1. Volcanic processes

2. Pyroclastic deposits & lava flows

3. Figure 4-18. Types of pyroclastic flow deposits
Figure 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 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
Lapilli
stone
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, b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1,

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-magmatic eruptions

21. Maar

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

51. The plinian column

52. Figure 4-15. Ash cloud and deposits of the 1980 eruption of Mt. St
Figure Ash cloud and deposits of the 1980 eruption of Mt. St. Helens. a. Photo of Mt. St. Helens vertical ash column, May 18, 1980 (courtesy USGS). b. Vertical section of the ash cloud showing temporal development during first 13 minutes. c. Map view of the ash deposit. Thickness is in cm. After Sarna-Wojcicki et al. ( 1981) in The 1980 Eruptions of Mount St. Helens, Washington. USGS Prof. Pap., 1250,

53. Ash fall

56. Pyroclastic flows

57. Lateral blasts

61. Mount Saint-Helens 1980 Eruption Sequence of events
Intrusion of magma: « cryptodome » and bulging
Early, minor phreatomagmatic activity
Flank destabilisation and collapse
Plinian column etc.
Aftermath: surface growth of the dome+local landslides+some block and ash flows

62. Summary of May 18, 1980 Eruption of Mount St. Helens (USGS)
Mountain
Elevation of summit 9,677 feet before; 8,363 feet after; 1,314 feet removed
Volume removed* cubic miles (3.7 billion cubic yards)
Crater dimensions 1.2 miles (east-west); 1.8 miles (north-south); 2,084 feet deep
Landslide
Area and volume* 23 square miles; cubic miles (3.7 billion cubic yards)
Depth of deposit Buried 14 miles of North Fork Toutle River Valley to an average depth of 150 feet (max. depth 600 feet)
Velocity 70 to 150 miles per hour
Lateral Blast
Area covered 230 square miles; reached 17 miles northwest of the crater
Volume of deposit* cubic miles (250 million cubic yards)
Depth of deposit From about 3 feet at volcano to less than 1 inch at blast edge
Velocity At least 300 miles per hour
Temperature As high as 660° F (350° C)
Eruption Column and Cloud
Height Reached about 80,000 feet in less than 15 minutes
Downwind extent Spread across US in 3 days; circled Earth in 15 days
Volume of ash* cubic miles (1.4 billion cubic yards)
Ash fall area Detectable amounts of ash covered 22,000 square miles
Ash fall depth 10 inches at 10 miles downwind (ash and pumice); 1 inch at 60 miles downwind; ¸ inch at 300 miles downwind
Pyroclastic Flows
Area covered 6 square miles; reached as far as 5 miles north of crater
Volume & depth* cubic miles (155 million cubic yards); multiple flows 3 to 30 feet thick; cumulative depth of deposits reached 120 feet in places
Velocity Estimated at 50 to 80 miles per hour
Temperature At least 1,300°F (700° C)

63. Mount Saint-Helens: The post-18 May dome

64. Calderas

65. Crater Lake, Oregon (USA)

69. Figure 4-16. Approximate aerial extent and thickness of Mt
Figure Approximate aerial extent and thickness of Mt. Mazama (Crater Lake) ash fall, erupted 6950 years ago. After Young (1990), Unpubl. Ph. D. thesis, University of Lancaster. UK.

72. Santorini