Mount St. Helens — From the 1980 Eruption to 2000

The Volcano Awakens

March 16 – May 17, 1980

The first sign of activity at Mount St.

Helens in the spring of 1980 was a series of

small earthquakes that began on March 16.

After hundreds of additional earthquakes,

steam explosions on March 27 blasted a crater

through the volcano's summit ice cap. Within a

week the crater had grown to about 1,300 feet

in diameter and two giant crack systems

crossed the entire summit area. By May 17,

more than 10,000 earthquakes had shaken the

volcano and the north flank had grown outward

at least 450 feet to form a noticeable bulge.

Such dramatic deformation of the volcano was

strong evidence that molten rock (magma) had

risen high into the volcano.

Cataclysmic Eruption

May 18, 1980

Within 15 to 20 seconds of a magnitude 5.1

earthquake at 8:32 a.m., the volcano's bulge

and summit slid away in a huge landslide —

the largest on Earth in recorded history. The

landslide depressurized the volcano’s magma

system, triggering powerful explosions that

ripped through the sliding debris. Rocks, ash,

volcanic gas, and steam were blasted upward

and outward to the north. This lateral blast of

hot material accelerated to at least 300 miles

per hour, then slowed as the rocks and ash fell

to the ground and spread away from the

volcano; several people escaping the blast on

its western edge were able to keep ahead of the

advancing cloud by driving 65 to 100 miles an

hour! The blast cloud traveled as far as 17

miles northward from the volcano and the

landslide traveled about 14 miles west down

the North Fork Toutle River.

The lateral blast produced a column of ash

and gas (eruption column) that rose more than

15 miles into the atmosphere in only 15 minutes. Less than an hour later, a second eruption

column formed as magma erupted explosively

from the new crater. Then, beginning just after

noon, swift avalanches of hot ash, pumice, and

gas (pyroclastic flows) poured out of the crater

at 50 to 80 miles per hour and spread as far as

5 miles to the north. Based on the eruption rate

of these pyroclastic flows, scientists estimate

that the eruption reached its peak between 3:00

and 5:00 p.m. Over the course of the day,

prevailing winds blew 520 million tons of ash

eastward across the United States and caused

complete darkness in Spokane, Washington,

250 miles from the volcano.

During the first few minutes of this

eruption, parts of the blast cloud surged over

the newly formed crater rim and down the

west, south, and east sides of the volcano. The

hot rocks and gas quickly melted some of the

snow and ice capping the volcano, creating

surges of water that eroded and mixed with

loose rock debris to form volcanic mudflows

(lahars). Several lahars poured down the

volcano into river valleys, ripping trees from

their roots and destroying roads and bridges.

The largest and most destructive lahar was

formed by water seeping from inside the huge

landslide deposit through most of the day. This

sustained flow of water eroded material from

both the landslide deposit and channel of the

North Fork Toutle River. The lahar increased

in size as it traveled downstream, destroying

bridges and homes and eventually flowing into

the Cowlitz River. It reached its maximum size

at about midnight in the Cowlitz River about

50 miles downstream from the volcano.

Explosions and Dome Growth

May 25 – October 16, 1980

Five smaller explosive episodes occurred

during the summer and fall of 1980. Each

produced eruption columns 8 to 9 miles above

sea level and pyroclastic flows down the

volcano's north flank. The episodes in June,

August, and October also erupted lava in the

crater to form a dome. Lava domes are moundshaped features that form when stiff, viscous

lava accumulates over and around a volcanic

vent. The June and August domes were

destroyed by subsequent explosive episodes.

The cataclysmic eruption of Mount St. Helens, Washington, on May 18, 1980

(inset), formed a deep north-facing horseshoe-shaped crater. Small eruptions

from 1980 to 1986 built a lava dome. The lava dome can be seen here

steaming within the crater. (Photos by Austin Post and Lyn Topinka, USGS)

U.S. Department of the Interior USGS Fact Sheet-036-00

U.S. Geological Survey March 2000

Mount St. Helens, Washington, isthe most active volcano in the

Cascade Range. Its most recent series

of eruptions began in 1980 when a large

landslide and powerful explosive

eruption created a large crater, and

ended 6 years later after more than a

dozen extrusions of lava built a dome in

the crater. Larger, longer lasting

eruptions have occurred in the

volcano's past and are likely to occur in

the future. Although the volcano seems

to have returned to a period of quiet,

scientists with the U.S. Geological

Survey and University of Washington

Geophysics Program continue to closely

monitor Mount St. Helens for signs of

renewed activity.

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS
October 16, 1980 – October 21, 1986

Beginning with the October 1980 eruption,

17 eruptive episodes built a new lava dome

that reached 876 feet above the crater floor.

Minor explosive activity and (or) lahars

accompanied several of the 1981 to 1986

episodes. Each of the dome-building episodes

added between 1 and 29 million cubic yards of

new lava to the dome. Most of the growth

occurred when magma extruded onto the

surface of the dome, forming short (650 to

1,300 feet), thick (65 to 130 feet) lava flows.

During a 12-month-long episode beginning in

1983, however, magma moved primarily into

the dome's molten interior, pushing its east side

outward by at least 250 feet. In addition to the

17 dome-building episodes, hundreds of small

explosions or bursts of gas and steam occurred,

sending ash a few hundred feet to several miles

above the volcano. The larger explosions

showered the crater with rocks and

occasionally generated small lahars.

November 1986 – December 1999

Since late 1986 several periods of increased

seismicity have occurred. Between 1989 and

1991 there were about 30 bursts of brief but

intense seismic activity lasting minutes to

hours. Several of these bursts were

accompanied by small explosions from the

dome. The explosions formed a new vent on

the north side of the dome and produced small

eruption columns that rose a few miles above

the volcano. A few explosions also hurled hot

rocks three feet in diameter at least 1⁄2 mile

northward from the dome, generated small

pyroclastic flows in the crater, and formed

small lahars. During 1995 and 1998 seismicity

increased for several months, but there were no

accompanying explosions.

Lava Dome Dimensions

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

Steven R. Brantley and Bobbie Myers

COOPERATING ORGANIZATIONS

U.S. Dept. of Agriculture, Forest Service

University of Washington, Geophysics Program

For more information contact:

U.S. Geological Survey

Cascades Volcano Observatory

5400 MacArthur Blvd., Vancouver, WA 98661

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

http://vulcan.wr.usgs.gov/

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov/

This Fact Sheet supersedes Mount St. Helens-From

the 1980 Eruption to 1996

(USGS Fact Sheet 070-97)

See also What are Volcano Hazards?

(USGS Fact Sheet 002-97)

Summary of May 18, 1980 Eruption of Mount St. Helens

Volcano

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

Volume removed* 0.67 cubic miles (3.7 billion cubic yards)

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

Crater floor elevation 6,279 feet

Landslide

Area and volume* 23 square miles; 0.67 cubic miles (3.7 billion cubic yards)

Depth of deposit Buried 14 miles of North Fork Toutle River Valley to an average depth of

150 feet (max. depth 600 feet)

Velocity 70 to 150 miles per hour

Lateral Blast

Area covered 230 square miles; reached 17 miles northwest of the crater

Volume of deposit* 0.046 cubic miles (250 million cubic yards)

Depth of deposit From about 3 feet at volcano to less than 1 inch at blast edge

Velocity At least 300 miles per hour

Temperature As high as 660 F (350° C)

Energy released 24 megatons thermal energy (7 by blast, rest through release of heat)

Trees blown down 4 billion board feet of timber (enough to build about 300,000 two-bedroom

homes)

Lahars

Velocity About 10 to 25 miles per hour (over 50 miles per hour on steep flanks of

volcano)

Damaged 27 bridges, nearly 200 homes

Effects on Cowlitz

River

Reduced carrying capacity at flood stage at Castle Rock from 76,000 cfs (cubic

feet per second) to less than 15,000 cfs

Effects on Columbia

River

Reduced channel depth from 40 to 14 feet; stranded 31 ships in upstream

ports

Eruption Column and Cloud

Height Reached about 80,000 feet in less than 15 minutes

Downwind extent Spread across US in 3 days; circled Earth in 15 days

Volume of ash* 0.26 cubic miles (1.4 billion cubic yards)

Ash fall area Detectable amounts of ash covered 22,000 square miles

Ash fall depth 10 inches at 10 miles downwind (ash and pumice); 1 inch at 60 miles

downwind; 1⁄2 inch at 300 miles downwind

Pyroclastic Flows

Area covered 6 square miles; reached as far as 5 miles north of crater

Volume & depth* 0.029 cubic miles (155 million cubic yards); multiple flows 3 to 30 feet

thick; cumulative depth of deposits reached 120 feet in places

Velocity Estimated at 50 to 80 miles per hour

Temperature At least 1,300 F (700 C)

Fatalities

Human 57

Wildlife Countless non-burrowing wildlife in blast area, including about 7,000 big

game animals; about 12 million salmon fingerlings in hatcheries.

* Volumes are based on uncompacted deposits

Magmatic Eruptions Since

May 18, 1980

Date*

Explosive activityPyroclastic flowsLava flows (dome)LaharsErupted volume**(million cubic yards)

05/25/80 X X 41.8

06/12/80 X X X 54.5

07/22/80 X X 13.1

08/07/80 X X X 7.2

10/16/80 X X X 3.6

12/27/80 X 2.1

02/05/81 X 4.7

04/10/81 X 5.4

06/18/81 X 5.4

09/06/81 X 5.1

10/30/81 X 4.7

03/19/82 m X X 4.4

05/14/82 X 3.5

08/18/82 X 6.0

02/07/83*** m X X 29.3

03/29/84 X 1.4

06/17/84 X 1.2

09/10/84 X 4.8

05/24/85 X 5.6

05/08/86 X m 7.6

10/21/86 X 8.0

m Minor

* Explosive activity usually lasted hours and

dome-building usually lasted days

** Volumes based on lava flows plus uncompacted airfall ash and pyroclastic flows

*** Dome growth continued for almost a year

Printed on recycled paper