Download Volcanism 1

Volcanic processes
Pyroclastic deposits & lava flows
Airborne
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).
a.
Vertical eruption
and column collapse
(Mt. Pinatubo,
Soufriere)
Flow
b.
Lateral blast
(Mt. St. Helens)
c.
Low pressure boiling over
(Mt. Lamington, Papua)
d.
Dome collapse
(Mt. Pelée)
Classification of Pyroclastic Rocks
Ash (< 2 mm)
Glass
Tuff
Lapillistone
Lapilli
Tuff
30
30
Vitric
Tuff
Lapilli -Tuff
Breccia
70
Crystals
Rock Fragments
(a)
70
Pyroclastic
Breccia or
Agglomerate
Crystal
Tuff
Lithic
Tuff
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.
Magma encounters
« external » water
Yes
Volcanic processes
and types
No
Magma contains
dissolved gas
Submarine, surtseyan,
phreatomagmatic types
Yes
No
Magma is viscous
Plinian eruptions
« grey » volcanoes
More explosive
Andesitic
Subductions
Yes
No
Domes and
block-and-ash flows
(Pelean)
Flows and scoria cones
(Strombolian, hawaian)
« red » volcanoes
Less explosive
Basaltic
Intra-plate
• 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)
Phreatomagmatic
eruptions
Maar
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
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
Yes
Volcanic processes
and types
No
Magma contains
dissolved gas
Submarine, surtseyan,
phreatomagmatic types
Yes
No
Magma is viscous
Plinian eruptions
« grey » volcanoes
More explosive
Andesitic
Subductions
Yes
No
Domes and
block-and-ash flows
(Pelean)
Flows and scoria cones
(Strombolian, hawaian)
« red » volcanoes
Less explosive
Basaltic
Intra-plate
• 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 SaintHelens, 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
Height (km)
b) 30
West
East
0845
20
0842
0840
0838
10
0832
40
30
10
20
Mt St Helens
10
0
20
30
40
50
Distance from Mt St Helens (km)
Canada
c)
Washington
0.05
0.25
0.10
Montana
1.0
2.0
10
2.0
Walla Walla
Mt St Helens
Isopachs in cm
0
Oregon
300 km
Scale
0.5
Idaho
Wyoming
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 SarnaWojcicki et al. ( 1981) in The 1980
Eruptions of Mount St. Helens,
Washington. USGS Prof. Pap.,
1250, 557-600.
Ash fall
Pyroclastic flows
Lateral blasts
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
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
•
Volume of deposit*
•
Depth of deposit
•
Velocity
•
Temperature
230 square miles; reached 17 miles northwest of the crater
0.046 cubic miles (250 million cubic yards)
From about 3 feet at volcano to less than 1 inch at blast edge
At least 300 miles per hour
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)
1 cm
Canada
5 cm
Washington
Montana
30
cm
Walla Walla
Oregon
Idaho
Crater Lake
Wyo.
California
Utah
Nevada
300 km
0
Scale
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.
Santorini