two cinder cones and erupted five basaltic 

andesite to andesite lava flows in 1666 C.E. (fig. 5; Sheppard 

Ash-covered

lava flows

Ash-covered

lava flows

Ash-free lava flows 

Ash-free lava flows 

Cinder Cone

and others, 2009). This eruption produced an oval-shaped tephra 

deposit covering about 100 km2 and ranging from 2.5 to 0.05 

meters thick (Heiken, 1978). An additional area of 90 km2 has 1–5 

centimeters of tephra. The five lava flows cover about 8.4 km2. 

The total volume of the Cinder Cone eruption is about 0.37 km3.

The approximately 1,050-year-old eruptions of Chaos Crags 

and the 1914–1917 eruption of Lassen Peak serve as models for 

the types and magnitude of explosive silicic volcanic activity that 

can be expected in the LVC in the future. The dacite to rhyodacite 

eruption of Chaos Crags began with explosive activity that emplaced 

two pyroclastic flows extending as much as to 5 km from the vent 

and built a small tuff cone (fig. 6). Effusion of a small lava dome 

then plugged the vent. Subsequently, a much larger explosive 

event violently destroyed the small lava dome, built a second tuff 

cone, and produced a pyroclastic flow that followed the drainages 

of Manzanita, Lost, and Hat Creeks for as much as 20 km. An 

accompanying ash fall can be traced for about 40 km to the northeast, 

but fine ash must have fallen at much greater distances. Continued 

effusive activity then built five more lava domes. Hot collapse events 

from two of the growing domes emplaced pyroclastic flows a few 

kilometers long. Total volume of the Chaos Crags eruptive sequence 

is about 1.19 km3, not including distal tephra that is not preserved.

The andesite to dacite 1914–1917 C.E. eruption of Lassen 

Peak comprised a year-long precursory period of intermittent steam 

(phreatic) explosions followed by a week-long period of intense 

eruptive activity in May 1915 (Clynne and others, 1998). Two 

years of waning activity, characterized by small steam explosions, 

completed the eruptive sequence. The most significant events 

occurred on May 19 and 22, 1915, when the eruptions devastated 

the northeast flank of Lassen Peak (fig.7). On May 19 an avalanche 

of hot rock and snow roared 6 km down the drainage of Lost Creek 

Figure 5. Aerial view 

looking northwest 

at Cinder Cone. All 

tephra and lava flows 

were erupted in 1666. 

High-relief gray areas 

with red to tan patches 

in the foreground and 

middle ground are early 

ash-covered lava flows 

from Cinder Cone. Dark 

areas in the middle 

ground and at the lower 

right are slightly later 

ash-free lava flows from 

Cinder Cone. Cinder 

Cone is 215 meters high.
Volcanism in the Lassen Region 15

when a large explosion disrupted a growing lava dome. The avalanche 

swiftly melted snow to generate a lahar that swept 16 km down 

Lost Creek to near the vicinity of its join with Hat Creek (plate 1). 

Water escaping from the lahar material immediately after deposition 

caused flooding farther downstream in the Hat Creek Valley. On May 

22, Lassen Peak explosively erupted a vertically directed column 

of pumice, ash, and hot gas that rose to an elevation of about 9 km 

(30,000 feet). Collapse of this eruption column generated a pyroclastic 

flow that swept across still snow-covered parts of the northeast flank 

of Lassen Peak. Melting of this snow transformed the pyroclastic flow 

into a lahar that followed the path of the May 19 lahar. The May 22 

lahar also came to rest in the vicinity of the confluence of Lost and 

Hat Creeks, dewatered, and caused a second flood in lower Hat Creek. 

Six additional small lahars descended the east, north, and west flanks 

of Lassen Peak but did not get beyond the base of the mountain. 

Pumice blocks and smaller fragments from the eruption column fell in 

a narrow northeast-directed lobe for a distance of about 25 km from 

Lassen Peak, and fine ash fell as far away as Elko, Nevada (about 450 

km distant). Despite the significant effects of the 1915 eruption, the 

total erupted volume was small, about 0.03 km3

 (table 1).

Composite tuff cone

Dome A

Dome B

Dome D Dome E Dome B

Figure 6. View of the 1,050-year-old Chaos Crags domes from southwest. Composite tuff cone formed by initial pyroclastic eruptions 

of Chaos Crags at right. Most of the area immediately surrounding the Chaos Crags domes is covered with as much as 1 m of air-fall 

pumice from the initial pyroclastic eruptions. Adjacent areas and drainages are filled with as much as 5 m of pyroclastic-flow deposits. 

Hot, partial dome-collapse pyroclastic deposits from domes D and E are present at the bases of those domes (not visible in the picture). 

Dome A is visible in the crater of the tuff cone. Large coulee sloping from the upper right to lower left is dome B. High craggy dome in 

center is dome D, and dome E is visible on the skyline behind it. Domes C and F are hidden behind domes D and E, respectively.

Geologic Record of Regional Volcanism

Regional volcanism in the area surrounding the Lassen Volcanic Center has been continuous during the lifetime of the Cascade 

arc in northern California. Dozens of cinder cones, lava cones, 

and shield volcanoes dot the landscape. A detailed geologic map, 

chronology, and discussion of regional volcanism at Lassen can be 

found in Clynne and Muffler (2010).

Regional Volcanism from 100,000 to 15,000 Years Ago

Approximately 56 small regional volcanoes in the Lassen region 

erupted between 100,000 and ~20,000 years ago (table 2). In general, 

these volcanoes were not strongly explosive, and their products consist 

of cinder cones, lava flows, and ash deposits of restricted areal extent. 

Although located all around the periphery of the LVC, most of them 

are concentrated in four relatively restricted areas. They are arranged 

as chains of vents parallel to the regional fault pattern: Tumble Buttes 

chain, Red Cinder chain, Bidwell Spring chain, and Sugarloaf chain 

(fig. 4A, B, plate 1). A fifth group includes a few small clusters of vents 
16 Volcano Hazards Assessment for the Lassen Region, Northern California

Table 2. Chronology of eruptions less than 100,000 years old for the mafic vents in the Lassen segment of the Cascade Range.

[Unit names in italics are from the geologic map of Lassen Volcanic National Park and vicinity (Clynne and Muffler, 2010) and unpublished mapping of the 

Lake Almanor and Burney quadrangles. Names in roman font are from unpublished reconnaissance mapping in the surrounding area. Ages with uncertainties 

were measured using 40Ar/39Ar or K-Ar. Others are estimates constrained by stratigraphy and geomorphology. Areas are based on mapped areas of lava flows, 

and volumes include additions for any associated tephra. For parts of lava flows buried by surficial deposits, areas and thicknesses have been estimated, and they 

are added to the area and volume. ka = thousands of years]

Eruption Sequence or cluster Age (ka) Area 

(km2

)

Volume 

(km3) Notes

Tholeiitic basalts of Big Lake Red Lake cluster 50–75 5.4 0.148

Basaltic andesite and andesite of Red Lake 

Mountain1

Red Lake cluster ~75 29.2 0.86

Basaltic andesite of Red Mountain Red Lake cluster 75–100 12.0 0.259

Andesite of Devils Rock Garden Tumble Buttes chain 10–15 7.5 0.57

Andesite of Bear Wallow Butte Tumble Buttes chain 35.1±3.1 12.5 0.453

Hall Butte Tumble Buttes chain 35–50 5.4 0.144

Hill 6795 Tumble Buttes chain 35–50 1.17 0.0224

Basaltic andesite of hill 6770 Tumble Buttes chain 35–50 2.96 0.085

Basaltic andesite of Tumble Buttes Tumble Buttes chain 35–50 2.21 0.074

Basaltic andesite of hill 5410 Tumble Buttes chain 35–50 1.14 0.0135 Includes 0.20 km2

 of buried lava flow

Basaltic andesite of Bear Wallow Butte Tumble Buttes chain 35–50 1.27 0.0211 Includes 0.32 km2

 of buried lava flow

Eiler Butte Tumble Buttes chain 35–50 7.0 0.150

Basaltic andesite of hill 6138 Tumble Buttes chain ~50 1.30 0.0258

Basaltic andesite of Section 5 Tumble Buttes chain ~50 1.94 0.051

Andesite of Tumble Buttes Tumble Buttes chain 50–75 5.9 0.059

Basaltic andesite of Mud Lake Tumble Buttes chain 75–100 2.27 0.0227

Andesite of Sugarloaf Peak Sugarloaf chain 46±7 32.4 2.64

Basaltic andesite of Little Potato Butte Sugarloaf chain 67±4 1.05 0.077

Andesite of Potato Butte Sugarloaf chain 77±11 5.7 0.280

Basaltic andesite of hill 4709 Sugarloaf chain ~80 0.94 0.0163

Andesites of Old Station Sugarloaf chain 75–100 0.418 0.0167

Hill 4041 Sugarloaf chain 80–100 0.305 0.0061

Hill 4899 Sugarloaf chain 80–100 0.55 0.0201

Highway 89 Sugarloaf chain 80–100 0.100 0.00100

Popcorn Cave Cinder Butte cluster 30–50 52 1.75

Cinder Butte Cinder Butte cluster 38±7 38.8 2.59

Six Mile Hill Cinder Butte cluster 40–50 30.8 0.50

Andesite of Bidwell Spring Bidwell Spring chain 25–45 3.75 0.114 Includes 1.2 km2

 of buried lava flow

Basaltic andesite of Pole Spring Road Bidwell Spring chain 25–45 2.69 0.072 Includes 0.30 km2

 of buried lava flow

Basaltic andesite of section 36 Bidwell Spring chain 25–45 1.20 0.0332
Volcanism in the Lassen Region 17

Table 2. Chronology of eruptions less than 100,000 years old for the mafic vents in the Lassen segment of the Cascade Range.—Continued.

Eruption Sequence or cluster Age (ka) Area 

(km2

)

Volume 

(km3) Notes

Basalt of Twin Buttes Bidwell Spring chain 46±3 19.8 0.80

Basaltic andesites of Black Butte Bidwell Spring chain ~50 10.7 0.334 Includes 2.30 km2

 of buried lava flow

Basaltic andesite of Red Cinder Cone Red Cinder chain 20–25 1.64 0.054

Basalt of Red Cinder Cone Red Cinder chain 20–25 1.65 0.120 Includes 0.57 km2 of buried lava flow

Basaltic andesite of Red Cinder Red Cinder chain 25–40 8.3 0.201 Includes 3.5 km2 of buried lava flow

Basalt of hill 8030 Red Cinder chain 25–40 7.3 0.178 Includes 2.0 km2 of buried lava flow

Basalt of Cameron Meadow Red Cinder chain 25–40 3.92 0.078 Includes 3.0 km2 of buried lava flow

Basalt of Ash Butte Red Cinder chain 40–70 1.10 0.062

Basalt of hill 2283 Red Cinder chain 40–70 0.88 0.0292 Includes 0.16 km2 of buried lava flow

Basalt of section 25 Red Cinder chain 40–70 1.08 0.0185

Andesite of Red Cinder Red Cinder chain 69±20 18.1 1.81 Includes 6.9 km2

 of buried lava flow and 

shield under Red Cinder edifice

Basalt east of Ash Butte Red Cinder chain 70–100 1.19 0.0178 Includes 1.0 km2

 of buried lava flow

Basalt of Widow Lake Red Cinder chain ~100 0.80 0.0295 Includes 0.6 km2 of buried lava flow

Basaltic andesites of Long Lake Red Cinder chain ~100 9.3 0.249 Includes 3.6 km2 of buried lava flow

Basaltic andesite of Caribou Wilderness Red Cinder chain ~100 1.20 0.0240 Includes 1.0 km2 of buried lava flow

Basalts of Triangle Lake Red Cinder chain ~100 2.70 0.050 Includes 0.75 km2

 of buried lava flow

Miscellaneous group

Silver Lake2 north of Miller Mtn. 10–15 8.3 0.249 Does not include area of scoria

Twin Buttes SE of Burney Mtn. 15–25 10.1 0.296

Basaltic andesite of Turnaround Lake Tuya chain 17–35 0.88 0.079

Hat Creek Basalt (tholeiitic) near Old Station 24±6 99 2.47

Basaltic andesite of section 32 SE of Twin Buttes 35-50 1.04 0.0228

Basalt of junction 4126 NE of Twin Buttes 35-50 0.300 0.0110

Basalt of Inskip Hill3 Inskip Hill ~50 62 2.51

Tholeiitic basalts of Buzzard Springs near Sifford Mtn. 65±45 11.1 0.137 Includes 0.30 km2 of buried lava flow

Tholeiitic basalt of Ice Cave Mountain near Sifford Mtn. ~65 13.5 0.162 Includes 2.6 km2 of buried lava flow

Basalt of Black Butte west of Shingletown ~70 6.5 0.137 Includes 3.8 km2 of ash

Basalts of Cold Creek Butte west of Mineral 75–100 7.1 0.275 Includes 1.2 km2 of buried lava flow

Whittington Place west of Magee Peak 75–100 7.9 0.079

1

 Includes basaltic andesite of Eskimo Hill.

2

 Includes flow at Buckhorn Lake.

3

 Includes Little Inskip Hill, vents at Paynes Creek and vents in Oak Creek.
18 Volcano Hazards Assessment for the Lassen Region, Northern California

Remnant of lava flow erupted May 19–20, 1915

Lassen Peak

(27 ka)

Rhyodacite flow

of Kings Creek

(35 ka)

Upper Devastated Area

Lithic pyroclastic flow 

from partial collapse of 

the dome of Lassen Peak

(27 ka)

Lost 

Creek

Hat Creek

and additional widely dispersed vents. At least 40 more vents 

erupted between 120,000 and 100,000 years ago.

The Tumble Buttes chain consists of a 13-km-long, northnorthwest-oriented linear array of 13 cinder cones and lava flows just 

north of LVNP. The vents range in age from nearly 100,000 to 35,000 

years (one vent is inferred to be younger and is discussed below). 

The Red Cinder chain consists of a north-northwest-oriented elongate 

area about 10 km long with 12 vents that range in age from ~100,000 

to ~20,000–25,000 years. Just north of the Red Cinder chain, the 

Bidwell Spring chain includes 5 vents aligned in a 10-km-long chain. 

The Sugarloaf chain includes 8 vents, arranged in a north-northwest 

oriented array about 12 km long, that range in age from ~80,000–

100,000 to 46,000 years (Turrin and others, 2007). The fifth group 

includes clusters of vents around Cinder Butte, the Red Lake cluster, 

Buzzard Spring-Ice Cave Mountain and eight other widely dispersed 

vents ranging in age from ~75,000–100,000 to ~20,000 years. 

Regional Volcanism in the Past 15,000 Years

Of the approximately 58 small mafic volcanoes in the Lassen 

region that erupted within the last 100,000 years (table 2), only a 

few are younger than 15,000 years old (table 2). One, the Devils 

Rock Garden flow, is the youngest vent in the Tumble Buttes chain 

(figure 4A). The others are vents located near Silver and Buckhorn 

Lakes. In general, these were not strongly explosive and consist of 

cinder cones, lava flows, and ash deposits of restricted areal extent. 

All are probably between 15,000 and 10,000 years old (fig. 4A).

Magma Beneath the Lassen Volcanic 

Center

The presence of magma stored in the shallow crust 

beneath a volcano is generally accepted as a precondition for 

future volcanic activity. At the LVC, the existence of a vigorous hydrothermal system and young silicic volcanism implies 

that an active magma reservoir lies beneath. Several unsuccessful attempts have been made to locate and delineate this 

reservoir (Iyer, 1984; Berge and Monfort, 1986; Berge and 

Stauber, 1987; Amato and Berge, 1988). These geophysical 

surveys found low-velocity zones that suggest the presence 

of magma in the crust beneath the Lassen Volcanic Center but 

Figure 7. Aerial view of the area devastated by the 1915 eruption of Lassen Peak looking southwest. Area with sparse trees marks the 

path of the avalanche and lahars of May 19–20, 1915, and the pyroclastic flow and lahars of May 22. Small dark crags just to the right of the 

summit of Lassen Peak are a remnant of the May 19–20 lava flow. The composite dacite dome of Lassen Peak (27,000 years, or 27 ka, old) 

dominates the upper part of the view. Lithic pyroclastic flow deposits from hot partial collapses of Lassen Peak during eruption of Lassen 

Peak 27,000 years ago, are exposed in the headwaters of Lost Creek. Area left of the large snow patch on the east flank of Lassen Peak is 

the dacite lava flow of Kings Creek (35,000 years old). Pyroclastic flows related to this eruption are exposed in the West Fork of Hat Creek 

(just off the photograph to the east). The area devastated by the 1915 eruption continues to the north off the bottom of the picture.
Lassen Hydrothermal System 19

failed to detect a discrete magma reservoir. The smallest bodies that these surveys could detect was about 5 km in diameter, 

and Clynne (1985) interpreted these studies to indicate that the 

Lassen magmatic system contains one or more small dispersed 

bodies of silicic magma in a larger but mostly solidified zone 

of silicic magma beneath the volcanic center. More recent 

petrologic studies support this model (Feeley and others, 2008; 

Underwood and others, 2012; Clynne and Muffler, 2010). The 

mineralogy of the young silicic rocks suggests that the magma 

bodies that produced the Lassen Peak and Chaos Crags 

eruptions reside at a depth of at least 8 km, but data required 

to evaluate quantitatively the depth and amount of magma 

beneath the LVC are not available.

Geologic models of the Lassen region indicate that 

regional mafic volcanism is derived from dispersed flux of 

mafic magma (basalt) that rapidly traverses the Earth’s crust to 

the surface. A higher and more focused flux of mafic magma 

and heat beneath the LVC leads to melting of crustal rocks 

(Guffanti and others, 1990), which produces silicic magma 

that accumulates beneath the volcanic center. Intrusions of 

mafic magma into an existing or mostly solidified silicic 

magma body can add sufficient heat and volatiles to regenerate 

the body and maintain it at least partially molten for thousands 

of years. If sufficiently large, these mafic magma intrusion 

events can trigger eruptions of the silicic magma. The presence of quenched mafic inclusions in most silicic lavas at 

LVC indicates that mafic magma was intruded into a silicic 

magma reservoir shortly before an eruption, thus documenting 

the presence of mafic magma that otherwise did not reach the 

surface (Clynne 1999; Underwood and others, 2012). Petrologic and chemical studies document that each intrusive event 

involves a different batch of mafic magma. More importantly, 

each batch of mafic magma has a significant effect on the 

composition of the silicic magma that it intrudes. Thus, the 

mostly liquid parts of the silicic magma body beneath LVC are 

probably relatively small, in agreement with the geophysical 

evidence discussed above. The mostly solid parts of the silicic 

magma are too viscous to be eruptible, but they are ductile 

enough that intrusions of mafic magma cannot pass through 

them. Instead such intrusions rejuvenate the silicic magma and 

provoke the type of silicic eruptions that have occurred at LVC 

in the past 100,000 years. The most significant implication of 

this interpretation of no large silicic magma chamber is that 

a large, caldera-forming eruption is not likely at the Lassen 

Volcanic Center in the foreseeable future.

Lassen Hydrothermal System

The Lassen Volcanic Center hosts far and away the most 

extensive hydrothermal system in the Cascade arc. The presence of the hydrothermal system indicates that hot rock or 

magma lies in the shallow subsurface beneath the LVC and 

indicates the potential for volcanic eruptions. The surface 

manifestations of the hydrothermal system include steam vents 

and hot springs, and alteration of the surface rocks by acidic 

water and gases creates unstable hazardous ground.

Steam vents and hot springs are the surface expressions 

of hydrothermal systems in which cold surface water percolates deep into the ground where it is heated by thermal energy 

from a heat source. The LVC is the host to such a system 

because it has the three required elements: abundant groundwater, permeable rock, and a heat source at depth.

The Lassen hydrothermal system consists of a shallow 

vapor-dominated reservoir underlain by a reservoir of hot 

water at a temperature of 240 ̊C (Muffler and others, 1982; 

Ingebritsen and Sorey, 1985, 1987; Clynne and others, 2003). 

Recharge of the system by meteoric water occurs primarily in 

the high-topography area of the Lassen domefield, especially 

in the vicinity of Lassen Peak. The cold water percolates to 

great depth, where it encounters and is heated by hot rock 

associated with the Lassen magmatic system. The heated 

water and dissolved volcanic gases emitted by the hot rock 

rise to a depth of about 1 km below the surface, where they 

accumulate in fractures beneath the area of thermal features. 

All thermal features in LVNP are driven by boiling and steam 

separation in this reservoir; the hottest and most vigorous 

features are at Bumpass Hell and Boiling Springs Lake, which 

mark the principal areas of upflow and steam discharge from 

the Lassen hydrothermal system (Janik and McLaren, 2010). 

Steam-dominated features occur at five additional areas in 

LVNP: Sulphur Works, Pilot Pinnacle, Little Hot Springs 

Valley, Devils Kitchen, and Terminal Geyser (fig. 8). Springs 

depositing travertine (CaCO3) occur on the periphery of some 

of the steam-dominated thermal areas in Little Hot Springs 

Valley (fig. 8). Morgan and Growler Hot Springs at lower 

elevations south of LVNP in Mill Creek mark the lateral discharge of near-neutral-pH, chloride-bearing hot water from the 

deep hot-water reservoirs under Bumpass Hell. Morgan and 

Growler Hot Springs deposit siliceous sinter, in contrast to the 

acid-sulfate alteration at Bumpass Hell, Sulphur Works, and 

Devils Kitchen. Hot water that deposits travertine at Drakesbad (fig. 8) is from a separate hydrothermal upflow centered 

in the Devils Kitchen or Boiling Springs Lake area (Janik and 

McLaren, 2010).

Boiling of the 240 ̊C hot water in the deep reservoir provides the steam that feeds the surface hydrothermal features of 

LVNP. This steam contains hydrogen sulfide (H2

S), which oxidizes in the near-surface environment to produce sulfuric acid 

that in turn reacts with the near-surface volcanic rocks, altering them to soft, light-gray to white slopes (fig.9) composed 

primarily of opal (SiO2) and kaolinite (a clay mineral). The 

major thermal areas within the LVNP display numerous thermal vents, ranging from superheated fumaroles with temperatures as high as 161°C at Bumpass Hell and 147°C at Little 

Hot Springs Valley (Janik and Bergfeld, 2010), steam vents at 

the boiling point for this altitude (~92°C), mud pots ranging 

in temperature from boiling to near ambient, and warm to hot 

ground commonly covered with orange and yellow sulfates. 

Native sulfur (S) is common on the walls of steam vents, and 

gray to black pyrite (FeS2

) is common as linings of the vents 
20 Volcano Hazards Assessment for the Lassen Region, Northern California

and discharge channels, as scum floating on the surface of 

pools, and as dispersions in mud pots.

The vigor of the surface hydrothermal features varies 

both seasonally and from year to year. To potentially provide 

early warning of impending volcanic activity, the thermal 

features in LVNP are chemically and physically monitored 

by the USGS and the National Park Service (Sorey, 1986) 

to detect changes that may be caused by renewed influx of 

magma into the LVC.

Seismicity and Ground Deformation

Other expressions of the active nature of the Lassen 

region include frequent earthquakes and slow deformation 

of the ground surface in response to changes at depth. 

Increased seismicity and ground deformation are often the first 

indication of a potential eruption in volcanically active areas. 

However, many volcanic areas located in tectonically active 

regions, like the Lassen region, experience earthquakes and 

deformation related to both tectonic and volcanic seismicity.

The Lassen region experiences both tectonic earthquakes 

and earthquakes of volcanic origin (long-period quakes). 

Tectonic earthquakes occur because Lassen is located along 

the western edge of a region of closely spaced NNW-trending 

normal faults that characterize the Basin and Range Province 

(Guffanti and others, 1990). The majority of earthquakes in 

the region occur in a broad NNW-trending seismic zone that 

extends from the Sierra Nevada through the Lassen region 

to about 20 km northeast of Lassen Peak (fig. 3). These 

earthquakes are tectonic in origin and are associated with the 

impingement of Basin and Range tectonism on the Cascade 

arc. The Lassen Volcanic Center is also within a developing 

zone of strike-slip faulting called the Walker Lane that is 

associated with interaction between the North American and 

Pacific tectonic plates (Faulds and Henry, 2008).

Tectonic Earthquakes

Tectonic earthquakes associated with plate movements 

are common in the Lassen region. Three vigorous earthquake 

sequences occurred in the region in 1936, 1945–1947, and 

1950 (Norris and others, 1997). These included main shocks 

as large as magnitude 5.5 and thousands of smaller events 

and were attributed to approximately east-west extension 

in the Lassen region localized on Basin and Range normal 

faults (Norris and others, 1997). Smaller bursts of seismic 

activity, generally with earthquakes as large as about 

magnitude 4–5 and including dozens of smaller shocks, 

occur every few years in the Lassen region. Basin and Range 

Figure 8. Map showing the location of the major thermal areas in Lassen Volcanic National Park and vicinity.

7600

8400

8000

8000

8800

7200

6800

6000

6000

6800

6400

6000

6000

6800

5200

7200

6800

89

Boiling Springs Lake

Cold Boiling Lake

Terminal Geyser

Devils Kitchen Drakesbad

Growler Hot Spring

Sulphur Works

Brokeoff

Mountain

Mount

Conard

Mount Diller

Pilot Pinnacle

Lassen Peak

Pilot Pinnacle

Morgan Hot Springs

Little Hot

Springs Valley

Bumpass

Hell

LASSEN

VOLCANIC

NATIONAL

PARK

Mill Canyon

0

0 1 2 3 KILOMETERS

1 2 3 MILES

Hydrothermal areas

Volcanic feature or remnant

Outline of Brokeoff Volcano

EXPLANATION

Contour interval 400 feet

-121°35’ -121°32.5’ -121°30’ -121°27.5’ -121°25’ -121°22.5’

40°22.5’

40°25’

40°27.5’

N
Seismicity and Ground Deformation 21

Big

Boiler

Figure 9. View looking east-southeast from trail at the west end of Bumpass Hell (see fig. 8). Bumpass Hell is the surface expression of 

a major upflow from the vapor-dominated reservoir of the Lassen hydrothermal system (Muffler and others, 1982; Ingebritsen and Sorey, 

1985, 1987; Janik and McLaren, 2010) and contains many fumaroles, mud pots, and acid hot springs (Muffler and others, 1983). Big Boiler 

is a superheated fumarole with temperatures as high as 161°C (Janik and Bergfeld, 2010).

faults in the Lassen region, including the Hat Creek Fault 

and faults with large offsets in the Lake Almanor area, are 

considered active and capable of earthquakes as large as 

magnitude 7 (Wills, 1990a, b). The Hat Creek Fault offsets 

the 24,000-year-old Hat Creek Basalt by as much as 30 m 

(Muffler and others, 1994; Turrin and others, 2007). Despite 

the lack of historical seismicity, displacement of outwash 

gravels overlying the Hat Creek Basalt shows that vertical 

offset on the Hat Creek Fault has averaged ~1.3 mm per year 

for the past 15,000 years, a figure similar to the longer-term 

average (Muffler and others, 1994).

About 25 percent of the seismic events in the Lassen 

region are associated with the Lassen hydrothermal system 

(Klein, 1979; Walter and others, 1984). These small 

earthquakes are clustered beneath the hydrothermal features 

at shallow depth and typically occur in episodes of 10–25 

events over a 1–3 day period. McLaren and Janik (1996) 

and Janik and McLaren (2010) interpret this seismicity to be 

related to hydrothermal alteration and brittle failure of rock 

in the hydrothermal system.

Long-Period Earthquakes

Pitt and others (2002) described long-period (LP) 

seismicity in the Lassen region. Between 1982 (when 

the Lassen seismic net was established) and 2002, 29 LP 

earthquakes at depths from 13 to 23 km were detected, 

primarily in the area about 5–8 km west of Lassen Peak near 

the northwest corner of LVNP. Accounting for some periods 

in which the seismic net was not operational, the average was 

about two LP earthquakes per year. The seismicity, however, 

is clearly episodic, as many as eight earthquakes in one year 

(1988) and no earthquakes in others (for example, 1991). 

Since 2002, improvements in the seismic net have led to 

increased detection of LP earthquakes. An average of 11 LP 

earthquakes per year were detected between 2003 and 2011, 

most in small clusters (A.M. Pitt, unpublished data).

These LP earthquakes are generally interpreted to reflect 

movement of mafic magma in the deep crust below active 

volcanic areas (Pitt and others, 2002). However, even high 

levels of long-period activity often do not result in immediate 
22 Volcano Hazards Assessment for the Lassen Region, Northern California

volcanic activity (for example, at Long Valley, Calif.; Pitt 

and Hill, 1994). The growing evidence that recharge of 

silicic magma bodies by injections of hot mafic magma 

(Clynne, 1999, Pallister and others, 1996) plays a role in 

triggering volcanic eruptions makes it important to monitor 

LP seismicity. An increase in the level of LP seismicity may 

provide the earliest indication that a volcanic system is being 

recharged. A subsequent sudden increase in small, similarmagnitude, high-frequency earthquakes and harmonic tremor

indicates unrest in the volcanic system that might evolve 

toward an eruption.

Ground Deformation

Measuring deformation of the ground surface plays 

an important role in detecting movements related to both 

tectonic and volcanic activity. Measurements on a regional 

scale permit interpretation of plate-tectonic movements and 

large-scale interactions. Measurements on a more local scale 

may detect the rise of fresh magma into the shallow magma 

system beneath a volcano and thereby indicate the potential 

for an eruption. Few ground deformation surveys have been 

undertaken at Lassen.

Field-measurement campaigns around Lassen Peak in 

1981, 1982, and 1984 detected no significant deformation 

(Poland and others, 2004). Leveling surveys designed to 

provide a baseline for detection of large-scale deformation 

in the Lassen region were conducted in 1991 by the USGS 

(Dzurisin, 1999). Leveling lines were run between Viola 

and Old Station and between Mineral and Chester, and 

comparison with surveys made in 1932 by the U.S. Coast 

and Geodetic Survey revealed no changes larger than 

the uncertainties in the measurements. Thus the area was 

apparently stable within the precision of the measurements 

between 1932 and 1991.

Interferometric synthetic aperture radar (InSAR) images 

spanning the 1996–2000 time interval indicate as much as 

about 1 centimeter/year subsidence over an area ~40 km in 

diameter and centered about 5 km southeast of Lassen Peak 

(Poland and others, 2004). More sensitive measurements 

conducted using the Global Positioning System in 2004 

confirm that subsidence has been taking place since at least 

1981. The large area of deformation and the presumed lack 

of a large magma chamber at Lassen led Poland and others 

(2004) to interpret the deformation as tectonic and resulting 

from Basin and Range extension.

Volcano-Related Hazards in the Lassen 

Region

This section discusses the character, hazards, and the 

most likely locations of future volcanic eruptions in the 

Lassen region. Mafic eruptions occur in the area surrounding 

the Lassen Volcanic Center, but generally not within it. This 

is because ascending mafic magma is intercepted by the 

partially molten silicic magmatic system under the Lassen 

Volcanic Center and mixes with it to produce andesite 

magma. Conversely, silicic eruptions occur only within the 

area of the surface expression of the LVC because generation 

of silicic magma is confined to beneath that area.

Hazardous areas are described as volcano hazard zones 

and are shown on figure 10 and plate 1. These hazard zones 

are defined on the basis of the location and character of 

volcanism in the past 100,000 years, with emphasis on the 

past 50,000 years. Most of these hazard zones are similar to 

those shown in Miller (1989), but they have been modified 

to take into account geologic data acquired since his report. 

One difference between hazard zones depicted herein and 

in Miller (1989) is his Pyroclastic Flow Hazard Zone. To 

define that zone, Miller used a hypothetical eruption similar 

to that of Mount Mazama (volume ~50 km3

) as an analogue 

for the largest possible eruption at Lassen. However, an 

eruption of that size is unprecedented at Lassen in the past 

100,000 years, and there is no indication that the magmatic 

system is evolving toward such a large eruption. The largest 

postglacial (<15,000 years) eruption at Lassen was the 

Chaos Crags eruption, which produced a total volume of 

~1.19 km3 (table 1), most of which erupted as lava. We 

estimate that the largest credible explosive eruption at 

Lassen would be no larger than the 1980 eruption at Mount 

St. Helens (total volume ~1.4 km3, ash volume ~1.1 km3; 

Brantley and Myers, 2000), and we use this volume to 

designate silicic eruption hazard zones (Combined Flowage 

Hazard Zone and Silicic Ash Hazard Zone).

The most likely volcanic event in the Lassen region is 

a short-lived mafic eruption, possibly lasting as long as a 

year, and producing a cinder cone, local tephra fall, and lava 

flows. The Mafic Vent and Lava Flow Hazard Zone is based 

on a few-kilometer buffer zone around the vents and adjacent 

valley bottoms of all mafic eruptions in the past 100,000 

years. The Mafic Ash Hazard Zone is based on the thickness 

of tephra produced by the 1666 C.E. eruption of Cinder Cone, 

a well-preserved eruption, and is extrapolated to the entire 

area of potential mafic volcanism.

The most likely silicic volcanic event would be an 

eruption of duration and magnitude similar to the 1915 

eruption of Lassen Peak or the 1,050-year-old Chaos Crags 

eruptive sequence. These two eruptions were of significantly 

different size and duration, and their effects were also very 

different, but in combination they are likely analogues for 

the range of volcanic eruptions expected at LVC. Such an 

event might include an extended period of precursory phreatic 

activity, explosive activity resulting in pyroclastic flows and 

(or) lahars, proximal and distal ash fall, and emplacement 

of silicic lava flows or domes. The most likely location of 

a silicic event is in the vicinity of Lassen Peak and Chaos 

Crags. An additional likely location for lava flows is the 

eastern part of the Lassen domefield in the vicinity of Hat 

Mountain and Cinder Cone. In the following sections, more 
Volcano-Related Hazards in the Lassen Region 23

detailed discussions of the volcanic and other hazards in the 

LVC and surrounding area are presented.

An eruption generating extremely large pyroclastic 

flows, for example a caldera-forming or “supervolcano” 

eruption on a scale of 10s to 100s of cubic kilometers, is 

not considered likely at LVC. Several such large-scale 

events have occurred in the Lassen region in the past 3 

million years, although none since the eruption of the 

Rockland ash-flow tuff and tephra about 611,000 years 

ago (Lanphere and others, 2004). However, the magmatic 

system of the LVC is not presently configured for this type 

of eruption, and the probability of such a high-consequence 

event is considered negligible in the foreseeable future 

(Nathenson and others, 2012).

Hazards Associated with Mafic Volcanism in 

the Lassen Region

This section discusses the likely locations and effects of 

explosive and effusive mafic eruptions in the Lassen region– 

primarily lava flows and ash fall. The effects of these types 

of eruptions were covered under the section titled “Hazards 

Associated with Mafic Eruptions.” Hazard zones for these 

types of eruptions are shown on figure 10 and plate 1.

Mafic Lava Flows

Mafic lava flows and associated cinder cones are the 

most common volcanic features in the Lassen region (figs. 

2, 3, 4, and 10). Two types of mafic lava flows associated 

with regional volcanism in the Lassen region are discussed 

together—cinder cones with small lava flows and larger 

volume, more fluid lava flows.

Eruptions producing cinder cones and small lava 

flows could occur anywhere in the region surrounding the 

Lassen Volcanic Center. Mafic lava flows are generally 

small, typically being limited to a few kilometers in length 

and covering a few square kilometers (fig. 11). Fifty-four 

eruptions that built cinder cones and (or) emitted lava flows 

and are known or estimated to be less than 100,000 years 

old are listed in table 2. The outlines of individual mafic lava 

flows or groups of such flows are shown in blue on figure 

10 and plate 1, and the locations of their vents are shown 

as asterisks. The distribution of these vents and lava flows 

defines a Mafic Vent and Lava Flow Hazard Zone (the solid 

blue line on fig. 10 and plate 1). This hazard zone indicates 

the area where new mafic vents are most likely to be located 

and adjoining areas downhill that new lava flows might be 

expected to cover.

The alignments of vents shown on figure 10 and plate1 

demonstrate that their location is controlled by fractures 

(faults) in the crust and that, although new mafic volcanism 

could occur anywhere in this zone, it is more likely in a few 

areas where the youngest mafic lava flows are concentrated. 

These are NNE-aligned chains or clusters of vents identified in 

table 2, for example the Red Cinder chain, the Bidwell Spring 

chain, vents in the area of the Hat Creek Valley, the Tumble 

Buttes chain and the Red Lake cluster. Together these and 

other alignments define several larger zones of likely locations 

for mafic volcanism: (1) a zone from the Red Cinder chain to 

Calif. Hwy 44, (2) a zone from south of Old Station to near 

the Pit River, (3) a zone from the south end of Tumble Buttes 

chain (approximately Calif. Hwy 44) to Burney Mountain, and 

(4) the Red Lake cluster (fig 4A, B, plate 1). Mafic volcanism 

in the other parts of the hazard zone shown in plate 1, for 

example west of LVC in the vicinity of Inskip Hill, Black 

Butte, or Silver Lake or south of the LVC around Mineral, is 

possible but less likely than the areas discussed above.

The largest mafic lava flows in the Lassen region are 

fluid basalt flows associated with Basin and Range volcanism 

(designated as tholeiitic in table 2). These erupt from fissures 

with little associated ash, and the lava is generally distributed 

through lava tubes. In the Lassen region, they most commonly 

erupt slightly east of the main Cascade arc axis. These fluid 

lava-flow eruptions occur much less frequently than the 

smaller cinder-cone and lava-flow eruptions (only 4 of the 

total of 58 eruptions listed in table 2 are fluid mafic lava 

flows). But these lava flows erupt much larger volumes (1–3 

km3

) than the small mafic lava flows and cover much greater 

areas, often as much as 100 km2, and voluminous eruptions 

could extend beyond the designated lava flow hazard zone 

(plate 1) in some valleys. The youngest large basalt lava flow 

in the Lassen region is the 24,000-year-old Hat Creek Basalt, 

which erupted from a fissure vent just south of Old Station 

(Muffler and others, 1994; Turrin and others, 2007). As much 

as ~2.5 km3 of lava from this vent was distributed for 20 km 

north in lava tubes and covered nearly 100 km2 of the Hat 

Creek Valley. Much smaller, young fluid lava flows include 

the vents near Big Lake north of LVNP and the Buzzard 

Spring and Ice Cave Mountain vents in the meadows along 

the North Fork Feather River (table 2).

Lava flows are not likely to advance rapidly enough 

to overtake fleeing people but would damage or destroy 

everything in their path, including forests and structures. 

During dry weather the advancing front of a lava flow in the 

forest would ignite fires. An eruption at the south end of the 

Tumble Buttes chain or the Red Lake cluster could easily 

block Calif. Hwys 44 and 89. An eruption in the Hat Creek 

Valley could block or disrupt traffic on Calif. Hwys 44 and 89. 

Wooden-pole power and telephone lines would be destroyed, 

and steel-pole power lines and other critical infrastructure 

could be destroyed, buried, or cut off by mafic lava flows.

Mafic Ash

The presence of mafic cinder cones in the area 

surrounding the Lassen Volcanic Center indicates explosive 

activity at those vents. Eruption columns associated with 

formation of cinder cones can rise several kilometers into 

the atmosphere (for example Luhr and Simkin, 1993; 

Hill and others, 1998). Close to vents, mafic ash (tephra) 
24 Volcano Hazards Assessment for the Lassen Region, Northern California

Andesite Vent and Lava Flow Hazard Zone

Area most likely for eruption of andesite vents and lava flows in

Lassen Volcanic Center. Also subject to thick accumulations of ash

Lahar Hazard Zones

Areas beyond the lava flows shown in the Silicic Vent, Lava Dome,

and Flow Hazard Zone and subject to inundation by lahars

Flood Hazard Zones

Areas beyond the Lahar Hazard Zones subject to floods.

VOLCANO HAZARD ZONES

[See text for additional explanation]

Mafic Ash Hazard Zone

Area surrounding Mafic Vent and Lava Flow Hazard Zone and 

subject to more than 5 cm of mafic ash

Mafic Vent and Lava Flow Hazard Zone

Area most likely for eruption of mafic vents and lava flows 

LASSEN REGION

LASSEN VOLCANIC CENTER

Combined Flowage Hazard Zone

Area adjacent to explosive volcanoes or vents and subject to

pyroclastic flow, blast, or surge eruptions. Also subject to deposition

of more than 15 cm of silicic ash.

Silicic Vent, Lava Dome, and Flow Hazard Zone

Area most likely for eruptions of silicic vents, lava domes, and flows.

Subject to thick accumulations of silicic ash. Also the area of initiation

of pyroclastic flows, blasts, surges, and lahars

Silicic Ash Hazard Zone

Area surrounding the Combined Flowage Hazards Zone and subject to

more than 5 cm of silicic ash

Mafic Lava Flows

Areas covered by mafic lava flows younger than 100,000 years

Andesite Lava Flows

Areas covered by andesitic lava flows younger than 100,000 years

Silicic Lava Flows

Areas covered by silicic domes and lava flows younger than 100,000 years

Major Power Lines

Gas Pipeline

Roads

EXPLANATION

INFRASTRUCTURE

Railroads 0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

Vents

Figure 10. Map of volcano hazard zones in the Lassen 

region (see also plate 1). The Mafic Vent and Lava 

Flow Hazard Zone is shown by the solid blue line, 

which encloses the area of regional mafic vents (black 

asterisks) and lava flows (blue) less than 100,000 years 

old. Mafic lava flows will likely cover similar areas 

depending on the topography in the immediate area 

of the vent and the volume of lava erupted. Although 

mafic volcanism is possible anywhere in the area, it 

is more likely in a few named areas (a zone from the 

Red Cinder and Bidwell Spring chains through the Hat 

Creek Valley, the area of the Tumble Buttes chain, and 

the area of the Red Lake cluster). The area covered by 

eruption of new flows will depend on the topography 

in the immediate area of the vent and the volume of 

lava erupted. Blue dotted line defines the Mafic Ash 

Hazard Zone and encloses area where more than 5 cm 

of mafic ash is possible. The boundary is drawn ~10 km 

from the potential vent areas and is based on the 5-cm 

isopach associated with the 1666 eruption of Cinder Cone 

(Heiken, 1978; Clynne and Muffler, 2010). The prevailing 

wind direction makes it most likely that ash deposits 

will extend east to northeast of new vents (see fig. 13). 

Red solid line denotes the Silicic Vent, Lava Dome and 

Flow Hazard Zone, which encompasses the area of 

silicic vents (black asterisks) for eruptions in the Lassen 

domefield less than 100,000 years old. The area covered 

by silicic lava domes and flows less than 100,000 years 

old is shown in red. The area covered by a new eruption 

will depend on the topography in the immediate area of 

the vent and the volume of lava erupted. The Andesite 

Vent and Lava Flow Hazard Zone is shown by the solid 

magenta line. It denotes the area of andesite and more 

mafic lava flows and vents (black asterisks) less than 

100,000 years old in the Lassen Volcanic Center. Vents of 

renewed andesite volcanism associated with the Lassen 

domefield will most likely be confined to this area. The 

magenta areas show the extent of andesite lava flows 

less than 100,000 years old. New andesite lava flows will 

likely cover similar areas depending on the topography 

in the immediate area of the vent and the volume of lava 

erupted. Ash fall thicker than 5 cm associated with these 

eruptions will most likely be contained within the Mafic 

Ash Hazard Zone (blue dotted line). The red dashed line 

encloses a Combined Flowage Hazard Zone that includes 

pyroclastic flows, blasts and surges generated by silicic 

eruptions, and the area that could be affected by more 

than 15 cm of silicic ash fall. Beyond areas within a few 

km of vents, pyroclastic flows and lahars will be confined 

mostly to valleys of the major drainages. Volcanic blasts 

and pyroclastic surges are less affected by topography 

and may extent out to the vicinity of the dashed red line. 

The Silicic Ash Hazard Zone shown by the red dotted line 

indicates the area that could be affected by more than 5 

cm of silicic ash. The prevailing wind direction makes it 

most likely that ash deposits will extend east to northeast 

of new vents (see fig. 13). Yellow indicates areas 

potentially subject to inundation by lahars generated by 

pyroclastic flows or other volcanic events originating 

in the Silicic Vent, Lava Dome and Flow Hazard Zone 

(Robinson and Clynne, 2012). Green indicates areas 

downstream of lahar deposition that are potentially 

subject to floods generated by dewatering of lahars. 

These areas are modified from Miller (1989). See text for 

further explanation of all hazard zones. (digital elevation 

model from Gesch, 2007)
Volcano-Related Hazards in the Lassen Region 25

Lassen Volcanic

National Park

Caribou

Wilderness

Area

5

36

89

99

44

299

89

44

COUNTY A6

COUNTY A21

USFS 17

Lake

Almanor

Lake

Almanor

Butt Valley

Reservoir

Butt Valley

Reservoir

Shasta

Lake

Lake Britton

Big Lake

PIT #3

DAM

CANYON

DAM

BUTT 

VALLEY

DAM

Mountain

Medows

Reservoir

North Fork Feather RiverNorth Fork Feather River

Sacramento RiverSacramento River

Deer CreekDeer Creek

Manzanita Creek anzanita Creek

Lost CreekLost Creek

North Fork Battle CreekNorth Fork Battle Creek

Bailey CreekBailey Creek

Battle CreekBattle Creek

Digger Creek er Creek

Mill CreekMill Creek

Warner CreekWarner Creek

Kings Creek

Hat CreekHat Creek

Shasta

Lake

Lake Britton

Big Lake

Pit River Pit River

Hat CreekHat Creek

South Fork Battle CreekSouth Fork Battle Creek

Butte

Lake

Black

Butte

Sugarloaf chain

Tumble Buttes

chain

Silver

Lake

Buckhorn

Lake

Cinder

Butte

Inskip Hill

Hat Creek

Basalt

Bidwell Spring chain

Red Cinder chain

Lassen Domefield Tuya chain

Red Lake

cluster

Buzzard

Spring/

Ice Cave

Mountain

Sunflower

Flat

Chaos

Crags

Cinder

Cone

Sugarloaf

Peak

Prospect

Peak

Hat

Mountain

Burney

Mountain

Big

Lake

Whittington

Butte Eiler

Butte

Twin

Buttes

Popcorn

Cave

Six Mile

Hill

Cold Creek

Butte

junction

4126

section

32

-121o45’ -121o30’ -121o15’ -121o00’ -122o00’

41o00’

40o45’

40o30’

40o15’

Burney

Shingletown

Manton

Viola

Mineral

Red

Bluff

Old

Station

Fall River

Mills

LASSEN

PEAK

Chester
26 Volcano Hazards Assessment for the Lassen Region, Northern California

deposits could be several meters thick, but such deposits 

typically thin abruptly away from their vents (fig. 12). 

The hazard zone for accumulation of mafic ash 5 or more 

centimeters thick is based on the 1666 C.E. eruption of 

Cinder Cone in LVNP. The total volume of Cinder Cone 

ash is calculated to be about 0.034 km3 using the method of 

Fierstein and Nathenson (1992). At Cinder Cone, the 5-cm 

contour describes an oval-shaped area covering about 250 

km2 and extending 10 km from the vent. The Mafic Ash 

Hazard Zone extends approximately 10 km (dotted blue 

line on figure 10 and plate 1) beyond the area of potential 

new vents shown in the Mafic Vent and Lava Flow Hazard 

Zone. Thinner accumulations of ash could extend at least 

another 10 km from vents, depending on the explosivity of 

individual eruptions and wind conditions. The most likely 

locations for new vents were discussed in the previous 

section. Eruptions producing mafic ash generally last from 

weeks to months or a year, but on rare occasions they can 

last as long as 10 years.

Eruption of mafic magma through bodies of water or 

interaction with groundwater can also produce explosive 

eruptions. The affected areas and thicknesses of ash 

accumulations are generally similar to those of cinder cone 

eruptions. However, if large plumes of steam are generated, 

ash can be carried much higher into the air and transported 

much farther (as much as hundreds of kilometers) than 

typical. Although this type of phreatomagmatic eruption

has occurred in the Lassen region in the past, it is not 

common, and none has occurred in the past 100,000 years. 

Explosions that occur when lava flows into a body of water, 

for example, a lake, generally affect only the immediate 

area of the interaction. Thus, separate hazard zones for 

phreatomagmatic activity and lava flowing into water are not 

drawn on figure 10 and plate 1.

Snag Lake

Butte Lake

Figure 11. Steep front 

of block lava flows from 

the 1666 eruption of 

Cinder Cone (telephoto 

view looking southwest 

from Sunrise Peak). The 

lava flow fronts at Cinder 

Cone range in height from 

10 to 50 m and typically 

are 20 to 30 m high. Blue 

arrows indicate the 

direction of advance of 

the lava flows. Snag Lake 

(barely visible at top left) 

was created by damming 

of drainage by the lava 

flows. Future lavas flows 

in the Lassen region 

can potentially also dam 

drainages.

Hazards Associated with Silicic Volcanism at 

the Lassen Volcanic Center

This section discusses the likely locations of silicic 

explosive and effusive eruptions at the Lassen Volcanic Center 

including lava domes and flows, pyroclastic flows, lateral 

blasts, airborne ash and ash fall, and lahars. Table 1 lists the 

eruptions that have occurred at the LVC in the past 100,000 

years. Five hazard zones for these types of events are shown 

in figure 10 and plate 1. They are: (1) Silicic Vent, Lava 

Dome and Flow Hazard Zone, (2) Andesite Vent and Lava 

Flow Hazard Zone, (3) Combined Flowage Hazard Zone, 

(4) Silicic Ash Hazard Zone, and (5) Lahar Hazard Zone. 

Downwind silicic ash hazards are also discussed, but a specific 

hazard zone is not designated. Hazards from floods resulting 

from lahars are also discussed. The effects of these types 

of eruptions were discussed in previous sections under the 

heading Hazards Associated with Silicic Eruptions.

Lava Domes and Flows

Silicic to andesitic lava domes and flows are common 

eruptive features of volcanism at the Lassen Volcanic Center 

and are discussed in this section. Lava domes occur primarily 

in the western part of the Lassen domefield in the northwest 

corner of LVNP. Thick andesite lava-flow complexes form the 

eastern part of the Lassen domefield.

Silicic lava flows and domes are most likely to occur in 

the immediate vicinity of Chaos Crags and Lassen Peak and are 

unlikely to occur outside the area of the western part of the Lassen 

domefield. The hazard zone for silicic lava domes and flows is 

shown on figure 10 and plate 1 as the solid red line (Silicic Vent, 

Lava Dome and Flow Hazard Zone). Lava domes and short, thick 
Volcano-Related Hazards in the Lassen Region 27

Prospect

Peak

Butte

Lake

Snag

Lake

200

10

50

25

10

5

0

40°35'

40°30'

-121°20' -121°15'

Lava flows between 

100,000 and 15,000 years 

Cinder Cone lava 

flows and cones 

Lava flows younger than 

15,000 years 

Ash Fall isopachs 

thickness in centimeters

EXPLANATION

0 0.5 1 1.5 2 MILES

0 0.5 1 1.5 2 KILOMETERS

Figure 12. Map showing thickness (isopachs in centimeters) of the ash deposits from the 1666 eruption of Cinder 

Cone (adapted from Heiken, 1978). Vent cinder cones are indicated with stipple pattern and the area covered by lava 

flows in buff (black lines indicate internal flow contacts in Cinder Cone and the older lava flows). (digital elevation 

model from Gesch, 2007)
28 Volcano Hazards Assessment for the Lassen Region, Northern California

lava flows associated with silicic volcanism in the LVC typically 

cover only a few square kilometers but can be as much as 1–2 

km3 in volume (figs. 5 and 6). Lava domes tend to occur in small 

groups or clusters that result from local shifting of the vent during 

dome growth. The viscous nature of silicic lava flows and the slow 

documented eruption rates at volcanoes such as Mount St. Helens 

indicate that such an eruption could extend over years to decades. 

Growth of lava domes is generally non-explosive, but periods of 

heightened activity resulting in explosions and pyroclastic flows 

(see section on “Combined Flowage Hazard Zone”) can occur if 

gas-rich magma reaches the vent.

Lava domes at Lassen are typically <1 km3 in volume but 

occasionally larger and are very steep, often unstable, and subject 

to collapse events. Collapse of growing lava domes can generate 

pyroclastic flows of hot dome rock called lithic pyroclastic flows or 

block and ash flows. Most lava domes grow by adding lava to the 

interior of the dome. The outer surface expands to accommodate 

the new lava being pushed up into the growing dome. The 

relatively cool and solid exterior of the dome cracks and portions 

of it can fail as small collapse events. Breaking of the solid but 

still hot rock during collapse events allows escape of contained 

gases that gives lithic pyroclastic flows considerable mobility. Both 

Lassen Peak and the Chaos Crags have generated lithic pyroclastic 

flows that traveled as far as 5 km from their sources. The hazard 

zone for dome collapse events is confined to an area of about 5 

km from any growing dome. This zone is not shown separately 

on figure 10 and plate 1, but it would be contained within the 

Combined Flowage Hazard Zone shown as the dashed red line.

At the LVC, andesite lava flows are most likely in the eastern 

part of the Lassen domefield in LVNP in the vicinity of Hat 

Mountain and Cinder Cone. The Andesite Vent and Lava Flow 

Hazard Zone is shown as the solid magenta line on figure 10 and 

plate 1. Andesite lava flows associated with the LVC are thicker 

and tend to have larger volumes than typical mafic lava flows in 

the surrounding area. Andesite lava flows in the eastern part of 

the Lassen domefield are as much as several cubic kilometers in 

volume and can cover several tens of square kilometers with lava. 

Four andesite lava-flow complexes have been emplaced on the 

Lassen domefield in the past 100,000 years (table 1). The largest 

of these, Hat Mountain, covers nearly 100 km2

 and has a volume 

of about 4.7 km3. The others are smaller, but still substantial and 

cover a combined area of ~40 km2. The length of time required to 

emplace these flows is unknown, but it is probably comparable to 

that for lava domes. Mild explosions that build cinder cones are 

associated with the vent areas of andesite lava flows. The effects of 

these eruptions are similar to those described for mafic ash.

Pyroclastic Flows and Lateral Blasts

Explosive silicic volcanism in the Lassen Volcanic Center is 

likely to generate pyroclastic flows. These will most likely occur 

in the early stages of silicic eruptions, but after precursory activity. 

Pyroclastic flows generated by eruption column-collapse events 

will flow off the high ground around the vent and be channeled by 

topography down valleys. Pyroclastic flows from the Chaos Crags 

eruption flowed down valleys for as much as about 20 km. The vents 

for explosive silicic eruptions will most likely be located in the area 

of youngest volcanism in the LVC, the area of the Silicic Vent, Lava 

Dome and Flow Hazard Zone. The valleys most likely to be effected 

are Manzanita Creek, Lost Creek and Hat Creek. The Combined 

Flowage Hazard Zone (dashed red line on figure 10 and plate 1) is 

for explosive silicic eruptions, including pyroclastic flows, similar 

to the magnitude of the Chaos Crags eruption, which is the largest 

known in the past 100,000 years. An eruption generating larger 

pyroclastic flows is much less likely and would be unprecedented in 

the Lassen region in the past 100,000 years.

Some types of more energetic explosive eruptions, for 

example lateral blasts and pyroclastic surges, are not as effected by 

topography as pyroclastic flows generated by collapse of eruption 

columns. These are pyroclastic blasts and surges that are directed 

away from vents at high speeds (for example, the blast at Mount 

St. Helens on May 18, 1980). They are less likely at Lassen than 

column-collapse pyroclastic flows, but a small pyroclastic surge 

occurred at Lassen in 1915. Lateral blasts and pyroclastic surges are 

expected to be confined to the Combined Flowage Hazard Zone.

Airborne Ash and Tephra Fallout

Tephra fallout accompanying a silicic eruption could be 

regionally widespread if the eruption column reaches at least 

several kilometers into the atmosphere, as would be likely. 

Winds in the Lassen region blow dominantly toward the east 

and northeast (fig. 13). Depending on plume height, wind 

direction, and duration and volume of the eruption, fallout could 

cause short-term disruption of activities in Redding, Chico, 

Red Bluff, the Chester-Lake Almanor area, Susanville, Alturas, 

and Reno, Nevada and perhaps as far away as rural northern 

Nevada and southern Oregon. Local communities, Lassen 

Volcanic National Park, adjacent Lassen National Forest, roads 

and highways, and utility corridors could be affected for days or 

weeks, depending on the duration of the eruption.

Past tephra eruptions are poorly represented in the 

geologic record at Lassen, probably because of removal of 

deposits by glacial erosion. The two postglacial explosive 

silicic eruptions, that of Chaos Crags and the 1915 eruption of 

Lassen Peak, have mappable tephra deposits, and it is likely 

that most of the 30 or more pre-glacial silicic eruptions also 

emitted ash. Tephra plumes produced in the Chaos Crags and 

1915 eruptions (fig. 14) were directed to the northeast by 

the most common prevailing wind direction to the northeast. 

Although a traceable deposit of the 1915 eruption is preserved 

for only about 25 km from Lassen Peak, ash fell as far away 

as Elko, Nevada, nearly 450 km from Lassen. Tephra fallout 

volume for this eruption is estimated to be at least 25,000,000 

m3

 (0.025 km3). The Chaos Crags eruption was much larger, 

but is traceable for only about 40 km because it is older and 

thus has been subjected to more erosion.

Near-vent accumulations of tephra from explosive silicic 

eruptions could be many meters thick, but no permanent 

habitations lie close to likely vent locations at Lassen. The 

hazard zone for 15 cm or more of silicic ash is confined to the 

area of the Combined Flowage Hazard Zone shown on figure 
Volcano-Related Hazards in the Lassen Region 29

Wind Velocity

(m/s)

≤10

>10 - 20

>20 - 30

>30

0

30

60

90 E

120

150

180

210

240

W 270

300

330

0% 2% 4% 6% 8% 10% 0% 2% 4% 6% 8% 10%

S

N

Figure 13. Rose diagram showing 

directions from which winds blow 

in the vicinity of Lassen Peak 

(Donnelly-Nolan and others, 2007). 

Diagram also shows relative 

frequencies of wind speeds (in 

percent of time) at an elevation 

of 9,360 m. Data from 1997–2006, 

comprising 14,608 data points from 

NOAA-ARL final archive (http://

www.arl.noaa.gov/ready-bin/fnl.pl, 

last accessed August 15, 2007).

Figure 14. Plume generated by 

the Lassen Peak eruption of May 

22, 1915. This plume reached a 

height of about 9 km, and ash 

from the it fell in a lobe directed to 

the northeast and as far as Elko, 

Nevada, about 450 km distant. 

Historic photograph taken by R.I. 

Meyers from Anderson in the 

Sacramento Valley about 65 km 

west of Lassen Peak. Eruption 

cloud photographically pasted onto 

a more photogenic foreground 

by Benjamin Franklin Loomis in 

1915, courtesy of the National Park 

Service.

10 and plate 1. The hazard zone for 5 cm or more of silicic 

ash is shown as a dotted red circle on figure 10 and plate 1 

(Silicic Ash Hazard Zone). Figure 15 shows the estimated 

annual probability of 1 cm or more of tephra accumulation 

from eruptions of the major Cascade volcanoes including 

the LVC (Hoblitt and Scott, 2011; Richard Hoblitt, written 

commun., 2012). In the area east of the LVC, the chance of 1 

cm of tephra falling in any given year is between 0.0002 and 

0.001 (1 in 5,000 to 1 in 1,000). Although 1 cm of ash may 

seem a very small amount, experience with Mount St. Helens 

eruptions indicates that as little as a few millimeters of ash is 

sufficient to slow vehicle traffic to a crawl, potentially disrupt 

electrical distribution, and to close businesses for as long as a 

week (Schuster, 1981, and references therein).

Numerous examples of engine failure, loss of visibility, 

and loss of navigational and operational instruments have 

occurred in aircraft encounters with airborne volcanic ash. 

The Lassen Volcanic Center sits directly below the primary 

air corridor between the southwestern and northwestern 

United States, and any eruption generating tephra is likely 
30 Volcano Hazards Assessment for the Lassen Region, Northern California

to cause temporary re-routing of commercial air 

traffic. Depending on the factors discussed above, 

operations at local airports, for example in Chester 

and Susanville, might be disrupted. Redding and 

Chico could also be affected if the winds were 

blowing to the west or south. Airports at greater 

distances, for example Reno, Sacramento, or the 

San Francisco Bay Area, might be affected under 

favorable circumstances or in larger eruptions. 

Eruption columns affecting commercial aircraft are 

not likely to persist in the atmosphere for more than 

a few days at a time.

Lahars and Lahar-Generated Floods

Any volcano with a significant cover of snow 

or ice is subject to lahars induced by sudden melting 

of snow and ice. Lahars are sparse in the geologic 

record at LVC, but events prior to 15,000 years ago 

have been obscured by the effects of glaciation. At 

Lassen, the most likely events to cause lahars are 

pyroclastic flows moving over snow or avalanches 

of hot rock that incorporate large amounts of snow. 

Both these types of lahars occurred during the 1915 

eruption of Lassen Peak (fig. 16). A pyroclastic flow 

that went down Manzanita Creek during the Chaos 

Crags eruption was transformed into a lahar by 

incorporating water from the creek. Torrential rains 

onto recently deposited loose volcanic or sedimentary material may also trigger a lahar, and in 1963 a 

small lahar of this type occurred on the lower slopes 

of Lassen Peak. Lahars may also be triggered by 

landslides of hydrothermally altered rock. Although 

the altered core of the Brokeoff Volcano has been 

the site of many landslides, some of which traveled 

considerable distances, there is no evidence that 

any transformed into significant lahars. Lahars were 

also initiated at Lassen by mobilization of recently 

deposited glacial sediments during the last deglaciation event about 8,000 years ago (Marron and 

Laudon, 1986; Christiansen and others, 2002), but 

this type of lahar is very unlikely under the present 

climatic conditions.

At LVC, lahars are not expected to be as large 

as those described at many Cascade composite 

volcanoes. Lassen Peak, the LVC’s largest edifice, 

is not large compared to most Cascade composite 

volcanoes, does not have any glaciers, and has 

only 1 small permanent snowfield. The winter of 

1914–1915 was an exceptional year for snowfall 

on Lassen Peak (Day and Allen, 1925) and probably approximates the largest volume of snow that 

could be available to generate lahars. Robinson and 

Clynne (2012) calculated the inundation areas for 

the lahars generated on May 19 and 22, 1915, with 

LAHARZ (Iverson and others, 1998), a computer 

Lassen Volcanic Center

Mount Hood

Mount Adams

Mount St. Helens

Mount Baker

Mount Shasta

Mount Meager

Mount Rainier

Mount Jefferson

Crater Lake

Glacier Peak

Medicine Lake Volcano

Mount Garibaldi

Newberry Volcano

Three Sisters

Mount Cayley

40°

45°

50°

-124° -120° -116° Annual Probability 

≥ 1cm Tephra Fall

0.0001

0.0002

0.001

0.002

0.01

0 60 120 KILOMETERS

0 40 80 MILES

Figure 15. Map showing annual probability of 1 centimeter or more of 

tephra accumulation from any major Cascade volcano (black triangles) 

(Hoblitt and Scott, 2011; Richard Hoblitt, written commun., 2012) using the 

method of Hoblitt and others (1987). The Lassen region has a probability 

of 0.0001 (0.01 percent) to 0.001 (0.1 percent)– 1 in 5,000 to 1 in 1,000– each 

year of receiving 1 centimeter or more of tephra. The area around Mount 

St. Helens dominates the probablity because it is the most active explosive 

volcano in the Cascade Range. Black lines are state and county boundaries.
Volcano-Related Hazards in the Lassen Region 31

Figure 16. Historic photographs taken in 1915 by Benjamin Franklin Loomis of the effects of lahars from Lassen Peak. Photographs 

courtesy of the National Park Service. A, View of logjam in Lost Creek after lahar of May 19, 1915. B, View of lower Lost Creek after 

lahars of May 1915.

program that uses a digital elevation model to estimate the 

extent of inundation by lahars of a given volume. They found 

that the inundation areas calculated using estimated volumes 

for the May 19 and 22 lahars corresponded closely with the 

mapped deposits.

At LVC, lahars can originate in any of the drainages 

heading in the high-elevation terrain around Lassen Peak, 

including Mill Creek, Bailey Creek, Manzanita Creek, Lost 

Creek, Hat Creek, and Warner Creek. However, they are 

most likely in Lost, Hat, and Manzanita Creeks. Robinson 

and Clynne (2012) used LAHARZ to calculate inundation 

zones for lahars originating in drainages heading on Lassen 

Peak, plus Bailey Creek. Model input parameters included 

water volumes available as snow in the various drainage 

basins. Lahar volumes modeled were 1, 3, 10, 30, 60, and 

90 million m3, values corresponding to small (1, 3), medium 

(10, 30) and large (60, 90) lahars. Maximum volumes were 

scaled to be appropriate for the maximum water content of 

snow in each drainage basin (Robinson and Clynne, 2012). 

Smaller volume lahars will be much more frequent than the 

larger volume lahars. The largest volume modeled lahars 

are the maximum size events that can be expected to be 

generated from Lassen Peak and require that the maximum 

water content of snow be available in a given drainage basin 

and that it all be melted in a single event. Medium-size 

lahars like those generated in 1915 (~15 million m3) are 

much more likely.

The results reported by Robinson and Clynne (2012) 

are used to delineate Lahar Hazard Zones for lahars in 

each of the valleys originating in the area of Lassen Peak. 

These are shown on figure 10 and plate 1. Lahars of the 

largest modeled volume will be confined to the area of the 

Combined Flowage Hazard Zone for Manzanita and Bailey 

Creeks (fig. 10 and plate 1). Only the largest volume lahars 

(unprecedented for Lassen) could extend as far as about 5 km 

beyond the Combined Flowage Hazard Zone in Hat Creek. 

Most modeled lahars in the Warner Creek drainage would 

be confined to the Combined Flowage Hazard Zone, but the 

largest (unprecedented at Lassen) would approach Chester. 

Only small-volume lahars in the Mill Creek drainage would 

be contained within the Combined Flowage Hazard Zone, 

and larger volume lahars could extend considerably beyond 

it. However, large-volume lahars in Warner and Mill Creeks 

are less likely than in the drainages heading on the north 

side of Lassen Peak, because vent locations are more likely 

to be on the north side of the volcano. Lahars starting at a 

lower elevation than the upper flanks of Lassen Peak would 

have a smaller potential water volume and a smaller drop in 

elevation to traverse and are expected to be smaller and less 

mobile than those generated in 1915. They will most likely 

be confined to the Combined Flowage Hazard Zone shown 

on figure 10 and plate 1.

Lahars can initiate flooding farther downstream than they 

travel. When the mass of water-saturated sediment comes to 

rest, water is released and continues downstream. Depending 

on the amount and grain size of sediment carried, these are 

hyperconcentrated stream flows or muddy floods. Both of the 

mudflows in Lost and Hat Creeks generated by the May 19 

and May 22, 1915, eruptions of Lassen Peak stopped in the 

vicinity of the confluence of Lost and Hat Creeks. The effects 

described in the historical literature farther down Hat Creek 

were muddy floods. In 1915, fish kills from muddy water 

were noted along the entire length of Hat Creek (Merrill, 

1916; Bryant, 1918; Shehley, 1922), as far as 50 km from 

Lassen Peak. Flood Hazard Zones that extend beyond the 

Lahar Hazard Zones for the valleys of Hat Creek, North Fork 

Battle Creek, Mill Creek, and the North Fork of the Feather 

River are shown on figure 10 and plate 1.

A B
32 Volcano Hazards Assessment for the Lassen Region, Northern California

Other Hazards in the Lassen Region

The Lassen region is mountainous and thus includes a variety 

of other geologic hazards associated with mountainous areas, especially landslides and rockfalls. In addition there are hazards associated with the hydrothermal system at the Lassen Volcanic Center.

Landslides and Rockfalls

In this section, we discuss landslides and rockfalls that are not 

directly related to volcanic eruptions but are the result of secondary 

processes related to volcanism at the LVC. Landslides and rockfalls 

are common hazards in any mountainous terrain, particularly those 

with volcanic or glacial landscapes. Many rockfalls occur without warning or are triggered by earthquakes. Volcanic landforms 

such as lava domes, edifices composed of fragmental deposits, or 

interbedded lava flows and volcanic breccias can be inherently 

unstable. Normal freeze-thaw processes attack weak and heavily 

jointed volcanic rocks, enhancing this instability. Thus, rockfalls of 

a variety of sizes are constant hazards in areas of exposed rock. The 

10,000-m3

 rockfall that occurred on the northeast flank of Lassen 

Peak in August 1993 (Norris and others, 1993) is typical of this type 

of rockfall. Talus cones, especially around the bases of Lassen Peak 

and Brokeoff Mountain, and in valleys below cliffs exposed in Mill 

Creek and Bailey Creek, attest to the rockfall hazards in LVNP. 

The hazard associated with typical small rockfall events normally 

does not extend beyond the base of talus cones. Large, precariously 

perched boulders near the summit of Diamond Peak also pose a 

hazard to vehicles driving on the Park road below, should they be 

dislodged by an earthquake or by normal erosion.

Hydrothermal alteration and oversteepening of valley walls 

and cirques by glacial erosion further contribute to instability 

of volcanic rocks. Human activity, for example road building 

in unstable hydrothermally altered deposits, also contributes to 

landslide hazards. Such landslide hazards are distributed throughout 

LVNP, but are particularly prevalent in the high, steep country of 

Brokeoff Volcano. In this area, small landslides have to be dealt 

with nearly every year, particularly where steep slopes are disrupted 

by road cuts and associated fill. The Park road (Calif. Hwy 89) that 

leads from Sulphur Works around Diamond Peak to the Bumpass 

Hell parking lot is particularly susceptible to continuing rock fall 

onto the road and to slumping of the road east and south into Little 

Hot Springs Valley. This rockfall and landslide hazard potential, 

however, is unlikely to affect vehicles or persons using the road, particularly because the road is closed over the winter.

In addition to these widespread, small landslide and rockfall 

hazards, LVNP is susceptible to hazards from larger landslides 

related to two specific geological environments: Chaos Crags and 

the core of Brokeoff Volcano.

Chaos Jumbles

The Chaos Crags are a complex of 6 volcanic domes that 

together form the youngest large volcanic landform in LVNP, having 

erupted about 1,050 years ago (Clynne and others, 2008). Approximately 340 years ago, dome C of Chaos Crags was the source of 

Chaos Jumbles (fig. 17), a complex of major rock avalanche deposits 

(Crandell and others, 1974; Eppler and others, 1987; Christiansen 

and others, 2002). The hazards of and potential for similar events in 

the future were evaluated by Eppler and others (1987).

Chaos Jumbles is made up of the deposits of three separate 

rockfall avalanches that in combination cover 6.8 km2. The deposits consist of a monolithologic breccia of dacite blocks in a matrix 

of pulverized dacite. The first rockfall avalanche was the largest 

and traveled 4.5 km with a 650-m vertical drop from the top of the 

Calif. Hwy 89

Lassen Peak

Dome B

covered by talus Dome B

Chaos Jumbles

Chaos Jumbles

Chaos Jumbles

Dome C

Dome D

Dome E

Dome F

Figure 17. View of Chaos 

Crags looking southeast from 

Calif. Hwy 89 where it crosses 

Chaos Jumbles. Hummocky 

surface of the rock-avalanche 

deposits of Chaos Jumbles 

(formed about 340 years ago) 

dominates the foreground and 

middle ground. The oval-shaped 

massive rock cliff (dome C) in the 

center of the view is the source 

of the Chaos Jumbles rock 

avalanches. The highest peak 

is dome D, and the high ground 

above and to the left of dome C 

is dome E. Small peak on the far 

left is dome F. The sparsely treecovered area (below the large 

snow patch) on the far right is 

the margin of dome B. Lassen 

Peak (formed 27,000 years ago) 

is visible on the far right skyline.
Volcano-Related Hazards in the Lassen Region 33

breakaway scar on dome C of Chaos Crags. The second and third 

avalanches were successively smaller and shorter, but thicker. The 

initial direction of each avalanche was west-northwest toward Table 

Mountain (fig. 4C), and the first avalanche deposit rode up nearly 

125 m onto Table Mountain before being deflected to the west.

Steep-sided volcanic domes such as Chaos Crags are 

clearly unstable and prone to collapse, but the trigger for the 

Chaos Jumbles avalanches is unknown. A number of mechanisms have been proposed, including an explosion at the base 

of dome C (Williams, 1932) and renewed volcanic activity on 

dome D, which was reported to be steaming in 1857 by the 

Brewer Survey (Brewer, 1930). However, there is no evidence 

to support either of these ideas, and the most likely explanation 

is triggering by a large earthquake. Approximately 90 percent 

of dome C collapsed in the 3 rockfall avalanches, leaving only 

about 10 percent of the dome still intact. Thus, Eppler and others (1987) suggested that an avalanche resulting from a further 

collapse of dome C would barely reach the Park road and thus 

would pose little danger to Park visitors. However, collapse of 

other domes at Chaos Crags is still a potential hazard. Largevolume avalanches from older domes in LVNP are less likely 

because those domes are less steep, glaciated, and eroded.

Brokeoff Volcano

Brokeoff Volcano is composed of two sequences of 

deposits (Clynne and Muffler, 2010). The younger sequence 

consists primarily of six thick, large-volume, lithologically 

similar, lava flows that erupted from flank vents. These 

flows generally lack interbedded pyroclastic deposits, are 

resistant to erosion, and are not prone to major landslides. 

The underlying older sequence, on the other hand, consists of 

dozens of small-volume lava flows and interbedded fragmental 

deposits erupted from a central vent. These thin lava flows 

and breccias in the central core of Brokeoff Volcano are very 

prone to landslides, primarily in areas where the rocks are 

hydrothermally altered (fig. 18).

The older deposits of Brokeoff Volcano have been 

subjected to intense and protracted alteration related to 

hydrothermal systems (Crowley and others, 2004; John and 

others, 2006, 2009). Centers of acidic hydrothermal alteration 

occur in the area between Diamond Peak, Brokeoff Mountain, 

Mount Diller, and Pilot Pinnacle, and a large area of alteration 

occurs in the area in and above the east side of Little Hot 

Springs Valley (fig. 19). The weak, altered rocks are prone 

to erosion by fluvial and landslide processes (Clynne and 

Muffler, 2010). Much of the core area of Brokeoff Volcano is 

covered today by landslide deposits (fig. 19).

Hydrothermal alteration in the core of Brokeoff Volcano 

is ongoing in the headwaters of Mill Creek, particularly 

in Little Hot Springs Valley and in the Sulphur Works and 

Pilot Pinnacle areas (figs. 8, 19). This ongoing hydrothermal 

activity promotes and exacerbates landslide activity in a 

positive feedback loop. Steam and acid gases rising from the 

underlying vapor-dominated reservoir (Muffler and others, 

1982; Janik and McLaren, 2010) alter the bedrock and existing 

landslide material to an unstable mixture of opal and clay. 

Landslides composed of hydrothermally altered rock

Diamond Peak Landslides in

Mill Creek

Figure 18. View looking south of active landslides in the area south of Pilot Pinnacle (off the photograph to the left) and west of 

Diamond Peak. Most of the central area of Brokeoff Volcano is covered by these young landslides. Landslides form in clay-rich 

hydrothermally altered lava flows and fragmental deposits and are most active during the annual spring snowmelt.
34 Volcano Hazards Assessment for the Lassen Region, Northern California

Landslides remove overburden from 

the hydrothermal conduits, thus making 

it easier for steam and gas to reach the 

surface and further alter the rock and 

landslide material. This feedback loop 

proceeds continuously, accelerating 

near-surface hydrothermal alteration 

and greatly enhancing removal of 

hydrothermally altered flows and breccias 

that form the core of Brokeoff Volcano. 

Weak, hydrothermally altered rocks 

and clay-rich deposits become saturated 

with water and move downslope under 

their own weight, particularly in the 

spring during periods of rapid snowmelt. 

Reactivation of older landslide deposits 

by this mechanism is common. The 

result is a complex of small, young 

landslides throughout the areas of active 

hydrothermal alteration.

Significant landslides can also 

occur in the areas of older hydrothermal 

alteration away from modern hydrothermal 

vents. Retreat of glaciers left areas of 

steep and slide-prone terrain in cirque 

headwalls and valley walls. Indeed, the 

south entrance station and the nearby 

visitor center of LVNP are located on a 

large landslide that moved 7 km down 

Mill Creek from the cliff at the base of 

Brokeoff Mountain 3,310±55 years ago 

(Clynne and others, 2008). Reactivation 

of a small part of this landslide in January 

1997 temporarily blocked the Park road 

just south of the entrance station. Such 

landslides can be significant events, 

especially in steep canyons, such as the 

canyon of Mill Creek. Lake deposits 

preserved on the canyon walls at the 

confluence of East and West Sulphur 

Creeks demonstrate that at one time a 

landslide temporarily blocked the drainage 

of East Sulphur Creek. Eventual failure of 

this blockage undoubtedly created a flood 

downstream in Mill Creek.

Hydrothermal Hazards

In addition to the landslide hazards 

discussed in the preceding section, the 

hydrothermal system at Lassen presents a 

variety of hazards to the infrastructure of 

LVNP, NPS personnel, and Park visitors. 

These are discussed in this section. An 

overview of the Lassen geothermal system 

was presented in a previous section.

Figure 19. Map showing the distribution of landslides in the eroded core 

of Brokeoff Volcano, adapted from sheet 1 of Clynne and Muffler (2010). The 

hollowed-out core of Brokeoff Volcano is the product of enhanced glacial and 

fluvial erosion of weak, hydrothermally altered rock, and much of the core area 

is now covered by material emplaced by landslides. The largest remnants of the 

Brokeoff Volcano—Mount Diller and Brokeoff Mountain—were protected from 

erosion by thick, unaltered lava flows on their outer flanks. (digital elevation 

model from Gesch, 2007)

Landslide deposits within and around Brokeoff Volcano. Within the erosional 

amphitheater, many of these deposits have a matrix of soft, clay-rich, 

hydrothermally altered, volcanic rock. 

Upper and outer part of Brokeoff Volcano, composed of seven large-volume and 

thick lava flows not hydrothermally altered and thus resistant to erosion. 

Core of Brokeoff Volcano, composed of small-volume, thin lava flows 

interbedded with fragmental deposits, many of which are hydrothermally 

altered clay-rich material easily eroded by glaciers and streams and prone 

to landsliding.

Rim of erosional amphitheater in Brokeoff Volcano

EXPLANATION

-121°32.5’ -121°30’

40°25’

40°27.5’
Volcanic Event Frequency, Probability and Magnitude 35

Hazards to People

The high-temperature fumaroles and mud pots in LVNP 

present potential burn hazards to persons straying from 

trails and boardwalks in hydrothermal areas. Thin crusts in 

the hydrothermal areas are susceptible to collapse under the 

weight of persons walking on them. First-, second-, thirddegree and sometimes fatal burns from boiling mud or water 

and steam have affected persons in similar thermal areas 

throughout the United States (particularly in Yellowstone 

National Park) and the world (probably best documented in 

New Zealand).

The gases hydrogen sulfide (H2S) and carbon dioxide 

(CO2) accompany steam in thermal areas throughout the 

world; their hazards are discussed extensively in Christiansen 

and others (2007, p. 34–36), from which much of the 

following is adapted.

Although CO2 is ubiquitous in the atmosphere at a 

concentration of 0.038 percent, both CO2

 and H2S can be 

lethal in higher concentrations; 0.5 percent is the threshold 

limit value for continuous 8-hour exposure to CO2. Signs of 

intoxication, however, appear in a 30-minute exposure at 5 

percent (Aero Medical Association, 1953), and a few minutes 

of exposure at 7–10 percent produces unconsciousness (Flury 

and Zernik, 1931; Hunter, 1975). H2

S at 0.005–0.01 percent 

causes mild conjunctivitis and respiratory irritation after 

1 hour, and 0.07–0.1 percent results in rapid unconsciousness, 

cessation of respiration, and death (Yant, 1930).

Typically, the gas released from fumaroles contains 

95–99 percent CO2 and 1–5 percent H2S. Normally, these 

gases dissipate safely in the surrounding air; concentrations 

at waist level are rarely above 0.1 percent and 0.0002 percent 

respectively, even adjacent to fumaroles (Christiansen and 

others, 2007, p. 34, based on unpublished data of Jacob 

Lowenstern and Henry Heasler). Both gases, however, are 

heavier than air and, under calm conditions, can collect in lowlying areas sheltered from the wind. In particular, these gases 

can collect in pits or caves that can form in deep snow around 

areas of gas venting. The hazard from CO2 is exacerbated 

because CO2

 is odorless and colorless and thus cannot be 

perceived readily. H2S has a conspicuous “rotten egg” smell at 

low concentrations, but at concentrations above 0.015 percent 

(150 parts per million) the olfactory nerve is overwhelmed and 

the sense of smell disappears, often together with awareness of 

danger (Mandavia, 2009).

Toxic gases from fumaroles thus pose a potential hazard 

to Park visitors and staff, and thermal areas are especially 

hazardous when buried by snow after a significant storm. 

Normally, the gases dissipate quickly into the atmosphere, 

and only rarely are visitors present where gas concentrations 

can cause harm. Nevertheless, fatal incidents such as those 

at Mammoth Mountain in eastern California (Hill, 2000; 

Becerra, 2006) and at Lassen in 1995 (Arthur and Packer, 

1995) are possible. Also, unless safety measures are utilized, 

incidents similar to the one that caused a worker’s death at 

Tower Junction in Yellowstone National Park (Whittlesey, 

1995, p. 67) could occur.

Hazards to Structures

The high-temperature, acidic, hydrothermal features of 

LVNP not only alter the volcanic rocks, but also affect manmade features, primarily roads and boardwalks. Boardwalks 

in Bumpass Hell, Devils Kitchen, and Sulphur Works have 

been re-located many times over the years as the acid vapors 

affected both wood and metal. In some cases, wood posts 

in the ground have served as conduits or “wicks” for steam 

and acid. Even boardwalks constructed on seemingly cool 

ground can be affected, because the steam vents and mud pots 

change in temperature and character and can migrate laterally 

with time. Roads can be affected and undermined by thermal 

activity, most prominently at Sulphur Works.

The temporal and spatial variation of fumaroles has 

been observed in all thermal areas of LVNP, but it has been 

most systematically documented in recent years at Sulphur 

Works, Little Hot Springs Valley, and Pilot Pinnacle. The 

boardwalks north of the LVNP road at Sulphur Works 

were so severely compromised by thermal activity that in 

2007 they were removed in the interests of safety to both 

visitors and NPS personnel. In recent years, fumarolic 

activity just north of Calif. Hwy 89 at Sulphur Works 

has migrated south towards the road and has become 

concentrated in a large, boiling mud pot adjacent to the 

sidewalk curb (fig. 20). In addition, borings by the Federal 

Highway Administration in April 2009 and in November 

2011 demonstrated that boiling temperatures have risen to 

a depth of 5–7 m under the north lane of the Park road and 

compromised the integrity of the roadbed.

Hydrothermal Explosions

Hydrothermal explosions present a significant hazard 

in many hot-spring areas (Browne and Lawless, 2001). Such 

hazards are more likely in thermal areas with active discharge 

of neutral, high-chloride waters, especially those hosting 

geysers, as in Yellowstone National Park, than in vapordominated areas with acid-sulfate alteration (Christiansen 

and others, 2007). In LVNP the near-surface hydrothermal 

systems are all of the vapor-dominated type. Although small 

explosions as vents or mud pots change in character or migrate 

have occurred and are likely in the future, there is no evidence 

of large craters in the Lassen thermal areas. Consequently, 

the likelihood of large hydrothermal explosions such as have 

occurred in Yellowstone is very low. However, intrusion of 

new magma into the volcanic system could alter the groundwater/hydrothermal regime and make hydrothermal explosions 

more likely.

Volcanic Event Frequency, Probability 

and Magnitude

Long-term probabilities of volcanic eruptions at the 

Lassen Volcanic Center and in the wider Lassen region have 
36 Volcano Hazards Assessment for the Lassen Region, Northern California

been estimated based on mathematical modeling of the eruption chronology and are presented in Nathenson and others 

(2012), along with the methodology and relative uncertainties 

for deriving those probabilities. The data contained in that 

analysis are also used to estimate the cumulative probability of 

the volume of eruptions and the size of area covered by lava 

flows (fig. 21).

For the LVC, the annual probability of a volcanic 

eruption is 0.00014 (0.014 percent), which translates to 

an average recurrence interval of 7,150 years. The largest 

eruption in the LVC covered an area of 39.6 km2 and had a 

volume of 4.72 km3. If an eruption occurs, the probability of 

it being larger than 2 km3 is about 0.1 (10 percent), whereas 

the probability of an eruptive volume greater than about 0.35 

km3 is about 0.5 (50 percent). Similarly, the probability that 

the area covered by a lava flow will be greater than about 17 

Figure 20. Dynamic nature of hydrothermal features at Sulphur Works shown by changes from 2004 to 2008. Photograph looking 

northwest across Calif. Hwy 89 was taken August 16, 2004. Dotted lines denote vents that either enlarged (C) or were new (L and M) as 

of October 12, 2005. Solid line represents the boundary of the large mud pot resulting from the collapse of the area between vent M and 

vent C, sometime between October 16, 2005 and June 20, 2008, probably in the winter of 2007–2008. Vents A, B, D, and E are essentially 

unchanged from August 2004 to June 2008.

Steaming

October 12, 2005

Vent D 

Vent E 

 Vent B 

 Vent L

October 12, 2005

and June 20, 2008

 Vent M

October 12, 2005

August 16, 2004

 Vent C

October 12, 2005

Vent A 

Peak 8,662’ 

Tree fallen between

October 12, 2005 and

June 20, 2008

Tree fallen

between

 October 12, 2005

to June 20, 2008

Tree vertical on

 October 12, 2005

but leaning to 

left on June 20, 2008

June 20, 2008 outline of 

major mudpot incorporating 

vents C and M

Vent C

km2 is about 0.1 (10 percent), and the probability that the area 

covered will be greater than about 7 km2 is 0.5 (50 percent).

For regional mafic volcanism in the Lassen region, the 

annual probability of a volcanic eruption is 0.00065 (0.065 

percent), which translates to an average recurrence interval 

of 1,550 years. The largest regional mafic eruption covered 

an area of 99 km2 and had a volume of 2.6 km3. If an eruption 

occurs, the probability of it being larger than about 1.3 km3 is 

about 0.1 (10 percent), whereas the probability of an eruptive 

volume greater than about 0.1 km3 is about 0.5 (50 percent). 

The probability that the area covered by a lava flow will be 

greater than about 30 km2 is about 0.1 (10 percent), and the 

probability that the area covered will be greater than about 

4 km2 is 0.5 (50 percent). Thus, the potential volumes of 

eruptions and areas covered by lava flows could be significant, 

but most eruptions will be small.
Volcano Monitoring and Response 37

Figure 21. Graph showing 

estimated cumulative 

probabilities of the volume 

and area of eruptions for the 

Lassen Volcanic Center and 

for regional mafic vents for 

the past 100,000 years. For 

data and methodology, see 

Nathenson and others (2012).

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1.0 

0.001 0.01 0.1 1 10 100 

Probability of larger eruptions

Volume, in cubic kilometers 

Area, in square kilometers 

Regional vents - Volume 

Lassen Volcanic Center - Volume 

Regional vents - Area 

Lassen Volcanic Center - Area 

EXPLANATION

It should be noted that the rate of volcanism at Lassen 

is not controlled by some simple predictable process. The 

values given here are long-term averages based on the known 

past eruptions in the surrounding region or at the LVC—an 

eruption cannot be considered “due” every 1,550 or 7,150 

years, and the hazard is not higher or lower because of the 

length of time since the previous eruption. Volcanism at 

Lassen is episodic in the long term, and events tend to occur 

in clusters or episodes on a variety of time scales (Clynne and 

Muffler, 2010). Volcanic eruptions are a complex function of 

evolution of the magma bodies in the shallow subsurface and 

recharge from greater depth and are not predictable in advance 

of premonitory activity. Thus, the average values stated are 

only a general guide to the expected frequency and magnitude 

of volcanic eruptions in the Lassen region.

The probabilities of eruptions stated above do not account 

for periods of unrest at volcanoes that do not culminate in 

an eruption. This is because the geologic record preserves 

no evidence of intrusive events that did not result in an 

eruption. The global experience of volcanologists is that unrest 

experienced at volcanic systems often does not lead to an 

eruption on the human time scale, but nevertheless can cause 

great anxiety and disruption locally. Any unrest at a volcano 

requires a significant effort by scientists to perform short- to 

long-term monitoring, data collection, and interpretation so 

that local managers can respond appropriately.

Volcano Monitoring and Response

Volcanic eruptions are typically preceded over periods of 

days to years by precursory phenomena related to the rise of 

magma toward the surface. Monitoring of changes in seismic 

activity, ground deformation, gas emissions, and changes in 

hydrothermal systems provide the opportunity to make forecasts 

of probable eruptions. However, it is not uncommon for volcanoes 

to have multiple discrete phases of unrest related to intrusions of 

new magma at depth that do not reach the surface as eruptions. 

Eruption forecasts are typically provided within the context of 

the known eruptive history at a volcano, although unprecedented 

events are always possible and need to be considered by scientists 

within the context of the changes occurring at the volcano. 

Detailed discussion of volcano monitoring and risk assessment 

and hazard mitigation can be found in Scarpa and Tilling (1996).

A network of eight seismic stations, including one shortperiod 3-component instrument and one broad-band instrument, 

are installed in the Lassen region. Data from these instruments are 

transmitted to USGS offices in Menlo Park, California. A report on 

seismic monitoring at Cascade volcanoes (Moran, 2004) assesses 

Lassen as one of the better-monitored volcanoes in the Cascades 

but recommends some improvements to increase capability and 

reliability. Given the large area in which potential volcanic activity 

might occur in the Lassen region, it is important to locate seismic 

events with high precision in order to forecast potential vent sites.
38 Volcano Hazards Assessment for the Lassen Region, Northern California

Monitoring of ground deformation at volcanoes has changed 

dramatically in recent decades. Ground movement has in the past 

been measured by precise leveling surveys (Dzurisin and others, 

1982; Dzurisin, 1999; Poland and others, 2004), but such studies 

are labor-intensive, performed on an intermittent basis, and can be 

done only in the summer when the area is free of snow. Recently, 

eight Plate Boundary Observatory (PBO) permanent GPS sites 

funded by the National Science Foundation were installed in 

LVNP or the immediate area (UNAVCO, 2009). At this writing, 

three more such sites are approved but not yet installed. Two 

more sites are nearby in northern California. These sites may be 

sufficient to detect initial stages of ground deformation associated 

with either volcanic or tectonic processes in the Lassen region on 

a real-time basis, and additional sites could be installed relatively 

quickly, snow conditions and weather permitting, should the 

existing network indicate deformation interpreted to be associated 

with potential volcanic activity.

According to the report of the proposed National 

Volcano Early Warning System (NVEWS) (Ewert and others, 

2005), in order to be well monitored in real time, the Lassen 

Volcanic Center would potentially need installation of 4 to 12 

Volcano Alert Levels Used by USGS Volcano Observatories

Alert Levels are intended to inform people on the ground about a volcano’s status and are issued in conjunction with the Aviation Color Code. Notifications are issued for both increasing

and decreasing volcanic activity and are accompanied by text with details (as known) about the nature of the unrest or eruption and about potential or current hazards and likely outcomes.

Term Description

NORMAL

Volcano is in typical background, noneruptive state 

or, after a change from a higher level,

volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORY

Volcano is exhibiting signs of elevated unrest above known background level 

or, after a change from a higher level,

volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCH

Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain,

OR

eruption is underway but poses limited hazards.

WARNING Hazardous eruption is imminent, underway, or suspected.

Volcano is in typical background, noneruptive state 

or, after a change from a higher level,

volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOW

ORANGE

RED

Eruption is imminent with significant emission of volcanic ash into the atmosphere likely

OR

eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].

GREEN

Color Description

Volcano is exhibiting signs of elevated unrest above known background level

or, after a change from a higher level,

volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain,

OR

eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

Aviation Color Code Used by USGS Volcano Observatories

Color codes, which are in accordance with recommended International Civil Aviation Organization (ICAO) procedures, are intended to inform the aviation sector about a volcano’s status and

are issued in conjunction with an Alert Level. Notifications are issued for both increasing and decreasing volcanic activity and are accompanied by text with details (as known) about the

nature of the unrest or eruption, especially in regard to ash-plume information and likely outcomes. 

additional seismic stations and some additional continuous 

GPS stations, in addition to airborne gas surveys and 

applications of remote sensing techniques. The information 

generated by these techniques would be evaluated by USGS 

scientists for any changes, such as an increase in seismicity, 

ground deformation, or changes in gas composition or 

flux. Monitoring would involve personnel at the USGS 

California Volcano Observatory (CalVO), based in Menlo 

Park, California. CalVO publishes a monthly status report for 

California volcanoes on the Internet (http://volcanoes.usgs.

gov/observatories/calvo/). The USGS National Volcano alertnotification system (Gardner and Guffanti, 2006), included 

here as table 3, is the system used to notify the public of 

significant changes in the activity status of U.S. volcanoes.

Additional related information can be obtained online 

at the Web site for the USGS Volcano Hazards Program 

(http://volcanoes.usgs.gov) and the USGS California Volcano 

Observatory Web site (http://volcanoes.usgs.gov/observatories/

calvo). Information about volcanic ash is available at http://

volcanoes.usgs.gov/ash/ and at http://www.ivhhn.org/, where 

two pamphlets (“The Health Hazards of Volcanic Ash; a Guide 

Table 3. Volcano Alert Levels and Aviation Color Code used by USGS volcano observatories
References Cited 39

for the Public,” and “Guidelines for Preparedness Before, 

During, and After an Airfall”) are available. Information about 

volcano monitoring can be obtained at http://volcanoes.usgs.

gov/About/Monitoring/monitor.html. Fact sheets describing 

volcano hazards (Myers and others, 2008) and the USGS 

Volcano National Early Warning System (Ewert and others, 

2006) are available on line at http://pubs.usgs.gov/fs/fs002-

097/ and http://pubs.usgs.gov/2006/fs2006/3142/, respectively.

Acknowledgments

Previous scientific work provides the essential foundation 

for any focused hazards assessment. In the Lassen region, this 

includes especially the pioneering work of J.S. Diller, A.L. Day, 

E.T. Allen, B.F. Loomis, and Howel Williams as well as the 

statewide volcano hazards assessment of California published by 

C. Dan Miller (1989). Critical to our understanding of the young 

volcanism is the insight of Robert L. Christiansen, who took 

the lead in geologic mapping and interpretation of the Holocene 

eruptive chronology of the Lassen Volcanic Center. Perhaps more 

importantly he mentored the senior author in the ways of young 

volcanic deposits. Colleague reviews by William E. Scott and John 

Ewert significantly improved the content and presentation of this 

contribution. Funding for all aspects of this work was provided by 

the U.S. Geological Survey Volcano Hazards Program.

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44 Volcano Hazards Assessment for the Lassen Region, Northern California

Glossary

This glossary provides definitions for a 

variety of technical terms commonly used in 

writing on volcanoes and volcanic geology. 

Some terms are not used directly in this report 

but are provided as synonyms or additional 

information that can be cross-referenced with 

other publications. Terms shown in bold type

within definitions are themselves separately 

defined in this glossary.

A

Acid-sulfate alteration Alteration of 

volcanic rock produced near the ground 

surface by sulfuric acid, which results from 

oxidation of hydrogen sulfide (H2S) that 

accompanies steam rising to the surface in 

fumarolic areas. The material produced by 

acid-sulfate alteration is composed primarily 

of amorphous silica (SiO2) and kaolinite (a 

clay mineral).

Agglutinate A welded pyroclastic deposit; 

term is commonly used for deposits of bombs

fused while hot and viscous. Agglutinate 

typically occurs as spatter cones or 

agglutinate cones.

Andesite Volcanic rock, typically gray to 

black, or shades of red when oxidized, with 

57–63 percent silica. Andesite lavas are 

moderately viscous and tend to form thick 

blocky lava flows.

Ash Fine fragments (less than 2 mm across) 

of volcanic rock formed and ejected in an 

explosive eruption.

Ash-flow tuff Deposit of a hot, chaotic 

mixture of pumice, ash, and gas (pyroclastic 

flow) that travels rapidly (as fast as tens of 

meters per second) away from a volcanic 

vent during an explosive eruption. Ashflow tuffs are often called ignimbrites in the 

literature.

B

Basalt Volcanic rock with about 47–53 percent 

silica, typically gray to black, or shades of red 

when oxidized. Basalt lavas are fluid and tend to 

form thin, widespread lava flows.

Basaltic andesite Volcanic rock, typically 

gray to black, or shades of red when oxidized, 

with 53–57 percent silica. Basaltic andesite 

lavas are fluid to moderately viscous and tend 

to form thin to thick lava flows.

Base surge A ring-shaped cloud of gas and 

suspended solid rock fragments that moves 

radially outward at high velocity from the 

base of a vertical eruption column. Can 

accompany phreatomagmatic eruptions.

Bomb (volcanic) Fragments bigger than 

3 inches across ejected explosively from 

a volcanic vent on an arcuate, ballistic 

trajectory. Typically found in, or peripheral 

to, cinder cones formed over mafic vents.

C

Caldera A large, basin-shaped volcanic 

depression, more or less circular in form; 

typically more than 2 km across. Commonly 

formed by collapse during withdrawal or 

ejection of a large volume of magma leaving 

the roof of a magma reservoir unsupported.

Cinder cone A conical hill formed by 

accumulation of solidified bubble-rich 

fragments (ash), clots (scoria), and blobs 

(bombs) of lava that fall around the vent of a 

single basaltic or andesitic eruption.

Cirque A steep-walled hollow formed by 

glacial erosion at the head of a glacial valley.

Composite volcano A large, long-lived 

volcano that erupts episodically for tens to 

hundreds of thousands of years from the 

same or closely-spaced vents, and displays a 

wide range of eruptive styles and explosivity. 

Generally andesitic to silicic in composition.

Coulee A short, thick lava flow with 

steep, block-covered sides, usually of silicic

composition.

D

Dacite Generally light-colored volcanic 

rock with 63–68 percent silica. Dacite lavas

are viscous and tend to form very thick, 

blocky lava flows or steep-sided piles of lava

called lava domes. Dacite magmas also tend 

to erupt explosively, thus ejecting abundant 

pumice and ash.
Glossary 45

Dike A tabular, commonly vertically 

oriented, igneous intrusion, typically much 

longer than it is wide.

Dome A steep-sided mass of viscous 

and often blocky lava extruded from a 

vent; typically has a rounded top and 

covers a roughly circular area. Typically 

silicic (dacite, rhyodacite, or rhyolite) in 

composition.

E

Effusive eruption An eruption that produces 

mainly lava flows and domes (as opposed to 

an explosive eruption).

Ejecta Material explosively ejected from 

a volcano (more or less synonymous with 

tephra).

Eruption column The ascending, vertical 

part of a mass of erupting debris and volcanic 

gases that rises directly above a volcanic 

vent. Higher in the atmosphere, columns 

usually spread laterally into plumes or 

umbrella-shaped eruption clouds. Synonym 

for eruption plume.

Explosive eruption An energetic eruption 

that produces mainly ash, pumice, and 

fragmental ballistic debris (as opposed to an 

effusive eruption).

F

Fallout A general term for all the ash and 

debris that falls to Earth (also known as ash 

fall or tephra fall) from an eruption cloud.

Fumarole A hydrothermal feature that 

emits primarily steam.

H

Harmonic tremor Continuous long-period 

volcanic ground vibration that indicates 

movement of magma or gas. It often, but not 

necessarily, precedes an eruption.

Holocene The youngest geologic time 

period, considered to include the past 

11,700 years.

Hydrothermal Of or relating to hot water.

Hydrothermal explosion Explosion that 

can occur when hot water within a volcano’s 

hydrothermal system flashes to steam, 

breaking rocks and throwing them into the air

I

Igneous Refers to rocks formed by 

solidification of magma.

L

Lahar A mixture of water and volcanic 

debris that moves rapidly down stream 

channels. Consistency can range from that of 

muddy water to that of wet cement, depending 

on the ratio of water to debris. Also called a 

volcanic mudflow or debris flow.

Lateral Blast A pyroclastic surge that is 

laterally or subhorizontally directed away 

from the volcano. A type of pyroclastic flow.

Lava Molten rock (magma) that reaches the 

Earth’s surface and maintains its integrity as 

a fluid or viscous mass, rather than exploding 

into fragments.

M

Mafic Describes magma that contains lower 

amounts of silica (<57 percent) and is generally 

less viscous and less gas-rich than silicic magma. 

Tends to erupt effusively, as lava flows. Includes 

basalt and basaltic andesite. Andesite, an 

intermediate composition magma, can create 

hazards associated with both mafic and silicic 

compositions.

Magma Molten rock beneath the Earth’s 

surface.

Magmatic system The generation, 

accumulation, evolution and storage of 

magma in the Earth’s crust beneath a volcano 

is its magmatic system.

Mud pot A hydrothermal feature that 

consists of a bubbling pool of hot, often 

boiling, mud.

P

Paleomagnetic studies Investigations 

of the orientation and (or) intensity of the 

Earth’s magnetic field in the past, as recorded 

in geologic materials. The magnetic poles 

wander about the Earth’s axis of rotation, and 

the paleomagnetic pole position at the time of 

cooling of a volcanic rock is “frozen in” by 

magnetic minerals. An empirical calibration 

of this secular variation over time allows 
46 Volcano Hazards Assessment for the Lassen Region, Northern California

eruption ages to be constrained and isolated 

outcrops to be correlated with one another.

Petrology The study of rocks, including 

their occurrence, composition, origin, and 

evolution.

Phreatic eruption An eruption that 

primarily involves steam explosions, usually 

ground-water flashed to steam by the heat of 

subsurface magma.

Phreatomagmatic eruption An eruption 

that involves both magma and water, which 

typically interact explosively, leading to 

concurrent ejection of steam and pyroclastic

fragments.

Pleistocene The geologic time period 

preceding the Holocene. Extends from 

2.6 million years ago to 11,700 years ago. 

Older works used 1.81 million years as the 

beginning of the Pleistocene. 

Postglacial Refers to the time since the end 

of the last major ice age (about 15,000 years 

ago in the Lassen region).

Pumice Highly vesicular (rich in gas 

bubbles) volcanic ejecta, typically silicic in 

composition. Pumice is essentially magma

that has been frothed up by escaping gases 

and then cooled and solidified during 

eruption. If sufficiently bubble-rich, pumice 

will float on water. Near a vent, pumice can 

accumulate to form a pumice cone.

Pyroclastic General term applied to 

volcanic products or processes that involve 

explosive ejection and fragmentation 

of erupting material. Literally means 

“fire-broken.”

Pyroclastic flow A hot, chaotic, turbulently 

flowing mixture of rock fragments, ash, and 

gas resulting from explosive eruptions of 

composite volcanoes. There are two types 

of pyroclastic flows, which can be described 

as pumiceous and lithic (sometimes called 

block and ash flows). Pumiceous pyroclastic 

flows are pyroclastic flows generated by 

gravitational collapse of vertically directly 

eruption columns of hot gas, pumice

fragments, and ash. They are hot (hundreds of 

degrees Celsius), move rapidly (as fast as tens 

of meters per second), and are highly mobile 

(depending on topography, they can travel 

tens of kilometers from the vent). When 

laterally or subhorizontally directed, are 

also called lateral blasts. Lithic pyroclastic 

flows are pyroclastic flows generated by 

gravitational collapse of growing lava domes. 

They consist of dense rock fragments, ash, 

and hot gas. They are also hot (as much 

as hundreds of degrees Celsius) and move 

rapidly (as fast as tens of meters per second), 

but they are not as mobile as pumiceous 

pyroclastic flows.

Pyroclastic surge A fluidized mass of 

turbulent gas and rock fragments that is 

ejected during some volcanic eruptions. It is 

a type of pyroclastic flow that has a lower 

density or contains a much higher proportion 

of gas to rock, which makes it more turbulent 

and allows it to rise over ridges and hills 

rather than always travel downhill as most 

pyroclastic flows do. When laterally or 

subhorizontally directed is also called a 

lateral blast.

R

Rhyodacite Volcanic rock with 68–72 

percent silica. Rhyodacite lavas are very 

viscous and tend to form very thick blocky lava 

flows or steep-sided piles of lava called lava 

domes. Rhyodacite magmas also tend to erupt 

explosively, ejecting abundant pumice and ash.

Rhyolite Volcanic rock with 72–76 percent 

silica. Rhyolite lavas are very viscous and tend 

to form very thick blocky lava flows or steepsided piles of lava called lava domes. Rhyolite 

magmas also tend to erupt explosively, ejecting 

abundant pumice and ash.

S

Scoria Vesicular volcanic ejecta, essentially 

magma that has been frothed by escaping 

gases. Scoria is typically more coarsely 

vesicular and more dense than pumice and 

usually forms from magma of andesite to 

basalt compositions.

Silica Silicon dioxide (SiO2), the most 

abundant component of volcanic rocks and 

magmas. In magma, silica polymerizes to 

form loose frameworks of molecules that 

are the dominant control on the viscosity. 

Basaltic magma has low SiO2 and is fairly 

fluid. With increasing SiO2

 content, basaltic 
Glossary 47

andesite, andesite, dacite, rhyodacite, and 

rhyolite magmas become progressively 

more viscous, thus retarding movement of 

the magma. Because it is more difficult for 

dissolved gas to escape from more viscous 

magma, higher silica magmas tend to erupt 

more explosively.

Siliceous sinter Amorphous SiO2 deposited 

on the ground surface by precipitation from 

hot water flowing from a near-neutral, 

chloride-bearing hot spring.

Silicic Describes an igneous rock or magma

that contains more than ~63 percent silica and 

is generally viscous, gas-rich and tends to erupt 

explosively. Includes dacite, rhyodacite, and 

rhyolite. Andesite, an intermediate composition 

magma, can create hazards associated with both 

mafic and silicic compositions.

Spatter cone A steep-sided cone constructed 

of agglutinate at a mafic vent. Most spatter 

cones are small (typically a few tens of meters 

or less in height) and commonly form in linear 

groups along a fissure.

T

Tectonic Relating to deformation of the 

Earth’s crust by regional forces such as those 

generated by plate tectonics.

Tephra Any type and size of rock fragment 

that is forcibly ejected from a volcano and 

travels an airborne path during an eruption, 

including ash, bombs, scoria, and pumice

(more or less synonymous with tephra).

V

Vent Any opening at the Earth’s surface 

through which magma erupts or volcanic 

gases are emitted. Also used for hydrothermal 

features (that is, hot-spring vents) that emit 

dominantly hot water or steam.

Produced in the Menlo Park Publishing Service Center, California

Manuscript approved for publication, August 28, 2012

Edited by Peter H. Stauffer

Layout and Design by Jeanne S. DiLeo
Clynne and others—Volcano Hazards Assessment for the Lassen Region, Northern California — Scientific Investigations Report 2012–5176–A