Volcanic processes Pyroclastic deposits lava flows Figure 4

Volcanic processes

Pyroclastic deposits & lava flows

Figure 4 -18. Types of pyroclastic flow deposits. After Mac. Donald (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 -18 d).

Classification of Pyroclastic Rocks Ash (< 2 mm) Tuff Lapillistone Lapilli Tuff 30 30 Lapilli -Tuff Breccia 70 Pyroclastic Breccia or Agglomerate 70 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 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 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.

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* • Depth of deposit depth 600 feet) • Velocity Lateral Blast • Area covered • Volume of deposit* • Depth of deposit • Velocity • Temperature 23 square miles; 0. 67 cubic miles (3. 7 billion cubic yards) Buried 14 miles of North Fork Toutle River Valley to an average depth of 150 feet (max. 70 to 150 miles per hour 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)

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