Physical Volcanology Flashcards
1
Q
What is the difference between a lava flow and a lava?
A
A lava flow is a moving body of molten rock, whereas a lava is the deposit.
2
Q
Does temperature (high) increase or decrease the viscosity?
A
Decrease
3
Q
Do dissolved volatiles increase or decrease the viscosity?
A
Decrease
4
Q
Does Si content increase or decrease the viscosity?
A
Increase
5
Q
Does the number of crystals increase or decrease the viscosity?
A
Increase
6
Q
Does the number of bubbles increase or decrease the viscosity?
A
Increase or decrease, depending on their form
7
Q
What is another name for magmas or lavas with high silica contents?
A
Polymers
8
Q
What are the main composition factors for a basaltic lava?
A
Low silica contents
Low viscosity
High temperatures
9
Q
How do flood basalts form?
A
These form from several lava flows which inflate to produce large deposits. The separate layers move over one another and upwards as they emerge. More layers build up on top to build up the deposit.
10
Q
What are ‘A’a lavas?
A
These are the most common type of basalt lava. They have surfaces made up of loose, irregular blocks with razor sharp asperities. Vesicles are abundant at a few cm’s scale, most of which are spherical.
11
Q
What are the two distinct zones within an ‘A’a lava?
A
An upper rubble part and a lower, massive part, consisting of solid lava which cooled slowly.
12
Q
What is pahoehoe lava and what is its typical surface appearance?
A
These are the least viscous lavas we are aware of and exhibit a range of fluid all surface structures. They have smooth glossy surfaces.
13
Q
What are the three types of pahoehoe lava?
A
Shelly - the surface overlies large cavities
Ropy - crumpled surface within channels
Entrail - glossy, bloated surfaces (these are common)
14
Q
Which type of lavas are block lavas typical for, and what do they look like?
A
Block lavas are typical for andesite lavas.
Their surfaces consist of large, smooth-sided blocks of lavas (up to several m’s) and the flow fronts are usually very steep, up to 100m high and made up of piles of huge angular blocks.
15
Q
What are autobrecciated textures and what lava type are they common for?
A
Blocks from the front of the flow are later incorporated within, causing a mass of angular fragments solidly welded together.
These are common for andesitic lavas.
16
Q
What are the main features for a dacitic lava?
A
They form at low temperatures, have high silica contents and high crystal contents (~50%). They are more sluggish than basalts and they form thick, steep intrusions. Dacite lavas are often more of a crystal mush than a melt.
17
Q
What is seen after eruption of dacite lavas?
A
Cones of ejects, due to explosive activity of the viscous lavas. Lava domes are also common for dacite lavas.
18
Q
Where are rhyolitic lavas commonly found?
A
Within calderas formed by earlier major explosive eruptions.
19
Q
What are pillow lavas and how do they form?
A
They are small packets or pillows of lava formed by submarine eruptions of pahoehoe lava, the outer edge of which is chilled by the cool seawater and the inside remains hot, allowing further build up and injection of lava. They build up over time and cumulatively form deposits that can be 100’s of m thick.
20
Q
What are hyaloclastites or hyaloclastite breccias?
A
As hot lava is abruptly chilled, extensive fragmentation takes place which produces fine grained angular detritus. The whole volume of the flow may be communited into debris consisting of small glassy fragments which we call a hyaloclastite breccia.
21
Q
What are festoons?
A
Looped ridges exposed on the surface of a lava flow. These form when the lavas surface crust is folded when in a semi solid state, as the viscosity increases as it cools.
22
Q
What are ogives?
A
These are similar to festoons. They are massive pressure ridges which are on a much larger scale than festoons. They can be many metres high and spaced 10’s of metres apart.
23
Q
What are the three key eruption styles relating to the ejected materials?
A
Magmatic - all juveniles, no accidentals
Phreatomagmatic - combination
Phreatic - all accidentals, no juveniles
24
Q
What are phreatic eruptions?
A
When groundwater comes into contact with hot volcanic rock, we get phreatic steam explosions that fragment the rock and shower the bird around.
25
What are phreatomagmatic eruptions?
Where greater amounts of groundwater interact with magma, we get violently explosive phreatomagmatic explosions. These will inject both juvenile and surficial rocks as they can quarry into the ground.
26
What are surtseyan eruptions?
These are eruptions involving the flooding of active vents by seawater or lake water.
27
What can drive volcanic eruptions?
Pressure drops, which allow the huge expansion of water.
Thermal energy, primary driver of eruptions.
In surtseyan and phreatomagmatic eruptions, the thermal energy from the magma is used to heat the water and turn it to steam, which fragments the magma as it explodes explosively.
28
What are fluid instabilities?
When magma and water, both liquids, come into contact, they do not touch as the water will boil so we get a vapour zone. Along this contact, we therefore instabilities and the more fluid magma there is, the more instabilities we see.
29
What are peperites and how do they form?
They are sedimentary rocks that contain fragments of igneous material. They form when swarms of glassy clasts detach from the magma before getting trapped. They do not heat the country rock as it is wet, so the vapour acts as an insulator.
30
What are the five key fragmentation mechanisms?
Magmatic explosivity (gas in the magma drives explosion)
Contact surface steam
Bulk interaction steam
Cooling contact granulation (hot glass in cool water)
Autobrecciation (outside cools and is deformed by hot interior)
31
Where is explosivity more important?
At shallow depths as this is where it is most common.
32
What is Surtseyan volcanism?
This is an explosive type of volcanism, where the explosivity results from water gaining access to the top of the vent and the explosions are characterised by jets of tephra, which are short lived. Is type shows periodic vigorous activity, the water causes a variety of fragmentation processes and the jets of tephra are usually a water-ash slurry.
33
What feature is characteristic for surtseyan eruptions?
Soft sediment deformation or slumping.
34
What grain size do phreatomagmatic and magmatic eruptions produce? (Think about their origins)
Phreatomagmatic produces fine material.
| Magmatic produces coarse material.
35
What type of eruption can ash suggest?
Plinian or Phoenix - as fine ash does not always represent low energy.
36
What is Ta'alian volcanism?
This is the volcanism exhibited by a volcano island Ta'al in the Phillipines, which exhibits phreatomagmatic eruptions. We therefore see a combinations of juveniles and accidentals from violently explosive eruptions. The main explosion known for this volcano is that seen in September 1965.
37
What are phreatoplinian eruptions characterised by?
They are characterised by extreme fragmentation of lithic and juvenile clasts, producing pyroclastic deposits dominated by wide,y dispersed fine grained unsorted ash.
38
What is a key example of a phreatoplinian eruption and what do the sedimentary structures tell us about the setting?
The Whorneyside phreatoplinian tuff in the Lake District.
This deposit shows an ash fall ~25m thick with multiple layers. The sedimentary structures indicate the presence of a lake.
39
How much material was ejected as during the famous eruptions of St Helens, Pinatubo, Canadas and Yellowstone?
St Helens - 0.25 km3
Pinatubo - 5-8 km3
Canadas - 10's km3 (predicted value - deposits lie in the sea)
Yellowstone - >1000 km3
40
What are some key characteristics of explosive plinian eruptions?
Include pumice, ash and volatiles which circle the Earth
High mass flux, sustained for minutes, hours or days
Volcanic explosive index (VEI) of 6-8
Can generate large pyroclastic density currents, can affect climate
Can be effected by climate, sea levels or ice levels
Can produce large subsidence structures - calderas ~5-500 km2
41
What type of deposit did Wright and Walker, 1977, find and how do they form?
They discovered co-ignimbrite lag-fall deposits.
These form at or near the site of eruptive column collapse and consist mainly of pyroclasts that are too large or heavy to be carried away in the resulting pumice flows. This indicates that the ignimbrite originated by continuous collapse of the eruptive column.
42
What did Branney and Kokelaar, 1997, state about the composition of ignimbrites?
They stated that ignimbrites can change composition from top to bottom as the currents can change their composition over time. Intergradational compositional zones within the bed record changes in the material supplied at the source over time.
43
What do plinian pumice falls represent and why are they often poorly preserved?
They represent a hiatus in density currents. They have an open work texture which is easily eroded so will not always be preserved.
44
Why do layers not necessarily mean pauses in the current?
The grain size can reduce at any time during an eruption, meaning the flow will be able to hold the grains and there will be no deposition at that time.
45
What is a typical steady eruption architecture for an ignimbrites wedge?
Thickness that increases with time but the depochrons remain the same thickness. The current is steady so the same amount of material is deposited within each time interval and the aggravation rate decreases downstream.
46
How does the mass flux and deposit architecture change over time for a waxing eruption?
The mass flux will steadily increase over time.
The depochrons show more material deposited within each successive time frame and they often prograde towards the distal end. As we move up through a sequence, the pumice clasts are normally graded and the lithics are inversely graded.
47
How does the mass flux and deposit architecture change over time for a waning eruption?
The mass flux decreases over time.
There is less material deposited within each successive depochron. The lithics will be normally graded and the pumices silk be reversely graded.
48
What are entrachrons?
These are entrainment isochrons and they are invisible surfaces along which the first appearance of clasts are correlated. However, we cannot use grain size to correlate these.
49
What is an intraplinian deposit?
This is an ignimbrite between two pumice falls.
50
If a unit shows all compositional layers the same as another locality but the sequence is thinner, what is the relative aggradation rate?
The thinner locality has a much lower aggradation rate but they would have been deposited on the same timescale.
51
Why is pumice more likely to remain in a current over lithics? What is pumice overpassing?
Pumice has a high buoyancy and litchis are heavier relatively so are more easily deposited.
Pumice overpassing is the process where pumice clasts bipass the lithic rich region until the current slows enough to deposit the pumice.
52
What are the key particle support mechanisms in density currents?
```
Fluid turbulence
Traction - rolling / sliding
Saltation - bouncing
Grain interaction - collisions
Gas fluidisation
Fluid escape
```
53
What is gas fluidisation?
In a gas-fluidised dispersion, an upward flow of gas exerts a drag force that partically supports the clasts so that the dispersion behaves like a fluid. The fluidisation is affected both by the viscosity of the fluid phase and by the density contrast between the particles and that fluid.
54
What did Druitt, 1995, state about fluidisation?
They stated that fluidisation can be caused by cementation. Dense particles travel one way and fluid travels the other and the fluids can carry and wash out small particles.
55
What is a fully dilute density current?
Within these currents, collisional momentum transfer between moving pyroclasts has little effect on the particle support, segregation and current rheology. Particle transport and support are dominated by effects of turbulence of the fluid phase at all levels in the current. Deposits are usually well sorted and the matrix and clasts both show grading.
56
What is a granular fluid based current?
Clast concentrations towards the lower flow boundary are sufficiently high for particle support to be dominated by collisional interactions between moving grains and/or fluid escape, and the currents may be density stratified. Here, grain interactions dominate the flow, the deposits are poorly sorted and matrix supported and the deposition is dominated by fluid escape and granular flow.
57
Deceleration is the primary cause of a current to deposit. What is the equation for this?
Substantive acceleration (du/dt) = 6u/6t + u.6u/6x
Where 6 is delta, u is local velocity, x is distance downcurrent.
6u/6t is the temporal acceleration at a fixed location
u.6u/6x is the spatial downcurrent deceleration
58
What factors affect the lithofacies of ignimbrites?
Whether the current is waxing or waning.
Whether the current is eroding or depositing.
The size of the current.
59
What are calderas and how do they form?
They are large subsidence structures that form after a magma reservoir empties so the overlying rocks subside. This process can be simplified to a piston sliding within its cylinder.
60
What is the key example for crater lake calderas and what are the stages of the eruption?
Mt Mazama, Crater Lake, Oregon.
9km diameter crater, 5700 BC, 50-60 km3 ejected material.
It initially had a plinian phase, ejecting material as far as Alberta, Canada. Then, the fully dilute pyroclastic density currents were erupted and flowed downslope, depositing stratified ignimbrites. During this eruption, the main mass of Mt Mazama subsided to form the caldera.
61
Smith and Bailey type calderas exhibit a sequence during their evolution. What are the four stages?
Post caldera collapse
Ignimbrite shields, where the ignimbrites spread outwards
Resurgent dome, where the dome comes up and then cracks as it is brittle.
Post collapse volcanoes, which grow up in the central crater.
62
What are resurgent calderas?
These are large secondary calderas that put any pre existing volcanoes into inexistence. Vertical uplift of up to 1km takes place, so there will be a broad depression.
63
What did Lipman, 1997, state about plate/piston calderas and topographic rims of calderas?
They involve subsidence of a coherent plate, bound by steeply dipping ring faults.
They also found that the topographic rim, which bounds the subsided area of a caldera, can be wider than the original caldera rim if it is sufficiently old to allow erosion. For young calderas, the topographic rim is the original caldera rim.
64
What did Kennedy and Stix, 2003, state about the angle of faults and caldera collapse?
If the faults dip inwards, they will tend to close as the caldera blocks subside.
If the faults dip outwards, they will open the system during subsidence.
65
What are ring intrusions and how do they form?
When a caldera forms through subsidence, faults form in a circular pattern around the caldera (ring faults). These faults allow magma to rise through the fractures, forming ring dykes that circle the caldera.
66
Where besides calderas has subsidence been found to occur, and what did Whittaker and Redding, 1989, find about subsidence?
Subsidence has also been found to occur above existing salt mines, I.e. Ashtan Salt Mines. The cracks dip outwards, and allow for intrusion of ring dykes, and the beds dip inwards so we will see subsidence.
Whittaker and Reddish studied subsidence above existing mines and found evidence to support that subsidence is structurally controlled.
67
What are the three facies used to identify old calderas?
```
Intra caldera (within) - thick welded turfs, mesobreccia, mega breccia, hydrothermal alteration, intrusions, lake sediments.
Extra caldera (outside) - thin poorly welded ignimbrites, columnar joints, pumice falls, little or no hydrothermal alteration.
Structural margin - not always round, abrupt ignimbrite thickness changes, complex arcuate faults and fault blocks, extension (graben), intrusions and alteration.
```
68
Why do we need to study the Scafell caldera?
To tell us more about the Ta'al caldera, thought to be the most volcano on earth.
69
What is the GVS for Scafell from top to bottom?
Lake sediments, deposited in the crater lake
Pumice fall, representing 100-1000 km3 ejected material
Ignimbrite, representing the caldera fill
Granite/andesite, representing the caldera floor
70
What are the three main structures that Scafell has?
Channels carved by small rivers - indicating a wet environment.
Instant unconformities - shaking from eruptions creates titling and slumping before the next eruptions.
Impact structures which increase in size closer to the volcano or along the dispersal axis.
71
What characteristic facies does the Whorneyside Phreatoplinian tuff show and how do the features form?
A subaqueous facies with sand or ash draped ripples, where ash from an eruption settles in water. There are also wave ripples so the water must be very shallow.
72
What are rheamorphic ignimbrites? Both Scafell and Ta'al have evidence for this.
Ignimbrites that are still extremely hot after landing so continue to move as a fluid.
73
What are fiamme and what can alter their orientation?
Pumice clasts which are conspicuously glassy and have been stretched to have a shape similar to candle flames.
74
What are piecemeal calderas and which caldera shows this?
This will be a caldera that shows slumping or wobbling. Scafell represents this well.
75
When was the key eruption of Pinatubo? How much material was ejected and how high was the eruption column?
June 15th 1991
5-10 km3 ejected
Eruption colum 40km high
76
What type of eruption was that seen at Pinatubo?
A phreatic eruption, so there were no juveniles, and the eruption created fissure aligned phreatic craters.
77
What are some key features produced by the Pinatubo eruption?
2.5km diameter crater
Crater lake ph of 2 from sulphur plumes in the water
Medial ignimbrite fan lies in self carved channels up to 200m deep
Flat top valley fill ignimbrite
Rootless crater which can provide cross stratified units
78
What are the four types of material ejected by a volcano?
Lavas
Volcaniclastic - pyroclastic rocks
Volcaniclastic - autoclastic rocks (i.e. Autobrecciated lavas)
Volcaniclastic - epiclastic rocks (formed by sedimentary processes)
79
What are the 6 key characteristics for volcaniclastic sediments that differ them from normal sediments?
Inherited grain shapes
Density variations - they are sorted both by size and density
Physically and chemically unstable
Undergo rapid diagenesis - sharp edges are easier to weather
Accretionary lapilli/pellets
Produced by a convulsive supply - catastrophic events
80
What are the 6 main types of gravity flow?
```
Soil creep
Slide
Slump
Debris flow
Turbidity current
Rockfall
```
81
What is a debris avalanche?
A fast moving, gravity driven current of partially of duly water saturated debris that is not confined to any particular channel. They do not include young magmatic material so the transported material consists of volcanic rocks and varying amounts of moisture.
82
Where has some of the most catastrophic debris avalanche deposits that we are aware of?
Tenerife
83
What does an avalanche deposit look like?
These are made up of large blocks of material that have one composition that are often micro fractured in thin section. A typical block size is 10s of m diameter, but they can be more than 1 km across.
84
What is the most important avalanche deposit?
Socompa, Chile.
The largest avalanche block is over 3km long and 400m high. Little lateral mixing takes place as the debris lies in areas of distinct composition, which can be traced along the debris trains back to the original volcanic cone.
85
What are three types of flows from volcanoes , aside from debris avalanches?
Debris flow - no sorting, depositing sediment from dust up to blocks. They deposit from the base first.
Hyperconcentrated flood flow deposit (HFF) - more poorly sorted but have better imbricated structures than a normal fluvial deposit.
Lahar - these are the deposit of any watery flow made up of volcanic material.
86
What did Smith and Lowe, 1991, state about lahars?
That they can change from dilute to concentrated along their paths and have a range of particle support mechanisms, such as turbulence and fluid buoyancy. If the current slows it is harder to keep a turbulent flow.
87
What did Pierson and Scott, 1985, state about downstream dilution of a lahar, based on Mt St Helens?
The rapid release of a large water flood from the crater caused a debris flow. As we move downstream, we see a transition zone before moving into a hyperconcentrated stream flow.
Debris flow zone - pebbles to v coarse sand, transition zone - v coarse to coarse sand, HFF - coarse to medium sand.
88
What are the two main controls on sedimentation on volcanoes?
Eruptions - cause fragmentation and erosion, supply sediment and water.
Volcano-tectonism - change topography, cause erosion through uplift, create soft state deformation.
89
What did Kokelaar and Romagnoli, 1995, find about the transport of grains on Stromboli?
Transport away from the volcano can round grains - the grains along the beaches were rounded and those at the top of the volcano were much more angular.
90
What are magmatic eruptions driven by?
Gas exsolution within the magma, not external processes. Viscous magmas prevent gas bubbles from escaping the melt, allowing pressure build up which will drive explosions.
91
From smallest to largest, what are the main types of volcanic activity?
Hawaiian <2km
Strombolian <10km
Vulcanian <20km
Plinian <55km
92
How do we characterise Hawaiian eruptions?
Low mass flux
Small eruption column
Spatter with low dispersal, these are short lived
Low viscosity magmas
Reticulite (extreme form of pumice where the bubbles have coalesced) and pele's hair (golden basalts extended into filaments) are common
93
How do we characterise Strombolian eruptions?
Involve basaltic lavas
Deposits are low in lithics, low explosivity means country rock cannot be fragmented instead of liquid
Scoria cones form around vents from an accumulation of ejected material and repose slope scoria cones have an angle of 34'
Good palaeowind indicators, subject to wind and produce asymmetric cones
94
What are typical deposits from a Strombolian eruption?
Scoria - fragments have a light frothy texture as they are highly macrovesicular, clasts are sharp edged and angular
Spindle bombs - stretched fluid ejecta
Cow pat bombs - form when material is still liquid when it hits the ground
95
What characterises a vulcanian eruption?
Discrete eruptions with no large umbrella clouds
Eruption cloud dissipates shortly after eruption
More violent than Hawaiian or Strombolian so produce craters
Bread crust bombs are common, which land before dis articulation took place as the fragments lie together.
96
What characterises a plinian eruption?
High mass flux and high eruption column
Jet produced initially is denser than atmosphere but mixing with air decreases the density
Clasts size decreases as you move away from the eruption column
Fall deposits are common, showing high sorting and numatic equivalence
