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Volcanic Debris AvalancheVolcanic Debris Avalanche
References:
Encyclopedia of Volcanoes, pp. 617-626Volcanic Successions, pp. 297-303
Types of Debris FlowsTypes of Debris Flows
• Dry rock avalanches
• Mud-flows and lahars
Dry Rock AvalanchesDry Rock Avalanches
• Magmatic eruption
– Bezymianny type (1956)
• Non-magmatic explosions
– Bandai type (1888)
• Cold avalanches
– Ontake type (1984)
PhysiognomyPhysiognomy
• Break-away scarp
• Hummocky terrain
• Water-filled depressions
• Steep flow margins
Sheveluch craterSheveluch crater Hummocks on debris flowHummocks on debris flowJocotitlan, MexicoJocotitlan, Mexico
Mount St. HelensMount St. Helens
• 2.3 km3 of material
• Amphitheater scarp 800 m high
• V > 50 ms-1
• Deposit ~ 45 m thick
HorseshoeHorseshoe--shaped Depressionsshaped DepressionsMount St. HelensMount St. Helens
• Crushed matrix with large blocks
• Hummocky surface
• Runout of 22 km
Mount St. Helens hummocksMount St. Helens hummocks
Mount St Helens
Topo change
Mount St Helen
Flow model
Socompa, ChileSocompa, Chile
• 500 km2 area covered
• Contains bread-crust blocks
• Cryptodome present?
• Movement by sliding
Socompa, ChileSocompa, Chile
• Toreva blocks up to 3 km long
• Maximum velocity of 300 ms-1
• 35 Km of Runout Distance
• Marginal levee
Debris Avalanche DepositsDebris Avalanche Deposits
• Contain huge blocks (>10 m)
• Broad stratigraphy preserved
• Retrogressive slope failure
Debris Avalanche DepositsDebris Avalanche Deposits
• Block facies
• Matrix facies
• Lower part fine-grained
• Boulders at the surface
La Soufriere debris avalancheLa Soufriere debris avalanche
Colima, hummock in 18,000 yBP Colima, hummock in 18,000 yBP debris avalanchedebris avalanche
Contact Between Debris AvalanchesContact Between Debris AvalanchesColimaColima
Bread crust bomb, Colima debris avalancheBread crust bomb, Colima debris avalanche
18.5 Ky debris avalanche, Colima18.5 Ky debris avalanche, Colima
Flow MechanismFlow Mechanism
• H/L characteristics
• Long runout (mobility)
• Sliding friction?
• Basal shear concentration
H/L = µ = tan β
Driving acceleration = g sin β; Retardation = µ g cos β
∆v/ ∆t = a = g sin β - g µ cos β = g (sin β - µ cos β )
Flow MechanismFlow Mechanism
• High yield strength
• Plug flow
• Grain dispersive forces?
• Acoustic fluidization?
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