1. Tuesday: bring calculator

2. Mt. St. Helens: A Case Study from 1980 Photo pre-eruption

3. Mt. St. Helens: Volcanic Precursors Tremors around the mountain

4. Mt. St. Helens: Volcanic Precursors Harmonic tremors are different from tectonic movements

5. Mt. St. Helens: March 20 Quakes triggered snow avalanches

6. Mt. St. Helens: March 21-26 Earthquake activity dramatically increases

7. Mt. St. Helens: March 27 Dark ash blankets the top of the mountain: Phreatic eruption

8. Mt. St. Helens: March 27 Phreatic eruption = steam related, with ash Magma+ groundwater = explosion

9. Mt. St. Helens: March 27 Crater is formed

10. Mt. St. Helens: March 28 12 explosions occur Ash landslides

11. Mt. St. Helens: March 31 Harry Truman refuses to leave

12. Mt. St. Helens: April 1 Huge crater develops: 200 feet deep and growing 4/3/80: 1500’ wide, 300’ deep

13. Mt. St. Helens: April 7-April 11 Now a bulge develops on the side of the volcano

14. Mt. St. Helens: April 7-April 11 And yet the crater keeps growing: 500’ deep

15. Mt. St. Helens: April 11-26 Bulge is still there, but explosions cease on 4/26… is it over, or is the pressure building? 4/29: nearly 1 mile across

16. The Bulge: March 29 through May 2

17. May 7: Explosions resume, end May 15 May 17…

18. Eruption Warnings: Volcanic Precursors Small changes in summit elevations or slope steepness can be observed by lasers of tiltmeters

19. Mt. St. Helens Case Study Last eruption: 1857 Activity begins in March Eruption occurred in May

20. Mt. St. Helens

21. 5/18/80

22. Volcanic Precursors Harmonic tremors Ground swelling or tilting Emitting steam and gases Increased ground temperatures Small phreatic eruptions (water)

31. Ground swelling