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

air and rained fine volcanic ash as far away as Winnemucca, Nevada, 200 miles to the east. In this photograph

taken from near the town of Red Bluff, 40 miles west of the volcano, the huge column of volcanic ash and gas

produced by the eruption rises to a height of more than 30,000 feet. (Photograph taken by R.E. Stinson; courtesy of

the National Park Service.)

DEVASTATION CAUSED BY THE MAY 1915 ERUPTIONS

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet-173-98

1999

Eruptions of Lassen Peak, California, 1914–1917

U.S. GEOLOGICAL SURVEY and the NATIONAL PARK SERVICE—OUR VOLCANIC PUBLIC LANDS

O

US SG

n May 22, 1915, an explosive

eruption at Lassen Peak, California, the southernmost active volcano in

the Cascade Range, devastated nearby

areas and rained volcanic ash as far

away as 200 miles to the east. This explosion was the most powerful in a

1914–17 series of eruptions that were

the last to occur in the Cascades before the 1980 eruption of Mount St.

Helens, Washington. Recent work by

scientists with the U.S. Geological Survey (USGS) in cooperation with the National Park Service is shedding new

light on these eruptions.

Lassen Peak is the largest of a group of

more than 30 volcanic domes erupted over the

past 300,000 years in Lassen Volcanic National

Park in northern California. These moundshaped accumulations of volcanic rock, called

lava domes, were created by eruptions of lava

too viscous to flow readily away from its

source. Eruptions about 27,000 years ago

formed Lassen Peak, probably in only a few

years. With a height of 2,000 feet and a volume

of half a cubic mile, it is one of the largest lava

domes on Earth.

When Lassen Peak formed, it looked much

like the nearby 1,100-year-old Chaos Crags

Domes, with steep sides covered with angular

rock talus. However, from 25,000 to 18,000

years ago, during the last ice age, Lassen’s shape

was significantly altered by glacial erosion. For

example, the bowl-shaped depression on the

volcano’s northeastern flank, called a cirque, was

eroded by a glacier that extended out 7 miles

from the dome.

Initial Rumblings (May 1914 to May 1915)

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 ground water. The hot

water rises under pressure through cracks and, on

nearing the surface, vaporizes and vents explosively.

By mid-May 1915, more than 180 steam explosions had blasted out a 1,000-foot-wide crater near

the summit of Lassen Peak. Then 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 from as

far away as the town of Manton, 20 miles to the

west. By the next morning, a growing dome of

dacite lava (lava containing 63 to 68% silica) had

welled up into the volcano’s crater and filled it.

Events of May 19–20, 1915

Late on the evening of May 19, a large steam explosion fragmented 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 snow-covered upper flanks of Lassen Peak.

The 1914-17 series of explosive eruptions at Lassen Peak, California, were some of the first volcanic

eruptions to be extensively photographed, largely by Benjamin Franklin Loomis, a local businessman and

amateur photographer. As shown in the left photograph, the northeastern flank of Lassen Peak was

covered by mature conifer forest before 1914. The photograph on the right, taken by Loomis in June 1915

from nearly the same location shows the devastation caused by the two most powerful eruptions in the

series, those of May 19 and 22 of that year. The large rock in the foreground is a piece of the lava dome that

filled Lassen’s summit crater during the week before the May 19 eruption and was carried to its present

location by the avalanche caused by this eruption. Today, this rock can be seen on the Devastated Area

Interpretive Trail. The appearance of Lassen’s northeastern flank was little changed for 50 years after the

eruptions, while seedlings struggled to regain a foothold, but today young vigorously growing trees are

obscuring the devastation and reclaiming the area as forest. (Photographs courtesy of the National Park Service.)

Before 1914 June 1915
Chaos Crags

domes

Chaos Crags

craters

Chaos Crags

pumice field

Crescent

Crater

Raker Peak

Parking

Lot

Emigrant

Pass

Rock

shown in

photo

DEVASTATED

AREA

Lassen Peak

Eagle Peak

Lassen Peak Trail

Parking Lot

Hat Creek

1 MILE

1 KILOMETER

0

0

Lost Creek

West Fork Hat Creek

89

89

GENERALIZED MAP OF DEPOSITS FROM LASSEN PEAK'S MAY 1915 ERUPTIONS

May 22, 1915 Deposits

May 19, 1915 Deposits

Lassen Peak dome

Tree blow-down limits

Later lahars

Pyroclastic flow and

 related lahar

Lava flow and dome

Avalanche and lahar

Lava dome

Crater

May 22

May 19

Hat Creek

Lost Creek

CALIF.

Area

of map

0 3 MILES

3 KILOMETERS

Old

Station

Lassen Peak

89

89

44

To Redding

 PARK

BOUNDARY

MAP

LOCATION

N

These falling blocks launched a half-mile-wide avalanche of snow and volcanic rock that roared 4

miles down the volcano’s steep northeast flank and

over a low ridge at Emigrant Pass into Hat Creek.

As the hot lava blocks broke into smaller

fragments, the snow melted, generating a mudflow of volcanic materials, called a lahar. The

bulk of this lahar was deflected northwestward at

Emigrant Pass and flowed 7 miles down Lost

Creek. Even after coming to rest, both the avalanche and lahar released huge volumes of water,

flooding the lower Hat Creek Valley during the

early morning hours of May 20. The lahar and

flood destroyed six mostly not-yet-occupied

summer ranch houses. Fortunately, the few people

in these houses escaped with only minor injuries.

Also during the night of May19–20, dacite

lava somewhat more fluid than that erupted on

the night of May 14–15 welled up into and filled

the new crater at Lassen’s summit, spilled over

low spots on its rim, and flowed 1,000 feet down

the steep west and northeast flanks of the volcano.

Climactic Eruption of May 22, 1915

Late on the afternoon of May 22, after two quiet

days, Lassen Peak exploded in a powerful eruption

that blasted rock fragments and pumice high into the

air, creating the larger and deeper of the two craters

seen near the summit of the volcano today. A huge

column of volcanic ash and gas rose more than

30,000 feet into the air and was visible from as far

away as Eureka, 150 miles to the west.

Pumice falling onto the northeastern slope of

Lassen Peak generated a high-speed avalanche of

hot ash, pumice, rock fragments, and gas, called a

pyroclastic flow, that swept down the side of the volcano, devastating a 3-square-mile area. The pyroclastic flow rapidly incorporated and melted snow in

its path. The water from the melted snow transformed the flow into a highly fluid lahar that followed the path of the May 19-20 lahar and rushed

nearly 10 miles down Lost Creek to Old Station.

This new lahar released a large volume of water that

flooded lower Hat Creek Valley a second time.

The powerful climactic eruption of May 22

also swept away the northeast lobe of the lava

flow extruded 2 days earlier. The eruption produced smaller mudflows on all flanks of Lassen

Peak, deposited a layer of volcanic ash and pumice traceable for 25 miles to the northeast, and

rained fine ash at least as far away as Winnemucca, Nevada, 200 miles to the east.

Continuing Volcanic Activity and Future Eruptions

For several years after the May 22, 1915,

eruption, spring snowmelt percolating down into

Lassen Peak triggered steam explosions, indicating that rocks beneath the volcano’s surface remained hot. Particularly vigorous steam explosions in May 1917 blasted out the second of the

Printed on recycled paper

Michael A. Clynne, Robert L. Christiansen, Tracey J. Felger,

Peter H. Stauffer, and James W. Hendley II

Graphic design by

Sara Boore and Susan Mayfield

Banner design by Bobbie Myers

COOPERATING ORGANIZATIONS

 Lassen Loomis Museum Association

National Park Service

U.S. Forest Service

For more information contact:

Lassen Volcanic National Park

P.O. Box 100, Mineral, CA 96063

(530) 595-4444

http://www.nps.gov/lavo/

or

http://volcanoes.usgs.gov/

See also Living With Volcanic Risk in the Cascades

(USGS Fact Sheet 165-97), What are Volcano Hazards?

(USGS Fact Sheet 002-97), and Pictorial History of the

Lassen Volcano by Benjamin Franklin Loomis (1926, republished by the Lassen Loomis Museum Association)

two craters now seen near the volcano’s summit.

Steam vents could be found in the area of these

craters into the 1950’s but gradually waned and

are difficult to locate today. Lassen Peak sleeps

again, but active steam vents, hot springs, and

bubbling pools of hot mud 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 May 1915 eruptions of Lassen

Peak destroyed a 3-square-mile area,

now simply called the Devastated Area,

on the flank of the volcano. On the evening

of May 19, a large steam explosion

fragmented 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 7 miles down Lost Creek,

where it released huge volumes of water, flooding the lower Hat Creek Valley. The avalanche, lahars, and floods produced

by this explosion uprooted and carried away 100-foot-tall trees with ease and destroyed six ranch houses.

During the night of May 19-20 new lava welled up into and filled the crater created by the steam explosion. 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 near the summit of the volcano today and generating a column of volcanic ash and gas. A highspeed avalanche of hot ash, pumice, rock fragments, and gas, called a pyroclastic flow, swept down the side of the volcano.

Trees that were still standing after the May 19 eruption were blown down, with their trunks left lying on the ground pointing

away from the volcano. 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 Creek to Old Station, releasing water that again flooded lower Hat Creek Valley.