Rich Flashcards
(1) Gas thrust region
Top = H=V2/2G V = velocity
• Gas thrust region can get it a few kms but pumice can reach 50km high
o Plume pushes through air, causing friction + turbulence
Brings air into it (mixes)
Allows it to rise by decreasing density
(1) Re-entraining material and heat transfer
Material falls but some is re-entrained
As material leaves it takes heat and mass out also
Heat stored in ash particles
• Heat transfer by diffusion
• Particle size is major factor
• Smaller grain size = faster equilibrium/diffusion
• Fine grained plume will transfer heat to air quicker
(1) Atmosphere
• Temperature of atmosphere is stratified
• Material rises, cools and thermally equilibrates with atmosphere
• If atmosphere warms, then it gets harder to rise
• Moisture in troposphere (10-20 thousand m)
• Sucking in moisture will release heat by latent heat of vaporisation which causes a rise
o Means latitude plays a role
(1) Umbrella clouds
- Plume spreads down wind = Reached neutral bouyancy
- Velocity decay
- When it falls below the terminal fall velocity the particles will fall out
Spread = 0.34 X height before being blown downwind
Different particle sizes have different terminal fall velocities
• Coarser materials out first
(1) Umbrella region
At Hb – material intrudes laterally as a gravity current, with radial velocity (ur)
“ur”=M/(2πRα (Ht-Hb))
Radial velocity component decays as 1/R
R radial distance; M mass flow rate of gas-particle mixture; α mean density of air between Hb and Ht
Particles fall when terminal fall velocity > transport velocity
(1) Factors affecting fallout
Shape – irregular (tumbling and spinning) Average shape (and size) of particles varies with grainsize (+ distance from vent).
Gravitational Instabilities
• Higher speed descending gravity currents of concentrated particles Manzella et al. (2015)
• increase sedimentation rate
• enhance sedimentation of fine ash
• Promote aggregation
• Associated with ice hydrometeor and mammatus cloud formation (Durant et al., 2009)
Aggregation
• Rarely separate out as individual particles – stick together due to bonding
• Moisture = hyrdrostatic bonds
Pyroclastic fall deposits • Use PF deposits to tell us • volume of deposit/erupted magma • wind-direction • column height • mean mass eruption rate • total grain size distribution (TGSD) • model impacts of future eruptions; damage to infrastructure etc.
Fall Deposit Geometry
• Fall deposits are cones or blankets thin and fine from source.
• Geometry is revealed through contouring thickness (isopachs).
• Represented on semilog plots of thickness [ln(T)] vs square root of isopach area √(A).
• If data fit straight line relationships, can define thinning half distances, bt, the distance required for the deposit to thin/thicken by a factor of 2x.
• Distal segments of Plinian fall deposits bt of 10-50 km.
• Exponential thinning models (e.g., Pyle, 1989) will give fall deposit volume.
(2) What is a pyroclastic density current?
• Biggest killer in volcanic eruptions
Why does it move?
• Moves because it is denser than the surrounding atmosphere – why it collapses back and fountains back into the vent
• Underlying physics is the same as a turbidity current – fluid medium is gas not water
How hot?
• Hottest found – pumice that had re-welded into glass – 800 to 1000 C
How fast?
• Up to a few 100s ms per second
How far?
• Can go further than 100kms from volcano
How much material?
• Ignimbrites (pyroclastic density current deposits) can have volumes of several thousand cubic kilometres
Formation of PDCs
- ‘Boiling over situation’ – feeds a sustained PDC where gas thrust fails and materials fountains back
- Partial fountaining of a buoyant Plinian column to produce small flows that come off the sides
- PDCs from single explosions that impulses material into the air that doesn’t become buoyant and fountains back to become a PDC
- Lateral blasts – lava dome intruded into volcano that depressurises the inside of the volcano – PDC created by explosion
- Collapse of lava dome to produce a ‘block and ash flow’
- Can be created through the interaction with water – groundwater, lake water, seawater etc – magma interacts with water – or chamber intruded by seawater – PDCs produced
(2) Vent diameter vs vent velocity
• As you widen the vent and decrease the velocity you make the eruption more unstable and therefore make more PDC
(2) Cause of Column Fountaining
Column Collapse
• Decrease in exit velocity (note, K=1/2mv2)
– Decrease in gas content of magma
– Vent widening
• Decrease in mass eruption rate (Q)
– Decrease in gas content
• Can be recorded in the ash and pumice
– Constriction of vent
• Decrease in thermal energy (T)
– Change in magmatic temperature
• Zonation of chamber – possible mechanism
– Inclusion of lithic clasts from vent walls
• Recorded in deposits
– Ingress of surface water (lake, sea) into vent
• Shown in chemistry of deposits
• These may result in a decrease in mass eruption rate
(2) Short lived Vulcanian eruptions
one-time eruption – material will not become a convective plume and will become a PDC - would only deposit a thin layer of ash and will not be seen in the geologic record
(2) Block and ash flow
collapse of lava dome – hot avalanche flow created
(2) Loft
when PDC deposited lots of material – lots of energy lost - ingested much air that’s been heated and becomes buoyant so starts rising rather than flowing – sheared by surface winds
(2) Geometry of PDC deposits
• As they as gravity deposits they are largely influenced by topography
o Valleys – nearly completely controlled by topography
o Overbank/veneer deposits – moderately controlled by topography – Tenerife
o Landscape burying – large amounts of volume of material so topography less important
(2) Hindered Settling Time
pyroclastic flow has a dynamic viscosity – in order for particle to settle through it it has to move through particles – ash coupled with gas etc – not depositing through a clean atmosphere – this process retards sedimentation
• Strongly controlled by particle size
• The more fine ash there is, the further the PDC will move out and the longer it will take for the material to be deposited
(3) A Definition of a Volcano
- ‘A volcano is a geologic environment that, at any scale, is characterised by three linked elements: magma, eruptions, and edifice.’ Borgia et al., (2010).
- Every eruption produces a volcano of some description, or adds to an existing volcano?
- It is useful to think of a volcano as the sum of all its parts – subsurface, surface, wind-dispersed, reworked, and chemically/physically altered (host rock, volcaniclastic sediments).
(3) Controls on Volcano Morphology, Size and Composition
Tectonic environment
• Melting conditions in upper mantle
• Crustal thickness
• Stress field
Magma composition
• Rheology of magma controls eruptions
• Large volcanoes erupt a range of compositions
Size of eruption
• Large explosive eruptions (>5 km3) tend to destroy edifices (form calderas)
• Effusive eruptions build edifices (outwards and upwards)
Lifetime of volcano
• Monogenetic or polygenetic?
Environmental conditions
• Eruption under sea or on land, or both
• Presence or absence of water at surface or in ground
• Slope and elevation
