4.2 - Eruption Styles

Question 1

What are the volcanic gases?

Answer 1

1% - 10% of magma may be dissolved gas

• Water (H20) is the most abundant gas
• Carbon dioxide (C02) is the second most abundant
• Sulfur dioxide (S02)

Question 2

What happens to the dissolved volatile content?

Answer 2

As the magma rises and the pressure on the magma decreases, the gas becomes less soluble in the melt and begins to ‘exsolve’ to form gas bubbles in the melt
(Eg. Champagne cork releasing pressure)

Question 3

Volatile content in magma

Answer 3

Volatiles such as H20, C02, C0, S02, HCl are dissolved in the magma from ,1 up to 10 wt%

• depressurising a magma causes volatiles to exsolve

Question 4

What happens to volcanic gases when expelled?

Answer 4

• The gases are expelled as magma rises (P drops)
• S02 reacts with water to form aerosol Sulfuric acid
• Gas bubbles in rock are called vesicles
• The style of gas escape controls eruption violence

Question 5

What eruption style was Mt St Helens in 1980?

Answer 5

It was a landslide uncorking volcano = pressure was released which causes a landslide of an eruption

1m3 of rhyolite with dissolved volatiles stored in a shallow magma chamber can expand to 670m3 of fragmental material and gas upon reaching atmospheric pressure

Question 6

What is fragmentation at an interface ?

Answer 6

The fragmentation of an interface volcanic eruption occurs when the molten magma within a volcano encounters a layer of solid rock or water.

The magma is unable to penetrate this layer and instead fractures, producing ash, pumice and other pyroclastic materials that are fragmented and expelled into the atmosphere.

This type of eruption is typically characterised by explosive and highly explosive activity and can pose a significant threat to nearby communities and infrastructure.

Question 7

What is the speed of ascent of magma in a pipe?

Answer 7

• It is how fast the magma is expelled from the volcano
• Speed can vary greatly depending on factors such as the viscosity of the magma, the pressure within the pipe (determined by a balance between the pressure of the magma and the weight of the overlying rock) , the size and shape of the pipe, and the presence of any obstructions (such as solidified magma or crystals) .
• Low viscosity magma can rise more quickly than higher viscosity magma and the speed of magma will be faster in larger, wider pipes.

Question 8

What are pyroclasts?

Answer 8

They are fragments of solidified volcanic material that are blasted into the air and dispersed during volcanic eruption.

Rocks are ripped from the pipe walls and propelled upwards by the expanding gas

Question 9

What controls whether near- surface magmas fragment or not?

Answer 9

Volatile concentrations and magma Rheology determine if near surface magmas fragment (blow apart into separate pieces)

• In exploding magmas, melt, crystals and accidental rock fragments are blown from the vent

Question 10

What happens if near surface magma is not fragmented?

Answer 10

Then the bubbly-bearing magmas turn into lava flows (effusively)

• Low viscosity and efficient degassing leads to effusions of magic magma (ropy and aa)

Question 11

Explosive eruptions of magic magma

Answer 11

• Low viscosity basaltic melt (typically), gas rising in conduit with magma
• Generates fire fountains, a spray of magma clots eg. Pyroclasts
• Generally low viscosity melt, gas coalesces and rises as slugs = Strombolian activity
  Bubbles burst and magma is fragmented, glowing magma pyroclasts from parabolic spray

Question 12

Fissure type (Or Icelandic) volcano

Answer 12

• The fissure type (or Icelandic) volcano has relatively gentle eruptions typically of basalt magma

Question 13

What are vulcanian style eruptions?

Answer 13

Vulcanian style = cannon like explosions which exit velocity of 200-300m/s

• Has high viscosity magmas (eg. Andesite and rhyolite) considerable dissolved gas so severley explosive
• In Papua New Guinea, the mount tavurvur volcano its blast compresses water vapour in front of it, creating temporary clouds

Question 14

What happens at volcanian style eruptions?

Answer 14

• Plug of rock on top of magma exsolving a lot of gas, causes very large gas pressure to develop in the magma
• Eventually the plug strength is exceeded and pyroclasts of magma and fragmented plug erupt violently
• This is followed by pyroclasts of principally ash and papilla (small stones)

Question 15

What are pilinian style eruptions?

Answer 15

They have a very large dissolved gas content, which violently explodes (paroxysmal/colossal) - rhyolite magma

• Sustained, continuous jet of gas and pyroclasts, rather than detonations. High flux of undegassed magma
• The eruption is sustained for a long time as fragmentation level migrates downwards with time into the chamber. The evolving has bubbles create a froth. When it erupts there is lots of pumice fragments in the eruption
• The exit velocity is up to 1000m/s

Question 16

What is an example of a pilinian volcano?

Answer 16

The pilinian eruption at Calbuco volcano in Chile, 2015

• The eruption was sustained and ash and pumice fell out

Question 17

What are Pelean style eruptions?

Answer 17

They have a large gas content but cannot sustain an eruptive column

• Typically associated with andesite/rhyolite magma, and produces ash/pyroclastoc density current (PDC) and ash fall
  ( such as at Mt St Helens)
• Temperature in flow 400-700 degrees
• 20% gas and 80% magma clots and solidified magma clasts

Question 18

Where was there a dome collapse and PDC’s?

Answer 18

At Mt Unzen, Japan there was a dome collapse and PDC’s ( pyroclastic density current)

Mt St Helens 1980 eruption

Question 19

Can there be a sudden transition in eruption style?

Answer 19

Yes

• Such as at volcan de colima where the lava dome overstepped the edge of the crater and it broke apart. This reduced the pressure on the magma which lead to explosive eruptions

Question 20

What affects eruption styles as well as Rheology and the dissolved volatile content?

Answer 20

The magma flux (ascent rate)

This explains transition from Plinian to Palean eruptions

Question 21

What is the degree of melt-vapour segregation controlled by?

Answer 21

It is controlled by dissolved volatile content and viscosity

Question 22

What are the characteristics of magic systems with low viscosity magmas?

Answer 22

• Gas pistoning
• Lava splattering - Strombolian
• Lava flows
• Lava fountaining
• Subplinian-plinian eruptions (rare)

Question 23

What are characteristics of silicic system eruptions with high viscosity magmas?

Answer 23

• Gas fluxing
• Lava dome collapse
• Vulcanian
• Lava flows-domes
• Subplinian-plinean eruptions

Question 24

What happens when there is an interaction with water

Answer 24

• Adding water increases the explosiivity of any eruption
• There is a huge difference in temperatures between liquid water and magma

The water heats up violently and turns to steam (expands)
The magma cools down violently to become solid (it contracts and shatters)

Question 25

What is a phreatic eruption?

Answer 25

Magma and groundwater interaction

It is an eruption driven by the heat from magma interacting with water. Phreatic eruptions pulverise surrounding rocks and can produce ash, but do not include new magma

Question 26

What is a phreatomagmatic eruption?

Answer 26

Magma interacting with surface water eg. Lakes and glaciers
It is an eruption resulting from the interaction of new magma or lava with water and can be very explosive

Question 27

What happened at the Eyjafjalljokull volcano in Iceland in 2010?

Answer 27

There was a very explosive basaltic eruption

• Water from the glacier above the volcano made it more explosive
• This created particularly fine ash (bad for aircraft engines and disrupted flights)

Question 28

When do the most explosive eruptions occur?

Answer 28

When large volumes of high viscosity magmas (eg. Rhyolites) with a high dissolved volatile content are brought to the surface rapidly (Plinian eruptions)

Question 29

What magmas are more efficient at degassing volatiles?

Answer 29

Low viscosity magic magmas are more efficient as degassing volatiles than high viscosity silicic magmas and hence tend to produce lava flows.