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Volcanic processes
Pyroclastic deposits & lava flows
Figure 4-18Figure 4-18. . Types of pyroclastic flow Types of pyroclastic flow deposits. After MacDonald (1972), deposits. After MacDonald (1972), VolcanoesVolcanoes. Prentice-Hall, Inc., Fisher and . Prentice-Hall, Inc., Fisher and Schminke (1984), Schminke (1984), Pyroclastic Rocks. Pyroclastic Rocks. SSpringer-Verlag. Berlin. pringer-Verlag. Berlin. a.a. collapse of a collapse of a vertical explosive or plinian column that vertical explosive or plinian column that falls back to earth, and continues to travel falls back to earth, and continues to travel along the ground surface. along the ground surface. b. b. Lateral blast, such as occurred at Mt. St. Lateral blast, such as occurred at Mt. St. Helens in 1980. Helens in 1980. c.c. “Boiling-over” of a “Boiling-over” of a highly gas-charged magma from a vent. highly gas-charged magma from a vent. d.d. Gravitational collapse of a hot dome Gravitational collapse of a hot dome (Fig. 4-18d). (Fig. 4-18d).
Flow
Vertical eruptionand column collapse(Mt. Pinatubo,Soufriere)
a.
Airborne
b.
c.
d.
Lateral blast(Mt. St. Helens)
Low pressure boiling over(Mt. Lamington, Papua)
Dome collapse(Mt. Pelée)
Classification of Pyroclastic Rocks
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.
Glass
Rock Fragments Crystals
VitricTuff
LithicTuff
CrystalTuff
(a)
Ash (< 2 mm)
Blocks and Bombs(> 64 mm)
LapilliTuff
Lapilli -TuffBreccia
TuffLapilli-stone
(b)
30 30
7070PyroclasticBreccia or
Agglomerate
Magma encounters « external » water
Submarine, surtseyan, phreatomagmatic types
Magma containsdissolved gas
Plinian eruptions Magma is viscous
Domes and block-and-ash flows
(Pelean)
Flows and scoria cones(Strombolian, hawaian)
Yes No
Yes No
Yes No
« grey » volcanoesMore explosiveAndesiticSubductions
« red » volcanoesLess explosiveBasalticIntra-plate
Volcanic processesand types
• 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
Magma/water interaction
Submarine eruptions and pillows
Pillow-lavas:ophiolitic pillows in the French alps
Moho
Surtseyan eruptions
Hyaloclastites
Réunion isl. (Indian Ocean)
Phreato-magmaticeruptions
Maar
Maar and tuff ring
Figure 4-6. a. Maar: Hole-in-the-Ground, Oregon (courtesy of USGS). b. Tuff ring: Diamond Head, Oahu, Hawaii (courtesy of Michael Garcia).
a
b
Phreatomagmatic deposits
Vertical fall deposits
• Dunes (horizontal surges)
• Blocks (« xenoliths »)
Eroded diatremes
Welded phreato-magmatic deposits(diatremes)
Bournac volcanic pipe, France
• NB: Kimberlites do also form diatremes (deep eruptions).
• Not clear whether they are phreato-magmatic
Magma encounters « external » water
Submarine, surtseyan, phreatomagmatic types
Magma containsdissolved gas
Plinian eruptions Magma is viscous
Domes and block-and-ash flows
(Pelean)
Flows and scoria cones(Strombolian, hawaian)
Yes No
Yes No
Yes No
« grey » volcanoesMore explosiveAndesiticSubductions
« red » volcanoesLess explosiveBasalticIntra-plate
Volcanic processesand types
• 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
Water solubility in magmas
Nucleation and growth of bubbles Fragmentation
Shape of pumices
Plinian eruption
Ignimbrites (pumice flow/fall)
« Ignimbrites », Turkey
Montserrat 1997
A classical example
The May 1981 eruption at Mount Saint-Helens, WA (U.S.A.)
Saint-Helens before the eruption
… and after
Mount Saint-Helens (2006)
Saint-Helens after
Spring 1980: early phreatic activity
Spring 1980: bulging of the flank
18 May 1980: Major eruption
• Flank collapse
• Plinian cloud
• Lateral blast
• Pyroclastic flows (column collapse))
Collapse caldera and debris flow
Debris avalanche
Avalanche
The plinian column
0842
08400838
0832 Mt St Helens
0 10 2030 40 50102040
30
20
10
Distance from Mt St Helens (km)
He
igh
t (km
)
0.25
0.510
1.0
0.05
Idaho
Montana
WyomingOregon
Washington
Walla WallaMt St Helens
Scale
0 300 km
Isopachs in cm
Canada
West Eastb)
c)
0845
30
0.10
2.02.0
Figure 4-15Figure 4-15.. Ash cloud and deposits Ash cloud and deposits of the 1980 eruption of Mt. St. of the 1980 eruption of Mt. St. Helens. Helens. a.a. Photo of Mt. St. Helens Photo of Mt. St. Helens vertical ash column, May 18, 1980 vertical ash column, May 18, 1980 (courtesy USGS). (courtesy USGS). b.b. Vertical section Vertical section of the ash cloud showing temporal of the ash cloud showing temporal development during first 13 minutes. development during first 13 minutes. c.c. Map view of the ash deposit. Map view of the ash deposit. Thickness is in cm. After Sarna-Thickness is in cm. After Sarna-Wojcicki Wojcicki et al.et al. ( 1981) in ( 1981) in The 1980 The 1980 Eruptions of Mount St. Helens, Eruptions of Mount St. Helens, Washington. USGS Prof. Pap.Washington. USGS Prof. Pap., , 12501250, 557-600. , 557-600.
Ash fall
Pyroclastic flows
Lateral blasts
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 dome+local landslides+some block and ash flows
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• Downwind extent Spread across US in 3 days; circled Earth in 15 days• Volume of ash* 0.26 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* 0.029 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)
Mount Saint-Helens:The post-18 May dome
Calderas
Crater Lake, Oregon (USA)
Figure 4-16Figure 4-16.. Approximate aerial extent and thickness of Mt. Mazama (Crater Lake) ash fall, erupted 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. 6950 years ago. After Young (1990), Unpubl. Ph. D. thesis, University of Lancaster. UK.
Washington
Walla Walla
IdahoOregon
CaliforniaNevada Utah
Montana
Canada
Crater Lake
Scale
0 300 km
Wyo.
30 c
m
5 cm
1 cm
Santorini
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