Volcanic processes

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Volcanic processes
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Pyroclastic deposits lava flows
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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).
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Classification of Pyroclastic Rocks
Ash (lt 2 mm)
Lapilli-
Tuff
Lapilli
stone
Tuff
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30
Lapilli -
Tuff
Breccia
70
70
Pyroclastic
Breccia or
Agglomerate
Blocks and Bombs
(gt 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.
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Volcanic processes and types
 grey  volcanoes More explosive Andesitic Subduc
tions
 red  volcanoes Less explosive Basaltic Intra-pl
ate
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• 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

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Magma/water interaction
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Submarine eruptions and pillows
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Pillow-lavasophiolitic pillows in the French
alps
Moho
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Surtseyan eruptions
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Hyaloclastites
Réunion isl. (Indian Ocean)
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Phreato-magmaticeruptions
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Maar
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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
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Phreatomagmatic deposits

• Vertical fall deposits

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• Dunes (horizontal surges)
• Blocks ( xenoliths )

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Eroded diatremes
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Welded phreato-magmatic deposits(diatremes)
Bournac volcanic pipe, France
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• NB Kimberlites do also form diatremes (deep
  eruptions).
• Not clear whether they are phreato-magmatic

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Cantal extinct volcano, France
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Structures and Field Relationships

• Figure 4-5. Cross sectional structure and
  morphology of small explosive volcanic landforms
  with approximate scales. After Wohletz and
  Sheridan (1983), Amer. J. Sci, 283, 385-413.

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Volcanic processes and types
 grey  volcanoes More explosive Andesitic Subduc
tions
 red  volcanoes Less explosive Basaltic Intra-pl
ate
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• 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

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Water solubility in magmas
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Nucleation and growth of bubbles Fragmentation
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Shape of pumices
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Plinian eruption
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Ignimbrites (pumice flow/fall)
 Ignimbrites , Turkey
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Montserrat 1997
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A classical example

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

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Saint-Helens before the eruption
and after
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Mount Saint-Helens (2006)
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Saint-Helens after
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Spring 1980 early phreatic activity
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Spring 1980 bulging of the flank
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18 May 1980 Major eruption

• Flank collapse
• Plinian cloud
• Lateral blast
• Pyroclastic flows (column collapse))

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Collapse caldera and debris flow
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Debris avalanche
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Avalanche
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The plinian column
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Figure 4-15. 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, 557-600.
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Ash fall
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Pyroclastic flows
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Lateral blasts
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Mount Saint-Helens 1980 EruptionSequence of
events

• Intrusion of magma  cryptodome  and bulging
• Early, minor phreatomagmatic activity
• Flank destabilisation and collapse
• Plinian column etc.
• Aftermath surface growth of the domelocal
  landslidessome block and ash flows

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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 0.67 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 0.67 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 0.046 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

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Mount Saint-HelensThe post-18 May dome
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Calderas
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Crater Lake, Oregon (USA)
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Figure 4-16. 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.
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Santorini