Summary of May 18, 1980 Eruption of Mount St. Helens (courtesy of usgs.gov)
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
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
.jpg)
.jpg){kind=link}