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?