Volcanic processes

Pyroclastic deposits & lava flows

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).

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, b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, Ash (< 2 mm) Blocks and Bombs (> 64 mm) Lapilli Tuff Lapilli -Tuff Breccia Tuff Lapilli- stone (b) Pyroclastic Breccia or Agglomerate

Yes No YesNo Yes No « grey » volcanoes More explosive Andesitic Subductions « red » volcanoes Less explosive Basaltic Intra-plate Volcanic processes and 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- magmatic eruptions

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

Cantal extinct volcano, France

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,

Yes No YesNo Yes No « grey » volcanoes More explosive Andesitic Subductions « red » volcanoes Less explosive Basaltic Intra-plate Volcanic processes and 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

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,

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 summit9,677 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 covered230 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* cubic miles (155 million cubic yards); multiple flows 3 to 30 feet thick; cumulative depth of deposits reached 120 feet in places VelocityEstimated 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 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