Eruption Column Dynamics

Question 1

Pyroclastic Surge Deposit

Answer 1

Low density mass of turbulent gas and rock fragments ejected during eruption (lower density) that travels down slope

Question 2

Pyroclastic Flow

Answer 2

Hot mixture of rock fragments, gas, and ash that travels rapidly away down slopes (high density)

Question 3

Three Categories of Pyroclastic Deposits

Answer 3

1. Pyroclastic Fall: blankets topography, well sorted, thin (e.g ash fall)
2. Pyroclastic Flow: fills topography, poorly sorted, thick, high temp. (e.g ignimbrites)
3. Pyroclastic Surge: dilute, searing cloud of ash, crystals and rock (not confined to topography) (cross-bedded deposits)

Question 4

Eruption Column

Answer 4

Droplets of molten and glass particles
Crystals
Lithic Fragments

All of these are dispersed in continuous gas phase.

Question 5

Stages of Eruption Column

Answer 5

1. Gas Thrust/Jet Region
2. Convective Ascent Region
3. Umbrella Region

Question 6

Gas Thrust/Jet Region

Answer 6

Pyroclastic and gas jetted into atmosphere—accelerated by expansion of magma gas.

Question 7

Convective Ascent Region

Answer 7

Material mixes with cooler surrounding air in column and rises due to buoyancy which decreases bulk density of mixture, creating a Plume.

Occurs when bulk density is less than surrounding atmosphere.

Question 8

Umbrella Region

Answer 8

Density of atmosphere continues to decrease due to height. Plume reaches neutral buoyancy and moves laterally. This forms a mushroom/umbrella shape.

Question 9

Co-Ignimbrite Plume

Answer 9

Pyroclastic flow heats up entrained air causing sedimentation to occur. Concentration and density of material decreases until it’s less than surrounding atmosphere. Buoyant Cloud/Plume develops.

E.g Mt. St. Helen’s 1980

Question 10

Vulcanian Eruption

Answer 10

Moderately explosive and produce moderate ash columns—consist of small volumes of magma and vigorous eruption columns of fragmented gas.
Propelled by solution of volatiles in magma beneath solid lava.

Pyroclastic flows are produced sometimes due to dome collapse.

Question 11

Plinian Eruptions

Answer 11

Very explosive; Ash columns that spread out into umbrellas shape and produced widespread deposits of falling ash. Eruption columns may collapse due to density to form pyroclastic flows. Lahars can be triggered, too.

E.g 1980 Mt. St. Helens