Mount St. HelensApril 10, 1980

On March 20, 1980, after a quiet period of 123 years,earthquake activity once again began under Mt. St. Helens.

March 27 , small steam explosions erupted.

Bulge Area

Mount St. Helens one day before the devastating eruption.View from Johnson’s Ridge six miles NW of the volcano.

Bulge AreaMay 17, 1980

The bulge was growing at a rate of 5-15 feet per day.Volcano’s north side had been pushed

upwards & outwards over 450 feet

Th

roat

Sill

VolcanoCrater

Sill

Fissure FlowLava

Dike

Cross section of a volcano. The throat of a volcano usually follows a crack inthe rocks. The Magma enlarges the opening and forces its way along cracksand joints to form dikes and sills and reaches the surface as lava flows.

Magma

Limestone

Sandstone

Dolomite

Conglomerate

Siltstone

Bulge Area

Mount St. HelensOn May 18, 1980, at 8:32 a.m.

• magnitude 5.1 earthquake

• bulge slid away in a gigantic rockslide and avalanche

• 1300 feet of peak collapsed or blew outwards

• 24 sq. miles of valleys filled with mud and sediment

• 250 sq. miles of recreation/timber damaged by lateral blast

• 57 people were killed

For more than 9 hours a vigorous plume of ash erupted, reaching12-15 miles (79,200 feet) above sea level. The plume moved eastwardat speeds of 60 mph, with ash reaching Idaho by noon.

For many weeks the volcanic ash coveredthe landscape around the volcano.

Mount St. Helens’ lost 1300 feet of elevation and the volcanohad a one-mile-wide horseshoe-shaped crater.

Over 4 billion board feet of useable timber were blown downby the lateral blast, enough to build 150,000 homes.

Spirit Lake, once surrounded by lush forest was destroyedby the area of the blast. Trees now float on surface of lake.Mount Rainier (14,410 feet) is in the distance. Oct. 4, 1980

Nearly 135 miles of river channels surrounding the volcanowere affected by the Lahars (volcanic mudflows) of May 18.

Note the person in the yellow circle.

More than 200 homes and over 185 miles of roads weredestroyed by the 1980 Lahars.

During May 18, 1980at 17 separate pyroclastic

flows descended the flanksof Mount St. Helens.

Pyroclastic flows move atspeeds of over 60 mphtemperatures of over

800 Degrees Fahrenheit

A U.S. Geological Survey geologist examines pumice blocksat the edge of a pyroclastic flow deposit.

Five more explosive eruptionsof Mount St. Helens occurred

in 1980, including thisspectacular event of July 22.

The eruption sent pumiceand ash 6 to 11 miles intothe air, and was visible in

Seattle, Washington, 100 miles to the north.