1. Mount St. Helens April 10, 1980

3. 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.

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

5. Bulge Area May 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

6. Throat Sill Volcano Crater Sill Fissure Flow Lava Dike Cross section of a volcano. The throat of a volcano usually follows a crack in the rocks. The Magma enlarges the opening and forces its way along cracks and joints to form dikes and sills and reaches the surface as lava flows. Magma Limestone Sandstone Dolomite Conglomerate Siltstone Bulge Area

7. Mount St. Helens On 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

9. For more than 9 hours a vigorous plume of ash erupted, reaching 12-15 miles (79,200 feet) above sea level. The plume moved eastward at speeds of 60 mph, with ash reaching Idaho by noon.

10. For many weeks the volcanic ash covered the landscape around the volcano.

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

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

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

14. Nearly 135 miles of river channels surrounding the volcano were affected by the Lahars (volcanic mudflows) of May 18. Note the person in the yellow circle.

15. More than 200 homes and over 185 miles of roads were destroyed by the 1980 Lahars.

16. During May 18, 1980 at 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows move at speeds of over 60 mph temperatures of over 800 Degrees Fahrenheit

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

18. Five more explosive eruptions of Mount St. Helens occurred in 1980, including this spectacular event of July 22. The eruption sent pumice and ash 6 to 11 miles into the air, and was visible in Seattle, Washington, 100 miles to the north.