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Mount St. Helens -- From the 1980 Eruption to
2000, Fact Sheet 036-00
U.S. Geological Survey Fact
Sheet 036-00
Online Version 1.0
Mount St. Helens –
From the 1980 Eruption to 2000
Mount St. Helens,
Washington, is the 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.
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.
The cataclysmic eruption of Mount St. Helens, Washington, on May 18, 1980, 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.)
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
1.2 miles (east-west);
1.8 miles (north-south);
2,084 feet 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; ¸ 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
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 mound- shaped
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.
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.
Magmatic Eruptions Since
May 18, 1980
Date*
Explosive activity
Pyroclastic flows
Lava flows (dome)
Lahars
Erupted 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***
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
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
By
Steve Brantley and Bobbie Myers
Banner by Bobbie Myers
Web design by Carolyn Donlin
For more information or copies of the paper version of
this fact sheet contact:
U.S. Geological Survey
David A. Johnston Cascades Volcano Observatory
5400 MacArthur Blvd., Vancouver, WA 98661
Tel: (360) 993-8900, Fax: (360) 993-8980
e-mail: cvo@usgs.gov
Also, visit the
Cascades Volcano Observatory on
the Web.
COOPERATING ORGANIZATIONS
U.S Department of Agriculture, Forest Service
University of Washington, Geophysics Program
Related Fact Sheets
What Are Volcano Hazards?
Living With Volcanic Risk in the Cascades
PDF version of this two-page fact sheet (352 KB)
U.S. GEOLOGICAL SURVEY–REDUCING THE RISK FROM VOLCANO
HAZARDS
Learn more about
volcanoes and the hazards they pose at the USGS
Volcano Hazards Program website
URL of this page:
http://pubs.usgs.gov/fs/2000/fs036-00/
Maintained by: Michael Diggles
Last modified: March 1, 2005 (mfd)
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