Understanding Volcanic Processes and Pyroclastic Deposits

1. Volcanic processes

2. Pyroclastic deposits & lava flows

3. Figure 4-18. Types of pyroclastic flow deposits. After MacDonald (1972), Volcanoes. Prentice-Hall, Inc., Fisher and Schminke (1984), Pyroclastic Rocks. Springer-Verlag. Berlin. a.collapse of a vertical explosive or plinian column that falls back to earth, and continues to travel along the ground surface. b. Lateral blast, such as occurred at Mt. St. Helens in 1980. c. “Boiling-over” of a highly gas-charged magma from a vent. d. Gravitational collapse of a hot dome (Fig. 4-18d).

5. Classification of Pyroclastic Rocks Ash (< 2 mm) Lapilli- Tuff stone Lapilli Tuff 30 30 Lapilli - Tuff Breccia 70 70 Pyroclastic Breccia or Agglomerate Blocks and Bombs (> 64 mm) (b) Figure 2-5. Classification of the pyroclastic rocks. a.Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b.Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

6. Volcanic processes and types « grey » volcanoes More explosive Andesitic Subductions « red » volcanoes Less explosive Basaltic Intra-plate

7. Dynamic types related to magma/water interactions • Dynamic types related to dissolved bubbles • Dynamic types related to domes growth and collapse • Dynamic types related to lava flows etc. • Destruction of volcanic edifices • Complex edifices

8. Magma/water interaction

9. Submarine eruptions and pillows

11. Pillow-lavas:ophiolitic pillows in the French alps Moho

15. Surtseyan eruptions

17. Hyaloclastites Réunion isl. (Indian Ocean)

19. Phreato-magmaticeruptions

21. Maar

22. Figure 4-6. a. Maar: Hole-in-the-Ground, Oregon (courtesy of USGS). b. Tuff ring: Diamond Head, Oahu, Hawaii (courtesy of Michael Garcia). Maar and tuff ring a b

23. Phreatomagmatic deposits Vertical fall deposits

24. Dunes (horizontal surges) • Blocks (« xenoliths »)

27. Eroded diatremes

28. Welded phreato-magmatic deposits(diatremes) Bournac volcanic pipe, France

29. NB: Kimberlites do also form diatremes (deep eruptions). • Not clear whether they are phreato-magmatic

30. Volcanic processes and types « grey » volcanoes More explosive Andesitic Subductions « red » volcanoes Less explosive Basaltic Intra-plate

31. Dynamic types related to magma/water interactions • Dynamic types related to dissolved bubbles • Dynamic types related to domes growth and collapse • Dynamic types related to lava flows etc. • Destruction of volcanic edifices • Complex edifices

32. Water solubility in magmas

33. Nucleation and growth of bubbles Fragmentation

34. Shape of pumices

35. Plinian eruption

36. Ignimbrites (pumice flow/fall) « Ignimbrites », Turkey

38. Montserrat 1997

39. A classical example The May 1981 eruption at Mount Saint-Helens, WA (U.S.A.)

40. Saint-Helens before the eruption … and after

41. Mount Saint-Helens (2006)

42. Saint-Helens after

43. Spring 1980: early phreatic activity

44. Spring 1980: bulging of the flank

46. 18 May 1980: Major eruption • Flank collapse • Plinian cloud • Lateral blast • Pyroclastic flows (column collapse))

48. Collapse caldera and debris flow

49. Debris avalanche

50. Avalanche