Rich Flashcards Preview

Volcanology > Rich > Flashcards

Flashcards in Rich Deck (44)
Loading flashcards...
1
Q

(1) Gas thrust region

A
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

2
Q

(1) Re-entraining material and heat transfer

A

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

3
Q

(1) Atmosphere

A

• 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

4
Q

(1) Umbrella clouds

A
  • 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

5
Q

(1) Umbrella region

A

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

6
Q

(1) Factors affecting fallout

A
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.

7
Q

(2) What is a pyroclastic density current?

A

• 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

8
Q

Formation of PDCs

A
  • ‘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
9
Q

(2) Vent diameter vs vent velocity

A

• As you widen the vent and decrease the velocity you make the eruption more unstable and therefore make more PDC

10
Q

(2) Cause of Column Fountaining

Column Collapse

A

• 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

11
Q

(2) Short lived Vulcanian eruptions

A

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

12
Q

(2) Block and ash flow

A

collapse of lava dome – hot avalanche flow created

13
Q

(2) Loft

A

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

14
Q

(2) Geometry of PDC deposits

A

• 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

15
Q

(2) Hindered Settling Time

A

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

16
Q

(3) A Definition of a Volcano

A
  • ‘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).
17
Q

(3) Controls on Volcano Morphology, Size and Composition

A

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

18
Q

(3) Monogenetic volcanism

A
  • A volcano that erupts only once (any magma composition)
  • Small volume of magma (0.001-0.1 km3)*
  • *not including flood basalt eruptions
  • Eruptions may last months to >year.
  • Small edifices (h<300 m)
  • Disperse lava a few kms to a few 10s km from volcano; ash up to several 100s km.
19
Q

(3) Strombolian and Hawaiian eruptions

A

Strombolian eruptions – Weak, small volume, transient explosions (no lava) lots of gas
Hawaiian eruptions - Sustained fire fountains

20
Q

(3) Scoria cones

A
  • Typically, mafic composition.
  • Generally composed of loose, vesicular, low density (1000-2000 kg/m3) porous pyroclastic material (scoria), but may include layers of lava, spatter and agglutinate).
  • Scoria – like pumice but a bit denser, vesicles tends to be more interconnected and larger
  • Typical Dimensions: Height (Hcone): <0.3 km, Width (Wcone): < 2.5 km, Volume: 0.0001 – 0.1 km3
  • Flank dips of <35° (angle of repose).
  • Can be more complex (e.g., coalesced, breached, elongate fissure vents).
  • Scoria clasts fallout around vent and build up cone over time
  • Clasts will roll or periodically form small grainflows down flanks of growing cone
  • Bedding is lenticular (grain flows deposits); deposits are well sorted.
  • Each bed is packages of material – lenticular – will pinch out up slope and down slope
  • Scattered or layers of bombs and spatter.
21
Q

(3) Spatter cones

A

are smaller and composed of dense (>2000 kg/m3), welded pyroclastic material.
• Can form at the start of a monogenetic eruption and then might have a scoria cone form over the top
• Forms from vigorous fountaining
• Smaller and composed of more dense material
• Steeper flanks (<45°).

22
Q

(3) Spatter ramparts

A

are linear spatter cones.

• Few metres to a few tens of metres high (below seismic resolution).

23
Q

(3) Dispersal of Pyroclasts during Basaltic Eruptions

A

• Dispersal:
o Small basaltic eruption (5-10 km2 -typical)
 Weak eruptions (mostly monogenetic) – column is bent over by wind – does not reach stratosphere
o Moderate basaltic eruption (25-135 km2)
o SubPlinian (150-500 km2).

Scoria fall deposits: well sorted sheets of angular scoria; massive or weakly bedded; thin and fine with distance from vent; thinning half distance of 100s m.

24
Q

(3) Fissure Volcanoes

A
  • Basaltic magma erupted from multiple sources along a long fissure (kms-100s km long). (Note that all basaltic eruptions start in this manner).
  • Erupted volume varies from 0.01-1000s km3.
  • Eruptions last from days to decades (or more).
  • Volume of lava far exceeds volume of pyroclastic material.

Very low-aspect ratio, lava-dominated sheet, fed from multiple sources.

25
Q

(3) Basaltic Shield Volcanoes

A
  • Low aspect-ratio, lava dominated, basaltic volcanoes
  • Flanks dip 4-8°; central steeper cone passes outwards in a flat lava apron.
  • «1-20 km3
  • 0.5-12 km diameter, < 500 m high.
  • Polygenetic shield volcanoes are morphologically similar.
26
Q

(3) Lava domes

A
  • Mainly intermediate to silicic lavas
  • Extrusion of typically viscous basaltic andesite to rhyolite composition lava (kimberlite known).
  • Up to several hundred metres in height.
  • Steep-sided; variable outline.
  • May sit in craters; surrounded by low-density tephra ring (pumice cone)
  • Can flow up to 15 km (termed a coulee)
27
Q

(3) Monogenetic volcanic fields

A
  • Monogenetic volcanic fields are composed of several to 100s (rarely 1000s) of various types of monogenetic volcanoes.
  • Eruption recurrence interval within fields 101–104 years.
  • Field may be active for millions of years.
  • Total magmatic output = a large composite volcano.
  • Useful for understanding basin evolution and reconstructing paleogeographic and paleoenvironmental conditions.
  • Volcano location may be tectonically controlled.
28
Q

(3) Phreatomagmatism

A
  • External Water (groundwater, lake, seawater) explosively mixes with erupting magma (commonly basaltic magma but can be silicic magmas).
  • Contrast with magmatic eruptions (no interaction with external water).
  • Phreatic eruptions – superheated steam (erupted products not derived from magma).
  • Can have a profound effect on the nature of an eruption, but very poorly understood - complicated 3-phase systems (gas, solids and liquids).
  • Typically enhances fragmentation of magma through explosive expansion of water to steam – so products have a higher fine ash content than those from magmatic eruptions
29
Q

(3) Hydro-volcanic Landforms

A

Small volume (monogenetic) basaltic eruptions (landforms are basically modified cinder cones)
• Essentially modified cinder cones
• Tuff ring – typically crater at surface
• Maar – crater sits below earth’s surface
• Tuff Cone – Littoral zone, river, lake etc – more water to interact with magma
• Deep water – water in more abundance than magma – hydrostatic pressure suppresses explosions by volatiles and by steam
• If you cut Tenerife, a seamount, the bottom layers would be made of pillow lavas and hyaloclastites

30
Q

(3) Surtseyan Eruptions – sea/lake water interaction

A
  • Eruptions that form tuff cones – Scenario C from previous diagram
  • Sea or lake – a lot of water to interact with magma
  • Magma is ascending, fragmenting and interacting with water – erupting in a series of pulsatory jets of slurry
  • Tuff cone sits atop submarine volcano – typically composed of hyaloclastite and pillow lavas.
  • Large quantities of surface water lead to eruption of wet slurries.
31
Q

(3) Dry Settings

A
  • Monogenetic basaltic fields:
  • Most eruptions produce scoria cones (non-phreatomagmatic eruptions of basaltic magma)
  • 0-30 % of volcanoes in fields are maars/tuff rings.
  • Magma:water interaction occurs in an aquifer.
  • What controls interaction?
  • Magma flux?
  • Eruption rate
  • Heterogeneous subsurface water table?
  • Structural control on groundwater (intersecting faults/joints)?
  • Variable subsurface geology (porosity, permeability)?
  • Climate variations (monogenetic fields active for 104-105 years)?
32
Q

(3) Tuff ring-/Maar-Diatreme Volcanoes

A

• Magma-water interaction with groundwater occurs in aquifer – low fluxes, dependent on permeability (i.e. mechanical properties of rock, faulting).
• Explosions happen at depth – fragment the country rock – cause a carrot shaped vent in the substrate – Diatreme
• Diatreme fills with material that does not get ejected out during the explosions
• Excavates a flared conduit (diatreme) 0.5–2 km depth.
• Eruptions build a broad, low cone around the crater (maar)
• Intermittent, repetitive explosions; small volumes of ejecta dispersed by PDCs and fallout.
• Ash clouds + PDCs are commonly produced but very short-lived – a few seconds
o An explosion will throw ash into air and produce a PDC but will not travel far
o Queso-steady
• PDCs + tephra and other fallout with build-up a tuff ring around the volcano that is not thick + low gradient slope

33
Q

(3) How to recognise phreatomagmatism

A

• Key characteristics of phreatomagmatic eruptions
1. Numerous, small explosions.
• Stratified and bedded deposits that relate to each of these explosions
2. Intense fragmentation of magma.
• Elevated abundance of fine ash
• Poorly sorted deposits
3. Explosions in aquifer.
• High lithic clast content – bits of country rock
4. Rapid chilling of magma by water.
• Juvenile clasts may be denser
5. Incorporation of water/steam into eruptive jets
• Presence of ash aggregates – wet ash that stuck together

34
Q

(3) Polygenetic volcanism

A
  • Volcanoes that erupt many times from a central vent and construct large volcanic edifices (excavational or constructional)
  • Active for 105–106 yrs.
  • Typically volcano grows through addition of material from numerous small-volume eruptions; individual eruption volumes up to 104 km3
  • Total volume of volcano 1–90 000 km3.
  • Sit upon volumetrically substantial intrusive complexes and altered rock.
35
Q

(3) Composite volcanoes

A
  • Large and long-lived volcanoes (> 1 Ma) comprised of lava and volcaniclastic rocks (500->3000 m high).
  • Common arc volcanoes; intraplate and early phases of rifting.
  • Result from eruption of evolved magmas (basaltic andesites, andesites, dacites, trachytes and ryholites).
  • Lava flows are more viscous; flow short distances and pile up around the vent.
  • Greater frequency of explosive eruptions and pyroclastic rocks
36
Q

(3) Composite volcano morphology

A
  • As magmatic system matures, more evolved magmas erupt.
  • Early magmas basaltic – form flat lying lava flows
  • Later magmas andesitic-rhyolitic – form stubby lavas, and steeper slopes
  • Thus, volcano has concave profile, accentuated by ongoing erosion.
  • Surrounded by extensive apron of volcaniclastic material (sediments, pyroclastic deposits).
37
Q

(3) Calderas

A
  • Calderas develop on composite volcanoes and seamounts during large volume (>5 km3) eruptions of differentiated magma.
  • Subside along steep faults; large volumes of tephra as ignimbrites and fall deposits.
  • Require development of a large shallow magma reservoir.
  • Steep-sided faults down throw massive low-density material against higher density lava-dominated flanks of volcano and/or crustal material.
38
Q

Seafloor deposits

A

Range of products produced from seafloor eruptions
• Less of a density difference between lava and water (rather than air) so tend to form pillow lavas – tubular structures of lavas that spread out across the seafloor
• Hyaloclastite – fragmented glassy margins of pillow lavas
o As there is such rapid cooling a glass forms – chills quickly – lava moves, and the brittle surface is fractured into small pieces of glass – forms this – chilled and granulated basaltic glass

39
Q

Ocean Island foundations

A
  • Islands are built upon layers of alternating strong lava flows (competent) and altered granulated basaltic glass that becomes like clay (weak)
  • Volcanos are therefore weak structures – pile of ‘rubbish’
40
Q

Emergence of a seamount

A
  • Persistence of a seamount as an island requires rate of lava output > erosion of lava by wave action.
  • Essentially if volcanism is persistent enough then a shield volcano will emerge from the ocean
  • Most likely will emerge and be eroded again by the ocean
  • Can remerge several times in different locations until it remains
  • Needs frequent eruptions so that more lava is produced, and a strong base established
  • Phreatic eruptions just fragment lava into ash and tephra that is easy to erode so these suppress emergences
  • Similarly with water reaching the eruption
  • Emergence generates features that do not occur in submarine environments:
  • Wave-cut platforms and terraces;
  • Smooth, flat tops;
  • Submarine canyons.
41
Q
  1. Reflect on the whole process of collecting data, creating isopach maps and estimating the eruptive parameters. Where are the uncertainties and errors? What assumptions have been made? How might this process be improved?
A

Collecting data
• Isopach collection – problem of collecting ultra-proximal and distal data (proximal too dangerous, distal – hard to resolve, removed quickly by rain, wind etc). Syndepositional reworking (clasts landing and tumbling downslopes results in overthickening at the foot of slopes (not always easy to recognise in field, esp on older, vegetated deposits). Erosion of fall deposits can result in thinning. Preservation of fall deposits decreases quickly with time due to erosion and soil formation etc.

  • Human error in measuring thickness, all sorts of general problems associated with collecting data outdoors, reworking. Poor exposure, etc.
  • Particles of different sizes exhibit different settling behaviours according to particle Reynolds number (which changes with altitude in atmosphere), thus, simple one-segment exponential thinning of fall deposits may not hold true – series of exponential segments (fallout from column, plume corner, and then umbrella cloud). May not be accurately capturing the volume in distal regions.
  • We haven’t taken into account lithic clasts, derived from erosion of the vent, in the fall deposit when calculating DRE of magma. This would reduce the volume of the magma, and thus the eruption rate etc.
  • We assume an average deposit density – no justification is given for that. Deposit density may vary from 1500 – 500 downwind.
  • Eruption duration – dodgy notes from pirates. Even with close monitoring, it is difficult to accurately ascribe a finite duration to particular phases of eruptions/fall out events; duration affects Q which affects HT estimations in this method.
  • Mass flux (Q): this is averaged in the calculations and may have varied widely during the eruption (would have affected column height);
  • Column height: calculated from an average Q value (see above) using an equation derived from empirical data – how accurate are measurements of column heights for historical eruptions? Very difficult to measure column height (triangulation, satellite measurements, aviation accounts) – could be considerable uncertainty in those values
  • We ignore atmospheric stratification. Variations in latitude affect the stratification of the atmosphere. Uncertainties in the estimation of column height. Effect of 10-20%.
42
Q

Formula for calculating true thickness from a borehole

A

depth (borehole thickness) = true thickness / (cos (dip angle))

43
Q

How accurate are your estimations? Outline the uncertainties, errors and assumptions in your calculations. What factors might contribute to overestimations of values, and what factors might contribute to underestimations?

A

These are just ball park figures using a limited dataset. There were lots of possible answers here and you didn’t need to get all of them to get score well.

Assume a perfect conical geometry; steady output of magma over the volcano’s lifetime; and that each eruption is the same size (volume); all material erupted is on the flanks of the volcano (some might have been dispersed further away (affects average output values); assume all the lavas sit on top of each other in sequence (i.e., there aren’t other lavas erupted <5000 yrs that cover other parts of the volcano).

Overestimations of age may come from some eruptions being large and contributing a lot to the volcano’s mass.
Underestimations of age may come from significant repose periods during the volcano’s life; erosion; flank collapse events that have removed material (which has then been rebuilt). Cyclicity in volcanic output may cause over or underestimations in age, average volumes etc.

Improvements – all revolve around more information: mapping of all lava flows; more realistic volume calculations; more radiometric dates; look for dispersed volcanic products (lavas, ash etc); investigate erosion on the volcano.

44
Q

Log interpretations

A

A. grainsize (x-axis) is a proxy for column height (a higher column will transport larger clasts further. So, if the grainsize increases in fall deposits, one can assume the column is getting higher; extra points if you mentioned that maybe the wind is changing direction – this can cause vertical grainsize increases as the umbrella cloud pivots (remember fall deposits thin and fine downwind and across wind – look at isopachs). Some people confused coarser grainsize for more proximal locations (and vice versa) – no evidence for this and all can be explained through changes to column dynamics.
B. Thickness (y-axis) represents time. Thus, if grainsize doesn’t vary with time (thickness), then the eruption is steady – conditions aren’t changing. If grainsize does vary (bedded parts of log) then the eruption is unsteady. Either the column is increasing and decreasing in height, or the umbrella cloud is being blown around in different directions.
C. Lithic clasts: These are bits of the existing volcano that are incorporated into the eruption. They can tell you interesting things about the eruption and were put in their specifically to see what you made of them. You should have considered their composition (what they are in each layer) and their abundance.
a. Hydrothermally altered clasts: some of you interpreted these as being altered by water during the eruption. This was wrong. It takes time to turn rock into gunky clays. The interiors of volcanoes are often strongly altered (rotten); The abundant hydrothermally altered clasts at the base of the log represent this plug of rotten material being blasted out by the first explosions.
b. Glassy lavas and vesicular lava lithic clasts. These do not relate to the erupting magma as some of you inferred, but instead represent bits of the volcano. Specifically, lavas erupted on the surface (hence glassy and vesicular).
c. Appearance in A5-A8 of finely crystalline rocks, cumulates and hydrothermally altered rocks indicates that the erosion of deeper parts of the volcano (go back to my Earth Materials lectures on crystal sizes of igneous rocks). Here you could have talked about potential caldera collapse.
d. Increases in abundance of lithic clasts can indicate collapse of the vent walls, or caldera collapse – especially if deep-sourced, coarsely crystalline lithic clasts appear.
D. Banded pumices: alongside the appearance of deep-derived lithic clasts, are banded pumices – indicative of magma mingling –ie. two types of magmas being erupted – common during caldera collapse as the subsiding caldera roof disrupts the magma reservoir. Indicates either 2 or more close, but separate, magma reservoirs of different composition, or a zoned magma chamber.