On May 18, 1980, Mount St. Helens, 

Washington, exploded in a spectacular and 

devastating eruption that brought the volcano 

to the attention of the world. Few people realized that Mount St. Helens had long been the 

most active volcano in the Cascade Range of 

the Pacifi c Northwest. It has a rich and complex 300,000-year history and has produced 

both violent explosive eruptions of volcanic 

ash and pumice and relatively quiet outpourings of lava. The volcano’s edifi ce was mostly 

built by lava domes and fl ows from numerous eruptions. Using evidence in these lavas 

and other deposits, U.S. Geological Survey 

(USGS) scientists have documented dozens of 

major individual eruptions of the volcano.

An extensive apron of ash and fragmented 

volcanic rocks surrounds Mount St. Helens 

and mostly fi lls the valleys draining its slopes. 

This material was transported by pyroclastic 

fl ows (searingly hot fl ows of ash and volcanic 

gases), lahars (volcanic mudfl ows), and debris 

avalanches (landslides). Farther away from 

the volcano, pumice and ash that fell during 

explosive eruptions form layers that bury the 

landscape to depths of 10 feet or more.

The pre-1980 eruptive history of Mount 

St. Helens is strongly episodic. Volcanologists have recognized and named four episodes of volcanic activity, called “stages”—

Ape Canyon, Cougar, Swift Creek, and Spirit 

Lake—separated by dormant intervals. The 

youngest stage, Spirit Lake, is further subdivided into six eruptive periods. Because the 

preservation of deposits and other geologic 

evidence is best for the youngest stages, the 

farther scientists look back in time the less 

detail they can infer for the history of volcanism at Mount St. Helens.

Ape Canyon Stage (300 to 35 ka)

The early history of Mount St. Helens is 

poorly known, and a long timespan is covered 

by the Ape Canyon Stage. During this stage, 

lava domes erupted west of the present edifi ce 

of the volcano in two distinct periods—one 

from 300 to 250 thousand years ago (ka) and 

a second from 125 to 35 ka. A number of ash 

layers, called the “C” set, are clearly related 

to volcanism during the younger phase of the 

Ape Canyon Stage, and a few ashes found in 

central Washington older than the C set are 

also from Mount St. Helens. Although some 

Ape Canyon-age lava domes are exposed in 

the area of Goat Mountain and Butte Camp, 

the best record of early Mount St. Helens volcanism is preserved in the Cougar-age debris 

avalanche (see below) and in glacial deposits 

and lahars in the Lewis River Valley.

Many Ape Canyon-age rocks were altered 

hydrothermally (by volcanically heated 

ground water), indicating that an extensive 

hydrothermal system existed during the latter part of the stage. Volcanism during the 

Ape Canyon Stage produced a small cluster 

of lava domes with maximum elevations of 

about 4,000 feet.

[Dormant Interval 35 to 23 ka]

Cougar Stage (23 to 17 ka)

During the Cougar Stage, Mount St. Helens produced lava domes and fl ows, as well as 

explosive eruptions that ejected large volumes 

of ash and generated pyroclastic fl ows. A 

huge debris avalanche and subsequent lahars 

were also emplaced. This avalanche (Cougar 

debris avalanche) is composed primarily of 

Ape Canyon-age rocks and was the most 

devastating event of the stage. The debris 

avalanche originated in the area of Butte 

Camp and emplaced a deposit 600 to 900 feet 

thick on the south fl ank of the volcano. This 

Mount St. Helens rises majestically above surrounding forests in this photo taken on May 17, 1980. In the devastating 

eruption the following day, 57 people were killed, most of the 

forest destroyed, and 1,300 feet of volcanic rock removed 

from the volcano’s edifi ce, lowering its summit elevation to 

8,364 feet (inset photo). During the volcano’s 300,000-year 

history, dozens of eruptions have repeatedly changed its appearance. (USGS photos by Harry Glicken and Lyn Topinka.)

Pre-1980 Eruptive History of Mount St. Helens, Washington

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2005-3045

2005

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

Mount St. Helens, famous for its 

explosive 1980 eruption, has 

long been the most active volcano in 

the Pacifi c Northwest. U.S. Geological Survey (USGS) scientists have 

documented the volcano’s 300,000-

year geologic history, including 

powerful explosions of ash, outpourings of lava, and huge landslides and 

volcanic mudfl ows. Understanding 

this history helps USGS scientists 

evaluate current activity at Mount 

St. Helens so that timely warnings of 

hazards can be issued to the public.
avalanche, which was at least twice as large 

as the huge debris avalanche that triggered 

Mount St. Helens’ 1980 eruption, dammed 

the Lewis River. Downcutting of the dam 

caused fl ooding downstream as far as the Columbia River and fi lled the lower Lewis River 

Valley with volcanic debris (ash and broken 

rock) at least 200 feet thick. 

The Cougar debris avalanche was followed 

by a large explosive eruption producing pyroclastic fl ows that buried the avalanche deposits with a 300-foot-thick sheet of dacite pumice (the “2-pumice pf”). (Dacite is volcanic 

rock containing 63 to 68% silica [SiO2

].) 

Continued explosive activity deposited ash 

sets “M” and “K” and more pyroclastic fl ows 

(the “white pumice”). The Cougar Stage culminated with the eruption of the largest lava 

fl ow in the history of Mount St. Helens (Swift 

Creek fl ow). The vent for this andesite (53 to 

63% SiO2

) lava fl ow, at an elevation of 6,000 

feet on the south fl ank of Mount St. Helens, 

marks the summit of the volcano at that time.

[Dormant Interval 17 to 13 ka]

Swift Creek Stage (13 to 11 ka)

During the Swift Creek Stage, explosive 

eruptions produced two widespread ash layers, and three extensive fans of volcanic debris 

were emplaced from growing, unstable dacite 

lava domes. The Swift Creek fan, composed 

of pyroclastic fl ows and lahars, buried Cougar-age deposits on the south fl ank of Mount 

St. Helens. The Crescent fan consists of pyroclastic fl ows at least 600 feet thick on the west 

fl ank of the volcano that were derived from the 

Crescent lava dome. The Cedar Flats fan completely fi lled the ancestral valley of Pine Creek 

and spilled into the Lewis River at Cedar Flats. 

It was at least 300 feet thick in the Cedar Flats 

area. This fan is dominated by lahar deposits, 

but also contains pyroclastic material produced by collapses of lava domes.

All three fans are associated with the deposition of ash set “S” dated at 13 to 12.5 

ka. The Swift Creek Stage culminated with 

deposition of ash set “J” at about 11.5 to 11 

ka. At the end of Swift Creek time, Mount St. 

Helens consisted of a cluster of dacite domes 

with elevations as high as 7,000 feet. 

[Dormant Interval 11 to 3.9 ka]

Spirit Lake Stage (3.9 ka to present)

The bulk of the pre-1980 edifi ce of Mount 

St. Helens was constructed by eruptions 

during the Spirit Lake Stage. Excellent preservation of deposits, as well as numerous 

ages from radiocarbon and tree-ring dating, 

provide considerable detail for this stage. Volcanism was intermittent, with lulls on the order of a few to about 600 years. As in earlier 

stages, Spirit Lake volcanism erupted mostly 

dacite, but signifi cant amounts of basalt (less 

than 53% SiO2

) and andesite were also erupted. The Spirit Lake Stage is subdivided into 

six eruptive periods—the Smith Creek, Pine 

Creek, Castle Creek, Sugar Bowl, Kalama, 

and Goat Rocks Eruptive Periods. 

Smith Creek Eruptive Period (3.9 to 3.3 

ka)—During this period, Mount St. Helens 

erupted mostly ash. Two periods of activity, 

about 3.90 to 3.85 ka and 3.5 to 3.3 ka, deposited set “Y” ashes. The second period was 

initiated with an eruption that produced “Yn” 

ash. This eruption, possibly the most voluminous in Mount St. Helens’ history, was about 

four times larger than the 1980 eruption. During late Smith Creek time, huge lahars swept 

down the Toutle River, and some probably 

reached the Columbia River. The primarily 

ash-producing eruptions of Mount St. Helens 

during Smith Creek time did not signifi cantly 

change the volcano’s shape. 

Pine Creek Eruptive Period (2.9 to 2.55 

ka)—During the Pine Creek Eruptive Period, 

Mount St. Helens erupted ash and produced 

pyroclastic fl ows and dacite domes, and two 

small debris avalanches occurred on its north 

fl ank. Repeated collapse of hot, growing lava 

domes produced an extensive and broad fan 

of volcanic debris as much as 600 feet thick 

on the south fl ank of the volcano. Similar 

deposits on the north fl ank can still be found 

as far downstream as the town of Toutle. Pine 

Creek-age dacite domes exposed in the walls 

of the crater left by the 1980 eruption show 

that at the end of Pine Creek time, the volcano 

was a cluster of lava domes with a maximum 

elevation of about 7,000 feet.

Castle Creek Eruptive Period (2.55 to 

1.895 ka)—The Castle Creek Eruptive Period 

produced many lava fl ows and domes, pyroclastic fl ows, and ash. Andesite lava fl ows and 

ash erupted from the summit were emplaced 

on all fl anks of Mount St. Helens between 

2.55 and about 2.50 ka. A lull of about 300 

This map, originally produced by the U.S. Geological Survey in 1919, shows the pre-1980 topography of Mount 

St. Helens. The gentle slopes on the fl anks of the volcano are fans of volcanic debris and consist of ash, pumice, and volcanic rock fragments. Dashed lines show the extent of these fans.

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CHRONOLOGY OF ERUPTIVE ACTIVITY AT MOUNT ST. HELENS

This simplifi ed chronology shows the volcanic history of Mount St. Helens 

from its earliest beginnings about 300,000 years ago (300 ka) to its devastating and deadly eruption on May 18, 1980. The major volcanic deposits of 

each stage and period are listed to the right of the time columns. Lava and 

ash erupted by the volcano are mostly dacite (volcanic rock containing 63 

to 68% silica [SiO2]), but also include andesite (53 to 63% SiO2) and basalt 

(less than 53% SiO2). Pyroclastic fl ows are searingly hot fl ows of ash and 

volcanic gases, and lahars are volcanic mudfl ows. “Dormant” intervals are 

time periods during which no volcanic activity is known. The diagram below 

shows how Mount St. Helens evolved from a small cluster of dacite lava 

domes to a moderate-size conical volcano.
0RINTEDONRECYCLEDPAPER

Michael A. Clynne, David W. Ramsey, 

and Edward W. Wolfe

years followed, and volcanism resumed at 

about 2.2 ka with eruption of andesite lava 

fl ows on the volcano’s north fl ank. Several 

thick dacite lava fl ows and domes, pyroclastic fl ows and ash, and lahars were produced 

at 2.0 ka. Castle Creek activity culminated 

with eruption of three groups of fl uid basalt 

lava fl ows that poured down all fl anks of the 

volcano as far as 8 miles. The Cave Basalt, 

erupted at 1.895 ka, was the most recent of 

these. Castle Creek lavas transformed the 

Pine Creek-age cluster of domes into a classic 

cone-shaped composite volcano, with a summit elevation of about 8,500 feet. 

Sugar Bowl Eruptive Period (1.2 to 1.15 

ka [A.D. 850 to 900, corrected radiocarbon 

dates])—During the Sugar Bowl Eruptive 

Period, three lava domes were built on the 

fl anks of Mount St. Helens. Explosive eruptions associated with growth of the Sugar 

Bowl Dome produced two “lateral blasts” 

that affected an area about one-tenth as large 

as that of the lateral blast in the 1980 eruption. Ash layer “D” and lahars were also 

emplaced. The Sugar Bowl period was short 

lived, produced a small volume of volcanic 

materials, and did not signifi cantly change 

the appearance of the volcano. 

Kalama Eruptive Period (A.D. 1479 to 

1720)—Activity during this period produced 

large-volume dacite ashes, pyroclastic fl ows, 

domes, lahars, and andesite lava fl ows. Mount 

St. Helens added about 1,000 feet of elevation and attained its pre-1980 form during the 

Kalama Period. The Kalama Eruptive Period 

is subdivided into three series of events—the 

early, middle, and late Kalama phases. 

The early Kalama phase began in 1479 

with a large pyroclastic eruption that deposited dacite ash layer “Wn.” In 1482, a smaller 

eruption produced ash layer “We.” Over the 

next 10 to 20 years, a number of lava domes 

grew in the volcano’s crater and were disrupted by explosive eruptions. Lahars and 

pyroclastic fl ows associated with early Kalama eruptions are abundant on the volcano’s 

west and south fl anks. 

The middle Kalama phase began about 

1510 with eruption of andesite as pyroclastic 

fl ows (which generated hot lahars), a few 

lava fl ows, and ash set “X.” The middle phase 

peaked about 1535 with eruption of the many 

thick andesite lava fl ows prominent on all 

fl anks of Mount St. Helens, including the 

Worm Complex fl ows, and ended by 1570. 

The most signifi cant event of the late Kalama phase was growth of a large dacite dome 

at the summit (Summit Dome). The Summit 

Dome took nearly 100 years to grow (1620 to 

1720) and gave Mount St. Helens its pre-1980 

form. During growth, it shed material as pyroclastic fl ows and lahars on all fl anks of the 

volcano. Mount St. Helens acquired its pre1980 cover of glaciers as a result of growth of 

the Summit Dome.

Goat Rocks Eruptive Period (A.D. 1800–

1857)—The Goat Rocks Period was short 

and relatively small. An explosive eruption 

in 1800 produced ash layer “T” and was followed in 1801 by an andesite lava fl ow, called 

the “Floating Island,” on Mount St. Helens’ 

north fl ank. Eruptions observed intermittently 

from 1831 to 1857 produced ash and the Goat 

Rocks Dome, whose growth also resulted in a 

small fan of volcanic debris and lahars. 

The last signifi cant eruption of Mount St. 

Helens before 1980 is generally considered 

to have occurred in 1857. Minor explosions 

reported in 1898, 1903, and 1921 were probably steam-driven and not magmatic (molten 

rock) eruptions. Eruptions of the Goat Rocks 

Period did not signifi cantly change the appearance of Mount St. Helens, but they 

added the fi nal pieces to the edifi ce and set 

the stage for the 1980 eruption. 

Recent work by scientists with the USGS 

in cooperation with the U.S. Forest Service is 

shedding new light on the 300,000-year history of Mount St. Helens Volcano. The work 

of these USGS scientists is only part of the 

USGS Volcano Hazards Program’s ongoing 

efforts to protect people’s lives and property in 

all of the volcanic regions of the United States, 

including the Pacifi c Northwest, eastern California, Wyoming, Alaska, and Hawaii.

For more information contact:

U.S. Geological Survey

David A. Johnston Cascades Volcano Observatory

1300 SE Cardinal Court, Bldg. 10, Suite 100

Vancouver, WA 98683-9589

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

e-mail: cvo@usgs.gov

http://volcanoes.usgs.gov/

See also What are Volcano Hazards? (USGS Fact Sheet 

002-97) and other USGS volcano Fact Sheets

http://volcanoes.usgs.gov/Products/sproducts.html

This Fact Sheet and any updates to it are available 

online at

http://pubs.usgs.gov/fs/2005/3045

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Edited by James W. Hendley II and Peter H. Stauffer

Graphic design by Susan Mayfi eld and Sara Boore

Banner design by Bobbie Myers

COOPERATING ORGANIZATIONS

U.S. Forest Service

By examining layers of volcanic ash (tiny jagged 

particles of volcanic rock and glass) and pumice 

deposited by successive explosive eruptions, U.S. 

Geological Survey scientists have deciphered the 

eruptive history of Mount St. Helens. Groups or 

“sets” of ash layers of similar age are designated 

with letters and signify explosive episodes in the 

volcano’s history. Ages of ash layers less than 

50,000 years old are determined by radiocarbon 

dating of wood or charcoal trapped in the ash. The 

20-foot-high cliff at “Stratigraphy Viewpoint” along 

the Muddy River (photo above) exposes a series 

of deposits from the past 13,000 years of Mount 

St. Helens’ history. The prominent yellow-brown 

layer in the middle is part of ash set “Y” from the 

Smith Creek Eruptive Period. The cliff is capped by 

THE HISTORY OF MOUNT ST. HELENS IS WRITTEN IN LAYERS OF ASH

deposits several feet thick from the volcano’s devastating 1980 eruption. Major explosive eruptions not 

only leave deposits near the volcano but also inject fi ne ash (see inset photo) high into the atmosphere, 

where wind can carry it great distances. The map shows the known distribution of recognizable ash 

layers from three fairly typical explosive eruptions of Mount St. Helens in the past few thousand years. 

The eruption that produced the “Yn” ash was about four times as large as that of 1980 and was probably the largest explosive eruption in the volcano’s history.