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