U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3119

December 2014

A Sight “Fearfully Grand”—Eruptions of Lassen Peak, California, 1914 to 1917

On May 22, 1915, a large explosive eruption at the summit of Lassen 

Peak, California, the southernmost 

active volcano in the Cascade Range, 

devastated nearby areas and rained 

volcanic ash as far away as 280 miles 

to the east. This explosion was the 

most powerful in a series of eruptions 

during 1914–17 that were the last to 

occur in the Cascade Range before 

the 1980 eruption of Mount St. Helens, 

Washington. A century after the Lassen 

eruptions, work by U.S. Geological 

Survey (USGS) scientists in cooperation 

with the National Park Service is 

shedding new light on these events. 

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

Lassen Peak in Lassen Volcanic 

National Park, northern California, is the 

largest of a group of more than 30 lava domes 

erupted in that area during the past 300,000 

years. These steep-sided accumulations of 

volcanic rock were created by the pushing 

up of lava too viscous to flow away from its 

source. Eruptions 27,000 years ago formed 

Lassen Peak, probably within a period of 

only a few years. Standing more than 2,000 

feet (600 m) above its surroundings and 

having a volume of half a cubic mile (2 km3), 

it is one of the largest lava domes on Earth.

When Lassen Peak formed, it 

must have looked much like the nearby 

1,100-year-old Chaos Crags domes do 

today, with steep sides covered by angular 

rock talus. However, from 25,000 to 

18,000 years ago, Lassen’s shape was 

significantly altered by glacial erosion. 

For example, the bowl-shaped depression 

on the volcano’s northeastern flank was 

eroded by a glacier that extended out 7 

miles (11 km) from the dome. Today, one 

tiny remnant of a glacier survives in a 

hollow on Lassen’s eastern flank. 

Lassen Peak in northern California produced a series of eruptions during 1914–17. The largest of those eruptions, on May 22, 1915, blasted rock fragments and pumice high into the air 

and rained fine volcanic ash as far away as Elko, Nevada, 280 miles (450 km) to the east. This 

photograph, taken from the town of Red Bluff, 40 miles (65 km) west of the volcano, shows the 

huge eruption column of ash and gas, which rose to a height of more than 30,000 feet (9,000 m). 

(Photograph by R.E. Stinson; courtesy of the National Park Service.)

The 1914–17 eruptions of Lassen 

Peak were among the first volcanic 

events to occur after photography became 

readily available to amateurs. Thus, they 

were documented and popularized by the 

northern California media, primarily the 

San Francisco Chronicle, and subsequently 

by the popular scientific media. A variety of 

accounts and interpretations of the eruptions 

were published in the years shortly after 

they occurred. Most of these accounts 

are flawed because of scientists’ lack of 

experience studying volcanoes at that time, 

failure to recognize crucial evidence in the 

deposits, and misinterpretation of eruption 

photographs. In recent decades, significant 

advances have been made in volcanology 

research, and these have enabled U.S. 

Benjamin Franklin Loomis (1857–1935) was a local businessman, Lassen Park supporter, and amateur photographer. Loomis used an 

8×10-inch view camera with glass-plate negatives. He set up a portable darkroom in a tent and made his own film by applying the 

silver emulsion to the glass plates just before exposing them. Many photographers recorded the Lassen Peak eruptions and their 

effects, but Loomis left the most complete and best documented photographic record of the events. Excited at seeing Lassen Peak 

erupt on June 14, 1914, Loomis wrote, “The sight was fearfully grand.” Many of his images were published in his book “Pictorial 

History of the Lassen Volcano” (1926, republished by the Lassen Association, 110 p.), and many of the original glass plates now 

reside in the archives of the National Park Service. Loomis took most of the historical pictures used in this Fact Sheet, and several 

of his photographs proved crucial to understanding the chronology and processes of the May 1915 eruptions.

Geological Survey (USGS) and other 

scientists to piece together a more accurate 

understanding of the events surrounding the 

Lassen Peak eruptions of 100 years ago. 

Initial Rumblings

On May 30, 1914, Lassen Peak 

awoke from a 27,000-year-long slumber 

when it was shaken by a steam explosion. 

Such steam blasts occur when molten 

rock (magma) rises toward the surface of 

a volcano and heats shallow groundwater. 

The hot water then rises under pressure 

through cracks and, on nearing the surface, 

vaporizes and may vent explosively. That 

first explosion created a small crater at the 

summit of Lassen Peak, and each of more 

Department

of the Interior
than 180 subsequent steam explosions 

enlarged it. Over more than 11 months, 

these steam explosions blasted out a crater 

1,000 feet (300 m) long.

In mid-May 1915, the character of 

the eruption changed dramatically. On the 

evening of May 14, 1915, incandescent 

blocks of lava could be seen bouncing down 

the flanks of Lassen Peak from as far away 

as Manton, 20 miles (32 km) to the west. By 

the next morning, a growing dome of dacite 

lava (dacite contains 63 to 68 percent silica 

[SiO2]) had welled up and filled the crater at 

the summit of Lassen Peak. 

Events of May 19–20, 1915

Late on the evening of May 19, a 

large explosion shattered the dacite dome, 

creating a new crater at the summit of 

Lassen Peak. No new magma was ejected 

in this explosion, but glowing blocks of 

hot lava from the dome fell on the summit 

and the deeply snow-covered upper flanks 

of Lassen Peak—the winter of 1914–15 

was the first recorded El Niño year in the 

western United States, and more than 30 

feet (9 m) of snow blanketed the upper 

flanks of Lassen Peak. These falling blocks 

of hot rock launched a half-mile-wide 

avalanche of snow and rock that roared 4 

miles (6.5 km) down the volcano’s steep 

flank and over a low ridge at Emigrant Pass 

into Hat Creek (see map). 

As the hot lava blocks broke into 

smaller fragments, the snow melted, causing 

the avalanche to transform into a giant 

mudflow of volcanic materials, called a 

lahar. The bulk of this lahar was deflected 

northwestward at Emigrant Pass and flowed 

another 7 miles (11 km) down Lost Creek. 

As they came to rest, both the avalanche 

and lahar released huge volumes of muddy 

water, flooding the lower Hat Creek Valley 

during the early morning hours of May 20. 

The flood damaged several ranch houses 

in the Old Station area. Fortunately, the few 

people in these houses escaped with only 

minor injuries. The flood continued down 

Hat Creek more than 30 miles (48 km) to 

the Pit River, where many fish were killed 

by the muddy water. 

Also during the night and early 

morning of May 19–20, dacite lava, 

somewhat more fluid than that erupted 

during the previous week, welled up into 

and filled the new crater at Lassen Peak’s 

summit. It spilled over low spots in the 

crater rim and flowed as two separate lobes 

1,000 feet (300 m) down the steep west and 

northeast flanks of the volcano. 

This generalized 

geologic map shows 

Lassen Peak and the 

deposits from its May 

1915 eruptions. Those 

eruptions destroyed an 

area of 3 square miles 

(7.7 km2

), now called 

the Devastated Area, 

on the northeast flank 

of the volcano. 

On the evening 

of May 19, a large 

explosion shattered 

the still-hot lava dome 

that had filled Lassen’s 

summit crater during 

the previous week, 

blasting out a new 

crater at the volcano’s 

summit and creating an 

avalanche of hot lava 

blocks, volcanic debris, 

and snow. As the 

snow melted, a huge 

mudflow of volcanic 

materials, called a 

lahar, was generated. 

The lahar rushed 11 

miles (18 km) down 

Lost Creek, releasing 

huge volumes of water 

and flooding lower 

Hat Creek Valley. The 

avalanche, lahars, and 

floods produced by 

this explosion uprooted 

and carried away trees 

100 feet (30 m) tall with 

ease and damaged

several ranch houses. 

During that night 

(May 19–20), lava 

welled up into the 

crater created by the 

explosion, overflowed 

the summit depression, 

and fed two lobes that 

flowed about 1,000 feet 

(300 m) down the 

northeast and west 

flanks of the mountain. 

On May 22, Lassen Peak exploded again in a powerful climactic eruption that blasted 

rock fragments and pumice high into the air, creating the larger and deeper of the two craters 

seen at the summit of the volcano today and generating a vertical eruption column that 

rose to 30,000 feet (9,000 m). Partial collapse of the eruption column generated a high-speed 

avalanche of hot ash, pumice, rock fragments and gas, called a pyroclastic flow, that swept 

down the northeast side of the volcano. Trees still standing after the May 19 eruption were 

now blown down. The pyroclastic flow rapidly melted snow in its path, creating a lahar that 

followed the path of the lahar 2 days earlier and rushed down Lost and Hat Creeks to Old 

Station, releasing water that again flooded lower Hat Creek Valley. 

DEPOSITS FROM LASSEN PEAK’S MAY 1915 ERUPTIONS

May 22, 1915, deposits

EXPLANATION

Pyroclastic flow

and related lahars

Lassen Peak lava dome

27,000 years old

May 19–20, 1915, deposits

Lava flows and dome

Avalanche and lahar

Tree blow-down limits

May 22, 1915

May 19–20, 1915

Area

of

map

Approximate

limit of 

May 19

avalanche

“Hot

Rock”

0 1 MILE

0 1 KILOMETER

Approximate

limit of 

May 19–20,

1915, lahar

Limit of May 22,

1915, lahar
A steam-explosion eruption at Lassen Peak 

on June 14, 1914. Such steam-blast eruptions 

ranged from a few minutes to several hours 

in duration and often sent plumes thousands 

of feet above the volcano. Photograph taken 

by B.F. Loomis from a point west of Manzanita 

Lake, about 6 miles (10 km) from Lassen Peak.

Climactic Event of May 22, 1915

After two quiet days, Lassen Peak 

exploded in a powerful eruption at about 

4:00 in the afternoon of May 22. That 

blast hurled rock fragments and pumice 

high into the air and created the larger and 

deeper of the two craters seen today near 

the summit of the volcano. Over the next 

30minutes, a huge column of volcanic 

ash and gas rose more than 30,000 feet 

(9,000m) into the air—it was visible 

from as far away as Eureka, California, 

150 miles (240km) to the west. Partial 

collapse of the eruption column that fell 

onto the northeastern slope of Lassen Peak 

generated a high-speed flow composed 

of hot ash, pumice, rock fragments, and 

gas—called a pyroclastic flow—that swept 

down the side of the volcano, devastating 

an area of 3 square miles (7.5 km2). This 

flow rapidly incorporated and melted snow 

in its path. The water from the melted 

snow transformed the pyroclastic flow into 

a highly fluid lahar that followed the path 

of the May 19–20 lahar and rushed 15 

miles (24 km) down Lost Creek nearly to 

Old Station. This new lahar again released 

a large volume of water that once again 

flooded lower Hat Creek.

DEVASTATION CAUSED BY THE MAY 1915 ERUPTIONS

Sequence of photographs taken by B.F. Loomis before and after the cataclyscmic 

eruptions of Lassen Peak in May 1915. 

Top: View of the 

northeast flank 

of Lassen Peak 

taken before 1914. 

Mature conifer 

forest covers the 

lower slopes. 

Middle: View taken 

at noon on May 

22, 1915, showing 

the destruction 

from the avalanche 

and lahar of May 

19–20. Trees 100 

feet (30 m) tall 

were uprooted 

and carried away 

with ease. The 

large rock in 

the foreground, 

labeled “hot rock” 

by Loomis, is a 

piece of the lava 

dome that filled the 

crater during the 

previous week that 

was carried down 

by the avalanche. 

Note the remaining 

tree-covered 

slopes at the left 

and right margins 

of the avalanche 

path and the dark 

gray tongue of new 

dacite lava below 

the volcano’s 

summit. That lava 

erupted on the 

night of May 19–20.

Bottom: View of Lassen Peak’s northeast flank taken in June 1915, showing the full 

devastation caused by the cataclysmic explosive eruption of May 22. Trees still standing on 

the morning of that day were blown down or snapped off and their trunks left lying on the 

ground, pointing away from Lassen Peak. The lava flow below the summit is gone, and the 

May 22 mudflows have smoothed the surface of the May 19 avalanche and lahar deposit. 

The “hot rock” remains—and can still be seen today on the Devastated Area Interpretive 

Trail. The appearance of the Devastated Area changed little for 50 years as seedlings 

struggled to gain a foothold. Today, however, young vigorously growing trees are obscuring 

the devastation and reclaiming the area as forest.
May 30, 1914, to mid May 1915: 

Steam explosions blasted a 

crater at the summit of Lassen 

Peak.

Mid-May 1915: 

Small lava dome filled the new 

crater; late in the evening of May 

19, a large explosion disrupted the 

dome, generating an avalanche 

and mudflow; the reopened vent 

erupted a lava flow.

Afternoon of May 22, 1915: 

A vigorous eruption of pumice 

and ash formed a vertical eruption 

column that collapsed to initiate a 

hot pyroclastic flow and mudflow; 

continued fallout of hot pumice 

on snow generated additional 

mudflows; wind spread ash fallout 

to the northeast.

May 1917: 

Renewed steam explosions 

blasted a new crater; vigorous 

steaming continued into the 

1920s and waned over the next 

few decades.

The powerful climactic eruption of 

May 22 swept away the northeast lobe 

of the lava flow that was extruded 2 days 

earlier, but it left the west lobe intact. It 

also produced smaller mudflows on all 

flanks of Lassen Peak, deposited a layer 

of pumice and volcanic ash traceable 

for 25 miles (40 km) to the northeast, 

and rained fine ash as far away as Elko, 

Nevada, 280 miles (450 km) to the east. 

Continued Volcanic Activity and 

Future Eruptions

For several years after the eruption of 

May 22, 1915, snowmelt percolating down 

into Lassen Peak, especially in the spring, 

triggered steam explosions, indicating 

that rocks beneath the volcano’s surface 

remained hot. Particularly vigorous steam 

explosions in May 1917 blasted out the 

northern of the two craters now seen at the 

volcano’s summit. Steam vents could still 

be found in these craters into the 1950s, but 

they gradually weakened and are difficult to 

locate today. 

The total volume of the 1915 

eruptions was approximately 0.007 cubic 

mile (0.03 km3)—tiny compared to a major 

View of Lassen Peak from the west taken by B.F. Loomis shortly 

after the climactic eruption of May 22, 1915. Fallout of hot pumice 

from that eruption onto the still partly snow-covered flanks of the 

peak melted snow and generated seven mudflows. In contrast to 

the large mudflow in the Devastated Area, however, these were 

of small volume and quite viscous, and they reached only to the 

base of the mountain.

eruption like that of Mount St. Helens 

in 1980 (0.24 cubic mile or 1 km3). The 

deposits from the Lassen Peak eruptions are 

rapidly becoming obscured by vegetation 

and erosion, and the small size and 

unconsolidated nature of the thin deposits 

make the 1915 eruptions unlikely to be 

preserved in the long-term geologic record. 

Today, Lassen Peak sleeps again, but active 

steam vents, hot springs, and bubbling 

mudpots are still found elsewhere in Lassen 

Volcanic National Park. 

No one can say when, but it is 

almost certain that the Lassen area will 

experience volcanic eruptions again. The 

USGS California Volcano Observatory 

operates a sophisticated sensor network 

to detect any increase in seismicity, 

ground deformation, or gas emissions that 

could indicate magma rising toward the 

surface in preparation for the next Lassen 

eruption. Early detection of such eruption 

precursors is essential to effective hazard 

mitigation and forecasting. The work of 

USGS scientists at Lassen is only part of 

the USGS Volcano Hazards Program’s 

ongoing efforts to protect people’s lives 

and property in all of the volcanic regions 

of the United States.

Michael A. Clynne,

Robert L. Christiansen,

Peter H. Stauffer, James W. Hendley II, 

and Heather Bleick

Graphics and layout by Jeanne S. DiLeo

COOPERATING ORGANIZATIONS

Lassen Association

 National Park Service

 U.S. Forest Service

For more information contact:

U.S. Geological Survey

California Volcano Observatory

http://volcanoes.usgs.gov/observatories/calvo/

This fact sheet and any updates to it are available 

online at http://pubs.usgs.gov/fs/2014/3119

For more of the geologic history of the Lassen 

region, see Volcano hazards of the Lassen 

Volcanic National Park area, California (USGS 

Fact Sheet 022-00); How old is “Cinder 

Cone”?—Solving a mystery in Lassen 

Volcanic National Park, California (USGS Fact 

Sheet 023-00); and “Hot water” in Lassen 

Volcanic National Park—Fumaroles, steaming 

ground, and boiling mudpots (USGS Fact 

Sheet 101-02). 

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143119

STAGES OF THE 1914–1917 ERUPTIONS OF LASSEN PEAK

The Lassen 

volcano 

awoke on 

May 30, 1914, 

when steam 

explosions 

blasted out a 

small crater 

at the summit 

of Lassen 

Peak. Wisps 

of steam seen at the edges 

of the crater were typical 

between explosions. Knoll 

on right skyline is the actual 

summit of Lassen Peak, a 

27,000-year-old dacite dome. 

Dark area in lower left is ashcovered snow. Middleground 

is covered with ash and rocks 

blasted out of the crater, whose 

wall shows such material to 

already be several feet thick. 

In this photograph taken by B.F. 

Loomis on June 20, 1914, the 

crater is about 250 feet (75 m) 

long; it eventually grew to 

about 1,000 feet (300 m) by mid 

May of 1915.