|
Eruption Summary -
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 (based on uncompacted
deposits): |
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 |
|
Eruption
|
Date: |
May 18, 1980 |
| Time of initial blast: |
8:32 a.m. PDT |
| Eruption trigger: |
A magnitude 5.1 earthquake about 1 mile beneath
the volcano |
|
Landslide -
Debris Avalance
|
Area and volume (volume based on
uncompacted deposits): |
23 square miles (0.67 cubic miles) (3.7 billion
cubic yards) |
| Depth of deposit: |
Buried North Fork Toutle River to average
depth of 150 feet (maximum 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 (based on uncompacted
deposits): |
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 degrees F (350 degrees C)
|
| Energy release: |
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) |
| Human fatalities: |
57 |
|
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. Blast and lahars
destroyed more than 185 miles of highways and roads and 15 miles of railways.
|
| 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 U.S. in 3 days; circled Earth
in 15 days |
| Volume of ash (based on uncompacted
deposits): |
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 and depth (volume based on
uncompacted deposits): |
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 degrees F (700 degrees C)
|
|
Other
|
Wildlife: |
The Washington State Department of Game
estimated nearly 7,000 big game animals (deer, elk, and bear) perished as well
as all birds and most small mammals. Many burrowing rodents, frogs, salamanders,
and crawfish, managed to survive because they were below ground level or water
surface when the disaster struck. |
| Fisheries: |
The Washington Department of Fisheries estimated
that 12 million Chinook and Coho salmon fingerlings were killed when hatcheries
were destroyed. Another estimated 40,000 young salmon were lost when forced to
swim through turbine blades of hydroelectric generators as reservoir levels along
the Lewis River were kept low to accommodate possible mudflows and flooding. |
| -- From: Brantley and Myers, 1997, Mount
St. Helens -- From the 1980 Eruption to 1996: USGS Fact Sheet 070-97 and Tilling,
Topinka, and Swanson, 1990, Eruption of Mount St. Helens - Past, Present, and
Future: USGS General Interest Publication |
| Table compiled by Lyn Topinka, USGS/CVO,
1997 |
