Pyroclastic Deposits

Question 1

What are the three phase of plume formation (within Fall deposits)? Characteristics?

Answer 1

Jet phase

• column density > atmosphere
• fragmented material ejected from vent is carried upwards by momentum.

Convective phase

• column density < atmosphere
• plume rises by convective buoyancy
• air is entrained into the plume and heated, causing it to be less dense and rise.

Umbrella region

• reached a level of natural buoyancy
• plume similar density to the atmosphere, plume will disperse

Question 2

Characteristics of Fall deposits?

Answer 2

Get deposits when clasts are denser than the plume mixture (e.g. tephra fallout).
Deposits mantle topography (uniform thickness)
Grain size decreases systematically with distance from source.
Well sorted

Question 3

How can sorting of particles be described? In terms of falling particles

Answer 3

Motion of a falling body:
velocity, u = kgt. (k = dimensionless constant)

Particle will accelerate as it falls due to gravity (Drag < mg).
But wont accelerate forever, reaches terminal velocity (Drag = mg).

Drag depends on Reynolds number.
Coarse particles fall inertially with high Re.
Fine particles fall with low Re.
In both cases, larger particles fall nearest volcano.

Question 4

What are the processes in estimating column height from wind speed?

Answer 4

Wind speed can be determined by plotting cross wind and down wind ranges.

Cross wind ranges effect column height. E.g. low crosswinds mean low column height.

Column height = k[Eruption rate]^1/4

Generally, greater column height means greater mass (or volume) eruption rate.

Question 5

What grading do Fall deposits have?

Answer 5

Normal Grading.

smaller clasts at the top.

Question 6

What causes fall deposits to have normal grading?

Answer 6

Reduction in column height.
Decrease in wind speed.
Change of wind direction.

Question 7

What are PDCs?

Answer 7

Pyroclastic Density Currents

Hot mixtures of pyroclasts and gas flowing along the ground downhill.
Can be pulses (blast/dome collapse) or sustained currents from the column.

Question 8

What are pumice?

Answer 8

Type of pyroclast. Is silicic
When in a PDC - can become rounded.
When in fall deposits - remain angular.

Question 9

Whats the equation for roundness, R?

Answer 9

R = 4Pi * A / P^2

Where P = perimeter of clasts

Question 10

Characteristics of PDCs?

Answer 10

Consist of volcanic gas + pumice + lithic deposits + ash + ingested atmospheric air.
Fill topographic lows (but can summit barriers).

Question 11

Internal structure of PDCs?

Answer 11

Heterogeneous.
Density-stratified clast concentration.
Velocity changes with density.
Vigorously turbulent in upper regions.

Question 12

PDCs depositional characteristics?

Answer 12

Clasts are deposited incrementally through the low flow boundary. Progressive Aggregation.
Lower flow boundary is what determines characteristics of deposits not upper.
Thickness of deposits is related to duration and rate of deposition.

Question 13

End member PDC currents? (two types)

Answer 13

1) Fully Dilute PDCs
- low concentration of particles in the flow
- particles dont collide
- turbulent transport
- deposits are poorly sorted, parallel and cross stratified, dune bedforms

2) Granular fluid based PDCs
- Particles supported by collisional momentum transfer
- upper portions vigorously turbulent
- deposits are massive and structureless, poorly sorted, thickly bedded

Question 14

What is the process that causes Welding?

Answer 14

Magma rises up conduit, strain rate is increasing.
Cross glass transition (liquid to glass).
Fragmentation occurs.
PDC formed from fragmented material erupted from the vent.
Cooling occurs.
Deposits have a low strain rate so can flow again.

The flowing of fragmented material in deposits is Welding.

Question 15

What is a product of Welding?

Answer 15

Fiammes

• Wispy lense shaped juvenile glassy clasts
• pumice flattened by welding

Question 16

How do Co-ignimbrite plumes form?

Answer 16

Density current coming from the vent.
Upper portion is turbulent and entrains cold atmospheric air.
Air is heated and expands, becomes less dense and rises by buoyant convection.
Plume spreads downwind once it reaches natural buoyancy.
Get Co-ignimbrite fallout deposits.

Question 17

Co-PDC deposit characteristics?

Answer 17

Fine grained - only fine grained ash is carried up into the plume, coarser dense grains remain in PDC deposits.
Co-PDC deposits are fine grained near and far from source.
Co-PDC deposits are depleted in crystals and enriched in fine glass shards.
This allows you to tell the difference between PDC and Co-PDC deposits.

Proximal Regions - clear layering (fine at top)
Medial Regions - vent and Co-PDC deposits mixed
Distal Regions - grain size of deposits all similar

Question 18

What are ignimbrite deposits?

Answer 18

Pumice (and lithic) clasts in a mixture of finer material, loose crystals and glassy ash.

Question 19

Why is pumice important in ignimbrite deposits?

Answer 19

Tells you the original crystal content of magma.

If the crystal to glass ratio is greater in the matrix than in pumice it means that the glassy ash has left (gone into ash clouds) and the crystal that are denser and have a higher terminal velocity remain (harder to carry into plume).

This is why ignimbrite deposits are enriched in crystals and depleted in fine glass shards. Opposite for Co-ignimbrite deposits.

Question 20

Co-PDC formation? In terms of density

Answer 20

Co-PDC formation will occur if PDC is less dense than air, plume rises (buoyant).
If ash + gas mixture is denser than cold air you get a PDC surge.

Density (mixture) = V(ash)Density(ash) + V(gas)Density(gas)

Question 21

What causes buoyancy reversal? PDC to Co-PDC

Answer 21

Get a decrease in density of mixture to lower than ambient air (get a plume) by:
- Sedimentation (of coarse particles)
- Entrain air and heat it
(both will lower the density of the mixture)

Question 22

What causes a buoyant convective plume to collapse?

Answer 22

• Widening of vent, leads to widening of the plume and lowers the efficiency of air entrainment.

Will collapse to form density current.