Pyroclastic FlowsPyroclastic Flows

References:

Smith, RL, 1960, As flows. Geol. Soc. America Bull., 71:795-842.

Sparks, RSJ, 1976, Grain size variations in ignimbrites and implications for the transport of pyroclastic flows: Sedimentology, 23:147-188.

Encyclopedia of Volcanoes, pp. 581-599

TopicsTopics

• Column collapse

• Emplacement

• Pumice flows

• Welding

TerminologyTerminology

• Ignimbrites• Ash-flow tuffs• Nueés ardentes• Block-and-ash flows

• Pumice flows• Scoria flows• Ash-cloud deposits• Welded tuffs

Column CollapseColumn Collapse• Energy: P.E. And K.E.• Column stability• Gas content, muzzle velocity,

conduit radius• Mass eruption rate• Sustained column • Unstable, collapsing column

Asama PlumeAsama Plume19731973

EmplacementEmplacement

• Mixtures of solid particles and gas

• High concentration (0.1< C < 0.6)

• Able to surmount hills and ridges

• Travel great distances 10s of km

• Can support large pumice blocks

• Fine ash matrix

Phoenix Cloud, after DobranPhoenix Cloud, after Dobran

AshAsh--flow Sheets, Smith (1960)flow Sheets, Smith (1960)

• Flow units

• Cooling units

• Welded tuffs

• Compound cooling units

FluidizationFluidization• Upward gas velocity supports particles• A large range of sizes supported• Very large sorting coefficient• Gas escape structures• Pumice concentration zones• Elutriation• Low viscosity

MorphologyMorphology• Controlled by topography• Fill depressions• Even upper surface• Valley ponded deposits• Veneer deposits• Multiple lobes and fans• Lateral levees

Pyroclastic Flows of 472 AD, VesuviusStandard Section (Sparks, 1976)Standard Section (Sparks, 1976)

• Layer 1 (ground layer or surge)

• Layer 2 (flow unit)

• Layer 2a (fine-grained basal)

• Layer 2b (main body of flow)

• Layer 3 (ash cloud)

Single Flow UnitSingle Flow Unit Base of Bandelier TuffBase of Bandelier Tuff

Gas PipeGas PipeKomagatake Tuff, JapanKomagatake Tuff, Japan

Welding (Ragan & Sheridan)Welding (Ragan & Sheridan)

• Degree of welding– non-welded

– partially welded

– densely welded

• Density is a good index

• Welding (density) profiles

Partly Welded Bishop TuffPartly Welded Bishop TuffPartly Welded Bishop Tuff

Vitrophyre, Vitrophyre, ArmeniaArmenia

Secondary CrystallizationSecondary Crystallization

• Vitrophyre

• Devitrification

• Lithophysae

• Vapor-phase crystallization

• Zeolitization

Sheet MorphologySheet Morphology

• Generally plateau forming

• Steep marginal escarpments

• Non-welded upper layers removed

• Erosion of soft material into teepees

Teepee Structures, BandelierTeepee Structures, Bandelier