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Eruption: May 18, 1980

 

The following summary table is provided as a courtesy by the U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington. We duplicated the table here to assist teachers, educators and students in developing their educational programs as part of the Mount St. Helens National Volcanic Monument Teacher's Corner.

If you use any of the material on this page, please credit the U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington. We thank them for their support of the Teacher's Corner.

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

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