eruption. By September 24 many 

hundreds of earthquakes had occurred. 

Earthquake rates peaked that day at 

about 2 to 3 per minute then gradually 

declined, and it appeared that the swarm 

was ending. But on September 25 

shallow earthquakes increased again in 

both size and rate. This reinvigorated 

level of seismicity prompted scientists 

to issue a notice of volcanic unrest for 

the first time since 1986. The release 

of energy by earthquakes increased 

dramatically during the next several 

days, and by October 1 earthquakes 

occurred nearly continuously. Peak 

sizes reached M3.5, and earthquakes 

larger than M2.5 occurred about once 

per minute. Concurrently, parts of the 

glacier south of the lava dome rose and 

cracked noticeably, creating a broad welt. 

The accelerated size and frequency of 

earthquakes and noticeable deformation 

of the glacier indicated that molten rock 

(magma) was rising toward the surface, 

Mount St. Helens Erupts Again

Activity from September 2004 through March 2005

ruptive activity at Mount 

St. Helens captured the 

world’s attention in 1980 when 

the largest historical landslide 

on Earth and a powerful 

explosion reshaped the volcano, 

created its distinctive crater, 

and dramatically modified the 

surrounding landscape. Over the 

next 6 years, episodic extrusions 

of lava built a large dome in the 

crater. From 1987 to 2004, Mount 

St. Helens returned to a period 

of relative quiet, interrupted by 

occasional, short-lived seismic 

swarms that lasted minutes 

to days, by months-to-yearslong increases in background 

seismicity that probably reflected 

replenishment of magma deep 

underground, and by minor 

steam explosions as late as 1991. 

During this period a new glacier 

grew in the crater and wrapped 

around and partly buried the lava 

dome. Although the volcano was 

relatively quiet, scientists with 

the U.S. Geological Survey and 

University of Washington’s Pacific 

Northwest Seismograph Network 

continued to closely monitor it for 

signs of renewed activity.

The Volcano Reawakens

September 23-October 1, 2004

 After 18 years of relative quiescence, 

Mount St. Helens volcano recaptured the 

world’s attention when it showed signs 

of reawakening in September 2004. On 

September 23 a swarm of small (less than 

magnitude (M) 0), shallow (less than 1.5 

miles below the surface) earthquakes 

began in and beneath the 1980-1986 lava 

dome. Initially this swarm resembled 

previous swarms that had not led to 

View westward of the steam-and-ash explosion of October 1, 2004. Note the dark-colored ash and 

white steam emerging simultaneously from a vent through the glacier. Also note the fractured and 

slightly uplifted glacier ice immediately east of the vent and south of the 1980-1986 lava dome. (USGS 

photo, October 1, 2004)

E

and this prompted scientists to issue 

several types of hazard notifications in 

collaboration with the Gifford Pinchot 

National Forest and Washington State 

Emergency Management Division. 

Steam and Ash Explosions

October 1-October 5, 2004

 On October 1, an explosion lofted 

steam and ash several thousand feet 

above a vent blasted through the 

fractured glacier, and hurled rock 

fragments at least 1⁄2 mile across the 

western half of the glacier and 1980-1986 

lava dome. Four more steam and ash 

explosions occurred through October 5. 

Of those five explosions, three produced 

noticeable fine-ash fallout downwind. 

Only the ashfall of October 5 affected 

populated areas: a light dusting of ash 

extended downwind (NNE) as much as 

60 miles to the northeast part of Mount 

Rainier National Park. The size 

Uplifted and

fractured ice 1980-86

lava dome

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3036

April 2005 Printed on recycled paper

U.S. GEOLOGICAL SURVEY — REDUCING THE RISK FROM VOLCANO HAZARDS
Topographic changes of the Mount St. Helens crater measured from aerial photographs and laser imaging (Lidar). Images show the rapid growth of the welt 

and new lava dome, the whaleback-shaped extrusion, and the deformed glacier. (October 14, 2004, image courtesy of NASA)

September 2003 October 14, 2004 February 1, 2005

1980-86

lava dome

1980-86

lava dome

Welt

Deformed

glacier

1980-86

lava dome

Initial spine

Whaleback

and frequency of earthquakes dropped 

following the first four explosions, but 

in each case built back within hours 

to pre-explosion levels. The level of 

earthquake activity dropped a final time 

after the largest explosion on October 

5. From October 5 through March 2005, 

earthquake rates and sizes waxed and 

waned but never regained the peak levels 

of September 29 through October 5.

Lava Dome Growth

October 6, 2004-March 31, 2005, and 

continuing

Following the brief series of steamand-ash explosions, the volcano settled 

into a quieter phase during which the welt 

continued to grow and a new lava dome 

appeared. Growth of the welt along with 

detection of volcanic gases (CO2

, SO2

, 

H2

S) in early October provided clear 

evidence that magma had risen to or close 

to the level of the 1980 crater floor. On 

October 11, a spine of solid, but still 

hot, lava punctured the surface of the 

welt. As this initial spine grew upward, 

several smaller spines appeared nearby. 

In late October, a larger whalebackshaped extrusion of solid lava emerged 

immediately southeast of the initial spine. 

The early spines plus the whaleback 

extrusion are referred to as the new lava 

dome. By early February 2005, the welt 

and new lava dome grew to a combined 

volume of about 50 million cubic yards, 

almost one-half the volume of the 

1980-1986 lava dome. By then, the top 

of the new lava dome reached nearly 

1,400 feet (taller than the Empire State 

Building) above the level of the 1980 

crater floor, and the welt and new lava 

dome together covered an area equivalent 

to about 60 city blocks.

 Unlike lava domes at many other 

volcanoes, which form when lava 

accumulates and slowly flows over and 

around a volcanic vent, the new dome at 

Mount St. Helens has extruded as a solid 

mass, much like a piston being pushed 

from the ground. In contrast, during 

the 1980s dome growth at the volcano 

occurred when magma extruded onto 

the surface and formed short (650-1,300

feet), thick (65-130 feet) lava flows.

 The new lava dome extruded from 

the ground rapidly. Global Positioning 

System (GPS) stations slung into place 

on top of the dome by helicopter showed 

that in November 2004 a point on the 

whaleback moved southward as much 

as 35 feet per day. In January 2005 the 

rate of movement had slowed slightly 

to about 30 feet per day, and by midFebruary 2005, motion had slowed to 

about 20 feet per day. 

Effects of the Eruption on the 

New Glacier

Deformation associated with intrusion 

of magma and extrusion of the new lava 

dome cleaved the crater glacier into two 

arms. The eastern arm of the glacier, 

pinched between the new lava dome and 

the east crater wall, bulged, fractured, 

View to southeast of Mount St. Helens’ crater. A whaleback-shaped extrusion (part of the new lava 

dome) emerges from the ground and moves southward (arrow). As it grows southward its edges 

fracture and crumble, creating a broad apron of blocky debris, over which the extrusion continues to 

move. Note how the glacier to east is being squeezed and fractured by the growing lava dome. (USGS 

photo, February 22, 2005)

Deformed glacier

Whaleback

Rockfall scar

Initial spine

1980-86 lava dome
and flowed northward rapidly (about 4 

feet per day) in response. At many other 

volcanoes where lava has intruded into 

glacier ice, large floods have occurred. 

Despite losing at least 5% to 10% of 

the glacier ice volume (as of February 

2005) to vaporization and melt, water 

flow from the crater of Mount St. Helens 

has changed little during the 2004-2005 

eruption. In addition to losing steam to 

the atmosphere, the highly porous and 

permeable crater floor has presumably 

absorbed melt water like a giant sponge 

and allowed it to pass into the groundwater system within the volcano rather 

than pond at the base of, or within, the 

glacier and burst forth in a large flood.

New Technologies for 

Monitoring Eruptions

 The remarkably rapid, but nearly 

steady, rate of dome growth and changes 

in the glacier surface, coupled with 

potential hazards from unheralded 

explosions, prompted development of 

new technologies to monitor the state 

of the volcano. Advances in digital 

technology allowed development of selfcontained instrument packages that could 

be slung to the volcano by helicopter and 

placed in a variety of settings. These 

packages have included seismometers 

to measure earthquakes, GPS units to 

track surface movement, and gas sensors. 

Such technological innovations have 

allowed scientists to gather critical data 

in the crater without compromising 

their own safety. In addition to the data 

gathered by these remotely deployed 

packages, conventional and digital 

photography, including vertical, oblique, 

fixed-station, and thermal imaging, have 

been used to document and quantify 

View of north side of the new lava dome from 

a location immediately south of 1980-1986 

lava dome. Circle shows the location of an 

instrument station on the dome. Inset shows the 

instrument station, a “tilt-leg spider” package 

named CDAN, on top of the north-face of the 

new dome. This station housed a GPS unit and 

a seismometer until it was destroyed by an 

explosion on January 16, 2005, 36-hours after 

placement. (USGS photos, January 14, 2005).

dome growth and surface changes. The 

various data are used not only to monitor 

the volcano but also to help refine ideas 

and models that explain how the volcano 

works. 

Potential Hazards Associated 

With Eruptive Activity

 Throughout its history Mount St. 

Helens has produced a wide variety of 

eruptive products, including lava flows, 

lava domes, and deposits of airfall 

pumice and ash, pyroclastic flows (swift 

avalanches of hot ash, pumice, and 

gas), landslides, and lahars (volcanic 

mudflows). The volcano is notorious 

for its gigantic landslide and subsequent 

powerful explosion on May 18, 1980, 

which claimed 57 lives, but smaller 

events are also potentially hazardous. 

 The 2004-2005 dome-growth eruption 

has been benign so far compared to 

the May 18, 1980, eruption. Between 

October 2004 and March 2005, however, 

it produced two more notable explosions 

(January 16 and March 8, 2005) that 

lofted ash plumes (one to nearly 7 miles 

above sea level, which drifted hundreds 

of miles downwind) and hurled rock 

fragments (to about 1 foot diameter) up 

to a mile across the crater. Continued 

growth and occasional collapse of the 

new lava dome may produce additional 

small explosions and also avalanches 

of hot debris that sweep out from the 

dome. Both explosions and hot rock 

avalanches can create ash clouds that 

drift downwind, which can endanger 

aircraft and affect people on the ground. 

Topographic profiles along a N-S axis through Mount St. Helens’ crater. Profiles show geometry of new 

lava dome relative to south crater rim, 1980 crater floor, 1980-1986 lava dome, and 2000 glacier surface.

New dome summit 

February 1, 2005

Crater rim

Old dome summit

Glacier surface 2000

Glacier bed

1980 crater floor

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rock avalanches or small explosions that 

spew hot gases and ash can also swiftly 

melt snow and ice and form floods or 

lahars that surge out of the crater. Such 

floods and lahars can endanger people 

along the upper North Fork Toutle River.

Status of the Eruption and 

Outlook

 As of March 2005, dome growth that 

began in late 2004 continues. Although 

the rate of lava extrusion has waned 

slightly between November 2004 and 

March 2005, the new dome continues 

to build at a remarkable rate. Previous 

periods of dome-growth at the volcano 

have persisted (intermittently) for years

to decades, punctuated by small-tomoderate explosive activity. Someday, 

episodic dome growth may eventually 

rebuild Mount St. Helens to a volcano 

resembling the near-perfect pre-1980 

cone.

Jon J. Major, William E. Scott, 

Carolyn Driedger, and Dan Dzurisin

COOPERATING ORGANIZATIONS

U.S. Department of Agriculture, Forest Service 

University of Washington

View southwest of Mount St. Helens’ crater minutes after onset of the March 8, 2005, explosion. The 

photograph was taken by a remote camera installed on the “Sugar Bowl” dome at the northeast 

margin of the crater mouth. The dark cloud to the left consists of a rain of ash and blocks (to about 

1 foot diameter) from the eruption plume. The light colored cloud to the right is a plume lofting from 

a small ash flow moving northward away from the new lava dome. The snow-clad 1980-1986 lava 

dome is visible in the foreground. The red streaks in the upper right corner are the result of camera 

imperfections. (USGS photo)

For more information contact:

U.S. Geological Survey

Cascades Volcano Observatory

1300 SE Cardinal Court, Bldg. 10, Suite 100

Vancouver, WA 98683-9589

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

http://vulcan.wr.usgs.gov

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov

See also Mount St. Helens—From the 1980 

Eruption to 2000

(USGS Fact Sheet 036-00)

Summary of volcano, lava dome, and glacier dimensions 1980-2005

Volcano

 Elevation of summit 9,677 feet before 1980; 8,363 after; 1,314 feet removed

 Volume removed by May 18, 1980, eruption 0.67 cubic miles (3.7 billion cubic yards)

 Crater dimensions 1.2 miles (east-west); 1.8 miles (north-south); 2,084 feet deep

 Crater floor elevation 6,279 feet

 Crater glacier area (September 2000) 0.4 square miles

 Crater glacier ice volume (September 2000) 105 million cubic yards 

 Maximum glacier thickness (September 2000) 650 feet

1980-1986 Lava Dome

 Elevation of top of dome 7,155 feet

 Height 876 feet above 1980 crater floor

 Diameter About 3,500 feet

 Volume 97 million cubic yards

2004-2005 Lava Dome (as of February 1, 2005)

 Elevation of top of dome 7,642 feet

 Height 1,363 feet above 1980 crater floor; 700 feet above

 2000 glacier surface

 Dimensions of “whaleback” About 1,550 feet long, 500 feet wide

 Diameter of lava dome and welt deformed by About 1,700 feet

 magma intrusion, excluding deformed east glacier arm

 Volume of lava dome and welt deformed by magma 50 million cubic yards

 intrusion, including deformed east glacier arm

 Approximate percentage of crater glacier ice removed 5% to 10%