1. Mount St. Helens, 1980 eruption
VOLCANISM
Mount St. Helens, 1980 eruption

2. Where are volcanoes located?

3. Volcanoes are located over divergent plate boundaries (mid-oceanic ridges)

4. Laki fissure, Iceland

5. Volcanoes are located over convergent plate boundaries (subduction zones)

6. Mount Rainier, Tacoma, Washington

7. Volcanoes are located over Hot spots

10. What are the products of volcanism?

12. Welded tuff (ash & flow deposits)

13. Vesicular Basalt

14. Pyroclastic eruption at Arenal Volcano, Costa Rica

15. Volcanic Bombs

16. Pyroclastic flow at Mount Unzen

17. Volcanic Breccia

18. WHY DO DIFFERENT VOLCANOES HAVE DIFFERENT SHAPES?

20. Gentle-slopes (Shield Volcano)

21. Non-explosive eruptions Basaltic flows Low-viscosity Lava
Gentle-slopes (Shield Volcano)
Non-explosive eruptions
Basaltic flows
Low-viscosity Lava
Low SiO2 content
Mafic composition

22. steep-slopes (Cinder-cone Volcano)

23. steep-slopes (Cinder-cone Volcano)
Pyroclastic material
High SiO2 content
More Felsic composition

24. steep-slopes (Composite Volcano)

25. steep-slopes (Composite Volcano)
Alternating layers of
Basaltic flows and
pyroclastic material
Mafic-felsic composition

26. Volcanic domes

27. Lava accumulates over the vent
Volcanic domes
Felsic lava
Very high viscosity
Lava accumulates over the vent

29. Evolution of
Crater Lake Caldera

34. Mount St. Helens (Before eruption)

35. Mount St. Helens (During eruption)

36. Mount St. Helens (After eruption)

37. Mount St. Helens (Before eruption)

38. Hot Spots and Plate movement

47. Volcanic Hazards

50. Pyroclastic Flows are
the most dangerous